This paper presents laboratory test results demonstrating the effects of aggregate type and gradation on fatigue and permanent deformation of asphalt concrete. Diametral fatigue tests and uniaxial incremental static creep tests were performed under varying temperatures and mixture variables, including aggregate type and gradation. The effects of aggregate type and gradation on permanent deformation were evaluated under test combinations with changing asphalt type, asphalt content, air voids content, temperature, and applied stress level. For the fatigue study, the effect of aggregate type was evaluated by changing asphalt content, air voids content, and temperature. The test results were analyzed using statistical analysis and graphical comparison of data. The analysis of variance (ANOVA) tests were performed to investigate the main effects and interactions of the test variables with the aggregate type or gradation. The analysis revealed that, with the size of experimentation used in this study, aggregate type has significant effects on fatigue resistance and permanent deformation of asphalt concrete, indicating better performance from the mixtures comprised of aggregates with a rough surface texture and an angular shape. Coarse gradation, meaning a larger proportion of coarse aggregates with the same nominal maximum aggregate size compared to medium gradation, did not show significant effects on permanent deformation. Interactions of aggregate type with gradation, asphalt type, air voids, and temperature were found to be significant for the permanent deformation of asphalt concrete, whereas no interaction appeared to be significant for fatigue with the given size of experimentation.
Three methods for classifying aggregate particle shape and texture—AASHTO TP33, ASTM D3389, and the flow rate method—were evaluated. These methods were used to rank four natural river sands and a crushed granite from good to poor performance based on the criteria established by each method. Test results indicate that all methods easily distinguished the crushed aggregate from the natural river sands. The AASHTO TP33 and the flow rate method were found to be somewhat less sensitive to slight differences in particle shape and texture than was ASTM D3398. All the test methods were found to be repeatable, each having low coefficients of variation for all the aggregates tested. To evaluate the effect of particle shape and texture and mineral filler content on mix performance in permanent deformation, one natural sand ranked as average performing was selected and blended with the crushed granite in the proportion of 20 percent natural sand and 80 percent crushed granite. Asphalt-aggregate mixtures containing 4, 6, 8, and 12 percent mineral filler were designed by using the Marshall procedure and were tested in repeated shear test at constant height. Within the range of mineral filler type and contents used, results indicate that mixtures containing 100 percent crushed granite show lower accumulation of permanent strain than does an 80/20 blend of crushed granite and natural sand, and an increase in the mineral filler content of a mixture was found to decrease its accumulated permanent strain while increasing the mixture shear resilient modulus.
Evaluation of a mixture’s moisture sensitivity is currently the final step in the Superpave® volumetric process. This step is accomplished by using AASHTO T-283, which tolerates a range of values in the test variables of sample air voids and degree of saturation. The tensile strength ratios determined for the mixes in this study varied with the air void level and degree of saturation. Although the levels of conditioning were within the specifications for AASHTO T-283, test results both passed and failed the 80 percent criterion, depending on the severity of conditioning. An alternative to measuring indirect tensile strength is a test that evaluates a mixture’s fundamental material properties. A relatively simple test is proposed that measures the cohesion and friction angle for asphalt mixtures. In addition, the Superpave shear tester (SST) was incorporated as a tool in evaluating moisture sensitivity. The proposed axial test determined the cohesion and angle of friction of the mix. The friction angle remained constant for the conditioned and unconditioned samples. Hence, conditioning of the samples had practically no effect on the mixture’s internal friction. The cohesion of the mix decreased when the mix was subjected to conditioning. The reduction in cohesion was greater in the case of the Fountain aggregate, which is known to be highly moisture susceptible. The shear tests to failure performed on the SST confirmed the results of the new apparatus, which provides a simple method for determining a mixture’s cohesion. The loss of cohesion due to conditioning can be used to determine a mixture’s moisture susceptibility. The three antistrip additives used in this study were hydrated lime, a liquid amine, and a liquid phosphate ester.
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