Evaluating Flatness and Levelness when Polishing Concrete
Credit: Peter Wagner
Flatness and Levelness are both factors for building owners
On your checklist, everything on a polished concrete project looks under control. But dangers are underfoot: The concrete floor wasn’t finished properly and doesn’t meet specifications for flatness and levelness. What are the visual consequences of a newly poured, out of spec floor?
As the polished concrete industry enters its second decade, what once passed muster now brings rejection, along with a need to better manage expectations and educate upstream to minimize damage downstream. While the structural aspect of the slab is the greatest concern of the engineer and is most important to the integrity of the project, the visual aspect will be under the magnifying glass of all parties, especially the owner and architect.
If you weigh flatness and levelness visually, flatness will generally be weighted more heavily. Both are engineering factors when they affect equipment operation, or the settings of doors and windows, or present a safety hazard.
When to evaluate the floor from a visual aspect
1. Consistency of finish between slabs: Not only do you have to deal with the visual appearance between slabs, you also must be aware of height differences at expansion joints. Poor strike-off or settling of the concrete could produce this variation. You will create a visual change when you grind down one side of the joint to level it out. Depending on your grinding steps, you will either end up with one side cream and the other with fines, or on the extreme, one side cream and the other showing large aggregate.
2. Hand-trowel finish adjacent to a hard steel trowel floor finish: This generally occurs at wall edges or around columns and footings. You receive a different consolidation of the concrete when finished by hand versus hard steel trowel. Your natural shine and clarity will not be consistent, and in instances of acid staining, you must be especially careful to fully neutralize and rinse your acids before densifying or you will have an acid/alkali reaction. A hard-to-remove white crust forms when the surface is improperly neutralized.
3. Birdbaths: It is rare not have some degree of depression, or birdbath, on a concrete floor. A depression that is 1/8 or 1/4 inch in any area greater than your machine head width will not cause a problem unless a super-flat floor has been specified. Numerous small depressions cause problems, either because your diamonds cannot reach the bottom, your metal bar diamonds dig into the side of the depression, or in eliminating the depression you expose more fines and/or aggregate than on the surrounding floor.
4. High spots: This is the reverse of a birdbath and exposes large aggregate more aggressively when grinding than the rest of the floor unless you commit to a total flattening of the floor. Both birdbaths and high spots will be accentuated by adding dye or acid stains, with birdbaths potentially pooling the color and high spots not retaining as much color.
5. Elevated metal decks: With an elevated deck the slab is generally only measured for flatness, due to inherent deflection in this type of pour. Your visual concern with an elevated deck is a tendency to have more map cracking, along with the potential for greater height variation at expansion joints. Metal decks tend to have greater undulations than a slab-on-grade. These undulations or waves will manifest themselves in a rippled reflection.
6. Dry shake and integral floors: Inconsistently applying a dry shake floor may affect the color. Dry shake also follows the flatness of the green, floated slab. So if the slab is left with high spots, then the dry shake itself becomes a high spot, and when ground, it can be completely removed, exposing the base concrete mix. With an integrally colored floor you do not expose a base gray concrete mix, but you do leave an area whose color is lighter than the surrounding floor. Both of these situations can often be visually blended using dyes.
Be aware of height differences at expansion joints
Options for fixing the floor
1. Localized grinding: This can flatten or level a floor, but there are visual consequences. How the floor was placed affects the control you maintain on the final finish. The floor can be ground and polished to provide cream, fines, or aggregate.
2. Rip it out and re-pour: This is rarely justified. Ripping out and replacing a concrete slab is generally between two and a half and three times the original cost.Often the end-user will reluctantly accept the floor, or in a worse case scenario, will decide to cover or patch with a coating, carpet, or tile. Either way you lose the project, along with time and money you have already invested.
3. Patching: A patch will always be a patch. You can try to blend the same mix appearance and color the mix to “match” the floor if it is colored. But once you have visually distorted the floor, the owner will find it. Sometimes you may achieve an initial match using an epoxy, but the epoxy and concrete will wear differently over time, either from different scratching or possible yellowing from UV exposure.
4. Overlayment: Only consider this when a large, contiguous area requires floating. Most overlayments are calcium aluminate-based. Except for one overlayment that I am aware of, they will not appear like a portland cement-based concrete, nor will they chemically interact with the densifiers.
How do we minimize flatness and levelness problems in the first place?
FF – Flatness is controlled by the finishing process. Ensuring that everyone is aware of the end use and final expectations for a polished concrete floor is your best deterrent to a floor not being level. From the owner to the architect, the general contractor, and subcontractor, understanding your process and needs helps ensure success.
FL – Forming and strike-off control the levelness. While level is usually not as imperative as being flat, it is a controllable factor.
Educating all parties, understanding how the finishing pieces complete the puzzle, and being conscientious and consistent are the best guarantees for a polished concrete floor that you and your customer will be proud of.