Concrete has the characteristics of thermal expansion and shrinkage. When the external environment or structure temperature changes, the concrete will deform. If the deformation is controlled, the structure will produce stress. When the stress exceeds the mixed tensile strength, temperature cracks will occur. In some long-span bridges, the thermal stress can reach or even exceed the live load stress.
The main characteristic of temperature crack is to expand or close with the change of temperature. The main cause of temperature change is annual temperature difference. During the year, the temperature of the four seasons changes continuously, but the change is relatively slow. The influence of bridge structure is mainly caused by the longitudinal displacement of bridge. Generally, bridge expansion joint, support displacement or flexible pier can be used to coordinate structural measures.
Only when the structural displacement is limited, temperature cracks such as arch bridge and rigid frame bridge will appear. The annual temperature difference in China is generally considered as the average temperature change in January and July. Considering the creep characteristics of concrete, the elastic modulus of concrete should be considered when calculating the annual temperature difference. Sunshine. The exposed temperature of bridge deck, main beam or pier side is significantly higher than that of other parts, and the temperature gradient is nonlinear.
Because of the self-restraint, the local tensile stress is large and cracks appear. Sunlight and sudden cooling are the most common causes of structural temperature cracks. The temperature suddenly dropped. The sudden downpour, cold air intrusion and sunset will cause the temperature of the external surface of the structure to drop suddenly, but the temperature gradient is relatively slow due to the relatively slow change of the internal temperature. When calculating the internal forces of sunshine and sudden cooling, the design criteria or actual bridge data can be used without considering the elastic modulus of concrete.