An investigation of the water to cementitious ratio of fly ash concrete
Abstract
The water to cement ratio (W/C) is recognized as an imported variable in understanding and controlling the quality of concrete mixes. If powder siliceous additives, such as fly ash, is used in concrete as cement replacement or addition, then there is a ratio, either by mass or volume, between water and cement. In addition, there is a ratio between water and cement plus pozzolan, usually referred to as the water to cementitious ratio (W/CM). The issue of equivalency between W/C and W/CM is of major interest to concrete technologists who are increasingly being faced with stringent demands for better economy and durability of concrete. The primary objective of this thesis was to find a correct way to evaluate the contribution of fly ash to the strength of concrete as indicated by the W/CM ratio. A rational mathematical model for W/CM ratio of the form W/CM = W/(C + K $\cdot$ F.A.) is presented, where W, C, and F.A. are water, cement, and fly ash contents per unit volume of concrete. K is a new fly ash cementing efficiency factor and is defined as the ratio between the total contribution of fly ash to concrete compressive strength and the compressive strength of a counterpart portland cement concrete. This factor is based on comparing the compressive strength of two concretes having the same workability and amount of cement. The secondary objective of this work was to evaluate separately the strength contributions of fly ash due to its physical effects from those due to its chemical effects. An experimental program was carried out to achieve the above objectives. The variables considered in this research were: three types of powder additives, four concrete slump categories, three addition levels, and five ages of concrete. A total of 40 different concrete mixtures were prepared and the generated data were analyzed using appropriate statistical methods. To measure dependency of the introduced cementing efficiency factor on the judiciously chosen variables, an analysis of variance was performed on all generated K values. To identify factors affecting strength development characteristics of fly ash concrete, compressive strength and K results were plotted versus age. This is followed by an evaluation of data related to measuring the physical and chemical contributions of fly ash to concrete strength. Finally, to test if the proposed mathematical model for W/CM ratio is suitable for use as an equivalent Abrams' law, W/CM ratio (using K values obtained from this research) versus compressive strength curves were obtained and compared to these suggested by Abrams'.
This paper has been withdrawn.