DETERMINATION OF MIX PROPORTION OF HARDENED CONCRETE CONTAINING FLY ASH OR LIMESTONE POWDER

Abstract

Nowadays, several techniques have been developed to identify mix proportion of hardened concrete. However, there is no universally applicable method for accurately determining mix proportion of hardened concrete because of the wide variety of materials from which concrete is made and the conditions to which it may have been subjected. The analysis is more complicated if fly ash or other admixtures are used in the concrete. Moreover, a promising method developed to identify the concrete mix design encompassing binder type, w/b, binder content and aggregate content does not exist. Even though standards such as ASTM C1084 and BS 1881: Part 124 described the methods for determining the cement content and other concrete compositions in the hardened concrete, they are applicable only for OPC concrete but not appropriate for concrete containing fly ash or limestone powder. In this study, a method for estimating mix proportion of hardened OPC concrete, fly ash concrete, and limestone powder concrete is proposed. Both physical and chemical methods were adopted for the analysis. The varying properties of fly ashes and limestone powders generally used in Thailand were taken into consideration. The degree of hydration of cement and the degree of pozzolanic reaction of fly ash were also taken into account in the analysis in order to cover the effect of age of concrete. The analytical procedures for evaluating fly ash and limestone powder existence and determination of water to binder ratio, fly ash replacement ratio, limestone powder replacement ratio and content of concrete compositions are described. In the analysis, three major investigation techniques are implemented. Image processing technique is used for determination of coarse aggregate content. Water to binder ratio can be calculated from data back analysis of the compressive strength by using the computer software, “FACOMP”. Cement, fly ash, limestone powder and sand contents are obtained from a set of mass balance equations. The proposed equations were applied to estimate mix proportions of concrete prepared in the laboratory with known mix design in order to verify efficiency of the proposed method. From the analytical results, it was found that the proposed method could be used to precisely estimate mix proportion of concrete with different types and replacement ratios of binder, different water to binder ratios, and different aggregate contents, at any age of concrete. The effect of age of concrete on determination of mix proportion of hardened concrete can be covered by taking into account the time-dependent degree of hydration in the analysis.