9 - Quality control

Abstract

The variations in a manufacturing process have serious technical, quality and commercial implications to a company if not monitored and controlled in some manner. Controlling the quality of readymixed concrete is similar to any process control system. The nature of the raw materials used in the manufacture and the large number of factors, which affect the strength of readymixed concrete, makes it a highly variable product. The coefficient of variation is typically between 10 per cent and 20 per cent. To make easier the task of controlling such mixes requires simplification of the data. A number of systems exist, such as Shewhart charts and Cusum system of quality control. Shewhart charts are used by some suppliers in the United Kingdom. They can be difficult to interpret when changes in more than one property of the concrete has occurred, e.g. simultaneous changes in mean strength and standard deviation of the concrete. Such objections can be overcome with complex analysis of the data. The Cusum is a manually calculated control system developed for application to readymixed concrete. Further, the control of concrete by the various statistical methods described relies on a large number of results to ensure compliance. Simplified conformity assessment systems involve comparing averages from a group of results against defined values.