SPECIAL TYPES OF CONCRETE

PRESTRESSED CONCRETEA prestressed concrete unit is one in whichengineered stresses have been placed before it hasbeen subjected to a load. When PRETENSION-ING is used, the reinforcement (high-tensile-strength steel strands) is stretched through theform between the two end abutments or anchors.A predetermined amount of stress is applied tothe steel strands. The concrete is then poured,encasing the reinforcement. As the concrete sets,it bonds to the pretensioned steel. When it hasreached a specified strength, the tension on thereinforcement is released. This prestresses theconcrete, putting it under compression, thuscreating a built-in tensile strength.

POST-TENSIONING involves a precastmember that contains normal reinforcing inaddition to a number of channels through which the prestressing cables or rods maybe passed. Thechannels are usually formed by suspending inflated tubes through the form and casting theconcrete around them. When the concrete has set,the tubes are deflated and removed. Once theconcrete has reached a specified strength, distressing steel strands or TENDONS are pulledinto the channels and secured at one end. They are then stressed from the opposite end with aportable hydraulic jack and anchored by one of several automatic gripping devices.Post-tensioning may be done where the member is poured or at the jobsite. Each membermay be tensioned, or two or more members maybe tensioned together after erection. In general,post-tensioning is used if the unit is over 45 ft longer over 7 tons in weight. However, some typesof pretensioned roof slabs will be considerably longer and heavier than this.When a beam is prestressed, either by pre-tensioning or post-tensioning, the tensioned steelproduces a high compression in the lower part of the beam. This compression creates an upwardbow or camber in the beam (fig. 7-19). When a load is placed on the beam, the camber is forcedout, creating a level beam with no deflection.Those members that are relatively small or that can be readily precast are normally pretensioned.These include precast roof slabs, T-slabs, floorslabs, and roof joists.

SPECIAL TYPES OF Concrete Special types of concrete are essentially those with unique physical properties or those produced with unusual techniques and/or reproduction processes. Many special types of concrete are made with portland cement as a binding medium;some use binders other than Portland cement.Lightweight ConcreteConventional concrete weighs approximately150 lb per cubic foot. Lightweight concrete weighs20 to 130 lb per cubic foot, depending on its intended use. Lightweight concrete can be madeby using either gas-generating chemicals.