What Is Prestressed Concrete?

Prestressed concrete is to compensate for the premature cracks in the concrete. Before the component is used (loaded), the concrete is preloaded in advance, that is, the steel bar is manually tensioned in the tensile area of the concrete. Using the retraction force of the steel bar, the concrete tensile zone is prestressed. This stored pre-stressing force, when the component is subjected to a tensile force generated by an external load, first offsets the pre-stressing force in the concrete in the tension zone, and then increases the load before the concrete is tensioned, which limits the elongation of the concrete. Delaying or preventing cracks from appearing is called prestressed concrete.

Prestressed Concrete

Prestressed concrete is to compensate for the premature appearance of concrete

Pre-compressive stress is used to reduce or offset the concrete tension caused by the load

Different classifications of the degree of control of cracks on member sections according to the prestressing value:

1. Full prestressed concrete (FPC, fully prestressed concrete ^[1]

Some requirements for prestressed reinforcement:

(1) The intensity should be high. The tensile stress of prestressed steel bars will cause various stress losses during the entire fabrication and use of the component. The sum of these losses can sometimes reach 200N / mm & sup; if the strength of the reinforcing steel used is not high, the stress established during tension may even be lost.

(2) Have better adhesion to concrete. Especially in the pre-tensioning method, there must be a high bond self-anchoring strength between the prestressed steel and the concrete. For some high-strength smooth steel wires, they need to go through "scoring", "pressing wave" or "kink", so that it forms scored wires, corrugated wires and kinked wires to increase the bonding force.

(3) It must have sufficient plasticity and good processability. The higher the strength of the steel, the lower its ductility. When the plasticity of the reinforcement is too low, especially under low temperature and impact load conditions, brittle fracture may occur. Good processability refers to good welding performance, and when the head anchor plate is used, the mechanical properties of the steel head will not be affected after upsetting.

Commonly used pre-stressed steel bars in China include cold-drawn grade III steel bars, cold-drawn grade IV steel bars, cold-rolled ribbed steel bars, heat-treated steel bars, and high-strength steel wires.

Some requirements for concrete in prestressed concrete:

(1) The strength should be high, and it must be compatible with high-strength steel bars to ensure that the prestressed steel bars fully function, and can effectively reduce the section size and weight of the members.

(2) Shrinkage and creep should be small to reduce the loss of prestress.

(3) Fast hardening and early strength, so that prestress can be applied as soon as possible to speed up the construction progress and increase the equipment utilization rate.