This article focuses on the practical, rather than the theoretical, aspects of statistical process control (SPC) and how statistics and SPC can improve the manufacturing process of building materials. At Granite Rock Co., Watsonville, Calif., SPC controls and improves production processes for aggregate, asphalt, and ready mixed concrete. In 1990, Granite Rock began using SPC to improve its ready mix batching accuracy. In two years, batching accuracy improved 300% to levels far exceeding state and federal specifications.
The SPC reports are straightforward and easy to understand. Actual weight data is collected fo 30 consecutive concrete loads for a single mix design. The data are placed into a computerized spreadsheet program where it is analyzed, comparing actual performance against mix design targets. The data are graphed allowing a visual evaluation of batching variability. Besides graphics, each report includes a simple summary sheet that lists SPC report results for the past year.