When is it worth ordering ready-mixed concrete instead of mixing it myself?

The decision is not really about money. It is about whether you can place the whole pour before any of it starts to stiffen. A slab poured in barrow loads across a day has cold joints through it and is a weaker, uglier thing than the same slab poured in forty minutes off a truck.

Safety

Step by step

1. Calculate the volume, then add to it.Length times width times depth in metres. Then add for the ground being uneven, the excavation being over-dug in places, and the base not being perfectly level — ten per cent is a normal allowance and on a dug base it can be more.
2. Compare honestly against mixing.A cubic metre of concrete is roughly forty barrow loads and, with a small mixer, most of a day for two people. Two cubic metres by hand is not a domestic proposition.
3. Decide between a barrow mix and a full truck.Volumetric or "mix on site" lorries batch what you actually use and charge accordingly, which suits an uncertain domestic quantity. A standard mixer truck delivers a fixed ordered volume, and you pay for what you ordered whether you use it or not.
4. Know what the supplier will ask.The volume, the strength or mix designation, the aggregate size, the slump or workability, whether you want fibres or an admixture, the access, and whether you need a pump or a chute. If you do not know the strength, tell them what it is for — a domestic drive, a shed base, a footing — and they will specify it.
5. Check the access properly, before ordering.A mixer truck is heavy, long and wide, and it needs a firm surface, headroom clear of trees and cables, and somewhere to turn. Measure the gate, look for a weak manhole cover on the drive, and ask the supplier what their vehicle needs.
6. Ask about a pump if the pour is not reachable.Concrete pumps make back-garden pours possible and add a significant cost with a minimum charge. Get the price before deciding, because for a small back-garden slab the pump can cost more than the concrete.
7. Understand the extra charges.Part-load charges below a minimum volume, waiting time beyond an allowance of usually half an hour or so, out-of-hours and Saturday premiums, and a charge for taking surplus away. Ask for all of them in writing when you get the price.
8. Be ready before the truck arrives.Formwork set, checked and braced. Membrane down. Mesh on chairs. Barrow route boarded. Tools, tamping beam, float and polythene to hand. Enough people. This is not the moment to discover the shuttering is not square.
9. Place, compact and screed continuously.Start at the far corner and work back. Fill in layers, compact each one, screed off the formwork as you go rather than at the end. The whole pour should go in without a break so there is no cold joint.
10. Cover and cure it.As soon as the surface will take it, cover with polythene or damp hessian and keep it damp and protected. This matters more with a good concrete than a poor one, because you have paid for strength you can still lose.

Tips

• Order for the morning. It gives you the day to finish the surface and to react if something goes wrong.
• Have somewhere agreed for surplus — a spare hole, a set of moulds, a path — because the last thing you want is a decision to make with wet concrete on the ground.
• Never wash concrete or washings into a drain, a gully or a watercourse. Let it go off in a lined pit or a tub and take the set lump to the tip.
• Two extra pairs of hands change this job completely. A cubic metre is not a solo pour.

Common mistakes

• Ordering before checking access and turning space — A truck that cannot reach the site still has to be paid for, and a load that cannot be discharged goes off in the drum. Suppliers will talk you through access if you ask before you book.
• Under-ordering to avoid waste — Running short mid-pour is far worse than a small surplus. The joint where the first load stopped and the second started is permanent, weaker and visible.
• Adding water at the truck to make it easier to place — Water added on site destroys the mix the supplier batched to a specification. Ask for a wetter slump when you order instead — that is a designed change, not a dilution.

Questions people ask