Why are many non-destructive tests used as “estimators” of concrete strength?

Q. Why are many non-destructive tests used as “estimators” of concrete strength?

A. No theoretical relationship exists between compressive strength and either rebound number, or penetration resistance. Empirically, it has been found that as concrete strength increases, the amount of rebound will increase and the amount of penetration will decrease. Either of these tests can be used to estimate compressive strength with a fair degree of accuracy, provided that the relationships with strength have been previously determined for the particular concrete in question. For concretes of equal compressive strength, the amount of rebound or the amount of penetration will be affected by the moisture content of the concrete, type and nominal maximum size of aggregate, age, and conditions contributing to hardness of the surface. Although the pullout strength and break-off strength are directly related to concrete strength, accepted theoretical relationships do not exist. Thus, pullout or the break-off tests also require pre-established empirical strength relationships. None of these tests is a substitute for tests of control cylinders or cores taken from the structure. ACI 228.1R provides guidance on the proper use of these indirect test methods to estimate in-place compressive strength.

References: SP-1(02); ACI 228.1R-19; ACI 228.2R-13; ASTM C803; ASTM C805; ASTM C900; ASTM C1150

Topics in Concrete: Nondestructive Testing; Testing of Concrete