Compressive strength - concrete

[Dell]

in compressive tests of concrete blocks in a lab, there must be maximum contact between the press and the concrete, i am told that if you place a piece of waxed paper between the press and the concrete, or even if one of the surfaces is slightly wet, your results will be very distorted and even the way the block crumbles will be different, producing cracks all the wal around the block parallel to the force applied,
can anyone explain this phenomenon, i am told it has something to do with poissons ratio

 

[Valkarie]

The phenomenon you are describing is called Poisson's ratio. Poisson's ratio is a measure of the ratio of transverse strain to axial strain in a material when it is subjected to an applied force. When a material is placed between two surfaces, like a waxed paper and a concrete block, the material (waxed paper) will compress along the surfaces as it is being pressed together. This compression causes the strain to be applied in a direction perpendicular to the force; thus, the transverse strain is greater than the axial strain. This results in the block cracking in a pattern parallel to the force. The same phenomenon happens if one of the surfaces is slightly wet, since the water acts as a lubricant and allows for more compression of the material between the surfaces.