When pouring a concrete slab, air or water can get trapped in the substrate during the finishing process, causing voids. If left untreated, voids or hollow spots lead to delamination, where a thin surface layer of concrete detaches from the slab and chips away. While time often reveals these hollow spots, they can also cause serious problems when trying to grind or polish concrete or lay down an epoxy or aspartic coating. Before taking on these tasks, it’s important to find and repair hollow spots on the concrete floor. Follow the steps below to learn how or watch our pros do it in the video below.

How to Find Hollow Spots in Concrete Floors

First, it’s important to know how to identify where the hollow spots are, and because they’re not visible on the surface of the concrete, the most effective way to do this is through sound. Using a hammer or prybar, lightly tap the concrete slab, working slowly and meticulously. Where the concrete is solid, you’ll hear a clear sound, whereas a hollow spot will produce a dull tone or an echo of trapped air.

If you find a hollow spot, use the claw side of the hammer or prybar to tap firmly against the slab. The concrete will easily chip away, exposing the hollow spot or void.

Cleaning the Concrete

After all hollow spots are revealed, the next step is to use a dust collector rather than a shop-vac to clean up all dust and debris from the slab. After using the dust collection system, go over the slab with a microfiber mop to catch any microscopic dust. Dust and debris left inside voids or on the slab will prevent any coatings or sealants from properly bonding.

Repairing Hollow Spots in Concrete

Once the hollow spots are exposed and cleaned out, and the slab is free of dust and debris, you can begin repairing them. We use a modified urethane joint filler, specifically Smith Paints Poly JF, because it’s a self-leveling, moisture tolerant material that’s semi-rigid to fill in the hollow spots in the slab. In addition to its durability, we recommend the Smith Poly JF because it dries to the point where it can be ground smooth in just a few hours after pouring it.

Order Your Concrete Surface Preparation Tools and Equipment Today

Whether you need a concrete grinder or concrete coatings, we have everything you need to get the job done right. To learn more about our products, find the right items for your next project, or get a free quote, call us today at 815-472-9754.