5 or more, this will cause the corresponding parts of the bridge deck to become thinner and reduce the bearing capacity of the bridge deck. On the other hand, because the maximum camber of the beam and slab is in the middle of the beam and slab span, the bridge deck is wavy in the longitudinal direction, which increases the local impact force of vehicle load, adversely affects the stress on the bridge deck and reduces the service life of the bridge deck.
2 Shrinkage deformation of concrete In the process of solidification and hardening of concrete in air, the phenomenon of volume reduction is called shrinkage. Although there is prestress loss due to concrete shrinkage, its more function is to make the beam produce upward pre-camber. Generally, the shrinkage of concrete is two weeks 1/4, one month 1/2, and about three quarters in three months, and it tends to be stable after two years.
The factors that affect the shrinkage of concrete are: A. Cement dosage B. Cement variety C. Properties of aggregate D.. Maintenance conditions e. production conditions f of concrete. When the volume/surface area of the module is large, the shrinkage rate is small. Creep of concrete is a phenomenon that the strain of concrete increases with time under long-term load (keeping the compressive stress constant), which is called concrete creep. Due to the creep of concrete, the pre-camber value of prestressed beam can be increased by 0.
5 ~ 1 time. There are many factors that affect the creep of concrete, which are generally divided into the following three categories: (1) Internal factors The composition and proportion of concrete are the internal factors that affect the creep. The greater the water-cement ratio, the greater the dosage of cement, the greater the creep, the harder the aggregate, the more the content, and the smaller the creep. (2) Environmental conditions Temperature and humidity under curing conditions are environmental conditions that affect creep. In the pre-stress stage, the higher the humidity, the higher the temperature, the more cement hydration and the smaller the creep. Steam curing can reduce creep by 20% ~ 35%.
(3) Creep under stress is the deformation of concrete that increases with time after loading. Therefore, the initial stress level and the age of concrete during loading are important factors affecting creep. The longer the age of concrete during loading, the more fully the concrete hardens and the smaller the creep. 4 Countermeasures When tensioning, strictly control the tensioning time and concrete strength, and at the same time control the tensioning stress to reach the design control stress to avoid excessive tensioning stress.
For pre-tensioned prestressed beams and slabs, the method of delaying the tensioning time can be adopted according to the construction period requirements, that is, the tensioning time can be appropriately extended on the premise that the concrete strength of beams and slabs meets the design requirements. For post-tensioned prestressed beams and slabs, the curing period can be appropriately extended. If the beam and slab cannot be installed within two months after tensioning, the beam top can be preloaded by increasing the load to reduce the camber of the beam.