Preparation and Water Stability of Permeable Polyurethane Mixtures

In response to the development of “sponge cities,” permeable pavements have been increasingly used in road construction to eliminate the adverse effects caused by traditional pavements. In recent years, permeable polyurethane mixtures have been considered as a novel material for permeable pavements. However, compared with traditional permeable pavement materials, the preparation process and material composition of polyurethane mixtures are not yet well understood, and their water stability has not been systematically analyzed. In this study, a polyurethane (PU) binder was synthesized, and the effects of isocyanate index (R value) on its performance were investigated. Based on this, a permeable polyurethane mixture (PPUM) was developed and its mixture proportion was designed. The effects of PU content, pH value, and immersion time on the water stability of PPUM were evaluated through immersion Marshall tests. Furthermore, the water damage mechanism in the PPUM was investigated using the water boiling method, water immersion method, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The results showed that the optimal R value for the PU binder was 2.3. The optimal mixture proportion was as follows: aggregate gradation was 80% of 2.36–4.75 mm aggregates and 20% of 4.75–9.5 mm aggregates, and PU content was 5%. The immersion Marshall test results revealed that water stability improved with higher PU content, while extended immersion time and extreme pH values (too high or too low) reduced the water stability of the mixture. The presence of water caused the hydrolysis of PU, weakening the adhesion between PU and aggregates and leading to water damage in PPUM. This study provides a novel approach for the preparation process of permeable polyurethane mixtures and offers a reference for research on their water stability.