Compressive Strength Characteristics Of Concrete Based On Site Production Practices

ABSTRACT

Concrete is the most common construction material in the world today. It is used as a material in structural elements of most buildings of the world and most building failures including collapse are associated with failure of concrete either as produced or as assembled. The variations in strength of concrete is as a result of several factors that could be the varying quality of materials, the variation in the mix proportion and the varying production processes that include batching, mixing, compaction, and curing. This study was aimed at appraising the characteristic strength of concrete from some selected construction sites in Lagos Metropolis and Ota with a view to ascertaining its conformance with specified standards and also to assess the concrete production practices employed on these sites. The research sought to know if the applied concrete margins were adequate for the concrete practices in Nigeria or if they need to be modified for design strength. This was accomplished through assessment of concrete production practices on some selected sites, identifying the possible strength deviations of concrete produced on site in relation to the expected design strength. The study also evaluated the effects of mix proportioning, methods of batching, mixing, compaction, and curing on the concrete compressive strength, identified the most appropriate production methods and materials combinations that can be employed for acceptable strength performance in structural concreting work. A model was developed to predict the compressive strength of concrete made with different combinations of mix proportioning, batching, mixing, and compaction investigated. A total of 15 sites were visited with 300 concrete cube samples collected. Compressive strength test was performed on the collected samples. Using three mix ratios of 1:2:4, 1:3:6, 1:11 /2:3, 288 concrete cube specimen were produced simulating the different site conditions under a controlled environment in order to determine the effects of batching, mixing and compaction, and curing on the final compressive strength of the concrete specimen. The results showed that there was variation between the compressive strength obtained from the respective site and the design strength. It was observed that the production methods of mix ratio, batching, mixing, compaction, and curing had a significant effect on the compressive strength and the interaction of all the different production processes had a combined effect on the compressive strength. The result showed that though some of the sites appeared to have attained the specified strength, the expected margin was not attained. It was recommended that the mix ratio, the methods of batching, mixing, compacting, and curing the concrete should be specified by the engineer. Batching by volume, which is common practice on Nigerian construction sites, should be replaced with absolute volume which will consume less materials and produce proportions of aggregates that are close to proportions measured out by weight batching.