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Significance Statement
Since the realization that the multiple stress creep and recovery (MSCR) test could be used as a performance-related test to estimate the susceptibility of asphalt binders to rutting, many scientific studies have been conducted. The purposes of these studies included the following: (a) to correlate the non-recoverable compliances of the MSCR test with the results obtained from asphalt mixtures in the laboratory or in the field; and (b) to evaluate the ability of this test in clearly showing the different rheological responses of asphalt binders modified with several types of additives, among other ones.
More recently, researchers have noticed that the recovery time of 9 s used in the standardized MSCR test procedure was not enough to fully capture the delayed elastic response of some modified binders, and also that the stress levels of 100 and 3,200 Pa could not accurately represent the actual levels experienced by the binder in the pavement. As a consequence, other test conditions – e. g., longer recovery times and higher stress levels – were used. In the present study, asphalt binders modified with the Elvaloy® reactive ethylene terpolymer and polyphosphoric acid (PPA) were prepared and a desired high-temperature performance grade (PG 76-xx) was selected for all the formulations. The MSCR tests were carried out at the creep-recovery times established in the ASTM D7405-10a standard (1-s creep time and 9-s recovery time), and also at 2- s creep time and 18-s recovery time. It was observed that asphalt binder modification with PPA alone (AC+PPA) and with Elvaloy+PPA (AC+Elvaloy+PPA) substantially increased the percent recovery (R) and markedly decreased the non-recoverable compliance (Jnr) of the base material at typical high pavement temperatures ranging from 52 to 76 °C, either at 1/9 s or at 2/18 s. These results indicate that the modified binders are less susceptible to the accumulation of permanent deformation and have a higher elastic response than the unmodified one. An increase in both creep and recovery times mainly increased the Jnr values and decreased the R values of the materials, as it can be inferred from their percent recovery ratios (RP) and non-recoverable compliance ratios (RC). In other words, the asphalt binders became more prone to rutting when the loading-unloading times were longer. The AC+Elvaloy+PPA showed the lowest variations in its rheological parameters (lowest RP and RC values), followed by the AC+PPA and the 50/70 base asphalt binder.
Figure Legend (AR-2000ex dynamic shear rheometer from TA Instruments).
Materials and Structures, December 2013.
Matheus David Inocente Domingos, Adalberto Leandro Faxina.
Department of Transportation Engineering, Sao Carlos School of Engineering, University of Sao Paulo, Avenida Trabalhador Sao-Carlense, 400, Parque Arnold Schimidt, Sao Carlos, Sao Paulo, 13566-590, Brazil.
Abstract
This study presents the results of a series of creep-recovery experiments that were conducted on asphalt binders modified with polyphosphoric acid (AC+PPA) and Elvaloy® terpolymer combined with polyphosphoric acid (AC+Elvaloy+PPA) at the temperatures of 52, 58, 64, 70 and 76 °C. The multiple stress creep and recovery (MSCR) test was used to determine the percent recoveries (R), the non-recoverable compliances (J nr) and the percent differences in non-recoverable compliances (J nr, diff) of these modified asphalt binders and the ones of the 50/70-penetration grade base material. The base material is graded as PG 64-xx and the formulations with PPA and Elvaloy+PPA are graded as PG 76-xx. Two pairs of loading–unloading times were used in the MSCR tests, namely, 1/9 s and 2/18 s. The AC+Elvaloy+PPA shows not only the highest R values and the lowestJ nr values, but also the lowest sensitivity to a sudden increase in the stress level inside the asphalt mixture. Although the results are not as promising as the ones found in the AC+Elvaloy+PPA, the AC+PPA may be taken as an alternative to replace the unmodified asphalt binder when the loading and/or the climate conditions are more severe.
