Relationship Between Compressive Strength of Different Shape and Thickness Specimens of Type S Mortar

Theodore James Moffett, Brigham Young University


Mortar is the cementitious binding material placed between masonry blocks to create a composite system. The American Society of Testing and Materials (ASTM), along with other organizations, have established the testing of prismatic mortar specimens for evaluating mechanical properties, like compressive strength. Mortar joints, however, possess vastly different characteristics compared to prismatic specimens, especially in terms of geometry and water content. These differences prevent a direct comparison of compressive strength between prismatic specimens and actual mortar joints in the assembly. The objective of this study was to analyze Type S mortar joints, with particular emphasis on specimen shape and water content, to draw accurate comparisons of compressive strength to ASTM prescribed mortar prismatic specimens.To examine the effect of water on mortar compressive strength, three different water contents were evaluated across nearly all testing series. Cubic (2-in) and cylindrical (2 by 4-in) mortar specimens were prepared and tested according to ASTM guidelines to verify the compressive strength relationship described by ASTM and to be used as comparative data. In addition, a small masonry wall was assembled and cured in a laboratory to simulate true properties of mortar joints. Mortar joints subjected to testing were a combination of thin slices cut from prismatic specimens as well as in-place bed joints cured between concrete masonry units (CMU). Two unorthodox test methods, the double punch test (DPT) and helix pull-out test (HPT), were selected as methods for assessing mortar joints. In addition, confining effects from neighboring material common to DPT were evaluated as a function of specimen face dimensions. The compressive strength ratio obtained through testing of 2-in cubic and 2 by 4-in cylindrical specimens was lower than ASTM recommendations. This may have been in part due to testing cylindrical specimens with rough surfaces and no capping material. DPT confinement on 2-in square and 2-in diameter circular specimens was found to be equivalent. Thinly sliced specimens tested in DPT showed increases in compressive strength as water content and specimen thickness decreased. As a whole, DPT results on thin mortar slices showed promise for accurate comparison to prismatic mortar specimens. In-place mortar joints tested in HPT showed moderate to high scatter. In addition, evaluation via HPT was determined to be more appropriate for qualitative rather than quantitative assessments of in-situ mortar.