A programme of research was carried out to examine the ability of selected near-to-surface tests to provide early, accurate measures of the 28-day in-situ strength of a range of cast concrete materials typical of those currently used in highway construction applications. Their ease of use, and ability to forecast the quality of cast materials on a broader scale than is currently provided by the 7-day cube test and core assessment procedures, was also included in the study. An analysis of the data has shown that each of the near-to-surface tests provides measures of early-life concrete strength broadly similar to those indicated by 3-day ambient-cured crushed cubes. Each of the tests is also sufficiently accurate to enable it to be used to indicate relatively large-scale variability in the 28-day strength of the material. For individual concretes, however, near-to-surface tests may predict strengths 30-40% greater than the in-situ strength indicated by crushing cores. Careful pre-test calibration could improve the accuracy of prediction association with each of the tests. From the point of view of their ease of use, and their output, the Schmidt Rebound Hammer appears to be the most suitable test to pursue. (A)
Abstract