Civil
Prestressed Concrete
Compression-tensioned concrete that resists bending tension
Prestressed concrete embeds high-strength steel tendons under tension, putting the concrete into compression before service loads arrive. Concrete is strong in compression but weak in tension; prestress counteracts the tension that develops under load. Two methods. Pretensioning: tendons stretched in casting bed, concrete poured around them, tendons released to compress concrete. Post-tensioning: tendons routed through ducts in cured concrete, then tensioned and anchored. Used for long-span beams, bridges, parking decks, nuclear containment, and silos.
- PrinciplePre-compress concrete to resist tension
- PretensioningTendons stretched before pouring
- Post-tensioningTendons tensioned after cure
- Tendon strengthUp to 1860 MPa (270 ksi) typical
- Concrete strength35 to 70 MPa typical
- Span advantage2 to 3x conventional reinforced concrete
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Why prestressed concrete matters
- Long spans. Bridges and parking decks 30 to 70 meters.
- Thin sections. Lighter members reduce dead load.
- Crack control. Members stay uncracked under service loads.
- Durability. Less cracking means less corrosion ingress.
- Precast efficiency. Mass-produced beams shipped to site.
- Pressure vessels. Containment for fluids and gases.
- Architectural freedom. Slender forms not possible with reinforced concrete.
Common misconceptions
- Same as reinforced. Reinforced uses passive rebar; prestressed uses active tensioned tendons.
- Tendons replace rebar. Most members combine tendons and conventional rebar.
- No losses. Creep, shrinkage, relaxation cost 15 to 25% of initial stress.
- Stronger concrete enough. Tension-resistance fundamentally requires precompression.
- Anchors trivial. Anchorage zones experience extreme local stress requiring careful detailing.
- Tendons last forever. Corrosion is the leading failure mode; grouting and protection matter.
Frequently asked questions
What is prestressed concrete?
Concrete with internal compressive stress applied before service loads, achieved by tensioning embedded steel tendons. Service loads cause tension in regions of the member; the precompression cancels or reduces this tension, preventing cracking and permitting much longer spans, thinner sections, and higher load capacity than reinforced concrete. Invented by Eugène Freyssinet in the 1920s.
How is pretensioning done?
In a precasting bed, high-strength steel strands are anchored at one end, run through forms, and tensioned at the other end with hydraulic jacks. Concrete is poured around the tensioned strands and cures. Once concrete reaches sufficient strength, strands are cut at both ends. Strands try to shrink back to original length but are bonded to the concrete, transferring compression into the member.
How does post-tensioning differ?
Concrete is cast with hollow ducts (steel or plastic) running through it. After concrete cures, tendons are threaded through the ducts and tensioned with hydraulic jacks at one or both ends, anchoring against bearing plates set into the concrete. Ducts are then grouted to bond tendons and prevent corrosion. Allows curved tendon profiles matching the bending moment diagram.
Why is prestressing necessary?
Concrete is weak in tension (about 10% of compressive strength) and cracks readily. Reinforced concrete relies on steel rebar to carry tension once concrete cracks. Prestressed concrete keeps the entire section in compression under service loads, preventing cracks. Result: smaller deflections, lighter members, longer spans, better durability, and resistance to fatigue.
What materials are used?
Tendons are high-strength low-relaxation steel strands, typically 7-wire 12.7 mm or 15.2 mm diameter, with ultimate strength of 1860 MPa. Concrete strengths of 40 to 70 MPa are common, higher than reinforced concrete to handle the prestress. Anchors are forged steel wedge assemblies. Ducts are corrugated steel or plastic for post-tensioning. Grout fills ducts after stressing.
What are losses in prestressing?
Initial stress decreases over time. Elastic shortening (concrete compresses immediately when tendons release). Creep (long-term concrete deformation under stress). Shrinkage (concrete dries and contracts). Steel relaxation (steel loses tension at high stress). Friction (tendons against ducts in post-tensioning). Total losses 15 to 25%. Designers anticipate these to ensure final stress meets requirements.
Where is it used?
Long-span bridge girders (prestressed I-beams or box girders dominate medium-span bridges). Parking structure floors (post-tensioned slabs span long unsupported lengths). High-rise buildings. Stadium roofs. Nuclear containment vessels. Pressure vessels and silos. Wind turbine towers. Most precast bridge beams in the US, UK, and Europe are prestressed.