George Lopp

Effects of interface condition characteristics on open-graded friction course top-down cracking performance

Top-down cracking is a distress mode that is of particular concern for pavements with Open-Graded Friction Course (OGFC) because open-graded mixture has considerably lower resistance to fracture (lower fracture energy limit and lower resistance to damage) than dense-graded mixture. This particular cracking phenomenon initiates on the pavement surface and propagates downward; so because the OGFC layer is thin, cracking performance relies on the properties and characteristics of three components near the pavement surface: OGFC, underlying structural layer, and the interface between. For this reason, to increase the durability of pavements surfaced with OGFC, it is significant to ensure a quality fracture resistant bond between OGFC and the structural layer. This research investigated top-down cracking performance of OGFC with different tack coats using a newly developed composite specimen interface cracking (CSIC) test. In addition, X-ray computed tomography (CT) was employed to analyze the interface characteristics between OGFC and dense-graded HMA. HMA fracture mechanics was employed to quantify the effect of polymer modified asphalt emulsion (PMAE) on pavement top-down cracking resistance enhancement. Results clearly indicated that PMAE-created bonded interface conditions greatly increased pavement top-down cracking resistance as compared with conventional tack coat.

Development of a binder fracture test to determine fracture energy properties

Based on knowledge and findings that existing binder tests may not accurately predict cracking performance at intermediate temperatures, this study embarked in the development a new binder fracture energy test to fill this void. Nonlinear 3-D finite element analysis (FEA) was used to identify and optimise an appropriate specimen geometry to allow for accurate determination of true stress and strain on the fracture plane, thereby ensuring accurate determination of fracture energy. Various binders were tested at multiple temperatures and loading rates using prototype specimens in order to evaluate the feasibility and validity of different specimen geometries. The researchers used nonlinear FEA results to establish a data interpretation system and develop a set of diagrams to simplify calculation procedures. After extensive observation of test results, the researchers concluded that the new fracture energy test and data interpretation system suitably measure unmodified and modified binder fracture energy density. The test results reveal that binder fracture energy is a fundamental property of binder and that characteristic of the true stress-true strain relationship is a good indicator of the presence and relative content of modifiers.

Performance evaluation of alternative polymer-modified asphalt binders

Styrene–butadiene–styrene (SBS) polymer-modified asphalt (PMA) binder is widely known to provide superior performance, particularly in terms of cracking, as compared to unmodified asphalt binder. In recent years, alternative PMA binders have been developed to meet PG 76-22 specifications and requirements from the multiple stress creep recovery (MSCR) test. However, existing tests/requirements may not be sufficient to ensure the quality of alternative PMA binders. This study evaluated seven types of alternative PMA binders using different tests, including existing Superpave PG binder tests, the MSCR test, and a newly developed binder fracture energy (BFE) test. Results showed that four alternative PMA binders can potentially have equivalent performance to SBS binder. Three deficient binders were identified in both the MCSR and the BFE tests. Superpave PG binder tests, however, distinguished only one as deficient. Compared to the MSCR test, which provides a qualitative assessment (pass/fail criterion), the BFE test brings the added advantage of providing a quantitative assessment of relative binder performance based on fracture energy density values. Findings appeared to indicate that the BFE test can be used as an effective tool for binder specification by state highway agencies.

Effects of an Asphalt Rubber Membrane Interlayer on Pavement Reflective Cracking Performance

AbstractWhen hot mix asphalt (HMA) overlays are placed on distressed flexible or rigid pavement, it is only a matter of time before cracks in existing pavement reflect to the new overlay surface. Asphalt rubber membrane interlayer (ARMI) is one type of interlayer system that has been suggested to mitigate reflective cracking in HMA overlay. However, in terms of reflective cracking performance, mixed results were reported. Also, the ARMI layer has the potential to contribute to instability rutting. In this study, composite specimen interface cracking (CSIC) tests were performed on composite specimens constructed with and without the ARMI layer to evaluate its effects on HMA overlay reflective cracking performance. HMA fracture mechanics was employed to characterize the mechanism of ARMI layer effects on HMA overlay reflective cracking resistance. Results indicate that ARMI not only cannot delay reflective cracking, but it also reduces reflective cracking resistance to some extent.

Development of a test method for evaluation and quantification of healing in asphalt mixture

It is now well recognised that microdamage healing may extend the fatigue life of asphalt pavement. In this study, a test was developed to quantify the healing of asphalt mixture, both dense-graded and open-graded, using the SuperpaveTM Indirect Tensile Test testing system. The healing test consists of repeated load damage tests (resilient modulus tests) during the damage phase followed by a healing phase during which resilient modulus tests are performed on a limited basis to measure modulus recovery (healing). Appropriate load level, as percentage of tensile strength, for the damage phase was found to depend on mixture brittleness index. A decrease in resilient modulus with an increasing number of load cycles is indicative of accumulation of microdamage during the damage phase. Recovery of resilient modulus during the healing phase is indicative of damage recovery or healing. Rate of healing was found to not be constant but rather changed with time at a decreasing rate for any given mixture. Therefore, a healing rate parameter was defined to allow for comparison between mixtures. Results showed that, in general, expected trends were observed. Mixtures tested at higher temperatures healed at a faster rate than those at lower temperatures and mixtures subjected to less oxidative ageing healed at faster rates than those subjected to more oxidative ageing.

Evaluation of cracking performance for polymer-modified asphalt mixtures with high RAP content

Fourteen reclaimed asphalt pavement (RAP) mixtures designed with different combinations of RAP sources, contents (up to 40%) and mixture conditioning levels were evaluated to determine the maximum allowable amount of RAP material in surface courses without jeopardising pavement cracking performance. Extracted RAP binder was blended with virgin polymer-modified asphalt (PMA) binder at various RAP binder replacement ratios. All blends behaved effectively as PMA binder as they met the multiple stress creep recovery (MSCR) %recovery requirement, and in addition they had satisfactory binder fracture energy density values. RAP gradation was found to significantly affect the fracture properties of RAP mixtures as it controls the distribution of RAP binder and potentially the degree of blending between virgin and RAP binder. Increased RAP content resulted in stronger (i.e. higher tensile strength) but more brittle (i.e. lower failure strain and lower mixture fracture energy) mixtures. However, after long-term oven ageing (LTOA) plus cyclic pore pressure conditioning (CPPC) which was used to simulate long-term field ageing conditioning, all RAP mixtures still exhibited dissipated creep strain energy to failure (DCSEf) values above 0.75 kJ/m3 and energy ratio (ER) values well above 1.0, indicating acceptable cracking performance. It must be emphasised that all RAP mixtures had good gradation characteristics as they all met Superpave design criteria and dominant aggregate size range and the interstitial component (DASR-IC) requirements. Therefore, satisfactory inclusion of up to 40% RAP was acceptable for well-designed PMA mixtures.

Long-term field evaluation and analysis of top-down cracking for Superpave projects

A field evaluation of cracking performance was undertaken for 10 pavement sections as part of a long-term Superpave monitoring project. It was found that top-down cracking was the major type of cracking. Also, moisture damage was identified in four of these sections. An analysis based on the energy ratio (ER) parameter showed that the mixtures that were affected by moisture generally exhibited a much faster reduction in the ER (indicating reduced fracture resistance) than those that were not. Further analysis of cracking performance was conducted using the enhanced hot mix asphalt fracture mechanics-based pavement performance model (HMA-FM-E). The HMA-FM-E is capable of predicting the entire process of top-down cracking from the onset of cracking until pavement failure, and thus provides more valuable information that may better assist material and pavement engineers to optimise their designs. The results showed that the predictions for sections not affected by moisture generally agreed well with field observations. More importantly, it was identified that the key to further improve the accuracy of the performance model is to more accurately predict changes of mixture properties affected by moisture in addition to oxidative ageing.

Development and Evaluation of Laboratory Conditioning Procedures to Simulate Mixture Property Changes Effectively in the Field

The study reported here involved the development and evaluation of laboratory conditioning methods and testing protocols to consider heat oxidation and moisture that would simulate more effectively asphalt mixture aging in the field and thereby help to assess asphalt mixture property changes properly over time. In this study, “aging” was defined as any detrimental effect on asphalt mixture properties during the pavement life. Three laboratory conditioning procedures were identified and further developed to evaluate the effects of heat oxidation and moisture: heat oxidation conditioning (HOC), cyclic pore pressure conditioning (CPPC), and a combination of HOC and CPPC. Cores from aged field pavements were used in the energy ratio approach, which could integrate various factors that affected cracking performance, including key mixture properties and pavement structure, to evaluate the effectiveness of the conditioning procedures. HOC was accomplished with the Superpave® long-term oven aging (LTOA) procedure, and CPPC was employed to induce moisture damage to the asphalt mixtures. Results indicated that asphalt mixtures subjected only to oxidative aging did not exhibit drops in mixture fracture energy (FE) comparable to the level observed in the field. The inclusion of CPPC was able to induce additional damage effectively through simulation of long-term moisture intrusion and repeated internal water pressure under traffic load. Further, the combination of LTOA followed by CPPC led to the most relevant changes in fracture properties with a level of reduction in FE consistent with field observation.

Development of a Composite Specimen Interface Cracking (CSIC) Test for Top-Down Cracking

Pavement top-down cracking performance not only depends on pavement layer material characteristics, but also on layer interface conditions. Interface conditions involve both the shear resistance along the interface and the cracking resistance across the interface provided by the interface bonding agents. Regarding hot-mix asphalt (HMA), currently available tests are mainly focused on pavement layer material properties. When thick polymer modified asphalt emulsion (PMAE) was applied at the interface between an open-graded friction course (OGFC) and a dense graded structural layer, a bonded interface was formed by the migration of PMAE up into the OGFC air voids. Shear strength tests, which can well characterize the adhesive film effect of interface bonding agents, cannot fully capture the effect of bonded interface on pavement cracking performance. To simulate the crack initiation and propagation process and evaluate the effect of bonded interface conditions on top-down cracking performance, a composite specimen interface cracking (CSIC) test was developed. The developed system involves repeated tensile loading and monitoring of the rate of damage development (reduction in stiffness) on composite specimens specifically designed for this purpose. Number of loading cycles to failure and damage rate results from the proposed test on three different interface conditions clearly indicate that this test method can be used to optimize bonding agents and application rates for enhanced cracking performance. This method may also provide a suitable specification test for evaluation of interface conditions on reflective cracking performance.

Predictive relationships for HMA fracture properties based on mixture component characteristics

This study was mainly focused on identifying appropriate relationships between asphalt mixture component characteristics and asphalt mixture properties known to control cracking performance. The current lack of material property models that can accurately describe the changes in material properties over time in the field is probably the greatest deficiency in our ability to accurately predict pavement performance. Therefore, there is a need to evaluate existing material property models, and develop improved models for use in the prediction of pavement performance. Relationships able to predict initial fracture energy and creep rate, which are the properties known to govern the change in material property over time and are also required for performance model predictions, were developed in this study. In addition, conceptual relationships were identified to describe changes in these properties over time (aging) by including the effect of the non-healable permanent damage related to load and moisture. This can serve as the foundation for further development of improved models to predict mixture properties as a function of age in the field based on additional field data and laboratory studies using more advanced laboratory conditioning procedures. The verified relationships will also serve to provide reliable inputs for prediction of service life using pavement performance prediction models.