CUSUM (cumulative sum control chart)
CUSUM (or cumulative sum control chart) is a sequential analysis statistical technique developed in the 1950s and widely used in the concrete industry.
Natural variations exist in the production of concrete due to variation in the raw materials (aggregate grading, moisture content etc), batching accuracy, plant performance, sampling and testing etc. Causes of variation, outside of the natural rannge, could be due to: changes in constituent materials being used; weigh-scales losing accuracy; problems with testing equipment or other influencing factors.
The most common application of control charts is as a means of continuously assessing compressive strength results in order to:
· check whether target strengths are being achieved;
· measure the variations from target (all products vary);
· identify magnitude of any variation;
· objectively define action required (e.g. change w/c ratio) to get the process back on target;
· identify periods and concretes where the strength was less than specified, so that investigations can be carried out and corrective action taken.
The use of control charts should not be treated in isolation from the rest of production control. For example routine maintenance and calibration of weigh equipment will minimise the risk of a weigh-scale failure.
For further information see:
Use of control charts in the production of concrete by Ian Gibb and Tom Harrison, October 2010 availabl;e from ERMCO / BRMCA.