The dispersion of concrete compressive strength of existing buildings

The correct estimation of the compressive concrete strength plays a key role in the evaluation of the structural performance of existing RC buildings. Both Italian (NTC 2008) and European (EC8) Standards define different levels of knowledge according to the number of tests carried out on a building. They indicate a reduced value to assume in the analysis, defined as the mean value of the compressive strength, divided by a confidence factor. However, such a procedure completely neglects the dispersion of the test data, as represented by the high values of the coefficient of variation. Instead, this aspect is treated by FEMA 356 where a limit to the coefficient of variation was introduced. In this paper, with reference to a significant number of existing buildings located in Tuscany, the coefficient of variation (cov) of concrete strength is evaluated and the frequency of high cov values is determined. The dispersion of compressive strength, obtained by SonReb method, using correlation curves calibrated ad hoc on single building, shows that increasing the number of data for each building the coefficient of variation does not necessary decrease. Moreover, the strength value considered by EC8 in the analysis for a single building, i.e. the mean value of compressive strength, is often not conservative, while the approach provided by FEMA 356 is safer since it dependent on the cov itself.