Concrete strength grade

Frequently asked questions

Why is a cube stronger than a cylinder of the same concrete?

Because of friction at the testing machine's platens, and the specimen's shape decides how much that helps it. As a specimen is squashed it tries to spread sideways, and friction against the steel platens restrains it at the top and bottom — which puts that material into a confined state that is stronger than plain compression. A cube is short relative to its width, so the restrained zones from both ends reach most of the specimen and the measured strength is flattered. A standard cylinder is twice as tall as it is wide, so its middle is far enough from both platens to fail in something much closer to pure compression. The result is that the same concrete reads roughly a fifth higher as a cube, which is why British grades carry both figures and why the ratio is not a constant — it drifts with strength, so a single conversion factor applied across the range is an approximation rather than an equivalence.

Why 28 days, and what happens after?

Twenty-eight days is a convention rather than a limit: it was long enough for ordinary Portland cement to reach most of its eventual strength and short enough to be a practical contractual milestone. Concrete keeps gaining strength for months and years as hydration continues — slowly, and only while moisture is available, which is why curing matters and why a slab left to dry out stops gaining. Two practical consequences follow. A 7-day result is not a failure: it is a partial result, typically a substantial fraction of the 28-day figure, and is used to predict rather than to accept. And modern mixes with fly ash or ground slag deliberately gain strength more slowly, so a mix specified for its durability can look weak at 28 days and meet its design strength at 56 or 90 — which is why the specification states the AGE as well as the strength, and why testing such a mix on the default schedule produces an argument rather than information.

What is characteristic strength, and why is the mix stronger than the grade?

Characteristic strength is a statistical floor, not an average: it is the value below which only a small proportion of test results — conventionally five percent — are expected to fall. Because real concrete varies from batch to batch, a supplier aiming AT the characteristic value would fail half the time, so they target a mean strength above it by a margin that depends on how consistent their production is. A plant with tight control needs a smaller margin; one with variable materials needs a larger one. Three things follow from this. Concrete routinely tests well above its grade, and that is the design working rather than generosity. A single low cube is not automatically a failure — conformity is judged on a rule about groups of results. And specifying a higher grade to be safe raises the cement content, which raises cost, shrinkage and heat of hydration, so it is a trade rather than a free margin.