Indexed on: 01 Jun '95Published on: 01 Jun '95Published in: Materials and Structures
Compressive strength data representing concrete used to slipform the water tower shaft were evaluated. The evaluation was performed utilizing the computer model, the complete quality control report (CQC Report), which was set up on the basis of the project specification, and statistical methods and criteria given in ACI 214-77. A continuing record of the strength data was maintained and analysed for individual strength tests, cumulative average, moving average, average required strength, overall standard deviation, coefficient of variation, ranges of individual tests, moving range, within-test coefficient of variation, and the adequacy of control over concrete and testing operations. The test data were used for three main purposes: (a) to assist the batch plant in the production of uniform, high quality concrete for slipforming; (b) to determine the adequacy of testing procedures, and (c) towards establishing and refining the statistical relationships for future similar projects.An early indication of the quality of concrete, which is vital for continuous slipforming, was obtained by estimating the predicted 28-day strengths using 1-, 3- and 7-day data and the strength-maturity relationship developed in accordance with ASTM C 918. Based on the total weighted percentage average errors and the integral absolute error it appears that all three sets of early strentth data were approximately equal in their validity as prediction bases. Analyses of test results show that the same prediction equation which was used to determine the potential 28-day strength can also be used, in the reverse form, to specify the early strength criterion for quality control purposes, provided that the data are normally or near normally distributed.Using CQC Report it was possible to process and analyse the vast amount of data in a very short time, and to provide the slipform contractor with key information regarding the probable quality of the concrete as placed. This allowed the contractor to note immediately a descending trend in quality of the concrete, to make corrective adjustments in the manufacturing process and to continue concreting with a high degree of confidence.