Ordering ready-mix concrete: quantity, wastage and what turns up

The drawing quantity and the delivered quantity are never the same number. Plan for the difference.

Working out the order

Start with the neat geometric volume from the drawings, then do the thing most people skip: measure the formwork that actually exists. Shutter positions, trench depth and blinding levels on site are not always what the drawing says, and the difference comes out of your order.

Wastage allowances

• Mass concrete footings and rafts on ground: 5% to 8%. Uneven subgrade, local depressions, absorption, over-excavation.
• Suspended slabs, beams and columns: 3% to 5%. Formwork deflection, pump line hold-up, chute and hopper losses.
• Piles and deep trench foundations: 10% to 15%. Over-boring, sidewall collapse, soil swelling.

Add 0.3 to 0.5 m³ on top for concrete that stays in the pump hopper and boom line at the end of the pour. It is real material and it is always there.

Why the real quantity differs

• Over-excavation and undulating subgrade. No machine cuts a flat bed. Local over-digs and soft pockets all take concrete.
• Formwork deflection. Wet concrete exerts serious hydrostatic pressure. Shutters that are flexible or under-braced bulge, and across a large surface that is real volume.
• Absorption. Mass concrete placed directly on dry unlined subgrade or aggregate loses water and slurry into the ground.
• Rebar congestion, which changes theoretical against practical yield slightly.
• Batching tolerance. Plant weighing tolerances of 1% to 2% per batch, plus air content variation, move the yield.

Individually these are small. Together they are why a 3% allowance on a ground-bearing element is optimistic.

The last load

The end of a pour is where money is either saved or wasted.

1. Hold the final load back. Order the bulk in full truckloads and keep the last one uncommitted until placement is 80% to 90% done.
2. Dip the remaining formwork. Tape or laser level the volume that is genuinely left, rather than estimating by eye across a busy deck.
3. Adjust the balance load with the batch plant — 3.5 m³ instead of a full truck.
4. Know the part-load surcharge. Suppliers charge below a minimum, typically under 4 to 6 m³. Ordering a small buffer, say 0.5 m³ extra on the balance load, is usually cheaper than the second trip you will need if you run short.

Checks when the truck arrives

• The ticket. Mix code and strength grade, aggregate size, cement type, admixtures, and the delivery address. Wrong grade delivered and placed is not recoverable.
• Batching time. Concrete should be placed within 90 minutes of batching, or 120 with a retarder, per ASTM C94 / BS EN 206. Check the stamp, not the driver's estimate.
• Temperature. A calibrated probe into fresh concrete. In hot climates the specification limit is commonly 32 °C.
• Slump. Abrams cone (ASTM C143 / BS EN 12350-2) against the specified range, for example 150 ± 25 mm for pumped concrete.
• Cubes or cylinders for 7 and 28 day strength, at your specified sampling frequency.

One rule with no exceptions: no water added on site to loosen a stiff load without the QC engineer's approval. Unmeasured water raises the water-cement ratio, and the strength and durability you lose does not show up until the cubes are crushed, three weeks after the concrete is buried in the structure.

Running short mid-pour

If fresh concrete sits while you wait for a delayed truck, it starts to set. Placing against it produces a cold joint: an unbonded plane that is weak in shear and leaks water. It is a permanent defect created by a temporary delivery problem.

If a delay looks likely:

• Stop at an engineered joint. Move placement to a proper construction joint location — around a third of the span in a beam or slab, where shear is lowest — rather than leaving a ragged sloping face wherever the concrete ran out.
• Form the joint properly, with timber or expanded metal stop-ends and dowels, so you get a clean vertical face.
• Keep the leading edge alive with a retarder or evaporation retarder, and cover it with wet hessian or plastic to stop surface drying.
• Re-vibrate the interface when the delayed truck arrives, blending old and new before initial set is complete.

Calculators for this work

• Concrete Volume Calculator — Work out the volume of concrete needed for a rectangular pour, with an allowance for wastage.
• Concrete Slab Calculator — Size a floor or ground slab: volume, ready-mix load, weight and area, including an edge thickening if there is one.
• Concrete Footing Calculator — Calculate concrete for strip footings or a set of identical pad footings.
• Concrete Bags Calculator — Convert a concrete volume into the number of pre-mixed bags needed, for small pours where ready-mix is not practical.

This guide describes general practice and typical figures. Your project specification, job mix formula and laboratory results govern the actual values on your job. Nothing here is a design or a substitute for the engineer of record.