A two stage procedure for assessment of in situ concrete strength using combined non-destructive testing

Test results of two different non-destructive test methods, the ultrasonic pulse velocity and Schmidt hammer, were used in conjunction with concrete mix proportions and density, to develop mathematical relationships using multiple linear regressions to obtain a first stage estimate of concrete compressive strength. These regressions covered both wet and air-dry concrete conditions. In each part of the work, a total number of 103 different mixes were tested at ages ranging between 7 - 90 days. Second stage (more accurate) concrete strength estimates are then obtained by linear correlation between the first estimates of concrete strength from the first part of the work and actual strengths obtained from limited number of cores cut from the structure itself This procedure was tested on two separate building cases and was found to yield good predicted values with standard errors of estimate of 2.95 MPa for a concrete with range of strength of (15.7 - 33.8) MPa for the first case study, and a standard error of only 0.91 MPa for a concrete with range of strength of (12.5 - 23) MPa for the second case study.