By Joe Zingale, Flooring Group Specialist, CTS Cement Manufacturing Corp., Garden Grove, Calif.
Contractors often face inconsistent results on polished concrete overlays. When this problem starts creeping up on jobs, it’s important to look back and review the necessary steps to ensure consistency in each project.
Use the proper mix proportions. It’s true – the mix can make or break a project. Contractors need to be cognizant of how much water they are using and how much is needed. Getting a consistent mix means starting with the correct water amount, which will keep the aggregate suspended and near the top.
Many contractors also have one person in charge of both the water and mix station on a jobsite.. This person will also perform slump tests and is responsible for hitting the desired target. These slumps tests are especially needed when colorants are being added.
Limit the size of your work area. The production capacity of your mix station should align with the width of the pour. Most contractors mix in larger batches (4-5 bags) to ensure consistency. Working with larger batches combined with controlling the width of the pour is more efficient at keeping a proper wet edge. The wet material is placed on top of the wet material. Then a gauge rake is used perpendicular to the way the material is placed. Typically, a spike roller is used in the same direction the material was first placed. Since large batch mixing is being used more on these types of pours, rather than pumps, there is no reason not to limit the width of the placement, since this doesn’t add any additional mixing time to the jobsite.
In one example of a larger batch mix, a contractor would use two large batch mixers for a 3/8-inch placement. 2 large batch mixers can process about 80-100 bags/hour, which is enough material for about 1,200 – 1,600 square feet of floor. Most contractors own multiple batch mixers, sometimes even as many as five, so it is easy to pour more than one area. Or use additional batch mixers if the project requires a wider pour.
So, why are contractors pouring a 40-foot-wide area when a 20-foor-wide area will get more consistent results? The slab will need joints anyway, so the prudent contractor should mix and place closer to what its mix station can actually produce.
Watch the surrounding temperature. Elevated temperatures mean contractors have reduced working time with materials. This means they need to shorten the width of the pour to keep the wet edge. When pouring indoors, climate-controlled buildings can keep temperatures consistent. No fans should be blowing – no air movement – for a better result. Since self-leveling concrete dries from the top down, to avoid cracking from premature drying, it is essential that fans are not used.
If you want to polish, control the water in the mix. If you have too much water in the mix, the aggregate will sink, and it won’t be visible when grinding, thus producing an inconsistent floor. A polished concrete surface should show aggregate consistently.
The truth is in the grinding. On the first step of the grinding stage, a polishing contractor can tell if the mixing and placement was done correctly. If done correctly, less grinding time will be needed. If the aggregate isn’t uniformly distributed in the overlay material, the polishing contractor may have to do multiple passes with the initial grinding steps to make it look uniform.
A healthy polished concrete overlay is about paying attention to the recipe. Don’t let too much water ruin the mix or place material in wider strips than your mix station can produce. It’s all in the attention to the details.
Read here for more detailed information on installing polishable concrete overlays.
