Mathematical modelling approach to determine flow time of asphalt mixtures modified with styrene butadiene styrene and polyphosphoric acid

This research recommends a mathematical modelling approach to determine flow time (FT). The scope of the work includes the collection and analysis of the static creep test data for asphalt mixtures produced using two different nominal maximum aggregate size (NMAS) values and two base binder grades. Modifiers of styrene butadiene styrene (SBS), polyphosphoric acid (PPA) and combination of them are selected to consider a wide range of testing times from 120 to 30,000 s. The FTs of the mixtures are determined using three techniques: the method developed in national cooperative highway research programme (NCHRP) Report 465, the Francken model, and the proposed mathematical model which is based on the ordinary tangent function. The results show that the NCHRP method is sensitive to the user-selected size of logarithmic scale unit of time, causing difficulties in the FT computation. It is also shown that the Francken model, often used to fit the dynamic creep curves, cannot completely fit the static creep test data, and so is not capable of estimating FT, properly. However, based on the limited data available in this study, the proposed tangent model performs very well in characterising all the three stages of the static creep curve and estimates the accurate value of FT in a reasonable time. Because of the simplicity, computational speed and accuracy of the proposed tangent model, it can be considered as an alternative to the NCHRP method to determine FT.