Why is the Amount of Water so Important for Concrete?

Mastour ReadyMix is a Ready mix concrete and cement products company Jeddah, Khamis Mushyat, Dammam, Abaha Kingdom of Saudi Arabia

An important item to understand in concrete work is the water-cement ratio. A minimum amount of water, approximately 25% of the weight of the cement, must be included to chemically hydrate the concrete batch. In the actual mixing process, though, it takes about 35% to 40% water to work through the mixing process, get to the actual cement, and cause effective hydration.

In practice, though, much more water gets added to increase the workability of the concrete. So why does it matter if there is lots of water in the concrete mix? Any water above the theoretical ideal of 25% doesn’t get used in the chemical hydration process. Therefore, the excess water remains in the concrete while the concrete cures. Over time, this excess water evaporates out of the concrete and voids remain. These voids weaken the concrete, causing less strength and more cracking.

The water-cement ratio matters to the engineer, but why does the Construction Supervisor care? Anyone who has placed concrete knows how much easier a flowing, more liquid concrete is to place than a drier concrete. There is a tendency to add water to the mix, as it is ready to be placed, to make the concrete flow better. In fact, if the concrete doesn’t flow well, it may not properly surround the rebar (causing a poor bond with the rebar) or it may not flow properly against the forms (causing voids and areas needing patching). Insert photo.

So, a conflict often exists on the jobsite:

1. Add water to the concrete mix to make it flow better, but weaken the quality of the concrete (both strength and crack resistance)

or

2. Don’t add water to the concrete mix to keep the proper water-cement ratio but work harder to place the concrete and possibly have significant voids.

The easy answer is never add water on the site to concrete, but that answer ignores the reality of the placement dilemma. This is often a complicated decision, with Structural Engineers, Building Officials, Specifications, Concrete Foreman and others all having input. It’s important the Construction Supervisor at least be aware of this issue for every concrete placement and understand how the decision to add water will be handled.