Concrete Strength Variations of Unwashed and Processed Coarse Aggregate Sizes

Abstract

This project aims to determine the effect of different coarse aggregate sizes and type on the compressive strength of concrete. The most important property of concrete is its compressive strength. The exceeding strength of concrete is as a result of the presence of coarse aggregate. Normal concrete is being produced from different types of aggregate and size and this imparts different property to the resulting concrete. The research has established that the coarse aggregates and their sizes play critical roles in the development of adequate strength in concrete. Fine aggregate is normal sand obtained from a borrow pit. Preliminary laboratory investigation was conducted to ascertain the suitability of using the aggregates for construction work. Tests conducted include sieve analysis, bulk density, and specific gravity. From the graph of the sieve analysis for fine and coarse aggregate, the coefficient of uniformity calculated was 1.5 and 3.45 respectively. It was observed that with proper mixing, the slump test results did not witness shear or collapse type of slump rather there were true slump in all cases of the test. The result of the slump test for 12.5mm, 19.5mm, and 25mm coarse aggregate were 60, 100, and 90. The workability decreased with slight differences when the coarse aggregate size was increased. Three different sizes of coarse aggregates with 25mm maximum size for both processed (granite) and sand stone (Local stone) were employed in the investigation, namely; 12.5mm, 19.5mm and 25mm. The grading and relative densities of the aggregates were studied. The mix ratio and water / cement ratio adopted for the study was 1:2:4 and 0.5 respectively. Twelve concrete cubes (72 total) (150mm× 150mm×150mm) were cast for each coarse aggregate size and type of which three were crushed at each maturity age namely; 7, 14, 21 and 28 days. The 28 day strength of the concrete made with granite and sand stone of 12.5mm, 19.5mm 25mm were 26.1N/mm^2 , 25.1N/mm^2, 25.2N/mm^2 for processed (granite) and 19.12N/mm^2, 19.10N/mm^2 and 19.90N/mm^2 for sand stone respectively. Consequently, the result confirmed that using processed (granite) as coarse aggregate yield higher strength compared to local stone coarse aggregate.