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.
Concrete grades correspond to nominal mix ratios — cement : sand : aggregate, 1:2:4 for C20 through 1:1.5:3 for C30 — and those ratios are by volume, which is how they are batched on site with a bucket. Dry, loose materials occupy more space than the concrete they become, because paste and water fill the gaps between particles; the standard correction is a 1.54× dry-volume factor. This page splits that dry volume by the ratio and converts each share to a weight using its own bulk density, so the three materials add up to exactly the dry volume quoted. Water is set by the water-cement ratio you choose, which is a stronger lever on final strength than the mix ratio is. Strength gain over time follows the Eurocode 2 curve, stretched by temperature through a maturity calculation.
A 0.475 m³ shed base at C20 (1:2:4) with a 10% allowance needs about 166 kg of cement (6.6 bags of 25 kg), 368 kg of sand and 690 kg of aggregate, plus 83 litres of water at a 0.50 ratio. That is 14 loads through a 40 litre mixer. At 12°C it will take about 6 days to reach the 70% you want before driving on it.
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.
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