Whether you should think more in terms of w/c ratios or the total amount of water in concrete isn’t as clear as it is with testing slump. It’s probably best to be aware of both and how they change in relation to each other with different concrete mixes. Here are some thoughts about the relevance of each measure when you want concrete to have certain characteristics.
Shrinkage and curling. It’s important to have concrete for industrial and commercial floor construction that is resistant to shrinkage and curling over time. Reducing the amount of cementitious material and water is a primary way to achieve that goal. Concentrating on the total water in the mix is a good method.
Durability. Several things that can affect the durability of concrete, but water content is a central concern. Water not required for hydration occupies space in fresh concrete that later becomes a void when the concrete is hard. The voids reduce the strength properties of concrete. Specifying w/c ratios is a good way to address durability issues. Under ideal conditions, a w/c ratio of approximately 0.25 is all that’s needed for hydration so anything over that is considered “water of convenience.”
Flowable concrete. When concrete must consolidate well in forms or fill highly congested steel-reinforced beams and columns, SCC-type mixes are important. Designing these mixes with water measured by w/c ratios provides relevant information.
Compressive strength. Determining the amount of water that’s best for a specified mix can be evaluated by either w/c ratios or total water.
Finishability. The amount of fines, cementitious material, and water in concrete all play a part in how well it can be finished. If there isn’t enough water, concrete can become sticky and hard to finish. Contractors experienced with low-shrinkage mixes think about total water requirements for their concrete. When several floor mixes are compared to each other, w/c ratios and total cementitious content may be more helpful.
Closing thoughts
In the past, it has been said that 33 gallons of water was needed to make concrete. But today with changes in technology and mix designs, the old rules no longer apply. Mixes can have as little as 29 gallons (27 gallons with rounded gravel aggregates) or more than 33 gallons to produce good concrete for an application. We used to think that the placeability of a mix stood at one end of a continuum and strength at the other. But this isn’t necessarily the case.
Today, the best concrete results from the interaction between specifying engineers and contractors. Engineers should specify the qualities that are important, such as strength and durability, and contractors should work with their ready-mix producers to develop the mixes that will meet the specification. In terms of water, it’s important that all parties understand the ways to measure it and which way provides the most useful information for the job at hand.
— Jack Gibbons is central region manager of Concrete Reinforcing Steel Institute, Schaumburg, Ill.
