Contribution to the Concrete Strength versus WaterCement Ratio Relationship
by Sandor Popovics, Ph.D., P.E., F.ASCE, (Prof. Emeritus, Dept. of Civ., Arch., and Envir. Engrg., Drexel Univ., Philadelphia, PA 19104. Email: popovics@ece.Drexel.edu) and Janos Ujhelyi, Ph.D., (Consultant, Honorary Dozent, Tech. Univ., Budapest, Hungary. Email: UjhelyiJ@mcsz.hu)
Journal of Materials in Civil Engineering, Vol. 20, No. 7, July 2008, pp. 459463, (doi: http://dx.doi.org/10.1061/(ASCE)08991561(2008)20:7(459))
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Document type: 
Journal Paper 
Abstract: 
The first rarely mentioned fundamental assumptions for the strength versus watercement ratio relationship are discussed, namely, that: (a) the strength of structural concrete is controlled by the strength of the cement paste in it; (b) the strength of a cement paste depends strongly on the porosity in it; and (c) the porosity (capillary) is a function of the watercement ratio. This is the foundation of the relationship between concrete strength and watercement ratio. Numerous empirical formulas, socalled strength formulas, have been developed for this relationship; the Abrams’ formula for instance. These formulas estimate the concrete strength from the watercement ratio only, and they are usually simple but have restricted limits of validity. For improvement, a new type of strength formulas is offered in this paper, formulas that have a second independent variable beside the watercement ratio, such as the cement content, or water content, or paste content, etc. Such augmentation (a) improves the accuracy of the strength estimation; (b) shows that if two comparable concretes have the same watercement ratio, the concrete having the higher cement content has the lower strength; and (c) shows that the magnitude of the changes in concrete strength depends on how the watercement ratio is changed, by changing the cement content or the water content… Experimental data support these expectations. A formula is also presented for the effect of the other kind of porosity, the air content on the concrete strength. The combination of this with any good strength formula gives good fit with experimental results both of airentrained and nonairentrained concretes. 
