For efficient use of slip-forming, the design of a structure must be allied to the technique. Height and regularity of cross-section decide whether or not slip-forming is economical. In general, the technique is believed to offer economies over the use of standard forms if a structure exceeds 60 to 80 feet in height. Two types of operation are practiced: continuous, on a 24 hour cycle until the structure is complete; or programmed, that is to provide a definite height based on a desired time cycle or story height. Jacks are usually hydraulically operated and mounted in banks. The choice of a jack system decides the type of form to be used. There are several preferred requirements: yokes should be designed so that legs and crossheads provide a self-contained, rigid unit and crossheads should be interchangeable to allow variations in wall size. Forms are faced with either steel or lumber. Steel is recognized to be heavier, more difficult to assemble and repair, and to have a low thermal insulation value. Lumber, however, has greater lightness and flexibility, and is easier to repair and handle. All jacking systems require leveling adjustments as the forms climb upwards. Several different types of level indicator are in use. The simplest consists of an optical level located at the center of the form for sighting on to targets located on each yoke. A water level system with the manometer tubes reporting back to another manometer tube on a central control panel is also used. Plumb is checked by conventional plumb bobs suspended in drums of oil. Bars of a reasonable size and length are necessary to allow the main vertical steel to be spliced into position. A first lift of steel is detailed in four or five lengths, such as 8,10,12, and 14 feet; all additional rods are then of constant length, say 10 feet, to maintain the stagger. Location of the vertical steel is set out on a template welded to the yokes at high level. The template both ensures that the steel is fixed in its correct position and has the correct concrete cover, and also acts as a stabilizer for the steel in high winds. Guides are also fixed to the yokes at low level to ensure that the reinforcement is not located at points where anchors are to be installed later. Slabs of expanded polystyrene foam are preferred for blocking out openings. This material is rigid, yet early worked and is flammable. Doorways and larger opening are formed with lumber cut to correct size. Hook bolts are positioned in the concrete first with the end of the hook about 3 feet below the required soffit. Concrete is transported from ground level by internal or external hoist, by tower crane or by pumping. The simplest and cheapest method is by means of a winch, located on the slip platform, which lifts the concrete from within the center of the structure. Curing is most commonly done by means of Hessian fixed to the face of the concrete.