Effect of Water Cement Ratio Variation and Coarse Aggregate Gradation on the Compressive Strength of Cement Concrete

The importance of gradation of coarse aggregate has often been neglected in the rigid pavement construction process resulting to pavement failures even before the design life of the pavements. This paper therefore aimed at investigating the effect of gradation and water cement ratio variation on selected properties of cement concrete such as workability and compressive strength. Three different grades of coarse aggregate (5mm, 13.2mm and 19mm) were sourced from the same location and combined in equal proportions before the commencement of laboratory studies. The mixed coarse aggregate was sieved and separated into four categories; well graded (all sizes present) category, gap 1 graded (minus 5mm) category, gap 2 graded (minus 13.2mm) category and gap 3 (minus 19mm) graded category. A constant design mix of 1:2:4 with variable water cement ratios of 0.45, 0.50 and 0.55 was adopted and used for experimental purposes. The workability of fresh concrete was determined through the slump test procedure and the compressive strength was also determined for 7, 14 and 28 days hardened concrete specimens. Results of this study revealed that the workability of fresh concrete increases with increment in water cement ratio irrespective of the gradation status of coarse aggregate. However, this increment is small in well graded coarse aggregate concrete in comparison to the gap graded counterparts. The compressive strength of concrete was reduced as the water cement ratio increased with the strength reduction been uniform and steady for the well graded coarse aggregate concrete. From the ageing effect analysis, the well graded coarse aggregate concrete proved to be more durable as the strength increment remained positive. Thus, well graded coarse aggregate should be used in preference to gap graded coarse aggregate in cement concrete production