Concrete is cement + water + aggregates — and the proportions decide everything.
The two numbers you'll hear about most on site are slump and air:
Slump — how workable the mix is. A typical general-construction mix runs 75–100 mm; pump mixes and some structural placements want more. Higher slump usually means more water — and more water means less strength, so the spec sets the range for a reason.
Air content — microscopic air bubbles that protect the concrete from freeze-thaw damage. In Ontario, exterior concrete commonly targets 5–8% air. Air is tested on the truck, not guessed.
Mix requirements come from the project spec (in Ontario, typically OPSS) and the engineer's design — the numbers here are typical ranges, not universal.
2Volume & ordering
Concrete is ordered by the cubic metre — and a little waste goes a long way.
Volume (m³) = Length (m) × Width (m) × Depth (m)
Example: a slab 12 m × 5 m × 0.15 m = 9.0 m³. Add waste and form/ground irregularities — 5–10% is common on earth-formed work — and you're ordering ~9.5–10 m³. A ready-mix truck typically hauls around 6–7 m³, so this pour is roughly a truck and a half.
Length × width × depth gives cubic metres — add waste before you call the plant.
Everything you do at the chute either helps the pour or costs you strength.
Don't add water on site. Water added at the chute to "loosen it up" changes the water-cement ratio and drops strength. If the mix isn't working, call the dispatcher — don't drown it.
Consolidate, don't over-vibrate. Internal vibration settles the concrete into the forms and around rebar, but running the vibrator too long can separate the mix and bleed out the fines.
Place it before it sets. Concrete has a working life measured in hours (and heat makes it shorter). Plan the pour so trucks aren't waiting and the finishers aren't racing the set.
Field tipCheck the ticket. The spec'd slump, air, strength and mix ID are all on the delivery ticket — verify them before the truck dumps, not after.
4Curing
Concrete doesn't dry to strength — it cures, and it needs water to do it.
Cement gains strength through a chemical reaction that needs moisture. If the surface dries out early, the top of the slab stays weak and dusts. Standard practice is to keep concrete moist for the first several days (specs commonly call for 7 days): wet burlap, a curing compound, or ponding on flatwork. In cold weather, protect fresh concrete from freezing until it has gained enough strength — freezing early is permanent damage.
5Strength & testing
Slump and air are checked on the truck — strength is proven with cylinders.
The slump test measures workability — the drop in mm after the cone is lifted.
Cylinders are cast from the same truckload, cured in the lab, and crushed at the specified age — usually 28 days, with 7-day breaks as an early check. A typical structural mix might be specified at 30–35 MPa at 28 days. The spec sets how many cylinders to take (often per set number of cubic metres or per pour).
Cylinders from the same load prove the concrete's strength weeks later.
Field tip — orderingOrder by the cubic metre and round up for waste. Coming up short mid-pour is a cold joint and a headache; a little extra concrete in the truck is easier to deal with.
Field tip — slumpA higher slump isn't "better" — it's usually extra water. If the mix is stiff, talk to the plant before anyone adds water.
Field tip — cylindersCast cylinders the moment the load arrives, label them with pour/date/location, and get them to the lab or curing setup — a cylinder that dries out proves nothing.
Put this guide to work.
Calculate the pour, track the deliveries, and keep the record with the workflow and Field QC.