PRACTICAL APPLICATION OF MATURITY CONCEPT TO DETERMINE IN SITU STRENGTH OF CONCRETE

Strength of field-cured concrete has been estimated by correlating the maturity of strength, the maturity being a function of age and curing temperature. Additional standard cylinders are cast, and the maturity of inplace concrete is determined from thermocouples cast into the concrete pour or maturity meters installed during placing of the concrete. Standard cylinder strength and field maturity are then correlated, and in-place strength is determined by using constants for the concrete used. To check the validity of this approach, a series of strength tests on 4-in. (10-cm) cubes at various maturities were conducted in the laboratory. These sets of cubes were cast from concrete obtained from various job sites in Toronto. Seven and 28-day cylinder strengths predicted from low-maturity cylinder strengths of the same set were evaluated. The constants of the strength-maturity equation were determined to suite the local normal concrete and to compare the results with those of Plowman. A satisfactory correlation was obtained. The method was used to predict the in situ strength of concrete slabs during construction of buildings at the University of Waterloo in 1971 and 1972. The contractor could then strip the forms early and determine if there were any inadequate curing conditions during winter construction. Tests were made during construction to check the actual strengths of the in situ concrete and to compare them with predicted theoretical values. The results were satisfactory. The gain of lightweight concrete floor slabs of a 37-storied tower recently completed in Toronto was monitored to determine the earliest time when posttensioning operation of the slabs could be carried out safely.