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The development of a cement strength uniformity requirement for the Illinois Department of Transportation's portland cement acceptance program is presented. The Department's historical test data indicate that while cement manufacturers have little difficulty producing cement meeting the minimum strength requirements of ASTM Specification for Portland Cement (C 150-85a), there is significant seasonal variation in strength for some manufacturers. Although cement strength variability is but one factor affecting concrete performance, it becomes of considerable practical importance in the design of economical concrete mixes. To determine the ability of manufacturers to produce cement of uniform strength, ASTM Method for Evaluation of Cement Strength Uniformity from a Single Source (C 917-82) test data was requested from each manufacturer furnishing cement to Illinois. Information on standard deviations, coefficients of variation, and 7- and 28-day average strengths is presented. The test information was studied, and a uniformity requirement was proposed wherein the manufacturer would submit 7- and 28-day strength levels at which he proposed to furnish cement, and control limits would be used to detect significant changes in the process average or variability. Information on standard deviations, coefficients of variation, and plots of average strengths are presented and compared to test information received before enforcement of the uniformity requirement. To verify test data reported by the manufacturer, split portions of selected ASTM C 917 samples are tested by the Department. Data obtained from these samples are also evaluated to determine differences in testing between the manufacturer and Department laboratories. The t-statistic is used to test whether the mean differences are significant. Data obtained are tabulated and presented. Results indicate that cement manufacturers are capable of furnishing cements which the user can be confident will vary in strength between known limits in the long run.
portland cement, compressive strength, standards, acceptance, variability, control limits
Cement technology engineer, Illinois Department of Transportation, Springfield, IL