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Ghosting: A Unique Problem For Concrete Flooring

by Tara Pena

Polishing a concrete floor can turn up a number of unexpected surprises. One of these surprises involves ghosting, a phenomenon that can leave unsightly discolorations on your finished concrete flooring. It's an effect most commonly noticed after pulling old tile in preparation for concrete polishing, although it can also happen to recently poured floors.

Read on to learn how ghosting happens, why it happens, and what can be done to correct the problem and prevent it from happening in the future.

What Is It?

Concrete constantly responds to changes in moisture content and humidity, among other factors. Any changes in the amount of water, cement, and aggregate used in concrete can also affect the curing process and the resulting product once it hardens. These changes can leave behind discoloration due to differences in curing and moisture absorption.

Ghosting, also commonly referred to as "shadowing," often appears as a lighter or darker discoloration in contrast to the rest of the concrete floor. The polishing and staining processes usually make the discoloration more prominent. 

Ghosting can be extremely hard to spot with the naked eye on unfinished concrete. One way to check for potential ghosting issues prior to sealing is to spray water on the concrete surface. The differences in moisture absorption rates will cause areas vulnerable to ghosting to appear darker than other areas. 

Why Does It Happen?

Ghosting often happens when reinforcing material is pushed into the concrete after the forms have been filled with concrete. The resulting pumping action displaces the cement paste, resulting in shadows following the direction of the forms. Rebar and other objects embedded inside the concrete can create ghosting if they're positioned too close to the surface. 

Ghosting can also happen after removing vinyl composition tile (VCT) and polishing the concrete floor underneath. The end result is a grid-like shadow left behind in the concrete, caused by differences in curing and moisture exposure between the covered portions of the concrete and the gaps caused by the tile. These differences can be exacerbated over time due to dirty water being pushed down into the concrete from washing and waxing.

Contamination by other materials can also result in ghosting. Sodas, coffee, and other acidic liquids can etch through unsealed concrete, resulting in a noticeable discoloration once the area is stained or sealed. Even a seemingly benign chemical like the adhesives used for tape can leave behind ghost lines. Pouring over previous construction can also cause elements of the prior work to shadow through the concrete. 

What Can Be Done About It?

Once ghosting appears in your concrete flooring, there's not much that can be done to get rid of it. Grinding the concrete surface won't remove ghosting, in most cases, since the discoloration usually penetrates deep into the concrete. In some cases, grinding may cause ghosted areas to become more pronounced. Covering the surface with a dark stain or opaque coating may help hide ghosting to some extent. 

Prevention is usually the best policy for avoiding ghosting. Here are a few tips that can help minimize the likelihood of ghosting on your concrete flooring:

  • Make sure there aren't any cans, tools, lumber, or other objects left on the floor during the initial curing process.
  • Use a resin or cement-based primer on your concrete flooring to prevent shadows from breaking through.
  • Avoiding placing tape directly on your concrete floor before or after it's primed.

Keep in mind that ghosting doesn't mean the concrete flooring is ruined. There aren't any structural effects associated with ghosting, so you won't have to worry about having your entire concrete floor redone. The above information can help you make the most of your concrete flooring in spite of the discoloration caused by ghosting. Contact a concrete repair professional for additional advice.