PERFORMANCE EVALUATION OF WARM MIX ADDITIVES – SASOBIT, EVOTHERM AND ZYCOTHERM

Abstract

The asphalt mix temperatures during the construction of bituminous pavements must be high enough to ensure desired workability and quality of the compacted pavement. These high temperature requirements add to the cost and environmental concerns associated with road construction. Nevertheless, the “Hot Mix Asphalt (HMA)” industry is constantly exploring technological improvements to enhance pavement performance, increase construction efficiency, and advance environmental stewardship. One way to achieve these goals is by incorporating methods to reduce HMA production temperatures. “Warm Mix Asphalt (WMA)” is one such technology that allows significant lowering in the mixing and compaction temperatures compared to HMA. However, with the lowering of temperatures, some concerns regarding the performance characteristics of asphalt binders and mixtures have been previously reported. Thus, to gain confidence in their performance, it becomes imperative to assess various WMA technologies. Therefore, the present research work is focused on appraising the effects of three WMA additives – Sasobit®, Evotherm®, and Zycotherm® on performance characteristics of asphalt binders and mixes prepared with base binder VG-30, with emphasis on high-temperature performance. For asphalt binder studies, three dosages of each WMA additive were blended with the control binder to give ten binder combinations. Firstly, the rotational viscosity of the binders was measured at high temperatures for workability estimation. Rheological tests were conducted on the binders, starting with “Superpave high-temperature performance grading (PG), Superpave rutting parameter G*/sinδ at varying temperatures" for unaged and short-term aged binders. “Multiple stress creep recovery (MSCR) test” was also performed on the short-term aged binders and creep compliance and recovery parameters were calculated. Further, the stress and recovery data was fitted with Burgers model. Next, frequency sweep test was conducted on unaged and short-term aged binders, and the complex viscosity data was fitted to “Carreau-Yasuda model to obtain zero shear viscosity (ZSV)”. Complex modulus master curves were developed for a reference temperature of 60°C, to study the viscoelastic behaviour of binders in detail. To assess fatigue performance of binders, intermediate temperature grading, stiffness-based parameter G*.sinδ, followed by “linear amplitude sweep (LAS) test” were conducted on the long-term aged binders. Based on the results of all rheological tests stated above, binders were ranked, and suitable WMA additive dosages were established. For the asphalt mixes characterization, mix design was done as per Marshall Method of mix design, Asphalt Institute Manual (MS-2). The asphalt mixes for the established WMA additives dosages were prepared with optimum binder content. The mixes were tested for Marshall stability and flow and indirect tensile strength (ITS) – wet and dry. The tensile strength ratio (TSR) was calculated to estimate the moisture susceptibility of the mixes. From the analysis of data and results, it was found that Sasobit addition resulted in reduced rotational viscosity and improved rutting resistance w.r.t. various parameters, indicating superior high-temperature performance. Evotherm addition helped in improving workability, however, it deteriorated the rutting performance of the binder, especially at higher dosages. Whereas, no appreciable change in high-temperature performance properties was found on addition of Zycotherm. W.r.t. fatigue evaluation, Sasobit addition increased the intermediate temperature and the damage factor “alpha” in LAS test, showing detrimental effects. However, a slight increment in the number of fatigue cycles at 5% strain level was observed. In case of Evotherm and Zycotherm, an improvement in fatigue performance was observed based on the parameters under study, which further improved on increasing additive dosages. Overall, from the rheological testing on binders, keeping more focus on high-temperature performance, 3% Sasobit addition binder outperformed other tested binders. Ranking of binders helped to establish the suitable dosages of Sasobit, Evotherm, and Zycotherm as 3%, 0.5%, and 0.1% respectively. The asphalt mix testing results revealed lower Marshall stability and ITS dry values for WMA mixes in comparison to HMA mix, indicating deterioration in strength, stability, and cohesion with WMA additives. However, the mixes with Sasobit and Zycotherm showed increased ITS wet values and better TSR, indicating enhanced moisture susceptibility. Overall assessment of WMA additives showed promising results and scope of their applicability in the industry as per specific requirements.