Double-punch compressive resistance of Eastern Canada's low-strength mortars

This study provides the first experimental characterization of low-strength mortars sampled from pre-code unreinforced masonry (URM) buildings in Eastern Canada using the double punch test (DPT) and develops a unique dataset of compressive strengths of existing mortars found in the region. The structural assessment of existing unreinforced masonry (URM) structures requires the accurate in-situ characterization of the masonry components’ mechanical properties. This proves challenging in the case of mortar due to the difficulties in extracting specimens from existing buildings that meet the dimensional requirements of standard testing techniques such as CSA A179, ASTM C109, and EN 1015-11. As such, alternative testing methods, such as the double punch test, have been developed to estimate the mortar compressive strength using specimens sampled from existing mortar joints. This paper presents an experimental campaign that uses the DPT to assess the compressive strength of six low-strength mortars, five sampled from existing URM buildings located in Montreal, QC (Canada), and one mixed and cast in a controlled laboratory environment. This study first compares the outcomes of the DPT for the five old mortars by focusing on the relationship between DPT compressive strength and specimen thickness, density, mortar chemical composition, and original joint type. A parametric study is then performed using mortar specimens prepared in a controlled laboratory setting to further delve into the effect of specimen thickness on DPT results and compare mortar compressive strength predictions from the DPT, penetrometer test, and prism compression test. The results indicate that, for the tested old mortars, DPT compressive strength is inversely proportional to specimen thickness and marginally affected by changes in specimen density. The experiments further suggest that mortar specimens sampled from head and bed joints yield similar DPT compressive strengths. The parametric study then indicates that the DPT tends to predict higher mortar compressive strengths compared to estimates obtained with the penetrometer and prism compression tests. A key contribution of the work presented herein is the development of an experimental dataset that characterizes the DPT compressive strength of some low-strength mortars extracted from existing pre-code URM buildings located in Eastern Canada, thereby providing a first step towards characterizing the mechanical properties of such materials. Another novelty of this study involves comparing the mortar compressive strength estimates obtained with standard experiments (e.g., cube compression test) and alternative experimental methods (e.g., DPT, penetrometer test) for a low-strength laboratory-prepared Type O mortar used locally in masonry restoration works. • DPT is used to estimate the compressive strength of six low-strength mortars, including five mortars sampled from old URM buildings typical of Eastern Canada • Compare DPT results for mortars extracted from URM buildings with different end uses • Evaluate the effect of mortar specimen thickness, density, chemical composition, and original joint type on DPT results • Compare outcomes of the DPT with other test methods such as the penetrometer, prism compression, and cube compression tests