Dense Graded Mix Research Articles

Modified asphalt-rubber green pavement (MARGPave) mixtures: Product development and performance assessment

The major objective of this study was to develop “Modified Asphalt-Rubber Green Pavement (MARGPave)” mixtures with dense- and gap-graded aggregate structures to accomplish performance attributes superior to the conventional asphalt-rubber mixtures. The scope encompassed production of one conventional dense-graded, one dense-graded MARGPave, one gap-graded MARGPave, and one gap-graded asphalt-rubber mixtures using the Superpave mix design, conduct of stiffness modulus tests, moisture sensitivity, and fatigue evaluation at varying temperature-frequency combinations, which generated over 300 data points. The dense-graded MARGPave mix exhibited three times higher fatigue life and increased durability than the conventional dense-graded mix. The gap-graded MARGPave mix had lower modulus at all temperatures by more than 50% than the dense-graded MARGPave mix due to 2% higher asphalt content and rubber inclusions. Likewise, the modulus of gap-graded MARGPave mix was higher than gap-graded asphalt-rubber mix across all temperatures and frequencies, indicative of the elastic nature of the MARGPave mix due to higher rubber content. Higher modulus at higher temperatures and lower modulus at lower temperatures were concurrently displayed by dense-graded MARGPave mix, which indicated that the material would be resistant against rutting and low-temperature thermal cracking, thereby able to outperform conventional materials with insignificant rate of change of viscosity with increasing temperatures, and the presence of rubber particles that would render the mix the necessary additional resilience to endure the mix performance for the entire design life. It is noteworthy that the procedure of blending asphalt binder, aggregates, and the modified asphalt-rubber product that replaces some portions of fine aggregates helped create MARGPave mixtures that was found akin to the semi-dry process technology. It is envisioned that the indigenously developed MARGPave mixtures will be best suited as a roadway construction and maintenance strategy that supports the engineering use of end-of-life tires, encouraging the waste-to-wealth concept.

Utilization of high contents of recycled tire crumb rubber in developing a modified-asphalt-rubber binder for road applications

Waste tires are generally mixed with asphalt materials to produce asphalt-rubber (AR) or rubberized asphalt through wet-, dry-, and terminal blending processes. However, the limitations such as long-term storage and stability issues of these special products have led to the need for advancing the AR technology capable of mitigating the production-related problems. Thus, the major objective of this research was to re-engineer AR in the form of a set of eighteen modified-asphalt-rubber (MAR) products comprising high contents of recycled tire crumb rubber (CR) through activation and pulverization. A set of MAR products was synthesized through different blending dosages, sequences, and material combinations. The material characterization techniques (attenuated total reflection-infrared spectroscopy and powder X-ray diffraction) revealed that the type of additives and the proportions of each material pose a significant effect on the physiochemical properties of MAR. It is noteworthy that this study emphasized the best use of CR emanating out of scrap tires in paving applications through incorporation of higher volumes (as large as 62%) in the preparation of MAR when compared to the conventional AR binder that currently includes about 15–20% CR by weight of the base asphalt binder. The fatigue performance of MAR-modified dense graded mix (MAR blended with natural aggregates) was compared to the conventional mix, which revealed that the MAR-modified dense graded product will have higher fatigue life by more than two times, and increased durability than the conventional mix. Practically, the MAR products can be used for instant application in pothole patching, rehabilitation, and new construction through dry-process technology by replacing a reasonable proportion of conventional aggregates in the asphalt mix. It is envisioned that the MAR binders will reduce the overall cost of pavement construction by utilizing the untreated (or unprocessed) CR in larger quantities by actively supporting the recycling mechanism and circular economy.

Evaluation of effect of moisture on fatigue performance of pavement designed with recycled asphalt mixtures

The moisture resistance of dense- and gap-graded bituminous mixes prepared with the replacement of virgin aggregates and fresh bitumen with reclaimed asphalt pavement (RAP) was evaluated using indirect tensile strength (ITS) and resilient modulus ( Mr). The mechanistic analysis was performed to estimate the effect of moisture resistance on the fatigue performance of thin and thick pavement sections designed with dense- and gap-graded bituminous mixes. The ITS ratio and Mr ratio results indicate that the resilient modulus ratio is more sensitive to moisture damage in bituminous mixes than the ITS ratio. In addition, the results of the fatigue life ratio indicated that the gap-graded mix prepared with different RAP content has better resistance to moisture-related cracking in thick pavement sections than the dense-graded mix. However, the effectiveness of gap-graded and dense-graded mixes prepared with different RAP content on moisture-related cracking depended on the subgrade strength for thin pavement sections.

Modeling of Asphalt Aging Based on Short-Term Aging Temperature and Time of Normal Asphalt Mixture for Quantifying Binder Aging

The binder in hot-mix asphalt (HMA) is aged (oxidized) in a short-time period during haul-and-queue in the field. Since the oxidative aging of asphalt is a complex chemical process, it is difficult to define the asphalt aging as a function of limited variables. When comparing the same types of asphalt mixes, however, the mix temperature (T) and time (t) kept at the T will be the primary source of variation affecting aging levels of the binder in the mix. Since the binder aging level is not easy to estimate without measuring a physical property, this study concentrated on developing an aging quantity (AQ) model for estimating aging levels of the binder in the mix based on T and t. The loose asphalt mixes were artificially short-term aged at various Ts for different t; 130, 160, and 180 °C for 1, 2, 4, and 8 h. The absolute viscosity (AV) values, which represent aging levels of the recovered binder after each short-term aging (SA) of normal dense-graded mix, were used for regression with AQ values computed by the AQ model. The best-fit AQ model was selected by trial-and-error regression iterations between measured AV and computed AQ. The AQ was then used to estimate AV (EAV) of the aged asphalts in various normal asphalt mixes. It was found that the AQ was useful for estimating AV of the binder in the SA-treated mix, and the EAV by AQ showed an excellent correlation with the measured AV with R2 > 0.99. Therefore, it was possible to conclude that the AQ could be used to predict aging level of various short-term-aged normal asphalt mixes if the materials sources were limited.

Laboratory Evaluation of Rubberized Binder and Mix Containing a Low Content of Devulcanized Rubber Modifier

This paper evaluates the mechanical properties of rubberized asphalt binder and mix containing 5% and 10% rubber. This rubberized asphalt binder was manufactured in a field-blend process using devulcanized rubber particles, finer than 250 microns, derived from waste tires. Comparison between the rubberized binder and the base binder test results showed that the rubberized binders had higher complex moduli and lower phase angles at the grade temperature. They also had a higher percentage recovery in the multiple stress creep recovery test, and a significant creep stiffness reduction in the bending beam rheometer test. Given the low rubber content and small rubber particle size, this rubberized binder can be used in dense-graded mixes, whereas asphalt rubber binders, with larger rubber particles and higher rubber content (>15%), must be used in gap- or open-graded mixes. This rubberized dense-graded mix met the volumetric design criteria at the same binder content as the control mix prepared with the unmodified base binder. Laboratory tests on the mix included repeated load triaxial, Hamburg wheel track, flexural dynamic modulus, and beam fatigue. The rubberized mixes had slightly lower stiffnesses than the control mix, but better resistance to moisture damage, rutting, and fatigue cracking. A strong linear correlation was found between the carbonyl area index and the rheological properties of the long-term aged binder and fatigue life of the mixes. Based on these findings, these rubber-modified binders can be considered for use in dense-graded mixes to improve overall performance and make use of waste tires.

Development and Performance Analysis of Asphalt-Rubber Gap-Graded Mixtures: Study on Aggregate Gradations

Abstract The objective of this research was to develop and evaluate asphalt-rubber gap-graded (AR-Gap) mixtures produced with varying gap aggregate gradations and designate a suitable aggregate gap gradation for local conditions, which could further be implemented at global level after necessary laboratory and field evaluations. A total of fifteen gap gradations were investigated, including seven specified by various highway agencies and eight trial gradations. The gradation studies recommended clustering seven gradations into two groups of similar characteristics. Furthermore, investigations on trial gradations presented the significance of each portion of the aggregate gradation in developing a superior performing gap gradation. Based on the refined gradation studies, three gap gradations were chosen: two from agency-specified, and one from the trial gradation for basic performance evaluation. Rutting, fatigue cracking, and moisture susceptibility investigations were accomplished on ten asphalt mixtures encompassing nine AR-Gap mixtures and one conventional dense graded mix. The results depicted superior performance of the proposed gradation indigenously developed in this study in respect of all distresses. The balance of coarse and fine aggregates in the proposed gradation facilitated higher strength at higher temperatures to resist rutting. Furthermore, the customized AR binder aided in providing additional resilience to the mixture, thereby improving the fatigue cracking resistance of the proposed AR-Gap mixture. Overall, it is envisaged that this study will help understand the effect of aggregate gradation characteristics on the performance of the AR-Gap mixture and aid in the development of AR-Gap mixtures as suitable long-lasting pavements.

Rutting performance of asphalt-rubber gap-graded mixtures: evaluation through statistical and reliability approaches

The objective of this study was to evaluate the rutting performance of asphalt-rubber gap-graded (AR-Gap) mixtures using wheel tracking tests and further explain the permanent deformation characteristics of those special materials through statistical and reliability approaches. A total of 28 asphalt mixtures covering over 200,000 data points were utilised, including: 27 AR-Gap mixtures and one conventional dense-graded (DG) mixture. Statistical analysis confirmed the significance of binder type, aggregate gradation, and binder content on the superior rutting resistance of AR-Gap mixtures than the DG mix. Selection of the best combination of all the three variables resulted in a significantly rut-resistant AR-Gap mixture. Reliability analysis on mixtures’ rutting performance was accomplished using Weibull distribution function. Weibull model parameters and derived factors aided in a better interpretation of the rutting performance of AR-Gap mixtures. Overall, it is envisaged that this research study would further the state-of-the-art in designing rut-resistant asphalt-rubber gap-graded mixtures.

Investigation of Cracking Performance of Asphalt-Rubber Gap-Graded Mixtures: Statistical Overview on Materials’ Interface

Abstract This study investigated the cracking mechanism of Asphalt-Rubber Gap-graded (AR-Gap) mixtures with various material properties. The research focused on using a monotonic semicircular bending (SCB) test to estimate the cracking potential of AR-Gap mixtures at 0°C and 25°C. A total of 28 asphalt mixtures, including 27 AR-Gap mixtures and 1 conventional dense-graded (DG) mix, covering over 220 data points were utilized for evaluation. The outcome parameter, fracture toughness, KIC, calculated from the SCB test was used to compare and contrast the mixtures’ resistance to cracking at low and intermediate temperatures. At both test temperatures, the results indicated that AR-Gap mixtures have higher fracture resistance (~60 %) than the DG mix. Furthermore, statistical analyses confirmed that asphalt binder type, aggregate gradation, and binder content had significant effects on cracking performance. Aggregate gradation had a more pronounced effect on cracking performance than binder type, followed by binder content. In addition, a KIC predictive model was developed that had excellent statistical goodness-of-fit measures (R2 ≥ 90 %), which can be conveniently utilized in case of unavailability of the test setup. Overall, the findings revealed superior performance characteristics of AR-Gap mixtures that had high potential to resist cracking. Based on the various outcomes, the study was able to recommend aggregate gradation, binder type, and binder content as the parameters that would exhibit higher resistance against cracking failure. It is envisaged that this research would further the state of the art in designing crack-resistant asphalt mixtures.

Evaluating fracture properties of crumb rubber modified asphalt mixes

Discarded vehicle tires pose a severe environmental problem in the United States. Incorporating shredded tire rubber as crumb rubber modifier (CRM) into asphalt paving materials has shown potential for great benefits. This study was undertaken to evaluate the effect of CRM, base virgin binder stiffness, total binder content, and aggregate gradation on fracture performance of CRM asphalt mixes using the semi-circular bend (SCB) fracture test. All mixes were long term oven aged (LTOA) before testing at the effective temperature for cracking resistance. The results were analyzed within the context of several response parameters: a modified version of Illinois flexibility index (FI), fracture energy, and the load at failure (peak load). Fracture performance of virgin and CRM binders were also evaluated using linear amplitude sweep (LAS) test and the Glover-Rowe (G-R) parameter obtained from dynamic shear rheometer (DSR). As expected, adding CRM into asphalt mixes increased the design binder content. In addition, design binder content increased as the CRM content increased in the mix. Fracture test results showed that among dense graded mixes, CRM mixes with the highest binder content showed superior flexibility and cracking resistance compared with those with lower binder content, in spite of the fact that CRM mixes delivered lower peak load (i.e. strength). It was also shown through this study that gap graded CRM mixes are preferred to dense graded CRM mixes as they deliver higher flexibility thus better cracking resistance. Finally, results from the binder tests demonstrated superior cracking resistance of CRM binders compared with virgin binders.

Design and evaluation of stone matrix asphalt using stiffer grade crumb rubber modified bitumen

Stone Matrix Asphalt (SMA) is primarily used as a surfacing layer material since it is found to perform well under heavy traffic conditions and at the same time is more cost-effective when compared to the conventional dense graded mixes. An attempt has been made in this study to incorporate a stiffer grade crumb rubber modified bitumen (CRMB) for design of SMA. Extensive laboratory studies on mixes prepared with different modified binders and normal binders (with fibers) were conducted. The results obtained indicate that SMA designed using CRMB-70 and CRMB-80 performs well under heavy traffic conditions and at the same time without significant drain-down. Further, CRMB-80 is found to be suitable at high temperatures since it yields a very low accumulated strain and is thus recommended for designing rut resistant durable mixes. In addition, CRMB-80 yields the least rut depth of 4.5 mm even after 20,000 load repetitions.

Use of reacted and activated rubber in ultra-thin hot mixture asphalt overlay for wet-freeze climates

This study investigated the use of crumb rubber in hot mixture asphalt pavement. The two main objectives of this study were to enhance pavement performance in wet-freeze climates and mitigate the environmental issues caused by waste tires. In this study, a reacted and activated rubber modified gap-graded mixture, abbreviated as “RAR-ThinGap”, was designed to improve performance in wet-freeze climates. The concepts behind this design were: an aggregate structure of stone-on-stone contact, a large asphalt film thickness, and elastic rubber fillers. Extensive laboratory tests and field noise measurement had been conducted to determine the properties of this mix. In general, the test results were favorable in regard to moisture damage resistance, low temperature cracking resistance, fatigue cracking resistance, rutting resistance, and noise reduction. The average fracture energy of the RAR-ThinGap mixture was at least four times higher than that of the control mixture at −24 °C. Higher fracture energy indicates an excellent ability to resist thermal cracking in wet-freeze climates. The RAR-ThinGap mix had a fatigue life over 50 times longer than that of the polymer modified SMA mixes or conventional dense-graded mixes. The stone-on-stone skeleton structure of the mixture enhanced the rutting resistance. After 20,000 wheel passes, the average rut depth was 6.80 mm at a test temperature of 50 °C. This RAR-ThinGap mixture may provide an appropriate and environmental-friendly pavement designed for wet-freeze climates.

Aggregate gradation effect on the fatigue performance of recycled asphalt mixtures

This paper presents the details of a laboratory study on the evaluation of the fatigue characteristics of asphalt mixtures containing Recycled Asphalt Pavement (RAP) material using controlled stress indirect tensile fatigue test (ITFT). The effect of different aggregate gradations on the fatigue performance of RAP mixes is the main focus of the study as this helps us to identify the proportion of RAP that can be used from fatigue performance consideration. Two dense mixes and three gap graded mixes, both with different percentages of RAP, were investigated. VG 30 viscosity grade virgin binder was used. The fatigue test was conducted at 25°C. Laboratory fatigue life determined under stress controlled mode increased with increase in the RAP content for all the five aggregate gradations. The corresponding ‘Initial strain vs fatigue life’ trends suggest a decrease (for any specific strain level) in the fatigue life with an increase in the RAP content. However, analysis of the fatigue performance of typical pavement sections with different RAP contents do not suggest any significant detrimental effect of increasing the RAP content for pavements with thick bituminous layers. Use of open graded mixes permits the use of larger binder contents and higher RAP contents in the mix. Fatigue lives of RAP mixes are longer for gap graded mixes compared to dense graded mixes. The fatigue lives of RAP mixes are found to correlate very well with fatigue parameters of the virgin-RAP binder blends prepared in suitable proportions.

Effect of rainfall on the compaction of dense-graded hot mix asphalt

Unpredictable tropical weather particularly rainfall can possibly interfere the compaction work of asphalt pavement. Heavy rainfall often leads the asphalt to cool and harden quickly and become difficult to compact. This study evaluates the effect of rainfall on the compaction properties of hot mix asphalt (HMA). A dense-graded mix of AC14 was prepared using 60/70 PEN bitumen and compacted at different temperatures. A handy steel roller was used to compact the slab samples with the size of 305 mm × 305 mm × 50 mm. The cooling rate and time available for compaction (TAC) were monitored using thermocouple under different levels of artificial rainfall and wind speed generated in the laboratory to simulate the wet conditions. The slabs were then cored at 100 mm diameter and tested for the volumetric properties. The result shows that environmental conditions and mix temperature determines the cooling rate and final density of the asphalt pavement. In addition, the TAC decreases as the compaction temperature reduces. Alternatively, working under light and moderate rainfall during the asphalt paving operation reasonably achieves the required degree of compaction (DOC). However, paving works under heavy rainfall reduce TAC, compactibility and density of the asphalt mat.

Investigation of the use of waste crushed glass in the production of asphalt mixes

Recent study at the Council for Scientific and Industrial Research in South Africa has revealed the potential to substitute depleting natural aggregates with waste crushed glass in asphalt mixes. This paper presents an investigation of the use of crushed glass as an aggregate replacement in a conventional dense-graded asphalt wearing course commonly used on highways in South Africa. The objective of this study was to design a dense-graded hot-mix asphalt that utilises 15% of waste crushed glass as a substitute material for sand, and to compare its performance with a conventional dense-graded mix. The mix design was based on current South African methods. The overall results indicate that the glass asphalt mix meets the South African criteria, thus the mix design is acceptable. The optimum binder content of the glass asphalt mix was 5·1%, compared with 5·0% for the conventional dense-graded asphalt mix. Further investigation of the stiffness and rutting performance characteristics indicated that the asphalt mix with crushed glass could outperform the conventional asphalt mix in terms of resistance to rutting. The measured performance data were used to develop semi-empirical dynamic modulus and permanent deformation models for the two asphalt mixes.

Examination of physical property degradation due to severe short-term ageing and effect of hydrated lime as antioxidant in asphalt mixture

If the short-term ageing (SA) level of asphalt mixture is intense before paving, the pavement service life will be reduced due to the highly stiffened binder from the starting time of service. However, since how much the ageing damage is induced by the severe SA is not known, this study was initiated to examine the physical property degradation of the binder in the SA treated mixes. Four SA times (1, 2, 4 and 8 h) at 160°C and 180°C were used for 3 mixes: a dense-graded HMA mix with a normal binder (PG64-22), the same mixes including a hydrated lime (HL; 1.5% by wt. of mix) and a polymer stabiliser (KL; 1.5% by wt. of binder). Increasing patterns of absolute viscosity (AV) and stiffness (G*/sin δ) due to ageing were found to follow the form of exponential function. They were highly increased in 2 h (h) SA, but exceedingly increased in 4 h and longer SA at 180°C. These AV and stiffness increase resulted in DSR critical temperature (CT) escalation and degradation of bending beam rheometer (BBR) CT. The AV, G*/sin δ, DSR CT and BBR CT of the normal mixes were found to have a very high correlation (R2 > 0.98) with large-molecular size (LMS) ratio (%) of gel-permeation chromatography. The binder AV and G*/sin δ of HL and KL mixes were estimated using LMS. In comparison of binder ageing level of mixes with and without HL or KL, it was found that both HL and KL retarded the ageing, but HL more significantly, for longer SA time at higher temperature. Therefore, it was evidenced that the use of HL in the HMA mix should be a highly beneficial choice for preserving durable pavement.

Characterization of Dense-Graded Asphalt Containing Virgin, Postconsumer, or Blended Polymer Systems

Abstract This paper contains data related to ground tire rubber (GTR), and binder and mixture performance information is presented that validates some aspects of current practice yet shows that other areas could be improved. While GTR has been utilized in asphalt paving since the 1960s and use in dense-graded mixes began in the late 1980s to early 1990s, proper formulations (including understanding GTR’s functional polymer content) have been lacking a considerable portion of the time. This paper’s primary objective was to review, evaluate, and suggest enhancements to GTR modified binder characterization. One finding was that parallel plate (PP) testing at a 2-mm gap is adequate for binders with 30 mesh (0.595 mm) and finer GTR particles, while cup and bob testing is more suitable for binders with coarser GTR particles. Enhancements were suggested for GTR’s use in modified binder systems related to incorporating hybrid binders containing styrene-butadiene-styrene (SBS) and GTR. Discussion is provided regarding specifying GTR based on virgin polymer replacement.

Improving cohesiveness of porous friction course 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 ravelling 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 ravelling 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 8 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.

Binder Rheological Relationships Depicting Wheel Tracking Rutting Parameter for Asphalt Mixtures: A Case Study

Abstract A number of wheel tracker devices are currently available worldwide to evaluate the rutting potential of the conventional and modified asphalt mixes. In the past, researchers have developed correlations between wheel tracker rutting parameter and binder–mixture characteristics. The main objective of this case study paper was to present the relationships established between the binder rutting G*/sinδ parameter and asphalt mixture permanent deformation rut depth characteristics obtained from a wheel tracker device. Sixteen asphalt mixtures encompassing virgin, polymer-, and rubber-modified binders with dense, gap-, and open-graded gradations were prepared with three replicate samples per mix for evaluation purposes. The wheel tracker device rut depth of the rubber- and polymer-modified gap graded mixes were about 12 %–20 % lower than that of the conventional dense graded mix produced with a virgin binder. Furthermore, a correlation of G*/sinδ of the three binders with rut depths of the corresponding three field practical mixtures revealed that the cumulative rut depth of a mix increased with decreasing G*/sinδ, with the modified gap graded mixtures best suited to counter rutting potential in comparison with the dense graded mixtures. The results of this case study investigation corroborated several previous laboratory and field findings that rubber- and polymer-modified mixes are best suited to reduce rutting problems in flexible pavements as simulated from the near-field wheel tracker device.

Evaluation of wearing course mix designs on sound absorption improvement of porous asphalt pavement

This paper describes a laboratory procedure to study the sound absorption characteristics of porous asphalt pavement wearing course mix designs, and estimate their contributions to tire-pavement noise reduction. The effects of the following mixture properties were considered: percent porosity of mixture, degree of clogging occurred, and layer thickness. Four porosity levels of porous asphalt were studied: 12%, 16%, 20% and 25%. The mix design with 20% porosity was tested for the effects of clogging and specimen thickness respectively. A dense graded mix was included to serve as a reference for comparison. The effectiveness of the sound absorption and tire-pavement noise reducing capability of the mixtures were evaluated using the laboratory data and field measurements of trial sections. An approximate noise reduction calculation based on laboratory sound absorption measurements is proposed for comparison of different mix designs. The study found that changing the percent porosity from 25% to 12% caused some changes in the frequency characteristics of sound absorption, but made negligible changes in their contributions to tire-pavement noise reduction. On the other hand, clogging of the pores in the mixtures resulted in noticeable decreases in both the sound absorption coefficients and tire-pavement noise reductions. The analysis estimates that about 23–33% of the total tire-pavement noise reduction achieved by the porous asphalt mixtures can be attributed to their sound absorption capability. Finally, changing the thickness of the asphalt mixture tested from 63mm to 200mm had no significant effect on sound absorption and tire-pavement noise reduction.

Overlay Tester Results from Dense-Graded Asphalt Concrete Mixes: Accuracy in Characterizing Crack Susceptibility

Several highway agencies have either implemented or considered implementing laboratory performance tests to estimate the cracking potential of asphalt concrete (AC) mixes during the mix design process. One such popular test, the overlay tester (OT), measures the number of cycles to failure of specimens that are caused by the repeated application of deformation. The major concern about using this test as a reliable characterization of the cracking susceptibility of AC mixes, especially for dense-graded mixtures, is the variability in the specified number of cycles to failure in the performance index. The main objective of this paper is to report a means for improving the consistency of the OT test results on dense-graded AC mixes. An assessment of a specimen preparation process that can yield more consistent results was conducted. The consistency of the traditional number of cycles to failure as well as of the load–displacement response and load reduction curves was investigated by using a modified specimen preparation process. The repeatability of alternative performance indexes, such as the critical fracture energy and crack progression rate, that can be measured from the OT test was also investigated and compared with that of the index for the number of cycles to failure. This study indicated that the raw data from the OT test seemed to be repeatable if the proposed specimen preparation process was consistently followed. Alternative performance indexes that yielded an acceptable degree of repeatability may be readily implemented in the OT test to assess the cracking characteristics of AC mixes.