10.057 Water/Cement Ratio vs Strength

Q Many published articles relate how changing the water-cement ratio has a large effect on concrete strength. Is there a simple explanation for this effect?

— Technician in San Leandro, California

A 2/09 – Response prepared by Terry Egland, a principle at Testing Engineers, Inc., and a registered engineer in California.

In general, there exists a fundamental inverse relationship between porosity and strength of solids. This strength-porosity relationship is applicable to a wide range of materials, such as iron, stainless steel and granite. Think of examining a concrete core, which exhibits voids created by a lack of consolidation. You can imagine, why with a lack of internal structure, the compressive strength would be lower than expected. On a much smaller scale, there is a theoretical volume of water (based on curing conditions) required to hydrate a given volume of cement. Once you have added more than that amount, it creates capillary porosity (i.e. microscopic cavities or voids). The higher the water-cement ratio, the more porous and the weaker the strength. Generally, to maximize strength and durability, the water-cement ratio should be the lowest possible to hydrate the cement while maintaining its workability.