What is Stress Concentration in Compression Test of Concrete?

Concrete have compressive strength several times more than tensile strength and our purpose is to capitalize compressive strength. To check whether hardened concrete comply with specification, we frequently test concrete specimen after different period of curing.
While making specimen, we follow ASTM standard to have laboratory strength that best represent actual concrete cast in any job. While testing the contact between machine and specimen is platen of selected testing machine.
Testing concrete specimen cylinder for planeness

Thus top surface have provided a finishing as smooth as possible and usually float is used. The texture of contact surface of platen and top surface of specimen is never found same as concrete is not cast against machine plate.

Thus surface of specimen have roughness to some extent and never found plane which permits concentration of stress leading test result somewhat less than actual. The uneven surface may be concave or convex; different curvatures produce different result.
Convex top surface subjected to more stress than concave ones; thus convex end of specimen results less compressive strength than that of concave. Again this reduction is different for concrete specimen of different strength. The strength loss is found more in high strength concrete.
The high spots of concave or convex not only influence strength but also presence of sand grains or any other debris may results premature failure; in some cases, sudden splitting was also reported.
The debris usually comes from the residue of previous tests. Now we will discuss about ASTM requirements for surface preparation during casting.
The requirements of ASTM C617 ensure plane surface at the contact point of specimen:
-The top surface of specimen cylinder should have planeness of within 0.002 in which should be checked by Feeler gauge and straight edge.
-The axis of specimen cylinder with perpendicular to top surface should have angular difference within 0.50.
Again planeness of platens of machine should also be ensured, ASTM C39 provides us such specification. Handbook of the U.S. Army Corps Engineers for Concrete & Cement provide us a method for testing concrete specimen cylinder for parallelism and planeness; but though this procedures are not very complex, this test is only used for research purposes.
The reduction in test result for this reason can be overcome by following means:
  • Capping
  • Grinding
  • Packing
We will discuss about these techniques in our upcoming post.

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