Effects of combined aggregate gradation on the compression strength and workability of concrete using fineness modulus

Aggregate gradation (particle size distribution) is a very important part of concrete production hence the need to combine coarse aggregate with fine aggregate in its simplest form. An improperly graded aggregate structure can have undesirable effects on the properties of concrete as it can produce weak, stiff or porous concretes. In this research, the properties of concrete in terms of strength, slump and density were studied by varying aggregate grades. Proportions of 12.7mm, 25.4mm, and 38.1mm and 50.8mm sizes of granite as coarse aggregates were varied in order to create diverse coarse aggregate grading and then combined with a constant fine aggregate gradation and a fixed water/cement (w/c) ratio of 0.7. The results showed that as the coarse aggregate was spread evenly across all four aggregate sizes the strength was maximum as compared to when the aggregates were concentrated towards the 50.8mm size. The workability was seen to be stiffer as more coarse aggregate sizes were introduced into the mix. When the 50.8mm granite size represented the total coarse aggregate content (60%) of the concrete mix, the mix recorded a slump of 40mm. The workability declined slightly to slumps of 30mm, 20mm and 10mm when the coarse aggregate content was produced by combining granite sizes of 50.8mm and 38.1mm; 50.8mm, 38.1mm and 25.4mm and finally 50.8mm, 38.1mm, 25.4mm and 12.7mm respectively. This indicated that the more coarse aggregate content in the mix the less workable the concrete. Finally the concrete density remained almost constant irrespective of the aggregate grading.Keywords: Aggregate, combined aggregate gradation, Fineness Modulus