Effects of combined aggregate gradation on the compression strength and workability of concrete using fineness modulus

D.C Ukala

doi:10.4314/jasem.v23i5.13

Abstract

Aggregate gradation (particle size distribution) is a very important part of concrete production hence the need to combine coarse aggregate with fine aggregate in its simplest form. An improperly graded aggregate structure can have undesirable effects on the properties of concrete as it can produce weak, stiff or porous concretes. In this research, the properties of concrete in terms of strength, slump and density were studied by varying aggregate grades. Proportions of 12.7mm, 25.4mm, and 38.1mm and 50.8mm sizes of granite as coarse aggregates were varied in order to create diverse coarse aggregate grading and then combined with a constant fine aggregate gradation and a fixed water/cement (w/c) ratio of 0.7. The results showed that as the coarse aggregate was spread evenly across all four aggregate sizes the strength was maximum as compared to when the aggregates were concentrated towards the 50.8mm size. The workability was seen to be stiffer as more coarse aggregate sizes were introduced into the mix. When the 50.8mm granite size represented the total coarse aggregate content (60%) of the concrete mix, the mix recorded a slump of 40mm. The workability declined slightly to slumps of 30mm, 20mm and 10mm when the coarse aggregate content was produced by combining granite sizes of 50.8mm and 38.1mm; 50.8mm, 38.1mm and 25.4mm and finally 50.8mm, 38.1mm, 25.4mm and 12.7mm respectively. This indicated that the more coarse aggregate content in the mix the less workable the concrete. Finally the concrete density remained almost constant irrespective of the aggregate grading.Keywords: Aggregate, combined aggregate gradation, Fineness Modulus

Highlights

Aggregate gradation is a very important part of concrete production the need to combine coarse aggregate with fine aggregate in its simplest form
This led research to focus on the effect of the aggregate size on the compressive strength of concrete (Walker & Bloem, 1960; Bloem & Gaynor, 1963; Cook, 1989; Zhou, Barr, & Lydon, 1995;) with results presented as though only one aggregate size was used for the experiment while disregarding the effect of the finer aggregates used in the mix
Most of the earlier works that focused on the effect of aggregate size on the compressive strength of concrete flawed in their methodology as they relied solely on the maximum size of aggregate to draw their conclusions disregarding the fine aggregate content which in itself modified the aggregate gradation of the concrete, a property that could have been responsible for the diverse conclusions reached

Summary

Introduction

Aggregate gradation (particle size distribution) is a very important part of concrete production the need to combine coarse aggregate with fine aggregate in its simplest form. Though it is believed that the denser the concrete the stronger it is, there is no empirical evidence to support that and this is shown typically in the 0.45 power curve where aggregate grades lying directly on the 0.45 most dense line are unworkable and harsh (Talbot and Richart 1923; Walsh, 1933; Besson 1935) and may require a lot of water in the mix To improve on this and limit the demand of water for the concrete mix, the fineness modulus was propounded as a method of representing the aggregate gradation with the mean size of all the aggregates in the mix (Abrams, 1918; Richardson 2005). It shows the distinction between the use of aggregate maximum size and fineness modulus in representing concrete having more than one aggregate size in it

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Results

Conclusion