Huge sections of form work complete with scaffolding dangle from a crane cable. Workers on the ground swing the form sections into position, guided by guy ropes, then by hand. Brent Byford, assistant project manager for Walsh Construction, Chicago, supervises as the gang forms are plumbed, then braced, for a retaining wall on a highway project on Chicago's west side.
Gang forms are several unit forms attached together, or a single large form, usually moved into position by crane because sections are too heavy to set by hand. The length of a gang form for walls is often determined by the distance between control joints or by the capacity of the jobsite crane.
Mike Miller, general product manager for Symons, Des Plaines, Ill., says that contractors use gang forms because of their versatility and capacity, the appearance of the finished work, and increased productivity. With gang forms, a forming and concrete placement cycle can be completed every day. In addition, the routine positioning increases safety and accuracy. Gang forms are often used for retaining walls, sound walls, bridge abutments, water treatment facilities, and building walls and columns. Formliners attached to gang forms can result in economical high-quality architectural walls.
Gang forms can be purchased from manufacturers of forming systems or built from basic materials on a jobsite. They can be steel, aluminum, wood, plastic, or some combination of these materials. A wide selection is available from the various form manufacturers. The system that you choose will be based on the number of reuses you hope to get, gang form weight, lateral pressures anticipated during concrete placement, cost, and the degree of architectural finish required.
Byford says Walsh built its own retaining wall gang forms for the I-294 highway widening project, placing standard plywood form panels over 2×6-inch lumber wales supported by manufactured strongbacks. There is a “haunch” formed into the top of the wall (a thicker concrete wall section that will become the footing for a traffic barrier wall) that had to stay at a constant elevation. But since the footing for the wall was at various elevations, Byford says, they designed the forms so that sections (face, wales, and strongbacks) could be attached to the bottom of the gang form in order to keep the haunches at a constant elevation. This wall will retain roadway subgrade on one side and won't be seen by the public on the other. The finish was therefore not critical, so Walsh used plywood form panels which typically get about 40 reuses. Plywood faces swell when they get wet, so the resulting concrete surfaces can be either positively or negatively imprinted.
Forming systems are rated by their ability to resist a certain pressure (in pounds per square foot—psf) that the concrete exerts against the form during placement. Form pressures will be low when only a few vertical feet of wall are placed each hour, or they can approach full liquid head when concrete is fresh from the bottom to the top of a placement.
A significant safety issue for contractors is the decision about the forming system for their project and the rate of concrete placement. Steffen Pippig, engineering manager for Meva Formwork Systems, Springfield, Ohio, says that formwork companies usually tell their customers what the maximum lateral form pressures are for a particular form-and-tie system. They cannot, however, tell them how fast they can place concrete to stay within those limits since too many factors influence lateral pressure in the field, including ambient and concrete temperatures, fluidity of concrete mixes, accelerating and retarding admixtures, and the use of pozzolans in the mix. Placing fly ash concrete in cold weather, for example, could easily result in full liquid head pressures on the forms. Guidance on a safe rate of placement can be found in ACI 347, “Guide to Formwork for Concrete.”
Self-consolidating concrete (SCC) can also result in high lateral pressures. The advantages of SCC, though, are that it does not require vibration, flows easily around congested reinforcement, and greatly reduces bug holes (see “Formwork for Self-Consolidating Concrete,” CONCRETE CONSTRUCTION, October 2004, p. 32). The form manufacturers are working with the American Concrete Institute to develop guidelines for placing SCC, including defining the maximum lateral pressures to be expected.
The basic elements of a gang form system include the sheathing (the surface that faces the concrete), studs, wales or walers, stiff backs or strongbacks, angle braces, ties, and safety equipment—including attached walkways and safety rails for workers to use during concrete placement and tie-offs for safety harnesses. Longer sections of gang forms require a spreader beam to better distribute lifting forces from the crane. Wales and strongbacks are not needed in some manufactured forming systems that incorporate their functions into the construction of the frame.