Prestressed concrete is composed of high-strength concrete and high-strength steel. Because of the high strength of these two materials, the resulting structures use much less concrete and steel and are, therefore, more economical. Concrete is an economical material for carrying compressive stresses, but has a very low tensile strength. In a prestressed concrete member, the concrete is precompressed before it is placed in use. As loads are applied, the areas of the structure which would normally go into tension simply lose some of their precompression. In this manner, concrete is made to function in either tension or compression. Prestressed concrete members can be divided into two basic types, pretensioned and post-tensioned. In a pretensioned member, the tendons are stretched to full load and then the concrete is placed around them. In a post-tensioned member, the tendons are encased in some sort of hose or wrapping and placed in the forms. Concrete is placed around the wrapping which protects the tendon from bond. After the concrete member has cured, the tendons are tensioned by jacks applied at the end of the member, after tensioning, the space around the tendon is pumped full of cement grout. Pretensioned members are invariably cheaper than post-tensioned member, but it is not always possible to use pretensioned member. Because of its extremely high strength, a prestressing strand is more susceptible to damage than ordinary structural steel and certain rules must be observed. Extreme heat will cause a strand or wire to lose up to 50 percent of its ultimate strength even though there is no visible evidence of the application of the heat. Use of electric welding equipment in the vicinity of prestressing tendons should be avoided because an arc to or from the tendon will generate enough heat to do serious damage. Nicks and kinks are stress raisers in prestressed concrete strands and, if at all severe, can cause failures under the tensioning loads normally used. It is standard practice to use temporary grips to hold the strands under tension in the casting bed until the concrete has been placed and cured.