A Sustainable Reinforcement Method for Recycled Road Subgrade Demolition Waste as Road Bases Using Waterborne Polyurethane and Fiber

The discard of recycled road demolition waste (RDW) leads to natural resource waste and environmental problems. The RDW is reused as road base, which is a sustainable application and promising construction technology with good environmental benefits and economic value. The performance of road base with RDW depends on the compressive strength, splitting strength, and ductility. However, owing to the poor mechanical properties, the RDW can only be reused after being treated. The traditional treatment method of waste as road base is to enhance mechanical performance using cement. However, when cement is used as a stabilizer, the road base cracks very easily, thereby damaging the pavement. Therefore, in this article, a new reutilization method of RDW as road base is proposed. The waterborne polyurethane (PU) was proposed to improve the strength of RDW, and basalt fiber (BF), polypropylene fiber (PF), and carbon fiber (CF) were used to improve the ductility of PU-stabilized RDW. The related mechanical behavior was investigated by unconfined compressive strength and splitting strength tests. Meanwhile, the mechanism of RDW enhanced by PU and PU + CF/PF/BF was revealed using scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier transform infrared reflection (FTIR) tests. The results showed that the PU could improve the strength of RDW. When BF, PF, and CF were added, the compressive strength, residual strength, splitting strength, failure strain, and ductility were further enhanced. When the fiber content was 6%, a maximum compressive strength was obtained, and the PF had the best improvement effect on the mechanical properties of PU-stabilized RDW. When the PF content was 0.6% and 0.8%, the 7-days compressive strength could meet the class II standard (3.0 MPa) of road subbase for heavy traffic expressways. The failure strain increased with the compressive strength increasing. The mechanical performance improvement effect of PU and PU + CF/PF/BF on RDW came from the bonding effect of PU, and the bridging effect and reinforcement effect of fiber. This study provides a favorable way for the recycling of RDW, which is conducive to the sustainable development of environment and the conservation and utilization of energy.