PRECAST CONCRETE

beam and girder bars, bends for reinforcement of vertical columns at or near floor levels, stirrup and column ties, slab reinforcement, and bars or wire for column spiral reinforcement. SPLICES.— Where splices in reinforcing steel are not dimensioned on the drawings, the bars should be lapped not less than 30 times the bar diameter, nor less than 12 in. The stress in a tension bar can be transmitted through the concrete and into another adjoining bar by a lap splice of proper length. The “lap” is expressed as the number of bar diameters. If using the No. 2 bar, make the lap at least 12 in. EXPANDED METAL AND WELDED WIRE FABRIC.— Expanded metal or wire mesh is also used for reinforcing concrete. Expanded metal is made by partly shearing a sheet of steel, as shown in figure 7-13, view A. The sheet steel has been sheared in parallel lines and then pulled out or expanded to form a diamond shape between each parallel cut. Another type is square rather than diamond shaped, as shown in figure 7-13, view B. Expanded metal is frequently used during plastering operations. Welded wire fabric is available both in rolls (fig. 7-14) for light building construction and sheets for highways and use in buildings when roll sizes will not give ample reinforcement. Wire Figure 7-13.-Expanded or diamond mesh steel reinforce- ment. Figure 7-14.-Welded wire fabric. fabric is furnished in both square and rectangular patterns, welded at each intersection. The rec- tangular sizes range from 2 by 4 in. to 6 by 12 in. The square patterns are available in 2 by 2 in., 3 by 3 in., 4 by 4 in., and 6 by 6 in. Both are furnished in a wide variety of wire gauges. The square pattern has the same gauge in both directions, while the rectangular type may have the same gauge in both directions or the larger gauge running longitudinally. Specifications and designs are usually used when wire fabric (mesh) is being lapped; however, a minimum of 2 in. between laps is usually sufficient. Reinforcing bars can be joined together by different types of ties. Figure 7-15 shows six types used by the SEABEEs. PRECAST CONCRETE Precasting is the fabrication of a structural member at a place other than its final position of use. It can be done anywhere, although this procedure is best adapted to a factory or yard. Jobsite precasting is not uncommon for large projects. Precast concrete can be produced in several different shapes and sizes, including piles, girders, and roof members. Prestressed concrete is especially well adapted to precasting techniques. Generally, structural members including standard highway girders, poles, electric poles, masts, and building members are precast by factory methods unless the difficulty or 7-10