Last reviewed 16 Sept 2026 · 6 min read
Principle
Prestressing applies a permanent compressive force to concrete before service loads act, so that the tension later produced by loads is cancelled or reduced. Concrete, strong in compression and weak in tension, then remains largely uncracked.
Three ways of looking at prestress:
- Stress concept — prestress and loads are combined elastic stresses ().
- Strength (internal couple) concept — like RCC, steel tension and concrete compression form a couple, but the steel is high-strength and pre-tensioned.
- Load balancing concept — a curved or draped tendon exerts upward forces that balance part of the gravity load (for a parabolic tendon of sag : ).
Advantages over RCC
- Section remains uncracked (or cracks are controlled) → better durability, watertightness.
- Smaller, lighter members for the same span → longer spans (bridges, roofs, sleepers, poles).
- High-strength steel and concrete used efficiently.
- Lower deflections (camber from prestress offsets load deflection).
- Better shear resistance (inclined tendons and compression).
- Higher fatigue resistance.
Disadvantages: needs high-quality materials, specialised equipment and skilled labour; higher unit cost for short spans; losses must be accounted for.
Pre-tensioning and post-tensioning
| Aspect | Pre-tensioning | Post-tensioning |
|---|---|---|
| Sequence | Tendons stretched between abutments → concrete cast → released after hardening | Concrete cast with ducts → tendons threaded and stressed after hardening → anchored |
| Transfer of force | Bond along the transmission length at the ends | End anchorages (bearing) |
| Typical use | Factory-made precast units: sleepers, poles, piles, hollow-core slabs | Cast-in-situ and large members: bridge girders, long-span beams, slabs, containment structures |
| Tendon profile | Usually straight (or deflected) | Curved profiles easy |
| Losses | Larger (elastic shortening full) | Smaller elastic shortening; friction and anchorage slip additional |
| Grouting | Not needed | Ducts grouted for bond and corrosion protection (bonded), or unbonded greased tendons |
Systems
- Pre-tensioning: Hoyer's long-line system — several units cast in a line between two abutments with continuous wires.
- Post-tensioning:
- Freyssinet — conical wedge anchorage for wire groups; flat jacks.
- Magnel–Blaton — wires anchored in pairs by flat metal wedges in sandwich plates.
- Gifford–Udall — each wire anchored by split conical wedges in a barrel.
- Lee–McCall — high-tensile bars with threaded ends and nuts.
- Modern multi-strand systems (VSL, CCL, etc.) use wedge anchorages for 7-wire strands.
Materials
- Concrete: minimum grade M40 for pre-tensioned and M30 for post-tensioned members (higher grades are common).
- Steel: high-tensile wires, 7-wire strands and bars (ultimate tensile strength roughly 1400–1900 N/mm²), low-relaxation strands preferred.
- Maximum initial prestress in the tendon is limited to a fraction of the ultimate tensile strength (about 80% during stressing).
Why high-strength materials: total losses are typically 150–300 N/mm². With mild steel stressed to, say, 150 N/mm², all the prestress would be lost; high-tensile steel stressed to 1000–1400 N/mm² retains most of it. High-strength concrete has lower creep and shrinkage, higher bearing strength at anchorages and allows smaller sections.
Losses of prestress
The initial force applied by the jack reduces with time. Effective prestress = initial prestress − losses.
| Loss | Pre-tensioned | Post-tensioned |
|---|---|---|
| Elastic shortening of concrete | Yes (full) | Only if tendons stressed one after another (average ≈ half) |
| Shrinkage of concrete | Yes | Yes (smaller — part has occurred before stressing) |
| Creep of concrete | Yes | Yes |
| Relaxation of steel | Yes | Yes |
| Friction (curvature and wobble) | — | Yes |
| Anchorage slip | — (small at abutments) | Yes |
Elastic shortening (pre-tensioned): , where and is the concrete stress at the tendon level due to prestress (and self-weight): (– self-weight moment term). Post-tensioned with tendons stressed in sequence: average loss ≈ half of ; zero if all stressed together.
Shrinkage (IS 1343): strain = for pre-tensioned; for post-tensioned ( = age of concrete at transfer in days). Loss .
Creep: loss ; creep coefficient = 2.2 (7 days), 1.6 (28 days), 1.1 (1 year) at the age of loading.
Relaxation of steel (IS 1343, 1000-hour values in the absence of test data): initial stress 0.5 → 0; 0.6 → 35 N/mm²; 0.7 → 70 N/mm²; 0.8 → 90 N/mm² ( = characteristic strength).
Friction (post-tensioned): — = coefficient of friction between tendon and duct, = cumulative angle change (rad), = wobble coefficient per metre, = distance from the jack.
Anchorage slip (post-tensioned): , = slip (typically a few mm), = tendon length — more significant for short tendons.
Typical total losses: roughly 15–25% for pre-tensioned and 10–20% for post-tensioned members (depends strongly on materials, age at transfer and tendon profile). Preliminary design often assumes about 20% and 15% respectively, then checks.