During construction, bridge deck concrete can be supported by reusable wood forms, permanent stay-in-place steel forms, or precast prestressed-concrete planks. Where permitted, contractors will generally use stay-in-place steel forms rather than removable wood forms. Allowance must be made in the design of the bridge for the extra weight of the steel forms, and for extra concrete where required. The forms are corrugated. Where the bottom transverse bar spacing can be made the same as the pitch of the corrugated form, the extra concrete in the valley below the nominal bottom of the slab line is compensated for by the concrete displaced by the peak of the corrugation above the bottom of the slab line, and the allowance can be for the weight of the forms only. If the spacing is different from the pitch, which is usually the case for curved bridges, a greater allowance will be required because extra concrete must provide the necessary cover. For long slab spans, the stay-in place forms are corrugated, but with a flat top plate. In this case, no extra concrete is required, and the extra weight allowance is for the forms only.
Prestressed-concrete planks can also be used as support forms. In this case, the planks also serve as a component of the structural slab. Some agencies have used prestressed planks for years with success, but others have experienced problems—particularly, longitudinal cracking through the cast-in-place top slab over the ends of the planks at the supporting beams—and have discontinued their use.
When stay-in-place steel forms or prestressed-concrete planks are used, the slab overhang beyond the outside beam is generally formed separately using conventional removable wood forms.
