Post-Tensioned Concrete: An Introduction

About this course.

Prestressing is the deliberate introduction of internal compression into a concrete member, using high-strength steel tendons, before the member carries its working loads. It helps because concrete is strong in compression but weak in tension: it can carry large squeezing forces but cracks readily when pulled. By precompressing the concrete, the designer ensures that the tensile stresses caused by loads must first cancel out this built-in compression before the concrete experiences any net tension at all. The result is a member that stays largely crack-free, deflects less, and behaves as a much stiffer and more efficient structural element than plain or conventionally reinforced concrete.

To appreciate why this matters, consider concrete's basic character. Its tensile strength is only a small fraction of its compressive strength — roughly of the order of one-tenth — so under bending, the tension face of an ordinary concrete beam cracks at quite modest loads. Conventional reinforced concrete accepts this: steel reinforcing bars are placed on the tension side to carry the tension after the concrete has cracked. The concrete is allowed to crack, and the cracks are simply kept small enough to be acceptable. Prestressing takes a fundamentally different approach. Instead of allowing the concrete to crack and letting steel take over, it keeps the concrete in compression so that cracking is prevented or greatly delayed in the first place.