The use of NDT to evaluate maturity for taking off formwork under hot curing

Abstract

The maturity concept assumes that the properties of concrete are directly related to its age and its previous temperature history. Irrespective of curing history, engineers use the maturity concept to stop curing or take off (remove) formwork of site concrete once strength is reached. Previous research concentrated on the use of maturity concept for strength evaluation but neglected other concrete properties. Although the maturity method is good indication for the strength of concrete, it may not be good indication for the durability and internal properties. The research investigates the applicability of the maturity of concrete to attain a reasonable level of both the strength, durability and internal properties of concrete, and hence recommends, for the first time, threshold limits for effective curing periods that attain both strength and durability. For this purpose, concrete samples were prepared and tested in the lab at three different temperatures, 20℃(68°F), 40℃(104°F) and 65℃(149°F). Several destructive and nondestructive tests have been implemented for this purpose. The tests included strength, pulse velocity, density, voids ratio, sorptivity and conductivity. Results show that if maturity-strength only is used as a threshold to stop curing, then durability and internal properties of concrete may not be attained. This is obvious for samples cured at high temperatures (>20 °C). Also, curing history is an important factor for achieving the required age to stop curing. Hence, further curing is necessary under hot weather conditions. Minimum curing periods to attain both strength and durability are suggested. The use of NDT is quite helpful in this respect. The results show that to attain the same durability (conductivity), a minimum maturity of 600℃.days is required irrespective of the strength attained. Higher maturities are required to attain other properties depending on the curing temperature. In practical applications, these findings are important for concrete under hot weather and steam curing.