Effects of water-cement ratio on bond strength of concrete

Abstract

It is vital that reinforced concrete structures develop and maintain a high level of composite action to achieve the purpose for which it is designed. This involves transfer of loads or stresses between concrete and reinforcing steel otherwise known as bond. Studies have shown that bond strength is affected amongst other factors, by grades of composite materials. There is paucity of information on influence of water-to-cement (w/c) ratio on the bond strength of concrete. Water is often arbitrarily added to concrete mix to achieve a higher workability at the construction site, it is therefore pertinent to determine the effect of w/c on bond strength of concrete and this is the focus of this study. Sieve analysis was carried out on aggregates and for tests on concrete, cubes and cylinders with reinforcing bars inserted were cast with w/c ratios of 0.4, 0.45, 0.5, 0.55 and 0.6 using mix ratio of 1:2:4. Slump and compaction factor tests were performed on the fresh concrete to determine the workability followed by the determination of compressive strengths of the concrete cubes at 7, 14, 21 and 28th day while pull-out test on the hardened concrete cylinders was carried out at 28th day only. Sieve analysis results showed that fine and coarse aggregates were uniformly graded; w/c ratios of 0.4, 0.45 and 0.5 gave low (stiff mix) workability while those of 0.55 and 0.6 gave medium (plastic mix) workability for slump and compacting factor tests. The compressive strength and the pull-out (bond) strength in N/mm2 at 28 day were (23.24, 5.11), (20.31, 5.34), (18.22, 5.87), (16.76, 4.53) and (16.09, 3.81), respectively for w/c ratios of 0.4, 0.45, 0.5, 0.55 and 0.6. The results clearly showed that increase in w/c ratio increases workability and reduces compressive strength and bond strength of concrete within the range of w/c ratios considered.