In this article, I provide details about two important specialty admixtures for concrete countertop mixes: superplasticizer and viscosity modifier. These admixtures work hand in hand to create the right mix consistency without compromising strength by adding water.
Polycarboxylate superplasticizers are a very powerful type of high range water reducer. This video shows how adding a small amount of a powerful water reducer, instead of water, can take a concrete mix from dry and crumbly to flowable.
Here is a video that demonstrates the dramatic effect superplasticizer has on mix consistency, then explains the science behind how it works.
The video uses a powdered superplasticizer, BASF’s Melflux 2651, in order to demonstrate that it is not the liquid in the superplasticizer that causes the increase in slump. Melflux 2651 is very difficult to find in reasonable quantities for a concrete countertop maker.
Buddy Rhodes Concrete Products offers a similar powdered superplasticizer, WR310. However, powdered superplasticizers are extremely powerful and very easy to overdose. For that reason, we recommend a liquid superplasticizer, Buddy Rhodes WR420.
The following seminar excerpt explains what viscosity modifying admixtures do. VMAs are used to reduce the slump of concrete, essentially performing the opposite of a high range water reducer (superplasticizer). They also help prevent segregation in aggregate-based mixes.
Fritz-Pak’s Super Slump Buster is an easy to use, powdered viscosity modifying admixture (VMA). It is used to reduce the slump of concrete, essentially performing the opposite of a high range water reducer (superplasticizer). It also helps prevent segregation in aggregate-based mixes.
It is available in 8-oz bags, which is enough for dozens of average concrete countertop projects. VMA should not need to be used for every project, as explained below.
How to Use/Dosage
VMA is one of the few concrete ingredients that does not need to be precisely dosed by weight. If your mix is too fluid or showing signs of segregation, simply sprinkle about 1/8 teaspoon (about 1/2 gram) into your mix, mix for 30 seconds, and observe the viscosity. Add more very gradually if needed.
If the mix becomes too stiff, you can counteract the effects of the VMA with superplasticizer.
Addition of small amounts of VMA and superplasticizer will not affect strength, set time or color of the concrete. However, if you add too much of either, then strength, set time and particularly color can be affected.
A common mistake is adding too much superplasticizer, because it has to be mixed for 2-5 minutes, and then having to add a lot of VMA. That can be avoided by mixing for 2-5 minutes and then judging consistency and whether VMA is needed.