Ballast is sand and aggregate pre-blended. One material to order and store, but you lose the ability to adjust the ratio on site.
Spillage, an uneven sub-base and material left in the mixer. Running out part-way through a pour is far worse than over-ordering — a cold joint is a permanent weak line.
Grades are nominal mixes of cement : sand : aggregate, 1:2:4 for C20 through 1:1.5:3 for C30, batched by volume with a bucket. This page turns each share into a weight with its own bulk density.
Paste and water fill the gaps between loose particles, so you need about 1.54× the finished volume in dry material. Skipping it is the commonest reason people under-order.
Set by the water-cement ratio you choose, and a stronger lever on final strength than the mix ratio. Each grade caps the water you can add and still reach it.
The Eurocode 2 curve for class N cement, stretched by temperature through a maturity calculation: cold slows the reaction, it does not stop it.
Relative final strength; strength falls roughly exponentially with the ratio. Nothing else you control on site moves the number that far. Wetter is easier to place and worse forever, and the damage is invisible until something cracks.
Cement is only 1 part in 7 of a C20 mix by volume. Sand and aggregate carry the load; cement binds them. More cement is not automatically stronger: the ratio matters more than the quantity, and an over-rich mix shrinks and cracks.
A shovel of damp sand and a shovel of dry cement are nothing like the same volume, and the error compounds over every load. Nominal ratios are volume ratios, so a bucket is the correct instrument. All-in ballast is one material instead of two, with the parts added together: 1:6 for C20, 1:5 for C25, 1:4.5 for C30.
At 5°C the hydration clock runs at half its 20°C speed, so the same slab takes twice as long to reach the same strength; at 10°C about two thirds. Freezing before it sets is different and permanent: that damage does not come back. The familiar “70% at seven days” is a British answer; at 20°C you get there in under five days.
Concrete poured onto a batch that has already stiffened does not bond to it. In a slab mixed load by load over several hours, those planes are where it will eventually crack, which is why volume, not effort, is the real argument for ready-mix. Around a cubic metre (roughly 25 loads through a small mixer) is where to stop mixing it yourself.
Concrete gains strength by reacting with water, not by losing it. A slab left to dry out in sun or wind stops gaining strength early. Cover it, keep it damp for the first week, and the 28-day figure means something.
Loose sand and aggregate have air gaps between the particles, and when cement paste and water fill those gaps during mixing the total volume shrinks. The 1.54× factor is the standard correction for that settling. It is an average: very coarse or very fine aggregate will shift it, which is part of why a wastage allowance is not optional.
More than anything else you control on site. Strength falls roughly exponentially with the water-cement ratio, so going from 0.50 to 0.60 — about two extra litres per 25 kg bag, which looks like nothing in a mixer — costs around a quarter of the final strength. At 0.70 you have lost nearly half. A wetter mix is easier to place and worse forever afterwards, and the damage is invisible until something cracks.
C20 handles the vast majority of domestic jobs — shed bases, patios, footings — and is the easiest to batch by hand. Step up to C25 anywhere vehicles will drive or park, and C30 only for structural elements like beams or columns, where hand-mixing consistency becomes hard to guarantee. Each grade also caps how much water you can use and still reach it, which this page checks against the ratio you have set.
Yes, and it is one material to order rather than two. Ballast is pre-blended, so the equivalent ratio is simply the sand and aggregate parts added together: 1:6 for C20, 1:5 for C25 and 1:4.5 for C30. Merchants often quote 1:5 as a general-purpose mix, which sits between C20 and C25 here. What you give up is the ability to adjust the sand fraction on site when the mix looks wrong.
Because cement hydration is a chemical reaction and cold slows it down. The page uses a maturity calculation with a −10°C datum, which is the standard approach: at 10°C the clock runs at about two thirds the speed it does at 20°C, and at 5°C roughly half. This is why the familiar “70% at seven days” figure is a British answer rather than a universal one — at 20°C you get there in under five days.
Around a cubic metre. That is roughly 25 loads through a small mixer, and every load has to be batched, mixed, barrowed and placed before the previous one has stiffened. Beyond that the real risk is not effort but cold joints, because a slab poured over several hours in separate batches has weak planes through it. The page flags the crossover as the volume climbs.
This page has changed in 161 archived releases, but each of those was a site-wide pass, so none is attributable to this tool on its own and none is listed here. That is not a claim that the tool never changed — a release that reworked many pages at once may well have altered this one too. The changelog has them.