Date of Award
Spring 1993
Document Type
Dissertation - Restricted
Degree Name
Doctor of Philosophy (PhD)
Department
Civil, Construction, and Environmental Engineering
First Advisor
Wenzel, Thomas H.
Second Advisor
Kueummell, David
Third Advisor
Naik, Tarun R.
Abstract
The water to cement ratio (W/C) is recognized as an important variable in understanding and controlling the quality of concrete mixes. If powder siliceous additives, such as fly ash, are 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 materials 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, along with improved strength and durability of concrete. The primary objective of this study was to determine a correct way to evaluate the contribution of fly ash to concrete strength as indicated by the W/CM ratio. A rational mathematical model for W/CM ratio of the form W/CM = W/(C + K * F.A.) is presented, where W, C and F.A. are water, cement and fly ash contents per unit volume of concrete, respectively. 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 port land 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 properties from those due to its chemical characteristics. Understanding how the fly ash influences concrete strength is very important in optimizing it's use in normal-strength and high-strength concretes. 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. Results from this work showed that the relationship between the proposed W/CM ratio model and fly ash concrete strength can be approximated by an exponential formula similar to that of Abrams. Also, it was proved that both Class C and Class F fly ashes contribute physically as well as chemically to concrete strength. Chemical contributions of fly ash should not be underestimated. Furthermore, this research revealed the importance of recognizing three basic criteria for proportioning fly ash concrete, namely; low water content, use of superplasticizers and early and long term moist curing.