The foreman gave the new boy one instruction before leaving him with the mixer: one, two, three. One part cement, two parts sand, three parts gravel.
The boy asked how many buckets that was. The foreman said that was the wrong question, and that this was why the recipe was given as a ratio and not as a number of buckets.
Six parts make a complete mix. If the job needs thirty buckets, one part is five buckets: five of cement, ten of sand, fifteen of gravel. If it needs twelve, one part is two. If it needs a lorry-load, the same three numbers still describe it. The ratio does not say how much concrete to make. It says what concrete is.
The boy asked what happens on a big pour, where nobody counts buckets at all. The foreman said the scales on the plant do the same sum in kilograms, and the three numbers do not change.
Then he explained what happens when the ratio is wrong, because that is the part apprentices remember. Too little cement and the mix will not set properly; it looks finished and crumbles under load a year later. Too much cement and it sets hard, then cracks as it dries, because there is not enough sand and gravel holding it together.
"Nobody will ever be able to see the ratio in the finished wall," he said. "That is why you have to get it right while it is still in the bucket."