Determination of Appropriate Mix Proportions for the Kenyan Blended Portland Cement Concrete Production

Abstract

The Kenya’s vision 2030 seeks to address the rising needs of its population through infrastructure development. Reinforced concrete being the most commonly used construction material forms an integral part of this development strategy. The direct substitution of the ordinary Portland cements with the cheaper, lower strength, locally available blended Portland cements could be responsible for the production of poor quality concrete and contribute to the failure of several concrete buildings in the country. This paper presents findings of an experimental investigation on the appropriate mix proportions for the Kenyan blended Portland cement concrete. Key variables used in this study included the water/ cement ratio (x1), the cement/ total aggregates ratio (x2) and the fine aggregates/ coarse aggregates ratio (x3). The response was measured in terms of slump, compressive strengths at 7days, 14days and 28 days and density. Minitab 17 software was used in the design of experiments and results analysis based on Central Composite Design method. The investigation revealed that for a workable concrete with slump of ≥ 30mm, the appropriate mix ratios for the Kenyan blended Portland cement concrete are: 1:2.2:3.4 (w/c 0.6) for strength class C15 and 1:1.3:2.2 (w/c 0.5) for strength class C20. It was further noted that the different brands of blended Portland cement in the country had varying properties and thus produced concrete with different wet and hardened properties. None of the brands achieved the target design strength for strength class C25 and above. Therefore, the blended Portland cements may not be suitable for producing structural concrete strength class C 25 and above.