Quantifying the Impacts of Warm Mix Asphalt. on Constructability and Performance

Transcription

1 Quantifying the Impacts of Warm Mix Asphalt on Constructability and Performance By: Andrew John Hanz A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Civil & Environmental Engineering) University of Wisconsin Madison 2012 Date of Final Oral Examination: 06/06/2012 The dissertation is approved by the following members of the Final Oral Committee: Hussain U. Bahia, Professor, Civil and Environmental Engineering Steven M. Cramer, Professor, Civil and Environmental Engineering Tuncer B. Edil, Professor, Civil and Environmental Engineering Daniel J. Klingenberg, Professor, Chemical and Biological Engineering Josè A. Pincheira, Associate Professor, Civil and Environmental Engineering

2 Copyright by Andrew Hanz, 2012 All rights reserved

3 i Acknowledgements I would like express gratitude to my advisor, Professor Hussain Bahia for his support and guidance in completing this study, the contribution of my doctoral committee, Professors Steven Cramer, Jose Pincheira, Tuncer Edil, and Dan Klingenberg is also recognized and greatly appreciated. I would like to thank my co-workers for creating such a diverse and enjoyable work environment and my colleagues for providing a forum to communicate results through interaction with external technical experts. I feel very fortunate to have had the opportunity to share my work and develop relationships with all those involved in this research both on and off campus. Most importantly I would like to express thanks to my family and friends, their support and encouragement made it possible to achieve this goal.

4 ii Abstract Joint efforts by the federal government and HMA industry to minimize cost and the environmental impacts of asphalt pavement construction have resulted in development of warm mix asphalt (WMA). This technology has gained such interest due to the potential to deliver pavements at lower temperatures, allowing for reduced energy consumption and emissions. In order to be effective WMA must meet the specified values of in-place density at reduced temperatures during construction and demonstrate sufficient resistance to pavement distresses while in-service. The overall objective of this research was to promote effective use of WMA through development of a procedure to recommend additive specific mixing and compaction temperature ranges that will provide adequate workability during construction, and an acceptable level of in-service performance. To pursue this objective an experiment was designed to evaluate the effects of various WMA technologies on the workability and performance properties of asphalt binders and mixtures using existing standards and new test methods developed during the study. The new test methods were pursued to better define the role of asphalt binder as a lubricant during compaction and to properly account for the effects of reduced production temperatures on asphalt binder performance and potential for moisture damage. Results found that use of WMA impacts both construction and performance properties. To account for these factors an evaluation framework to select appropriate production temperatures for WMA was introduced that is based on evaluation of mixture volumetrics, compactability, resistance to moisture damage, and rutting performance. Furthermore, to facilitate the mixture design and temperature selection process surrogate test methods to evaluate workability and performance properties of the asphalt binder as well as the integrity of the bond at the asphalt/binder aggregate interface were proposed and verified through relationships with mixture performance.

5 iii Executive Summary Joint efforts by the federal government and HMA industry to minimize cost and the environmental impacts of asphalt pavement construction have resulted in development of warm mix asphalt (WMA). This technology has gained such interest due to the potential to deliver pavements at lower temperatures, allowing for reduced energy consumption and reduced emissions. In order to be effective WMA must meet the specified values of in-place density at reduced temperatures during construction and demonstrate sufficient resistance to pavement distresses while in-service. This study was designed to evaluate the ability of SuperPave mixture design and asphalt binder grading procedures to account for the effects of WMA. The experiment included a wide range of WMA additive types a range of production temperatures to assess the combined effects of reduced temperatures and WMA additives on the workability and performance properties of asphalt binders and mixtures. Two levels of aggregate gradation and asphalt binder modification were included to assess interactions between the effectiveness of the WMA additives and properties generally varied in the mix design. Impacts of WMA on Workability In mix design for conventional HMA selection of appropriate mixing and compaction temperatures is based on the relationship between asphalt binder viscosity and temperature. Use of this approach for estimating the effects of WMA was deemed insufficient as the viscosity based methods severely under-predicted production temperatures. Furthermore, viscosity was found incapable for use to establish threshold values of mixture density as wide variation in values of viscosity that correspond to air void content at a given compactive effort were observed.

6 iv As a replacement or compliment to viscosity the concept that it is necessary to evaluate the role of the asphalt binder as a lubricant in thin films under pressure was introduced. To support this effort the Asphalt Lubricity Test was developed as a means to measure the internal friction of asphalt binders. It was demonstrated that the test is repeatable and sensitive to asphalt binder modification, presence of WMA additives, and lower testing temperatures. However, due to the use of smooth surfaces during testing and other factors the test method was proven to be limited to the evaluation of material behavior in the transition from mixed to hydrodynamic lubrication. In mixtures it was proposed that this type of behavior relates to compaction at lower temperatures and/or higher air void contents. The measurements of the Asphalt Lubricity Test were correlated to mixture workability at compaction temperatures of 85 C and 115 C, therefore, it was deemed a potential surrogate measure for estimating the change asphalt mixture density due to lower compaction temperatures. To limit this change a maximum threshold of 1.25 was established for the ratio the coefficient of WMA at reduced temperatures to conventional HMA. Using this approach the Asphalt Lubricity Test was deemed a viable test method to screen combinations of WMA additive types, WMA additive concentrations, and compaction temperatures. It was not recommended as a replacement to mixture testing, as aggregate gradation was also found to significantly influence workability. Two modifications to the current WMA mix design guidelines presented in NCHRP Report 691 were recommended. The requirement to measure the density vs. temperature relationship for conventional HMA compacted at lower temperatures was added to better establish the need for WMA additives and define the ranges in compaction temperatures at which they are most effective. Also, a modification to the compactibility requirement was proposed to

7 v better align the mix design procedure with the requirement that WMA meets or exceeds the performance of HMA by ensuring that the WMA compacted at the selected range of temperatures achieves a level of density similar to that of HMA compacted at conventional temperatures. Impacts of WMA on Moisture Damage In this study the potential that reduced bonding due to lower production temperatures contributes to increased moisture sensitivity for WMA was investigated. To measure this effect, the Bitumen Bond Strength (BBS) test was used to apply conventional and WMA modified binders to aggregate substrates heated to different temperatures. Results found that reduced application temperatures and the presence of WMA additives significantly impacted wet bond strength but had no effect on dry strength. WMA additives demonstrated an ability to offset the loss of strength caused by lower application temperatures. To best communicate the loss in bond strength due to moisture conditioning the bond strength ratio (BSR) as defined by the ratio of wet to dry bond strength was introduced as the parameter selected to evaluate materials. In comparison to mixture performance, a general relationship between the BSR and the ratio of conditioned to dry dynamic modulus (ESR) was observed, however, in some instances the BSR and ESR produce different ranking of materials. Although wet bond strength as measured by the BBS test was deemed insufficient to fully explain the reduced moisture damage resistance observed for WMA results found that it was sensitive to the presence of WMA additives and capable of measuring their potential to offset the detrimental effects of conventional binders produced at lower temperatures. Based on this finding a minimum BSR value of 0.65 was established to ensure selection of moisture resistant combinations of WMA additives and production temperatures. Similar to the Asphalt Lubricity

8 vi test, it was recommended that the BBS test be applied as a tool to screen WMA additives, thus verification of performance through mixture testing is still required. Impacts of WMA on Performance Assuming adequate density is achieved and other construction requirements are met the in-service performance of the pavement is in part dictated by the performance properties of the asphalt binder. It was found that the changes to conventional practice used in construction of WMA, specifically, reduced temperatures and use of additives, caused reduced age hardening, resulting in asphalt binders that have lower stiffness relative to those used in conventional HMA. As a result, resistance to permanent deformation was identified as the behavior dictating binder selection guidelines, as softer binders decrease rutting resistance. Conversely, marginal improvements in performance were observed for intermediate and low temperature properties, indicating that WMA does not have a detrimental effect on mixture cracking performance, thus no changes to current grading protocols were deemed necessary. The need for revised high temperature performance evaluation procedures was confirmed through correlation of mixture and binder rutting resistance as measured by the Flow Number and MSCR tests respectively. A strong linear relationship was observed with increase in J nr of approximately 1 kpa -1 causing a reduction in Flow Number of approximately 150 cycles. The ratio of performance of HMA aged under standard conditions and WMA aged at reduced temperatures was defined as a way to communicate the performance impacts of using WMA. The MSCR test was applied in a similar fashion to estimate mixture performance using asphalt binder properties. Based on the requirement that WMA must meet or exceed the performance of conventional HMA, a J nr ratio of 1.0 was proposed. In addition, recommended modifications to

9 vii the current binder grading protocols specified in AASHTO M320 were provided to promote use of the MSCR test after aging at standard conditions and short term aging at 130 C. Recommendations It was recommended that selection of appropriate WMA additives and production temperatures be based both on constructability and performance. In addition, various tools to evaluate asphalt binder properties were introduced as a compliment to generally accepted mixture tests to support proper design of WMA additives. In regards to constructability it was deemed necessary to evaluate WMA based on the ability to meet volumetric criteria at the design compaction temperature and compactability at lower temperatures. The Asphalt Binder Lubricity Test was introduced as a surrogate measure of compactability useful for screening WMA additives types and temperatures. Resistance to moisture damage and rutting at high pavement temperatures were identified as limiting performance properties that must be considered in WMA mixture design. In mixture testing this study recommends the use of the Dynamic Modulus Ratio to assess moisture damage and the Flow Number to measure resistance to rutting. Surrogate or complimentary test methods were introduced to estimate both aspects of mixture performance through use of the BBS test and the MSCR test on binders aged at conditions representative of the productions temperatures used for WMA.

10 viii Table of Contents Acknowledgements... i Abstract... ii Executive Summary... iii 1 Introduction Background and Problem Statement Hypotheses Objectives Research Methodology Literature Review Review of WMA Technologies Overview of WMA-Related Research Impacts of WMA Additives on Workability Impacts of WMA on Performance Potential Impact of WMA on Reduction of Energy Consumption and Emissions Experimental Methods Overview Materials Selection Evaluating the Impacts of WMA Additives on Asphalt Binder Workability Evaluating the Impacts of WMA on Moisture Damage Evaluating the Impacts of WMA Additives and Reduced Binder Aging on Performance Impacts of WMA on Asphalt Binder and Mixture Workability Asphalt Binder Workability Asphalt Mixture Workability and Verification of the Asphalt Lubricity Test Moisture Susceptibility of WMA Overview Application of the BBS Test to Evaluate the Bond at the Asphalt/Aggregate Interface Evaluation of Impact of WMA on Moisture Damage Using Mixture Dynamic Modulus (E*) Relationship Between Bond Strength and Mixture Performance Effects of WMA on Asphalt Binder Performance Development of a Short Term Binder Aging Method Applicable to WMA Impacts of WMA on Asphalt Binder Performance Summary of Findings Impacts of WMA Additives on Workability Impacts of WMA Additives on Moisture Damage Impacts of WMA on In-Service Performance Conclusions and Recommendations Overview Conclusions Recommendations References Appendix A: Detailed Mix Design Information

11 ix Appendix B: Supporting Data for Development of the Asphalt Lubricity Test Appendix C: Supporting Data for the Application of the BBS Test to WMA Appendix D: Supporting Data for Effects of WMA Additives on In-Service Performance

12 x List of Figures Figure 2-1: Fuel Consumption and CO 2 Emissions for the Heating of One Ton of Wet Aggregates (10)... 9 Figure 2-2: Effect of Foamant Water on Expansion Ratio and Foam Half Life for a Typical Asphalt (11) Figure 2-3: Relationship between Expansion Ratio and Asphalt Binder Viscosity (11) Figure 2-4: Transition from Bulk to Thin-Film Lubricating Properties (39) Figure 2-5: Conceptual Representation of the Stribeck Curve Identifying Four Regimes of Lubrication (40) Figure 2-6: Impact of Using Conventional Stribeck Diagram to Evaluate the Behavior of Non-Newtonian Fluids (40) Figure 2-7: Effect of Surface Texture on the Stribeck Diagram (40) Figure 2-8: Friction Maps and Dependence on Applied Contact Stress (41) Figure 2-9: Photographs of the new Lubricity Testing Fixture Machined for the TA DSR: (a) Lower cup that holds lubricant and lower balls, (b) Screw Assembly is used to clamp the lower balls into place Figure 2-10: Hogentogler Compaction Stages of Wetting (42) Figure 2-11: Relationship between Air Voids and Compaction Temperature for HMA and Various WMA Additives and Concentrations (3) Figure 2-12: Schematic Showing the PDA Plate and Definition of the Construction Force Index Figure 2-13: Effect of WMA Additive Type and Compaction Temperature on Mixture Workability CFI (4) Figure 2-14: Variation in SARA Fractions due to Asphalt Binder Source SHRP Core Asphalts (45) Figure 2-15: Relationship Between Asphaltenes and Viscosity and the Effects of Asphalt Binder Source and Level of Aging (46) Figure 2-16: Dynamic Viscosity vs. Aging Time for SHRP Asphalts TFO Aged, then PAV Aged at 60 C (46) Figure 2-17: Effect of Reduced Binder Aging on HT Continuous Grade (50) Figure 2-18: Reduction in High Temperature Asphalt Performance due to Reduced Production Temperatures for WMA Figure 2-19: Summary of Aging Index for Various WMA Additives (50) Figure 2-20: Effect of WMA Additive Type and Concentration on Wet Hamburg Test Results Using Dry Aggregate (6) Figure 2-21: Influence of Reduced Production Temperatures and Aggregate Moisture on Indirect Tensile Strength and TSR (7) Figure 2-22: Effect of Mixture Short Term Aging Temperature, WMA Additive Type, and WMA Concentration on the FN Parameter (7) Figure 2-23: Effects of Aging Temperature and Increased Asphalt Binder Grade on %Permanent Strain vs. Number of Cycles (50) Figure 2-24: Qualitative Effects of Reduced Aging Temperatures on Mixture Rutting Resistance (50) Figure 2-25: Existing Models to Capture the Relationship Between Mix Temperature and Fuel Consumption (1).. 70 Figure 2-26: Carbon dioxide emissions at HMA plants for various mix temperatures, generated using the World Bank greenhouse gas emissions calculator. Shifts in emissions are shown for different fuel types and aggregate moisture contents (MC). (1) Figure 3-1: Gradation Curves for Fine and Coarse Mix Designs Plotted on the 0.45 Power Maximum Density Graph Figure 3-2: Effect of the α Parameter on Gradation Modeled Using the Weibull Distribution (60) Figure 3-3: Effect of the β Parameter on Gradation Modeled Using the Weibull Distribution (60) Figure 3-4: Gyratory Compactor Troxler Model 5850 ( 88 Figure 3-5: Schematic of Gyratory Compactor Figure 3-6: Example of Compaction Curve and Identification of SuperPave Requirements Figure 3-7: Bitumen Bond Strength Testing Device Figure 3-8: Picture and Schematic of Newly Designed BBS Pull-Off Stubs Figure 3-9: Schematic of BBS Test Assembly at the Asphalt/Substrate Interface Figure 3-10: Effects of Moisture Conditioning on the Pull Off Pressure vs. Time Relationship and Definition of the POTS Figure 3-11: Example of Cohesive Failure (Left) and Adhesive Failure (Right) on Granite Substrate Figure 3-12: Configuration of Sample Pans and Aging Times to Assess Impacts of Oven Position on Oven Temperature and Asphalt Binder Performance (65) Figure 3-13: Validation of DSR to Estimate BBR Measured vs. Predicted Stiffness

13 Figure 3-14: Validation of DSR to Estimate BBR Measured vs. Predicted Stiffness Figure 3-15: Schematic of the SENB Testing Geometry (70) Figure 4-1: Use of Viscosity Ratio to Evaluate the Effect of WMA Additive Type and Concentration on the Viscosity Temperature Profile Unmodified Binders (PG 64-22) Figure 4-2: Use of LSV Viscosity Ratio to Evaluate the Effect of WMA Additive Type and Concentration on the Viscosity Temperature Profile Modified Binders (PG 76-22) Figure 4-3: Effect of WMA Additives with Testing Temperature PG Series Binders Figure 4-4: Effect of WMA Additives with Testing Temperature PG Series Binders Figure 4-5: Estimated Stribeck Curve for PG Series Binders Figure 4-6: Estimated Stribeck Curve for PG Series Binders Figure 4-7: Effect of WMA Additive Type and Concentration Coefficient of Friction vs. Temperature for PG Series Binders Figure 4-8: Effect of WMA Additive Type and Concentration Coefficient of Friction vs. Temperature for PG Series Binders Figure 4-9: Effect of Gradation and Binder Type on Sensitivity to Compaction Temperature of Control Mixes - % Air Voids at N des Figure 4-10: Effect of Gradation and Binder Type on N92 Control Mixes Figure 4-11: Effect of Additive Type and Concentration - Summary of Air Voids at Ndes Figure 4-12: Effect of WMA Additives and Compaction Temperature on Air Voids at Nd es PG 64 Series Binder and Coarse Gradation Figure 4-13: Effect of WMA Additives and Compaction Temperature on Air Voids at Nd es PG 76 Series Binder and Coarse Gradation Figure 4-14: Effect of WMA Additive Type, and Compaction Temperature on N92 PG 64 Binder/Coarse Gradation Figure 4-15: Effect of WMA Additive Type, and Compaction Temperature on N92 PG 76 Binder/Coarse Gradation Figure 5-1: Application of the BBS Test to WMA Dry Pull Off Strength Figure 5-2: Application of the BBS Test to WMA Wet Pull Off Strength Figure 5-3: Effects of Application Temperature and WMA Additives on Bond Strength Ratio (BSR) Figure 5-4: Effects of Aging Temperature, Aging Time, and WMA Additives on Unconditioned Dynamic Modulus Figure 5-5: Effects of Aging Temperature, Aging Time, and WMA Additives on Unconditioned Dynamic Modulus Figure 5-6: Measured vs. Predicted Conditioned Dynamic Modulus Figure 5-7: Relationship Between Bond Strength Ratio and Mixture Performance Figure 6-1: Effect of Aging Method and Temperature on Asphalt Binder HT Performance Properties Figure 6-2: Effect of WMA Additives on Short and Long Term Aging Indices PG 64 Series Materials Figure 6-3: Effect of WMA Additives on Short and Long Term Aging Indices PG 76 Series Materials Figure 6-4: Effect of Reduced Oxidation on Change in HT Continuous Grade PG 64 Series Binders Figure 6-5: Effect of Reduced Oxidation on Change in HT Continuous Grade PG 76 Series Binders Figure 6-6: Evaluation of the Impacts of WMA Additives and Reduced Aging Temperatures on J nr vs. % Recovery for PG Series Binders Figure 6-7: Consideration of the Combined Effects of Reduced Short Term Aging Temperature and WMA Additives on Binder Fatigue Performance after PAV Aging Figure 6-8: Combined Effects of Reduced Short Term Aging Temperature and WMA Additives on Binder Low Temperature Performance after PAV Aging S(60) Figure 6-9: Combined Effects of Reduced Short Term Aging Temperature and WMA Additives on Binder Low Temperature Performance after PAV Aging m(60) Figure 6-10: Effect of WMA Additive Type and Short Term Aging Temperature on SENB Load vs. Displacement Curves Figure 6-11: Effect of WMA Additives and Short Term Aging Temperature on Fracture Energy (G f ) after PAV Aging Figure 7-1: Measured vs. Predicted N92 for the Coefficient of Friction and Gradation Model Figure 7-2: Air Voids at N des vs. Asphalt Binder Viscosity Figure 7-3: Relationship Between N92 Ratio and Coefficient of Friction Ratio Figure 7-4: Relationship Between Bond Strength Ratio and Mixture Performance xi

14 xii Figure 7-5: Relationship Between Mixture Flow Number and Asphalt Binder J nr (50) Figure 7-6: Binder Selection Criterion Based on Change in Asphalt Mixture Performance Relative to Conventional HMA Figure A- 1: Fine Gradation Plotted on 0.45 Max Density Graph Figure A- 2: Measured Gradation vs. Weibull Predicted Gradation Figure A- 3: Summary of Mixture Volumetrics vs. Asphalt Content- Fine Gradation Figure A- 4: Coarse Gradation Plotted on 0.45 Max Density Graph Figure A- 5: Measured Gradation vs. Weibull Predicted Gradation Figure A- 6: Summary of Mixture Volumetrics vs. Asphalt Content- Coarse Gradation Figure B- 1: Measured vs. Predicted - % Air N ini Figure B- 2: Measured vs. Predicted N Figure B- 3: Measured vs. Predicted - % Air N des Figure C- 1: Dependence of G* on Short Term Aging Temperature PG 64 Series Materials Figure C- 2: Dependence of G* on Short Term Aging Temperature PG 76 Series Materials Figure C- 3: Relationship between Asphalt Binder Stiffness and Dry Pull Off Strength Figure C- 4: Relationship between Asphalt Binder Stiffness and Wet Pull Off Strength Figure D- 1: Effects of WMA Additive Type and Aging Temperature on the HT Performance PG 64 Series Binders Figure D- 2: Effects of WMA Additive Type and Aging Temperature on the HT Performance PG 76 Series Binders Figure D- 3: Effect of WMA Additives and Reduced Aging Temperature on Relationship between Jnr and Stress Level PG 64 Series Materials Figure D- 4: Effect of WMA Additives and Reduced Aging Temperature on Relationship between Jnr and Stress Level PG 76 Series Materials *W-4% tested at 70 C Figure D- 5: Effect of WMA Additive Type and TFA Aging Temperature prior to PAV Aging on SuperPave Fatigue Parameter PG 64 Series Binders Figure D- 6: Effect of WMA Additive Type and TFA Aging Temperature prior to PAV Aging on SuperPave Fatigue Parameter PG 76 Series Binders Figure D- 7: Normalized G*sinδ vs. Strain PG 64 Series Materials Figure D- 8: Normalized G*sinδ vs. Strain PG 76 Series Materials Figure D- 9: Effect of Short Term Aging Temperature on Long Term Aged Low Temperature Performance Properties PG 64 Series Binders Figure D- 10: Effect of Short Term Aging Temperature on Long Term Aged Low Temperature Performance Properties PG 76 Series Binders

15 xiii List of Tables Table 2-1: Summary of Mechanisms and Comparison of Production Temperatures for a Variety of Warm Mix Additives... 8 Table 2-2: % Mass Loss for Advera as a Function of Production Temperature (15) Table 2-3: Legend for Figure Table 2-4: Concept of Effective Viscosity Table 2-5: Aggregate Properties Influencing Mixture Workability Table 3-1: Summary of Selected WMA Additives and Concentrations Table 3-2: Summary of Aggregate Source Properties for Selected Aggregate Sources Table 3-3: Experimental Plan for Sensitivity Analysis and Evaluation of Repeatability of the Asphalt Lubricity Test Table 3-4: Experimental Design for Evaluation of the Effects of WMA Additive Type and Concentration on Asphalt Binder Lubricity Table 3-5: Experimental Design for Mixture Workability Testing to Validate the Asphalt Lubricity Test Table 3-6: Asphalt Binder Properties Influencing Mixture Workability Table 3-7: Models Required and Threshold Values for Determining Binder Property Limits Table 3-8: NCHRP 9-43 Criteria to Verify Mixing and Compaction Temperatures for WMA (20) Table 3-9: Experimental Design Application of the BBS Test to Evaluate the Effects of WMA Additives and Reduced Application Temperatures Table 3-10: Summary of Mixes for Performance Testing and Sample Preparation Conditions (Same for both PG 64 and PG 76 Series Binders) Table 3-11: Material Combinations for Evaluation of Impacts of WMA on Asphalt Binder Properties Table 3-12: Asphalt Binder Performance Testing to Evaluate the Effects of WMA and Reduced Oxidation using the Dynamic Shear Rheometer Table 4-1: Estimated Mixing and Compaction Temperatures for Control and WMA Binders AASHTO T Table 4-2: Estimated Mixing and Compaction Temperatures for PG Control and WMA Binders LSV Procedure Table 4-3: Summary of Evaluation of Repeatability of the Asphalt Lubricity Test Based on Pooled Standard Deviation Table 4-4: Summary Analysis of Variance for Development of the Asphalt Lubricity Test Table 4-5: Summary of Tukey Comparison for the Interaction between Binder Grade and Testing Temperature All Material Types Table 4-6: ANOVA Analysis of the Effect of WMA Additive Type and Concentration PG and PG Binders Table 4-7: Proposed Revisions to WMA Mix Design Guidance for Determination of Compaction Temperatures. 148 Table 4-8: Estimated Compaction Temperatures for WMA Using Revised Mix Design Procedure Coarse Gradation Table 4-9: Results of Regression Analysis to Identify the Factors Contributing to % Air Voids at Nini, N92, and % Air Voids at Ndes for all Mix Types and Compaction Temperatures N= Table 4-10: Application of Best Subsets Regression to Define the Impacts of Asphalt Binder Viscosity and Coefficient of Friction on Regression Model Precision Table 5-1: BBS Testing Results ANOVA for Dry and Wet Pull of Tensile Strength Table 5-2: Application of Best Subsets Regression to Identify Experimental Factors Influencing Unconditioned Dynamic Modulus Table 6-1: Summary of Critical TFA Aging Temperatures for Control and WMA Modified Binders Table 6-2: Effect of WMA Additive and Thin Film Oven Aging Temperature on MSCR Performance at a Stress Level of 3.2 kpa -1 and AASHTO MP19 Performance Grade Table 7-1: Verification of the Asphalt Binder Lubricity Test to Assess the Impacts of WMA on Mixture Workability Table 7-2: Recommended Mix Design Procedure to Estimate WMA Compaction Temperatures Table 7-3: Asphalt Binder Grading Specification for Conventional and WMA Modified Binders at HMA Aging Temperatures Table 7-4: Additional Tests Required for WMA Modified Binders at WMA Aging Temperatures Table 8-1: Framework for Selection of Appropriate WMA Production Temperatures

16 xiv Table A- 1: Summary of Aggregate Gradation and Properties Fine Mix Design Table A- 2: Summary of Weibull Parameters and Fitting Statistics Table A- 3: Granite Fine Mix Design Information Table A- 4: Summary of Volumetric Properties at Optimum Asphalt Content Table A- 5: Summary of Aggregate Gradation and Properties Coarse Mix Design Table A- 6: Summary of Weibull Parameters and Fitting Statistics Table A- 7: Granite Coarse Mix Design Information Table A- 8: Summary of Volumetric Properties at Optimum Asphalt Content Table B- 1: Results of Tukey Comparison for the WMA Additive Type/Testing Temperature Two Way Interaction Table B- 2: Summary of Model Fitting Parameters and Evaluation of Accuracy for Stribeck Curve Fitting Based on Experimental Data Table C- 1: Summary of Complete Data Set Collected for the BBS Test Table D- 1: Summary of Evaluation of the Effect of Oven Position on Asphalt Binder Performance Properties (65) List of Equations Equation 2-1: Definition of the Gumbel Number Equation 2-2: Calculation of Asphalt Binder Coefficient of Friction Equation 3-1: Weibull Distribution to Model Aggregate Gradation Equation 3-2: Relationship between Shear (DSR) and Creep (BBR) Testing Conditions Equation 3-3: Estimate of BBR Stiffness Based on DSR Shear Properties Equation 3-4: Estimate of BBR m-value Based on DSR Shear Properties Equation 4-1: Polynomial Model Used to Model Stribeck Curve Equation 4-2: Proposed Revision to the Compactability Requirement for WMA Equation 5-1: Linear Regression Equation for Prediction of Unconditioned Dynamic Modulus Equation 6-1: Short Term Aging Index (AI) Equation 6-2: Long Term Aging Index (AI) Equation A- 1: Calculation of Voids in Mineral Aggregate (VMA) Equation A- 2: Calculation of Voids Filled with Asphalt (VFA)

17 1 1 Introduction 1.1 Background and Problem Statement In recent years the escalating cost of energy and the green building movement have created a great interest in introducing new technologies in the road building industry. Joint efforts by the federal government and HMA industry to minimize cost and the environmental impacts of asphalt pavement construction have resulted in development of warm mix asphalt (WMA). This technology has gained such interest due to the potential to deliver pavements at lower temperatures, allowing for reduced energy consumption and reduced emissions. Quantitatively, World Bank estimates indicate that for every 10 decrease in production temperature, savings of nearly 1 L of fuel oil and 1 kg of CO 2 emissions are realized per ton of HMA mix produced (1). Furthermore, results of numerous field trials indicate that a variety of WMA additives have allowed for production at temperatures 15 C - 40 C lower than conventional HMA with no premature failures (2). These field trials cite the potential for WMA to benefit performance in the following ways: More uniform mat compaction: due to the increased workability and reduced temperature susceptibility of WMA. Increased mixture durability: Lower production temperatures lead to reduced oxidative aging of the asphalt binder during mixing, in turn providing additional long term durability of the WMA mix. Higher Recycled Asphalt Pavement (RAP) contents in mixes: Enhanced workability allows for improved constructability of high RAP mixes. Successful field trials coupled with the perceived performance related and environmental benefits prompted a need to facilitate the use of WMA through development of appropriate test

18 2 procedures and specifications. Significant barriers to implementation of WMA exist in terms of evaluation of mixture workability and quantifying the impacts of the reduced production temperatures on performance. In regards to workability, the effect of reduced temperatures on the ability to achieve density and the specific influence of different WMA additives remains undefined. Conventional practice for HMA requires mix production at a narrow range of temperatures defined based on a simplistic relationship between binder viscosity and temperature. Asphalt binder viscosity is a material property while the production temperature, asphalt binder content, and gradation are defined in the mix design process. Recent research studies reported that the reduction in production temperature due to the use of WMA additives predicted using conventional viscosity based methods underestimates the workability and the densification behavior observed in the field (3), (4). It is also observed by many experts that aggregate gradation and mixture volumetrics play a significant role in densification. These observations indicate that viscosity is not the only material property contributing to mixture workability and that there is a need to further define the role of the asphalt binder as a lubricant in mixture compaction (5). As a result of these shortcomings current test methods and mix design methodologies are unable to quantify the effects of WMA additives. Due to this lack of understanding state agencies are unable to provide compaction temperature guidelines in specifications for the use of WMA. Also materials producers lack the technical information required to optimize amount of additives needed, necessary changes in asphalt binder content, and role of mixture gradation to supply a cost effective mix that meets density requirements at the recommended lower WMA temperatures.

19 3 In regards to performance of mixtures under service traffic, the effects of reduced production temperatures is known to reduce short term oxidative aging and possibly increase moisture retained by aggregates. Furthermore the presence of WMA additives could change the resistance of mixtures to moisture damage and modify the effect of less binder aging on pavement performance. Increased potential for moisture damage has been cited as a potential concern in WMA through results of numerous laboratory studies involving evaluation of mixture performance (6), (7), (8), (9). However, the mechanisms driving this decrease in performance remain undefined, thus an opportunity exists to provide more clarity to this issue through evaluation of the relationship between development of asphalt/aggregate adhesion and production temperature. Defining this relationship will provide a means to determine if it is necessary to incorporate an adhesion test into current WMA specifications and provide recommendations for production temperatures and WMA additives that decrease the potential for WMA moisture susceptibility. Asphalt binders are currently accepted according to the SuperPave binder grading system. The thresholds used in this system were developed under the assumption of material production at conventional HMA temperatures. The reduced production temperatures associated with WMA and potentially the presence of WMA additives result in less oxidation of the asphalt binder during mix production. Conceptually, decreased oxidation has the potential to be a detriment to early performance due to increased potential for rutting, but a benefit in the long term due to improved durability. To properly implement the use of WMA, consideration of these effects must be integrated into the design process through a revised binder grading and mixture design system for WMA that addresses both the presence of WMA additives and the

20 4 effects of aging temperature. This will enhance the reliability of the WMA mix designs specified by state agencies to adequately resist mixture rutting and low temperature cracking. In evaluating the sustainability of WMA relative to HMA the reduced energy consumption and emissions realized through use of WMA during production alone are inadequate. Successful implementation requires mix design procedures that properly define appropriate temperature ranges that allow for mixture compaction at no detriment to short term performance in terms of both rutting resistance and moisture susceptibility. Furthermore, the benefits of reduced oxidation to long term mixture durability must be quantified to determine if these effects extend pavement service life. Use of an approach that establishes the relationship between mixture workability and performance as a function of temperature reduction allows for complete assessment of the environmental impacts of WMA. 1.2 Hypotheses The following hypotheses are proposed: Viscosity of binders is not sufficient to explain effects of warm mix additives on workability of mixtures. Measurement of asphalt binder internal friction (lubricity) using an adaptation of the Dynamic Shear Rheometer currently used in the standard testing protocols is possible and necessary to quantify the impacts of WMA additives on mixture workability. Internal friction measured in thin films and viscosity of binders dictate the optimum temperature at which mixture density is achieved. These two measures can be used to optimize the type and amount of additives, and determine the appropriate production and construction temperatures.

21 5 The reduced temperatures used in WMA and presence of WMA additives will result in reduction of aging, which significantly impacts the performance properties of WMA. A relationship between performance properties and aging temperature can be developed based on the aging index of the asphalt binder with and without use of the WMA additive. This relationship is needed to adjust grade selection to accommodate the effects of reduced production temperatures. The reduced production temperatures associated with WMA could negatively impact the development of the adhesive bond at the asphalt/aggregate interface, potentially increasing the moisture susceptibility of the WMA mix. Application of the Bitumen Bond Strength (BBS) Test to conventional and WMA modified binders applied to aggregate substrates heated to different temperatures will provide a means to quantify the loss of adhesion due to reduced temperatures and allow for consideration of resistance to moisture damage in WMA mix design. 1.3 Objectives The overall objective of this research is to develop a procedure to recommend additive specific mixing and compaction temperature ranges for WMA that will provide adequate workability during construction, and an acceptable level of in-service performance. This objective will be achieved through completion of the following tasks: Constructability: Define tests and analyses to determine relative effects of binder viscosity, lubricity, and aggregate gradation on typical mixture densification behavior in the lab. This will lead to establishing a protocol to estimate appropriate mixing and compaction temperature ranges for a given WMA additive.

22 6 In-Service Performance: Based on the range of reducing production temperatures, establish guidance as to when binder grade change or use of adhesion promoting modifiers is necessary to ensure that the intended performance of the asphalt binder and mixture is maintained. 1.4 Research Methodology The objectives of this research will be met through completion of the following tasks. Task 1: Literature Review o Mechanisms used by WMA additives for temperature reduction. o Overview of WMA additives currently in use. o Energy and emissions reduction associated with reduced production and construction temperatures. Task 2: Experimental Design and Testing Task 3: Data Analysis o Modeling of Allowable Temperature Reductions based on performance and workability. Task 4: Development of Recommendations for Implementation.

23 7 2 Literature Review 2.1 Review of WMA Technologies Overview There are three mechanisms by which warm mix additives are believed to allow for reduced production temperatures: foaming, viscosity reduction, and use of chemical additives to change the surface tension or internal friction properties of the binder. Table 2-1 provides a summary of some of the widely used warm mix additives (Products), the mechanisms by which they are known or claimed to operate, and the WMA production temperatures recommended by suppliers. These technologies come with numerous potential benefits and risks related to both construction and performance. As a result, many of the WMA technologies in practice utilize a combination of these categories to optimize the benefits and control potential risks of the additive.

24 8 Table 2-1: Summary of Mechanisms and Comparison of Production Temperatures for a Variety of Warm Mix Additives Mechanism Foaming: Using water bearing minerals Foaming: By water injection or mixing in binder process Wax: Viscosity Reduction Chemical Additive: Increase Internal Lubrication Chemical Surfactant: Reduction in Surface Tension Chemical Surfactant and Wax: Combination of Surfactants and Viscosity Reduction Product Mineral Additives ( Zeolite D, Asphamin, Advera) WAM Foam Low Energy Asphalt Asphalt Foaming by Injection Sasobit FT Wax Thiopave Sulfur Based Additive Oxidized Polyethylene Developmental Chemical surfactants: CECA Base, Evotherm 3G (Revix) Pelletized Additive: Rediset Production Temperatures >120 C 80 C -120 C 85 C C >120 C >135 C >120 C 100 C -120 C 100 C -120 C The information presented in Table 2-1 indicates that based on temperature reduction allowed, WMA additives and processes can be separated by their ability to reduce production temperatures below the boiling point of water. In practice, these categories are defined as warm mix asphalt and half warm mix asphalt for production temperatures above and below 100 C, respectively. This differentiation relates directly to potential for energy reduction as the amount of energy required for vaporization of water is significant. A schematic of the energy and emissions associated with warm and half warm mix asphalt is provided in Figure 2-1.

25 9 Figure 2-1: Fuel Consumption and CO 2 Emissions for the Heating of One Ton of Wet Aggregates (10) The half-warm mix asphalt processes, LEA and WAM Foam were developed in Europe and have not gained wide-spread acceptance in the United States due to the complex process required in production. Both methods require incorporation of different asphalt binder types or aggregate fractions at varying moisture contents at different stages of production as well as modifications to conventional mixing plants (10). These requirements have led to concern that use of the half warm technologies would significantly reduce production rates, preventing their viability for use in conventionally bid construction projects. Conversely, many of the WMA additives are pre-blended with asphalt binders or introduced during mixing with minor plant modifications, making the technologies more suitable for use in a plant that produces both WMA and HMA. Due to the remaining concerns with half warm technologies and the increasing use of WMA additives, the scope of this study is limited to evaluation of WMA technologies that are produced above 100 C. Warm mix asphalt is required to allow for coating and compaction at lower temperatures and to exhibit similar performance to conventional hot mix asphalt. The presence of WMA additives significantly impacts all three of these functions. Different additives achieve

26 10 constructability through different mechanisms and have varying impacts on in-service performance in terms of both resistance to distress and moisture damage. Due to these complexities, in evaluating WMA technologies it is necessary to define the mechanisms by which constructability is achieved in terms of both coating and compaction and to define potential impacts on performance properties Foaming In applications to WMA asphalt is foamed by both injection and use of water-bearing mineral additives. The temperature reduction required for WMA is achieved by using foaming as a means to temporarily reduce asphalt binder viscosity. Both technologies assume that after the foam has dissipated, the rheological properties of the asphalt binder are similar to those prior to foaming. The success of foaming in application to WMA depends on the amount of volume expansion, defined as the expansion ratio, and the rate at which the foamed asphalt collapses (half-life). These two factors influence the viscosity reduction and the time available for mixing respectively. The effect of water concentration on expansion ratio and half life is provided in Figure 2-2. Figure 2-2: Effect of Foamant Water on Expansion Ratio and Foam Half Life for a Typical Asphalt (11)

27 11 Water concentration has opposite effects on expansion ratio and half life, with foams of higher expansion ratio, being less stable. In development of practical guidelines for use of asphalt foamed by injection for paving applications, Jenkins introduced the concept of replacing the single point measure of half-life with a parameter that allowed for consideration of the change in expansion ratio as a function of time (11), (12). The need for this modification was justified due to practical considerations. In order to be suitable for mixing at reduced temperatures a foamed asphalt must have an expansion ratio sufficiently high to reduce asphalt binder viscosity and must be adequately stable to allow time for mixing with the aggregates. The relationship between expansion ratio and foamed asphalt binder viscosity was established using a handheld viscometer to evaluate laboratory produced foam. Guidelines for the recommended viscosity range for mixing using conventional asphalt was then used to recommend a minimum expansion ratio of 4. The relationship between expansion ratio and viscosity is provided in Figure 2-3. Figure 2-3: Relationship between Expansion Ratio and Asphalt Binder Viscosity (11)

28 12 In foamed asphalts, a force balance between the surface tension of the asphalt film and the steam pressure developed during the foaming process allows for the foam to remain in equilibrium for a finite time. As demonstrated in Figure 2-2 this time scale is on the order of seconds. It has been established that the foam collapse with time can be modeled using an exponential functions similar to those used to model isotope decay (12). The expansion ratio reduces as air bubbles collapse, the physical mechanisms of collapse include: temperature differentials between the foamed asphalt and environment, steam generation from large water droplets which causes the tensile stresses in the bubble to exceed the tensile strength of the asphalt film, and foamed particle size distribution. The most mechanically stable size distribution in foamed systems is a single size that allows the walls of the bubbles to meet at an angle of 120 C, variation of bubble size distribution prevents this orientation, reducing stability. The foam index, represented as the area under the expansion ratio (ER) vs. time curve for ER>4 has been proposed as the parameter to properly evaluate the collapse of foam systems for different material combinations. Jenkins identifies many material and operational factors impacting the foam index including (12): Asphalt: o Viscosity, chemical composition, temperature and spray rate during foaming. Water: o Water quality, temperature, application rate, quantity Foam: o Particle size and distribution, presence of additives to increase foam stability. In regards to asphalt binder composition, it has been noted that the ability of the asphalt binder to foam is asphalt source dependent, however there have been no studies to date that

29 13 identify the specific chemical components within asphalt binders that influence foam characteristics (11). The production process used to manufacture the asphalt also has an impact on foamed asphalt properties. Different crude oil sources contain varying levels of paraffinic wax, which remains present in the asphalt by-product (13). Limited data suggests that the increased presence of paraffin wax in the asphalt has a positive effect on the Foam Index. Conversely, silicones are occasionally used in the oil refining process; presence of these materials in the asphalt tends to impede foam development (11). To date, there is no direct relationship between the physical properties of the asphalt binder and foaming performance. For example, lower asphalt binder viscosity is not directly related to improved foaming performance (11). Furthermore, the effect of polymers or other asphalt binder modifiers used to enhance in-service performance on foaming characteristics is unclear. To mitigate the impact of source variation and the production process on asphalt foam quality additives are used either within the asphalt binder or the foaming liquid to stabilize the foam. In the asphalt binders, surfactants or other additives are used to reduce the asphalt binder surface tension, thus reducing the steam pressure required for foaming and increasing the expansion ratio. Super-plasticizers are used in the foaming fluid, reducing surface tension of the water and allowing for finer water droplets to be injected into the asphalt resulting in smaller particle size distribution in the foam, thus extending half life (11). Although there has been a considerable amount of research related to improved characterization of foamed binders for application to paving materials, the foam index has yet to be correlated with mixture properties such as compaction, density, or workability. Therefore, the practical significance of changes in the foam index due to asphalt binder source or production variation remain unclear and criteria for use of additives to improve performance are unavailable.

30 14 Due to these technical challenges the aforementioned guidelines for evaluation of foamed asphalt have not been implemented by industry or required by state agencies in applications to WMA. Instead, foaming technology is applied to WMA solely based on the recommendations of the equipment manufacturer. For most processes water concentrations between 1% - 2% by weight of asphalt binder are used for foaming, resulting in expansion ratios ranging from (2). However, published information providing the expansion ratio as a function of time was unavailable. Conceptually, there is potential for the half life and foam index of the foam used for WMA applications to increase due to the higher temperature of the aggregates used in production. The foam produced by water-bearing mineral additives serves the same purpose in providing a temporary reduction in viscosity to promote aggregate coating and mixture workability. The additives are generally referred to as zeolites. These minerals are crystallized hydrated aluminum silicates that have the ability to store up to 20% of water by weight of the additive in their chemical structure. The chemical structure of the zeolite minerals allows for gradual continuous release of water vapor that creates a temporary foaming effect which allows for improved mixture workability at lower temperatures. The additive is incorporated into the mixing drum before the asphalt binder at a concentration of 0.25% % by weight of the asphalt mixture (4%-8% by weight of binder), based on an estimated water content of 20%, this additive concentration corresponds to approximately the 1% - 2% range for injection foaming. There are two commercially available additives with this classification: Asphamin and Advera. The additives have the same composition, with Asphamin being marketed in Europe and Advera prevalent in the United States. In laboratory studies, Advera demonstrated the

31 15 ability to reduce compaction temperature by 30 C using an additive concentration of 0.3% by weight of the mix, resulting in production temperatures of approximately 120 C (14). Although they serve the same general purpose, there are differences between foaming by injection and foaming through use of the additive, related to the amount of water available to the system at the time of foaming. Using injection, the entire water concentration is used and the reaction time is on the order of seconds. Conversely, the chemical structure of the hydrated mineral additive impedes water release, requiring at least 25 minutes for 66% of the water to be released. After 66% water loss, the rate at which the remaining water is released decreases substantially as the mineral becomes more stable (15). The mass loss after 25 and 67 minutes for 10 g of the zeolite heated to different temperatures is provided in Table 2-2. Table 2-2: % Mass Loss for Advera as a Function of Production Temperature (15) Temperature ( C) % Mass 25 min. % Mass 67 min 99 60% 70% % 80% % 90% Due to the availability of less water for foaming when water bearing additives are used, the properties of the foam, specifically the expansion ratio and half life are significantly altered. Conceptually, the decrease in water content results in a lower expansion ratio and a more stable foam (longer half life). The reduced expansion ratio is verified by Yin, who demonstrated that the change in volume due the addition of a hydrated mineral to asphalt binder is on the order of 5-10% (15), considerably less than the 400% minimum threshold specified for mixing by Jenkins (12). Although there are no publications to date that specifically address the half life of the foam produced by water-bearing minerals, the conditions imposed by time released water at lower

32 16 concentrations have potential to be characterized as foam with slow expansion gradual decay behavior. Foams exhibiting this behavior have considerably longer half lives and very fine bubbles with a single size distribution (11). This behavior is consistent with manufacturer descriptions that detail a fine bubble size as micro-foaming and suggest that the effects on mixture workability are present in the mix for up to 5 hours after production (2). Due to the differences in the foam produced by these two alternatives, it is clear that their interaction and potential impacts on the mix are different, necessitating that the two delivery systems be treated differently in evaluation of WMA technologies and mix design. In contrast to foaming by injection, which operates under the assumption that once the foam dissipates there are no adverse effects on asphalt binder properties, mineral based foaming additives have performance impacts due to the filler effects of the mineral remaining in the asphalt binder after foaming is complete slightly stiffening the binder. The increased stiffness has potential to improve high temperature performance and decrease resistance to fatigue and thermal cracking at low and intermediate temperatures (14). Based on the concentration of the additives used, the presence of the mineral filler does not impact the performance grade of the asphalt binder. By nature, the additive has an affinity to water presenting the potential to increase moisture susceptibility of the asphalt mixture (6) Viscosity Reduction Using Synthetic Wax Additives Paraffinic wax occurs naturally in asphalt or is an artifact of the crude oil refining process. The concentration of the wax ranges from 0-4% and is crude oil source and manufacturing process specific. This type of wax has melting temperatures ranging from C and a relatively coarse crystalline structure (13). Due to these properties the increased presence of wax in asphalt was perceived as a detriment to performance due to the potential for

33 17 melting of wax in the range of high in-service temperatures and crystallization at lower temperatures. This behavior compromises pavement performance in terms of both resistance to permanent deformation and thermal cracking (13). The negative performance characteristics of paraffinic waxes were considered in development of a synthetic wax for applications to warm mix asphalt. This wax, commercially referred to as Sasobit, is produced using the Fischer Tropsch synthesis process of polymerization and has shown promise for applications to warm mix asphalt (16). It is classified as a crystalline long chain aliphatic hydrocarbon manufactured from natural gas. The additive is introduced to the asphalt binder in pelletized form at concentrations ranging from 1-4% by weight of the asphalt binder (6). In synthesis of the wax, the developers of the additive approximately doubled the length of the carbon chain relative to paraffinic waxes and reduced the size of the crystalline structure to address performance concerns (17). As a result, the melting temperature was increased to a range of C and the negative effect on intermediate and low temperature performance was reduced (16). Above the melting point, the synthetic wax acts as a flow improver, reducing asphalt binder viscosity. In addition to reduced asphalt binder viscosity, research indicates that aggregate coating is improved due to the ability of the wax additive to reduce the surface tension of the asphalt binder, thus improving the spreading coefficient and reducing the contact angle relative to conventional materials (18). As a result of these combined effects, the manufacturer recommend mixing and compaction temperatures range from 120 C (16) to 138 C (5). At in-service temperatures, the wax crystallizes and acts as a stiffening agent, increasing both high and low temperature rheological properties. At high temperatures, this enhances pavement performance by increasing resistance to rutting. Conversely, stiffening is a detriment

34 18 to pavement durability as it has the potential to result in increased susceptibility to fatigue and low temperature cracking (16). The stiffening effect of the synthetic wax additive decreases with increasing binder grade, thus the effect is more drastic for a PG relative to a PG (19). Through analysis of Fourier transform infrared (FTIR) spectroscopy and rheological performance data there is also an indication that use of the wax as an asphalt modifier significantly decreases the rate at which the wax modified asphalt ages relative to conventional asphalt. In the FTIR, peaks at a wavenumber1705 cm -1 are used to identify the presence of carbonyl compounds as a chemical measure of aging. For three asphalt sources containing differing levels of paraffinic wax content and concentrations of 0%, 3% and 6% of synthetic wax the IR absorbance of carbonyl compounds decreased 5% - 20% relative to the base asphalt due to the presence of 3% wax, with the variation in absorbance due to different asphalt sources. Furthermore, carbonyl absorbance consistently decreased with increasing wax concentration (19). These results are consistent with the change of the Superpave G*/sinδ due to short term aging, referred to the aging index. Synthetic waxes were reported to cause significant reductions in the aging index with less aging occurring at higher wax concentrations. The magnitude of the reduction relative to un-modified material increased with increasing additive concentration. Furthermore, the slope of the change in performance due to short term aging temperature drastically decreases when synthetic wax is introduced to the system (20) Surfactants Surface active agents or surfactants have been applied as a warm mix asphalt technology as a means to both improve aggregate coating and promote compaction at lower production temperatures. Surfactant molecules consist of a polar hydrophilic head group and a non-polar hydrophobic tail that is usually composed of long chain hydrocarbons. Surfactants are generally

35 19 categorized as anionic (negatively charged), cationic (positively charged), or nonionic (no charge) based on the charge of the molecules that compose their hydrophilic, or head group (21). There are currently two surfactant based WMA additives available, Evotherm 3G manufactured by Meadwest Vaaco and the Arkema product Cecabase. For both products the additive is delivered in liquid form at a concentration of 0.3% - 0.6% by weight of the asphalt binder, the materials are mixed with a low shear mixer for approximately 10 minutes (22). Field and laboratory studies indicate that the products are capable of reducing compaction temperatures by approximately 40 C, resulting in an operating range of 100 C C. Previous research has shown no effect of the additive on asphalt binder viscosity and performance properties (4), (22). In application to warm mix asphalt, the polar and non-polar components of the surfactant are used in conjunction with the chemistries of the asphalt and aggregate in the mix. In general, the surfaces of the aggregate are considered polar and the asphalt molecules non-polar. Given a proven chemistry, surfactant concentration is the key component to application to WMA because in addition to providing coating, an adequate quantity of surfactant must be available to promote creation of reverse micelles, which in theory provide the lubrication of the mix Use of Surfactants to Improve Aggregate Coating Aggregate coating depends on the surface tension and viscosity of the asphalt binder and the compatibility between the binder and aggregate particle. The production of WMA at lower temperatures implies that particle coating is achieved at increased binder viscosity. Therefore, the mechanisms available for surfactants to allow for coating at lower temperatures include reduction of asphalt binder surface tension and increasing the compatibility between the asphalt and aggregate. For purposes of this description surface tension is defined as the amount of work required to increase the surface area of the liquid (23). The use of surfactants reduces

36 20 surface tension due to adsorption of the surfactant molecules into the asphalt film. The reduction in surface tension increases with increasing surfactant concentration until the surface is saturated. At surfactant concentrations beyond the point of saturation, the additional surfactant is not adsorbed and is available for other behavior such as micelle generation (23). In a system composed of solely asphalt and aggregate, coating requires adsorption of the asphalt onto aggregate surface. Due to the opposing polarities of these materials a considerable amount of heat and mechanical energy is required to overcome chemical repulsion and achieve coating. Even under these extreme conditions, certain asphalt and aggregate combinations are unable to be coated properly as manifested by moisture damage due to stripping of the asphalt film from the surface of the aggregate. Aggregate can be considered a polar surface with discrete surface charges that are dependent on mineralogy. For calcareous aggregates such as limestone, the fracture of calcium carbonate results in electro-positive unsatisfied surface charges consisting of calcium and carbonate ions. Conversely, the fracture of siliceous materials, i.e. quartz and granite results in unsatisfied electro-negative charges (24). At the interface, the polar head of the surfactant aligns with the surface of the aggregate, with the non-polar tail attracted to the asphalt binder. Adsorption is dependent on the orientation of the surfactant and chemical characteristics of the materials at the interface. The end result is a fully coated aggregate particle that is stable due to the adsorption of the surfactant into the aggregate surface. Furthermore, the chemical energy contributed to the system by these reactions reduces the dependence of viscosity on obtaining quality coating, thus reducing the temperature at which the asphalt must be heated.

37 Use of Surfactants to Improve Compaction Surfactants are used in WMA to both improve mixture workability through use of concentrations beyond the critical micelle concentration. When the adsorption into the aggregate is complete the remaining surfactant molecules are available to form micelles. In applications to asphalt, the free surfactant molecules exist in a non-aqueous solution and orient themselves with their non-polar tails toward the bulk fluid, as a result the polar heads of the molecules aggregate together, forming reverse micelles. This orientation is favored to reduce unfavorable reactions between the ionic head group of surfactant molecules and the non-polar asphalt (23). Chemical bonding interactions cause strong forces that promote a tendency for available surfactant molecules to aggregate and create a micelle shape that is small and spherical. During asphalt mixture compaction, aggregate particles move past each other and reorient in a closer packing density due to the compactive load, with the binder properties providing resistance to movement of the aggregates. In applications of surfactants to WMA it has been proposed that the reverse micelles generated using the un-adsorbed surfactant molecules are distributed between the aggregate particles and cause slip planes to promote compaction due to the low viscosity and resistance to shear deformation of the particles (25). Similar effects have been published in the research related to the science of tribology that summarized the different mechanisms of lubrication at the micro and nano-scale. In one application, it was demonstrated that the use of reverse micelles significantly reduced the coefficient of friction and the wear accumulated after 2000 cycles relative to an unmodified lubricant. Specifically, the coefficient of friction was reduced from 0.7 to 0.15 and wear reduced from 450 µm to 150 µm (26).

38 Impact of Surfactants on Asphalt Binder Performance Properties Based on the need for the surfactant concentration to be above the critical micelle concentration for WMA potential performance considerations arise, specifically related to the potentially negative effect of the surfactant on resistance to permanent deformation and aging susceptibility and the potential benefits of the adsorbed surfactant to increased resistance to moisture damage. In regards to high temperature performance, there is potential that the reverse micelles used to lubricate the mix at compaction temperatures will be a detrimental to mixture performance at high service temperatures due to softening of the asphalt binder. Previous research indicates that the effects of liquid surfactants on performance are a function of the concentration and chemistry of the surfactant as well as the chemical composition of the asphalt binder (27). In addition, the presence of surfactant has been shown to soften the binder and also reduce susceptibility to aging as demonstrated decreased viscosity (27) and a decrease in the G*/sinδ parameter. Consistent with softening behavior observed using other measures, use of surfactants also marginally increased estimated fatigue life using the ratio of dissipated energy indicates (28). Surfactants also caused a significant decrease in the change in the G*/sinδ parameter due to short term aging, indicating that theses additives are altering the composition of the base asphalt (27). Many surfactants have been classified as adhesion promoters and are marketed to improve mixture resistance to moisture damage. At the asphalt/aggregate interface there are three fundamental properties that correspond to the mixture: the work of adhesion, the work of debonding, and the work of cohesion (29). The work of cohesion represents the energy required to separate the asphalt film. The work of adhesion and debonding represent the energy required to separate the bond between the asphalt and aggregate in the dry condition and in the presence

39 23 of water, respectively. Given these definitions, the conditions that relate to increased moisture resistance are a high work of adhesion and low work of debonding (29). The magnitude of the work of adhesion is desired to be high because it represents a strong bond between the asphalt and aggregate was developed in dry condition. Furthermore, a low strength of debonding implies that the asphalt/aggregate bond is stable in the presence of moisture because it is thermodynamically preferred for the asphalt film to stay intact rather than to be replaced by water. Surfactants have been shown to lower the surface tension of the asphalt binder and the solid at the liquid solid interface (23). Although the effects of these changes on resistance to moisture damage are material specific, the general concept is that based on work of adhesion and debonding there is potential for surfactants to increase moisture damage resistance Functionalized Polyolefin Polymers Functionalized polyolefin polymers have been recently introduced to the paving materials market as both a warm mix additive and asphalt binder modifier. As asphalt modifiers, polymers are classified as either elastomers or plastomers based on their ability to enhance the elastic response of the binder after loading. Using this definition, polyolefins are plastomers in that they increase stiffness, particularly at high temperatures but do not substantially impact the elastic response of the material. In practical applications, plastomers are used as a means to improve resistance to permanent deformation and thermal cracking. However, of the 52 state agencies surveyed only six regularly used plastomers, all using ethyl vinyl acetate (EVA). In comparison, the number of states using a variety of different elastomeric polymers ranged from (30). Use of plastomers was limited to EVA and sporadically polyethylene, both modifiers proved successful in producing higher grade asphalt, but were generally not used due to the perception that superior modified binder performance required increased elasticity and

40 24 because the workability of the mixtures prepared using these plastomers was relatively poor in comparison to other modifiers (30). Exploration of alternatives to SBS by the asphalt industry found applicable technologies from the hot melt adhesive industry. Specifically, properly functionalized polyolefins were identified as a viable asphalt modifier as they had potential to improve the wetting, adhesion, and strength characteristics of asphalt binders allowing for both improved workability and performance (31). Low density polyethylene functionalized by oxidation was identified as one promising technology. Oxidation of this molecule results in an increase in polarity, as measured by the acid number (31). The additive has a Mettler drop point of 137 C and an acid number of 25 mg KOH/g. The Mettler drop point indicates the temperature at which the additive changes from a semi-solid to liquid state. It is delivered in a pelletized form at concentrations ranging from 1% - 4% by weight of the asphalt binder. The additive is pre-blended with the asphalt binder at temperatures above the Mettler drop point to ensure complete blending. In applications to hot and warm mix asphalt the oxidized low density polyethylene essentially acts as a wax. In the liquid and semi-solid state, the additive acts as a flow improver, in the solid form a stiffening effect of the asphalt binder is realized, increasing the high temperature performance grade of the asphalt binder with minor influence on fatigue and low temperature properties. In general, additive concentrations of 2.0% and 3.5% are associated with high temperature grade increases of 6 C (one grade) and 12 C (two grades), respectively. Asphalt binder viscosity is unaffected by the presence of the additive or additive concentration, meaning that for a given increase in in-service performance grade, the viscosity remains unchanged. In contrast, considerable increases in asphalt binder viscosity are realized when elastomeric (SBS) modifiers are used to increase asphalt binder grade to the same level (32).

41 25 At concentrations greater than 1.0% significant increases in mixture workability due to the presence of the additive are realized (33). Increased density at no change in viscosity indicates that the additive has a lubricating effect that is not captured through conventional measures of asphalt binder workability. The use of oxidized polyethylene as a lubricant is evident in other industries, particularly the processing of Polyvinyl Chloride (PVC). In applications to PVC processing oxidized polyethylene is considered an external lubricant due to its ability to reduce the friction between the PVC melt and forming machines. Due to these effects, the fusion time of the PVC is delayed and the fusion torque as measured by the Haake Torque Rheometer is reduced (34). In PVC the oxidized polyethylene acts as an external lubricant because the two materials are incompatible. In application to asphalt, the oxidized polyethylene is dissolved in the asphalt, thus acting as an internal lubricant, facilitating movement between the asphalt particles (35). The role of the oxidized polyethylene additive as an internal lubricant for asphalt mixtures was confirmed in work by Teymourpour, which found that additive at concentrations ranging from 1%-3.5% significantly improved mixture workability and that lower density values were observed at reduced compaction temperatures (33). As previously mentioned, oxidation of polyethylene causes the molecule to become more polar relative to the un-oxidized state. The oxidation creates a random distribution of acid, alcohol, and ester groups placed along and at the end of the polymer chains. In the asphalt/aggregate system the presence of these polar groups promotes bonding of the asphalt to aggregate surface, both in the dry state and in the presence of moisture. As a result the cohesive strength of the asphalt binder and resistance to moisture damage is improved (31). These effects are confirmed by Moraes which found that the use of the additive at a concentration of 1%

42 26 increased the dry and wet bond strength as measured by the bitumen bond strength test by 20% and 65% respectively. As a result of this increase in bond strength, mixture resistance to moisture damage as measured by the ratio of wet to dry indirect tensile strength increased from 0.50 to 0.99 due to the use of the additive (36). The mechanisms causing the gain in strength are an increase in the work of cohesion and decrease in work of debonding Combination Additives Surfactants and Synthetic Waxes In certain instances, WMA additive manufacturers have developed chemical additive packages that use a combination of the aforementioned technologies in an effort to optimize the benefits and risks associated with use of a single WMA classification. A specific additive in this category is Rediset, developed by Akzo Nobel. This additive is delivered in pellet form and preblended with the asphalt binder at concentrations ranging from 1-2% by weight of the binder (2). The additive has two components, the range of proportions of each of these components is provided in parenthesis: an amine or modified amine surfactant (10-60%) and a combination of a synthetic wax and resin (20-90%) (37). The synthetic wax in the product is a blend of the wax previously mentioned as used in the Sasobit additive produced by Fischer Tropsch synthesis process and a polyethylene wax. The waxes used are required to have a melting point ranging from 60 C C. The resin component is from vegetable or petroleum oil, the physical properties required are a melting point <60 C and a penetration at 25 C<50 (37). In serving as a WMA additive Rediset uses the mechanisms previously stated for both the surfactant and wax components to allow for coating and compaction at lower temperatures as it reduces asphalt binder viscosity above the melting point the melting point of the wax component and makes use of the surfactant to promote aggregate coating and compaction. Also due to the surfactant, mixture performance data suggests that the additive has potential to improve moisture

43 27 resistance as shown by tensile strength ratios in the range of 0.9 for a granite mix that was known to have low moisture damage resistance in conventional applications (37). As previously noted there is potential for the wax component of the additive to produce a stiffening effect in the asphalt binder at temperatures below the melting point and also to reduce the sensitivity of binder performance to short term aging. However, based on the composition of the additive detailed in the patent the Fischer Tropsch wax content ranges from approximately 0.40% and 0.60%, at these concentrations, the wax component was found insufficient to cause a stiffening effect at high or low temperatures. In fact, binder performance grading indicates that the use of the Rediset package resulted in a slight softening of the asphalt binder. Furthermore, there was no effect of the additive on susceptibility to short term aging using the RTFOT test (37). 2.2 Overview of WMA-Related Research Numerous research projects at both the state and national levels have been completed related to the construction, performance, and environmental aspects of warm mix asphalt. Most of these studies are categorized as applied research, evaluating WMA with the overall objective of identifying differences between WMA and conventional HMA as a means to support development of WMA specific specification language and best practices. A majority of initial studies applied the technology developed for HMA through the Strategic Highway Research Program (SHRP) as a means to evaluate WMA technologies. Research findings indicated that due to the presence of the various additives and deviations from production temperature relative to HMA warm mix asphalt could not be properly accounted for using solely conventional test methods. A testament to the complexity introduced by WMA and need for improved practice is funding by the National Highway Cooperative Research Program (NCHRP) at a level of $4.5 million to address the technical issues related to the construction, performance, and

44 28 environmental impacts of WMA. Approximately 86% of this funding is allocated to projects that are in-progress or yet to be awarded, thus the availability of detailed specifications for use of WMA is lacking. It is anticipated that as the in-progress research projects come to completion and the use of WMA technology becomes better understood, specifications will be updated to reflect the implementable aspects of the research. 2.3 Impacts of WMA Additives on Workability The current specification framework adopted by many agencies base acceptance of asphalt mixtures on in-place air voids (density). Thus in terms of current practice the goal of warm mix asphalts is to improve mixture workability in order to allow contractors to meet inplace air void criterion at lower construction temperatures. Previous efforts have been made to characterize the effects of warm mix additives on workability in terms of both asphalt binders and mixtures Asphalt Binder Workability Initial WMA research focused on application of rotational viscosity in an attempt to explain the effect of WMA technologies on the mixture behavior. Findings indicate that in both the HMA and WMA temperature ranges there is no significant difference between the viscosity of conventional asphalt binders and those modified with mineral zeolite or surfactant based WMA additives (3), (4), (22). Results published for the WMA additives including synthetic wax used both alone and in combination with other chemicals indicate reductions in viscosity relative to neat binders ranges ranging from 10% - 20% (5), (32). The reduction in viscosity increases with concentration and with use of softer binder grades, thus there is potential that the effect of the additive diminishes with use of binders modified to enhance performance properties. For all WMA additives evaluated, the observed reductions in viscosity did not correspond to a drop in

45 29 predicted mixing and compaction temperatures consistent with what is observed for mixtures placed in the field (3). In addition to being insensitive to the presence of WMA additives, the equi-viscous concept has been found inappropriate for modified binders and tends to over-estimate mixing and compaction temperatures. This result is due to the differing properties of conventional and modified asphalts in the mixing and compaction temperature range. In this range, conventional asphalt binders exhibit Newtonian behavior. Conversely, the viscosity of modified binders is shear rate dependent, a property that must be considered in an appropriate method to estimate mixing and compaction temperatures due to the cyclic nature of mixture compaction. Two different test methods have been developed to apply more complex rheological concepts to evaluate the workability of modified binders. NCHRP Report 459 recommends use of low shear viscosity (LSV) to estimate mixing and compaction temperatures based on estimates of the bulk shear rate experienced during laboratory compaction. The test method involves use modeling to estimate LSV based on the relationship between viscosity and shear rate. The LSV for each temperature is plotted on a loglog viscosity vs. log temperature scale and mixing and compaction temperature thresholds are defined as the temperatures at which the LSV is 3000 cps and 6000 cps respectively (30). A test method presented NCHRP Report 648 addresses the non-newtonian behavior of modified binders by using the DSR to identify the transition between visco-elastic and viscous flow (38). The phase angle (δ) is used to evaluate the consistency of the asphalt, defining the cross over frequency between visco-elastic and purely viscous material behavior as the frequency corresponding to 86. Mixing and compaction temperatures are then estimated using the cross-over frequency and coefficients from an empirically developed power law. However,

46 30 application of both new methods did not improve sensitivity to the presence of WMA additives as the predicted mixing and compaction temperatures for binders modified with both viscosity reducers and surfactants were not statistically different than those predicted for conventional asphalt binders (3), (4), (38). The common theme between the previously presented test methods is that they employ the use the rheology to characterize asphalt binder workability in terms of bulk properties. However, it is generally accepted that during construction a thin film of asphalt coats the aggregate with the mix subjected to normal and shear forces as it is being compacted in the field. Under these conditions of high stresses and small distances between aggregate particles, lubrication is governed by thin film behavior, or tribology, rather than bulk properties. Work conducted by Kavehpour and McKinley demonstrated that characterization of both rheology and tribology of a fluid can be achieved on the same apparatus through use of a modified fixture in the DSR (39). The test involved subjecting the fluid to increasing shear rates at a constant testing gap and monitoring the torque using the parallel plate geometry. The test was conducted at decreasing gaps, and to the point at which the plates came in contact, causing a normal force to develop. Results for different testing gaps are presented in Figure 2-4.

47 Figure 2-4: Transition from Bulk to Thin-Film Lubricating Properties (39) Table 2-3: Legend for Figure 2-4 Symbol Film Thickness (µm) Δ , Normal Force = 5N X 0, Normal Force = 15N The symbols corresponding to different film thicknesses are provided in Table 2-3. For tests conducted at film thicknesses of 50µm - 300µm the plot of torque vs. shear rate varied linearly throughout the entire range of angular velocity, given the relationship between torque and viscosity, this implies Newtonian flow similar to the behavior observed for bulk viscosity (39). However, when the testing gap reached 20 µm or a normal force was applied to the system, the torque measured was independent of shear rate, revealing a behavior that cannot be

48 32 captured using solely rheological measurements. This change in regards to angular velocity dependence marks the transition from behavior governed by bulk rheology to behavior defined by tribology. Furthermore, the behavior measured by the Tribo-rheometer is consistent with lubrication theory which is described by the Stribeck Curve. For rotational systems, the Stribeck curve is represented by a plot of coefficient of friction vs. Gumbel Number on a log-log scale. The Gumbel Number is defined in Equation 2-1. Equation 2-1: Definition of the Gumbel Number Where: Gu = Gumbel Number η 0 = Zero Shear Viscosity (Pa s) Ω = Rotational Velocity (rad/s) σ = Normal Stress (Pa) A conceptual representation of the Stribeck Curve is presented in Figure 2-5. Gumbel Number, Gu Figure 2-5: Conceptual Representation of the Stribeck Curve Identifying Four Regimes of Lubrication (40)

49 33 The Stribeck Curve describes three lubrication regimes: boundary, mixed, and hydrodynamic lubrication. These are indicated as Zone 1, 2, and 3 Figure 2-4 and a, b, and d on Figure 2-5, respectively. For boundary lubrication (Zone 1/a) there is sufficient contact between surfaces such that a significant portion of their irregularities come into contact and torque remains high and relatively constant. As the thickness of the film between surfaces increases, less surface irregularities come into contact and less torque is required to maintain a given angular velocity, this is defined as mixed lubrication (Zone 2/b). The third lubrication regime, hydrodynamic lubrication, occurs when a sufficient film thickness exists such that there is no contact between surfaces (Zone 3/d). In this regime, lubrication is governed by the viscosity of the fluid and material behavior tends towards the bulk properties of the fluid. Comparison to Figure 2-4 demonstrates that the tribo-rheometer fixture developed Kavehpour captures behavior very similar to the conceptual representation of the Stribeck Curve presented in Figure 2-5. The tribo-rheometer device has been used successfully to characterize bio-lubricants, finding significant differences between two fluids used to lubricate hip and shoulder joints (39). Based on these results there is potential to apply these concepts in evaluation of WMA additives influence the internal friction behavior of asphalt binders. Based on the behavior presented in the Stribeck Diagram, improved lubrication could be realized through either a reduction in the minimum friction coefficient or a more gradual transition between minimum friction and hydrodynamic lubrication. Prior to application of the Stribeck Diagram to evaluation of asphalts the properties of the fluid must be addressed. In regards to asphalt binder properties, there is potential that the elasticity and non-newtonian behavior of asphalt will influence friction behavior. The Stribeck Curve assumes that the lubricant is Newtonian, thus the fluid is fully viscous and demonstrates viscosity independent of shear rate. It is well established that asphalt

50 34 binders exhibit both visco-elasticity and dependence of viscosity on shear rate. The impact of attempting to use the conventional Stribeck Diagram to evaluate non-newtonian lubricants is presented in Figure 2-6. Figure 2-6: Impact of Using Conventional Stribeck Diagram to Evaluate the Behavior of Non-Newtonian Fluids (40) As shown in Figure 2-6 the Gumbel Number successfully normalizes the behavior of two different Newtonian lubricants; however, behavior of the visco-elastic material varies significantly. In applications to evaluation of asphalt binders, this result has the potential to impact comparison of materials with significantly different rheological properties. Examples include comparison of friction behavior of conventional and modified binders or evaluation of WMA additives that alter the rheological properties of the binder. A majority of studies related to the field of friction and lubrication apply to the design of bearings and machine parts, as a result the friction behavior of machined surfaces that are inherently smooth are investigated. In contrast, many state specifications require that crushed aggregate be used as a means to control aggregate interlock and ensure in-service mixture

51 35 stability. As a result, the aggregate surfaces in asphalt mixtures are rough relative to smooth machined parts, the impacts of surface roughness on the Stribeck curve is presented in Figure 2-7. Shift in Stribeck Curve due to increasing surface texture. Figure 2-7: Effect of Surface Texture on the Stribeck Diagram (40) A downward and shift to the left along the Stribeck curve is observed in Figure 2-7 due to increasing the surface roughness. The practical implications of this are realized in both the Gumbel number and coefficient of friction. Given a constant speed, a decrease in the Gumbel number associated with the minimum value of coefficient of friction dictates that fluid viscosity must be decreased or stress increased to reach optimum friction conditions. Based on the observed effects of surface roughness, there is potential that it is necessary to increase compaction temperature due to increased surface roughness. Furthermore, the range in coefficient of friction values for rough surfaces dramatically increases due to a decrease in the minimum value of coefficient of friction. This behavior implies that the Stribeck behavior is

52 36 more sensitive for rough surfaces, indicating that the contribution of WMA additives to lubrication are more clearly realized as the surface roughness of the substrate increases. Subsequent work by Hupp used the device to develop necessary modifications to constitutive equations governing Newtonian flow to allow for consideration of Non-Newtonian. Specifically, modification of the Gumbel Number to include shear rate dependency and relaxation time were proposed, also mathematical relationships were developed to determine the effects of surface texture on the transition between lubrication regimes for a fluid (40). Numerous research results indicate that the bulk viscosity does not adequately characterize the behavior of thin films, requiring the definition of effective viscosity. Experimental data on lubricating systems indicates that effective viscosity can be up to 7 orders of magnitude higher than bulk viscosity. Friction mapping is a related concept that provides full assessment of lubricant behavior from bulk properties to thin film behavior. Similar to the dependence of viscosity on gap presented by Kavehpour and provided in Figure 2-4, friction mapping defines the parameter of effective viscosity to characterize three regimes of behavior: thin films, transition zone, and bulk viscosity behavior (41). Effective viscosity is defined in Table 2-4. Table 2-4: Concept of Effective Viscosity For Thin Films: In the Transition Zone Thick Films and, Where: Ω= Rotational Speed (rad/s) a h = Gap dependent shift factor R = Contact Radius H = Gap Height = Surface roughness Where: γ = shear rate (1/s) n = exponent ranging from (39) At sufficiently thick films effective viscosity is equal to bulk viscosity

53 37 Based on these three fluid behavior regimes a friction map can be developed for any fluid as a function of shear rate. A conceptual representation of the friction map and the dependence of the relationship on applied stress is provided in Figure 2-8 Figure 2-8: Friction Maps and Dependence on Applied Contact Stress (41) In the thin film regime, the relationship provided in Figure 2-8 is consistent with the concepts previously presented related to the coefficient of friction in that higher applied contact stresses relate to more friction, thus higher effective viscosity. However, use of friction maps builds on the concept of coefficient of friction by introducing the transition zone and the relationship between contact pressure and the shear rate required to achieve Newtonian flow. Furthermore, Luengo has developed empirical relations to describe viscosity in both thin and thick films for visco-elastic fluids, potentially allowing for similar friction maps to be developed for asphalt binders using coefficient of friction and conventional viscosity measurements (41).

54 38 Based on application of these concepts, the authors and others hypothesized that thin film behavior provides insight to the enhanced lubrication observed in WMA. The concept of using tribology to evaluate the impacts of WMA on asphalt binder workability has been implemented through two separate research efforts. The previously presented small gap testing has been applied to asphalts in a test developed by Mathy Technology and Engineering (MTE). At small gaps the sample is subjected to rotational speeds ranging from rad/s with normal force, torque, and viscosity monitored. Torque increases with increasing speed, and then reaches a maximum before dramatically decreasing due to slippage between the top plate of the DSR and the sample. The maximum torque that the material can sustain before slippage occurs is used to evaluate the potential for enhanced workability. Comparison to mixture workability data has shown similar ranking between the MTE Lubricity test and measurements of asphalt workability taken both during mixing and compaction (3). A second approach has been pursued as part of the current research that uses the coefficient of friction as measured by the Asphalt Lubricity Test to evaluate the impacts of the presence of WMA additives on asphalt binder workability. The Asphalt Lubricity Test is based on a test used for lubricating oils, ASTM D (Standard Test Method for Determination of the Coefficent of Friction of Lubricants Using the Four Ball Wear Test Machine) the apparatus consists of three lower balls which are clamped in a cup; a fourth ball secured by chuck is loaded against them, with a sufficient amount of binder added to produce a film. The dimensions of the apparatus provided in the standard were reduced to fit the TA AR-2000 Dynamic Shear Rheometer, thus the sample cup was scaled down to a diameter of 38.1 mm and a height of 25.4 mm. The ball bearings used in the test have a diameter of 12.5 mm. Pictures of the testing apparatus in the DSR are provided in Figure 2-9.

55 39 (a) (b) Figure 2-9: Photographs of the new Lubricity Testing Fixture Machined for the TA DSR: (a) Lower cup that holds lubricant and lower balls, (b) Screw Assembly is used to clamp the lower balls into place. During the test the chuck is rotated at a constant speed, under a prescribed normal force at a constant temperature. The coefficient of friction is then calculated using Equation 2-2. Equation 2-2: Calculation of Asphalt Binder Coefficient of Friction Where: μ = Coefficient of Friction C = Constant related to Four Ball testing geometry T = Torque (N) P = Normal Force (N) D = Diameter of Balls (m) In this test the conditions necessary for evaluation of thin film properties are imposed through application of normal force during testing (4). Similar to the tribo-rheometer procedure, the test involves subjecting the sample to different angular velocities under a constant normal force. Previous publications cited the need for further procedure development and evaluation of

56 40 a wide range of WMA additives before the test could be confirmed as appropriate in characterization of asphalt binder workability (4) Asphalt Mixture Workability The role of asphalt binder in the compaction and workability of asphalt mixtures is synonymous with the role of water as a lubricant in the compaction of soils. Both the moisture content of soils and the binder content of asphalt mixtures impact the ability to achieve density for a given level of compacted effort. The typical range of gradations used in asphalt mixture construction is related to a United Soil System classification of well graded gravel to well graded sands. For this soil classification a S-Shaped moisture-density relationship is observed with high density achieved at very low moisture contents, as moisture content increases the density trends to a minimum, subsequent addition of moisture is required to reach optimum density, and after optimum further addition of water causes a net decrease in density (42). There are many theories discussing the mechanisms of evolution of compaction including consideration of the role of water, pore water and air pressures, and the microstructure of cohesive soils. For the purposes of the comparison between the roles of asphalt binder and water in the compaction of soils and asphalt mixtures the compaction theory presented by Hogentogler is most applicable. This theory proposes that the presence of water films around soil particles have different effects based on film thickness. Furthermore, the interaction between the water and soil particles changes with distance from the water/soil interface, becoming less cohesive. Based on these concepts four stages of wetting were proposed (42): 1. Hydration: The water is absorbed into aggregate particles and forms a thin cohesive film. 2. Lubrication: The water facilitates movement of particles under a compactive load, allowing the soil to achieve optimum density. 3. Swelling: Excess water causes the soil mass to swell.

57 41 4. Saturation: The soil because saturated causing a decrease in density and an approach to the condition of zero air voids. The location of the aforementioned stages on the dry unit weight vs. moisture content plot and their position relative to the zero air void line is provided in Figure Figure 2-10: Hogentogler Compaction Stages of Wetting (42) In applications to the compaction of asphalt mixes, swelling is not of concern due to the well graded cohesionless structure of the aggregate gradation used to prepare mixes. Furthermore, the properties of the lubricating fluids used in compaction of asphalt mixes and soils are drastically different. As a lubricant water has been shown to exhibit both low viscosity and low coefficient of friction independent of the conditions such as the speed and normal stresses generated when particles experience re-orientation (41). As a result the density of soils is only a function of water content. Conversely, the previously introduced concepts of tribology and friction mapping indicate that for asphalt binders and other non-newtonian materials both the coefficient of friction and effective viscosity are dependent on the film thickness, speed, and normal stress. Based on these conditions, it is hypothesized that in addition to asphalt binder content the position of a given asphalt mixture relative to optimum density is a function of the

58 42 properties of the asphalt binder and therefore is impacted by compaction temperature, WMA additive type, and WMA additive concentration. In addition to the role of asphalt binder as a lubricant there a number of aggregate source and blend properties that influence the densification of the mixture and its sensitivity to changes in temperature or asphalt content. These factors are summarized in Table 2-5, furthermore a description of how some of these parameters are measured is provided in Section Table 2-5: Aggregate Properties Influencing Mixture Workability Aggregate Property Gradation and Packing Characteristics Mix Design Property Angularity Mix Design Property Mineralogy Source Property Hardness Source Property Absorption Source Property Evaluation Parameter Alpha and Beta CA Ratio (Bailey Parameter) FA C Ratio (Bailey Parameter) FA F Ratio (Bailey Parameter) Coarse Aggregate Fractured Faces Fine Aggregate Fine Aggregate Angularity (FAA) ph of Mineral Filler LA Abrasion % Water Absorption Potential Impacts on Compaction for HMA and WMA Weibull Distribution Parameters are indicators of the shape of the gradation curve. Describe packing of coarse aggregates. Describe packing of the coarse portion of the fine aggregates. Describe packing of the fine portion of the fine aggregates. Angular coarse and fine aggregates reduce mixture workability. Most WMA additives are chemical packages, the interaction with aggregate chemistry has the potential to impact the effectiveness of the WMA additive. Soft aggregates allow for more densification to occur due to degradation of the aggregate. Absorptive aggregates have the potential to impact mix design volumetrics at WMA temperatures

59 43 Given the contribution of the aggregate blend and source properties to mixture densification it is apparent that a generalized definition of the role of the asphalt binder as a lubricant during mixture compaction is unavailable. Instead it is proposed that the lubricating properties of the asphalt binder and its potential to influence densification are related to the properties of the aggregate blend. Therefore, an opportunity exists to influence asphalt mixture workability through modification of asphalt binder properties, changing aggregate source, or by changing the properties of the aggregate blend. In application to WMA the need to consider the influence of both asphalt binder and aggregate properties on workability is directly related to establishing the sensitivity of the mix to compaction temperature, thus determining the effectiveness of WMA technologies. The conventional measure of asphalt mixture workability is air voids for a given level of compactive effort, numerous publications reviewed used this parameter for comparison of WMA and HMA for samples compacted with both the SuperPave Gyratory Compactor (SGC) and using the Marshall method. The inadequacy of the concept that the ability of asphalt mixes to achieve density is only a function of asphalt binder viscosity, thus density decreases with decreasing compaction temperature is supported by the findings of Bennert et. al presented in Figure 2-11.

60 44 Figure 2-11: Relationship between Air Voids and Compaction Temperature for HMA and Various WMA Additives and Concentrations (3) Results presented in Figure 2-11 indicate that even for the extreme temperature range of 85 F (50 C) the hypothesis that air voids increase (i.e. decrease in density) with increasing temperatures is not an appropriate method to characterize asphalt mixture workability. Furthermore, the results reinforce the concepts that a) an optimum level of density exists for the compaction of asphalt mixtures and b) the optimum density achieved and position relative to the optimum is dependent on the properties of the fluid other than solely viscosity. This behavior is shown to be dependent on WMA additive type and additive concentration. As warm mix asphalt has become more prevalent, other measures of workability using different analysis methods for compaction data or new devices have been developed in both the United States and Europe. In the United States new devices/methods to monitor the performance of the mix during both mixing and compaction have been introduced. To monitor workability during mixing, the Asphalt Workability device (AWD) has been developed by UMass Dartmouth. The AWD operates at a constant speed, the temperature and the torque required to

61 45 maintain that speed as the mix cools is recorded (6). Mixture workability is characterized by the amount of torque required to maintain a constant mixing speed. The device is meant to simulate the mixing and lay down process of WMA. However, the AWD has proven insensitive to the presence of WMA additives or additive concentration at temperatures consistent with mixing and lay down operations, with differentiation between material types observed only at temperatures below 220 F (105 C) (6). A new volumetric criterion, N92 has been proposed to evaluate asphalt mixture workability using volumetric data routinely collected during current mix design and quality control testing (20). N92 is defined as the number of gyrations required to reach 92% Gmm (8% air voids). The level of 8% air voids was selected because it corresponds to the density requirement commonly used for acceptance in the field. To move away from using solely volumetrics to characterize mixture workability, a device called the Gyratory Pressure Distribution Analyzer (GPDA) was introduced. This device monitors the resistive forces of the mix during compaction. The GPDA Plate was designed to fit into the gyratory mold oriented in the direction of loading during compaction and contains three load cells to monitor the load and eccentricity as the mold gyrates during compaction. These measurements are used to calculate the resistive force of the mix for each gyration, allowing for definition of the Construction Force Index (CFI) which is the area under the resistive effort curve from N ini to 92% G mm for a specific mixture (43). A schematic of the PDA plate and a visual representation of calculation of CFI are provided in Figure 2-12.

62 46 Figure 2-12: Schematic Showing the PDA Plate and Definition of the Construction Force Index Initial laboratory generated data indicate that both the volumetric parameter N92 and the direct measurement of resistive forces during compaction as measured by the CFI are sensitive to the presence of WMA additives and compaction temperature. Furthermore, a strong linear relationship exists between the N92 and CFI parameters. To demonstrate the effect of temperature, results for a fine graded mix compacted at three different temperatures with and without WMA additives are presented in Figure CFI Compaction Temperature [C] PG PG % 3G PG % Rediset Figure 2-13: Effect of WMA Additive Type and Compaction Temperature on Mixture Workability CFI (4)

63 47 Results indicate that the largest difference between the conventional binders and those modified with WMA is observed at temperature of 90. Furthermore, WMA mixes exhibit less temperature sensitivity relative to conventional HMA. These relationships are consistent with results presented for the AWD mixture workability test, a test method that evaluates workability before compaction. For a similar asphalt binder grade (PG 76-22) AWD test results indicated that no significant difference in the Workability Index between the HMA control mix and a variety of WMA additives exists until testing temperatures below 105 C. In practice, the threshold temperature at which minimum density is gained for a given compactive effort for conventional HMA has been reported as 80 ο C 90 ο C (44). Based on the test methods reviewed the compaction parameters of N92 and CFI and mixture torque measurements produced using the AWD are consistent with this practical guidance. Furthermore, the larger differentiation between HMA and WMA at lower temperatures indicates that use of WMA additives extends the temperature range at which adequate mixture densification can still be achieved. However, to date the impacts of WMA additives and compaction temperature on asphalt mixture workability have been empirical in nature and based on trends observed in data generated in the laboratory or field. A majority of these relationships have been developed using concepts and technologies adopted from conventional practice in design and construction of conventional HMA. Relative to HMA, the workability of WMA presents an added level of complexity due to the use of WMA additives and varying compaction temperatures. Proper consideration of these effects requires an improved understanding of the role of the asphalt binder as a lubricant and how the properties of the lubricant relate to density.

64 Impacts of WMA on Performance Overview In general, asphalt binder consists of four chemical groups: saturates, aromatics, resins, and asphaltenes. The chemical composition of asphalt binders is defined based on the relative concentrations of these components, referred to as SARA fractions (45). Saturates, aromatics, and resins are generally referred to maltenes and asphaltenes are considered the viscosity building component of the asphalt. As the asphalt binder ages the lighter compounds are converted to resins, which are eventually converted to asphaltenes, thus viscosity increases with aging. Also due the effect of asphaltenes on viscosity, it is possible for un-aged materials from different sources to demonstrate a wide range of physical properties. The variation of SARA fractions due to asphalt binder source is presented in Figure Figure 2-14: Variation in SARA Fractions due to Asphalt Binder Source SHRP Core Asphalts (45) The applicability of SARA analysis to characterization of asphalt binders provides a conceptual basis for understanding composition, based on SARA fractions it can be stated that

65 49 asphalt structure is a chemical continuum with a gradual increase in molar mass, aromatic content, and polarity from saturates to asphaltenes (45). In further modeling of the chemical structure based on use of maltenes and asphaltenes, the chemical composition of the asphalt binder was modeled both as a dispersed polar fluid and as a colloidal model with asphaltenes dispersed in a maltene phase (45). The stability of the asphalt chemical structure can be defined by the Colloidal Index, which is the ratio of asphaltenes and flocculated maltenes to the resins that serve as a peptizing agent. For paving grade asphalts the colloidal index ranges from , with lower colloidal index relating to a more stable system (45). A physical representation of the effects of maltenes as a dispersing agent is provided in Figure 2-15, which demonstrates the effects of changes in composition and aging on viscosity. Figure 2-15: Relationship Between Asphaltenes and Viscosity and the Effects of Asphalt Binder Source and Level of Aging (46)

66 50 The effect of naturally occurring asphaltenes present in a binder source is demonstrated in Figure 2-15 as the asphaltenes increase the tendency of agglomeration and development of microstructure in both the un-aged and aged condition. The effects of aging are also apparent, with oxidation causing an increase in asphaltenes for both asphalt binder sources Mechanisms and Test Methods for Short Term Aging During construction, the asphalt binder ages through two mechanisms, volatilization and oxidation. Volatilization occurs in the lighter fractions of the asphalt, specifically the saturates and to some extent the aromatics, as the boiling point of these components is exceeded during production (47). As a result of the loss of light oils the asphalt binder becomes more viscous and exhibits higher stiffness. Concurrently, oxidation reactions occur within the binder, providing the second mechanism of hardening. Short term aging due to oxidation is referred to as an oxidation spurt by Peterson due to the drastic change in properties realized in a short period of time (46). Physically, viscosity is increased by a factor of 2-4 during short term aging (45). The cause of this increase is due to contact of the asphalt binder with the super-heated aggregates during production; the surface area of the aggregates is large and are contacted by the asphalt in a thin film leading to rapid volatilization and oxidation. Oxidation also causes a change in the distribution of SARA fractions with the aromatics turning to resins and resins turning to asphaltenes, which increase by 1-4% and cause the increase in viscosity (19). It was also found through sampling asphalt binder at different time periods during laboratory short term aging that the rate of increase of asphaltenes is linear with aging time (45). Laboratory simulation of short term aging is conducted using two standard test methods, the Thin Film Oven Test (TFO) (AASHTO T179) and the Rolling Thin Film Oven Test (RTFO)

67 51 (AASHTO T240). The TFO is a static test, aging an asphalt film of 3.2 mm thickness at 163 C for 5 hours. The RTFO test ages the samples in a rotating carriage for 85 minutes at the same temperature, as the carriage rotates pressurized air is introduced at regular intervals to promote a faster rate of oxidation. The rotation of the sample container also allows for a higher oxidation rate by reducing film thickness to 5-10 microns. For unmodified asphalts the two test methods produce asphalts with virtually equivalent physical and rheological properties (48). However, it was shown that the TFO was unable to adequately age modified binders due to development of a polymer film or skinning during the test. This skinning effect prevented diffusion of oxygen into the binder, causing a significant decrease in the rate binder of oxidation. In practice the RTFO test is preferred because it is faster, easier to perform, and ages films with thicknesses more representative of what is experienced during HMA production. To date, there has been a lack of research investigating the impacts of reduced short term aging temperatures on asphalt binder performance, however there is potential that the ability of the TFO method to isolate the effects of oxidation has use in applications to WMA. The shortcomings of the current TFO standard method include a prolonged aging time and a film thickness that is not representative of the material placed in the field. Glover, et. al., investigated the potential to reduce film thickness in the TFO test as a means to reduce the testing time required for the aged binder to demonstrate properties equivalent to RTFO aging. Results found that for all asphalts tested a linear relationship between carbonyl area and viscosity existed and the slope of the relationship did not change with film thickness, therefore, it was concluded that that aging with thinner films only impacted the rate of oxidation and not the oxidation reaction mechanism, with higher rates exhibited for thin films due to a smaller diffusion barrier (48).

68 Mechanisms and Test Methods for Long Term Aging Long term aging in asphalt occurs during the service life of the pavement and is caused by reaction of the of the asphalt binder with atmospheric oxygen. The reaction is caused by diffusion of oxygen into the pavement layer, causing the conversion of lighter fractions to asphaltenes and increasing the concentration of polar functional groups within the asphalt binder. The increase in polarity causes a propensity for agglomeration, increasing asphalt binder viscosity and causing embrittlement of the material (46).. The extent of oxidation experienced during short term aging is asphalt binder source dependent. However, after the initial oxidation spurt, all asphalt binders age at a similar rate when aging is conducted at temperatures similar to the conditions experienced while the pavement is in-service. This behavior is clearly demonstrated for the relationship between dynamic viscosity and aging time for the eight SHRP core asphalts aged at 60 C provided in Figure Figure 2-16: Dynamic Viscosity vs. Aging Time for SHRP Asphalts TFO Aged, then PAV Aged at 60 C (46)

69 53 As demonstrated in Figure 2-16 the parallel relationship between aging time and viscosity holds for all asphalt binder sources, indicating that the reduced rate of oxidation at longer aging times observed is independent of asphalt binder source or physical properties. The similar behavior with aging time for asphalts with drastically different physical and chemical properties is caused by the temperature dependence of formation of micro-structure within the asphalt (46). At higher aging temperatures the effect of asphalt binder chemical composition becomes more prominent as molecular mobility becomes more favorable. This is supported by data presented by Peterson, which demonstrated that the variation in the change in asphalt binder viscosity increased with increasing long term aging temperature (46). In practice, the Pressure Aging Vessel (PAV) as specified in AASHTO R-28 is used for long term aging of asphalt binders, subsequent performance tests are conducted at intermediate and low temperatures to evaluate asphalt binder properties associated with durability. The test consists of aging a 3.2 mm film thickness of asphalt binder for 20 hours under a pressure of 300 psi at temperatures ranging from 90 C C. The user is required to select an appropriate aging temperature based on the climactic conditions of the area where the pavement is being placed, with the laboratory aging temperature increasing as the climate changes from cold to hot. The decision to increase aging temperature and age the material under pressure was driven by the practical need to reduce the time required for long-term aging of the material. Based on comparison of laboratory aged material to that extracted from the top surface of field cores, PAV aging demonstrates properties equivalent to that of material that has been in service 4-8 years in locations such as Wyoming and California (45). Further complicating the use of laboratory asphalt binder and aging characterization methods to model field performance is the influence of climatic conditions and mixture

70 54 properties on asphalt binder aging. In the field, the extent of oxidation is influenced by the location in the pavement structure and the permeability of the asphalt mixture. For both factors, the extent of oxidation increases in areas with the most exposure to atmospheric oxygen, thus the most aging occurs at the pavement surface and decreases with increasing depth within the layer. Also, more aging occurs within the depth of the pavement structure for coarse or open graded mixtures due to an increased opportunity for oxygen to react with the asphalt binder. To a lesser extent, hardening of the binder in asphalt pavements is also caused by photo-oxidation due to reaction between the asphalt binder and ultra-violet light (45). It was also found through work by Peterson and Barbour that the presence of mineral aggregates had a negligible effect on asphalt oxidation reaction mechanisms of kinetics (46) Synthesis of WMA Research - Asphalt Binder Performance Initial research efforts focused on the impacts of WMA additives on asphalt binder performance (3), (6), (22). In these studies, WMA modified binders were subjected to the same tests and aging conditions of those used for conventional asphalts. The studies included use of viscosity reducers, foaming by mineral additive, and surfactants. Results consistently found that WMA additives in general do not significantly impact asphalt binder performance properties with the exception of water bearing minerals and waxes which caused an overall stiffening of the binder. For water bearing minerals the stiffening is caused by a filler effect in the binder; however the change in performance properties were insufficient to result in a change in binder performance grade (6). Conversely, the changes in binder performance grade were significant for the wax based viscosity reducing additive, which was found to significantly impact both high and low temperature properties. At high temperatures, Sasobit improved the continuous grade

71 55 by approximately 2 C 5 C. Conversely, the additive is a detriment to low temperature performance causing decreases of up to a one performance grade (-6 C) (3), (6). With the exception of wax additives results in initial studies indicate that the main impact of WMA additives on binder performance observed in laboratory could be due to reduced binder aging when temperatures are reduced, but not the physical presence of WMA additives. Previous research has attempted to modify the standard RTFO short term aging procedure to study this impact by reducing the standard aging temperature by up to 50 C and account for the lower production temperatures associated with WMA. In a research study conducted by Clemson University, the rheological properties of the WMA additives were evaluated at RTFO aging temperatures of 130 C-140 C and compared to the base binder aged under standard conditions of 163 C. All materials were long term aged using the Pressure Aging Vessel (PAV) at standard conditions. Results showed that reducing the short-term aging temperature, caused a significant reduction in the SuperPave rutting parameter G*/sinδ, however the impacts on the performance of PAV aged material was found to be insignificant. Furthermore, significant differences between asphalt sources were observed for all performance properties (49). Similar efforts evaluating the effect of lower aging temperatures were conducted by the FHWA and as part of this study (20), (50). Results were consistent throughout all three studies in regards to the impact of short term aging temperature on both short and long term performance properties. An example of the relationship between asphalt binder high temperature grade and short term aging temperature is provided in Figure 2-17, in the figure binders that include WMA additives are represented by dashed lines.

72 56 Change in HT Cont. Grade RTFO Aging Temperature ( C) FH Control FH + 2%RS FH + 1.5% VR-1 FH + 6% Advera Figure 2-17: Effect of Reduced Binder Aging on HT Continuous Grade (50) The FHWA study identified the sensitivity to aging temperature demonstrated in Figure 2-17 and the susceptibility of the asphalt binder to short term aging as the two prominent factors in predicting the reduction in binder performance for a given production temperature. The susceptibility of the binder to short term aging is referred to as the aging index (AI), which is the ratio of high temperature performance after short term aging at standard conditions to the performance of the un-aged binder. The sensitivity of the asphalt binder to short term aging temperature is characterized as the slope of the change in high temperature performance grade and aging temperature relationship presented in Figure For the purposes of the FHWA analysis, this relationship was assumed to be linear (20). To generalize the model, experimental data is plotted as the slope of the change in high temperature grade vs. aging index and fit with a power law. Using the power law, the study proposed a methodology for estimating the allowable reduction in production temperature based on a user defined decrease in asphalt binder performance grade as

73 57 a function of aging index. An example of this analysis is presented in Figure 2-18, in this figure temperature thresholds based on reductions of one full (-6 C) and one half (-3 C) grade are represented by dashed lines. Target Temperature Reduction for WMA ( C) Aging Index -2 PG -3 PG -4 PG -6 PG Figure 2-18: Reduction in High Temperature Asphalt Performance due to Reduced Production Temperatures for WMA The short term aging protocol developed by FHWA was further evaluated in NCHRP Report 691. In addition to conducting a similar laboratory binder aging study, the NCHRP project team performed a validation study to assess if the laboratory results related to reduced performance in the field. A total of four WMA technologies were evaluated from mixes produced on six different projects. Temperature reductions in the field ranged from 15 C to 40 C. To validate the grade bumping recommendations, asphalt binders from the field produced mix were extracted and continuous grades determined. Results indicated that the reduced temperature realized during field production caused approximately half the reduction in PG grade predicted using reduced RTFOT temperatures (51).

74 58 The information presented in Figure 2-18 represents the first attempt to incorporate consideration of the impacts of selection of WMA production temperatures on binder performance at high pavement temperatures. The methodology of change in HT grade can be used by a specifying agency or contractor the ability to define an appropriate range of production temperatures based on the Aging Index of the particular asphalt binder/wma combination that is intended for use. However, further research into this concept is needed because the models developed in the FHWA study were based on a limited number of modified binders, and did not account for the effects of WMA additives on binder aging. Also no models were developed for intermediate and low temperature performance properties because the material properties measured after PAV aging were in general found to be insensitive to reduced short term aging temperature. Furthermore, subsequent research has shown that the presence of some WMA additives significantly influences the aging index of the binder, thus different additives would result in different acceptable temperature ranges, an example is presented in Figure Aging Index Neat 2% Rediset 1.5% VR-1 6% Advera Binder Flint Hills NuStar Figure 2-19: Summary of Aging Index for Various WMA Additives (50)

75 59 While the current state of knowledge presented provides a method to consider the effects of WMA production temperatures on rutting resistance, research needs remain in developing similar models for intermediate and low temperature properties due to aging. Given that the mechanisms for fatigue and low temperature cracking are related to binder embrittlement, it is expected that reduced aging could result in extension of service can be considered in design and asphalt binder grading protocols. There is also a need to investigate laboratory short term aging methods capable of isolating the effects of short term aging temperature on oxidation from viscosity effects, and that can be used for aging modified binders with high viscosities at lower temperatures Synthesis of WMA Research - Asphalt Mixture Performance Prior to the advent of WMA, reduced performance of conventional HMA mixtures prepared at lower temperatures has been reported (38), confirming the concerns related to mixture performance issues related to rutting and moisture damage. Lower production temperatures introduce the potential for incomplete drying of the aggregate, causing moisture damage through incomplete aggregate coating and due to entrapped moisture in the aggregate. Limited investigations of the potential changes in moisture damage resistance, mixture fatigue and low temperature cracking have been completed Moisture Damage In general, tests used to evaluate the potential for moisture damage in HMA are either strength tests conducted in a split tensile mode or wheel tracking tests, in which a cylindrical or lab compacted slab specimen is subjected to continuous loading cycles. The specification criteria for wheel tracking tests is the rut depth measured at a predetermined number of cycles or

76 60 level of deformation. Most laboratory samples were prepared using dry aggregate; however some studies investigated the influence of aggregate moisture. For the samples prepared using dry aggregate, it is assumed that any decrease in performance was caused by the reduced oxidation related to lower sample aging temperatures. In recent studies, the indirect tensile strength and tensile strength ratio (TSR) was evaluated using both American and European test methods for one of the water bearing minerals additives. In cases in which dry aggregate used, statistically significant differences were not observed in dry tensile strength or TSR between mixes including WMA additives and conventional HMA (5), (14). This finding was in contrast to results presented in NCHRP Report 691 which presented decreases in dry tensile strength and TSR value ranging from 20%-30% and 10%-20% respectively (51). Consistent with the findings of the NCHRP project, wheel tracking results using the Hamburg test in two separate studies found that performance decreases ranging from 50%-90% were realized due to the use of WMA (5), (6). Results from one of these studies are provided in Figure 2-20.

77 61 Figure 2-20: Effect of WMA Additive Type and Concentration on Wet Hamburg Test Results Using Dry Aggregate (6) For the data presented in Figure 2-20, the HMA control sample was compacted at 150 C and the WMA samples using the additives Advera and Sasobit at varying concentrations were compacted at 110 C, all tests were conducted in the Hamburg Wheel Tracking device in the wet condition. Results indicate that performance as measured by rut depth is dependent on compaction temperature, WMA additive type, and WMA additive concentration. The negative influence of reduced temperatures on performance was verified in a study by Mogawer (8). In this study the Hamburg Wheel Tracking device was used in the wet condition at three aging temperatures and two aging times. For both aging times similar 50-90% decreases in performance were realized with decreasing aging temperature, reinforcing the concept that reduced asphalt binder oxidation influences asphalt mixture performance (8). In addition, at both aging times, the addition of WMA additives was found to significantly influence resistance to

78 62 moisture damage with some additives improving performance relative to the control mix and others reducing it. The combined effect of aggregate moisture and lower production temperatures without the use of WMA additives was investigated by Bennert (7). The study included two compaction temperatures, 157 C to represent HMA, and 130 C for WMA, and three levels of aggregate moisture content, 0%, 3%, and 6%. The effect of reduced compaction temperatures and the presence 3% moisture on indirect tensile strength and the TSR value are provided in Figure % Indirect Tensile Strength (psi) % 63% 64% 52% 90% 80% 70% 60% 50% 40% TSR 0 HMA 0% MC WMA 0% MC HMA 3% MC WMA 3% MC 30% Dry Wet TSR Figure 2-21: Influence of Reduced Production Temperatures and Aggregate Moisture on Indirect Tensile Strength and TSR (7) Results presented in Figure 2-21 indicate that both the presence of aggregate moisture and reduced production temperatures cause significant decreases in the tensile strength ratio, mostly due to a decrease in wet indirect tensile strength (7). As a frame of reference, most state specifications require TSR values ranging from 70% 80% for a mix design to be approved,

79 63 based on this requirement both reduced temperatures and the presence of moisture result in a failing mix. These findings were supported by previous studies which evaluated the impact of a residual aggregate moisture content of 0.5% by weight of the aggregate on indirect tensile strength values. Test results showed that the presence of moisture in the aggregate resulted in a reduction in dry and wet tensile strength regardless of the presence of WMA additives (14), (52), as a result the WMA mixes produced at lower temperatures using three different aggregate sources failed the TSR due to both reduced production temperatures and the presence of moisture. In addition, mixes using the water bearing mineral additive, failed during moisture conditioning in a 60 C water bath for two of the three aggregate sources used in the one study (52). These results indicate that if entrapped aggregate moisture exists due to incomplete drying before mixing, an anti-stripping additive or hydrated lime is required to reduce the risk of premature pavement failure due to moisture damage. This finding is significant because the use of these additives increases the unit cost of the mix Resistance to Permanent Deformation Similar to moisture damage, pavement rutting is evaluated using both wheel tracking and strength tests. To evaluate the strength of stability of the mix the repeated load deformation test has been recommended by recently developed mix design guidelines specific to WMA (20). The test applies a haversine load pulse of 0.1 seconds loading and 0.9 seconds unloading on an unconfined test specimen. Permanent strain is monitored during the test and plotted as a function of loading cycles. The evaluation parameter for the test is the Flow Number (FN), defined as the transition from secondary to tertiary flow. Evaluation of mixture rutting using both the Hamburg Wheel Tracking Test and the Flow Number test in various studies have revealed that WMA mixes are more susceptible to rutting

80 64 relative to conventional HMA (53), (54). Specifically, results published in NCHRP Report 691 and in other work by Bennert indicate reductions in the FN parameter 40%-60% due to the use of warm mix asphalt (7), (51). It is proposed that the majority of this reduction is due to the lower mix aging temperatures used for WMA to simulate production conditions in the field. The study by Bennert investigated the effects of three aging temperatures, WMA additive type, and WMA additive concentration. Results are presented in Figure Figure 2-22: Effect of Mixture Short Term Aging Temperature, WMA Additive Type, and WMA Concentration on the FN Parameter (7) Similar to the trends presented for resistance to moisture damage, the data presented in Figure 2-22 indicates that for a given short term conditioning temperature, the FN parameter varies considerably due to WMA additive type and concentration. The relationship between FN and temperature for a given material indicates that for a majority of the samples tested the largest decrease in rutting resistance is observed in the first temperature reduction which relates to the transition from conventional HMA to WMA production temperatures. The significance of this result is that the in most cases even a moderate reduction in production temperature to for WMA

81 65 will cause decreased rutting resistance, thus it must be accounted for in mixture design procedures to ensure that the quality of the WMA is similar to that of the HMA. Further insight into the effects of aging temperature and WMA additive type was provided by Hanz and co-workers (50) using the relationship between between loading cycles and accumulated strain as provided in Figure The effect of reduced aging temperatures on high temperature mixture performance was also documented visually, as demonstrated in Figure % Permanent Strain Number of Cycles 105 2%RS 163 2%RS % VR % VR-1 Figure 2-23: Effects of Aging Temperature and Increased Asphalt Binder Grade on %Permanent Strain vs. Number of Cycles (50)

82 66 2% 105 2% 163 Figure 2-24: Qualitative Effects of Reduced Aging Temperatures on Mixture Rutting Resistance (50) Results support the FN values reported by Bennert, clearly indicating that reduced aging temperatures cause mixtures to accumulate permanent strain at a significantly higher rate relative to those aged at conventional HMA temperatures. Furthermore, the effect of aging temperature overwhelms the potential benefits of using WMA additives that increase the performance grade of the asphalt binder. In Figure 2-24 these effects are also observed visually by the increased crack quantity and width demonstrated by the sample aged at 105 C. Similar relationships between aging temperature and development of permanent strain were also presented by Lee and co-workers in a publication submitted to the International Society of Asphalt Pavements 2010 conference (55). The importance of the sensitivity of asphalt binder performance to aging temperature was further confirmed in a VDOT study which through comparison of WMA performance compacted at temperatures ranging from found rutting susceptibility decreased with increasing compaction temperature (54). As previously mentioned, the negative impacts of reduced aging temperatures on asphalt mixture performance are prevalent throughout literature

83 67 related to evaluation of moisture damage and permanent deformation. Similar behavior in terms of mixture stiffness, as measured by the dynamic modulus are noted by Mogawer, indicating a significant decrease in mixture stiffness (E*) at low frequencies due to reduced aging temperatures, signifying reduced high temperature performance (8) WMA Impacts on Durability Fatigue and Thermal Cracking The previous overview of mixture performance focused on the risks associated with the use of warm mix asphalt, however warm mix asphalt could also have potential benefits related to pavement durability. In recent studies, two different mechanical test methods have been used to evaluate the effects of reduced aging and additives on fatigue life. The cycles to failure (Nf) as determined by the Texas Overlay Tester and visco-elastic continuum damage (VECD) modeling based on uni-axial cyclic direct tension-compression testing were used to quantify the fatigue life of conventional HMA and WMA mixes. In both studies, various additives were used and all WMA mixes were produced at temperatures ranging from 110 C C, with the HMA control mixes produced at 160 C. Fatigue life as estimated by the Texas Overlay tester after short term aging of the mixture was found to increase with decreasing aging temperature, with the most substantial gain in fatigue life realized within the range of WMA temperatures. There was also a significant difference between the performance due to WMA additive type and concentration (7). Conversely, fatigue life estimated based on VECD modeling after long term mixture aging using AASHTO R30 indicates that there is no significant difference in the fatigue performance of WMA and conventional HMA (56). Fatigue cracking of asphalt mixtures and pavements is a complex phenomenon that demonstrates dependence on test parameters such as mode (stress control vs. strain control) and amplitude of loading cycles, and material properties, which could explain the differences in performance.

84 68 To date the potential effects of WMA on thermal cracking resistance has not been well documented. One study conducted in China used a mixture three point bending test conducted at -20 C to measure the fracture energy of HMA and WMA mixtures made with conventional and polymer modified asphalt binders as a means to estimate potential impacts on thermal cracking. Results indicate that the fracture toughness of the WMA mixes was the same or marginally better than that of the HMA control mixes (57). 2.5 Potential Impact of WMA on Reduction of Energy Consumption and Emissions To quantify the potential environmental benefits of WMA recent work has focused on investigations of energy consumed over the life cycle of asphalt pavements. Results indicate that the production of hot mix asphalt (HMA), consumes significant energy resources compared to other life cycle processes. One estimate suggests that 91 percent of all life cycle energy is consumed during the material production (58), with the remaining 9 percent consumed during aggregate production and pavement construction. Detailed analysis of the production process indicates that approximately 53 percent of energy is used to mix and dry aggregates; 43 percent of energy is used in producing bitumen; and 4 percent of energy is used to store bitumen at high temperatures (1). While research has focused on estimating total life cycle costs and overall impact, relatively little research has been conducted to evaluate the critical processes in HMA production from an energy and emissions perspective. Heat used in the production of HMA is one of the main targets in reducing its energy and environmental impact. While reducing heat and increasing use of recycled asphalt are obvious techniques to make asphalt pavements more sustainable, quantifying their benefits and impact on service life are not well defined. To date, field studies and pilot projects suggest that lowtemperature mixes demonstrate equivalent performance to HMA.

85 69 It is clear that in the context of warm mix asphalt, the energy used in production of the asphalt binder has the potential to marginally increase energy consumption due to the addition of warm mix additives. However, a significant opportunity exists to reduce energy consumption in mixing and aggregate drying activities through reduction of production temperatures. As noted in Figure 2-1, many studies suggest that an energy barrier exists in heating aggregates sufficiently to allow for vaporization of remaining water, however quantitative data sets to substantiate this hypothesis are lacking. Several spreadsheets based tools for estimating energy consumption, emissions, and raw materials consumption have been developed. Examples include a plant diagnostic tool developed by the Pennsylvania Asphalt Pavement Association (PAPA) and a model for calculation of energy and emissions develop by the World Bank (1), (59). Representative outputs of the model using various input variables such as fuel type and temperature are shown in Figure The outputs are also compared to the estimates of PAPA tool and the hypothetical plots provided by FHWA.

86 70 Figure 2-25: Existing Models to Capture the Relationship Between Mix Temperature and Fuel Consumption (1) All tools demonstrate a consistent trend between mix production temperature, energy consumption and aggregate moisture content. Mixes produced at higher temperatures and mixes containing aggregates with more moisture demand more energy. Fuel type is also shown to influence energy demand. Discrepancies between hypothetical trends and estimated values are also observed, indicating that more data are needed to verify some of the assumptions made in the models. The effect of changing production temperature on emissions is shown in Figure The estimates shown are based on the World Bank model and demonstrate that similar to energy consumption, fuel type and moisture in the aggregates can have a significant effect on emissions.

87 71 Figure 2-26: Carbon dioxide emissions at HMA plants for various mix temperatures, generated using the World Bank greenhouse gas emissions calculator. Shifts in emissions are shown for different fuel types and aggregate moisture contents (MC). (1) These models and estimates provide an indication of the potential environmental benefits due to production at lower temperatures and the effect of controlling aggregate moisture or choosing the type fuel type on energy and emissions. Based on the information presented in Table 2-1 of this thesis, the operating range of WMA is between 110 and 135. Under the assumption that HMA is produced at 150, use of WMA alone allows for a reduction in energy consumption of approximately 40%. Furthermore, current practice in mixture production involves super-heating aggregates to temperatures exceeding 200 C prior to mixing with hot asphalt binders to drive out excess moisture. Estimates indicate that if super heating is reduced or eliminated additional energy savings can be realized. Currently there is little research published indicating that small amount of residual moisture in the aggregate has negative impacts on HMA performance.

88 72 While the previously summarized models present methods to estimate the environmental benefits associated with lower production temperatures, their application is beyond the scope of this study due to the complexity of translating laboratory measurements to field performance. Field production of HMA and WMA introduces a multitude of additional variables and practical considerations that are not adequately represented by the factors investigated in this study. Furthermore, there is a level of uncertainty present in these models as additional validation through environmental monitoring of production facilities is required.

89 73 3 Experimental Methods 3.1 Overview The goal of this study is to propose modifications to current mixture design and asphalt binder grading protocols to allow for consideration of the effects of WMA additives and reduced production temperatures on the construction and in-service properties of warm mix asphalt. Based on review of available WMA technologies and the current state of the art related to characterization of WMA workability and performance four hypotheses were developed to achieve this objective. These hypotheses were selected to address a diverse range of knowledge gaps present in the current understanding of WMA, separate experimental plans developed to address each area. 3.2 Materials Selection To isolate the factors unique to WMA and leverage the base of knowledge established in development of current specifications, the study will focus on the effects of WMA additive type, WMA additive concentration, and production temperature for conventional and polymer modified binder grades from a single asphalt source. Also, one aggregate source using two gradations representative of the range in specification limits used by state agencies were included in the study Asphalt Binder Selection The asphalt binder source selected is from Venezuela and is a blend of Boscan and Bachquero crude oil sources. The asphalt has been selected as a core material for the Asphalt Research Consortium project (ARC-1) and is similar in chemical composition to SHRP core asphalt AAK. As shown in Figure 2-14, this asphalt contains a relatively high level of

90 74 asphaltenes, ~20%, and also approximately 40% resins. The asphalt was supplied by NuStar Energy and has performance grade of PG To allow for consideration of the impacts of asphalt binder modification styrene butadiene styrene (SBS) polymer at one polymer concentration was used. In asphalt modification SBS is considered an elastomer and is most commonly used to improve resistance to permanent deformation and cracking. The SBS polymer was used in sufficient quantities to increase the performance grade of the unmodified asphalt binder by two grades on the high temperature end, from a PG to a PG The modification was accomplished by laboratory blending of a polymer modified concentrate provided by the supplier with the neat asphalt. Based on laboratory formulation it was determined that a blend consisting of 60% of the polymer modified concentrate and 40% of the neat binder were required to achieve the PG performance grade WMA Additive Selection Warm mix additives were selected from the summary of WMA additive types referenced in Section 2.1 to represent the range of different mechanisms by which they allow for production at lower temperatures. Specifically, four additives were chosen inclusive of the mechanisms of foaming, viscosity reduction, and decrease in internal friction through use of chemicals and surfactants. To investigate the impacts of additive concentration, high and low concentrations of the additive were defined based on manufacturer recommendations. For all asphalt binder testing, additives were pre-blended with the asphalt binder using low shear mixing for a time period of at least 10 minutes. For use in mixtures, the additive was introduced according to manufacturer recommendations and included either pre-blending with the asphalt binder or incorporation into the aggregate immediately before preparation of the asphalt mix. The WMA

91 75 additives selected, their concentrations, and how they will be incorporated into mixture preparation are summarized in Table 3-1. Table 3-1: Summary of Selected WMA Additives and Concentrations WMA Mechanism Foaming: Additive WMA Additive and Sample Code Advera HZ Concentration (by Wt. of Binder) 4% 8% Delivery to Mix Additive delivered to mix immediately before asphalt binder. Wax: Viscosity Reduction Sasobit W 2% 4% Pre-blended with asphalt binder. Chemical Additive Increase Internal Lubrication Oxidized Polyethylene PE 1% 3.5% Pre-blended with asphalt binder. Chemical Surfactant and Wax: Combination of increase in internal lubrication and viscosity reduction. Rediset AN 1% 2% Pre-blended with asphalt binder Selection of Aggregate Source and Gradation Aggregate from a granite source commonly used for HMA mix production in Wisconsin was used in the study. The source properties, as measured biannually, by the Wisconsin Department of Transportation are provided in Table 3-2. Table 3-2: Summary of Aggregate Source Properties for Selected Aggregate Sources Source Aggregate Type Location Soundness (%) LA Wear 100 LA Wear 500 Spec. Gravity Absorption (%) Cisler Granite NC WI

92 76 The source properties of the aggregate provided in Table 3-2 indicate that it is resistant to wear and demonstrates freeze thaw durability as shown be relatively low values of LA Wear and soundness testing. The aggregate soundness is test is specified in AASHTO T104 which evaluates durability by measuring the weight loss in the aggregate after five cycles of immersion of the aggregate sample in a saturated solution of magnesium sulfate or sodium and drying. The use of the solution simulates formation of ice crystals by promoting growth of salt crystals in aggregate pores, thus the test provides an estimate of freeze thaw durability. For use in HMA, WisDOT requires that the aggregate demonstrate a maximum loss in weight of 12%. In application to WMA, soundness is particularly important as the potential for incomplete aggregate drying due to lower production temperatures causes entrapped water in the aggregate, in turn leading to formation of ice crystals in northern climates. The low absorption of the selected aggregate source implies that a low amount of asphalt will be absorbed during mixing and compaction. Absorbed asphalt is considered in evaluation of mixture volumetrics. For WMA there is potential that due to the relationship between viscosity and temperature asphalt absorption into the aggregate will decrease with decreasing production temperatures, causing a change in overall mixture volumetrics. The low absorption of the aggregate source reduces the impact of changing absorption on volumetrics, eliminating a potential reason for variation in density for WMA. Previous research findings indicate that aggregate gradation significantly influences mixture workability (50). To capture these effects fine and coarse gradations were selected, specifically the material passing the No. 4 sieve is 65% and 40% for the fine and coarse respectively. Plots of the gradations with respect to the 0.45 maximum density line along with the control points specified by WisDOT are provided in Figure 3-1.

93 77 % Passing Sieve Size Raised to the 0.45 Power Fine Coarse Max Denisty Control Points Figure 3-1: Gradation Curves for Fine and Coarse Mix Designs Plotted on the 0.45 Power Maximum Density Graph Both aggregate gradations are achieved through blending of five aggregate stock piles, four of which are different size fractions of crushed granite aggregate ranging from ¾ stone to manufactured sand, the fifth stock pile is a natural sand imported from a source near the location of the quarry. The fine and coarse gradations contain 90% and 96% crushed material respectively. Use of predominantly crushed aggregate in mix design is required to ensure that the aggregate blend demonstrates the appropriate angularity to provide aggregate interlock while in service. In practice, aggregate angularity is controlled by limits on fractured faces for the coarse aggregate and fine aggregate angularity, with the specification thresholds increasing for increasing design traffic level. To evaluate the impacts of gradation on mixture workability and performance the mixture gradations selected were quantified through modeling using the Weibull Cumulative Distribution (60). The function is expressed mathematically in Equation 3-1.

94 78 Equation 3-1: Weibull Distribution to Model Aggregate Gradation,, 1 Where: x = Sieve size α and β = Regression constants In the model governs the slope of the gradation curve, thus is most appropriate to differentiate between dense and open graded mixtures. The parameter shifts cumulative distribution curve horizontally, representing the fineness of the aggregate gradation (60). These differences are presented visually in Figure 3-2 and Figure 3-3. The success of the Weibull Distribution in fitting the proposed graduations is evaluated using the sum of square errors (SSE) between the model and actual gradation. Figure 3-2: Effect of the α Parameter on Gradation Modeled Using the Weibull Distribution (60)

95 79 Figure 3-3: Effect of the β Parameter on Gradation Modeled Using the Weibull Distribution (60) The fine and coarse gradations for the granite aggregate blends used in this study had values of α of 0.75 and 0.97 and β of 4.28 and 7.81 respectively. The function fit to each gradation had acceptable values of SSE, a detailed summary of the fitting is provided in Appendix A. These numerical parameters allow for aggregate size distribution to be incorporated into regression models predicting asphalt mixture workability and performance. Both mixes were designed to conform to WisDOT E-10 mix design specifications. Asphalt mixes designated as E-10 are appropriate for traffic loadings up to 10 million equivalent single axle loads (ESALs) throughout the service life of the pavement (61). The design level of gyrations (N des ) represents the level of compactive effort in the SuperPave Gyratory compactor after which the mix design must achieve 4% air voids and other volumetric criteria, for the E10 mixes N des is 100 gyrations. For the mix designs used in this study, both mixes met the volumetric criteria at a design binder content of 5.0%. Detailed mix design information is provided in Appendix A.

96 Evaluating the Impacts of WMA Additives on Asphalt Binder Workability Overview The objective of this task is to justify the need for consideration of the internal friction of the asphalt binder, as measured by the Asphalt Lubricity Test, in evaluation of WMA effects. The objective is achieved by first evaluating the sensitivity of asphalt binder viscosity to the presence of WMA additives and then by introducing the Asphalt Lubricity Test as a method to measure thin film behavior of asphalt binder under normal stress. Measurement of thin film behavior provides an opportunity to apply concepts prevalent in the lubricating oils industry to evaluation of asphalt binder internal friction (called here lubricity), allowing for an improved understanding of the role of asphalt as a lubricant in compaction of Hot Mix Asphalt. Validation of the Asphalt Lubricity Test usefulness is pursued through comparison to mixture workability data collected for conventional and WMA modified mixes Conventional Measures of Asphalt Binder Workability Viscosity The standard property to evaluate asphalt binder workability is viscosity as measured by the Brookfield Rotational Viscometer. Current practice, controls workability through specification of mixing and compaction temperature ranges based on values obtained from the viscosity temperature profile. For un-modified binders the procedure for determining mixing and compaction temperatures is providing in AASHTO T316. In this procedure asphalt binder viscosity is measured at a shear rate of 6.8/s at different temperatures, mixing and compaction temperatures are then defined as the temperatures associated with viscosity ranges of 170 +/- 20 cps (for mixing) and 280 +/- 30 cps (for compaction).

97 81 The AASHTO T316 procedure predicts unreasonably high mixing and compaction temperatures for polymer modified asphalts due in part to the shear rate sensitivity of polymer modified systems (30). To properly account for shear rate sensitivity a low-shear viscosity method based on the results published in NCHRP Report 459 was selected to determine the mixing and compaction temperatures of the polymer modified control and WMA binders used in this study. Specifically, the method estimates viscosity at a shear rate of 0.001/s and uses viscosity thresholds of 3000 cps and 6000 cps for mixing and compaction temperatures respectively. In application to this study, these thresholds were reduced to 1000 cps and 2000 cps to provide estimated mixing and compaction temperatures that were representative of current practice and to serve as a baseline for temperature reductions in assessing the workability of WMA. Due to torque limitations of the Brookfield RV, the range of shear rates used in the testing protocol vary for different test temperatures. Testing included both the high and low concentrations of the WMA additives provided in Table 3-1. The effect of WMA additives was represented as the viscosity ratio to represent the change in viscosity due to the presence of WMA additives and through comparison of mixing and compaction temperatures for WMA binders relative to control binders Asphalt Binder Lubricity Test Repeatability Development An experiment was designed to develop the Asphalt Lubricity Test procedure, establish test repeatability, and develop analysis methods based concepts from the oils and lubricants industry. The procedure development phase is focused on establishing the repeatability of the test and sensitivity to test parameters. Test parameters identified include test temperature, testing speed, and normal force. To evaluate the sensitivity of the test to changes in materials two

98 82 WMA additives and two levels of binder modification were included in the experimental design, which is provided in Table 3-3. Table 3-3: Experimental Plan for Sensitivity Analysis and Evaluation of Repeatability of the Asphalt Lubricity Test Factor Levels Value Asphalt Binder Grade 2 WMA Additive Type 3 Testing Temperature ( C) 3 Normal Force (N) 2 Testing Speed (rad/s) 3 Replicates 3 PG PG Control Rediset (AN) 2% Oxidized Polyethylene (PE) 1% The testing procedure was developed based on work by Hupp and Kavenpour, in which thin film behavior of lubricants was evaluated using a constant normal force and varying speed (39), (40). In these procedures tests were conducted at a constant temperature and two levels of normal force, 10N and 20N. For a given sample the normal force was held constant, with two levels selected as a means to verify the test procedure (40). In regards to testing speed, the procedure by Hupp recommends testing in a range from rad/s 100 rad/s (0.009 RPM 900 RPM) (40). For the testing geometry used for the Asphalt Lubricity Test these conditions were deemed too extreme, thus speeds ranging from 0.5 rad/s 5 rad/s were selected to ensure testing safely remains within the limits of the DSR. The other modification to the published test

99 83 procedure was the variation of testing temperature to coincide with the range in temperatures realized during production of HMA and WMA. For a given temperature and speed, torque and normal force were recorded for three minutes with data collected at 12 second intervals. Values were monitored to ensure that consistent readings were achieved throughout the test. The data collection interval was based on the recommendation by Hupp to allow for at least one full cycle of rotation between data points due to the potential for shear stress measurements to vary by as much as 5000 Pa during a given cycle (40). Given the sampling interval test responses are collected after 1, 4, and 10 cycles for the selected testing speeds. The matrix provided in Table 3-3 was used to develop the final test procedure and assess the feasibility of using conventional analysis methods from the lubricating oils industry to evaluate asphalt binders. Statistical methods were used to identify the significance of changes in testing parameters and material types, allowing for selection of appropriate testing conditions and establishing the ability of the test to differentiate between material types. Previous applications of the Asphalt Lubricity test have compared the performance of materials using the coefficient of friction value measured at one speed (4). As presented in Section 2.3.1, this approach may not be suitable due to the dependence of coefficient of friction on speed, normal force, and fluid viscosity demonstrated by the Stribeck Curve. The literature review also indicates that a DSR based system can be successfully applied to evaluate the friction properties of both Newtonian and visco-elastic lubricants (39), (40). Based on these findings analysis methods adapted from the lubricating oils industry were used to identify the region of Stribeck behavior that the Asphalt Lubricity Test and how it is influenced by changes in materials.

100 Effects of WMA Additives and Concentration on Asphalt Binder Lubricity The final Asphalt Binder Lubricity Test procedure and analysis methods were implemented to assess the impacts of WMA additive type and concentration on the asphalt binder coefficient of friction. The experimental design is presented in Table 3-4. Table 3-4: Experimental Design for Evaluation of the Effects of WMA Additive Type and Concentration on Asphalt Binder Lubricity Factor Levels Value Asphalt Binder Grade 2 WMA Additive Type 4 PG PG Control Rediset (AN) Oxidized Polyethylene (PE) WMA Additive Concentration 2 Synthetic Wax (W) High Low Replicates 2 For the test matrix in Table 3-4 the high and low levels of concentration vary depending on the additive type, as previously provided in Table 3-1. The hydrated mineral additive Advera was omitted from the experimental design due to difficulties in running the test caused by foaming of the asphalt at higher temperatures. Analysis of Variance was applied to the results to identify the effects of additive concentration and the testing conditions at which the performance of the selected WMA additives significantly differs from that of the control binders Impacts of Asphalt Binder Properties on Mixture Workability To validate the internal coefficient of friction measured by the Asphalt Binder Lubricity test and identify other factors contributing to mixture workability mix designs were prepared

101 85 with control binders and selected WMA additives. The relationship between mixture air void content and temperature was established as means to evaluate the current WMA mix design procedure presented in NCHRP Report 690 and estimate production temperatures for the mixes selected. The summary of mixes used and temperatures of compaction is provided in Table 3-5. Table 3-5: Experimental Design for Mixture Workability Testing to Validate the Asphalt Lubricity Test Binder Grade WMA Additive Type WMA Additive Concentration Gradation Compaction Temperature ( C) Control N/A Fine X X X Coarse X X X PG PE 1% Fine X X X Fine N/A X X 2% Coarse X X X 3.5% Coarse X X X AN Control 1% 2% N/A Fine X X X Coarse X X X Fine X X X Coarse X X X X X X PG PE 1% X X X Coarse 1% X X X AN 2% N/A X X Total Data Points for Regression

102 86 All mixes evaluated were prepared at the optimum asphalt content. The variables selected for this experiment include temperature of compaction, binder grade, WMA additive type, and WMA additive concentration. The compaction temperatures selected are based on the range of temperatures realized in mixture design and placement of conventional HMA and WMA. It has been well established through previous research that both asphalt binder and aggregate properties influence mixture workability (38), (62). Analysis of variance was used to quantify the influence of these components and to evaluate the need for the Asphalt Lubricity Test. The measurable properties considered in this study are provided in Table 3-6. The properties were not selected arbitrarily but rather were based on review of the literature and hypothesizing how these measures could logically affect densification. Table 3-6: Asphalt Binder Properties Influencing Mixture Workability Mix Component Aggregate Property Gradation and Packing Characteristics Evaluation Parameter Beta (β) Justification The Weibull Distribution Parameter (β) represents the shape of the gradation curve for dense graded mixes. Asphalt Viscosity (η) η as function of temperature Conventional workability parameter, previous research indicates that viscosity does not impact laboratory compaction below 50 Pa s Lubricity Coefficient of Friction (µ) as a function of temperature. Evaluate lubricating properties of conventional and WMA modified binders. The study was limited to one aggregate source to negate the potentially confounding effects of variation in aggregate source properties in evaluating the effects of gradation and WMA additives on the mixture density vs. temperature relationship. In the study, aggregate packing characteristics are represented by gradation through use of the β parameter, two

103 87 gradations were used based on the identification of gradation as a significant contributor to workability in past research (32). Although by weight the amount of asphalt binder present in mixes is small relative to the quantity of aggregate, the asphalt is the mixture component that is temperature sensitive. Therefore, it is proposed that the success of WMA is contingent upon both defining the asphalt binder property that controls mixture workability at lower temperatures and the ability to modify this property through the use of WMA additives. To support this effort both the asphalt binder viscosity and coefficient of friction were selected as for consideration in the experimental design. In current practice laboratory compaction of asphalt mixtures is achieved through use of the SuperPave Gyratory compactor (SGC). To compact the sample, the device applies a normal pressure of 600kPa and rotates the gyratory mold at a speed of 30 RPM. During compaction the mold is tilted such that an internal angle of 1.16 is achieved. The gyratory compactor was selected over the standard Marshall method of compaction to better simulate compaction in the field as it imposes an internal angle in conjunction with rotation of the mold during compaction to create a kneading action representative of what is experienced during the vibratory compaction used in construction. An example of the gyratory compactor and a schematic are provided in Figure 3-4 and Figure 3-5.

104 88 Figure 3-4: Gyratory Compactor Troxler Model 5850 ( Mold R e Ram HMA Sample GLPA o Figure 3-5: Schematic of Gyratory Compactor

105 89 During compaction the SGC monitors the height of the sample after each gyration. Evaluation of mixture densification requires measurement of the bulk specific gravity of the compacted sample (G mb ) and the theoretical maximum specific gravity of a loose sample (G mm ). Procedures to measure these values are provided in AASHTO T331 and AASHTO T209 respectively. The density of the sample at a given level of gyrations is defined as the ratio of bulk specific gravity to maximum specific gravity (%G mm ), with the level of air voids in the sample found by subtracting the %G mm from 100%. Using the relative density a compaction curve is generated to evaluate the density of the mix as a function of SGC gyrations. An example densification curve, including the effect of compaction temperature and the current SuperPave criteria for E-10 mix designs is provided in Figure % Gmm N ini = 8 %Gmm < 89% N des =100 %Gmm = 96% N max =160 %Gmm < 98% Log Gyrations PG C PG C Figure 3-6: Example of Compaction Curve and Identification of SuperPave Requirements The SuperPave Mix design method is specified in AASHTO R35. As demonstrated in the Figure 3-6 the mix design method has established volumetric criteria at three levels of

106 90 compactive effort to characterize the performance of the mix throughout the design life of the pavement. The level of traffic that the mix is designed for is accounted for by changing the compaction levels at which density is evaluated. For example, the mix presented in Figure 3-6 is designed for a traffic level of 10 million equivalent single axel loads (ESALs) and thus N ini, N des, and N max are defined 8, 100, and 160 gyrations respectively. For a mix designed to perform at a reduced traffic level of 3 million ESALs the compactive effort at which density is evaluated is reduced to 7, 75, and 115 gyrations. In relation to field performance the levels of compactive effort have been empirically related to the following aspects of pavement performance: N initial : Represents the constructability of the mix, a maximum value is required to ensure the mix has proper aggregate structure and to prevent tenderness. Mixes that do not meet this requirement are prone to over compaction during construction and potential instability under traffic loading (63). N design : Represents the air void content that the pavement is expected to experience throughout a majority of service life. The level of air voids at N des is the most closely controlled (4% +/- 0.5%) and is used as a basis to calculate other volumetric properties of the mixture. N max : A maximum density is specified to ensure that the mix will remain stable at the end of the design life when traffic levels approach the maximum value. As demonstrated above the mix design procedure in AASHTO R35 requires that higher quality aggregate structures be used for high traffic mixes by increasing the compactive effort to adjust for increasing traffic loads while holding the volumetric requirements constant. At the design level of compactive effort additional volumetric parameters are calculated to better describe the packing characteristics and control the quantity of effective asphalt in the mix.

107 91 Specifically, these properties are voids in mineral aggregate (VMA) and the voids filled with asphalt (VFA). The equations used to calculate these properties are provided in Appendix A. Both VMA and VFA are related to the volume of effective binder content, which is defined as the total binder content less the binder absorbed by the aggregate. Physically, the VMA represents the voids created by the aggregate structure that are available to accommodate both the required air voids for the mixture and the asphalt that is not absorbed by the aggregate. VMA is the volumetric property most related to performance as by definition it allows for consideration of both the volume of air and effective asphalt in the mix. The effective asphalt and air have varying influences on mixture performance. In regards to rutting resistance these components are the most easily deformed upon loading, thus a lower VMA is required to ensure rutting resistance. Conversely, for a given level of air voids more effective asphalt content is needed to resist fatigue and thermal cracking, thus a higher VMA is required to ensure durability (63). To account for both these effects recent mix design procedures recommend that both minimum (to ensure durability) and maximum (to prevent rutting) limits on VMA be imposed. VMA is also related to asphalt mixture workability, as aggregate structures that are too closely packed do not have sufficient void space to accommodate the asphalt binder required to achieve adequate compaction in the field. VFA represents the voids in the aggregate structure filled with asphalt and is used to indicate that there is a sufficient amount of asphalt binder in the mix and that the aggregate void structure selected is appropriate for use in asphalt mixture design. A mix that is designed with poor packing characteristics will result in a low VFA for high amounts of asphalt binder content, indicating that the void spaces created are too large. State agencies specify a range in VFA, the lower limit (65%) is in place to ensure that there is sufficient asphalt in the mix for durability and

108 92 the upper limit is used to ensure the mix isn t over-asphalted. Recent research has found that the effective binder content is a better indicator than VFA of mixture fatigue resistance, however VFA provides an additional means for state agencies to ensure there is adequate asphalt binder in the mix (63). The previously defined volumetric parameters and a measure of asphalt mixture workability were used as the basis to develop predictive equations to define the roles aggregate gradation and asphalt binder properties in the ability of the mix to achieve volumetric performance. The specific models required and the corresponding performance criteria are provided in Table 3-7. Table 3-7: Models Required and Threshold Values for Determining Binder Property Limits Property Parameter Criterion % N ini >11% Air Voids 4% +/- 0.5% Air % Gmm@N des Voids Volumetrics %Gmm@N max * >2% Air Voids VFA 65% - 75% VMA >13% Change due to a 30 C decrease in Compactability N92 compaction temperature >25% *Predicted from compaction curve Potential Model Parameters Aggregate Weibull Paramter (β) Asphalt Viscosity Coefficient of Friction In addition to providing a means to validate the asphalt binder coefficient of friction as a material property related to mixture workability, the experimental plan presented in Table 3-5 was applied to the mix design guideline presented in NCHRP Report 691 to evaluate the proposed procedure and assess the impacts of the WMA additives selected. The mix design

109 93 procedure and specification limits proposed by NCHRP Report 691 are provided in Table 3-8 (51). Table 3-8: NCHRP 9-43 Criteria to Verify Mixing and Compaction Temperatures for WMA (20) Test Testing Temperature Evaluation Criteria Aggregate Coating (AASHTO T195) Volumetric Mix Design WMA Mixing Temperature (MT) WMA Compaction Temperature (CT) Compactibility WMA CT- 30 C % Aggregate Coating>95% Air Voids at N ini, N des, N max VFA VMA Evaluating the Impacts of WMA on Moisture Damage Overview As summarized in Section , numerous asphalt mixture performance tests indicate that the risk associated with premature failure caused by distresses related to moisture damage in WMA pavements is significant. The causes of increased moisture susceptibility are an artifact of the changes to conventional construction procedures to accommodate the lower production temperatures required for WMA. In relation to performance there is potential that WMA additives and reduced production temperatures impact the individual components of the mix, asphalt binder and aggregate, as well as bonding at the asphalt aggregate interface. Due to lower production temperatures and the presence of WMA additives the binder experiences less age hardening which has potential to be a detriment to performance as the asphalt binder can be more readily stripped from the aggregates in the presence of moisture. Furthermore, based on current construction practices lower production temperatures can cause

110 94 incomplete aggregate drying prior to incorporation of the asphalt binder in conventional drum mixing plants. As a result moisture is entrapped in the aggregate presenting a mechanism to deteriorate the bond between the asphalt and aggregate that is assumed to be inconsequential in conventional HMA. In addition to impacting the components of the mix use of WMA also has potential to influence interactions at the asphalt/aggregate interface. As discussed in Section coating of the aggregate surface with asphalt and the subsequent creation of the bond is a function of viscosity and surface energy. Lower production temperatures cause an increase in asphalt binder viscosity, thus more mechanical action is required to ensure aggregate coating. In addition higher viscosity impedes absorption of the asphalt into the aggregate potentially reducing the strength of the bond between the materials, presenting yet another circumstance that increases the moisture susceptibility of WMA. WMA additive manufacturers were cognizant of the aforementioned challenges regarding potential for increased moisture damage in WMA, introducing additives that include surfactants or other chemicals meant to offset these negative effects. Specifically, these additives are aimed at modifying the interaction between the aggregate surface and binder such that bonding to the asphalt is preferred over creation of bonds with water. An experiment was designed to quantify the factors causing increased moisture susceptibility for WMA and define the potential benefits of WMA additives through evaluation of the bond at the asphalt/aggregate interface and mixture performance after moisture conditioning Impacts of WMA on Bonding at the Asphalt/Aggregate Interface The recently developed Bitumen Bond Strength (BBS) test was selected to assess the impacts of reduced production temperatures, WMA additives, and the effects of moisture

111 95 conditioning on the bond at the asphalt aggregate interface. The BBS test is a pneumatic adhesion test adapted from the paint and coatings industry (ASTM D4541) for application to evaluation of the bond created at the asphalt/aggregate surface. The procedure developed for this application has recently been accepted as a provisional AASHTO Test Method (TP-91). The new test method was established based on modifications to the device, sample preparation, and analysis procedures recommended in recent research efforts to evaluate moisture damage in hot applied binders and the rate of curing of fresh emulsions (36), (64). The BBS test involves subjecting a pull stub adhered to an aggregate substrate to a normal force created by increasing pneumatic pressure. The bond strength is defined as the maximum pull off pressure exerted by the machine. After testing, the failure surface is visually examined to determine if the failure mode was adhesive or cohesive. The testing apparatus consists of a machine that regulates and monitors the pneumatic pressure, a loading piston and air bladder to generate the normal force, and a pull stub to bond the asphalt to a given substrate. A key component of the new procedure was the development of a new testing stub capable of controlling the thickness of the asphalt binder film placed on the substrate. Control of the film thickness was achieved through machining of a step at the edge of the stub and creation of a rough surface to promote bonding. The testing device and pull stub are provided in Figure 3-7 and Figure 3-8.

112 96 Figure 3-7: Bitumen Bond Strength Testing Device Figure 3-8: Picture and Schematic of Newly Designed BBS Pull-Off Stubs To conduct the test, the stub is adhered to the substrate by asphalt binder and the loading piston is attached to the stub. The pneumatic pressure during the test is created by inflation of the air bladder (pressure ring) located inside of the loading piston. A schematic of the loading piston and pull stub attached to the aggregate surface is provided in Figure 3-9

113 97 Pullout Stub Pressure Plate Pressure Ring Pullout Stub Ring Support Asphalt Binder Substrate Figure 3-9: Schematic of BBS Test Assembly at the Asphalt/Substrate Interface During the test the air pressure is increased at a constant rate until it is sufficient to remove the stub from the substrate. The BBS test allows for quantitative and qualitative evaluation of the bond at the asphalt/aggregate interface through use of the pull off tensile strength (POTS) and visual examination of the failure surface to determine if the failure mode is cohesive or adhesive. Cohesive failure occurs within the asphalt binder film and indicates that the internal strength of the asphalt binder is the weakest component of the asphalt/aggregate system, thus there is no aggregate surface exposed on the failure surface. This type of failure is most often observed in testing of dry samples prior to moisture conditioning. Moisture conditioning promotes stripping of the asphalt from the aggregate surface, thus the failure mode is a combination of adhesive and cohesive, as for the adhesive failure mode there is a portion of the aggregate exposed on the failure surface. The effect of moisture conditioning on both POTS and pull off pressure vs. time and an example of differentiation between failure modes are provided in Figure 3-10 and Figure 3-11.

114 98 Pull Off Pressure (psi) Loading Rate (psi/s) Maximum Pull Off Time (s) Dry Moisture Conditioned Figure 3-10: Effects of Moisture Conditioning on the Pull Off Pressure vs. Time Relationship and Definition of the POTS Figure 3-11: Example of Cohesive Failure (Left) and Adhesive Failure (Right) on Granite Substrate The capabilities of the BBS test to quantify the strength of the bond at the asphalt/aggregate interface and differentiate between cohesive and adhesive modes of failure were applied to evaluation of the potential impacts of WMA on moisture damage. To maintain consistency with conditions experienced in the field, the experiment was designed to assess the