Concrete behavior usually degrades due to freeze-thaw cycles, fire, or both. Existing studies on the degradation behavior of concrete due to exposure to high temperatures were primarily focused on unfrozen concrete. In this paper, the degradation behavior of damaged concrete, after different freeze-thaw cycles (25, 35, 45, and 55 cycles), exposure temperatures (20°C, 300°C, 400°C, and 500°C), and cooling methods (water-cooled and air-cooled), was tested with seventy-five prism specimens. The degradation behavior of the damaged concrete, such as surface characteristics, weight loss, compressive strength, peak strains, and elastic modulus, was studied and analyzed. Results show that (i) the surface color of the concrete does not change significantly throughout the test. As the number of freeze-thaw cycles and temperatures increase, the weight loss of the concrete specimens increases gradually. (ii) After freeze-thaw cycles, the relative strengths and elastic modulus of the concrete specimens significantly degrade compared with those of the unfrozen ones at same temperatures. (c) At elevated number of freeze-thaw cycles and exposure temperatures, the peak strain of the concrete increases gradually. (d) Cooling methods have different effects on the degradation of concrete under different number of freeze-thaw cycles. Finally, a uniaxial compression constitutive model for concrete after freeze-thaw cycles and then exposure to high temperatures was established and a good agreement was observed with test results.

Three 1/10-scale bridge pier specimens were tested under quasi-static test. The specimens included two precast specimens (PC1 and PC2) and one cast-in-place reference specimen. The two precast bridge pier specimens were connected with prestressing threaded steel bar and steel flange at the connection between precast pier column and the foundation, and non-socket assembly scheme and socket assembly scheme are adopted, respectively. They were tested to verify the seismic performance of prefabricated piers connected by prestressed threaded steel bars and steel flanges and study which assembly scheme is better for non-socketed and socketed piers. The results show that the prefabricated pier with the combination of the prestressed threaded steel bars and steel flange has higher cracking load and smaller residual displacement, which indicates that it has good service performance and good self-resetting ability. Compared with the non-socket assembly scheme, the socket assembly scheme is superior due to its higher ductility, higher overall initial stiffness, and higher energy dissipation capacity. Therefore, the prefabricated assembled pier with the socket connection scheme of the combination of the prestressed threaded steel bars and steel flange has good service performance and seismic performance. After that, a hysteretic model for the precast assembled columns was proposed, which has a good agreement with the test results.