DETERMINATION OF STRENGTH MARGIN FOR CONCRETE PRODUCED AT CONSTRUCTION SITES IN AKURE, ONDO STATE, NIGERIA

Abstract

This research was carried out to assess the quality of concrete produced at various sites in Akure, Nigeria with a view to determining the current margin of the concrete. For this purpose, Akure metropolis was mapped into seven (7) zones based on the magnitude of the building construction going on in the various zones. Building projects having minimum contract fee of two hundred million Naira (N200,000,000.00) were indentified. From each of the seven zones, one construction site was randomly selected as the research site. In each of the seven sites that emerged, standard concrete cubes of 150x150x150mm dimension were cast from mixes prepared for each of the major structural elements (columns, beams, and slabs). For each concrete batch, nine (9) concrete cubes altogether were cast on each day of the sampling, three of the cubes being cast at a time at intervals of 2 hours. This was done for 10 different concreting days, bringing the total number of cubes cast at each site to ninety. Since only the average compressive strength of every three samples of the cubes was recorded for further analysis, the number of compressive strength results available for further analysis was thirty (30) for each of the seven sites or two hundred and ten (210) cubes for all the seven sites. All the concrete cubes were cured, and at the standard maturity age of 28 days, they were crushed for compressive strength. The statistics of the mean strengths and standard deviations were computed for concrete produced at each site and for all the sites taken together. Based on a probability factor of 1.65, the standard deviations were used to compute the (current) strength margin for concrete produced at each site and then for all the sites considered together. The results of the investigation revealed the statistical parameters for concrete produced at various sites in Akure metropolis to be: characteristic strength - 13.9N/mm², standard deviation - 2.47N/mm², and (current) strength margin - 4.06N/mm². The implication is that the current practice of specifying 21 N/mm² by concrete designers for concrete with mix ratio 1:2:4 produced from normal granite aggregate is rather ambiguous under the current local concrete practice condition. For such specified value to be meaningful, site supervision must be firm, otherwise the value of the specified design characteristic strength, should have an upper limit of 15N/mm² for normal granite aggregate and mix ratio of 1:2:4.