Assessment of the in-situ concrete compressive strength in existing structures - First-hand hints from expert engineers

The assessment of the in-situ concrete compressive strength is a common task during projects concerning existing structures and during damage analysis. The standard approach given in DIN EN 13791 is quite disputed, since it might give unsafe results for existing structures [1, 2]. Alternatives are provided by DIN EN 1990 and classical statistics. To find out which approach gives the best results by means of safety and economic efficiency, engineers from our team conducted numeric simulations for different circumstances and evaluated the results according to DIN EN 13791, DIN EN 1990 and by classical statistic methods for the estimation of the 5 %-fractile with a confidence level of 75 %. Result was on the one hand, that the approach given in DIN EN 13791 has, as expected, considerable weaknesses, but beyond, that each of the approaches may be susceptible to errors. Hence they are generally not suitable for replacing the wits and the experience of an engineer. As a support for the engineer at work a reasonable procedure for the estimation of the in-situ concrete compressive strength is suggested, taking into account the results of the numeric simulation and containing hints from the longstanding experience of the authors to avoid typical mistakes.