Unmodified Asphalt Binder Research Articles

Rutting response of hot-mix asphalt to generalized dynamic shear moduli of asphalt binder

In this paper, a stiffness indicator of Generalized Dynamic Shear Modulus (GDSM) of asphalt binders is introduced to study the rutting resistance of unmodified and polymer-modified asphalt binders. Seven binders of two unmodified and five SBS-modified binders are tested. At 60 °C, the data of peak shear stress and corresponding peak shear strain are obtained for each binder by the Dynamic Shear Rheometer (DSR). In the linear viscoelastic region of binders, the GDSM is calculated on the basis of the constitutive shear stress–strain equation that is obtained by fitting the Box–Lucas model to experimental data. Two kinds of bituminous mixtures classified by the type of aggregate gradation are used to evaluate the rutting potential. For each gradation, the aggregate is blended with the seven binders. For each mixture, single binder content is employed so that differences in rutting depth can be investigated, depending on various GDMS values of the binders. The rutting depth of mixes is tested with the Asphalt Pavement Analyzer (APA) at 60 °C and the correlation between the average rutting depth and the GDSM is established. Results indicate that for two kinds of mixtures, there exists a good correlation between the GDSM and the average rutting depth. It is confirmed that GDSM is useful as a stiffness indicator for the evaluation of rutting resistance of binders. The constitutive equation of shear stress–strain can be obtained by fitting the Box–Lucas model up to the peak shear stress, which represents an explicit mechanistic implication.

Performance of Modified Asphalt Binders with Identical High-Temperature Performance Grades but Varied Polymer Chemistries

The rutting resistances of mixtures that contain polymer-modified asphalt binders with identical performance grades but varied polymer chemistries were evaluated. Eleven asphalt binders were obtained for this study: two unmodified asphalt binders, an air-blown asphalt binder, and eight polymer-modified asphalt binders. Five binders used in a prior study also were included. Asphalt binder properties were measured with a dynamic shear rheometer. Mixture rutting resistance was measured by ( a) the shear modulus, G*, and the Superpave® binder rutting factor, G*/sinδ; ( b) cumulative permanent shear strain (CPSS); ( c) the rut depths from the French pavement rutting tester (French PRT); and ( d) the creep slopes from the Hamburg wheel-tracking device. CPSS and rut depths from the French PRT were the primary mixture tests because they were specifically developed to measure rutting resistance under dry conditions. The high-temperature properties of the 11 asphalt binders had a high correlation to mixture rutting resistance as measured by the CPSSs. A weak correlation was found using the French PRT. Both correlations were high when the data from all 16 asphalt binders and mixtures were analyzed. The number of discrepancies between the high-temperature properties of the asphalt binders and mixture rutting resistance was low. A change in high-temperature performance grade from 70 to 76 significantly increased rutting resistance based on both mixture tests.

Effect of Silicone Rubber Molds on Low-Temperature Binder Grading Parameters: Bending Beam Rheometer S[60] and m-Value

A study was conducted to determine whether test specimen produced using silicone (Si) rubber molds affect the Superpave low-temperature grading parameters, S(60) and m-value, determined using the bending beam rheometer (BBR; AASHTO-TP1). Six laboratories participated in this study. Four unmodified asphalt binders with low-temperature performance grades between -6°C and -30°C were tested. The thickness of each test specimen was measured using the “flip” technique. The results showed that the Si rubber molds produce significantly thinner beams than the required 6.35 ± 0.05-mm thickness. This affects the calculated S(60) values but not the m-value. Aluminum (Al) molds were also used in this study according to AASHTO-TP1. It was found that the stiffness values S(60) calculated in the BBR test are significantly lower when Si rubber molds are used to produce test specimen as compared with those test beams produced using Al molds. The cause is the peculiarity of the calculation process specified in AASHTO-TP1. AASHTO-TP1 requires that the thickness of the test beam must always be assumed to be 6.35 mm for computing S(60). This study suggests that the test beams produced using the Si rubber molds are significantly thinner; therefore, assuming a thickness of 6.35 mm to calculate S(60) for these beams produces a negative bias. It is shown that when thickness is measured using the flip technique and the measured thickness is then used to calculate S(60), there is no statistically significant difference between the S(60) values of the test beams produced using either the Al or Si rubber molds.

A new look at rubber-modified asphalt binders

The high-temperature rheological characteristics and the low-temperature fracture properties of asphalt binders containing crumb and devulcanized rubber waste have been investigated. Asphalt binders containing crumb rubber of different mesh sizes, with and without surface modification, and a commercially available binder containing devulcanized rubber, were tested and compared with an unmodified asphalt and three commercially available polymer-modified binders. Interfacial modification of asphalt systems containing crumb rubber was found to give binders that were far superior in their low-temperature performance to commercially available products. The data suggest that a crack-pinning or crack-blunting mechanism is responsible for the increase in toughness found in these systems. A commercially available binder containing devulcanized rubber showed reasonably good high-temperature properties; however, its low-temperature fracture performance was disappointing in that it was not significantly better than that of unmodified asphalt binders.

Polymer Modification of Paving Asphalt Binders

Abstract • Polymer modification of asphalt binders has become a more accepted method for addressing pavement distresses. The heavier vehicle loads, higher traffic volumes and increased tire pressures have forced user agencies to explore polymer modification for asphalt pavement applications. • The compatibility between the asphalt and polymer depends on many factors. The most significant of these, based on microscopy, are the asphalt crude source, polymer microstructure and the thermal/mechanical history of the polymer-modified asphalt binder. • Classical methods and methods derived specifically for measuring the effect of polymers in the asphalt have poor correlation to mixture performance. The tests also seem to be specific to the different polymers tested. The test conditions make it difficult to extract basic information about the binder's mechanical properties. • Considerable work has been done on the rheology of asphalt and polymer-modified asphalt binders over a wide range of temperatures and rates of loading. Time-temperature superposition has been used to describe the effect of rate of loading on the complex shear modulus (G*) of both polymer modified and unmodified asphalt binders. The addition of polymers has been found to dramatically change the properties at high temperatures or low rates of loading. This has been correlated with varying degrees of success to permanent deformation in the asphalt mixture. • The bending beam rheometer and the direct tension test are ideally suited for measuring the low temperature properties of polymer-modified asphalt binders. Good correlation was found with bending beam results and the fracture temperature of the mixture using the TSRST method. Failure strains, measured for polymer-modified asphalt binders with the direct tension test, were up to ten times greater than that observed for unmodified binders. Polymer-modifiers generally decreased the fracture temperature of the mixture by 6–10°C. • Polymer modifiers for asphalt binders which contain a large percentage of butadiene (50% or greater), exhibit improved low temperature properties. This was observed as a decreased Tg for a polybutadiene modified asphalt measured using dynamic mechanical analysis. Also direct tension results for SB (50% butadiene) -modified asphalt binders showed a marked increase in low temperature failure strains. • The performance-based specifications (SHRP) show good correlation with mixture performance. The best correlations were observed between the binder's Theological properties and the load-associated fatigue and low-temperature thermal cracking resistance. For permanent deformation, it was observed that the aggregate plays a significant role in the resulting rutting. Further testing and field studies are required to validate these laboratory measurements. • Asphalt-rubber mixtures have been shown to have useful properties with respect to distresses observed in asphalt concrete pavements. Most notably a large increase in viscosity and improved low-temperature cracking resistance have been measured. Only a limited body of test results exists and further testing is required to fully understand the contribution of asphalt-rubber to the mixture's performance.