Assessing the Effect of Different Mortar Mixes on Strength of Burnt Clay Brick Masonry Wall

Abstract

The population of Uganda grew from 12.6m in 1980 to 34.6m in 2014. This has been matched by an increase in housing demand. The estimated annual demand is 200,000 units per year. The National Housing and population census of 2014 showed that burnt clay bricks are the most commonly used walling material at 36.4% followed by mud and poles at 33.6%. However, there is no data on the strength of burnt clay brick walls and the relationship between strength of bricks and that of mortar that would inform design. A survey of 24 Structural Engineering consultancy firms was carried and it was found that none of these design masonry walls made from burnt clay bricks. A survey of 102 sites revealed that the batching method employed was a bag of cement to a number of wheelbarrows. Thus a mix of one bag of cement to three wheel barrows would be specified as 1:3, yet the average volume of a wheel barrow is about 0.089m 3 and the volume of a 50Kg bag of cement is about 0.036m 3. Tests revealed that the water absorption of the bricks varied from 9.23% to 16.68%. The strength of the bricks varied widely from 1.62N/mm 2 to 8.49N/mm 2. The source of sand used on most sites determined the source of sand used in making mortar cubes. The strength of mortar cubes varied from 17.8N/mm 2 at 1:3 to 1.87N/mm 2 at 1:14. Masonry walls of 1m wide by 1m high were constructed with mortar mixes to match those in the field and the walls were tested under compression. Tests revealed that mortar joints fail before bricks. The strength of the wall constructed varied from 1.85N/mm 2 at 1:7 mortar mix to 1.49N/mm 2 at 1:14 mortar mix. This was 69% to 77% of the designed wall strength. The failure of the mortar before bricks eases maintenance because mortar is more flexible and cracks in mortar tend to be smaller and easier to repair.