AbstractThe mechanical properties of rigid polyurethane foam (PUF) will be significantly affected because of the existence of water when applied in dam repair and road maintenance. The related studies generally consider the effects of water and matrix on the mechanical properties of rigid PUF. However, PUF is a three-phase composite consisting of water, air, and matrix in practical engineering. Its mechanical properties are subject to the combined effects of these three composites at the same time. In this case, an improved three-phase viscoelastic model was proposed to quantify the effects of water, air, and matrix on the compressive mechanical behavior of rigid polyurethane foam immersed in water (WRPF). Based on the improved viscoelastic model and piecewise method, a compressive strength prediction model of WRPF was developed to describe the change of WRPF’s compressive strength during the full-service life. Then uniaxial compression tests and electron microscope tests were conducted to verify the rationality of the model. Results show that water and air have reinforcing effects on the compressive property of WRPF in the short-term, unsaturated absorption stage. In contrast, water has weakening effects on its compressive property in the long-term, saturated stage. Finally, at different stages of water absorption, a semitheoretical and empirical formula was developed from the prediction model and the experimental data from compressive tests.