Concrete takeoff: footings, walls, slabs, reinforcing and formwork

Cast-in-place concrete is the backbone of most buildings' substructure and often their structure too, and it is the trade general contractors most often self-perform. A concrete takeoff produces four related quantities for each element: the volume of concrete, the contact area of formwork, the mass or weight of reinforcing steel, and the area of finishing. Each is bought, built and priced differently, but all come from the same dimensions. This lesson takes off the common elements of a building foundation and slab and shows how the quantities connect.

Organizing a concrete takeoff

Concrete work is taken off element by element, following the structural drawings: footings, piers and pile caps, foundation walls, grade beams, columns, slabs on grade, suspended slabs, beams, stairs and miscellaneous items such as equipment pads, curbs and sumps. Each element is listed separately because each has its own formwork method, placing method and productivity, and therefore its own unit price. A wall poured from a pump in large lifts costs much less per cubic metre or cubic yard to place than a small equipment pad poured by hand.

• Concrete volume: the net volume in place, measured from dimensions, with no deduction for reinforcing steel and usually none for small embedded items or openings below a stated size.
• Formwork: the contact area of concrete surfaces that need forms, by element type, measured in square metres or square feet of contact area (SFCA in US practice).
• Reinforcing: bar lengths by size, converted to mass or weight, with laps, hooks and supports, often taken off from the structural drawings or later from the rebar fabricator's shop drawings.
• Finishing and curing: the area of exposed horizontal surfaces to be finished, such as slab tops and wall tops, and the area to be cured or protected.
• Accessories: anchor bolts, embedded plates, waterstops, expansion joint filler, vapour retarder under slabs, form ties and chamfer strips.

Strip footings and walls

Strip footings and foundation walls are measured along their centreline, which avoids double-counting at corners. For a rectangular building with walls of uniform section, the centreline length is the perimeter measured to the middle of the wall. Exterior corners add a little and interior corners subtract a little relative to outside dimensions, and the centreline method balances them automatically. Footings under walls of a different width have their own centreline.

Formwork for footings is usually the two side faces, since the bottom is cast against soil; formwork for walls is both faces over the full height, plus bulkheads at pour joints and blockouts for openings. Corners, pilasters and steps in the wall add labour and are often counted separately, because they slow the forming crew far more than their area suggests.

Reinforcing steel

Reinforcing steel is bought and priced by mass or weight, but taken off by counting bars and measuring their lengths. For each bar mark, the estimator calculates the number of bars from the spacing and the length of the element, the length of each bar including hooks and laps, and the total length, then multiplies by the mass or weight per unit length for the bar size.

Laps are needed where bars are spliced because they are longer than the stock lengths delivered, commonly 12 m or 18 m in Canada and 20, 40 or 60 ft in the United States. Lap lengths are shown on the drawings or in the general notes, often as a multiple of bar diameter. Dowels from footings into walls, corner bars and bars around openings are counted separately. Supports such as chairs and bolsters, tie wire and a small waste allowance are added in pricing.

A ratio check catches gross errors: divide the reinforcing mass by the concrete volume. Here 1,776 kg ÷ 46.08 m³ is about 39 kg/m³, or in imperial terms 4,321 lb ÷ 62.4 yd³ is about 69 lb/yd³, which is reasonable for a lightly reinforced foundation wall. Heavily reinforced columns, transfer beams and shear walls can exceed 150 kg/m³ or 250 lb/yd³. A ratio far outside the expected range suggests a missed bar mark or a units error.

Slabs on grade and suspended slabs

Slabs on grade are measured by area, with volume calculated from the specified thickness. Thickened edges, thickened slabs under walls, drop panels at columns and depressions for finishes or showers add concrete and must be taken off separately. The estimator also takes off the granular base and compaction, the vapour retarder with laps, insulation where specified, reinforcing mesh or bars, edge forms, construction and control joints, and finishing and curing by area.

Concrete placed on grade usually needs a larger waste allowance than concrete in forms, because the subgrade is never perfectly level: a slab specified at 125 mm or 5 in often averages several millimetres or a fraction of an inch thicker. Many estimators allow 4 to 7 % on slabs on grade against 2 to 3 % in formed elements.

Suspended slabs add formwork for the soffit, measured by area, and shoring and reshoring, which are usually priced per unit of slab area with allowances for the number of floors of reshores required. Beams and drop panels add soffit and side forms. Openings larger than the stated deduction size are deducted from concrete and soffit forms but add edge forms.

Columns, stairs and miscellaneous concrete

Columns are counted by type and height, with volume from the section and height and forms from the perimeter and height. Round columns are formed with fibre or steel tubes priced per unit length by diameter. Stairs are measured by volume for concrete but priced largely on labour, because forming treads and risers is slow; many estimators price them per riser or per flight. Equipment pads, housekeeping pads, curbs, sumps and trenches are small in volume but labour-intensive, and they are often omitted from takeoff because they appear only on mechanical or electrical drawings.

Pricing concrete work

Each concrete element's unit price combines several cost items, built up from the quantities above.

• Concrete material: purchase quantity × delivered price, plus winter, short-load and admixture charges, from Lesson 6.
• Placing: labour and placing equipment such as pump or crane, priced per cubic unit by element and pour size.
• Formwork: labour per unit of contact area from Lesson 4, plus form materials or rental, with reuse factors for systems used several times.
• Reinforcing: usually a subcontract or supply-and-place price per tonne or ton, from the fabricator and placer.
• Finishing, curing and protection: labour and materials per unit area, plus winter heating and hoarding where needed.
• Testing and inspection: cylinders and field tests, if the contract assigns them to the contractor.

Formwork reuse deserves attention. Prefabricated wall forms or panels used ten times on a repetitive building cost far less per use than plywood forms built for a single pour. The estimator plans the number of form sets, the pour sequence and the number of uses, and spreads the form material cost over the total contact area formed.