Question: I've heard many times that changing the water-cement ratio in concrete has a large effect on its strength. The question is, why?

Answer: We posed the question to Cornell University's Ken Hover who also is a popular speaker each year at World of Concrete. He explains that the volume of water in fresh cement paste, and therefore fresh concrete, is related directly to the volume of empty pore space in hardened cement paste, and therefore hardened concrete. Similarly, the volume of cement in fresh concrete is related directly to the solid volume in hardened concrete. Water-cement ratio is therefore a measure of the void volume relative to the solid volume in hardened cement paste, and its strength goes up as the void volume goes down. So, the lower the w/c, the lower is the void volume/solid volume, and the stronger the hardened cement paste.