Asphalt mixture design for porous ultra-thin overlay

In order to design a porous ultra-thin overlay (PUTO) asphalt mixture, two high-viscosity modifiers and two blended asphalts were used to prepare high-viscosity asphalt. Dynamic shear rheology (DSR) tests, 60 °C dynamic viscosity tests, Fourier transform infrared spectroscopy (FTIR) tests and physical properties tests were used to study the modification effect and modification mechanism of high-viscosity asphalt. Next, based on existing gradation design methods, two types of gradation, coarse and fine, were designed for PUTO. In addition, based on the results of draindown and Cantabro tests, the optimal asphalt-aggregate ratio was determined. Finally, the performance of the designed PUTO asphalt mixture was verified. The results show that with the increase of the amount of high-viscosity modifier, the complex shear modulus and anti-rutting factors of the four kinds of high-viscosity asphalt gradually increase, while their phase angles gradually decrease. Only SBS-I (with high-viscosity modifier contents of 14% and 16%) and SBS-II (with high-viscosity modifier contents of 14% and 16%) high-viscosity asphalt have a 60 °C dynamic viscosity exceeding 100,000 Pa·s. The main component of the high-viscosity modifier for the modification of the original asphalt is thermoplastic rubber, which forms a polymer network structure between the polymer in the high-viscosity modifier and the asphalt component, which is reflected in the infrared spectrum. That is, there is a significant characteristic absorption peak at 966 cm−1 in the fingerprint vibration spectrum region (1500–600 cm−1). After the high-viscosity modifier content reaches 14%, the penetration of SBS-I and SBS-II high-viscosity asphalt decreases, and their growth trends of ductility and softening point also slow. Considering the total cost, when the SBS-modified asphalt-high-viscosity modifier ratio is 86:14, the overall performance of the high-viscosity asphalt is optimal. According to the results of Cantabro and draindown tests, the optimal asphalt-aggregate ratios of PAC-1-I, PAC-1-II, PAC-2-I and PAC-2-II asphalt mixtures are 4.6%, 4.5%, 5.3%, and 5.3%, respectively. The performance of the four asphalt mixtures designed was verified and meets the requirements.