WATER-CEMENT RATIO LAW AND SILICA FUME CONCRETE MIX DESIGN

摘要

New generation concretes almost invariably incorporate mineral admixtures for a variety of reasons such as improvement of strength, durability, economy and ecological factors. Out of the different types of admixtures used nowadays, inspite of the very high price of silica fume, its addition seems to be the only answer when extremely high strengths and durability are required. The Abrams law, which was originally formulated for conventional concrete containing cement as the only cementitious material, is not directly applicable to these new generation concretes. Abrams formulation of the water-cement ratio law in 1918 is still considered a milestone in the history of concrete technology. The concept of concrete making has undergone a paradigm shift since the formulation of Abrams law. Originally, concrete was made by mixing cement, aggregates and water and the use of admixtures was unknown. The only cementitious material was cement and the largest single factor that governed the strength of concrete was the water-cement ratio. The application of condensed silica fume as a mineral admixture in concrete is almost a routine nowadays for the production of tailor made High Performance Concretes. Hence the water-cementitious material ratio should be considered instead of the water-cement ratio and Abrams formulation needs to be modified. Apart from the water-cementitious material ratio, other important factors that govern the strength of concrete are total cementitious material content and the admixture-binder ratio. Based on extensive experimentation on silica fume concrete over a wide range of water-binder ratios and cement replacement percentages, modified relationships have been proposed in the present paper, relating the 28-day strength to a number of fundamental parameters apart from the water-binder ratio. These modified relationships might find useful application in proportioning concrete mixes incorporating silica fume.