Planning a Concrete Pour: From Mix Design to Sign-Off

The Pour That Set Us Back a Month

Years ago, on a culvert job in Warrington, we poured 45 cubic metres into a base slab on a Friday afternoon. The concrete was late arriving — first wagon didn't show until 14:30 instead of 12:00. The slump was all over the place, the vibrating poker packed in halfway through, and by the time we finished it was dark and raining. Monday morning, the test cubes came back weak at 7-day, and by 28 days we knew we were in trouble. The whole slab got condemned and broken out. Six weeks lost, £38,000 written off, and a very uncomfortable conversation with the client. Every one of those problems was avoidable with better planning.

Start With the Specification

Before you think about logistics, nail down the specification. What strength class? What exposure class? What cement type? What maximum water/cement ratio? What admixtures? These aren't decisions for the site team — they come from the designer and the spec, usually BS 8500-1 for the UK. But the site team needs to understand them, because they determine the mix design, and the mix design determines how the concrete behaves on the day.

A C35/45 with a maximum w/c ratio of 0.45 for an XA2 exposure class (sulfate conditions, common on water industry sites) behaves very differently from a GEN3 going into a blinding layer. The first one will be stiffer, set faster in warm weather, and have a tighter placement window. If you're planning the day without knowing this, you'll get caught out.

Volume, Rate, and Timing

Calculate the volume properly. Measure the formwork, allow for over-excavation, and add 10% for waste. Don't rely on the drawing dimensions alone — measure what's actually on the ground, because it's never exactly what the drawing says. Ebrora's Concrete Volume Calculator handles the geometry for standard shapes including slabs, columns, beams, and circular sections, so you're not doing it on the back of a fag packet.

Then work backwards from the volume to determine how many wagons you need and at what intervals. A standard ready-mix truck carries 6m³. If you're pouring 30m³, that's five loads. If each wagon takes 20 minutes to discharge and you want continuous placement with no cold joints, you need a wagon arriving every 20 minutes. Factor in the travel time from the batching plant, the queuing time at the site entrance, and the time it takes to wash out between loads. If the batching plant is 45 minutes away, you need to stagger the orders so the second wagon arrives just as the first one finishes.

Ebrora's Concrete Pour Planner works all of this out for you. You enter the volume, wagon capacity, plant distance, and discharge rate, and it produces a delivery schedule. Takes the guesswork out of it.

Pre-Pour Checks

The morning of the pour is not the time to discover problems. Here's a checklist you should be running through at least 24 hours before:

Formwork. Is it clean, oiled, aligned, and braced? Has it been checked by the TWC if it's a designed element? Are the kickers true and the shutters plumb? Check the formwork pressure calculations if you're doing a deep wall pour — the lateral pressure at the base of a 3-metre wall pour at a high pour rate can blow formwork out if it's not adequately tied.

Reinforcement. Is it complete, tied, covered with the right spacers, and signed off by the clerk of works? Any clashes with box-outs or cast-in items? Has it been inspected and released? Don't start pouring on a verbal OK — get the inspection recorded.

Access. Can the wagon get to the pour location? Is the ground firm enough? Do you need a concrete pump, and if so, is it booked, is there a lifting plan for the boom, and is there enough reach? Pump lines cleaned and tested?

People. Who's vibrating, who's levelling, who's sampling, who's directing wagons, who's doing the slump tests? Have they all been briefed? Does everyone know the stop/start signals?

Contingency. What if a wagon is rejected for slump? What if the pump blocks? What if it rains? Having answers to these before they happen is the difference between a controlled pour and a disaster.

On the Day

Test the slump of the first wagon before it goes in. If it's out of spec, reject it. Yes, it's painful. But accepting out-of-spec concrete is always worse. Record every wagon: time of arrival, delivery ticket number, slump result, batch time, and any observations. Our Concrete Pour Register gives you a structured template for exactly this.

Vibrate properly — it's the single biggest factor in concrete quality after the mix itself. Under-vibration leaves voids and honeycombing. Over-vibration causes segregation. The poker should go in at regular centres, be left until the air bubbles stop rising (usually 10–15 seconds), and be withdrawn slowly. If you can see grout around the poker and the aggregate has settled, you've over-done it.

Cure it. Concrete reaches its design strength over 28 days, but the first seven are critical. Cover it with polythene, damp hessian, or a curing compound to prevent rapid moisture loss. In cold weather, insulate it. In hot weather, keep it damp. Curing is the bit that everyone forgets, and it's the bit that determines whether your 7-day cubes pass or fail. Check our Curing Estimator for temperature-adjusted curing guidance.

A good pour is 90% planning and 10% doing. Get the preparation right and the pour day runs itself. Skip the prep and you're gambling with a material that can't be un-poured.