Ask any builder about the jobs that go wrong, and concrete comes up more often than you might expect. Not because the material is unreliable, but because the mix so often is. From cracked garage floors to sinking shed bases, a surprising number of domestic building failures trace back to the same root cause: the wrong concrete for the job.
Concrete is deceptively simple. Cement, sand, aggregate and water, combined in the right proportions, produce one of the most durable building materials ever devised. Change those proportions even slightly, however, and the result can be dramatically weaker. Too much water is the classic error on DIY projects, because a sloppier mix is easier to pour and spread, yet every extra litre dilutes the cement paste and reduces the final strength.
This is why the industry works to standardised ratios rather than guesswork. A general purpose concrete mix for paths and non-structural slabs typically uses one part cement to two parts sand and four parts aggregate, while foundations and driveways call for stronger designated mixes with more cement and carefully graded stone. British Standards classify mixes by compressive strength, from C10 at the light end through to C40 and beyond for structural and industrial work, giving builders a common language for specifying exactly what a job needs.
For homeowners, the practical question is usually whether to mix on site or order ready mixed. Hand mixing suits small jobs, a few fence posts or a repair, where the total volume is under a quarter of a cubic metre. Beyond that, consistency becomes the enemy. Each barrow load mixed by hand will vary slightly, and on a large slab those variations show up later as weak spots and cracks. Volumetric and ready mix suppliers batch to precise ratios, and for anything over about half a cubic metre the cost difference against bagged materials narrows sharply once labour is counted.
Timing matters as much as proportions. Concrete does not dry, it cures, a chemical reaction between cement and water that continues for weeks. It reaches roughly two thirds of its strength within seven days but needs around twenty eight to hit its rated figure. Pouring in frost can halt the reaction entirely, while pouring in hot sun can pull moisture out before the reaction completes, which is why professionals cover fresh slabs with polythene or damp hessian.
The consequences of getting it wrong are rarely immediate, which is part of the problem. An underspecified base may look fine for a year before hairline cracks appear, and by then the remedy is usually to break out and start again. Trade bodies consistently advise that the cost of upgrading from a borderline mix to the correct one is trivial against the cost of replacement.
Suppliers have responded to the knowledge gap. Most reputable firms will advise on the right concrete mix for a stated use, whether that is a shed base, a driveway or an extension footing, and volumetric lorries can even adjust the mix on site so customers pay only for what is poured.
The lesson for anyone planning a project this year is simple. Decide what the concrete has to do before deciding what to buy, and treat the mix specification with the same seriousness as the design itself. Concrete forgives very little, but specified correctly, it will outlast almost everything built on top of it.