Effect of Coarse Aggregate Gradation on Workability and Flexural Strength of Cement Concrete

The construction of rigid pavements and other concrete structures in Nigeria without consideration of the gradation status of the coarse aggregate used served as a prerequisite for this study. This project thus investigated the effect of gradation of coarse aggregate on the workability and flexural strength of cement concrete. Coarse aggregates of nominal size of 5mm, 13mm and 19mm were sourced, mixed together and subjected to sieve analysis. The coarse aggregate were then grouped into well graded (consisting of all three aggregate sizes), gap 1 graded (minus 5 mm), gap 2 graded (minus 13mm) and gap 3 graded (minus 19mm) coarse aggregates. Fresh concrete test in form of workability tests was carried out on all concrete specimens and flexural strength test was performed for all hardened concrete specimens. The concrete specimens were cured for 7, 14, 21 and 28 days. From results of experiments, the workability of fresh concrete specimens increases with poor gradation pattern. The workability of well graded aggregate concrete was small in comparison to the other gradation patterns (gap 1, gap 2 and gap 3) and for the gap graded aggregates, the workability increased with removal of the larger aggregate size. For the flexural strength, the gap 1 graded aggregate concrete recorded the highest early strength due to presence of larger aggregate sizes. As the concrete ages, the percentage strength increment was noticed to be higher for the well graded aggregate concrete. That is, the well graded aggregates, gained more strength increase as the concrete ages. This indicates that the well graded aggregates will produce a more durable concrete in comparison to the gap graded aggregate concrete. The use of well graded aggregates in cement concrete construction should be encouraged as less binder content will be required and a more durable concrete produced.