Transportation Research Board Technical Activities Division

Transcription

1 Transportation Research Board Technical Activities Division Committee Research Problem Statements Design and Construction Group (AF000) Bituminous Materials Section (AFK20) Characteristics of Bituminous Materials Committee (AFK20) Preface An important function of the Transportation Research Board (TRB) is to stimulate research that addresses problems facing the transportation community. In support of this function, TRB technical committees identify problems, and develop and disseminate research problem statements for use by practitioners, researchers, and others. The problem statements listed below were developed by the TRB committee noted above. These problem statements should not be considered comprehensive; they may only represent a portion of overall research problems identified by committee members. Statements Problem Number Priority Problem Statements Date Posted 1 1 Acid/Alkaline Modification of PG Asphalt Binders 06/ Effect of Binder Rheologic Type on Fatigue, Fracture and 05/05 Healing 3 3 Binder Contribution to Hot-Mix Asphalt Fatigue Performance 06/ Test for Binder Resistance to Moisture Damage 06/ Oxidative Aging Tests and Models for Asphalt Binders 06/ Non-Oxidative Hardening of Asphalt Binders 06/ Characterization and Performance of Bituminous Mastics 06/ Advanced Rheological Models for Asphalt Binders 06/ Using Nanoindentation to Investigate the Environmental 06/05 Deterioration of Asphalt Binders Improved Purchase Specifications for Asphalt Binders 06/03 I. Problem 1: Acid/Alkaline Modification of PG Asphalt Binders State highway agencies in the western region of the U.S. have known for 30 years the benefits associated with polymer modification of asphalt binder, and continue to specify asphalt binders modified with elastomeric polymers to minimize rutting and fatigue cracking in asphalt pavements. A majority of asphalt binder formulators/suppliers is now using polyphosphoric acid (PPA) and/or other types of acids, sometimes in combination with decreased percentages of polymers, to meet the requirements of high performance binders under the SHRP PG grading Page 1 of 12

2 system. However, laboratory research studies and observations of the field performance of these materials suggest that there may be a number of problems with mixtures containing acidmodified asphalt binders. These problems include interactions with hydrated lime (used as an antistripping agent) that can reduce or eliminate the effect of both the acid modification and the hydrated lime; interactions with other antistripping agents that reduce the effectiveness of the agent and/or the acid modification; an increase in moisture sensitivity of mixtures made using acid-modified asphalt binders; loss of effectiveness of acid-modification over time. Suppliers providing PPA modified binders claim that the observed problems are limited to binders modified using orthophosphoric or green acid. The purpose of the proposed research is to thoroughly evaluate the performance and potential problems in the use of acid-modified binders, especially in conjunction with antistripping agents and various types of polymer modification. 1. Determine how asphalt binders modified with different types of acid modification perform relative to each other. Are there real differences in performance among these materials? 2. Compare the performance of acid-modified binders with that of binders modified with a combination of acids and polymers, and with binders modified with polymers only. The comparison should include an evaluation of the performance of mixtures made with these materials, including changes in performance that occur over several months. 3. Evaluate interactions between acid modified binders and hydrated lime and other antistripping agents, including changes in the effectiveness of the antistripping agents and changes in the performance of the binders. 4. Are their trends in the field performance of acid-modified binders indicating that there may be real problems with these materials? 5. Evaluate binder recovery methods to determine which are most likely to provide accurate properties for acid-modified binders. 6. Develop practical guidelines for state highway agencies for specifying, monitoring and controlling the use of acid-modified asphalt binders. The estimated cost is $500,000 over 3 years. The use of acid-modified binders is increasing, but it is not clear if the performance of these materials warrants this increased use. If there are real performance problems associated with all or some acid-modified binders and their use is not properly controlled, the resulting loss in performance of our paving systems could dramatically increase maintenance costs. The successful completion of this study will help prevent the use of poorly performing binders and the associated waste of pavement maintenance funds. I. Problem 2: Effect of Binder Rheologic Type on Fatigue, Fracture and Healing Page 2 of 12

3 During the 1950 s and 1960 s, paving engineers began to suspect that asphalt cements with disperse relaxation spectra at that time generally meaning asphalt cements that were heavily oxidized or blown during production were often prone to premature failure due to severe early fatigue cracking. Several methods were developed for identifying these materials and proscribing their use in high way construction, the most widely used being the establishment of minimum ductility values relative to the penetration grade. The ductility test was widely misunderstood by many engineers; it was meant to eliminate the use of heavily oxidized asphalt cements, and not to ensure that asphalt cements had adequate ductility. During the development of the SHRP binder specification, this test was eliminated because of its empirical nature. Although not widely discussed, it was felt that the requirements for binder creep modulus and m- value (log-log slope of the creep curve) would prevent the use of heavily blown asphalt binders. It is however not clear that this approach has been effective in controlling the rheologic type of asphalt binders. Furthermore, it is not even clear that oxidized asphalts are in fact more susceptible to fatigue damage than other binders; in most laboratory testing, oxidized asphalts show better fatigue resistance than non-oxidized binders. It is however possible that the poor field performance of oxidized binders might be the result of failure to heal damage after it occurs, rather than the result of a more rapid accumulation of damage. Recently, NCHRP has initiated a study on top-down surface cracking of asphalt concrete pavements (Project 1-42), because of the perceived increase in surface cracking with the advent of the Superpave system including implementation of SHRP performance grading of asphalt binders. It is possible that changes in the rheologic type of asphalt binders brought about by the implementation of the SHRP grading system have contributed significantly to this problem. Unfortunately, there is not enough information concerning the inter-relationships among binder rheology, fatigue and fracture properties, healing rate and field performance to evaluate this hypothesis. This project is meant to collect and analyze such information and determine if revisions are needed in the SHRP binder specification. 1. Review literature concerning the relationships among binder rheology (especially rheologic type), fatigue and fracture properties, healing rate and field performance, and explain these relationships with a working theory that can be evaluated through laboratory testing and an analysis of existing data on field performance. 2. Develop and execute an laboratory test plan to evaluate the effect of binder rheologic type and related factors on fatigue, fracture and healing properties 3. Develop and execute a plan for analyzing the effect of binder rheologic type and related factors on fatigue resistance in the field using existing data from studies performed using accelerated loading facilities 4. Determine what changes are warranted, if any, in the SHRP binder grading system to ensure that asphalt binders will not be prone to premature failure due to poor fatigue and/or fracture resistance, or slow healing rates in the field. Page 3 of 12

4 The estimated cost is $500,000 over 3 years. Tens of billions of dollars are spent each year building and maintaining asphalt concrete pavements in the U.S. Premature surface distress of asphalt concrete pavements is a serious problem that substantially increases these costs. The proposed project will help to reduce the incidence of premature surface cracking and reduce the maintenance required on asphalt concrete pavements, potentially saving taxpayers hundreds of millions of dollars a year in pavement repair costs. The results of the research would be simple to implement, since this would most likely involve a few simple changes in the existing SHRP binder grading specification. I. Problem 3: Binder Contribution to Hot-Mix Asphalt Fatigue Performance The primary factor affecting the fatigue performance of asphalt pavements is its structural design the thickness of the various layers comprising the pavement system. However, other factors, including the characteristics of asphalt binders can significantly affect the fatigue performance of a flexible pavement. If the factors influencing binder fatigue resistance can be identified, characterized and specified effectively, it should be possible to improve bituminous pavement economics by building thinner pavements while using asphalt binders with improved fatigue resistance. The fatigue contribution of asphalt binders is not well understood. It is believed that the current Superpave fatigue parameter (G*Sin Delta) is not directly representative of binder fatigue performance. Furthermore, it is known that (1) mixture healing has a strong impact on its fatigue performance, (2) that this heeling is largely a function of binder properties, and (3) that asphalt binders can heal at dramatically different rates. The factors that relate to heeling are not currently incorporated into the binder specification. This project would look at the fatigue contribution of asphalt binders, with the goal of defining the factors that affect mixture fatigue, and incorporating those factors into the binder specification. 1. Test mixtures produced from a wide range of binder types, in order to characterize their fatigue performance over a range of strains and temperatures 2. Identify the physical characteristics of binders that have the greatest influence on fatigue behavior Page 4 of 12

5 3. Develop an integrated model for the binder's impact on fatigue, including the impact of heeling 4. Propose an alternate binder specification parameter and specification limit which could be used to realistically control the binder's potential contribution to mixture fatigue A subcontract under WRI's asphalt chemistry project, conducted at Texas A&M and NC State, is examining the heeling characteristics of mixtures and binders. The estimated cost is $400,000 over 2-1/2 years. A better understanding of the fatigue performance of mixtures and binders will improve our ability to produce durable pavements, and may allow us to produce thinner (lower cost) pavements without reducing pavement life. I. Problem 4: Tests for Binder Resistance to Moisture Damage Water damage is a significant problem affecting asphalt pavements. There are at least two potential mechanisms of water damage: stripping (debonding) at the asphalt/aggregate interface, and softening of the binder due to water absorption. Water damage probably occurs through a complex interaction of binder characteristics (binder chemistry and rheology), aggregate characteristics (surface chemistry and morphology), mixture characteristics (voids content, asphalt content, voids connectivity), and structural characteristics (drainage, and "pumping" due to traffic induced strains). Although there are many factors that can affect the water resistance potential of asphalt pavements, it is believed that a highly water resistant binder can minimize or eliminate the potential for water damage. The Superpave binder specification does not address this aspect of performance. However, the resistance of asphalt binders to moisture damage may be a greater issue than ever before, due to the increased use of modified binders of various types. Some of the currently used modification techniques, including certain polymers, are believed to be beneficial, while others, such as acid modification, are believed to be detrimental for water resistance. This project would focus on gaining a better understanding of the factors that influence asphalt binder water resistance, with a goal of developing either a screening or a specification test to assess the level of moisture resistance provided by the binder. Page 5 of 12

6 1. Synthesize existing information on moisture damage of asphalt pavements through a comprehensive review of literature 2. Conduct experiments to eliminate gaps in the existing knowledge 3. Develop a model that incorporates each mode of potential water damage 4. Investigate the impact of various modifiers on the water resistance of asphalt binders, with a focus on identifying well performing modifiers 5. Develop and validate a screening test for the water resistance of asphalt binders WRI is conducting fundamental studies on moisture resistance as part of a larger project to examine the chemistry of asphalts. The estimated cost is $500,000 over 3 years. V. Urgency, Payoff Potential, and Implementation Moisture damage is a major failure mechanism for asphalt pavements, and implementation of a binder specification parameter for moisture resistance could significantly reduce the costs associated with this damage. I. Problem 5: Oxidative Aging Tests and Models for Asphalt Binders Binder aging can have a significant effect on the performance of asphalt pavements. This process generally results in a hardening of the asphalt, which can improve rut resistance, but will degrade resistance to thermal cracking, and might also reduce resistance to fatigue cracking. Aging occurs during mixing, transport and placement of hot mix asphalt concrete, and also throughout the service life of the pavement. Currently, the rolling thin-film oven test (RTFOT ) is used in the Superpave system to simulate hot-mix plant aging. This procedure was adopted by SHRP, without modification or validation, for use in the specification. While the test is believed to provide a reasonable simulation for unmodified binders, this may not be the case for modified binders, due to the low shear nature of the test The PAV test was developed during SHRP and is used in the Superpave binder specification to simulate long-term aging. Again, this test has not been validated for modified binders. In addition, the test is conducted at temperatures significantly above pavement service temperatures, possibly resulting in altered reaction kinetics, and an inaccurate characterization of the aging tendency of asphalt binders. This project would be focused on examining the validity of the existing aging tests, and if Page 6 of 12

7 necessary the development of alternate tests. In addition an effort could be included to synthesize, develop and validate binder aging models, which describe the effect of pavement depth, air voids, porosity, and temperature. 1. Assess validity and limitations of existing aging tests (RTFOT and PAV) 2. Identify and evaluate alternative procedures for characterizing the aging properties of asphalt binders 3. Develop and validate aging models 4. If necessary, select one or more promising methods for characterizing age hardening in the laboratory, and refine and validate these methods for use in the binder specification as potential replacements for the RTFOT and/or PAV Limited studies on modifying the RTFOT to improve its suitability for modified binders have been conducted by the SEAUPG, as well as NCHRP NCHRP 9-36 is beginning a related study. IV. Estimated of Problem Funding and Research Period The estimated cost is $300,000 over 3 years. This project will have more value for binders used in high temperature climates as opposed to low temperature climates, and urgency should increase over time, as modified binders see greater use. I. Problem 6: Non-Oxidative Hardening of Asphalt Binders Non-oxidative hardening is reversible hardening that occurs due to molecular structuring of the binders. There appear to be two types of non-oxidative hardening: stearic hardening and physical hardening. Stearic hardening occurs at intermediate temperatures, is due to self-assembly of polar molecules, and is reversible with higher temperatures and with shear. Physical hardening occurs below a relatively low threshold temperature, is at least partially due to crystalizable components, and is rapidly reversible with a temperature increase above the threshold temperature. Both phenomena have been observed in pure binders, however evidence of their impact in asphalt mixtures is limited. This project would attempt to develop models describing the rate and extent of this type of hardening, and ascertain the effect and importance of non-oxidative hardening in asphalt mixtures. Page 7 of 12

8 1. Conduct experiments to determine the extent to which non-oxidative hardening affects bituminous mixtures. 2. Develop generalized models which describe the rate and impact of non-oxidative hardening 3. Develop rapid screening tests that will predict the rate of non-oxidative hardening 4. If appropriate, propose modifications to the binder specification which account for the impact of non-oxidative hardening Fundamental studies on these topics are underway at WRI as part of the study on the chemistry of asphalt. The WRI investigations are focused on pure binders, as opposed to the binder's effect in mixtures. The estimated cost is $200,000 over 2 years. This study would increase fundamental understanding of the performance of asphalt binders, and may facilitate improved binder specifications. I. Problem 7: Characterization and Performance of Bituminous Mastics When asphalt binders are combined with mineral fillers, a mastic is formed. This mastic can be viewed as the component which glues the aggregate together and which undergoes deformation when the pavement is stressed during service. While there are detailed specifications for both the binder and the aggregate, use of filler specifications is limited. However the characteristics of the filler can significantly influence the properties of the mastic, and thus can have a significant effect on mixture performance. A study is needed to examine the effect of binder/filler interaction on the characteristics of the mastic, and to examine the effect of mastics on the performance of asphalt mixtures. This study should consider the impact of filler characteristics (gradation, surface morphology, shape and surface chemistry) as well as binder characteristics (rheology, chemistry), with a goal of developing a predictive model for mastic performance, and if appropriate, a specification for mineral fillers and/or mastics. 1. Conduct an experimental program to determine the rheological properties of mastics produced with various combinations of asphalt binders and fillers Page 8 of 12

9 2. Determine the extent to which chemical interactions between binder and filler affect mastic properties 3. Develop models describing the rheology of mastics based on physical and chemical properties of both components 4. Utilize results to evaluate the development of prototype specifications for mastics and/or fillers No major projects are currently underway. The estimated cost is $200,000 over 2 years. A better understanding of filler and mastic performance is necessary for fundamental understanding and modeling of pavement performance. I. Problem 8: Advanced Rheological Models for Asphalt Binders The Christensen/Anderson rheological model is the most commonly used binder rheological model, and appears to provide an excellent description of the flow properties of asphalt binders over a wide range of conditions, with a minimum use of parameters. Furthermore, the parameters used in this model appear to have a physical meaning; they are not simply curve-fitting parameters. The primary limitations of the Christensen/Anderson model are that it applies only to the linear region, and that it applies primarily to unmodified asphalt binders. These factors may limit its applicability as a constitutive model in micromechanical modeling, and perhaps more importantly, may limit our ability to make generalizations about the performance of a binder based on a limited number of rheological measurements. Several alternate approaches to modeling asphalt binders have been considered, however there has not been an integrated effort to consider all the options and compare their performance in modeling both modified and unmodified binders, in both the linear and nonlinear regions. The proposed project would address this deficiency. 1. Survey existing asphalt rheological models 2. Define advantages and limitations of each model as it relates to asphalt binders 3. Investigate non-linear models that may be appropriate for binder use 4. Recommend an advanced rheological model and educate the industry on its utility and Page 9 of 12

10 application Efforts are underway at PennState to address the limitations of existing models. The estimated cost is $200,000 over 2 years. This project will provide improved constitutive models for use in micromechanical modeling, and urgency will increase as the use of micromechanical modeling increases over time. I. Problem 9: Using Nanoindentation to Investigate the Environmental Deterioration of Asphalt Binders. In the past several years more and more researchers have started to acknowledge the significant effect of asphalt binder film thickness on the mechanical properties of asphalt mixtures. Asphalt binder properties are considered the main factor influencing the low temperature fracture resistance of the asphalt concrete mixtures, and therefore the determination of the mechanical properties of asphalt films is critical in providing a fundamental insight into the behavior of asphalt mixtures at low temperatures. One of the most powerful methods to obtain the properties of thin films in composite materials such as asphalt concrete is instrumented nanoindentation. This technique is similar to classical indentation, but is performed at a much smaller scale and uses extremely low indentation forces. During a nanoindentation test, a mechanical probe penetrates the specimen surface under either load or displacement control and the corresponding variation of load as a function of displacement is recorded during both loading and unloading. Nanoindentation is commonly used to measure two properties of a bulk or thin film material: the elastic modulus and the hardness. More recently, nanoindentation has been used to determine creep and relaxation modulus, dynamic modulus and fracture toughness. This project will investigate the use of nanoindentation to characterize the rheological and fracture properties of asphalt binders at low temperatures. 1. Characterize the rheological and fracture properties of asphalt binders at low temperatures using nanoindentation methods 2. Characterize the rheological and fracture properties of asphalt binders at low temperatures using traditional methods 3. Compare the results of the characterization of asphalt binders at low temperatures using nanoindentation and using traditional methods Page 10 of 12

11 The estimated cost is $300,000 over two years. Pavement engineers and researchers have recognized for many years that film thickness has a significant effect on the properties of asphalt binders. It is likely that nanoindentation can provide a useful means for investigating this phenomenon and applying the results in developing more effective asphalt concrete mix designs. I. Problem 10: Improved Purchase Specifications for Asphalt Binders The Strategic Highway Research Program (SHRP) was a large federally funded research program, which resulted in an integrated set of performance-based specifications for both asphalt binders and mixtures. These specifications describe a range of binder types, which can be chosen to match specific climate and traffic conditions. In contrast to earlier specifications, which were implemented on a state-by-state basis, the SHRP specifications have been nationally implemented, with only minor changes from the original proposal. The implementation of these specifications was aided, in part, by an intensive outreach effort, and by development of an industry-wide consensus around these specifications. Because of the focused nature of the SHRP project, and the need to finalize the project on a timetable, the current SHRP specifications are believed to have some flaws. Some of these flaws have been addressed by relatively small research efforts, both funded and "volunteer" work by interested and/or affected parties in the industry. Other issues remain unaddressed. In particular, the specification was primarily developed based on unmodified asphalts, and there a consensus that it does not adequately reflect the performance of certain modified asphalts. Some work on this issue has been done by NCHRP 9-10, however the project scope and funding are limited, and NCHRP 9-10 may not be able to fully address this issue. This project proposed here envisions a large scale, multi-researcher, coordinated program, to revise, update and validate the binder specification system, as well as an effort to reach out to the industry to develop consensus around the new proposal and the need to implement it. 1. Synthesize the developments in asphalt binder research and technology that have occurred subsequent to the completion of the SHRP program. 2. Conduct additional research to fill gaps in the data. 3. Utilize available information to develop an enhanced, performance based, purchase specification for asphalt binders. Page 11 of 12

12 4. Conduct testing to determine how existing binders will perform under this specification. 5. Validate the proposed specification with mixture and/or field testing. 6. Update and enhance the binder grade selection algorithms. 7. Wok with users and producers to obtain consensus, and implement the specification on a national basis. NCHRP 9-10 recently completed a study to consider potential improvements in the binder specification. The estimated cost is $5,000,000 over 5 years About $3 billion is spent in the U.S. each year on asphalt binders, with most of the spending by state agencies using the Superpave specification. Because binder characteristics can have a disproportionately large impact on the performance of pavements, even an incremental improvement in the binder specification can have a major impact on the performance and cost effectiveness of asphalt pavements. Page 12 of 12