Prestressed concrete is a structural method that makes concrete beams stronger and more useful by squeezing them before they carry service loads. Concrete is very strong in compression but weak in tension, so ordinary reinforced concrete often cracks where bending creates tensile stress. By adding a built-in compressive force, engineers can reduce or even eliminate tension under normal loads.
This allows bridges, floors, parking structures, and roof systems to span farther with thinner members.
Understanding Engineering: Prestressed Concrete
A loaded beam does not feel the same stress everywhere. In a simple beam supported at both ends, the top is squeezed while the bottom is pulled apart near the middle. Near supports, the stress pattern can change because shear forces become important.
Engineers place and shape the steel tendons so their force acts where it is most useful. A tendon may run close to the bottom at midspan, then rise toward the ends.
This curved path creates an upward effect that helps balance part of the downward load. The tendon layout is therefore as important as the amount of steel used.
Pre-tensioned members are often made in factories on long casting beds. Steel strands are pulled tight between strong anchors. Concrete is poured around them and allowed to gain enough strength.
When the strands are released, they try to shorten. Their grip on the hardened concrete transfers the force gradually along the member. Hollow-core floor slabs and railway sleepers are common examples.
Post-tensioned work is useful when a structure is built on site. A contractor places ducts, pours the concrete, pulls the tendons with hydraulic jacks, then locks them at anchorage plates. Grout may be pumped into the ducts to protect the steel and bond it to the concrete.
The initial force does not stay exactly constant for the whole life of a structure. Concrete slowly shortens under sustained compression. This is called creep.
It can lose moisture and shrink. Steel can relax slightly when held at high stress. In post-tensioned members, friction between a tendon and its duct reduces the force farther from the jack.
Anchorages can settle by a small amount during stressing. Engineers estimate these losses before construction, then apply extra initial force when needed. They must avoid applying too much force because high compression near anchors can crush concrete or cause splitting cracks.
Students can see the value of this method in parking garages, apartment floors, stadium roofs, water tanks, and long bridge decks. Thin slabs can provide open rooms with fewer columns, but service behavior still matters. A beam may be safe against collapse yet perform poorly if it sags too much, vibrates, or develops visible cracks.
When studying a design, track the load path from the floor or deck into the beam, then into supports. Pay close attention to tendon position, concrete strength at stressing, anchorage zones, and the expected long-term losses. These details explain why prestressed structures need careful design, accurate construction, and regular inspection.
Key Facts
- Prestress force creates compression: stress = P/A, where P is tendon force and A is concrete area.
- Bending stress in a beam is sigma = My/I, where M is bending moment, y is distance from the neutral axis, and I is moment of inertia.
- Combined stress is sigma = P/A plus or minus My/I, depending on whether prestress and bending act in the same or opposite directions.
- Pre-tensioning means steel tendons are stretched before concrete is cast, then released after the concrete hardens.
- Post-tensioning means tendons are stressed after the concrete hardens, usually through ducts or sleeves inside the member.
- The purpose of prestressing is to keep service-load tensile stress low so cracks, deflection, and required beam depth are reduced.
Vocabulary
- Prestress
- Prestress is an intentionally applied internal force that compresses concrete before the structure carries its working loads.
- Tendon
- A tendon is a high-strength steel strand, wire, or bar used to apply prestressing force to concrete.
- Pre-tensioning
- Pre-tensioning is a method in which tendons are stretched before concrete is placed and the hardened concrete grips the tendons when they are released.
- Post-tensioning
- Post-tensioning is a method in which tendons are stressed after the concrete has hardened and then anchored at the ends.
- Service load
- Service load is the normal load a structure is expected to carry during use, such as people, vehicles, furniture, and the structure's own weight.
Common Mistakes to Avoid
- Thinking prestressed concrete has no tension anywhere is wrong because local tensile stresses can still occur, especially near supports, anchors, openings, or overload conditions.
- Forgetting the sign of bending stress is wrong because prestress compression and load bending may add on one face of the beam and subtract on the other.
- Treating pre-tensioning and post-tensioning as the same process is wrong because pre-tensioning relies mainly on bond after release, while post-tensioning relies on stressing and anchoring after curing.
- Ignoring prestress losses is wrong because elastic shortening, creep, shrinkage, relaxation, and anchorage slip reduce the effective tendon force over time.
Practice Questions
- 1 A concrete beam has a cross-sectional area of 0.18 m^2 and is compressed by a tendon force of 900 kN. What average compressive stress does the prestress create in MPa?
- 2 A rectangular beam has a bending moment of 120 kN m, moment of inertia 0.0040 m^4, and distance from neutral axis to bottom fiber 0.30 m. What bending stress occurs at the bottom fiber in MPa using sigma = My/I?
- 3 Explain why a prestressed beam can span farther than a similar non-prestressed reinforced concrete beam, even if both use concrete and steel.