DEVELOPING A CONCRETE MIX DESIGN FOR A SIMULATED AGGRESSIVE ACIDIC SEWER ENVIRONMENT USING RICE HUSK ASH AS AN ADMIXTURE

Abstract

When sewage flows through closed concrete pipes, sulphuric acid (H2SO4) generated from the sewage interacts with elements of concrete leading it to corrode. This leads to reduced durability and reduction of lifetime of sewer pipes and pumping stations. Interventions that have been used to mitigate this phenomenon have been found to result into high maintenance costs, thus expensive and unsustainable. The study aimed to develop a concrete mix design for use in acidic sewer environments using Rice Husks Ash (RHA) as an admixture with minimum water cement ratio. The required characteristic strength for such environment according to BS 8500 Part I is 35N/mm2. Two variables, namely percentages of RHA and water to cement ratio (W/C) were used to alter the properties concrete. The properties evaluated were workability, compressive strength, surface texture, mass loss and porosity. The two water cement ratios 0.4 and 0.35 were used with 0%, 5%, 10%, 15% and 20% RHA in cement to produce concrete. Slump test was carried on fresh concrete for all the categories. The cubes were exposed to acidic solution, which was used to simulate the aggressive acidic sewer environment. The effect the acidic solution had on surface roughness, mass loss, porosity and compressive strength of the cubes was then evaluated. The presence of RHA diminished the workability of concrete as the slump values reduced with increased presence of RHA in cement. Samples with 0.35 W/C were observed to have slightly more compressive strengths and also performed better in terms of surface texture, mass loss and porosity than those with 0.4 W/C. The highest compressive strengths, for the two water cement ratios, were achieved with 5% RHA in cement and thereafter the strengths reduced with increasing presence of RHA. The mass loss and porosities, for both water cement ratios, decreased with increasing presence of RHA in cement. As the various categories performed differently on the properties investigated, Analytic Hierarchy Process (AHP) was used to develop a scoring system to evaluate the performance of various percentages of RHA and thus determine the optimum % RHA replacement. The process determined a mix of 10% RHA and 0.35 W/C as the optimum with a compressive strength of 35.29N/mm2 slightly higher than the recommended characteristic strength by 0.8%. This translated to a mix proportion of cement, fine aggregates and coarse aggregates of 1: 0.98: 1.67, with 10% RHA as an admixture.