Graphical Construction Glossary >> Concrete and Concreting. >> Compression test
Compressive strength is a measure of the concrete's ability to resist loads which tend to crush it. It is measured in Megapascals (MPa) and the job specifications will state that the characteristic strength of concrete is say 25MPa measured at 28 days after mixing.
In certain cases the engineer may require a test to be done after seven days.
At the same time that the slump test has been done the same person from an approved testing laboritary takes samples that will later be used back at the laboritary for compression tests.
For routine sampling he takes three samples spaced throut the load.
The concrete is placed in these steel testing cylinders, compacted and then left on site in an out of the way spot for it to set. If you ever come across cylinders like this do not move them if the concrete has not set. Careless handling can upset test results.
The cylinders are taken to the lab the next day and stored in moist conditions to cure the concrete.
Just as an aside I have seen a few of the concrete cyinders tested (and quite a few hundreds of them later used as garden bed edgings) and it is not the shattering "myth busters" type happening that you would expect. Placed under a hydraulic ram the concrete cylinder (stripped from the metal casing,) has slowly increasing pressure applied to it. All of a sudden the dial flips backwards and that's it. Loss of pressure denotes the miniscule amount of movement that shows failure. Rarely do the cylinders of concrete split or have obvious cracks.
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