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Information About: Concrete Floors
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  • Flatter floors have dramatically improved the consistency and quality of polish concrete finishes
  • It is crucial to maintain a consistent concrete slump for uniform finishing.
  • Getting the appropriate finish is critical. Overburnishingm the floor can make polishing difficult.
  • Riding the machine up over the slab edge produces a more uniform finish than hand finishing.
  • Screws stuck in lift truck tires can scratch up and mar finished floors.
  • Damage from nail marks are very difficult to repair.
  • A sample polishing mockup with a uniform salt-and-pepper aggregate exposure.
  • Protection of the floor during construction is now mandatory on most projects.

Recently I was asked which changes or advancements have had the biggest impact on polished concrete floors, primarily in the industrial and commercial market sectors. Although we could discuss equipment improvements or how diamond tooling technology has significantly advanced through the years, the biggest impact by far is how concrete slabs are poured and treated from the ground up.

This first became apparent seven years ago, when a high-end client engaged our services to evaluate their polished concrete program. While touring four of their stores that had been in service for nearly a year, it was immediately obvious to us that the floors were not being poured flat due to the random pockets of coarse aggregate exposure. To improve the overall quality of the polishing, we needed to start by improving the quality of the concrete placement, especially how the slabs were being finished, cured and protected during the construction phase.

Today, there is now a heightened awareness of all of the team players involved in producing these floors, and it has dramatically helped to improve the overall quality of polished concrete. That said, it can still be very challenging to work with the various trades and recommend that they perform their work in a way that deviates from the norm and their standard routine.

A new game planConcrete finishers can be notorious for being set in their ways, and often they don’t like outside sources asking them to finish a slab a certain way. Being a third-generation finisher myself, I appreciate this mentality and I love how most finishers consider themselves artists and take an attitude of ownership of the slab they are finishing. Finishing crews are accustomed to burnishing the slab to the point of darkening the concrete and obtaining a high shine, and they will often wet cure the slab with blankets for three to four days to increase the overall strength of the floor. This combination of wet curing and a burnished slab can wreak havoc with polishing contractors, who will typically need to use their most aggressive diamond tooling combined with wet cutting to expose the appropriate amount of aggregate. We have found that producing a “haze” or “smoke” finish, with little if any sheen or burnishing, is more conducive for initial grinding. But this must be done without compromising floor flatness and levelness numbers.

As an independent consultant on many of these projects, my involvement comes in after the slab has been placed and finished to help produce a representative jobsite mockup, which becomes the standard for the rest of the finished floor. Usually we are given an area of roughly 300 to 400 square feet, and the polishing contractor must complete all the steps involved, including joint filling, grinding, honing, coloring, densifying, polishing and the application of topical guards. In order for us to produce a beautiful sample and, more importantly, one that will be accepted by the owner or owner’s agent, we need to start with a canvas that is free of imperfections. Obviously, slab placement and finishing directly impacts the quality of the polishing.

Winning strategies for polished floorsRecently while working on a project together with my friend Denny Bartz, a consultant with Structural Services Incorporated, I asked him about the dynamics of industrial slab placements specifically for the intent of polishing. His many responsibilities include consulting on concrete floor design, construction and repair on large commercial slabs. Often, he is involved on three to four large pours a week at different geographical locations across the country. Here are some strategies that he recommends for producing high-quality polished floors:

Go back to the basics
Harris: With recent improvements on finishing slabs for polished concrete, have you seen the mix design change?

Bartz: We have reverted back to simplified mix designs, where in the past specifications called for ultra-high-performance mixes consisting of many different admixtures. By getting back to the basics and specifying well-graded mixes, we are able to produce a uniform slump which minimizes differential setting times of the concrete which helps in producing a much more consistent finish without surface mottling.

Control the playing field
Harris: I know you are overseeing slab placements in many different environments, including harsh temperatures. What are a few considerations?

Bartz: Controlling the placing environment can be one of the most challenging considerations when you take into account subzero temperatures on some slab placements. Obviously this means controlling the concrete temperatures from the batch plant to the jobsite. Also, it is mandatory to maintain a consistent minimum ambient and subbase temperature of 55° F while the concrete needs to be a minimum of 60° F at the point of placement. Since temporary heating is being used, proper ventilation is a must and air quality is continually being monitored.

Be flexible
Harris: Have you found it to be challenging to recommend to the concrete placement crew to finish the slab a certain way?

Bartz: Sometimes yes and sometimes no. I have found that most finishing crews are receptive to recommendations at the preslab placement meetings. Occasionally, I will come across a crew that has been finishing for years and does not embrace change with open arms. Typically, finishers are told to burn a floor in as tight as possible without scratching the surface, which is not the most desirable finish if the intent is to polish it. To eliminate mottling, smears and other surface imperfections, a less aggressive, lighter finish using minimal hand tooling appears to produce a better canvas for a homogeneous polished surface.

Pay attention to the details
Harris: I have seen the methodology on how theses slabs are being cured change over the years. What are your thoughts on the curing of concrete slabs specifically for polished concrete?

Bartz:Choosing a curing product and method that minimizes or eliminates surface discoloration has been a huge challenge. Originally we thought liquid-applied curing membranes were the most effective. However, on several projects, the applicators were not using the proper tip, which created drips and speckles over the entire slab. Considering the liquid was not atomized at the time of application, the heavier areas of the curing compound resulted in differential cure. Even after grinding, we still ended up with darker speckles across the slab. The end result was the polished concrete contractor needed to grind deeper to remove the blemishes even though the project was not originally bid to grind that deep. For the last year or so, we have been using the wet cure method with curing blankets left on the slab for three days on the polished concrete areas. This system has seemed to improve the other issues. However, if it is not installed properly (stretched tight and wrinkle free), you can be left with blanket lines as a result of the blanket not being uniformly in direct contact with the concrete.

The importance of a preslab construction meetingAs you can see, it takes a lot more than just the polishing contractor to produce a stunning polished concrete floor. Many clients are now implementing preslab construction meetings and holding the entire construction team accountable for everything from slab placement to the time the store opens. This has dramatically improved the final appearance of the polished concrete.

If you have never attended a preslab construction meeting specific to the polishing of concrete floors, you’d be surprised at the scope of the participants who attend. In addition to the key players, including the owner’s representatives, general contractor, concrete subcontractor, and polishing contractor, other members of the team that can have an impact are present, from the ready-mix supplier and concrete testing agency to plumbers and electricians.

At these meetings, which typically last several hours, many topics are covered in great detail, all the way down to the specifics of how the slab will be protected both before and after polishing:

  • Pour schedule
  • Test panel location and requirements
  • Placement environment
  • Soil support system
  • Concrete materials and specific mix design
  • Concrete testing and observation
  • Floor flatness/Floor levelness requirements
  • Slab reinforcement
  • Joint detail
  • Placing and finishing procedures
  • Curing
  • Moisture testing
  • Joint filling and materials
  • Polished concrete surface
  • Slab protection, pre and post polish

It is now mandatory for GCs to submit a pre- and post-slab protection plan and have all of the subs sign off on the sheets during their weekly meetings. On one project I was involved with recently, there was a worker whose sole responsibility was to pick up nails and screws so they did not end up in the tires of the lift trucks. On a few projects, I have seen screws stuck in these tires scratch the floor every 10 inches or so as the wheels turned, which is very difficult to fix, especially if the floor has already been polished. Now it is not uncommon for the GC to mark off the sections of the floor to be polished concrete with orange cones to delineate where the exposed concrete is from the areas of the floor that will receive a covering, such as carpet or tile. The sections to be polished are routinely cleaned throughout the day.

Related: Concrete Polishing Buyer’s Guide
Concrete Polishing Equipment