Prestressed concrete

A concrete (reinforced in the special method described below with cold drawn steel wires of high tensile strength) which is subjected to compression (at the time of manufacture) in those parts which under load will be subjected to tensile forces so that the concrete will be nowhere in a state of tension under the working load. Prestressing induces compression on the lower or tension side of a beam and when the design load is applied, tension is produced on the lower side which neutralizes the compression already set up by prestressing. The aim in prestressing thus is to completely neutralize the stresses due to the design load. It has been stated earlier that it is not economical to use high tensile steel in ordinary reinforced concrete works.

Fig.

The prestress is set up in a concrete beam by stretching several wires of high tensile strength in the concrete. There are two methods in general use. The wire are stretched before the concrete is cast (called “pretensioning”) and the stretching force subsequently released. After the concrete set, the wires are cut and prestress is created in the concrete due to the prevention of steel from contracting to its original length. The concrete through the bond between wires and concrete. The steel is prevented from returning to its original condition by the concrete and this induces compression in the concrete. A good bond between the steel and concrete is ensured because several wires of small section (as compared with the common mild steel rods) are used, and due to the slight lateral expansion of the wires in the surrounding concrete on release of the wires from strectching.

In the second method of prestressing, which is called “post-tensioning” the wires are stretched after the concrete has hardened : which are either encased in pipes or sheathes, or holes are left in the concrete through which wires are subsequently threaded. The wires in this method have to be held stretched permanently be mechanical means. i.e., anchors. There is no bond between wires and concrete. The reinforcement in the post-tensioning method consists of a few large or several small cables made of high tensile steel wires laid in one or more rings round a core. Pretensioned bonded type is more suitable for small structures particularly of the precast variety and the post-tension bondless type for heavy structural members of long lengths such as in bridges.

For the same design load the weight of concrete in a prestressed concrete is about 50 percent less than in ordinary reinforced concrete and the weight of the high tensile steel used is about 50 to 75 percent less than the quantity of mild steel. The working stresses adopted for prestressed concrete are much higher than the common reinforced concrete, thus permitting the use of much smaller and lighter section for the same load.