Review of ODOT s Overlay Design Procedure

Transcription

1 Final Report Review of ODOT s Overlay Design Procedure Volume 2 of 2 PCC Overlays of Existing Composite Pavements Jagannath Mallela, Leslie Titus Glover, Michael I. Darter, and Eddie Y. Chou Applied Research Associates, Inc. 100 Trade Centre Drive Suite 200 Champaign, IL for the Ohio Department of Transportation Office of Research and Development and the U.S. Department of Transportation Federal Highway Administration State Job Number November 2008

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3 1. Report No. FHWA/OH-2008/8 4. Title and Subtitle 2. Government Accession No Review of ODOT s Overlay Design Procedures: Volume II PCC Overlays of Existing Composite Pavements 7. Authors Jagannath Mallela, Leslie Titus Glover, Michael I. Darter, and Eddie Y. Chou 9. Performing Organization Name and Address Applied Research Associates, Inc. 100 Trade Centre Drive, Suite 200 Champaign, IL Sponsoring Agency Name and Address Ohio Department of Transportation 1980 West Broad Street Columbus, OH Supplementary Notes 3. Recipient s Catalog No 5. Report Date November Performing Organization Code 8. Performing Organization Report No. 10. Work Unit No. (TRAIS) 11. Contract or Grant No Type of Report and Period Covered Final Report July 2005 November Sponsoring Agency Code 16. Abstract ODOT initiated this research study to determine (1) the impact of milling off portions of the existing pavement on the structural capacity of the remaining pavement and (2) whether currently recommended HMA structural coefficients adequately reflect the structural properties of new HMA overlay materials. The study mainly focused on the impact of milling on the design of HMA overlays over existing flexible pavements and composite pavements. However, the impact of milling of composite pavements on unbonded overlay design was also another objective of this study. This volume deals with the latter objective and presents findings of a detailed evaluation of four composite pavement projects (eight pavement test sections) located in Southeast Ohio. The report presents descriptions of the data collected, data analysis, observations, and recommendations for improvements of the current ODOT overlay design procedure. 17. Key Words Pavement design, distress, nondestructive deflection testing, maximum deflection, unbonded overlay design, effective thickness. 18. Distribution Statement No restrictions. This document is available to the public through the National Technical Information Service, Springfield, Virginia Security Classif.(of this report) Unclassified Form DOT F (8-72) 20. Security Classif. (of this page) (20) No. of Pages Unclassified 33 Reproduction of completed pages authorized 21. Price i

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5 REVIEW OF ODOT S OVERLAY DESIGN PROCEDURES VOLUME 2 OF 2: PCC OVERLAYS OF EXISTING COMPOSITE PAVEMENTS Prepared by: Jagannath Mallela, Leslie Titus Glover, and Michael I. Darter Applied Research Associates, Inc. 100 Trade Centre Drive, Suite 200 Champaign, IL Eddie Y. Chou Engineering Consultant Prepared in cooperation with the Ohio Department of Transportation and the U.S. Department of Transportation, Federal Highway Administration November 2008 iii

6 DISCLAIMER The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Ohio Department of Transportation or the Federal Highway Administration. This report does not constitute a standard, specification or regulation. iv

7 TABLE OF CONTENTS CHAPTER 1. INTRODUCTION... 1 Background... 1 Overview of ODOT Procedure for Unbonded PCC Overlays... 2 Investigation Scope and Objectives... 4 CHAPTER 2. STRUCTURAL EVALUATION OF SELECTED COMPOSITE PAVEMENT PROJECTS... 7 Selection of Pavement Projects for Structural Evaluation... 7 CHAPTER 3. DATA ANALYSIS AND FINDINGS Analysis of Field and Laboratory Testing Results Observations from Dynaflect Data Analysis Observations from FWD Data Analysis CHAPTER 4. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS29 Summary Conclusions Recommendations REFERENCES v

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9 LIST OF FIGURES Figure 1. Summary of observed trends in maximum deflections for HMA-overlaid composite pavements Figure 2. Summary of observed trends in SPR for HMA-overlaid composite pavements Figure 3. Summary of observed trends in AREA for HMA-overlaid composite pavements Figure 4. Summary of observed trends in E P for HMA-overlaid composite pavements Figure 5. Summary of observed trends in k-value for HMA-overlaid composite pavements Figure 6. Summary of observed trends in D eff for milling off all of the HMA overlay of an HMA-overlaid composite pavements Figure 7. Summary of observed trends in FWD first sensor (maximum) deflections for HMA-overlaid composite pavements Figure 8. Summary of observed trends in FWD third sensor deflections for HMAoverlaid composite pavements Figure 9. Summary of observed trends in FWD last sensor (subgrade) deflections for HMA-overlaid composite pavements Figure 10. Summary of observed trends in E P for HMA-overlaid composite pavements Figure 11. Summary of observed trends in k-value for HMA-overlaid composite pavements vii

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11 LIST OF TABLES Table 1. General description of the pavement projects selected for evaluation Table 2. Timeline showing significant historical construction and maintenance and rehabilitation (M&R) events for FAY-71 (NB and SB) Table 3. Timeline showing significant historical construction and M&R events for FAY/MAD-71 (NB and SB) Table 4. Timeline showing significant historical construction and M &R events Table 5. Timeline showing significant historical construction and M &R events for MOT-4 (NB and SB) Table 6. Summary of analysis of variance (ANOVA) results for Dynaflect deflection testing Table 7. Summary of analysis of variance (ANOVA) results for FWD deflection testing ix

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13 CHAPTER 1. INTRODUCTION Background The first generation of rehabilitation of existing portland cement concrete (PCC) pavements in Ohio consisted of repairing the deteriorated PCC using a combination of various concrete pavement repair (CPR) treatments followed by the placement of a 3 inch hot mix asphalt (HMA) overlay on the repaired pavement, resulting in what is known as a composite pavement. With the aging of repaired composite pavements, the need for additional rehabilitation has become increasingly important. There are several feasible alternatives for rehabilitating composite pavements including the placement of an unbonded overlay. Although an unbonded overlay can be placed without repairing the existing pavement, in most instances, the condition of the HMA surface makes it impossible to place the PCC overlay without some form of surface preparation or repair. Surface preparation for unbonded overlays typically takes the form of: Milling off portions of the existing HMA layer to prepare a level surface for PCC overlay placement. This is acceptable if the HMA surface and the underlying PCC pavement is in a relatively good condition and the existing HMA layer is thick enough. Removing the entire HMA layer, repairing deteriorated joints and cracks, and placing a 1-in or more or a binder-rich or high mastic HMA interlayer prior to PCC overlay placement. Typically, this is done for existing composite pavements in an advanced stage of deterioration. The Ohio Department of Transportation s (ODOT s) procedure for unbonded PCC overlay design is based on nondestructive deflection testing (using Dynaflect deflection testing equipment). Pavement deflections obtained through nondestructive testing (NDT) are used to estimate the existing composite pavement effective thickness (D eff ). This procedure has been implemented ODOT s overlay design software, DOITOVER. The ODOT unbonded PCC overlay design procedure does not directly address the impact of milling off portions of or the entire existing HMA surface layer on the computed D eff. ODOT initiated this research study to determine the impact of milling off portions of the existing composite pavement on the structural capacity of the remaining pavement. This report (volume 2 of 2) focuses on understanding and quantifying the impact of milling off portions of the existing composite pavement on the structural capacity of the remaining pavement and presents a detailed description of the projects evaluated, field testing conducted, analysis of field test and other collected data, structural evaluation 1

14 results, analysis of results, and recommendations for improvements of the current ODOT unbonded PCC overlay design procedure. Overview of ODOT Procedure for Unbonded PCC Overlays ODOT defines an unbonded PCC overlay as the placement of a new PCC layer on top of an existing deteriorated PCC pavement with a thin layer of asphalt in between the existing and new PCC layers to act as a bond-breaker. The thin layer of asphalt is called an interlayer. Unbonded PCC overlays can also placed on existing composite pavements in which the existing asphalt surface layer is partially removed or or completely and replaced with an HMA interlayer. The thickness of the unbonded PCC overlay (D OL ) is derived from the required thickness of a new PCC pavement (D req ) reduced by an amount based on the effective thickness of the existing PCC pavement or composite pavement (D eff ) in accordance with equation 1. D OL = D D Eq. 1 To determine the effective thickness of composite pavement (D eff ), the existing pavement is compared with a new composite pavement having the same layer thickness as the existing composite pavement. The effective thickness of the new composite pavement can be estimated as follows: 2 req 2 eff D = eff ( E E ) eff D new / P Eq. 2 D new is the effective PCC thickness of the composite pavement and is calculated as follows: h D + h AC new = PCC 2 Eq. 3 where, h AC is the thickness of the existing bituminous layers and h PCC is the thickness of the existing PCC layer. E P is the effective elastic modulus of the existing pavement (combination of all layers above the subgrade) and is determined through backcalculation using Dynaflect deflection test data and the AREA method. No temperature correction is applied to the raw deflection test data. AREA is computed using all five Dynaflect sensors (see equation 4) and, thus, reflects the structural contributions of both the HMA and PCC layers, along with any other layers in the pavement system: 2

15 1 AREA = (2.81* S * S * S * S * S5 ) Eq. 4 S 1 E eff is the effective elastic modulus of the combined HMA and PCC layers and is determined based on the equivalent rigidity concept, as illustrated in the following discussion. Modulus of rigidity of a plate or slab, R, is defined as: EI R = 1 v 2 3 Eh = 12(1 v 2 ) Eq. 5 Where, E, ν, I, and h are the modulus of elasticity, Poisson s ratio, moment of inertia, and plate thickness. For a bonded two-layer system (HMA/PCC), the rigidity of each layer is as follows: R R 1 2 = = E E AC PCC h 12 E E b = 3 AC h 12 AC PCC 3 PCC + h h AC + h ( 0.5h + h b) 1 v PCC 1 v AC AC 2 AC PCC ( b 0.5h ) 2 PCC PCC ( 0.5h + h ) E E AC PCC AC h AC PCC + h where, R 1 = rigidity of the HMA layer R 2 = rigidity of the PCC layer h AC = thickness of the HMA layer h PCC = thickness of the PCC layer E AC = elastic modulus of the HMA layer E PCC = elastic modulus of the PCC layer ν AC = Poisson s ratio of the HMA layer ν PCC = Poisson s ratio of the PCC layer 2 PCC h PCC Eq. 6 Eq. 7 Eq. 8 3

16 The combined rigidity of the two bonded layers can be calculated as: 3 E eff R t = R 1 + R 2 = 2 12(1 h v ) Eq. 9 where h = h AC + h PCC and ν is the equivalent Poisson s ratio for the composite layer. Therefore, E eff can be calculated as follows: 2 12(1 v )( R1 + E eff = 3 h R 2 ) Eq. 10 E eff is determined from equation 10 using assumed properties for new materials (i.e., 450,000 psi for HMA and 5,000,000 psi for PCC). Finally, as shown in equation 1, the PCC unbonded overlay thickness is determined using D req and D eff. Investigation Scope and Objectives The primary objectives of this study is to evaluate ODOT procedures for characterizing existing composite pavement structural capacity for unbonded PCC overlay design. Based on a review of the current ODOT unbonded overlay design procedure, it was determined that the following aspects of the procedure need further investigation: Issue #1: E p Computation. The effective pavement thickness (D eff ) is determined based on the entire composite pavement structure because deflection testing to estimate E p is done on the composite pavement prior to milling. Since in most situations, the existing asphalt layer is partially or completely removed by milling prior to the placement of the unbonded PCC overlay and the deflection data are typically collected prior to removal of the asphalt layer, the effective thickness of the existing pavement determined by DOITOVER may need to be adjusted to account for the subsequent milling operation. The extent of the adjustment will depend on (1) the thickness and condition of the remaining asphalt layer and the existing PCC layer (if the asphalt layer is partially removed) or (2) the existing PCC layer condition plus the new HMA interlayer if the entire HMA layer is removed. Issue #2: Impact of Milling on E eff. It is obvious that milling the HMA layer can affect the computed E eff a key input for estimating the existing composite pavement s effective thickness. This is because, for a composite pavement E eff is expressed as a function of the rigidity of the HMA and PCC layers (see equation 10). If the HMA layer layer is completely milled off prior to unbonded PCC 4

17 overlay placement, then the rigidity of the HMA layer will not be relevant to the analysis. The following hypotheses were formulated with respect to these two parameters a priori based on engineering judgment: E p will increase with milling if the underlying concrete layer is relatively sound (concrete pavement modulus is higher than composite pavement modulus) but will decrease if the concrete layer is in poor condition. E eff will not be affected much since in a composite system the most rigid layer, i.e., the PCC layer, can be expected to provide a bulk of the load response and it is not subject to milling. Moreover, typically an HMA leveling course is recommended to be placed on the existing PCC layer prior to the placement of the unbonded PCC overlay which can offset some of the impact of milling off the old HMA layer. This, in effect, further reduces the need for accurately quantifying the impact of milling off the HMA layers in unbonded overlay design particularly when the existing HMA layers that are being milled off are relatively thin or badly deteriorated. These hypotheses will be tested in the data analysis conducted in this study the extent possible. 5

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19 CHAPTER 2. STRUCTURAL EVALUATION OF SELECTED COMPOSITE PAVEMENT PROJECTS Selection of Pavement Projects for Structural Evaluation To achieve the project objectives and to address the issues noted with the existing ODOT unbonded overlay design procedure, it is important that data from ODOT projects be available that: Typify local practices for this overlay type in terms of composite pavement projects selected for this type of rehabilitation, pre-construction repairs, layer thicknesses, etc. Have adequate amounts of coring and deflection test data (i.e., data before milling the HMA overlay, after milling the HMA overlay on bare PCC slab, after level course is placed, and after PCC overlay is placed) to enable the team to perform an in-depth analysis and evaluation of each aspect of the overlay design process. However, it was recognized at the outset that ODOT did not have such data from any of its previous unbonded overlay projects that could be used. Therefore, it was decided that the next best option would be to use data from any ODOT overlay project that will allow a quantification of the impact of milling off the entire HMA layer on structural capacity of the underlying PCC pavement. Based on a review ODOT s research projects, it was determined that detailed data pertinent to this study are available from an ODOT study conducted to investigate the effectiveness of the break-and-seat rehabilitation option and documented by Minkarah and Arudi (1995). In this investigation ODOT monitored four sites (eight sections in all if both North and South bound pavements on each site are considered as different sections) located in Southeastern Ohio (see volume 1, figure 3 for a location map). A brief overview of these projects is given below: FAY 71 Northbound and Southbound (composite pavement prior to milling the HMA overlay and rigid pavement after HMA overlay is milled off). FAY/MAD 71 Northbound and Southbound (composite pavement prior to milling the HMA overlay and rigid pavement after HMA overlay is milled off). GRE-4 Northbound and Southbound (composite pavement prior to milling the HMA overlay and rigid pavement after HMA overlay is milled off). MOT-4 Northbound and Southbound (composite pavement prior to milling the HMA overlay and rigid pavement after HMA overlay is milled off). 7

20 More details regarding the location of the selected pavement projects are presented in table 1. Project No. Table 1. General description of the pavement projects selected for evaluation. Site ID ODOT DirectionDistrict No. County Route Site Begin Station Site End Station Original Pavement Type Pavement Type (After Milling) 1 FAY-71 (N& S) 6 Fayette I Composite Rigid 2 FAY/MAD-71 (N& S) 6 Fayette & Madison I Composite Rigid 3 GRE-4 (N& S) 8 Greene SR Composite Rigid 4 MOT-4 (N& S) 7 Montgomery SR Composite Rigid For the identified pavements, deflection testing was done to characterize pavement structural capacity at various stages of rehabilitation (i.e., before milling the HMA overlay, after milling the HMA overlay on bare PCC slab, after level course is placed, and after PCC overlay is placed) in order to obtain the data required for analysis to meet the project objectives. Tables 2 through 5 present the layer types and thicknesses of the pavement sections evaluated at various stages of rehabilitation. It is noted here that layer information presented in the tables were obtained primarily from ODOT plans and cores extracted as part of the 1992 rehabilitation. Project Data Availability Dynaflect and Falling Weight Deflectometer (FWD) deflection test data were available for the each of the northbound and southbound pavement sections identified in table 1 for the following stages of rehabilitation: On intact pavement prior to milling off the HMA overlay (pre-milling). On bare PCC pavement after milling off the HMA overlay (post-milling). On broken and seated jointed reinforced concrete (JRC) pavement after breaking and seating (post break and roll). After HMA base layer placement (post-301 placement) After HMA surface layer placement immediately after construction and annually for several years after construction. 8

21 Table 2. Timeline showing significant historical construction and maintenance and rehabilitation (M&R) events for FAY-71 (Northbound and Southbound). Original Construction Rehabilitation (1978) Rehabilitation (1992) Stage B (Placement of upper and lower HMA overlays) Placement of a 2.9-in HMA 3-in HMA (846) New JRCP 2.9-in HMA Stage A (Milling off the existing HMA layer) 5.5-in HMA (301) 9.6-in JRC 9.6-in JRC 9.6-in JRC 9.6-in JRC 5.5-in Aggregate Base 5.5-in Aggregate Base 5.5-in Aggregate Base 5.5-in Aggregate Base Subgrade Subgrade Subgrade Subgrade Table 3. Timeline showing significant historical construction and M&R events for FAY/MAD-71 (Northbound and Southbound). Original Construction Rehabilitation (1978) Rehabilitation (1992) Stage B (Placement of upper and lower HMA overlays) Placement of a 2.9-in HMA 3-in HMA (846) New JRCP 2.9-in HMA Stage A (Milling off the existing HMA layer) 5.5-in HMA (301) 9.6-in JRC 9.6-in JRC 9.6-in JRC 9.6-in JRC 5.5-in Aggregate Base 5.5-in Aggregate Base 5.5-in Aggregate Base 5.5-in Aggregate Base Subgrade Subgrade Subgrade Subgrade 9

22 Table 4. Timeline showing significant historical construction and M &R events for GRE-4 (North and Southbound) Original Construction Rehabilitation (1973) Rehabilitation (1993) Stage B (Placement of upper and lower HMA overlays) Placement of a 2.5-in HMA 3-in HMA (846) New JRCP 2.5-in HMA Stage A (Milling off the existing HMA layer) 3-in HMA (301) 9-in JRC 9-in JRC 9-in JRC 9-in JRC 3-in Aggregate Base 3-in Aggregate Base 3-in Aggregate Base 3-in Aggregate Base Subgrade Subgrade Subgrade Subgrade Table 5. Timeline showing significant historical construction and M &R events for MOT-4 (Northbound and Southbound). Original Construction Rehabilitation (1973) Rehabilitation (1993) Stage B (Placement of upper and lower HMA overlays) Placement of a 2.5-in HMA 3-in HMA (846) New JRCP 2.5-in HMA Stage A (Milling off the existing HMA layer) 3-in HMA (301) 9-in JRC 9-in JRC 9-in JRC 9-in JRC 3-in Aggregate Base 3-in Aggregate Base 3-in Aggregate Base 3-in Aggregate Base Subgrade Subgrade Subgrade Subgrade 10

23 The pre- and post-hma milling Dynaflect data from these projects were provided by ODOT and were used to determine the impact of milling on the effective PCC pavement thickness (D eff ) calculation. Detailed coring data collocated with deflection test locations were not available for any of these projects. As noted earlier, layer thicknesses were established from ODOT records. Also, the stationing for the pre- and post-milling Dynaflect test points did not always match up (the same was the case with the FWD data). To make the valid comparisons, the project team considered only preand post-milling deflection testing stations that were within 10-ft of each other as being collocated. Finally, the following data limitations are recognized at the outset which will have a significant bearing on the conclusions that can be drawn from this work: 1. The cross-sections of the pavement projects evaluated provide a very narrow HMA and existing PCC layer thickness range to develop a rigorous understanding of the impact of milling. 2. The data analyzed does not provide an opportunity to understand the impact of the leveling course that is generally used in unbonded overlay design in defining the structural capacity of the substructure (note that some of the lost structural capacity due to milling is regained due to the placement of the leveling course). 3. Lack of detailed coring data introduces additional source of variability into the data analysis process. 11

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25 CHAPTER 3. DATA ANALYSIS AND FINDINGS Analysis of Field and Laboratory Testing Results Data analysis was divided into two parts: 1. Preliminary analysis of field deflection data reviewing trends in raw deflection data along with computed structural indices to determine the effect of milling on pavement structural capacity. The following deflection indices were computed based on the before milling and after milling (on bare PCC) Dynaflect deflection data and used in the preliminary evaluation: o Maximum deflection a measure of overall pavement structural capacity. o Spreadability rating (SPR) an indication of pavement upper layer structural capacity. 2. Detailed analysis of field data backcalculating layer moduli using ODOT s DOITOVER program and determining the effect of milling on pavement structural capacity. o Composite elastic modulus, E P (for all layers above the subgrade). o Modulus of subgrade reaction, k-value. o Effective pavement thickness, D eff. Detailed descriptions of what is included as part of the preliminary and detailed analyses were presented in volume 1, chapter 1 of this report under the subsection titled Analysis of Field and Laboratory Testing Results. A summary of analysis results was also presented in volume 1 for each of the pavement sections being discussed here. The reader is referred to chapter 3 for the FAY 71 project, chapter 4 for the FAY/MAD 71 project, chapter 5 for the GRE-4 project, and chapter 7 for the MOT-4 project. The intent of the evaluation was to (1) understand the impact of milling on E p and E eff, (2) identify possible deficiencies in the current ODOT procedure for characterizing the structural capacity of the existing pavement after milling, and (3) develop solutions to the identified deficiencies. This chapter presents the findings of this evaluation. Observations from Dynaflect Data Analysis Figures 1 through 6 present the results of analyzing Dynaflect structural evaluation data using ODOT s DOITOVER software. Note that the north and southbound data are plotted separately for each of the four projects identified in table 1. Results of the statistical analysis of variance performed to identify trends in the pre-milling and postmilling structural indices on each project are presented in table 6. Observed trends, identified deficiencies, and suggested solutions are presented as follows: 13

26 Maximum deflection S 1, mils FAY71N FAY71S FMAD71 N FMAD71 S GRE4N GRE4S MOT4N MOT4S Before Mill After Mill Figure 1. Comparison of the maximum Dynaflect deflections (S 1 ) before and after milling off the HMA layer SPR FAY71N FAY71S FMAD71N FMAD71S GRE4N GRE4S MOT4N MOT4S Before Mill After Mill Figure 2. Comparison of the SPR computed using Dynaflect deflections before and after milling off the HMA layer. 14

27 30 25 AREA FAY71N FAY71S FMAD71N FMAD71S GRE4N GRE4S MOT4N MOT4S Before Mill After Mill Figure 3. Comparison of the AREA term computed using ODOT s Dynaflect-based calculation procedure before and after milling off the HMA layer.. 4,000,000 Backcalculated Ep, psi 3,500,000 3,000,000 2,500,000 2,000,000 1,500,000 1,000, ,000 0 FAY71N FAY71S FMAD71N FMAD71S GRE4N GRE4S MOT4N MOT4S Before Mill 1,187,094 1,460,336 1,205,145 1,304,524 2,145,673 1,986,062 2,233,454 2,079,904 After Mill 3,238,009 3,564,599 2,313,734 2,487,260 1,718,759 1,620,987 1,927,355 2,009,409 Figure 4. Comparison of the backcalculated E p output from DOITOVER before and after milling off the HMA layer. 15

28 Backcalculated subgrade k, psi/in FAY71N FAY71S FMAD71N FMAD71S GRE4N GRE4S MOT4N MOT4S Before Mill After Mill Figure 5. Comparison of the backcalculated k-value output from DOITOVER before and after milling off the HMA layer 10 Effective thickness, in FAY71N FAY71S FMAD71N FMAD71S GRE4N GRE4S MOT4N MOT4S Before Mill After Mill Figure 6. Summary of D eff output from DOITOVER before and after milling off the HMA layer. 16

29 Table 6. Summary of analysis of variance (ANOVA) results for Dynaflect deflection testing. Project ID Parameter Rehabilitation Value N Duncan s Grouping Max. (sensor 1) After mill A deflection, mils Before mill B Sensor 5 deflection, mils After mill A Before mill B AREA After mill A Before mill B FAY-71N SPR, percent After mill A Before mill B E P, psi After mill 3,238, A Before mill 1,187, B Subgrade k, psi/in Before mill A After mill B D eff, in After mill A Before mill A Max. (sensor 1) After mill A deflection, mils Before mill B Sensor 5 deflection, mils After mill A Before mill B AREA After mill A Before mill B FAY-71S SPR, percent After mill A Before mill B E P, psi After mill 3,564, A Before mill 1,460, B Subgrade k, psi/in Before mill A After mill B D eff, in After mill A Before mill A Max. (sensor 1) After mill A deflection, mils Before mill A Sensor 5 deflection, mils Before mill A After mill A AREA Before mill A After mill A FMAD71N SPR, percent Before mill A After mill B E P, psi After mill 2,313, A Before mill 1,205, B Subgrade k, psi/in Before mill A After mill A D eff, in After mill A Before mill B 17

30 Table 6. Summary of analysis of variance (ANOVA) results for Dynaflect deflection testing, continued. Project ID Parameter Rehabilitation Value N Duncan s Grouping Max. (sensor 1) After mill A deflection, mils Before mill A Sensor 5 deflection, mils Before mill A After mill A AREA, in Before mill A After mill A FMAD71S SPR, percent Before mill A After mill B E P, psi After mill 2,487, A Before mill 1,304, B Subgrade k, psi/in Before mill A After mill A D eff, in After mill A Before mill B Max. (sensor 1) After mill A deflection, mils Before mill B Sensor 5 deflection, mils After mill A Before mill B AREA Before mill A After mill B GRE-4N SPR, percent Before mill A After mill B E P, psi Before mill 2,145, A After mill 1,718, B Subgrade k, psi/in Before mill A After mill B Effective thickness, in After mill A Before mill B Max. (sensor 1) After mill A deflection, mils Before mill B Sensor 5 deflection, mils After mill A Before mill B AREA Before mill A After mill A GRE-4S SPR, percent Before mill A After mill A E P, psi Before mill 1,986, A After mill 1,620, A Subgrade k, psi/in Before mill A After mill B Effective thickness, in After mill A Before mill B 18

31 Table 6. Summary of analysis of variance (ANOVA) results for Dynaflect deflection testing, continued. Project ID Parameter Rehabilitation Value N Duncan s Grouping Max. (sensor 1) After mill A deflection, mils Before mill B Sensor 5 deflection, mils After mill A Before mill B AREA Before mill A After mill A MOT-4N SPR, percent Before mill A After mill A E P, psi Before mill 2,233, A After mill 1,927, A Subgrade k, psi/in Before mill A After mill B Effective thickness, in After mill A Before mill B Max. (sensor 1) After mill A deflection, mils Before mill B Sensor 5 deflection, mils After mill A Before mill B AREA Before mill A After mill A MOT-4S SPR, percent Before mill A After mill A E P, psi Before mill 2,079, A After mill 2,009, A Subgrade k, psi/in Before mill A After mill B Effective thickness, in After mill A Before mill B Maximum Deflection, S 1 : S 1 is an indicator of overall pavement structural capacity. There was an increase in the magnitude of after milling maximum deflection (S 1 ) when compared to the before milling S 1 for all the sections analyzed. However, only 6 of the 8 projects evaluated experienced a statistically significant increase in S 1. It can also be noted from figure 2 that the post-milling deflections for the GRE-4 and MOT-4 sections were 103 to 144 percent greater than the pre-milling deflections. This is rather unexpected because a small change in the composite pavement structure should not produce such a large change in deflection. This finding brings the deflection measurements into question and does not allow one to simply infer that the changes in S 1 were due to a milling alone. 19

32 Last Sensor Deflection, S 5 : S 5 is an indicator of subgrade strength (graphic not shown). This followed the same trend as S 1. There was an increase in the magnitude of after milling S 5 deflection when compared to the before milling S 5 deflection for all the sections analyzed. Seven of the 8 projects evaluated experienced a statistically significant increase in S 5. It can also be noted that the post-milling deflections for the GRE-4 and MOT-4 sections were again very high 70 to 123 percent greater than the pre-milling deflections. Again, a small change in the composite pavement structure should not produce such a large change in deflection. Spreadability, SPR: SPR is an indicator of pavement upper layer strength. Comparing the before milling SPR to the after milling SPR it is observed from figure 2 that SPR increased for 2 of the 8 projects while it decreased for the remaining 6. In total, only 5 of the 8 projects reported a significant change in SPR from a statistical standpoint. The actual magnitude of change in SPR was low with percent change in SPR before to after milling ranging from 0.8 to 7.4 percent. It must be noted that although, the ANOVA points to a statistical difference (mostly due to consistency of the mean and small along project variation of SPR), from an engineering standpoint, the pre- and post-mill SPR values for each section can be considered to be the same. AREA: The before milling to after milling AREA term (computed from deflection ratios) on 3 out of the 8 projects evaluated were statistically different. Of these 3 projects, 2 exhibited an increase in AREA post-milling (unexpected). All of the remaining 5 projects that exhibited no statistically significant change in AREA pre- and post-milling registered a decrease in AREA. The percent change in AREA was up to 10 percent. As with SPR, although the ANOVA points to a statistical difference in AREA calculations before and after milling, from an engineering standpoint, the pre- and post-mill SPR values for each section can be considered to be the same. Backcalculated E P : The E P increased as expected for the FAY71 and FAY/MAD71 projects. However, it decreased for the MOT4 and GRE4 projects indicating potential deterioration of the underlying concrete layer. However, this could also be a result of the potentially anomalous high deflections after milling. Backcalculated k-value: Unexpectedly, the k-values decreased (by as much as (50 percent in some cases) after milling.in most instances. This is a cause for concern as milling off the HMA layer should not have a significant impact on the k-value on any of these projects. Effective thickness (D eff ): For the before milling to after milling rehabilitation stage, D eff changed significantly for 6 of the 8 projects evaluated. For all projects, change in D eff decreased as expected. The D eff change varied from 0.2 in to 3.3 in with the FAY71 and FAY/MAD71 projects showing a change in the range of 0.5 in and the GRE4 and MOT4 projects in the range of 3 inches. 20

33 The GRE4 and MOT4 projects exhibited unexpected changes in E p, and D eff far greater than expected (lower E p values after milling and lower than expected D eff ). One reason for this is could be the inconsistencies in the deflection measurments. However, another reason, which could contribute to this phenomenon is if underlying PCC pavement is badly deteriorated. An examination of the reflective cracking photos sent by ODOT on this project point to evidence of extensive secondary cracking. The crack patterns suggest either a durability problem (e.g., D- cracking) or other forms of joint deterioration which need to be investigated further (see figures 7 through 9). Figure 7. Photograph showing extensive secondary cracking around a reflection crack on the GRE4 project (photo 1). Figure 8. Photograph showing extensive secondary cracking around a reflection crack on the GRE4 project (photo 2). 21

34 Figure 9. Photograph showing extensive secondary cracking around a reflection crack on the GRE4 project (photo 3). There was a clear and reasonable trend in observed S 1, S 5, SPR, and D eff all indicating that majority of the projects experienced a decrease in structural capacity with milling. The magnitude of change in structural capacity, however, varied. It was, however, concluded that there were was considerable changes in pavement structural capacity as a result of milling off the HMA overlays. The reason for change in structural capacity, however, could not all be attributed to milling of the HMA overlay. The exact reasons could not be ascertained due to inadequate data regarding the HMA overlay condition and the condition of the underlying PCC materials. To confirm the findings from the Dynaflect data, it was decided to conduct further evaluation of the changes in pavement structural condition using FWD deflection data collected on this project by ODOT. It was surmised that the heavier loads imparted by the FWD may result in a higher deflection data resolution and potentially better estimates of E P and k-values. Results of the FWD based evaluation are presented in the following sections. Observations from FWD Data Analysis Patterns of changes in maximum deflection, i.e., deflection at the center of the loading zone or plate (D 0 ), deflections just outside the load plate (i.e., at 8 in from the load plate center, D 8 ), last sensor deflection, i.e., deflection at a distance of 60 inches from the load center (D 60 ), backcalculated composite pavement modulus E P, and backcalculated k- values, were examined Note that because the pavement type changes from composite to rigid from the pre-milling to the post-milling phase, the AREA5 and AREA7 methods were used to compute E P and k for each of these phases respectively. The significance of the D 8 sensor is that it allows an examination of the deflections without 22

35 having to take into account the potential compression in the asphalt layer induced by the heavier FWD loading (note that the Dynaflect does not have this problem since the deflection sensors are not immediately under the load plate in this device). Figures 10 through 14 present information gathered from the structural evaluation (using FWD) of pavements analyzed. The north and southbound data are plotted separately for FAY/MAD71 and FAY71 projects. Only northbound data could be analyzed for the GRE4 and MOT4 projects Results of the statistical analysis of variance performed to identify trends in the pre-milling and post-milling structural indices on each project are presented in table 7. Maximum Deflection D 0, mils FMAD71NB FMAD71SB FAY71NB FAY71SB GRE4NB MOT4NB Before mill After mill Figure 10. Summary of observed trends in FWD first sensor (maximum) deflections for HMA-overlaid composite pavements. 23

36 Deflection Outside FWD Load Plate D 8, mils FMAD71NB FMAD71SB FAY71NB FAY71SB GRE4NB MOT4NB Before mill After mill Figure 11. Summary of observed trends in FWD third sensor deflections for HMAoverlaid composite pavements. Last Sensor Deflection D 60, mils FMAD71NB FMAD71SB FAY71NB FAY71SB GRE4NB MOT4NB Before mill After mill Figure 12. Summary of observed trends in FWD last sensor (subgrade) deflections for HMA-overlaid composite pavements. 24

37 Backcalculated Composite Modulus E P, ksi FMAD71NB FMAD71SB FAY71NB FAY71SB GRE4NB MOT4NB Before mill 3,341 3,740 3,249 3,833 2,826 2,561 After mill 6,489 6,876 5,419 5,809 2,533 4,511 Figure 13. Summary of observed trends in E P for HMA-overlaid composite pavements. Backcalculated k-value, psi/in FAY/MAD 71 NB FAY/MAD 71 SB FAY 71 NB FAY 71 SB GRE 4 NB MOT 4 NB Before Mill After Mill Figure 14. Summary of observed trends in k-value for HMA-overlaid composite pavements. 25

38 Table 7. Summary of analysis of variance (ANOVA) results for FWD deflection testing. Project ID FM71NB FM71SB FY71NB Parameter Rehabilitation Value N Duncan s Grouping First sensor (maximum) Before mill A deflection, mils After mill B Third sensor deflection, After mill A mils Before mill B Last sensor deflection, mils After mill A Before mill B E P, psi After mill 6,489, A Before mill 3,341, B Subgrade k, psi/in Before mill A After mill A First sensor (maximum) After mill A deflection, mils Before mill A Third sensor deflection, After mill A mils Before mill A Last sensor deflection, mils Before mill A After mill A E P, psi After mill 6,876, A Before mill 3,740, B Subgrade k, psi/in After mill A Before mill A First sensor (maximum) After mill A deflection, mils Before mill B Third sensor deflection, After mill A mils Before mill B Last sensor deflection, mils After mill A Before mill B E P, psi After mill 5,419, A Before mill 3,249, B Subgrade k, psi/in Before mill A After mill B 26

39 Table 7. Summary of analysis of variance (ANOVA) results for FWD deflection testing, continued. Project ID FY71SB GRE4NB MOT4NB Parameter Rehabilitation Value N Duncan s Grouping First sensor (maximum) After mill A deflection, mils Before mill B Third sensor deflection, After mill A mils Before mill B Last sensor deflection, After mill A mils Before mill B E P, psi After mill 5,809, A Before mill 3,832, B Subgrade k, psi/in Before mill A After mill B First sensor (maximum) After mill A deflection, mils Before mill B Third sensor deflection, After mill A mils Before mill B Last sensor deflection, Before mill A mils After mill A E P, psi Before mill 2,826, A After mill 2,533, A Subgrade k, psi/in Before mill A After mill A First sensor (maximum) After mill A deflection, mils Before mill B Third sensor deflection, After mill A mils Before mill B Last sensor deflection, Before mill A mils After mill A E P, psi After mill 4,511, A Before mill 2,560, B Subgrade k, psi/in After mill A Before mill A 27

40 Observed trends and identified deficiencies are presented as follows: Maximum Deflection, D 0 : There was a noticeable increase in the magnitude of after milling maximum deflection (S 1 ) when compared to the before milling S 1 for 4 of the 6 projects evaluated. One project showed a marginal increase in the deflection and another showed a marginal decrease in deflection after milling. The percent increase in D 0 ranged from 1.6 to 23.8 percent. The change in FWD D 0 values was considerably less than that reported for Dynaflect deflection testing. Third Sensor Deflection, D 8 : There was a noticeable increase in the magnitude of after milling D 8 deflection when compared to the before milling D 8 for 4 of the 6 projects evaluated. The increase in D 8 experienced by the remaining 2 projects was not significant. The percent change in D 8 ranged from 4.6 to 24.8 percent. Last Sensor Deflection, D 60 : There was an increase in the magnitude of after milling D 60 when compared to the before milling D 60 for 3 of the 6 projects evaluated. The decrease in D 60 experienced by both projects was not significant. The percent change in D 60 ranged from 0.6 to 20.7 percent. The change in FWD D 60 values was considerably less than that reported for Dynaflect deflection testing. Backcalculated E P : Along expected lines, E P increased for all projects with the exception of GRE4. For GRE4, the E P decreased slightly. Backcalculated k-value: Although 2 of the 6 projects showed a significant change in k-value after milling, it can be surmised that the magnitude of change in k- values is not significant for any of the projects along expected lines. 28

41 CHAPTER 4. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS This study provides a review and evaluation of the ODOT unbonded PCC overlay design procedure. Understanding and quantifying the impact of milling off portions of the existing composite pavement on the structural capacity of the remaining pavement is addressed in this report. The key findings are summarized in this chapter. This chapter also presents the research team's recommendations on improving ODOT unbonded PCC overlay design procedure. Summary Dynaflect analysis. o There was an increase in the magnitude of after milling maximum deflection (S 1 ) when compared to the before milling S 1 for all the sections analyzed. However, only 6 of the 8 projects evaluated experienced a statistically significant increase in S 1. It can also be noted from figure 2 that the post-milling deflections for the GRE-4 and MOT-4 sections were 103 to 144 percent greater than the pre-milling deflections. This finding brings the deflection measurements into question and does not allow one to simply infer that the changes in S 1 were due to a milling alone. o Last sensor deflection, S 5 followed the same trend as S 1. There was an increase in the magnitude of after milling S 5 deflection when compared to the before milling S 5 deflection for all the sections analyzed. Seven of the 8 projects evaluated experienced a statistically significant increase in S 5. It can also be noted that the post-milling deflections for the GRE-4 and MOT-4 sections were again very high 70 to 123 percent greater than the pre-milling deflections. Again, a small change in the composite pavement structure should not produce such a large change in deflection. o Backcalculated E P increased as expected for the FAY71 and FAY/MAD71 projects. However, it decreased for the MOT4 and GRE4 projects indicating potential deterioration of the underlying concrete layer. However, this could also be a result of the potentially anomalous high deflections after milling. o Backcalculated k-value unexpectedly decreased (by as much as (50 percent in some cases) after milling. In most instances. This is a cause for concern as milling off the HMA layer should not have a significant impact on the k-value on any of these projects. o Effective thickness D eff for the before milling to after milling rehabilitation stage, changed significantly for 6 of the 8 projects evaluated. For all projects, change in D eff decreased as expected. The D eff change varied from 0.2 in to 3.3 in with the FAY71 and FAY/MAD71 projects showing a 29

42 change in the range of 0.5 in and the GRE4 and MOT4 projects in the range of 3 inches. o There was a clear and reasonable trend in observed S 1, S 5, SPR, and D eff all indicating that majority of the projects experienced a decrease in structural capacity with milling. The magnitude of change in structural capacity, however, varied. It was however, concluded that there were was considerable changes in pavement structural capacity as a result of milling off the HMA overlays. The reason for change in structural capacity, however, could not all be attributed to milling of the HMA overlay. The exact reasons could not be ascertained due to inadequate data regarding the HMA overlay condition and the condition of the underlying PCC materials. FWD Analysis o There was a noticeable increase in the magnitude of after milling maximum deflection (D0) when compared to the before milling D0 for 4 of the 6 projects evaluated. One project showed a marginal increase in the deflection and another showed a marginal increase in deflection after milling. The percent increase in D 0 ranged from 1.6 to 23.8 percent. The change in FWD D 0 values was considerably less than that reported for Dynaflect deflection testing. o Third Sensor Deflection, D 8 : There was a noticeable increase in the magnitude of after milling D 8 deflection when compared to the before milling D 8 for 4 of the 6 projects evaluated. The decrease in D 8 experienced by the other 2 projects was not significant. The percent change in D 8 ranged from 4.6 to 24.8 percent. o Last Sensor Deflection, D 60 : There was an increase in the magnitude of after milling D 60 when compared to the before milling D 60 for 4 of the 6 projects evaluated. The decrease in D 60 experienced by both projects was not significant. The percent change in D 60 ranged from 0.6 to 20.7 percent. The change in FWD D 60 values was considerably less than that reported for Dynaflect deflection testing. o Backcalculated E P : Along expected lines, E P increased for all projects with the exception of GRE4. For GRE4, the E P decreased slightly. o Backcalculated k-value: Although 2 of the 6 projects showed a significant change in k-value after milling, it can be surmised that the magnitude of change in k-values is not significant for any of the projects along expected lines. o There was a clear and reasonable trend in observed D 0, D 8, and D 60 all indicating that majority of the projects experienced a decrease in structural capacity with milling. The magnitude of change in structural capacity, however, varied. It was however, concluded that there were was considerable changes in pavement structural capacity as a result of milling off the HMA overlays. 30

43 Conclusions o The magnitude of change reported by the FWD test results was more reasonable on the whole than that reported by Dynaflect. The following conclusions were drawn from the work performed under this project: The project database needs to be expanded to enable a more thorough quantification of composite pavement structural response to milling off portions of or all of the existing HMA layer prior to the placement of an unbounded overlay. This will enable researchers to develop better guidance on the impact of HMA overlay milling on unbonded overlay design. Deflection testing of ODOT s composite pavement structures (which have gradually evolved to be multilayer pavements of substantial structural thickness) using FWD produced more stable and reasonable results than Dynaflect. The unreasonableness of the Dynaflect results may be partly due to low-magnitude surface deflections produced by the test equipment, which causes instability in the backcalculated pavement modulus of subgrade reaction, k-value, and effective pavement moduli (E p ) (the two key inputs used to determined PCC overlay thickness). Recommendations A review of ODOT s unbonded PCC overlay design procedure identified areas requiring further investigation and improvements. Two levels of recommendations one for medium- to long-term implementation consideration and the other for immediate implementation (short term) were developed based on the research findings. The recommendations are as follows: For the short-term, it is recommended that ODOT consider: o Adopting the AASHTO 1993 Pavement Design Guide approach for unbonded overlay design. o Developing a catalog of unbonded PCC overlay designs using AASHTO s Mechanistic-Empirical Pavement Design Guide (MEPDG), Interim Edition: A Manual of Practice (AASHTO 2008). The catalog will be based on current ODOT existing PCC and composite pavement designs, ODOT site conditions, and PCC pavement design features. For the medium- to long-term, it is recommended that ODOT evaluate the rehabilitation design methodologies presented in the new AASHTO Interim Mechanistic-Empirical Pavement Design Guide (MEPDG) (AASHTO 2008) and adopt them if they are found to be reasonable for Ohio conditions. 31