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Tips for Estimating Concrete Formwork08/11/2009 by Stephen Plotner, Senior Engineer
Concrete is one of the most adaptable and versatile materials used in the construction industry and is one of the most widely used materials in construction projects. In a previous article on concrete, we discussed the fact that the estimator must consider each of the following major components individually: formwork, reinforcing steel, ready-mix concrete, placement of the concrete mix, finishing of the surface, and curing. Subsequent articles will deal with each of these topics in turn. This article will deal with concrete forming.
Formwork is essential to concrete construction — forms determine the size, shape, alignment and position of the finished concrete work. Formwork can also serve as a temporary support structure for materials, workers and equipment.
The concrete subcontractor must achieve 3 goals in his formwork workmanship:
Quality – formwork must be designed and built so that the resulting cast concrete attains the desired size, shape, position and finish.
Safety – formwork must be designed built strong enough to support all dead and live loads without blowouts, collapse, danger to workers, and risk of damage to the structure.
Economy – formwork must be designed, built and re-used in an effort to save time and money for the general contractor and the owner.
The estimator must keep these subcontractor goals in mind when estimating concrete formwork.
Formwork for spread footings, strip footings, pile caps, slabs on grade and sidewalks typically consist of some type of edge forms, usually wood, that is staked into the ground and braced. Formwork for walls and columns typically consist of steel-framed plywood forms, ties, adjustable column braces, walers, strong-backs and bracing. Formwork for elevated slabs and beams typically consist of a plywood deck and edge forms that are supported by some type of shoring system and bracing.
Described so far are removable and re-usable wood forms that must be protected with form oil or a release agent in order to facilitate easy removal. There are also removable and re-usable metal and fiberglass domes and pans, as well as form liners used to achieve architectural texture.
There are some permanent type forms such as insulated concrete forms (ICF), metal floor decking that is corrugated, open rib, composite or cellular, and metal roof decking used as a form for a lightweight concrete roof deck.
The estimator must not overlook other items such as inserts, dovetail slots, blockouts, construction and expansion joints, waterstops, chamfers and rustication grooves.
Wood formwork is usually estimated by the square foot of contact area (SFCA) — the surface area of the forms that will come into contact with the concrete. Inserts and other formwork accessories are usually estimated by the lineal foot or the each.