Variations in the strength of Portland cement

Summary Data on the variation in compressive strength of ordinary Portland cement have been collected from tests carried out by a number of investigators. The tests were made on mortar cubes made and tested as described in B.S. 12: 1947 or on 4 in. concrete cubes made and tested as described in B.S. 1881:1952. The principal conclusions, based on tests at 3, 7 and 28 days, are as follows. (1) When several vibrated mortar cubes are made from a blended sample representing a single batch of cement, the variation in the compressive strengths obtained will be primarily due to the experimental errors arising from unknown variations occurring in the making and testing of the specimens. The standard deviation of the compressive strengths obtained from a single batch increased from 150 lb/in2 at 3 days to about 280 lb/in2 at 28 days. The coefficient of variation was approximately 4%. (2) The variation between batches of cement from any one works corresponds to a standard deviation generally greater than 400 lb/in2 ...