Effect of bed joint mortar thicknesses on the flexural strength of non-reinforced burnt clay brick walls

Abstract

Mortar binds the masonry units together, it is specified in most standards that the mortar joints should be within 10-12mm, though this has not been followed in some cases. This research looked into determining the effect of bed joint mortar thicknesses on the flexural strength of non-reinforced burnt clay brick walls to improve on supervision considerations for Civil Engineers to always take note of mortar batching, placing and mortar thicknesses. This improves on quality and durability of masonry structures in District Local Governments (DLGs). This was achieved through; ascertaining the common masonry construction practices in DLGs considering Amudat, determining the effect of bed joint mortar thicknesses, mortar mix ratio and curing periods on masonry flexural strength with a greater output being suggestion of a safe range of mortar thicknesses for brick masonry works. The researcher thought of the idea after observing that the DLGs have always struggled with the challenge of substandard (shoddy) work in constructions, with masonry structures having bed joint mortar thicknesses beyond the recommended sizes; mostly from the local contractors who use cheap labor and unqualified masons yet occasionally, high speed winds occur amidst poor workmanship, causing cracks that appear at times so early even before handover of these masonry structures. This has led to collapse of some masonry buildings and wall fences. The research method employed was scientific. This involved actual measurement of mortar thicknesses used in the districts (considering Amudat), ascertaining mortar mix ratios and curing periods commonly used. Flexural strength tests on masonry wall panels was carried out at Makerere University materials’ laboratory while varying the mortar thicknesses, mortar mix ratios and curing periods. However, there was a challenge of inability to incorporate field variation of temperature and wind conditions in the laboratory experiment and difficulty of using boundary conditions in the laboratory setup. ABAQUS 6.13 (FEA software) was used to carry out some simulations for comparison and validation of laboratory work. The field and laboratory data was analyzed using SPSS to ascertain the effect of various mortar thicknesses on the flexural strength of the clay brick-walls. The commonly used bed joint mortar thicknesses for burnt clay brick masonry walls in DLGs are in the range of 46-55mm which is more than 4 to 5 times the recommended mortar thicknesses. According to the field observations, 50% of masonry walls in DLGs are built using mortar mix ratio of 1:6 (batched by volume) and the batching for this ratio is for a 50kg bag of cement to 4 non-standardized wheelbarrows of sand which makes the quantity of sand; one & half times the recommended volume and only 8.3% of the masonry structures used the right mortar mix ratio of 1:3. This is coupled with the fact that 54.2% of Masonry walls in DLGs are not cured during construction. The safe range of the bed joint mortar thickness suggested by the researcher for masonry work is 8-17mm and on average half inch. The research was done using burnt clay bricks, another researcher can build on the findings of this research using concrete blocks as masonry units. It is also prudent to have the same work carried out under actual field environment and using standard equipment for the masonry flexural test and more intensive work on FEM considering different scenarios.