Quality characteristics of fine aggregates and controlling their effects on concrete

Abstract

For the aggregate producer, the concrete aggregates are end products, while, for the concrete manufacturer, the aggregates are raw materials to be used for mix designs and successful concrete production. The aim of this study was to identify which fine aggregate characteristics are important, and additionally to relate the extent of the effect that the aggregate has on the concrete as compared against the effect of the changes in mix design.

The testing programme contained six different mix designs in which 21 fine aggregate products were studied in 215 castings. The mix designs consisted of two cement amounts (300 or 350 kg/m3) corresponding to low and high paste volumes, with three admixture possibilities: no admixture, superplasticizer or air-entraining agent.

The results of the experiments show that the effect of the fine aggregate characteristics on the flow value was greater than the effect of the cement amount. Furthermore, the entrained and entrapped air amounts correlated strongly with the flow value of the concrete. The compressive strength for the same mix design differed according to the fine aggregate characteristics, and the difference ranged between 8.4...18.8 MPa on the six mix designs. The superplasticizer enhanced the quality differences of the aggregate products. The effect of the porosity of the fine aggregate on the drying shrinkage exceeded twice the effect of the mix design changes, i.e. amount of paste and/or the admixtures.

A program for predicting the interaction between the fine aggregate and concrete was developed. The Bayesian method was used for statistical processing, and non-parametric non-linear Gaussian process models were applied for the interaction models. The program includes four concrete properties; flow value, air %, bleeding and compressive strength.

The most important fine aggregate characteristics in the models are: shape and particle porosity related semi-coarse fraction parameters (flow value and air %), flakiness and Los Angeles value of the semi-coarse fraction (compressive strength) and particle porosity of the semi-coarse and surface area of the fines fractions (bleeding).

As an example of the model predictions, six different fine aggregate combinations were modelled; two gravel products representing past and future gravel types and four crushed rock products representing different strength qualities and shape characteristics.