AbstractCrumb rubber is one of the waste materials used to improve the behavior of binders. The use of this recycled rubber as a common cheap additive can also help in preserving the environment. Modifying the binder with this additive can consequently increase its viscosity. The primary aim of this research is to identify the factors that influence the viscosity of the crumb rubber–modified binders and determine their optimal values by modeling the viscosity of the samples. To this end, central composite design (CCD) method of response surface methodology (RSM) was utilized to evaluate the influence of the selected factors including mixing temperature, time, speed, and blending content of the modifier on the viscosity of the crumb rubber–modified binders, which was identified as the main response. Based on the CCD method, 30 samples were prepared, and a rotational viscosity (RV) test was conducted on them. The results showed that crumb rubber content and mixing temperature are more effective than the other factors and their interactions. To yield the minimum viscosity, the optimal values for the influential factors were 4% for the blending content of the crumb rubber, 160°C for the mixing temperature, 57 Hz for the mixing speed, and 35 min for the mixing time. In this optimal case, the results of Fourier-transform infrared (FTIR) spectrum analysis showed that adding crumb rubber to the neat binder does not create a new factor group in the binder spectrum.

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