On the calibration dependencies designing for concrete strength testing by sclerometric nondestructive metods

Abstract

Sclerometric methods of testing concrete, in particular, the rebound hammer testing, retain their positions as the most common indirect methods for concrete strength determining in situ. They are used as a means for technological testing during the buildings construction, as well as during their inspection during maintenance. Indirect parameters of current sclerometric devices are usually point-values: the rebound index, the diameter and depth of the print, the duration of the indenter insertion process, and the maximum interaction force. However, the lack of an unambiguous relationship between these parameters and the strength of concrete leads to additional uncertainty of the test result, which is one of the problems of sclerometric methods of concrete control. The instability of the calibration dependencies of the method is also observed. Even strict compliance with the requirements of the procedure does not guarantee the repeatability of the obtained dependencies.The article considers the possibility of improving metrological indicators of calibration dependencies of sclerometric methods by introducing an additional parameter-concrete humidity. The possibility of combining two sclerometric methods – the elastic rebound method and the shock pulse method is also considered. Data were obtained on concrete samples with a curing time of 25..60 days. Two-parameter calibration dependencies, including concrete humidity as an additional parameter, allowed for all experimental series of samples to reduce the residual mean square deviation of the calibration dependence of elastic rebound and shock pulse methods by 10%..16%, and to consistently provide the minimum acceptable value of the correlation coefficient r = 0.7 for the studied concrete compositions. The influence of concrete heterogeneity on the readings of sclerometric test methods is considered.The developed method of constructing two-parameter calibration dependencies can be recommended for inclusion in the current standards governing the determination of the strength of concrete structures under construction.