What is prestressed concrete?
Prestressed concrete is concrete with reinforcement of which part has been put under tensile stress, so that the concrete is placed in compression in advance. Concrete is strong in compression but weak in tension. By applying this compressive stress beforehand, little or no tension develops in the concrete under load. The element cracks less, deflects less and can be made more slender or span further.
Methods of prestressing
- Pre-tensioned steel: in the factory, steel strands are tensioned along a long prestressing bed, after which the concrete is cast around them. Once the concrete has hardened, the strands are released and the bond transfers the stress to the concrete. This is the most widely used form, for example in hollow-core slab floors, precast beams and precast concrete piles.
- Post-tensioned steel with bond: the steel lies in ducts in the concrete and is only tensioned after the concrete has hardened. The ducts are then injected with grout, so that steel and concrete work together. This is used on large projects, such as bridges.
- Post-tensioned steel without bond: the steel lies in a greased sheath and remains separate from the concrete.
- External prestressing: the cables run outside the concrete cross-section, for example in a box girder bridge.
Due to creep and shrinkage of the concrete and relaxation of the steel, the prestress decreases slightly over time. The structural engineer allows for these prestress losses in the calculations.
| Pre-tensioned steel | Strands tensioned in the factory before casting, then released so the bond transfers the stress. Most widely used: hollow-core slab floors, precast beams, precast concrete piles. |
|---|---|
| Post-tensioned steel with bond | Steel in ducts, tensioned after the concrete has hardened; ducts injected with grout so steel and concrete work together. Used on large projects, such as bridges. |
| Post-tensioned steel without bond | Steel in a greased sheath, remains separate from the concrete. |
| External prestressing | Cables run outside the concrete cross-section, for example in a box girder bridge. |
Applications and points to watch
- You mainly see prestressed elements in floors, beams, bridges, piles and car parks.
- Never simply drill, saw or chisel into prestressed concrete. If you hit a prestressing strand, the element can lose part of its load-bearing capacity. For an opening in a hollow-core slab floor, first ask the supplier or a structural engineer for the permitted position and size.
- Fire resistance and concrete cover to the steel are particularly important for prestressed elements; follow the manufacturer’s product data.
Related terms
- Prestressing
- Reinforced concrete
- Reinforcement
- Reinforcing steel
- Hollow-core slab floor
- Creep
- Pile
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