Concrete Cube Testing
In order to prove the strength of a freshly mixed concrete cube samples are taken in either 100mm or 150mm cube moulds. A sample of the mix is taken in accordance with BS EN 12350-1 and a set of six cubes are made in accordance with BS EN 1881-108 or its replacement BS EN 12390-2. The cubes are then cured in a water bath for 28 days and crushed in a hydraulic press to determine the compressive strength. The cubes are filled in three layers with the wet concrete with each layer being compacted with a standard tamping bar and a minimum 35 strokes. The tops of the cube samples are floated off level with the top of the moulds using a polished float. Protected from the worst of the weather the samples are allowed to set overnight and are carefully removed from the moulds the following day. After the cubes have been allocated a unique number which is written on the top face along with the casting date the cubes are placed in a curing tank for 28 days at a temperature of 20o
The cubes are subsequently tested in a compression machine where they are subjected to increasing load until failure. The resultant crushing strength can then be compared to the design strength of the content. Cubes are sometimes crushed at 7 days in order to gain an indication of the 28 day strength as 7 day cubes generally achieve two thirds of the ultimate strength.
In some extreme cases many cubes are taken and the tests carried out every few days but as a general rule the 28 day strength is taken as the target. For some concrete not reaching the desired 28 day strength, the crushed cube remains are submitted for chemical testing to confirm the cement content and water/cement ratio.
Test cores can be drilled from hardened concrete enabling strength tests to be carried out on older or suspect material. These can be drilled vertically or horizontally and tested in a similar manner to cubes. Visual examination of concrete cores can be used to confirm the aggregate type and estimate the void content.
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