Concrete Cylinder Compressive Strength Test

Reference

The number the whole structure waits for. Cylinders cast at the truck become the legal record of what strength the concrete actually reached.

the deliverable: fc at 28 days
StandardsASTM C31 and C39, CSA A23.2 in Canada
Cylinder sizes100 by 200 mm standard in Canada, 150 by 300 also used
Initial curing24 to 48 hours on site, protected, roughly 15 to 25 C
Standard curingmoist room or lime bath at 23 C until test
Test ages7 and 28 days, sometimes 56 for slow mixes
7 day rule of thumbabout 65 to 75 percent of the 28 day strength
A testthe average of two or three companion cylinders
Typical acceptance shapeaverages at or above fc, no single test more than 3.5 MPa below

How the test works

Sample from the middle of the load, cast the cylinders in rodded or vibrated layers, and protect them on site through the first day or two, because that early window sets the ceiling on everything after. Then standard curing until test day, ends ground or capped plane, and a controlled loading rate to failure in the press. The reported strength is the average of companions, and the pattern of breaks across a pour matters more than any single number.

Where it goes wrong

Most low breaks are curing stories, not concrete stories. Cylinders that spent their first night freezing on a tailgate or baking in a truck box will test low no matter what was in the drum, which is why insulated cure boxes earn their cost many times over. When a low break lands, investigate in order: the cylinders themselves, the curing record, the sampling, and only then the concrete. If it comes to the structure, cores are the referee, judged against the reduced thresholds the codes set for in place strength rather than the full fc.

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