Q.: In our area it seems like there is more and more pressure to use very low water-cement ratios, which our finishers say is making the concrete hard to finish. What is the lowest water-cement ratio where we can still expect good finishability?
A.: The water-cement ratio is only one variable in a concrete mix design. While it is an important indicator, it is not independent of other factors, such as total paste content and use of admixtures. Beyond the mix design, the equipment and techniques used to place and finish the concrete also play a role. And then there is the expertise of the finishing crew.
Each of the ingredients in the concrete, the proportions of those ingredients, and how the concrete is mixed, placed, and finished all affect the outcome. It’s this wonderful complexity that is at the heart of the National Ready-Mixed Concrete Association’s P2P (prescription to performance) initiative. NRMCA says that engineers and contractors should specify performance requirements, such as strength and finishability, and leave the mix design details to the producer. That way the producer will have the flexibility to adjust proportions of the various ingredients, and include admixtures as appropriate, to ensure that the concrete suits the application.
The keys to making this work are building good relationships with your ready-mix producer and then maintaining good communication. Help the producer understand your specific needs on each job. That way they can supply the right mix for each application, and they’ll begin to appreciate how well you know the material you are placing. And then you’ll also be able to discuss more than just water-cement ratios with them.