Polished Concrete Challenges
can be Overcome with Overlays

Ghosting images on a concrete floor.
When it comes to existing concrete slabs, designers must realize “industrial chic” looks, such as the ghosting images on this floor, can be a little too crude for some customers’ tastes. In those instances, a polished overlay could offer a solution. Photos courtesy of Craig Morris

The aesthetics of polished concrete is often one of the primary reasons why it’s chosen as the finished flooring on many projects. The material’s natural beauty that’s expressed through varying aggregate reveals, the color of the cement binder and the clarity of the polished surface makes it a singularly unique flooring choice.

While the beauty of a fully polished concrete surface isn’t debatable, the specifier must be aware of the unique challenges that come with polished concrete. When presented with an existing concrete slab, both the designer and the customer should be aware of some of the challenges that may be encountered.

A concrete floor that has binder colors that become more pronounced when polished.
Differences in binder colors are more pronounced after the concrete is polished.

Polishing existing concrete slabs

Varying aggregate reveal – Mechanical processing will reveal varying degrees of aggregate and at different concentrations. With an existing slab, it’s common to have the cement fines and fine and coarse aggregate all appear within the installation.

Additionally, areas of high aggregate reveal will accompany areas where there’s little to no reveal, many times in an adjacent area. This is to be expected and appreciated for the unique aesthetic that’s created. Both the specifier and the customer must understand that polished concrete is not terrazzo!

A concrete floor that has slab curl, making it challenging to achieve uniform aggregate reveal.
When slab curl is present, it can be challenging to achieve uniform aggregate reveal.

Surface imperfections – With existing slabs, the areas where there were trenches, spalls, patches and repairs made using a cementitious material will show and typically appear more pronounced in the final polished product.

Repaired areas can’t be processed out via grinding and should be considered as part of the final aesthetic.

Ghosting – Polishing concrete where modular floor coverings were removed will result in ghosting. Ghosting is the result of dirt and debris migrating over time into the concrete at the modular product’s seams.

No matter how much grinding is done, it will not reduce the ghosting effect.
Aggressive grinding won’t reduce the ghosting effect and, in many cases, makes the grid lines more pronounced.

The net effect is that when the concrete is polished, the grid pattern will appear in the polished surface. Aggressive grinding won’t reduce this effect and, in many cases, makes the grid lines appear more pronounced.

Slab curl – Slab curl occurs as a result of moisture and temperature gradients in the slab during curing and drying that results in warped edges. When processed, the warped edges often will show a deeper aggregate reveal than the center of the slab as more material is being removed in those areas.

When using a mechanical process, it will reveal varying degrees of aggregate concentration.
Slab curl reveal with repaired crack:
Mechanical processing will reveal varying degrees of aggregate concentration.

The presence of slab curl can make it challenging to achieve uniform aggregate reveal. This must be considered when a specific aggregate exposure is desired.

Cracks Dormant or nonmoving cracks in the existing concrete must be repaired from both a serviceability and safety perspective as well as to improve the final aesthetic. As with the other surface imperfections listed, repaired cracks can’t be processed out and will be apparent in the polished surface.

Areas that have been repaired will not be processed out via grinding
Repaired areas can’t be processed out via grinding.

Mock-up expectations

It’s critical to understand that it’s your responsibility to set expectations for the finished surface. This is typically achieved with a 10-by-10-foot mock-up on the slab that will be polished.

But be forewarned. While the mock-up can provide an estimation of the finished surface, it shouldn’t be used as a measuring stick by which the entire area to be processed is evaluated. In areas beyond the mock-up, it’s possible that different batch material was used and the areas were finished by different crews on different days using different finishing techniques.

These differences naturally affect how the concrete polishes and may be dramatically different than what is reflected in the mock-up. If the expectation is that the mock-up will be reproduced over the entire area, the floor may be rejected if it fails that test.

Specifying polishable overlays

The aesthetic created by each of the considerations noted earlier may be entirely acceptable to your customer. With polishing an existing concrete slab, beauty often is in the eye of the beholder.

Ghosting, patchwork, trenching and spalls are often seen as “industrial chic” and fit the larger aesthetic the designer is seeking to capture for the space. If all parties accept the challenges associated with polishing the existing slab — to include a degree of uncertainty regarding the final product — then the process can move forward.

If the customer isn’t willing to accept those considerations, using a polishable overlay is one solution. Polishable overlays remove those variables detailed above by providing a reproducible outcome through the following equation: Product Control + Process Control = Predictable Results

A finished polished overlay in a large warehouse space
If a customer isn’t willing to accept an “industrial chic” concrete floor that displays ghosting, patchwork, trenching and spalls, a polishable overlay will remedy those areas and create a shiny new expanse free of flaws.

In other words, using a polishable overlay allows you to control everything from how the material is mixed and placed to the way it’s processed and protected. This results in a finished surface that’s reproducible, and which leaves little unknown regarding the final aesthetic.

Polishable overlays will still absolutely require a mock-up, but the mock-up will fairly reflect how the overall installation will appear. When poured and processed correctly, polishable overlays can achieve a cement fines reveal (CPC Aggregate Exposure – Class A) up to a coarse aggregate reveal (Class C).

Polishable overlays are also suitable to receive a full polish (1,500 – 3,000 grit polish and guard) or may also be processed as a grind and seal.

While polishable overlays and polished concrete are within the same family of flooring surfaces, overlays create an aesthetic different from polished concrete. Polished concrete is truly unique given that every slab will process differently. Every slab is different from the next and will yield a different result. Overlays, on the other hand, are much more reproducible.

This doesn’t mean to imply that polishable overlays can’t create a unique look — quite the contrary. Ultimately, overlays provide the designer considerably more design options. Creating a signature project by including aggregates, integral color, topical staining, terrazzo strips and similar elements allow designers and customers to create with a fresh palette. With polishing an existing slab, the designer is limited in what’s presented by that concrete.

Whether polishing existing concrete or installing a polishable overlay, the end effect is the same … a unique, naturally beautiful, low-maintenance floor surface that’s environmentally friendly and sustainable.

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