What irregularities are to be avoided in the test specimen?

Q. What irregularities are to be avoided in the test specimen?

A. Uneven ends, especially convex ends, ends not parallel, or ends not perpendicular to the axis of the cylinder should be avoided (ASTM C39). A convexity of 0.25 mm (0.01 in.) in the cap on the end reduces the strength about 25%. Ends out of parallel by more than 6 mm (0.25 in.) will reduce the strength even though a spherical bearing block is used. Incorrect assumptions about diameter result in incorrect end areas being used in calculations. Low strengths will result from physical disturbance of specimens after molding, as in the case where specimens before setting were shaken by a passing train. Inserting a foam rubber mattress between the plywood on which the cylinders rested and the ground solved the problem.

References: SP-1(02); ASTM C39

Topics in Concrete: Testing of Concrete