Comparison of outlier detection approaches for compressive strength of cement-based mortars

Abstract

EN 196-1 states that the compressive strength result of testing of standard mortar specimens corresponds to the average of the six individual measurements. If one of the six individual measurements varies more than +/- 10 % from the average, it must be excluded, and a new average is calculated with the five remaining values. This methodology might be interesting for quality control or characterization of cement; however, it may not be the best approach for research or scientific purposes. Applying EN 196-1 to non-standard cement-based mortars with multiple and even unconventional supplementary cementitious materials may induce even more variation in the result. Also, mixtures with higher mechanical strength generally present the highest absolute variation. The main goal of the present work is to compare different methodologies, such as Tails of Normal Distribution, Thompson-Tau, Box-plot, T-test, Dixon Criteria, and other methods provided by engineering standards, to evaluate the existence of discrepant values and the effect of their exclusion (or not) on compressive strength experimental values, namely for science purposes. A sample of compressive strength of 338 mortar specimens was used The main findings revealed that excluding data points as a result of applying 8 of the 13 methods under study did not affect the mechanical strength result by more than +/- 5 %. The method described in the Brazilian standard that guides preparation, control, receipt, and acceptance for Portland cement concrete, would invalidate almost 46 % of all mixtures produced and did not prove to be reliable. Simplified methods of excluding outliers in relative or absolute percentages of 6 % around the average value can be considered an interesting option for professionals involved in laboratory testing of building materials. Otherwise, the authors would recommend applying the either Dixon Criteria or T test of the ASTM E178 for scientific purposes.