In December 2007, a portion of State Route 11 in Deland, Florida, was milled and repaved with 45% reclaimed asphalt pavement (RAP). These high RAP mixes were produced at lower than normal hot-mix temperatures and with foamed warm-mix asphalt (WMA) technology. This project was the first large production in which the Florida Department of Transportation (DOT) allowed the use of high RAP in combination with WMA. FHWA, in cooperation with Florida DOT and the National Center for Asphalt Technology, was on site for production and placement of the high RAP-WMA. Plant-produced mix was collected by FHWA for performance testing evaluation. Two mixes were produced: a high RAP–hot-mix asphalt (HMA) control mix and a high RAP-WMA mix. Performance tests conducted by FHWA included performance grade (PG) determination of binders, dynamic modulus, and flow number. PG results of the binders indicate that the high RAP-WMA mix is softer than the high RAP-HMA control mix. This is further confirmed by flow number results, where the high RAP-WMA mix had a lower flow number than the high RAP-HMA control mix did. Dynamic modulus results indicate that the high RAP-WMA mix is slightly softer than the high RAP-HMA control mix, especially at intermediate temperatures. Comparison of measured dynamic modulus results with those predicted using the Hirsch and Witczak models confirm that complete blending occurred in the high RAP-HMA control mix. However, incomplete mixing of RAP and virgin binders may have occurred in the high RAP-WMA mix.
Flow number determined with the repeated-load test is used as a criterion to characterize the rut resistance of hot-mix asphalt. The repeated-load test is performed with the asphalt mixture performance tester (AMPT). The algorithm currently used in the AMPT to determine flow number was found to be extremely sensitive to noise in the data and identifies erroneous flow number results, especially for modified mixes. A new algorithm that uses the Francken model to fit the flow number data is considered for implementation. The robustness of the Francken model is verified in this study by fitting flow number data obtained from field projects. The flow number test is time-consuming, especially for high-stiffness binders and, in many cases, can take as long as 6 h. In this study, other parameters are examined to determine whether the test can be terminated early without loss of information on rut resistance. Steady-state slope and slope at 2% strain are found to correlate well with flow number; the correlation indicates that they may be robust indicators of rut resistance. The flow number test can be terminated once steady-state slope or slope at 2% strain is reached; this substantially reduces test time. This paper discusses the findings from analysis of flow number data from the Francken model and the development of a refined, more expedient method for determining flow number.
The rheological and failure properties of eight asphalt-filler mastics were characterized using new testing techniques being developed within the SHRP program for testing asphalt binders. The mastics were prepared using four SHRP core asphalts and two fillers (calcite and quartz). The rheological measurements were used to construct rheological master curves and temperature shift functions. The failure properties were measured at different temperatures and strain rates and were shifted using time-temperature superposition to construct stress and strain-to-failure master curves. A subset of the mastics was also tested after oxidative aging with the pressure aging vessel (PAV) and after isothermal aging at low temperatures. The properties of the mastics were compared with the asphalt binder properties to describe the changes resulting from adding the fillers. The fillers were found to change the shape of the rheological master curves and to significantly increase the failure stress at all combinations of temperatures and loading times. The changes in rheological properties were observed only in the time dependency while the temperature shift functions showed only slight changes at the highest temperatures. The relative stiffening effects were observed to be asphalt-specific, especially at the lower frequencies or higher temperatures. Oxidative aging was shown to change the rheological type of the mastics, to be asphalt-specific, and to be independent of filler type. Physical hardening was observed to result in shifting the master rheological curves to longer loading times without affecting the shape of the master curves. The anticipated influence of mastic properties on the main distress types of asphalt pavements was considered. The influence of fillers on low-temperature cracking was hypothesized to be independent of filler type. Therefore, it is expected that the asphalt binder will control the cracking mechanism. Based on the energy concept, the addition of the filler is expected to improve resistance to stress-controlled fatigue while for strain-controlled fatigue the improvement, if any, is expected to be minimal. Fillers are expected to significantly improve rutting resistance as a result of increasing moduli (viscous component). Effect on rutting is expected to be asphalt-filler-specific.
The rheological properties of asphalt binders modified by styrene–butadiene–styrene (SBS) depend on formulation variables. The most sensitive of them may be listed as polymer amount, cross-linking agent amount (percentage), and other additives such as polyphosphoric acid (PPA). The dispersion of SBS in an asphalt binder depends on the time and temperature of blending and the base asphalt binder compatibility. In this study an incompatible binder and a compatible base asphalt binder were selected and modified with various amounts of SBS. Elemental sulfur was used as a cross-linking agent in different proportions. Other additives, such as PPA at 0.5% concentration, were also used. High shear blends of SBS-modified asphalt binders were made in the laboratory by varying blending time until an optimum dispersion of polymer was obtained. The dispersion of the polymer was studied with a fluorescence microscope. A multiple stress creep and recovery (MSCR) test was used to study creep and recovery behavior of these modified binders. MSCR test results ( Jnr and percentage recovery) were able to characterize the extent of dispersion of SBS in these polymer-modified asphalts (PMA). This implies that a fundamental test method is now available to discriminate between the dump-and-stir types of PMAs and those that have been optimally dispersed. This presentation discusses the effect of SBS dispersion and other additives on the MSCR test results.
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