What is the water-to-cement ratio and why does it matter?

The water-to-cement ratio (w/c ratio) is the weight of water in a concrete mix divided by the weight of cement — and it is the single most important variable controlling concrete strength and durability. For NB exterior concrete, the maximum w/c ratio is 0.45 per CSA standards for freeze-thaw exposed concrete.

Here is the fundamental relationship: the less water relative to cement, the stronger and more durable the concrete. A 0.40 w/c ratio produces denser, stronger concrete than a 0.55 ratio. More water means more pores as the excess water that does not participate in hydration evaporates, leaving capillary voids behind. More pores mean more pathways for water, salt, and damaging ions to enter the concrete. In NB's freeze-thaw environment, this porosity directly determines how quickly the concrete surface deteriorates.

The CSA A23.1 standard specifies a maximum w/c ratio of 0.45 for Exposure Class C-1 — which covers all NB exterior concrete exposed to freeze-thaw cycles and de-icing chemicals. Many quality NB contractors push lower, to 0.40, for driveways and high-exposure surfaces. The 32 MPa (4,600 PSI) air-entrained mix typically ordered for NB driveways is formulated at or near this ratio.

The critical jobsite problem: every time a driver or worker adds water to the ready-mix truck at the site, the w/c ratio increases. Adding just 5 litres of water per cubic metre increases the w/c ratio by approximately 0.05 — enough to meaningfully reduce strength and dramatically increase porosity. This is the most common way that properly specified concrete becomes improperly placed concrete. The concrete leaves the plant at the right specification and arrives on-site correct, then someone adds water to make it easier to work, and durability drops.

If the concrete seems too stiff to work at the correct slump, the solutions are:

• Contact the batch plant to discuss plasticizer addition (achieves better workability without water addition)


• Accept a slightly lower slump — concrete at 75–100 mm slump is more difficult to finish but produces better durability


• Work faster with more crew

The w/c ratio also affects curing. Higher water content means more water is available for evaporation. On a hot, windy August day in Fredericton, a high-w/c mix loses surface moisture rapidly, increasing the risk of plastic shrinkage cracking. A properly low w/c ratio mix is less vulnerable to this.

For homeowners overseeing a concrete project, asking the contractor 'what w/c ratio is specified in the mix design?' is a legitimate quality question. Any experienced NB contractor should be able to answer immediately.