Chapter 6: Estimating the in situ strength of existing concrete structures

For the structural adequacy assessment of existing concrete structures, one of the most important tasks is to estimate f_ck of the existing concrete structures. A series of NDTs and core tests on the existing structure have to be carried out to estimate the in situ compressive strength of concrete. The in situ compressive strength, f_ck, of existing concrete structures can be estimated using a lower and upper bounds approach using a combination of the rebound hammer test or Windsor probe test and UPV test or core test. In the lower and upper bounds approach, a bias factor for the core test and each of the NDTs is needed to account for the overestimation of characteristic in situ compressive strength by rebound hammer test and Windsor probe test and underestimation of characteristic in situ compressive strength by UPV test and core test. Ranges of each of these bias factors have been derived from in situ concrete testing results of 34 existing concrete structures aged 4 to 73 years. Illustrative examples of using this lower and upper bounds approach to estimate f_ck of existing concrete structures are presented.