What does a high coefficient of variation in compressive strength test results usually indicate?

A high coefficient of variation (COV) in compressive strength test results typically indicates a lack of consistency and uniformity in the concrete production process. The coefficient of variation is a statistical measure of the dispersion of data points around the mean, expressed as a percentage. It is calculated by dividing the standard deviation by the mean. In the context of compressive strength testing, a high COV means that there is a wide range of strength values among the individual test specimens, relative to the average strength. This variability can be caused by several factors: inconsistencies in batching, mixing, placing, consolidating, or curing the concrete; variations in the properties of the constituent materials (cement, aggregates, admixtures); errors in testing procedures; or inadequate quality control measures. For example, if the target compressive strength is 4000 psi, and the COV is 15%, it suggests that the individual test results are scattered over a wider range (e.g., from 3400 psi to 4600 psi) compared to a situation with a lower COV (e.g., 5%), where the results would be clustered more closely around the mean. A high COV raises concerns about the reliability of the concrete and its ability to meet the specified performance requirements. It necessitates a thorough investigation to identify and correct the underlying causes of the variability.