Impact of Variability in Mechanical Properties of Local Aggregates on Concrete Strength

Abstract

Sedimentary rock is one of the commonest rock aggregates used in Nigeria for concrete production. Its properties vary significantly from one quarry site to the other due to differences in production technique and variability in physical properties of sedimentary rocks in general. The Acquisition of the skill of knowing a good aggregate by visual inspection becomes necessary for users of these aggregates. This report, therefore, demonstrated the relative influences of aggregate strength, porosity, grading and clay/silt contamination on the strength of concrete based on the experimental results from three aggregate samples obtained from three different quarry sites, namely Umunya, Umuaga and Nkwere-Ezunaka quarries. The samples from Umunya quarry had a very high percentage of clay/silt contamination of 37.5% and very poor grading and therefore produced concrete of lowest strengths. The samples from Nkwere-Ezunaka and Umuaga had very good grading, much lower silt/clay contaminations, lower porosity and good aggregate strength and therefore produced concrete of strengths much higher than that of Umunya sample. The presence of clay in all the samples affected the strengths of the concrete generally as a result of the fact that stress must be transferred from the mortar phase to the aggregate phase, it is only when the mortar phase is strong enough that the effects of the aggregate phase become apparent. For this reason, the strengths of concretes from the three samples were generally lower than expected. Stake holders were, therefore, advised to avoid aggregates with contaminations above 5 percent. Very strong aggregates with crystalline texture were also recommended. For all in aggregates, good grading was also recommended.