Tilt-up construction utilizes concrete in its hardened form rather than in its plastic form. It involves the casting of wall panels, usually at the job site, and their erection by a crane or other means to a vertical position for attachment to a floor slab. Tilt-up is well suited to one-story structures- warehouses, industrial, commercial and business buildings, and schools- and it has proved successful in some types of multi-stored buildings. Wall panels for tilt-up construction are usually 6 to 8 inches thick. Aggregate should be at least 1 and one-half inches and may be as large as 2 to 3 inches. The standard practice is to use 5 sacks of cement per cubic yard to obtain a 28 day strength of 2,500 psi. Lightweight concrete composed of expanded slag or shale aggregate may be used for wall panels, but it should be noted that lightweight costs more than conventional concrete. In planning wall panels provision must be made for pickup insert eyebolts, brackets, or hairpin units, which are generally welded to the reinforcement and the protruding parts removed as the building is finished. These units may be purchased or fabricated by the builder. Placing and finishing concrete for the wall panels is much the same as for floor slabs. Usually concrete is placed in the forms directly from the ready mix truck, although some builders prefer to use a crane and bucket. Space problems may dictate this procedure. Vibratory screeds are generally not suitable for wall panel finishing, as the surfaces are small and there are numerous projections. At the same time, because panels are cast on a sealed surface, there can be no downward movement of water. Once the wall panels are finished and properly cured, a critical stage of the building schedule begins, for the process of lifting the panels to a vertical position places them in some jeopardy. Truck cranes are the most common means for erecting wall panels, particularly panels for large buildings. Other means include lifting masts, truck or ground mounted A-frames, special leverage devices mounted on wheels, fixed cranes with booms, and telescoping hydraulic jacks.