Recently I got the following question:
I have been making concrete counters for a while, but I always have used mixes with aggregate in them. Recently, I started using sand based mixes. The water/cement ratio always ended up being more than desirable. I watched your video on superplasticizers, and although I used the max amount of CounterFlo (by Fritz-Pak), the .34 to 1 water to cement ratio I used produced a really stiff mix. We ended up adding a lot more water and the mix was still fairly stiff.
Here’s the video:
Sand mixes are very versatile mixes, in fact most of the countertops I’ve made have been cast using an all-sand mix, even before I started using GFRC. (GFRC always uses all-sand mixes, except in the case when there is a face coat that contains decorative aggregate for aesthetic purposes.)
By their very nature sand mixes are best for stiffer, hand-packed finishes. They’re not the best choice when making a flowable mix due to the high surface area of the sand. Aggregate based mixes are best for flowable concrete.
With a good mix design that yields high early strengths it’s possible to hone the surface of an aggregate mix and not expose the aggregate. Usually it takes a great deal of effort to expose the aggregate, in large part due to the strength and flowability of the mix.
Getting an all sand mix to become flowable can be a challenge, in part because of the type of superplasticizer required but more so due to the fact that sand has much more surface area than gravel does. 1 lb of sand has a lot more particle surface to cover with a fixed amount of cement paste than 1 lb of gravel, so there’s less cement paste separating the sand grains. That means there’s less lubricant, so more friction. That’s why you’ve always needed to add more water to your sand mixes.
When you add water you’re increasing the cement paste volume, thus increasing the particle spacing and adding lubricant. Adding extra water isn’t always bad, provided you know what the w/c ratio is and that it’s below a tolerable level. I’d recommend staying below 0.4.
Fritz-Pak’s Counterflo is a good water reducer, but it’s a very weak one. You’ll have a very hard time making flowable conrete with it. It’s not meant to do that, it’s simply a mild water reducer. It’s only meant to reduce the amount of water needed, not radically change the mix characteristics.
Both my all-sand mix and my aggregate based mix are available in my Precast Mix Calculator.