Low-strength concrete properties in existing structures using NDT and core test results

Appropriate decisions about the assessment of building safety and the degree of structural strengthening depend heavily on conducting an accurate structural assessment study of existing old concrete buildings. The compressive strength and modulus of elasticity of concrete are two major parameters to be determined quickly and accurately when assessing the structural per-formance of existing concrete buildings. One of the main factors contributing to the severe damage or collapse of existing concrete structures during earthquakes is low concrete strength, specified as fc & PRIME; <17 MPa = 2.5ksi, in this study, which is mainly caused by using poor quality building materials and poor construction practices. Modern building standards, however, are almost silent for evaluation of the low-strength concrete parameters, particularly the in-place compressive strength and the static modulus of elasticity. This study provides recommenda-tions to practicing engineers on how to estimate in-place strength and modulus of elasticity for the low-strength concrete. First, it is suggested that additional relevant drilling damage factors from published literature be taken into account when evaluating the low-strength concrete core test results in compliance with ACI 214.4R [1]. Second, based on the outcomes of both non-destructive and compressive strength tests, the SonReb regression method is suggested to combine the findings of core tests and non-destructive testing (UPV and R). Then, a methodology is suggested for predicting the static modulus of elasticity based on the non-destructive and compressive strength tests. It is also suggested that the static modulus of elasticity equation in ACI 318 M [34] for normal-strength concrete be modified to include the low-strength concrete characteristics. For a better understanding of the technique and to enable the discussion of the ideas, real case studies with numerical examples are provided.