Concrete is the most-used building material on earth and one of the most commonly got wrong on domestic projects. Not because it is complicated to pour — it is not — but because almost every decision that determines whether a slab performs has to be made before delivery, over the phone, by someone who may never have ordered it before.
The consequences are unforgiving. Concrete that is short by half a cubic metre leaves a cold joint through the middle of a slab. Concrete specified at the wrong strength cracks under a load it was never designed for. And concrete that arrives on a day nobody is ready for it starts going off in the drum while a decision is being made.
Here is what is worth understanding first.
Working out how much you need
Volume is length multiplied by width multiplied by depth, in metres, giving cubic metres. A garden shed base of 3m by 4m at 100mm deep is 3 × 4 × 0.1, or 1.2 cubic metres. That part is simple arithmetic.
What catches people out is that the hole is never quite the shape they measured. Sub-bases are rarely perfectly level, edges bulge slightly when formwork flexes, and a slab that averages 110mm rather than the intended 100mm has quietly consumed ten per cent more material than the calculation said. Anyone experienced will tell you to allow between five and ten per cent over, and for anything with an irregular profile — footings following a trench, or a slab over uneven ground — considerably more.
This is where the way concrete is supplied starts to matter a great deal.
Barrel-mixed versus mixed on site
Traditional ready-mixed concrete is batched at a plant and arrives in a rotating drum. The mix is fixed at the point of batching, the clock is running from that moment, and you are buying a set volume whether you use it all or not. Order too little and you have a serious problem; order too much and you have paid for concrete you will be shoveling into a corner of the garden.
Volumetric or on-site mixing works differently. The lorry carries the raw materials — cement, aggregate, sand, water — separately and mixes them at the point of discharge. Three practical consequences follow. You pay only for what actually comes out of the chute, so over-ordering costs nothing. The mix can be adjusted on the spot if the job turns out to need a different strength or a wetter consistency. And nothing is going off in a drum while you work, because it has not been mixed yet.
For domestic projects, where the exact quantity is genuinely uncertain until the pour is underway, that flexibility is usually worth more than the headline price comparison suggests. Most suppliers of ready mix concrete in Essex and across London now offer both, and it is worth asking which is being quoted.
Getting the strength right
Concrete is specified by compressive strength, expressed as a C figure — C20, C25, C30, C35 and so on — representing the strength in newtons per square millimetre it should reach after 28 days.
As a rough guide for domestic work: C20 suits non-structural applications like kerb backing and general fill. C25 is a common choice for domestic slabs, shed and garage bases, and light footings. C30 is the usual specification for driveways, garage floors carrying vehicle loads, and paths taking regular traffic. C35 and above move into heavier structural territory, and if a structural engineer has specified a grade, use exactly what has been specified rather than the nearest convenient equivalent.
Two related points. If the concrete will contain reinforcement or is exposed to de-icing salts or persistent wet-and-freeze cycling, the durability requirements change and the mix should reflect that. And if you are laying concrete in cold weather, additives can be introduced to help it gain strength before frost gets to it — but they must be added at mixing, not afterwards.
Also specify the slump, which describes how wet and workable the mix is. Too stiff and it will not flow around reinforcement or into corners. Too wet and the strength drops, because excess water weakens the cured concrete. Tell the supplier what you are pouring and let them recommend.
Access is the thing people forget
A fully loaded concrete lorry is a heavy vehicle with limited manoeuvrability and a fixed chute reach. Before ordering, walk the route it will take and check the practicalities: width of access, overhead cables and branches, turning space, ground conditions if it needs to leave the road, and how far the discharge point is from where the concrete actually has to go.
If the pour location is behind a house, across a garden, or up a level, the chute will not reach and the options are barrowing or pumping. Barrowing is viable for small volumes with several people available; it is exhausting, slow, and slow is a problem when the material is curing. A concrete pump moves material accurately over distance and height with a fraction of the labour, and for anything more than a couple of cubic metres in an awkward location it usually pays for itself in time and reduced risk. Booking the pump at the same time as the concrete avoids the classic scenario of one arriving without the other.
Also confirm where the lorry will wash out. It needs somewhere to do this, and the answer should be agreed in advance rather than improvised.
Preparation, weather and curing
Have everything ready before the delivery window opens: formwork fixed and braced, reinforcement positioned on spacers rather than sitting on the ground, damp-proof membrane laid if required, sub-base compacted, tools and enough people on site. Concrete does not wait.
Weather matters more than most people assume. In hot, dry or windy conditions the surface loses water rapidly and can crack before it has gained strength. In cold conditions, hydration slows dramatically, and frost on fresh concrete causes permanent damage. Pouring below about 5°C without precautions is asking for trouble.
Curing is the stage most often neglected. Concrete does not dry — it hydrates, a chemical reaction that requires moisture and continues for weeks. Covering a fresh pour with polythene or a curing compound and keeping it damp for the first several days makes a substantial difference to final strength and surface durability. Keep foot traffic off for at least 24 to 48 hours and vehicles off for a week or more, longer in cold weather.
Ask before you order
A good supplier will ask what you are building before quoting, and will push back if the grade you have asked for does not suit the application. That conversation is free and takes five minutes. Getting it wrong is neither.
