The effect of cyclic wetting and drying of brick masonry on the compressive strength of brick walls

Abstract

As a result of alternating wet and dry seasons experienced in Uganda throughout the year, masonry buildings, mostly in the low-lying areas like Bwaise, Katanga, Ndeeba among others are subjected to higher and more sustained moisture content levels due to flooding, along with experiencing an increased severity of wetting and drying cycles (WDC). This study presents the impact of such cyclic wetting and drying on the compressive strength of brick walls in the above-mentioned areas. The reported results are obtained from a series of tests carried out on clay, mud bricks and brick wallettes. Bricks used in the study include burnt clay bricks obtained from Nakawuka brick making site, Uganda Clays Kajjansi and burnt mud bricks obtained from Kiwuunya site in Kikoni. Compressive strength tests were carried out on the walling elements and panels, where six brick specimens from the commonly made clay and mud bricks and three factory made clay bricks were tested, to establish their strengths before exposure to WDC. A total of nineteen wallettes, comprising of eight mud, eight commonly made clay and three wallettes made of factory made clay bricks were constructed, and exposed to seven (7) days of curing. One Wallette from each category was then tested for compressive strength, to provide the reference data set for comparison with the strength of units subjected to WDC. The remaining Wallettes were then subjected to cyclic wetting and drying, testing was done after every single cycle (four days of wetting and four days of drying) for over (7) weeks. Bricks were also subjected to two days of wetting and two days of drying to make a single cycle, and testing was done every after two WDC. Water absorption tests were also carried out on the brick units to establish their response to the water absorption effect. Results revealed that the water absorption capacity for all the brick samples lay within the acceptable range, below 20% in accordance with BS EN 771-1, (2003) and 18% by mass according to ASTM C-67 (2000), but the water absorption values for both commonly made brick categories did not comply with US EAS 94: (2020), which gives a water absorption value of at most 10% by mass. Also, between the first and the 10th cycle, the compressive strength of the different brick categories reduced averagely by; 11% for mud bricks, 10% for commonly made clay bricks and 1% for Kajjansi factory made clay bricks. For brick wallettes subjected to similar cycles, the strength between the first and last cycle reduced averagely by 12% for mud wallettes, 8% for common clay wallettes and 3% for factory made clay brick wallettes. This shows that WDC reduce the compressive strength of masonry walls. However, the effect varies from one brick category to another. It is therefore prudent, that thorough inspection of the brick making process, and approval of bricks used in general construction by Uganda National Bureau of Standards (UNBS) is done, to ensure that they fit within the recommended set standards.