Pre-stressing concrete seeks to mitigate concrete's natural weakness in tension by imposing a permanent compression load on the structural members. There are two methods of pre-stressing the concrete, pre-stressing and post-tensioning. Pre-stressing is a method that consists of stressing the reinforcing and then casting the concrete around the reinforcing steel. This method can only be done at a pre-cast manufacturing facility and requires the final product to be transported to the job site and then assembled. The other method of pre-stressing is called post-tensioning. Instead of stressing the reinforcing at a manufacturing plant, the reinforcing is simply installed on the job site after the concrete has been cast. The reinforcing steel is housed in a sheathing that prevents the tendons from bonding to the concrete so that it can be stressed after the concrete cures.
Using the post-tensioning method of pre-stressing enables a builder to get all the advantages of pre-stressed concrete while still having the freedom to construct the structure on the job site. This section will focus on the post-tensioning system.
In the post tensioning system, high-strength steel tendons, in combination with reinforcing steel bars, are embedded and anchored in the concrete. When the concrete has acquired adequate strength, usually three or four days after placement, the tendons are tensioned, imposing a compression force on the concrete. The tendons remain stressed throughout the life of the structure, offsetting the future tension loads. The presence of these tendons, in combination with conventional rebar reinforcing, enables the engineer to design shallower structural members with increased load capacities and less deflection. When selecting a structural building system, it is important that the engineers and architects understand the appropriate applications of post-tensioned concrete and the effects that may result. If properly analyzed and constructed, concrete structures from high quality materials can provide a superior combination of durability, sound control and fire safety needed in today's building market. Considering the current market factors of cost options, concrete is often selected as the more cost effective material over a steel structure.