Concrete cube testing 1

[Ginger]

I have recently been testing 150mm x 150mm x 150mm concrete cubes with an upper surface sloping from front to back. The cubes have been loaded onto the crushing rig with the loading platten centered on the high front edge. The purpose of this is to encourage the leading edge to begin to crush so that the top of the leading edge flattens and the breadth of the loaded edge increases as the concrete continues to crush locally. This should continue until the front face fails through bursting or a limiting deflection of 5mm (measured as the platten moves down) is reached. However, when we measure the height of the front face after taking it out of the test rig we find that the cube has only crushed approx 1.5mm. It appears to have elastic recovery. Anyone else tried this test? Any ideas why the cube exhibits such a recovery?

 

In using this test procedure, you are assuming concrete has a significant plastic strain range. It does not. In compression, it truly acts as an elastic material, but you are likely to get multi-axial stress increases that will fail the sample prior to the onset of localized crushing, usually in a shear or shear-tension mode.

 

[Ginger]

rwoods<br>
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Thank you for your advice. I understand the suggestion that multi-axial stresses are generated in conventional cubes loaded uniformly on the top flat face and this results in the typically conical type failures seen in text books. To further test my model of concrete crushing would you suggest say a cube with a raised central ridge or prehaps a square based pyramid at the top face to reduce this multi-axial effect?<br>
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Regards Ginger