About Polished Concrete
Polished Concrete Overview
Polished Concrete Summary
Polished concrete is leading the way when it comes to architecturally specified floor finishes. The many unique features of polished concrete floors are attracting more than just architects and designers. Home owners have placed polished concrete as one of the most desirable and must have flooring surfaces. The high demand for polished concrete floors over the past ten years has resulted in advanced techniques, tools, and materials creating faster and more consistent results to meet the expectations of the residential and architectural community.
Concrete is a fundamental building block in almost every construction project, whether new construction or re-modeling of an existing building. As rigorous standards for sustainable green building continue to rise, and the construction industry's quest for environmentally responsible construction continues, finding affordable, sustainable building materials is becoming a necessity for new construction and renovation projects. Natural, durable, beautiful, and sustainable, polished concrete ticks all the right boxes from every aspect.
Sustainability aims to reduce the environmental impact of construction, including environmental, economic, and social benefits. Sustainable building practices take advantage of renewable resources, bringing together a vast array of practices, techniques, skills, and materials to reduce the impacts of construction on the environment and long-term performance of the project.
Polished Concrete at a Glance
Polished concrete and polished concrete technology have evolved quickly over the past seven to ten years. Below is a brief overview of the key stages involved in polishing concrete. In addition, the basic terminology and language used throughout the polished concrete industry will help gain a better working knowledge without the need for a Ph.D. in concrete technology. More in-depth and detailed resources can be found through industry associations and publications.
Polished Concrete Process
Concrete polishing is a process of diamond grinding, chemical treatments with further diamond honing and polishing. Most polished concrete floors are rated by one or more of the following areas: reflectivity, smoothness, flatness, gloss reading, and coefficient of friction. Each of these categories is specific to the individual flooring requirements, usually defined by the client or architectural practicing principle.
Polished Concrete Diamonds
The diamonds used for polishing concrete are synthetic diamonds embedded in metal, resinous, or ceramic binders. In some cases, the binding material is a hybrid of two components to meet the required density. The diamonds and the binders are engineered to resist the different abrasive properties of concrete. Most diamond tools are produced to three standards: hard, medium and soft. Most concretes fall into one of the categories at some part of the concrete grinding and polishing process.
Polished Concrete Equipment
There are many different types of polishing equipment, from small hand held grinders to large ride on machines. All follow the same basic principles of grinding, honing, and polishing concrete. The different equipment configurations are balance and the speed in which the equipment works, including the more detailed aspects such as the motion and points of contact between the diamond tools and the concrete.
Polished Concrete Chemicals
Concrete turns from a solidified liquid into a hardened stone like material. This is the hydration process, called precipitation, and is a reaction between the cement and the water in the concrete matrix. During the hydration process up to thirty percent of the cement particles can be left un-hydrated, along with other active materials that result in a weaker and less stable concrete surface.
Chemicals such as Sodium, Lithium, and Potassium Silicates, along with the more recent Colloidal Silica, are all used to saturate the surface of pre-hardened concrete. These materials, through chemical interaction, develop the non-hydrated cement particles and other active imbalances in the concrete surface structure. This results in a more dense and less permeable concrete that, when polished, will achieve longer lasting performance.
Coloring Polished Concrete
Concrete can be colored using many different methods, before, during, and after placement. The two primary methods of coloring new concrete are adding powdered or liquid iron oxide pigment during the concrete manufacturing process, and/or dry-shake color hardeners that are broadcast and worked into the surface of freshly placed concrete. Both of these methods result in a permanent UV stable color.
Coloring pre-hardened concrete can be achieved using reactive stains also known as acid stain or chemical stain. These materials are an acid based material that react with the free-lime particles at the surface of the concrete, offering a UV stable color. The chemical reaction permanently changes the color of the cement paste and active aggregates, such as limestone. Acid Stains are limited to a small range of earth-tone colors and can be problematic when polishing concrete. The grinding process removes most of the free-lime, leaving little active material for the acid to react with.
Water or acetone based dyes are the most commonly used products for coloring pre-hardened concrete for polishing. Micronized dye particles are catalyzed in acetone or water, which in turn penetrate into the concrete surface. The more porous the concrete, the deeper the dye color will saturate. The use of dyes opens a wide spectrum of vivid colors not available with iron oxide or acid stains. It is important to note that these dyes are not UV stable and should only be used indoors. The dyes are non-reactive and moisture can affect the longevity of the color, therefore it is highly recommended that moisture tests should be carried out and a topical sealer used for additional protection.
Polished Concrete Sealers
In most cases, it is not advised to use a film forming topical sealer unless the project requires the additional features of such a product. There are many types of sealers and protective materials designed specifically for the protection and maintenance of polished concrete floors. Hybrid epoxy silicates, silanes, siloxanes, acrylic waxes, metalized waxes cover the main generalization of sealers used for polished concrete. No one sealer is perfect for all project environments, performance criteria will need to be addressed to determine which sealer type is best suited to the individual flooring requirements.
Polished Concrete Maintenance
As with all flooring products, polished concrete requires maintenance for continue performance over long periods of time. Unlike most other flooring materials polished concrete requires, at most, basic and minimal maintenance. In most cases, polished concrete needs to be cleaned with uncontaminated detergent free water on a regular basis. Certain sealer types will require different levels of maintenance and re-application or conditioners may need to be used to maintain sealer performance. Should a polished concrete floor become damaged or stained in any way, it is most likely that the damage will be in an isolated area that can easily be ground, re-polished, and treated without having to replace or disrupt the remainder of the floor.
Polished Concrete Cost
When considering a polished concrete floor, the question of cost always needs to be considered. Unlike manufactured products or pre-determined flooring systems that offer a fixed cost per square foot, polished concrete needs a much deeper evaluation to quantify the price per square foot. Collaboration at all levels is critical to achieve a successful polished concrete floor. The client, architect, designer, general contractor, sub-contractors, along with the concrete and polishing contractors, all need to understand the requirements and expectations of each other’s roles to manage and consequently reach the same goal.
Once the expectation is outlined and understood by all parties, the half-way point is reached to determine the cost for the polished concrete floor. Now the distinguishing factors need be addressed. There are two kinds of concrete’s that can be polished, first, existing concrete and second, newly specified concrete. Both require different thought processes to meet the outlines given in the pre-contract discussions. Here are the factors to consider and conclude to reach the final stages of price per square foot for concrete polishing.
Newly Specified Polished Concrete
Concrete Considerations – Starting with the concrete itself, the size and type of aggregate may need to be incorporated into the physical properties of the concrete, should the aggregate be required for a reveal in the polished concrete finish. In some cases an exotic or specialist aggregate may be required to suit the design aspects of the floor. This can be cost prohibitive to use throughout the entire concrete slab and may require the specific aggregate to be seeded into the freshly placed concrete. It is worth noting that seeding aggregate into concrete requires high levels of experience. Even then it is possible to see aggregate contamination from the underlying concrete and/or the rate of seeded aggregate could vary in areas throughout the floor.
The overall concrete strengths and aggregate properties will have a dramatic impact on the time required for heavy grinding and diamond usage. Discussing the properties of the concrete and aggregate to find compromise can influence the overall cost in materials and labor.
Concrete Placement – When placing concrete it is important to pre-determine the floor flatness FF and FL levelness. There is a balance to consider when choosing the flatness. Super flat floors can be expensive to install due to the specialized equipment and skills required. Medium flatness concrete is often more than acceptable for concrete polishing, and more cost effective for placement. It is only highly uneven concrete placement that can become problematic for concrete polishing, resulting in uneven aggregate exposure and distorted light reflectivity. Please refer to ASTM E1155 for the floor flatness specification guidelines and standards.
It is important that the concrete contractor uses the correct placement and finishing techniques best suited to the concrete polishing goals. Excessive vibration causes aggregate segregation that will become visible when polished. Should steel reinforcement be used in the concrete it must not be too close to the surface, as the outline of the steel can cause shadows or ghosting. Simple communication can prevent these errors.
Concrete Finishing – The level of concrete finishing can be beneficial or detrimental to the grinding or polishing process. There are several stages of finishing concrete, that range from early loose finishing, medium, and hard-trowel finishes. Each plays a part in how the concrete will polish.
Loose concrete finishes are easier to grind, but in most instances the concrete will reveal pinholes and potentially some of the smaller aggregates can pop-out, leaving holes that will need to be grouted and filled several times, adding another timely application to the process. Foot prints from operators of the mechanical trowels can also be visible if finishing begins to soon.
Medium concrete finishes are, in most cases, simple to grind at a fast rate. Still micro-pin-holes will be present but much easier and quicker to grout.
Limitations of Polished Concrete
Polished concrete is not without its limitations and drawbacks. There are two main reasons for residential or architectural polished concrete becoming part of the project. The sustainable green features or simply a desire for a fashionable material. Regardless of the interest in polished concrete as a flooring material, here are some considerations that need be address before committing to a floor that may not satisfy your expectations.
Noise – Noise can be an issue with all concrete floors unless softer materials are used to reduce sound deflection. Hard surfaces such as polished concrete create sharp echoes, which in turn produce poor sound quality.
Fatigue – Like other hard surface flooring, polished concrete can cause discomfort and fatigue for those who have to stand on it for long periods of time. Office or retail environments should consider softer rest areas to allow more comfortable break periods for staff.
Concrete Condition – Some concrete floors are in such poor condition that quality polishing is not an option. In this case there are concrete overlays that are designed to become a fully bonded concrete veneer. Polishable overlays can offer many features and greater control for guaranteed results on multiple projects. Concrete overlays are applied at varying thicknesses, from a quarter of an inch to three inches, and can be mixed with local or exotic aggregate to produce a terrazzo like appearance.