What is the water-cement ratio and why is it important?

The water-cement ratio (w/c) is the mass of water divided by the mass of cement in a concrete mix. It directly affects concrete strength - lower w/c ratios produce stronger concrete (typically 0.35-0.45 for structural concrete). It also affects durability, permeability, and workability. Minimum water for hydration is about 0.25 w/c, but more is added for workability. The relationship between w/c ratio and strength follows Abram's law.