Increasing the Cost-Effectiveness of Interlaboratory Studies and Routine Comparative Testing: A Practical Example Involving Masonry Bond Strength

Under the auspices of ASTM Committee C01 on Cement, an interlaboratory study was conducted to determine precision and bias for bond-wrench testing carried out under the requirements of ASTM Specification for Mortar Cement (C 1329-98) and ASTM Test Methods for Evaluating Masonry Bond Strength (C1357-98a). Another paper at this symposium [1] describes in detail that interlaboratory study and the results of the corresponding analysis for precision and bias. One objective of that analysis is to determine a “repeatability limit,” r, and a “reproducibility limit,” R. The repeatability limit quantifies what is commonly called the “within-lab variability,” whereas the reproducibility limit quantifies what is commonly called the “lab-to-lab variability.” The goal of this paper is to take the data from that interlaboratory study and analyze in more detail the possible sources of within-lab variability, including batch-to-batch, prism-to-prism, and bond-to-bond variability. This detailed analysis enables the rediction of variability for different test protocols (that is, with different numbers of batches, prisms and bonds). It also enables the prediction of how much time each test protocol would take to complete, which is directly related to the cost of testing. Finally, by comparing the probable variability and probable cost of each test protocol, it is possible to identify those test protocols that are most cost-effective for routine comparison of two mortars. This type of analysis is also useful in setting up other interlaboratory studies and interpreting their results.