It is both erroneous and costly to work on the assumption that capping concrete cylinders is a panacea; that it will convert up and compensate for sloppy workmanship and carelessness in the making and handling of the cylinders. Good cylinders are a prerequisite to good capping. Among the essential requirements are good concrete to begin with, careful molding and rodding, proper curing, and above all ends that are flat, smooth, square with the sides, and parallel with each other.
Bad caps are a common cause of specimen failure. Air pockets and voids beneath the surface will cause the load to be applied to only a percentage of the total area instead of to all of it; the result is usually a shear break and low reading. Before a capped specimen is tested, put a straightedge across each end and if the cap is not absolutely flat, break it off and apply a new cap.