P-Wave Velocity and Energy Evolution Process of Concrete in Uniaxial Loading–Unloading Tests

P-wave velocity ($V_{\mathrm{p}}$) is a significant parameter in characterizing the physical and mechanical properties of rock. The $V_{\mathrm{p}}$ of rock under different stress conditions will vary greatly. This study investigated the transformation of $V_{\mathrm{p}}$ under a single load and unload condition. Six concrete specimens were used. The unload level was 0 (uniaxial compression test), 0.1, 0.3, 0.5, 0.7, and 0.9 times the uniaxial compressive strength. Ultrasonic tests were carried out on the test processing. The loading and unloading process was analyzed as an integral part, and the process of secondary loading to destruction was divided into three sections—the compaction section, the elastic section, and the crack extension section—and the test results were analyzed using the least-squares regression method. Quadratic, linear, and logarithmic function relationships were found between axial stress and $V_{\mathrm{p}}$ at different sections throughout the test, and different polynomial relationships were found between the energy and $V_{\mathrm{p}}$ before and after the peak of $V_{\mathrm{p}}$. A polynomial relation was found between the energy and $V_{\mathrm{p}}$. In addition, the total input energy, elastic energy, and dissipated energy had a linear/quadratic function relationship.