Variation of concrete strength in existing buildings

Synopsis Ultrasonic-pulse-velocity measurements from 1145 concrete columns in 15 existing buildings were used as data to indicate variability of concrete strength. The factors considered were: differences between columns. location within column height. column size. storey location. and differences between buildings. Coefficients of variation of concrete strength are used to indicate control of quality: five buildings with values of less than 10% are regarded as having excellent control; five with from 10 to 15%. good control; three with from 15 to 20%. satisfactory control; and two with values of 26 and 30%, poor control. Isolated cases of very-low-strength concrete (below 7 N/mm 2 ) were found and verified by coring. It is suggested that the information provided illustrates the value of such measurements for inspection purposes and that the distribution of concrete strength provides for the statistical evaluation of strength of concrete columns.