Establishing Threshold Level for Gravel Inclusion in Concrete Production

The paper investigated the threshold level for gravel inclusion in concrete production. Concrete was produced using granite/gravel combination in varying percentages of 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, 20/80 and 10/90. Concrete made from 100 % granite and 100 % gravel served as controls while other constituents of the concrete were kept constant. Two different mix ratios of 1:2:4 and 1:3:6 were employed. Sieve analysis was carried out on the aggregates while slump and compaction factor tests were carried out on fresh concrete. Compressive and splitting tensile strength tests were performed on hardened concrete. Specimens were produced using 150 mm cubes and 150 mm × 300 mm cylinders for compressive and tensile strength tests respectively. The size analysis results show that the coefficient of uniformity for sand, gravel and granite are 2.29, 2.95 and 4.07 respectively an indication that the fine and coarse aggregates were well graded. The slump and compactive factor increased with increasing gravel content. Compressive strength tests showed that 60/40 and 70/30 percentage of granite/gravel combination with values of 21.15 N/mm2 and 15.17 N/mm2 respectively for mix ratios 1:2:4 and 1:3:6 at 28 days was quite satisfactory. The minimum requirement of 20 N/mm2 and 15 N/mm2 for 1:2:4 and 1:3:6 mix ratio respectively as specified by BS 8110:Part 1 (1997), EN 1992-1-1 (2004) were satisfied . The splitting tensile strength of 70/30 percentage of granite/gravel combination for 1:2:4 and 1:3:6 mix ratios were 10.50 N/mm and 4.70 N/mm2 respectively. The study concluded that the thresholds corresponding to 60 % and 70 % granite contents are suitable for 1:2:4 and 1:3:6 concrete mix proportions respectively. Keywords: Threshold level, granite, gravel, compressive strength and splitting tensile strength