Filling the voids: Differentiating between concrete coating and polishing

by sadia_badhon | June 17, 2021 8:06 pm

by Chris Bennett, CSI, iSCS, CDT, Bill Dubois, AIA, CSI, CCS, John Guill, FCSI, AIA, IIBEC, SCIP, CCS, CCCA, Keith Robinson, RSW, FCSC, FCSI, Rae Taylor, PhD, and Jonathan Ware, AIA, LEED AP, LEED CxA

Photo courtesy Bay Area Concretes[1]
Photo courtesy Bay Area Concretes

Concrete slab surfaces have voids. They can be big or small. Voids might come in the form of visible cracks, a dislodged piece of aggregate, or much smaller spaces on the micro or nano scale. Voids might be produced later in a slab’s life due to floor traffic over a curled joint. When joints break down, visible voids are created, wear and tear at uneven portions of the slab is increased, and additional maintenance costs are incurred. These voids are the weak points that reduce a slab’s life cycle and increase permeability—thereby allowing dirt and abrasives to quickly wear away the surface—as well as the probability to be negatively affected by moisture, contaminants, and other negative traits that facility owners do not want for their exposed concrete walkway surfaces.

For existing concrete, one can either deal with these voids through coating (sometimes referred to as sealing) or by refining and changing the surface via polishing. Unfortunately, not enough members of the project team are fluent in differentiating between the coating products[2] of Division 09 and the cementitious products of Division 03. The terms “coatings” and “polishing” are often confused, especially when it comes to the many clear coating technologies available.


There are many types of concrete coatings from polyuria, polyaspartic, and polyurethane to epoxy and acrylic. Concrete coating types are many. Polyurethane usually does not bond well to concrete, but epoxies do. This means these coating technologies are often used together. Epoxies fill voids and polyurethane coatings are used as the finish coat to establish slip-resistive, durable, and aesthetically pleasing floors.

Image © Mike Smith[3]
Image © Mike Smith

Water-based coatings, such as acrylics, are quick and easy to apply, but require frequent maintenance and repairs, making them a less sustainable option when compared to solvent-based coatings. The latter generally lasts longer, but can turn yellow or amber when exposed to ultraviolet (UV) light (in the case of epoxies) or with oxygen reactions to the solvent (in the case of acrylics, polyaspartic, and polyurethane). Two- and single-part products containing no solvents have become more readily available in recent years as technologies advance to combine various hardening and adhesive properties, which reduces many of the concerns for appearance retention.

Siloxane and silane coatings are derived from the silicon molecule and can penetrate deeper under the surface than other coatings. They are applied by dipping, spinning, or spraying. The downside of these types of coating is their lack of chemical resistance[4] and the need for reapplications. Floor coatings with high solid content may need to be avoided in certain industrial and commercial applications, as they will easily become tarnished from fork lift and foot traffic.

Some coatings can enjoy high gloss if burnished. However, this can cause confusion with polished concrete installations. Gloss benchmarks[5] for coatings include distinctness of image (DOI), similar to the auto coatings industry. As coatings tend to be film-forming by nature, maintaining higher traction levels, and thus a higher coefficient of friction (COF), can be more difficult. Epoxy coatings, including thin-mil coated floors made by resin-tool transfer, can decrease COF. Anti-slip broadcast agents may need to be used to ensure safer walking surfaces. With slips, trips, and falls listed as one of the “fatal four” injuries, and 67 percent of those happening on the same level, an appropriate COF is an important component to user safety.


Using a densifier does not mean the floor is being polished. It is all about application. A concrete densifying agent can be employed in either coated floor systems or concrete polishing. They can be applied like a shell coating to the top of a slab or processed into a floor to aid in refinement and strengthen inside the slab. Densifiers typically do not enhance substrate color, but can produce some sheen as a coating or with polishing. Densifier performance is based on a number of factors, including pH, percentages of solids, molecule size, and application.

There are two basic types of densifiers—silicate and silica. Examples of silicates are sodium, potassium, lithium silicates, or magnesium flourosilicates. Some silicate technologies have been around since the time the Model T was in production, but still have some uses in modern construction as a floor coating. In the presence of moisture, densifiers become chemically reactive with calcium hydroxide (CH) to produce calcium silicate hydrate (C-S-H). Silica densifiers are a newer, more reactive type. Some of these products are referred to as colloidal, which describes the suspended state of silica.

Microsurface benchmarks are measured in microinches (µin) or in micrometers (μm) by a device called a profilometer. Additional readings for gloss or gloss clarity may be taken in conjunction with surface micro texture readings. Referenced from A New Concrete Glossary by Keith Robinson, Bill DuBois, Rae Taylor, Chris Bennett, and John Guill. Images courtesy Chris Bennett[6]
Microsurface benchmarks are measured in microinches (µin) or in micrometers (μm) by a device called a profilometer. Additional readings for gloss or gloss clarity may be taken in conjunction with surface micro texture readings. Referenced from A New Concrete Glossary by Keith Robinson, Bill DuBois, Rae Taylor, Chris Bennett, and John Guill.
Images courtesy Chris Bennett

Nano silica densifiers are the smallest category of silica densifiers in which the silica size must be below 100 nanometers, allowing for better capillary penetration. When applied, billions of nano-scaled silica flood the substrate to fill the larger micro-sized pores and channels, creating new C-S-H that bonds not only to existing C-S-H, but also to itself and other silica to reduce porosity and voids on the surface of concrete. Nanoengineering-based technologies are not only small, but also offer advantages with reactive surface areas, genomic code (stacking shape), and other features that industry professionals are beginning to understand, thanks to advances in nanoindentation technologies[7].

Polished or refined concrete

Polished concrete is the act of mechanically processing a floor to achieve a concrete surface with high durability and physical resistance to chemical and stain attacks within an aesthetic framework. Benchmarks for polished concrete are quantified by readings taken from the concrete microsurface texture. Microsurface benchmarks are measured in microinches (μin) or micrometres (μm) with the American Society of Mechanical Engineers (ASME) B46.1-2009 (R2002), Surface Texture (Surface Roughness, Waviness, and Lay), available as an easy way to test and measure average roughness (Ra) on a surface. A microinch is equivalent to 25.4 nanometers.

Polished concrete does not rely on application of a resinous coating or topical sealers to achieve performance benchmarks. Full refinement and consistent Ra is easy to achieve with polishing systems. Ra can help ensure voids are completely being filled by the densifier, and the floor is physically refined. Ra is inconsistent with shiny coatings applications that mimic (or even advertise as) polished concrete finishes because the surface preparation involved with those systems create micro fractures while creating a profile for a topical coating.

Densifiers should not be used to coat the floor when polishing, but rather as a part of the physical processing of the new floor surface to aid in greater C-S-H formation, thereby reducing permeability and increasing physical resilience. The processing or folding of the slab back into its own matrix with a densifier is a wet process where final levels of refinement can be performed without creating slurry. Gloss and DOI readings may also be taken from a polished concrete floor and can be helpful in selecting a reflective aesthetic, but the latter alone cannot determine if a floor has received a polishing process. It is important to ensure the selected polishing systems can achieve a consistent and repeatable surface micro texture (Ra) as well as high-traction DCOF ratings, such as from the American National Standards Institute (ANSI) B101.3, Test Method for Measuring Wet DCOF of Common Hard-surface Floor Materials, and to avoid coatings that have a similar appearance.

A refined and polished concrete floor brings many resilient performance benefits, but it can be susceptible to certain chemical attacks. If one wanted an exposed concrete floor finish in a water treatment plant’s fluoride tank room, a coating is likely the best finish. However, for heavy floor traffic, long-term durability, and a more permanent solution in void mitigation in commercial and industrial settings, polished concrete is likely the preferred option.

The big picture

For a concrete floor to be as maintenance-friendly as possible and retain its original appearance, it must be non-absorptive to resist moisture, dirt, and movement from volume changes. The process of curing and finishing a slab is key to constructing a dense and durable surface. A facility cannot maintain its clean and healthy status when the floor is dirty or unable to be resilient to abuse.

While each project will be unique, there is often overlap in slab requirements. Floor flatness and levelness (Ff/Fl), once only a factor in logistics and distribution facilities, has become a concern in many other sectors due to the increased use of robots. Durability remains important as it reduces maintenance and replacement costs.

Hitting a curled joint takes its toll on both the equipment and operator. It can slow down material handling as operators reduce speed when approaching floor joints. Wear and tear at these locations is increased. This not only incurs maintenance costs, but also the finished appearances of the repairs are often inconsistent with the rest of the slab.

In the case of food facilities, a durable surface that holds up over time with minimal cracking (a larger void type in the concrete matrix) is critical as it is less likely to create harborage for chemical, biological, physical, or allergenic contaminants. “Joint-less” slabs or those with limited shrinkage can also be helpful in this regard as additional maintenance and work at control and cold joints is not required.

This photo shows a magnified view of a profilometer’s stylus. The stylus moves across a surface to measure the peaks and valleys of its surface micro texture. Image © Eric Traffie[8]
This photo shows a magnified view of a profilometer’s stylus. The stylus moves across a surface to measure the peaks and valleys of its surface micro texture.
Image © Eric Traffie

New slabs present excellent opportunities to reduce these problems and create durable surfaces that hold up over time. Allowing concrete the maximum possible amount of hydration and cure can help not only reduce undulation, shrinkage, and inconsistent Ff/Fl issues that may later create integrity issues at joints as volume change, but also in the ability of the surface to start with fewer voids, and improve the ability to resist new voids.

Internally curing[9] with natural fibers and similar agents can decrease early autogenous shrinkage of cement paste. As with concrete polishing, nano-sized particles[10] significantly increase reactivity and promote a degree of hydration (DOH) that decreases permeability (voids), which strengthen the entire slab, with or without joints.

Today the use of nano-silica, high-dosed macro synthetic fibers, and other admixtures, along with advanced placing and finishing technology, is changing the entire behavior of slabs on grade for the better,” says Jim Raffin, PE, co-owner of Raffin Construction Co., a concrete contracting firm in Chicago, Illinois.

Lastly, simple, mechanical processes can also help with surface void reduction by trowel finishing with proper techniques, such as high psi blades and ensuring that edge machines work within the same psi parameters to avoid inconsistency[11] with field and edge areas.


Performance not only varies between the two terms of coatings and polishing, but also specifically within the coating categories as well. These differences are reflected in not only final floor finish performance, but also installation methods, and certainly cost and time. Coatings cover the void-filled surface and polishing reprocess the void-filled surface, but sometimes the answer may be to incorporate both processes for a final solution. However, relevant benchmarks must be included to ensure proper installation.

[12]Chris Bennett, CSI, iSCS, CDT, is a critical voice in the development of sustainable concrete solutions to replace expensive, outdated methods. His firm, Bennett Build, leads project teams in lowering the economic and environmental costs of designing and implementing stronger concrete systems. He can be reached at[13].

[14]Bill DuBois, AIA, CSI, CCS, is an architect with a passion for working with the entire construction project team, which includes owners, designers, constructors, and suppliers. DuBois assists in the decision-making processes necessary for efficient implementation of powerful design solutions and the creation of construction specifications. He can be reached at[15].

[16]John Guill, FCSI, AIA, IIBEC, SCIP, CCS, CCCA, is a registered architect, specifier, and principal at DTR Consulting. Guill has practiced architecture and engineering in a wide variety of civic, educational, and institutional projects and helps clients develop construction specifications, identify building materials/products, and quality control to protect clients and improve projects. He has received numerous awards from CSI, the American Institute of Architects (AIA), and the Coalition for Adequate School Housing (CASH).

[17]Keith Robinson, RSW, FCSC, FCSI, is an associate at Dialog in Edmonton, Alberta, Canada. Robinson also instructs courses for the University of Alberta, acts as an advisor to several construction groups, and sits on many standards review committees for ASTM and the National Fire Protection Association (NFPA). He can be reached at[18].

[19]Rae Taylor, PhD, holds a doctorate in civil engineering and materials science from the University of Leeds, and a postgraduate certificate in technology management from the Open University. Her principal research interests lie in the field of materials science and improving the environmental impact of construction materials, with a focus on the effect of cement replacement materials and additives on cement microstructure. She can be reached at[20].

[21]Jonathan Ware, AIA, LEED AP, LEED CxA, is senior design manager at Gray Architects and Managers. He has over 25 years of experience as a designer and a construction project manager in the commercial industry. He is passionate about sustainable design and construction, continuous improvement, and developing individuals, teams, and relationships. Ware can be reached at[22].

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