Application of Intermediate Temperature Semi-Circular Bending (SCB) Test Results to Design Mixtures with Improved Load Associated Cracking Resistance
This paper describes the development and practical application of regression equations to estimate the relative load associated cracking resistance of asphalt mixtures from properties that are included in specifications and controlled through current quality control and acceptance procedures. This work was part of a research study performed through the Wisconsin Highway Research Program to evaluate changes to the composition of asphalt concrete mixtures that should be considered to improve the durability of flexible pavements. Based on the findings of a synthesis of current research, a laboratory experiment was conducted to quantify the effects of: (1) effective binder volume, (2) virgin binder low temperature performance grade, (3) recycled binder content, and (4) polymer modification on the resistance of typical Wisconsin mixtures to aging and load associated cracking. For the types of mixtures normally used in Wisconsin, the laboratory experiment found mixture composition had little effect on aging; however, load associated cracking resistance was significantly affected. The laboratory experiment produced a regression equation that was used to recommend revised volumetric criteria that provide equivalent load associated cracking resistance and allow producers the flexibility to design mixtures using a range of effective binder contents, recycled binder contents, and virgin binders.
Reflective Cracking Relief Interlayer for Asphalt Pavement Rehabilitation: From Development to Demonstration
Reflective cracking is one of the major types of distresses associated with the use of asphalt concrete (AC) overlays for rehabilitating deteriorated asphalt pavements. This paper briefly describes the research efforts completed in Nevada to identify the best promising techniques to mitigate reflective cracking in AC overlays under Nevada’s climatic and materials conditions. The main outcome of the study was the development of a preliminary performance-based mix design specification for an experimental stress relief course (referred to as ESRC) to be placed between the AC overlay and the milled AC surface. Consequently, in summer of 2015, the Nevada Department of Transportation (NDOT) implemented the developed specification on a demonstration project on US 95 in southern Nevada. The project included test sections with three different strategies: (1) AC overlay on top of the ESRC mixture, (2) AC overlay on top of the NDOT fine-graded Type 3 mixture (SRC-T3), and (3) AC overlay without any stress relief layer. An extensive laboratory evaluation was conducted on all three mixtures using both laboratory-mixed laboratory-compacted (LL) and field-mixed laboratory-compacted (FL) mixtures. Core samples were also collected after construction and were evaluated for in-place thickness and air voids as well as resistance to reflective cracking. The data showed promising results for the ESRC mixture in terms of its resistance to fatigue and reflective cracking while maintaining an acceptable level of stability and resistance to rutting. A windshield survey conducted approximately 13 months after construction showed no distresses in any of the surveyed sections. In summary, the study showed a very good performance for the ESRC mixture thus far, making it a promising technique for reflective cracking mitigation in Nevada.
Optimizing Water Content in Cold Recycled Foamed Asphalt Mixtures
During cold recycling, water is added to facilitate the dispersion of foamed asphalt in the mixture and to achieve uniform mixing and help compaction by providing sufficient lubrication. Too little water may cause difficulty in workability and compaction of the mixture, but too much water may extend the curing time and reduce density and strength. Therefore, the optimum water content is considered as one of the most important factors in mix design procedures for cold recycling. Currently, mix design procedures for cold recycled foamed asphalt mixtures suggest adding water to the mixture at an optimum content to facilitate mixing and compaction. However, there is no standard method for determining the optimum total water content (OTWC) for cold recycling mixtures. Several empirical relationships have been developed to determine the OTWC based on modified Proctor test results for RAP/aggregate. However, the compaction effort in the modified Proctor test for RAP/aggregate may not match that for mixtures, which are compacted using the Superpave gyratory compactor or Marshall hammer. A study is underway to improve the design method for cold recycled foamed asphalt mixtures with 100% RAP. The purpose of this paper is to optimize the design procedure by developing a new method to determine OTWC. A Superpave Gyratory Compactor was used to compact RAP instead of the modified Proctor test to match the compaction effort recommended for foamed asphalt mixtures. A regression model was developed to calculate the OTWC for a mixture based on the determined optimum water content of the RAP, foamed asphalt content, and binder type as factors. The method for determining OTWC for a mixture was validated using six different mixtures and was found to correlate well with the measured OTWC, even though two of six mixtures had underestimated OTWC due to use of a different binder source. Further comparisons with two other OTWC determination methods showed the mixtures at the proposed OTWC had improvement in indirect tensile strength.
Application of Laboratory Asphalt Cracking Tests to Cold In-Place Recycled (CIR) Mixtures
Cold In-Place Recycling (CIR) is an asphalt pavement rehabilitation technology which is rapidly gaining popularity. This technique consists of milling the top 75-100 mm of an existing asphalt pavement, mixing the crushed material with a recycling agent, and reusing it to repave the roadway. Hence, there is a need to identify laboratory tests for developing CIR mixtures with improved resistance to cracking. This study investigates the applicability of Semi-Circular Bend (SCB) and Disk-Shaped Compact Tension (DCT) tests to CIR mixtures. Tests were performed on five similar laboratory and field prepared mixtures. Since the CIR mixtures were relatively softer and more brittle than typical Hot Mix Asphalt (HMA), minor modifications to the testing parameters and data analysis approach were found necessary. Fracture energies computed from SCB and DCT data resulted in being statistically similar and detected considerable differences among the five mixtures. Overall, both fracture tests showed good repeatability and potential applications in mix design, engineering and optimization of CIR mixtures, quality control (QC) and field validation usages.
Using Electrical Resistance to Evaluate the Chip Seal Curing Process
Chip seals are among the most cost-effective surface treatments available for pavement preventive maintenance. However, frequent issues associated with asphalt emulsion early mechanical strength development, resulting in premature surface treatment failure, have led to the need to improve the characterization of the chip seal curing process. As such, the use of an electrical resistance measurement has been studied to develop a sound construction methodology that prevents common failures that occur soon after construction. This paper presents a novel approach, based on electrical resistance measurements, to determine when a chip seal has developed enough binder adhesive strength to bond to the existing pavement while keeping the aggregate chips in place. The electrical resistance measurements provide a rapid, non-destructive indication of the amount of curing that has occurred. By implementing this methodology, the user can determine when a chip seal has gained sufficient mechanical strength to allow for brooming or opening to unrestricted traffic without an undue loss of cover aggregate. Laboratory and full-scale field trials were conducted using a variety of materials. The electrical properties of the fresh seal coats were quantified by employing a handheld electrical device with a two-point probe resistance measurement. The experimental results suggest that chip seal systems have gained significant mechanical strength when the initial electrical resistance measurement increases by a factor of 10. As a result, this study establishes that electrical resistance measurements can be used to determine when a fresh chip seal has sufficiently cured to withstand the shear forces of brooms and uncontrolled traffic. The implementation of the technique could potentially impact chip seal construction quality, as well as service life performance.
Improving Accuracy of Asphalt Content Determination by Ignition Test
Accurate determination of asphalt content and aggregate gradation is critical for controlling the quality of asphalt mixtures during construction. Most state specifications require quantitative evaluation of the asphalt content of mixes as a criterion for acceptance. The ignition oven test procedure specified in AASHTO T 308 is required or allowed by most state DOTs for determining the asphalt content and aggregate gradation of asphalt mixtures. The ignition oven test specified in AASHTO T 308 requires the determination of asphalt content correction factors for each asphalt mix and for each ignition oven used. However, in some instances when numerous asphalt mix designs and several ignition ovens are available, correction factors (CFs) are shared between ignition units, even when that practice is not allowed by the standard. There is a need to identify the consequences of sharing correction factors between units/mixes and also to identify testing parameters that affect the measured CFs. By identifying possible causes of variation, the test procedure could be adjusted to make the CFs more consistent between ignition ovens. Also, it should be possible to reduce the amount of difference in CFs between all types of equipment. A study was conducted to assess the variability of ignition oven CFs for different ignition oven unit brands and mixes to better understand the implications of sharing CFs. Twenty-three laboratories used various brands of ovens to test four mixes containing aggregates with varying CFs. The results indicated that CFs were significantly different for the different mixes even when same brand of unit was used. The within lab and between lab precision developed in this study suggests that different precision statements are necessary for aggregates with high breakdown potential and that the current precision included in AASHTO T 308 was likely developed for low weight loss aggregate making it unacceptable to use for aggregates with higher CFs. It was also found that the addition of lime caused no significant difference in the measured asphalt content when the CF was correctly measured. However, it was cautioned that a change in amount of lime during mixture production would affect the measured asphalt content. In addition, ways to minimize variability in asphalt CF were evaluated. It was determined that ignition tests conducted at lower temperature (427°C) proved to be effective in reducing the variability in measured asphalt content since the lower temperature reduced the asphalt correction factors for asphalt mixes.
Evaluation of Various Hamburg Wheel- Tracking Devices and AASHTO T 324 Specification for Rutting Testing of Asphalt Mixtures
The Hamburg Wheel Tracking (HWT) device is a laboratory-controlled rut depth test that uses loaded wheel(s) to apply a moving load on asphalt mixture specimens to simulate traffic loading on asphalt pavements. Currently, different machines with various degrees of compliance with the current AASHTO T 324-14 requirements are used by highway agencies. The objective of this study was to assess the capabilities of commercially available and representative HWT equipment in the US and to evaluate different analysis and reporting methods for rutting and stripping performance assessment from the HWT test. After performing a comprehensive evaluation of HWT devices from different vendors, considerable discrepancies among different equipment were identified. Furthermore, the available HWT machines do not meet all the requirements set forth in AASHTO T 324 including those for the wheel position waveform, the temperature range, and the reporting parameters. Evaluation of the analysis methods was conducted by applying six analysis procedures to the test data of two asphalt mixes with contrasting rutting resistance and moisture susceptibility. For the poor-performing mix, substantial differences were observed among the different methods, especially in the reporting of the Stripping Inflection Point (SIP). For the good-performing mix, only two of the six methods successfully identified this mix as a non-stripping mix. It is recommended that an approach similar to the one adopted by Iowa DOT be implemented in the revised AASHTO T 324 specification.
Simulation and Validation of Asphalt Foaming Process for Virtual Experiments and Optimization of WMA Production
The asphalt foaming process, as one of the major warm mix asphalt (WMA) technologies, can significantly increase the volume of asphalt binder with large surface area in the unit volume and thus generate a strong coating with high shear strength of the mix and improved workability. It allows lower production and construction temperatures and less greenhouse gas and other emissions. Foaming asphalt has generally been achieved by introducing water as a foaming agent into hot asphalt flow before mixing with aggregate at a certain temperature. The water will evaporate and expand the asphalt volume and reduce its viscosity. However, both the formation and decay of the foamed asphalt binder is a highly thermodynamic process, which makes the characterization of the foamed asphalt binder extremely difficult. A high fidelity model to simulate the foaming process will provide a powerful tool to conduct virtual experiments of WMA production and optimize the asphalt foaming process and WMA design. To this end, a numerical model using smooth particle hydrodynamics (SPH) is developed to simulate the asphalt foaming process. A self-developed nozzle-based foamer was used to generate foamed asphalt binder at different water contents. Three primary parameters, i.e., expansion ratio, half-life and foam index, that have been widely applied to evaluate the foaming characteristics of foamed asphalt, have been studied. It was found that the simulation results agree well with the experiments. Parametric studies were further conducted by using the numerical model to evaluate the effects of environmental controlling parameters on the foaming characteristics of the foamed asphalt binder.
Effect of Aging on Asphalt Binders in the State of Qatar: A Case Study
Aging of asphalt binders has a significant impact on the rheological properties of asphalt binders and thus affects the performance of asphalt pavements. The rate of binder aging is accelerated at high temperatures. This study presented a case study on the effect of aging of the asphalt binder properties in the State of Qatar where asphalt mixtures experience harsh environmental conditions of elevated temperatures. The researchers collected field cores from five-year old trial pavement sections in Qatar, recovered the binder and tested to evaluate the change in the rheological properties of the binders. Test results indicated that there is a significant effect of aging on the rheological properties of asphalt binder in Qatar. This effect was more prominent in the wearing course of pavements, diminishing with an increase in pavement depth. Binders along the wheel path experienced less aging compared to the shoulder. Aggregate type and mix design methods were also found to be affecting the extent of aging of asphalt binder in the field. Furthermore, the researchers found that the current practice of aging asphalt binders in the PAV to simulate aging in the field is not sufficient for Qatar’s conditions. It is recommended to extend the PAV aging period from the standard 20 hours for a hot climate like Qatar and it might go up to about 70 hours based on the results of this study.
Non-Isothermal Kinetic Analysis of Reversible Aging in Asphalt Cements
The AASHTO M 320 specification for thermal cracking—meant to limit damaging temperatures to only a 1 in 50 risk in any given winter—stipulates that asphalt cement chemically (irreversibly) aged in the rolling thin film oven (RTFO) and pressure aging vessel (PAV) be tested after minimal cold conditioning. Creep stiffness and creep rate (m-value) are determined with specimens rapidly cooled to 10°C above the pavement design temperature and then conditioned for only an hour prior to testing. As a result, materials are often tested in a non-equilibrium state and pavements end up underdesigned for thermal cracking; adjacent sections can show from best-case to worst-case performance. Several Ontario user agencies have recently implemented a specification to effectively limit cracking due to reversible aging: “Determination of Performance Grade of Physically Aged Asphalt Binder Using Extended Bending Beam Rheometer (EBBR) Methodâ€ (Ministry of Transportation of Ontario designation LS-308 and recently adopted by AASHTO under designation TP 122-16). This empirical protocol tests beams after 72 hours of cold conditioning to determine a low temperature grade as well as a grade loss from the 1 hr results specified in AASHTO M 320. Grade losses are sensitive to the presence of deleterious additives (waxes, air blown residues, recycled engine oil bottoms (REOB)). Further, by testing recovered material from paving mixtures, the protocol also provides a secure way to account for the presence of recycled pavement (RAP), and to monitor for overheating during production. The extended BBR method has several limitations: (1) it requires a relatively large quantity of 150 grams of aged material; (2) it takes a relatively long 72 hours to complete; and (3) it quenches the binder from room temperature to two cold temperatures, namely 10°C and 20°C above the pavement design limit (Td+10 and Td+20), and therefore, because of its empirical nature, it is unclear if the results obtained are relevant for other thermal histories. Since these drawbacks have slowed implementation, current research is focused on the following improvements: (1) modeling the aging/hardening processes within the Ozawa theoretical framework; (2) reducing sample requirement to less than 12 grams; (3) shortening testing time to less than 24 hours; (4) conditioning at more temperatures between ambient and the pavement design limit; and (5) automating the procedure. Modeling efforts are based on an analysis of non-isothermal phase transformation kinetics. This paper presents results obtained from differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) three-point bending tests on a number of Ontario asphalt cements under varying cooling rates. The application of the Ozawa theory provides an improved understanding of how thermal history and variable aging tendencies can explain vast performance differences between pavement sections of identical AASHTO M 320 grades. It is suggested that by adding an upper limit on the Ozawa exponent to the low temperature asphalt cement specification, in order to reduce the rate of reversible aging, user agencies will be able to better control this type of cracking.
Comparison of Asphalt Binder and Mixture Cracking Parameters
Cracking is one of the most prevalent types of distresses in asphalt pavements. There are different cracking index parameters that are determined from tests conducted on binders and mixtures to assess cracking potential. The objective of this study is to compare binder and mixture parameters and evaluate the similarities and differences between the rankings and values obtained. This study includes binder- and mixture testing on 14 plant produced mixtures including three different binder grades, three binder sources, three aggregate gradations, and mixtures containing a range of RAP and/or RAS contents. Testing included PG grading and 4-mm DSR testing on the extracted and recovered binders that were long-term aged. Mixture testing included complex modulus, SVECD fatigue, and DCT testing on short-term aged mixtures. Parameters evaluated included high and low PG temperatures, Tcr, Glover-Rowe parameter (binder- and mix-based), R value, dynamic modulus, phase angle, number of cycles to failure from SVECD and LVECD analysis, and fracture energy. The results show that generally the binder parameters correlate well with each other but the mixture parameters do not. Good correlation was observed between binder and mixture stiffness-based parameters, but there was generally low correlation observed between binder and mixture cracking parameters for the mixtures evaluated in this study, possibly a result of differences in aging level. Recommended future work includes non-linear statistical analysis, incorporation of field performance, and testing on long-term aged mixtures.
Effect of Cooling Medium on Low Temperature Properties of Asphalt Binder
The characterization of asphalt binder at low temperature is of fundamental importance for selecting and designing asphalt materials with good and durable performance in regions experiencing severely cold climates. The current specification addresses this issue based on the Performance Grading (PG) system, developed during the Strategic Highway Research Program, and on low temperature creep tests performed on asphalt binder with the Bending Beam Rheometer (BBR). Recently, an alternative experimental method was proposed to relate the complex modulus, obtained with the Dynamic Shear Rheometer (DSR) at low temperature, to the BBR creep stiffness. However, while DSR tests are performed in air, the BBR relies on an ethanol bath for conditioning the binder specimens, making the relation between complex modulus and creep stiffness dependent on the specific cooling medium. In this paper, the effect of cooling medium on the low PG and on the rheological properties obtained from DSR and BBR tests is experimentally investigated and modeled. First, DSR and BBR tests, in ethanol and air, are performed on a set of different asphalt binders. Then, a relationship between the complex modulus in the time domain and the creep stiffness obtained both in ethanol and air is derived and the low PG for both cooling media is estimated. Finally, 2 Springs 2 Parabolic Elements 1 Dashpot (2S2P1D) and the Huet models are used to compare the effects of ethanol and air on the rheological properties of the asphalt binders. It is found that air results in higher creep stiffness and smaller m-values compared to ethanol. The two rheological models indicate that complex modulus and creep stiffness present the same kernel model parameters only in the case of air. This suggests that the low performance grade, obtained from BBR tests in ethanol, is strongly affected by the cooling medium, as well as the recently proposed procedure based on DSR tests. Based on the finding of the present research, the use of air for BBR creep tests is recommended.
Improving Cohesiveness of Porous Friction Course (PFC) Asphalt Mixtures
The use of porous friction course (PFC) provides numerous safety benefits and improves the noise quality of surrounding areas. Many agencies once used PFC for these reasons, but have since stopped using PFC due to performance issues. PFC pavements have reportedly been prone to raveling and cracking which leads to reduced service life. In addition, PFC is also typically more expensive than a dense-graded mix due to its use of high quality aggregates, modified asphalt binder and higher asphalt binder contents. Research is needed to extend the service life of PFC pavements in order to encourage agencies to start, or continue, use of PFC for its safety benefits. The objective of this research is to address the raveling and cracking distresses commonly seen by adjusting the asphalt and dust content of PFC mixes to improve durability. This was accomplished by using an array of performance tests to evaluate the effect of additional fine aggregate passing the 0.075 mm (P-0.075) sieve on two PFC mixtures: one that had good field performance (up to 18 years) and one that had poor field performance (less than eight years). It was found that the Cantabro test was a good indicator of mix performance and a maximum loss of 20% is recommended. The study revealed the importance of increased percent passing the 0.075 mm sieve to provide more durable PFC mix designs. An increased P-0.075 content had a positive effect on almost all of the results; thus, it is recommended that the current P-0.075 gradation band be expanded.
Characterizing the Long-Term Rejuvenating Effectiveness of Recycling Agents on Asphalt Blends and Mixtures with High RAP and RAS Contents
Although the use of high reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) contents in asphalt mixtures is desirable for environmental and economic reasons, these mixtures are prone to cracking, raveling, and other durability-related pavement distresses mainly due to the heavily aged recycled binders. Highway agencies and the asphalt paving industry have been exploring the use of recycling agents (RA) in order to produce these mixtures with desirable performance. This study focused on characterizing the long-term rejuvenating effectiveness of RA on asphalt blends and mixtures with high RAP and RAS contents. Materials from two field projects were used to prepare a number of asphalt blends and mixtures with various combinations of base binder, recycled material, and RA. These blends and mixtures were subject to various aging protocols prior to being characterized for their oxidation kinetics, rheological properties, and cracking resistance. The test results indicated that the RA evaluated in this study were effective in partially restoring the properties of recycled materials, but their rejuvenating effectiveness diminished with aging. Nevertheless, the recycled blends and mixtures with RA achieved equivalent or even better rheological properties and cracking resistance than those with an allowable amount of recycled materials per agency specifications but without RA. In addition, adding RA had no significant effect on the oxidation kinetics of the recycled blends, but increased their susceptibility to physical hardening in response to oxidation. Finally, the correlation between laboratory aging protocols for asphalt blends and mixtures were determined; the laboratory long-term oven aging protocols of five days at 85°C on compacted specimens and one day at 135°C on loose mix yielded binders with equivalent rheological properties to those subjected to rolling thin film oven (RTFO) plus approximately ten and 40 hours of pressure aging vessel (PAV), respectively.
Performance Evaluation of Plant-Produced Warm Mix Asphalts Containing RAP and RAS
In the present study, rutting performance, cracking resistance, and durability of five plant-produced asphalt mixes containing 12% reclaimed asphalt pavement (RAP) and 3% recycled asphalt shingles (RAS) and produced with different warm mix asphalt (WMA) technologies were evaluated through characterization of extracted asphalt binder and performance tests conducted on asphalt mix specimens. For all of the extracted and recovered asphalt binders, continuous grades and the difference between critical low temperatures, ΔTcr parameter, were determined from the performance grading and a Glover-Rowe (G-R) damage zone was evaluated at 45°C and 10 rad/s. It was revealed that application of a rejuvenating agent (RA) significantly reduced the high PG grade of the overall asphalt binder. The binder test results also indicated that the chemical-additive-based WMA technology had an advantage over the foaming-based technology in terms of asphalt binder durability. Furthermore, application of RA improved the asphalt binder durability. Comparison of G-R parameters of the recovered asphalt binders showed that using RA and lowering the virgin binder high PG true grade improves the overall binder ductility and damage resistance. Asphalt mix performance tests included dynamic modulus, flow number, and semi-circular bend. The dynamic modulus test data indicated that application of RA and lowering the virgin binder high PG grade lowered the mix stiffness at low frequencies. Comparison of the methods to compensate the effects of highly aged RAP and RAS binders indicated that the mix prepared with RA and one level high PG grade drop performed better than the mix prepared by dropping the PG grade by two levels in terms of cracking resistance and rutting performance. The findings give credence to utilization of rejuvenating agents and softer virgin binders in balanced RAP/RAS mix design approaches.
Impact of High Recycled Mixes on HMA Overlay Crack Development Rate
Illinois has many years of experience using various reclaimed materials in highway construction, and in recent years, recycled asphalt shingles (RAS) have been adopted for use in hot-mix asphalt (HMA), along with much higher amounts of reclaimed asphalt pavement (RAP). These reclaimed asphalt materials usually contain aged asphalt binders, which may increase the mix brittleness and hence, pose a challenge for maintaining a flexible pavement and ensuring good performance. To counter these hard asphalt binders, softer asphalts are incorporated into the HMA. The goal is for the final mix to provide acceptable mix properties for the life of the pavement. To determine the impact of recycled materials on pavement performance, this study monitored nine field projects in terms of the testing, construction, and performance of surface mixes that have a variety of asphalt binder replacement (ABR) levels from RAP and RAS which used different virgin asphalt binder grades. Simple performance tests (Hamburg wheel tracking test and the Illinois flexibility index test (I-FIT)) were used to evaluate the mix designs. Flexibility index (FI) from the I-FIT showed good correlation with field crack development, especially after first year performance of the mix. Early-age field performance showed that placing the HMA overlay directly over existing bare concrete pavement or milling off all the HMA and placing the new overlay on concrete pavement results in higher extents of cracking in early age than the sections that left an HMA layer in place. Regardless of which mix type is designed and what material sources are used, the performance of the mix should be evaluated to ensure it has sufficient flexibility to resist cracking before the mix is used in road construction. This allows owners and contractors to use low-cost reclaimed and recycled materials to the extent possible without negatively impacting pavement performance.
Evaluation of Foamed Warm Mix Asphalt with Reclaimed Asphalt Pavement: Field and Laboratory Experiments
Warm mix asphalt (WMA) technology and reclaimed asphalt pavement (RAP) materials have been increasingly used in asphalt paving mixtures due to environmental and cost benefits. Combining WMA and RAP offers more economic and environmental benefits compared to using either alone. To evaluate the combined effect of WMA and RAP, two WMA mixtures with 20% and 30% RAP were produced using water-injection plant foaming, and they were compared with two comparable HMA mixtures with 20% and 30% RAP. The four mixtures were paved on I-70 near Eagle, Colorado, in May 2013. Laboratory performance properties of plant-produced mixes and field performance of the four test sections after 13, 25, 38 months were evaluated. The research results showed that the effect of water foaming WMA and RAP was not significant on the laboratory performance properties, construction quality and field performance. Thus, water-injection plant foaming WMA could be used to produce WMA mixtures with 20% and 30% RAP at a lower production temperature. These sections will continue to be monitored to evaluate their long-term performance and compare with the laboratory test results.
Assessing the Effects of RAP, RAS, and Warm- Mix Technologies on Fatigue Performance of Asphalt Mixtures and Pavements Using Viscoelastic Continuum Damage Approach
The objective of this study was to evaluate the effects of incorporating recycled materials, i.e., reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), and the use of warm-mix technologies on fatigue performance of asphalt mixtures and pavement structures using the viscoelastic continuum damage (VECD) approach. For this purpose, ten mixtures from the full-scale test lanes constructed at the Federal Highway Administration (FHWA) Accelerated Loading Facility (ALF) were acquired and characterized in the laboratory. The dynamic modulus test and direct tension cyclic fatigue test were employed to assess the linear viscoelastic property and fatigue characteristics, respectively. Within the VECD framework, two parameters, namely material fatigue sensitivity (MFS) and structure fatigue sensitivity (SFS) were developed to represent the fatigue resistance of asphalt mixtures and their performance in pavement structures, respectively. Both parameters can be easily obtained via the strain-based fatigue simulation using the experimental data without extra requirements on the specimen failure location and the number of cycles to failure in the cyclic fatigue test. The validity of MFS was verified by the critical strain energy release rate obtained from semi-circular bend (SCB) testing at intermediate temperature. Based on the obtained dynamic modulus and MFS data, an increase in the content of recycled materials was able to enhance the stiffness of hot-mix asphalt (HMA) mixtures while compromising the fatigue resistance. Implementation of warm-mix technologies benefited materials’ fatigue performance but the improvement was not substantial. Use of soft binder yielded pronounced fatigue benefits for HMA mixtures with high RAP content. The proposed SFS parameter resulted in the same performance ranking of the ALF lanes as the measured fatigue lives. Moreover, quantitatively a power-law function was found to adequately correlate SFS with the field measurements.
Development of a New Methodology to Effectively Predict the Fracture Properties of RAP Mixtures
The existing design method for RAP mixtures assumes virgin and RAP binders fully blend. However, full blending may not occur and the impact of partial blending on mixture cracking performance is still unclear. A previous study revealed that RAP gradation, which is not currently considered from the standpoint of binder blending, controls the distribution of RAP binder within a mixture and consequently affects cracking performance. The objective of this study was to develop a methodology that overcomes the uncertainty of blending and effectively predicts fracture properties of RAP mixtures. This methodology is based on the evaluation of the interstitial component (IC) of a mixture (i.e., the fine portion that governs cracking performance) by means of a direct tension test named ICDT test. Two RAP sources and four RAP contents were considered. ICDT specimens were produced by blending the fine portion of RAP and virgin aggregate with virgin binder in the same way and corresponding proportions as in RAP mixtures. Binder and mixture fracture properties from the previous study were used for comparison. Mixture and IC exhibited almost similar reduction in fracture energy density (FED) with increasing RAP content, whereas fully blended binder exhibited a less pronounced reduction. This indicated that IC better simulated the actual blending that occurred in mixtures. Mixtures with coarsely graded RAP (less RAP content in the IC) exhibited better fracture properties; thus, the key to satisfactory cracking performance appears to be minimizing the amount of RAP in IC. Consequently, the stiffening effect of RAP on the fine portion that controls cracking performance should be directly evaluated, instead of placing focus on the fully blended binder or the whole mixture. The ICDT test was proven to be a valuable tool to predict fracture properties of RAP mixtures.
Alternative Methodology for Assessing Cracking Resistance of Hot Mix Asphalt Mixtures with Overlay Tester
Several highway agencies have either implemented or considered implementing performance tests to predict the cracking potential of asphalt concrete (AC) mixes in the laboratory setting. One such test, the overlay tester (OT), simulates the opening and closing of cracks induced by daily temperature variations and tensile strain generated by traffic loads. The variability of the OT results is expressed as a major concern in reliably characterizing cracking potential of AC mixes. A more fundamental analysis process and more mechanistic performance indicators were implemented that consider the two stages of the cracking mechanism (i.e., crack initiation and crack propagation). The repeatability of the proposed performance indices, critical fracture energy and crack progression rate, seems to be better than the current criterion based on the number of cycles to dissipate 93% of the initial maximum peak load. The proposed cracking methodology and associated preliminary failure limits seem to characterize and satisfactorily discriminate the cracking resistance of AC mixes. Given its promise in this study, the proposed OT test method is recommended as a routine test during the mix design process of AC mixes to predict and screen their cracking susceptibility.
Development of an IDEAL Cracking Test for Asphalt Mix Design and QC/QA
The focus in recent years has been to make asphalt mixes more affordable, and this has led to the increased use of recycled materials and binder modifications. Consequently, one often-heard complaint is that the recent mixes are more susceptible to cracking. There is an urgent need for a practical cracking test for routine use in the process of mix design, quality control, and quality assurance testing. This paper develops an indirect tensile asphalt cracking test (IDEAL-CT). The IDEAL-CT is typically run at room temperature with 150-mm diameter and 62-mm high cylindrical specimens with a loading rate of 50 mm/min. The IDEAL-CT is a simple (no instrumentation, cutting, gluing, drilling, or notching of specimens), practical (minimum training needed for routine operation), and efficient test (test completion less than one minute). The test can be performed with regular indirect tensile strength test equipment. As described in this paper, the IDEAL-CT is sensitive to key asphalt mix components and volumetric properties including reclaimed asphalt pavement and recycled asphalt shingles content, asphalt binder type, binder content, aging conditions, and air voids. The proposed test also has a much lower coefficient of variation than traditional repeated load cracking tests. Furthermore, the IDEAL-CT results were compared with field cracking data collected from the Federal Highway Administration’s accelerated load facility, Texas SH15 and SH62, and MnROAD. The IDEAL-CT characterization correlated well with field performance in terms of fatigue, reflective, and thermal cracking. Last but not the least, the ruggedness test performed in this study indicated that the IDEAL-CT, after combining both statistical and practical views, could be considered as rugged with all four variables: specimen thickness, loading rate, test temperature, and air voids.
Relationships between Simple Asphalt Mixture Cracking Tests Using Ndesign Specimens and Fatigue Cracking at FHWA’s Accelerated Loading Facility
Current methods and criteria for asphalt mix design and quality assurance testing are unable to discern the true impact of recycled materials, such as reclaimed asphalt pavement, recycled shingles, ground tire rubber, re-refined engine oil bottoms, or products aimed at enhancing performance such as polymer modified binders, fibers, or even warm- mix asphalt technologies. There is an urgent need to implement reliable tests to assess the cracking resistance of asphalt mixtures. This study used test specimens compacted to Ndesign in four simple tests: the Cantabro test, a modified version of the Texas overlay test (OT), the semi-circular bend (SCB) test, and the indirect tensile (IDT) test from which IDT strength and two other index parameters were obtained. Relationships between results from these tests and the fatigue cracking of the FHWA’s Accelerated Loading Facility (ALF) test lanes were used to assess how well test methods could differentiate actual performance. The study also examined variability data for test results and how that impacted the utility of the tests to discern mixtures from one another. Results of this study indicate that the IDT Nflex factor and the Cantabro test provide the strongest relationship to fatigue cracking in the field and are able to detect differences among common mixture variables. Both tests are simple, quick, and have very low costs to implement.
Evaluation of Asphalt Mixture Cracking Performance Using Monotonic Direct Tension Test in the AMPT
This study investigated the feasibility of using a monotonic direct tension test in the AMPT with small scale specimens to evaluate the crack properties of the asphalt materials. Ten loose asphalt mixtures collected from the FHWA’s Accelerated Loading Facility (ALF) test lanes combining various recycled asphalt pavement (RAP) and recycled asphalt shingles (RAS) percentages and warm mix (WMA) technologies were tested at three loading rates and two aging conditions. Various mechanical parameters and indexes were developed to analyze the experimental data. The laboratory monotonic testing results were compared to and statistically correlated to the ALF field cracking performance. The coefficients of variation from the experimental results indicated that the monotonic test has satisfactory repeatability. Test loading rates were found to have significant impact on the behavior of the test materials, and an actuator rate of 10mm/min is considered to be ideal for all tested materials, including both short-term oven aging (STOA) and long-term oven aging (LTOA) conditions. Aging conditions were also found to dramatically influence the monotonic testing results. The experimental results illustrated that the components (percentages of RAP/RAS and binder PG) of the asphalt mixtures and the warm mix technologies have clear effects on the monotonic testing, indicating the test is capable of differentiating such asphalt mixtures. The parameters of the total fracture energy and the total energy divided by the inflection point slope from the laboratory monotonic test were found to have very strong correlations with the field performance. As a general conclusion, this study shows that the monotonic direct tension test in the AMPT is a very promising tool to evaluate the crack properties of asphalt mixtures due to the ease in the sample preparation, compatibility with the AMPT, strong correlation with field performance and feasibility for testing field specimens.
Comprehensive Evaluation of Low Temperature Fracture Indices for Asphalt Mixtures
For performance based specifications and design processes, a number of cracking related index parameters have been proposed for asphalt mixtures in recent years. A number of these parameters have been developed to utilize results from fracture tests. This study conducted a comprehensive evaluation of various fracture index parameters including fracture energy, Illinois flexibility index, stress intensity factor and toughness index. Over 200 tests from 61 distinct test data sets representing 21 asphalt mixtures are included. The focus of this study is on low temperature cracking and all indices were evaluated using test results from the disk shaped compact tension (DCT) fracture tests conducted at low temperatures. The objective of this study was to determine if there is a relationship between various fracture index parameters as well as to determine typical measurement variability associated with each parameter. Comparisons were made between different indices and correlations were determined for the mix rankings provided by individual indices. The results indicate that fracture energy successfully captured the mix rankings and showed strong correlation with other indices. In order to better capture post-peak softening behavior of asphalt mixtures the flexibility and toughness indices have been utilized, however these parameters were found to have high variability. A new index called fracture strain tolerance (FST) has been proposed that was shown to provide the same level of distinction between mixtures as the flexibility and toughness indices while having considerably lower variability. Finally, several areas were identified for future extension of this research.
Prediction of Phase Angles from Dynamic Modulus Data and Implications on Cracking Performance Evaluation
The need for viscoelastic characterization of hot mix asphalt (HMA) is increasing as advanced testing and modelling is incorporated through mechanistic-empirical pavement design and performance based specifications. Viscoelastic characterization includes measurement of the mixture stiffness and relative proportion of elastic and viscous response. The most common method is to measure the complex modulus, where dynamic modulus represents the stiffness and the phase angle represents the relative extent of elastic and viscous response. Determination of phase angle from temperature and frequency sweep tests has been challenging, unreliable, and prone to error due to a high degree of variability and sensitivity to signal noise. There is also a large amount of historical dynamic modulus data that is either missing phase angle measurements or has poorly measured phase angle data that inhibits their use in further evaluation. This paper evaluates the robustness of phase angle estimation from stiffness data for asphalt mixtures. The objectives of the study are to: (1) evaluate phase angle determination from the slope of log-log stiffness master curve fitted with a generalized logistic sigmoidal curve and compare it with lab measurements and the Hirsch model; (2) assess the effect of measured and predicted phase angle on mixture Black space diagram; (3) evaluate the effect of using predicted phase angles on S-VECD fatigue analysis, particularly on damage characteristics curves and fatigue coefficients; and (4) evaluate the impact on LVECD pavement fatigue performance evaluation due to the use of predicted phase angles. Three sets of independent mixtures were evaluated in this study comprising a wide range of mixture conditions. The results indicate a good agreement between measured and predicted phase angle values in terms of shape and peak master curve values. In terms of magnitude, the values from both matched very well for a certain sets of mixtures and subsequently manifested in similar performance predictions. However, for other sets of mixtures a considerable difference was observed between measured and predicted phase angle values as well as S-VECD and LVECD results. The differences may be attributed to the use of different types of LVDTs (loose core versus spring loaded). Another possible explanation of the difference could be the contribution of plastic strain, which may create a difference in phase angles of 1 to 2 degrees.
Fatigue Investigation on Asphalt Mixture Layers’ Interface
In this paper the interface shear fatigue performance is investigated based on a new fatigue test protocol, which includes both cyclic shear and normal static compressive load. Several asphalt structure types are tested at five temperatures (from -10°C to 50°C) and three normal stress levels (from 0 MPa to 0.50 MPa). Based on fatigue tests at three different shear amplitude levels unique fatigue functions of the layers’ interface were identified. It was found that the interface shear fatigue performance is highly dependent on normal stress state and increases with increasing normal stress. This is a direct consequence of the enhanced friction, adhesion, and interlock at the shear interface. Observing the fatigue lives at different test temperatures, the best resistance was observed at lowest temperature. From a practical viewpoint, it is highly recommended to perform the test without normal stress application, since this can better reflect the research and practice experience, while keeping the testing protocol to a reasonable level of complexity.
International Forum: Hot Recycling Technologies: Experiences and New Challenges in Costa Rica and Central America
The use of recycled pavement technologies presents a new, economical and environmentally efficient alternative for road maintenance and construction. For this reason, the implementation of recycled pavement technologies has become a necessity worldwide. Recently, hot recycling technologies have been actively implemented and promoted in Central America. The main applications consist of reclaimed asphalt pavement (RAP), stabilization of base courses with foamed asphalt and/or asphalt emulsion, and warm mix asphalts. Recognizing the existence of diverse recycled pavement techniques that look to comply with different environmental goals, generating economic benefits, and promoting improvements in quality of life of the local citizens, to date Nicaragua, Guatemala, El Salvador and Costa Rica have been researching and implementing some of these technologies. Furthermore, the National Laboratory of Materials and Structural Models of Costa Rica established a group of researchers dedicated to the implementation of sustainability in the development of road infrastructure. Their main purpose is to achieve the Carbon Neutrality goal that Costa Rica set for 2021 (bicentennial celebration of independence). The document presents an overview of projects constructed in Central America by applying environmentally friendly technologies, as well as R&D from Costa Rica promoting the implementation of these techniques.
AAPT Symposium: Rubberized Asphalt Use is Up: Developments, Opportunities and Challenges