Prestressed concrete is a form of concrete used in construction. It is substantially prestressed (compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service. It was patented by Eugène Freyssinet in 1928. This compression is produced by the tensioning of high-strength tendons located within or adjacent to the concrete and is done to improve the performance of the concrete in service. Tendons may consist of single wires, multi-wire strands or threaded bars that are most commonly made from high-tensile steels, carbon fiber or aramid fiber. The essence of prestressed concrete is that once the initial compression has been applied, the resulting material has the characteristics of high-strength concrete when subject to any subsequent compression forces and of ductile high-strength steel when subject to tension forces. This can result in improved structural capacity or serviceability, or both, compared with conventionally reinforced concrete in many situations. In a prestressed concrete member, the internal stresses are introduced in a planned manner so that the stresses resulting from the imposed loads are counteracted to the desired degree. Prestressed concrete is used in a wide range of building and civil structures where its improved performance can allow for longer spans, reduced structural thicknesses, and material savings compared with simple reinforced concrete. Typical applications include high-rise buildings, residential concrete slabs, foundation systems, bridge and dam structures, silos and tanks, industrial pavements and nuclear containment structures. First used in the late nineteenth century, prestressed concrete has developed beyond pre-tensioning to include post-tensioning, which occurs after the concrete is cast. Tensioning systems may be classed as either 'monostrand', where each tendon's strand or wire is stressed individually, or 'multi-strand', where all strands or wires in a tendon are stressed simultaneously. Tendons may be located either within the concrete volume (internal prestressing) or wholly outside of it (external prestressing). While pre-tensioned concrete uses tendons directly bonded to the concrete, post-tensioned concrete can use either bonded or unbonded tendons.
Pre-tensioned concrete
Pre-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned prior to the concrete being cast. The concrete bonds to the tendons as it cures, following which the end-anchoring of the tendons is released, and the tendon tension forces are transferred to the concrete as compression by static friction.
Pre-tensioning is a common prefabrication technique, where the resulting concrete element is manufactured off-site from the final structure location and transported to site once cured. It requires strong, stable end-anchorage points between which the tendons are stretched. These anchorages form the ends of a casting bed which may be many times the length of the concrete element being fabricated. This allows multiple elements to be constructed end-to-end in the one pre-tensioning operation, allowing significant productivity benefits and economies of scale to be realized. The amount of bond (or adhesion) achievable between the freshly set concrete and the surface of the tendons is critical to the pre-tensioning process, as it determines when the tendon anchorages can be safely released. Higher bond strength in early-age concrete will speed production and allow more economical fabrication. To promote this, pre-tensioned tendons are usually composed of isolated single wires or strands, which provides a greater surface area for bonding than bundled-strand tendons.
Unlike those of post-tensioned concrete (see below), the tendons of pre-tensioned concrete elements generally form straight lines between end anchorages. Where profiled or harped tendons are required, one or more intermediate deviators are located between the ends of the tendon to hold the tendon to the desired non-linear alignment during tensioning. Such deviators usually act against substantial forces, and hence require a robust casting-bed foundation system. Straight tendons are typically used in linear precast concrete elements, such as shallow beams and hollow-core slabs; whereas profiled tendons are more commonly found in deeper precast bridge beams and girders. Pre-tensioned concrete is most commonly used for the fabrication of structural beams, floor slabs, hollow-core slabs, balconies, lintels, driven piles, water tanks and concrete pipes.
Post-tensioned concrete
Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast. The tendons are not placed in direct contact with the concrete, but are encapsulated within a protective sleeve or duct which is either cast into the concrete structure or placed adjacent to it. At each end of a tendon is an anchorage assembly firmly fixed to the surrounding concrete. Once the concrete has been cast and set, the tendons are tensioned (stressed) by pulling the tendon ends through the anchorages while pressing against the concrete. The large forces required to tension the tendons result in a significant permanent compression being applied to the concrete once the tendon is locked off at the anchorage. The method of locking the tendon ends to the anchorage is dependent upon the tendon composition, with the most common systems being button-head anchoring (for wire tendons), split-wedge anchoring (for strand tendons), and threaded anchoring (for bar tendons).
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