Abu N.M. Faruk

Laboratory Hot-Mix Asphalt Cracking Testing: Evaluation of Three Repeated Loading Crack Tests

Cracking is one of the predominant distresses occurring in hot-mix asphalt (HMA) pavements and often costs highway agencies millions of dollars in road maintenance and rehabilitation activities. One way to minimize this distress is through material screening and selection of appropriate mix designs that are representative of crack-resistant HMA. However, there are no universally adapted standardized laboratory cracking test methods for routine mix design and screening purposes for HMA mix crack resistance. In this study, three repeated loading cracking test methods—namely, the repeated loading overlay tester (OTR), repeated loading indirect tension (R-IDT), and repeated loading semicircular bending (R-SCB)—were evaluated and compared for their potential application as surrogate tests for routine crack evaluation and screening of HMA mixes in the laboratory. The evaluation criteria, based on commonly used Texas HMA mixes, included (a) the potential to differentiate and screen mixes readily, (b) the sensitivity to mix design variables such as variations in the asphalt binder content, (c) repeatability and statistical variability, (d) correlation to field performance data, and (e) the practicality of implementation. Overall, the OTR test, with a proven correlation to field performance data, exhibited statistical superiority as a practical repeated loading crack test method for routine HMA mix design and screening, followed by the R-IDT test. However, although the R-IDT test exhibited superiority concerning sample fabrication simplicity, validation with field data remains one of the key challenges to be addressed with this test. The R-SCB test was the least repeatable, particularly at high asphalt binder content levels, and appears to be better suited for low-temperature testing.

Comparison of Fracture Cracking Parameters from Monotonic Loading Tests: Indirect Tension and Monotonic Overlay Tester Tests

Cracking is one of the major structural distresses that occur in hot-mix asphalt (HMA) pavements and is undesirable. Cracking often leads to costly maintenance and rehabilitation activities. Various laboratory test methods, including the indirect tension (IDT) test, semicircular bend test, Texas Overlay Test (OT), and bending beam fatigue test, are typically used to characterize and quantify the resistance of HMA to cracking. Recently, the OT test in monotonic loading configuration was explored as a supplement or surrogate to the traditional repeated loading OT test for evaluating the cracking resistance potential of HMA mixes in the laboratory. The basic analysis parameters from the monotonic OT test include the tensile strength, failure tensile strain, fracture energy (FE), and FE index, which are essentially the same fracture parameters obtainable from the IDT test. In this study, the crack resistance potentials of 15 different HMA mixes were comparatively evaluated with two monotonic loading tests: the IDT and monotonic OT. Statistical analysis with analysis of variance and Tukey’s honestly significant difference methods was performed to characterize the test methods’ comparative potential to discriminate and screen mixes. For the mixes evaluated and the test conditions considered, the corresponding results indicate that the FE index was the most effective fracture parameter in capturing the test loading history and differentiating the crack resistance potential of the HMA mixes. The statistical comparisons between the IDT and monotonic OT tests showed plausible correlations for all the fracture parameters that were evaluated, with the FE index being the best.

Laboratory Hot-Mix Asphalt Performance Testing: Asphalt Mixture Performance Tester Versus Universal Testing Machine

Reliable and repeatable laboratory testing of hot-mix asphalt (HMA) mixes is of paramount importance to ensure proper HMA mix-design characterization and ultimately to ensure satisfactory field performance. The study reported in this paper was undertaken to evaluate the comparability, repeatability, and reliability of two testing systems, namely, the asphalt mixture performance tester (AMPT) and the traditional universal testing machine (UTM), and to assess if the two systems could be used concurrently or in lieu of one another for dynamic modulus, flow number, and repeated load permanent deformation (RLPD) testing. For the HMA mixes evaluated to accomplish these objectives, the test results from the AMPT and UTM—on the basis of dynamic modulus, flow number, and RLPD testing—were statistically comparable and acceptable at the 95% confidence level. The test repeatability and variability in the AMPT and UTM systems also were statistically acceptable with low coefficient of variation values in the test results. Thus either system could be used confidently in lieu of the other to generate results of similar quality and reliability and with a comparable statistical degree of accuracy with acceptable variability for dynamic modulus, flow number, and RLPD testing. The choice or preference is up to the user, given that each system has its own merits and demerits. However, operator and technician proficiency and equipment calibration are some of the most critical factors never to ignore in laboratory testing work of this nature.