OPTIMISATION OF SELECTED POZZOLANIC CONCRETE MIXES FOR IMPROVED STRENGTH AND DURABILITY OF CONCRETE

Abstract

The use of pozzolans has been recently discovered to improve the particle packing which in turn improves the compressive strength, durability characteristics and other properties of Portland cement pastes, mortars and concretes. It also reduces the quantity of cement needed by the construction industry. This research work has been able to evaluate the effectiveness of groundnut shell ash (GSA), locust beans pod ash (LBPA), sugarcane bagasse ash (SBA) and bamboo leaf ash (BLA) in the reduction of Portland cement fraction in concrete through improvement in the particle packing. Their chemical analyses were carried out and the pozzolanic activity was studied with the aid of Minipal 4 Energy Dispersive X-Ray Fluorescence Spectrometer (EDXRF), an X-Ray Diffraction machine and scanning electron microscopic techniques. The results show that all the ashes belong to the group of pozzolans. These pozzolans were used for concrete production to test their effects on the particle packing as well as the compressive strength of concrete. A mix ratio of 1: 2: 4 (cement to fine aggregates to coarse aggregates) and 55mm slump were used and a total of 430 concrete cubes were cast. The cement was partially replaced at various percentage intervals from 1% to 25% for each of the pozzolans. All the cubes were cured in water by immersion for hydration period of 7, 14, 28 and 56 days. Average of three cubes was crushed for any % replacement. The slump, compaction, air entrainment and compressive strength tests were carried out on the concrete. The maximum compressive strength was obtained at 8% replacement of OPC with either of GSA, LBPA, SBA and BLA using a sieve size of 50μm. The results further revealed that at 12% replacement of OPC with either of the above pozzolans, the resulting compressive concrete strength is equal to the compressive strength of the control samples. Packing densities of some selected concrete constituents were determined alongside with their other properties.The effect of the various packing densities on the compressive strength of the different mix constituents was also determined. The results obtained from the Regression analysis of the data for 7 days, 14 days and 28 days using Microsoft Excel shows that compressive strength depends on packing density, workability and duration of curing. The models for the relationship are presented in the work. Also, the research work presented some new concrete mixes obtained by proportioning the aggregate particles of various sizes. This was carried out by widening the particle size range of aggregate particles and inclusion of powder content of size smaller than 50μm through the use of the selected pozzolans. The result shows that these new concrete mixes with better packing are less permeable and therefore more durable than the traditional mix. Further accelerated durability tests conducted on the pozzolanic concrete show that they are more durable in terms of chloride ion penetration and carbonation depth than the OPC concrete mix.