Basic Principles of Concrete Prestressing and its Types

Let’s get back to the basics

A homogenous concrete beam has very low flexural strength and bends under the effect of heavy-weights and with the passage of time. In order to do away with this deficiency of the concrete structures, the beams are provided with steel reinforcements near the bottom to take care of the tensile stresses. Yet, a considerable part of concrete below the neutral axis merely maintains the reinforcement in position, but its tensile strength is neglected in the computation for the flexural strength.
This is where prestressing comes in! Tensile stresses are applied to the tensile reinforcement before applying the external loads, resulting in the generation of compressive stresses in the concrete of the beam. Generally, the tensile stresses instigated by the loads are totally counteracted or absorbed by the compression stresses in the concrete, as a resultant of prestressing.

Principle of prestressing

The most significant principle of prestressing is to counterpoise the stress generated in the tension-bearing portion of a beam generated due to loading. In prestressing technology, prestressed tendons provide a clamping load that generates a compressive stress to balance the tensile stress that the concrete compression member would have else experienced owing to the bending load. Simply put, the internal pressure is the tensile stress produced when a pre-tensioned rod is included between the concrete in order to keep the rod from returning to its original position. Chemical bond between the concrete and the steel makes the process possible.
In order to better understand the functioning of prestressing, consider a scenario. Take a rubber band, stretch it a little and put some weight on it; the rubber band sags considerably. Now, stretch the rubber band further, as much as you can (without breaking it) and place the same load on it. This time, the rubber band sags very less (almost negligible). This is the exact same case with prestressing.
Basically, there are two main types of prestressing:

  1. Pre-tensioning
  2. Post tensioning

Let’s discuss both the types of prestressing in brief.
Pre-tensioning: Usually done with precast, pre-tensioning involves tensioning tendons against some abutments before pacing concrete. After the concrete hardens, the tension is released and the tendons try to shrink back to their original lengths. Quite obviously, this isn’t possible as the concrete resists the tendons getting back shorter because of the bong between them. Thus, a compression force is induced in the concrete.

Post-tensioning: On the other hand, metal or plastic ducts are placed inside the concrete before casting. In this process, the tendons are tensioned after the concrete is hardened and has enough strength. The tendon is placed inside the duct, is stressed, and anchored against the concrete. Grout may be injected into the duct later. This can be done either as a precast or cast-in-place.