Concrete takeoff

Concrete volume is the placed solid, ordered as ready-mix in cubic yards or cubic metres. Formwork is the area of concrete face that touches a form. Reinforcement is taken by weight, and precast by piece. These four do not scale from one another in any fixed way, so each is taken off on its own terms. The plan outline you trace is only the starting geometry; joints, finishes, embedments, and any blinding layer become further items measured by length, area, or count.

Concrete is measured net as fixed in position, meaning the actual placed solid. For formed work that solid runs to the outside face of the formwork, not the inside of the forms, the structural centreline, or the architectural finished face. Tracing to the inside of the forms understates volume by roughly 2 to 5 percent on a slab, and by more on thin members. Where concrete is cast against earth or hardcore, the boundary is the neat excavated or blinded line.

Every standard agrees here. RICS NRM2 measures concrete net and allows nothing for deflection of the formwork, the Australia and New Zealand standard measures net as fixed in position, and CESMM4 and the German VOB with DIN 18331 bill to the actual member dimensions. The regional divergence is all downstream, in the deduction thresholds and the treatment of waste.

Concrete is net, but small voids such as a single pipe penetration, an anchor pocket, or a sleeve are not deducted, because forming them costs more than the concrete they save. Every method of measurement sets a minimum void size below which nothing comes out, and that threshold is one of the clearest regional differences in the trade. Under RICS NRM2 in the United Kingdom, no deduction is made for voids smaller than 0.05 cubic metres, except in troughed and coffered slabs. Under the German VOB with DIN 18331, openings, penetrations, and embedments are deducted only when an individual one exceeds 0.5 cubic metres by volume, or 0.1 cubic metres per metre run for slots and channels; by area, only openings over 2.5 square metres are deducted. The United States, with no legal method of measurement, follows the British pattern: ignore single penetrations, drains, anchor pockets, and small bases, but deduct large pits, sumps, stair and elevator openings, and full block-outs.

One rule holds everywhere: reinforcement, embedded structural steel, and cast-in accessories are never deducted from concrete volume. Australia and New Zealand practice adds one nuance, that embedded hollow sections do displace concrete and are deducted.

A flat slab is priced at its constant thickness. Thickened edges, turn-downs, haunches, integral footings, and grade beams are taken off separately, because each has a different depth, its own edge formwork, and its own reinforcement. The flat field is measured at one thickness, then the extra concrete in the thickenings is added as a separate linear or volume item.

RICS NRM2 splits concrete further into a matrix of location, orientation, and thickness: mass concrete (unreinforced bulk), horizontal work, sloping work at 15 degrees or less, sloping work over 15 degrees, and vertical work, each banded at 300 millimetres thick or less versus over 300 millimetres. An upstand is classed as vertical only when its height exceeds three times its width. United States practice splits more loosely by element type (slab, footing, wall, column) with less formal banding; that element split is estimating convention rather than a method-of-measurement clause, since the MasterFormat Division 03 scheme organises specification sections, not quantities.

Formwork is its own item, measured as the area of concrete face actually in contact with a form. In the United States this is square feet of contact area, or SFCA; the German term is the developed area of the formed surfaces. It covers slab edges, wall faces (both sides where both are formed), beam sides and soffits, column sides, and stair risers. Surfaces cast against earth, and open top surfaces left struck or trowelled, are not formwork. Contact area bears no fixed relationship to plan area: a six-inch slab edge contributes its perimeter length multiplied by half a foot of height, not the slab field.

Openings are handled differently by region. RICS NRM2 keeps the wall face measured gross and bills each opening as an extra-over item banded at 5 square metres or less, 5 to 10 square metres, and over 10 square metres, with the labour of forming it included. The German VOB instead deducts openings over 2.5 square metres and bills the reveal or jamb contact area separately. Either way, small openings stay in the form area.

Reinforcement is priced and ordered by weight everywhere: the total bar length for each size is multiplied by the fixed nominal weight per length for that size, then summed to a tonnage. In the United States, bar sizes run in eighths of an inch (number 3 up to number 18), with nominal weights set by ASTM A615 and A615M. A number 3 bar is 0.376, a number 4 is 0.668, a number 5 is 1.043, a number 6 is 1.502, and a number 8 is 2.670 pounds per foot, so one ton of number 4 bar is roughly 2,994 linear feet. Metric standards use millimetre diameters. Bars are separated by size, and welded-wire fabric or mesh is taken by area, not by bar weight, with the weight per square metre stated.

Laps are where the regions diverge. Bars come in stock lengths and overlap at splices, so the real steel exceeds the centreline run-out. A tension lap is typically a Class B splice, which ACI 318 sets at 1.3 times the development length, often approximated as about 40 bar diameters. Under detailed United States bar-schedule practice the lap is added explicitly to each bar so the tonnage carries it. Under RICS NRM2 the unit rate is deemed to include laps, hooks, tying wire, cutting, and bending, so the takeoff uses net bar lengths. The German VOB likewise excludes tying wire, rolling tolerance, and bar off-cut waste from the measured weight, except that mesh or fabric waste above 10 percent of the installed mesh mass is paid additionally. Where bar cutting scrap is added on the ordering side it runs about 5 to 10 percent; laps and a separate scrap allowance should not be double-counted.

The measured volume is the in-place solid, but the ordered ready-mix volume adds a contingency for spillage, drum retention, settlement, form deflection, and especially over-excavation of an uneven subgrade. ASTM C94 and C94M enumerate these contingencies for the purchaser without stating a percentage. By common practice the flat-slab allowance starts near 5 percent, rises to about 7 to 8 percent for irregular or multi-strip pours, and reaches about 10 percent over a porous or over-dug subbase. This factor belongs only to the ordering quantity; it never inflates the net bid or billing measure.

Segmental and unit paving instead carries a pattern-driven waste, since diagonal cutting drives breakage: running bond around 7 to 10 percent, basketweave about 12 to 15 percent, herringbone or 45-degree patterns about 15 to 20 percent, and circular or fan patterns about 20 to 25 percent. These pattern bands are estimating practice rather than published clauses. Cast flatwork is traced as plan area to the outer edge, then converted to volume by thickness.

The slab outline also drives several items that are neither volume, formwork, nor reinforcement. Surface finishes such as power-floating, steel-trowelling, and broom finish, along with curing and sealing, are a top-of-slab area item measured in square feet or square metres; RICS NRM2 lists trowelling and floating to top surfaces as their own area item. Construction, contraction, and expansion joints, together with waterstops, are linear items measured by length and separated by joint type, with their sealant and dowels stated. Joint spacing is a design input that ACI 360R addresses, but the takeoff deliverable is run length by type. Cast-in accessories such as anchor bolts, embed and base plates, dowels, sockets, and bearing pads are enumerated by type and size, and are never deducted from volume.

Blinding, the thin lean-mix levelling layer under slabs and foundations, is a separate horizontal concrete item at its own thickness and a lower-grade mix; RICS NRM2 lists it within horizontal work, so it must not be merged into the structural slab. Formwork material is not one-to-one with contact area either, since form-ply is reused across several pours; ACI 347 treats the number of reuses as the cost lever, with the reuse count left as a project input.

Precast and prestressed concrete break the cast-in-place model: there is no site formwork item, and the dominant measure is each, or per piece. PCI guidance is explicit that erection is bid per piece rather than per ton or per square foot, since handling and crane cost track the number and size of lifts. Double-tees, hollowcore planks, wall panels, columns, and beams are counted by type and size, and cross-reported as erected area for cladding or deck scope and as weight for transport and crane sizing.

Finally, keep the three purpose-based quantities apart, because the same wall produces three different numbers. The net measured quantity, for bidding, progress billing, and cost control, is the in-place solid to the form face with the void thresholds applied and no waste. The ordered quantity, for procurement, is the net plus waste and over-order, rounded up to the ready-mix increment, with reinforcement laps and scrap added. The measured-for-payment quantity, used for civil and highway progress billing, is the net amount placed to date by pay item, with waste excluded because it sits in the contractor's rate. Reporting an order quantity as a billing quantity over-bills the owner; reporting a net quantity as an order under-supplies the pour.