What happens when steel rods in pre-stressed concrete are released after setting?

When steel rods in pre-stressed concrete are released after the concrete has set, they actively compress the beam. This process is fundamental to the concept of pre-stressing, where the steel rods (tendons) are initially tensioned (stretched) before the concrete is poured. Once the concrete hardens and gains sufficient strength, the tension in the steel rods is released.

This release causes the rods to try to return to their original length, which in turn applies a compressive force to the surrounding concrete. This compression counteracts the tensile stresses that the concrete may experience under service loads, enhancing the structural performance and load-bearing capacity of the beam. The compression reduces the likelihood of cracking and improves the overall durability of the concrete structure.

In summary, the interaction between the steel tendons and the concrete upon release creates a beneficial state of internal compression, which is a critical aspect of pre-stressed concrete technology.