Environmentally friendly high-content polyurethane modified asphalt: Workability evaluation and early performance evolution

High-content polyurethane modified asphalt (HPMA) has ignited the enthusiasm of researchers due to its low- carbon characteristics and excellent performance. The workability and service performance of HPMA are both affected by complex curing reaction. The viscosity of HPMA changes over time at simulated construction temperature, and the evolution of the chemical structure and rheological properties at different environmental temperatures at the early stage were systematically investigated in this work. After thorough exploration, the construction tolerance time for HPMA-50 % (with isocyanate pre-polymer content of 50 %) and HPMA-30 % (with isocyanate pre-polymer content of 30 %) at a construction temperature of 130 degrees C are 46 minutes and 72 minutes, respectively, which can basically meet the construction time requirements. The complex modulus of HPMA gradually increases with the increasing curing time, and the phase angle temperature dependence of HPMA-50 % is significantly weakened compared to HPMA-30 %, its viscoelasticity gradually transforms to thermoset. Meanwhile, the high-temperature rutting factor improvement rate of HPMA-50 % reaches 142.1 % after curing for 14 days, indicating a tremendously improved rutting resistance. The high-temperature creep recovery rates of HPMA-50 % and HPMA-30 % increased from 65.9 % and 3.6-96.1 % and 7.6 %, respectively, after 14 days of curing. This reveals that increasing the isocyanate pre-polymer content and extending the curing time all can significantly improve the entropy-elastic properties and creep recovery ability of HPMA. Fluorescence microscopy analysis showed that the polyurethane particles in HPMA changed from a granular structure to a network structure during the curing process, and gradually transitioned to a continuous phase as the prepolymer content increased. Further research also revealed a good correlation between the conversion rate of isocyanates and the rheological performance index of HPMA. Additionally, a functional model between the rheological performance index of HPMA and curing times was successfully established, which accurately predicted the evolution law of the rheological performance of HPMA under different ambient temperatures. The evolution mechanism of the early performance of environmentally friendly HPMA was investigated in this work, which laid the methodological foundation and theoretical basis for determining the construction tolerance time of HPMA and accurately evaluating its performance at the early stage.