Concrete floors are often functional, but sometimes they even need a facelift. Fouling, spalling, cracking and mold can detract from the appearance and function of concrete. Sometimes the pour is poor and the concrete is uneven or slippery. If the floor is structurally sound but the finish is substandard, there are options other than breaking it up, tearing it up and starting from scratch. Consider installing a concrete overlay. You can apply concrete overlay with a 1/16-inch finish coat or even a concrete slab several inches thick over an existing floor.
We'll focus primarily on thinner overlays designed to trim countertops or floors.
When applying concrete overlay, you can usually use one of two tools. Standard floaters have oval ends and a flat or slightly rounded middle section. The float allows you to push and pull the concrete in thin layers as needed to spread it over a wide surface.
Another tool is more similar to a trapezoidal spatula. Some call it a "magic trowel" or a knockout knife. It has a flexible blade and requires some skill to use it.
The two tools are available in different finishes – this will determine which one you will use. Similar to using a trowel to apply a layer of stucco or mud, the magic trowel gives you a great way to apply a thin coat of concrete.
Ready to Apply Your Concrete Overlay
The first thing you need to do is clean the existing floor with a suitable cleaner. Apply with a push broom, then pressure wash the concrete. You'll then need to use the acid used for concrete resurfacing to contour the existing floor, or you can use diluted hydrochloric acid. Apply acid with a push broom, wear appropriate clothing, and wear respirator and eye protection. This is especially important when you plan to apply a thin concrete cover rather than a new slab.
You might also consider using a thickener like this one to reduce the porosity of your concrete. Thickeners prevent moisture from migrating through existing floors. Basically, a concrete thickener is a chemical that is applied to the surface of concrete to fill in the pores. This increases surface density.
Next, you'll need to touch up large flaking or scaling. These areas lack bulk material that needs to be filled. Make sure the existing floor is as flat and smooth as possible to create a better finished product. As with acid finishes, this is even more important when you are applying thin concrete overlays compared to new slabs.
Consider using adhesives. Pros debate this step, but the difference of opinion may lie in the smoothness or roughness of the concrete you're resurfacing. Rough surfaces generally do not require adhesives.
Pour and float concrete cover
Next comes the part you've been waiting for – pouring and applying a new concrete cover over your surface. This could be floors, sidewalks, or even concrete countertops. Thinner decorative overlays may contain resins or other additives that require a special primer (or even two coats of primer).
Use a float, trowel, or concrete broom to level wet concrete. As mentioned above, different effects can be achieved with different tools. It just depends on what works for you and what kind of look you want to achieve.
When applying overlays, it's important to get a nice, even coat. You'll also want to make sure the new coat gets into the corners and edges — completely covering the old surface. Otherwise, you risk ending your project with a very uneven look. We usually start at the edge and move back and forth inwards and towards the area behind you (assuming you are kneeling to work on a larger area). Not only does this help us make sure we don't get bogged down, but it often makes the most efficient use of the material as you get a good idea of how much extra mix might be needed to get the job done.
after specific settings
After the new concrete has set, you'll need to sand or scrape the floor until it's smooth. Use a diamond disc or concrete scraper to smooth the surface to desired smoothness, or prepare for a second coat if necessary. Most of the time this involves a floor sander or similar tool designed to remove any ridges and smooth the surface in preparation for any finish coats.
If you are sanding a product designed to be a multi-coat process, you will usually need to re-prime after sanding. Then go ahead and apply the last coat. When applying new concrete to repair a driveway, sidewalk, or yard, you'll often finish with a brush or other means to create a nice level surface.
Finally, you need to seal the new surface. Sealants help ensure that new surfaces will last for years to come.
We hope that by understanding how to apply concrete overlay, you can save yourself and your clients some time and effort. If you're a pro and have specific overlay tips, add them in the comments below – or connect with us on Facebook, Instagram or Twitter!