EFFECT OF CONSTITUENT MATERIALS ON THE QUALITY OF CONCRETE: A CASE OF MATERIALS IN KAKAMEGA COUNTY, KENYA

Abstract

The role of the quality of concrete in structural applications cannot be overemphasized. Quality of concrete in terms of workability, compressive strength and durability is influenced majorly by the properties and mix proportions of the individual constituent materials: cement, fine aggregates, coarse aggregates and water. The properties of aggregates are dependent on the parent rock formation which further depends on the geographical location resulting into a complex interaction in determination of quality of concrete. In order to address this complex interaction, there exist standards stipulating the required properties and the mix proportions of constituent materials for quality concrete. However, in Kenya, most building construction works are implemented based on materials which are sourced and used without characterization and/or modification. The practice leads to poor quality concrete in structural works and has been documented to be one of the reasons for building collapses. The aim of this research was therefore, to characterize the locally sourced concrete constituent materials and establish how they affect the quality of concrete. As a starting point, this research identified materials used for structural concrete in Kakamega County as the study area. The cement of different brands was sourced from local hardware stores while the fine aggregates were sourced from river beds, quarries and roadside. Furthermore, the coarse aggregates were sourced from two different quarries with significant spatial difference and difference in crushing methods: machine crushed and hand crushed. The materials were characterized to determine their properties. Each of the three-sourced fine aggregates were separately combined with the two-sourced coarse aggregates. Trial mixes were done using Portland Pozzolana Cement (PPC) and Ordinary Portland Cement (OPC) using mix designs that were developed based on the criteria outlined by British Research Establishment (BRE) for concrete classes C20/25 and C25/30 for OPC based concrete in conjunction with BS 8500 and BS EN 206-1 in order to accommodate the deficiencies of BRE in mix design of PPC. A total of 1008 concrete cubes were cast and cured under water for 7 days’ intervals up to 98 days as quality tests were being conducted. The results showed that aggregates vary in their characteristics. Coarse aggregates had values of 2.28 and 2.02 for specific gravity and 1.58% and 12.72% for water absorption respectively. The fine aggregates were all well graded. For RS, QD and FS the silt contents were 12.82%,5% and 33.33% respectively and specific gravity values of 2.57,2.51 and 2.35 respectively. Three out of the four cement brands had characteristics that were within the recommended standards. The findings also showed that the characteristics of aggregates affects the mix ratios with the calculated mix ratios differing from the empirical ratios applied in the field case of C20/25 for FSCA2 that was 1:1.8:2 vs 1:2:3. Furthermore, it was shown that a high silt content present in roadside sand (FS) led to high water absorption and reduced the compressive strength of concrete. In terms of cement type, it was found that OPC based concrete had a slightly higher strength than PPC based concretes but they both continue to gain strength beyond 28 days. The results also showed that PPC combined with machine crushed coarse aggregate and quarried fines or river sand can only be used to produce concrete class C20/25: higher concrete class is not feasible with the PPC. This is because it has a slower setting time hence the concrete may take more time before reaching its minimum characteristic strength. Further research on cylinder compressive strength and strength tests on aggregate should be carried out.