Prestressed concrete is a construction material designed to handle heavy loads without cracking as easily as ordinary concrete. Concrete is strong in compression but weak in tension, so engineers use steel tendons to squeeze the concrete before it carries major loads. This built-in compression helps bridges, parking garages, floors, and beams span longer distances with less sagging.
The idea matters because it lets builders make strong, efficient structures using less material.
In a prestressed beam, high-strength steel cables are stretched by jacks and then anchored so they pull inward on the concrete. This inward pull creates compressive stress that counteracts the tensile stress caused when the beam bends under weight. In pretensioning, the steel is stretched before concrete is poured, while in post-tensioning, the steel is tightened after the concrete hardens.
Construction machines such as hydraulic jacks, stressing beds, anchor plates, and grout pumps make the process accurate and safe.
Key Facts
- Concrete is strong in compression but weak in tension.
- Prestressing adds compressive stress before the beam carries its service load.
- Bending stress can be estimated by σ = My/I, where M is bending moment, y is distance from the neutral axis, and I is moment of inertia.
- A prestressing force creates stress in the concrete: σ = P/A, where P is tendon force and A is cross-sectional area.
- Net stress is the combined effect of prestress and load stress: σnet = σprestress + σload.
- Prestressed beams can span longer distances and reduce cracking compared with ordinary reinforced concrete beams.
Vocabulary
- Prestressed concrete
- Concrete that has been compressed by tensioned steel tendons before it carries major loads.
- Tendon
- A high-strength steel cable or strand used to apply prestressing force to concrete.
- Compression
- A squeezing force that pushes particles of a material closer together.
- Tension
- A pulling force that stretches a material and can cause concrete to crack.
- Post-tensioning
- A prestressing method in which tendons are tightened after the concrete has hardened.
Common Mistakes to Avoid
- Thinking concrete is equally strong in tension and compression. This is wrong because concrete cracks much more easily when pulled than when squeezed.
- Confusing tendon tension with concrete tension. The steel tendons are pulled tight, but their pull creates compression in the surrounding concrete.
- Ignoring the direction of bending stresses. A loaded beam usually has compression near the top and tension near the bottom, so tendon placement is chosen to oppose those stresses.
- Assuming prestressing removes all cracking risk. It reduces tensile stress and cracking, but poor design, overloads, corrosion, or construction errors can still cause damage.
Practice Questions
- 1 A tendon applies a prestressing force of 600,000 N to a concrete beam with a cross-sectional area of 0.30 m2. What average compressive stress is produced in the concrete using σ = P/A?
- 2 A rectangular concrete beam has I = 0.020 m4, y = 0.25 m, and a bending moment of 160,000 N m. What bending stress is produced using σ = My/I?
- 3 Explain why placing tensioned tendons near the bottom of a simply supported beam helps the beam resist cracking when a heavy load is placed on top.