Polished concrete: Not just shiny

Defining polished concrete

To achieve the aesthetic and refinement requirements of this large aggregate concrete floor, the specification required a 0.4 μm (16 μin) in roughness average in addition to gloss requirements. Image © Concrete Intelligence Agency
To achieve the aesthetic and refinement requirements of this large aggregate concrete floor, the specification required a 0.4 μm (16 μin) in roughness average in addition to gloss requirements.
Image © Concrete Intelligence Agency

Specifiers are not the only ones affected in this seemingly confusing world of attempting to quantify polished concrete. In attempts to gain flooring market share, many coating companies joined trade organizations and relevant committees to lobby for the expansion of the definition of polished concrete to include all materials that could make a floor shiny. As an example, the current definition of polished concrete, according to the Concrete Polishing Council (CPC) and the American Society of Concrete Contractors (ASCC) is, “Polished concrete is the act of changing a concrete floor surface, with or without aggregate exposure, to achieve a specified level of gloss using one of the listed classifications—bonded abrasive polished concrete, burnished polished concrete, or hybrid polished concrete.”

It is not that they have ruled out polished concrete created through refinement of the material itself, but they have also included liquid-applied coatings and classified it all as “polished concrete.” As the character Inigo Montoya famously said in William Goldman’s the Princess Bride, “You keep using that word. I do not think it means what you think it means.”

The preferred method to quantify polished concrete from CPC and other groups is again limited to gloss and reflectivity—this misses the qualitative attributes necessary to provide full performance criteria relating to the floor’s use. As an example, groups will use reflectivity as a benchmark via distinctness of image (DOI) gloss. DOI is an aspect of gloss characterized by the sharpness of images of objects produced by reflection at a surface, per ASTM E284, Standard Terminology of Appearance. This is helpful to describe the aesthetic appearance, and has been used in the automobile coating industry to define reflective car coat quality for some time—hence, the attempts to use similar benchmarks for polished concrete.

The devices used to carry out these tasks are called gloss meters, with the degrees of radiant light measured in lumens (lm). These tools are useful for various applications and are accurate for reporting the amount of gloss reflected (or light quality) off the floor, but they do not remotely begin to tell how the light is reflected. Was the concrete properly refined, meaning is the finished surface durable, sustainable, and enjoying some measure of COF? Or did a resinous coating product from Division 09 get installed, and will not be discovered as an apparent substitution until the floor fails?

A profilometer is used to take floor measurements. Micro-surface benchmarks are measured in microinches (μin) or micrometers (μm). Polished concrete does not rely on application of a resinous coating or topical sealers to achieve performance benchmarks. Image © Chris Bennett, Bill DuBois, John Guill, and Keith Robinson
A profilometer is used to take floor measurements. Micro-surface benchmarks are measured in microinches (μin) or micrometers (μm). Polished concrete does not rely on application of a resinous coating or topical sealers to achieve performance benchmarks.
Image © Chris Bennett, Bill DuBois, John Guill, and Keith Robinson

Over the past decade, building professionals have been working hard to create testing methodologies that are easy, repeatable, and speak to physically making concrete slabs tough as well as shiny and beautiful. Bruce Nicholson, one of the earliest polishing contractors in North America, along with the late Harry Gressette, were some of the first, skilled tradespersons to begin measuring the floor itself and not just reflected light. Along with craft persons like Chris Swanson and pioneering architectural firms like DIALOG, Laith Sayigh’s DFA, Johnson McAdams Group, Gensler, SRG Partnership, and Thrailkill Associates, empirical research at the University of Alberta, Alberta, Canada, and the University of Akron, Ohio, and training events around North America began to prove through millions of square feet benchmarks for polished concrete can be quantified by readings taken from the concrete micro-surface texture with simple stylus devices (e.g. profilometers), while mechanically processing a floor to achieve a surface with high physical durability and chemical and stain resistance within an aesthetic framework. However, despite the evidence supporting using quantifiable and qualitative methods like measuring micro-surface texture there is yet to be an accepted standard for project team members to clearly communicate definitions and check polished concrete benchmarks.

The Concrete Sawing and Drilling Association (CSDA) published the first standard known as ST 115, Measuring Concrete Micro Surface Texture, to measure concrete surfaces and their surface texture value to evaluate how the concrete itself (versus mere reflected light) influenced the finishing process with regards to gloss, friction, and sustainability. It was an amazing success when compared to previous standards from the Concrete Polishing Association (CPAA) and similar groups where gloss considerations were used for benchmarks. However, due to ST 115’s attempt to create a micro-surface texture grade scale that equaled an approximate ‘grit scale,’ this well-intentioned standard was unable to achieve a clear path on a methodology to achieve consistent roughness average (Ra) readings and measure accordingly.

The National Center for Education and Research on Corrosion and Materials Performance (NCERCAMP) at the University of Akron continues the study of polished concrete, abrasion resistance, COF, and even the installation and maintenance of carbon footprint for this floor finish. In April 2020, it will host the third annual National Concrete and Corrosion Symposium in partnership with Kent State University, Ohio, as well as other schools and organizations to further research. Additionally, the National Concrete and Corrosion Symposium Research Fund has been created to allow owners, architects, academia, constructors, and manufacturer team members to participate and help develop a standard and installation curriculum that can be utilized to define quantifiably, not only what is to be achieved as a condition of the construction documents, but also how to do so with the goal of aiding design professionals reach net-zero emissions in their projects.

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2 comments on “Polished concrete: Not just shiny”

  1. In addition to the sheen and the level of grinding for a polished concrete, what is the language that should be included on the specifications about the staining proof and COF? are there any ASTMs to address this?

    Thank you

    1. Luis,

      A good place to start is with ANSI B101.3 for dynamic coefficient of friction as well as ASME B46.1-2009. Feel free to contact directly for more information.

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