Water-cement ratio
The water-cement ratio (typically 0.45-0.65) is the key lever on concrete strength and durability: lower is stronger and less permeable but harder to place.
Ask Chalkline about this →The water-cement ratio (w/c) is the ratio of the mass of water to the mass of cement in a concrete mix, typically in the range 0.45 to 0.65. It is the single biggest lever on concrete strength and durability: a lower water-cement ratio gives stronger, less permeable concrete but a stiffer mix that is harder to place, while a higher ratio is easier to work but weaker and more porous.
Permeability is the durability half of the story. A low water-cement ratio produces a denser paste that resists the ingress of water, chlorides, and CO₂, which is why high-exposure elements (marine, aggressive soils) call for a tightly controlled w/c. The classic site failure is raising the ratio after the mix leaves the plant: adding water to the truck to “loosen” it, or pouring into a trench with standing water, both raise the water-cement ratio, drop the strength, and void the supplier’s guarantee. If the mix is too stiff to place, the fix is a superplasticiser (a water reducer) at the plant, or a controlled dose with the supplier’s written agreement, not a hose on site.
Dewater trenches before the pour, do not add water on site, and let the slump and any admixture be set by the supplier, not the gang on the day. The water-cement ratio you order is part of what delivers the characteristic strength and the grade. See concrete grades.
Also known as: w/c ratio, water-to-cement ratio, water-binder ratio.
Category: Materials / Concrete.
Related
See also
References
- AS 1379-2007 Specification and supply of concrete, Standards Australia (verified 2026-05-13)
- ABCB NCC 2022 Housing Provisions Part 4.2 Footings and slabs (verified 2026-05-13)
Last updated: 2026-05-30. Verified: 2026-05-13. Quarterly review for currency.