*This article by INSTALL executive director David Gross was originally featured in the latest edition of Construction Specifier.

Polished concrete is highly durable, versatile, and low-maintenance, making it a popular choice for commercial and industrial flooring. With the non-residential polished concrete market size1 at more than $1 billion in 2021, and a predicted growth of 4 percent over the next four years, flooring contractors are actively increasing their capabilities for concrete polishing. This means construction specifiers must be extra vigilant in vetting and specifying flooring contractors who perform the work.

With the increase in demand and the number flooring contractors adding concrete polishing to their service offerings, leading product manufacturers and accredited training organizations have united to develop curriculums specifically dedicated to concrete polishing. This relatively new area of quality training includes classroom instruction and hands-on instruction for both contractors and workers. Proof of training helps specifiers and facility managers identify concrete polishing contractors with advanced education and skills that reflect industry standards and best practices.

To help understand the complexity of concrete polishing and emphasize why training is essential, consider two key aspects of the work: modern designs and polishing techniques. These exemplify the art and the science of the concrete polishing process and demonstrate why specifying a highly qualified contractor is essential.

Staining, dying, and stenciling

One of the main reasons polished concrete has become so popular is the growing number of stains and dyes used in conjunction with stencils to create custom looks. Overarching sustainability trends in the construction and interior design industry are also driving demand for commercial concrete polishing, particularly for the resurfacing and polishing of existing concrete floors, which is predicted to represent more than 15 percent of market share by 2028.2

As more design options make polished concrete appealing to a broader clientele, new products are demanding more advanced skills that build upon concrete polishing fundamentals. Three design categories that are popularizing polished concrete, especially in commercial spaces, are staining, dying, and the use of elaborate stenciling. While these design options are not new, their level of visual sophistication and material technology have grown, and as have the skills contractors must possess to deliver the respective effects.

Here is a quick overview of what stains, dyes, and stenciling bring to the design table.

Stains

Staining is the action of applying a colored liquid material to change the appearance of the concrete surface. The products used can be acid stains, pigmented water-based stains, or penetrating dyes. An acid stain is a reactive solution of one or more metal salts stabilized by acid to produce coloration in a concrete substrate. Water-based stains are non-reactive and impart color by penetrating the concrete surface and filling the pores with pigment. Acid stains are typically limited to earth tones, blues, and greens; while water-based stains offer a broader palette and produce a more uniform color effect. Most concrete stains are UV stable, making them suitable for indoor and outdoor use when direct sunlight is a factor. Some new formulations are also accepted by the LEED program, making them appealing for sustainable projects.

Dyes

Dye is a colorant for concrete, once it is applied, it penetrates while it is in a solution form, producing a stained appearance. Concrete dyes do not produce color via a chemical reaction as they are usually a synthetic organic compound and can degrade when exposed to light or alkalinity. Concrete dyes include water- and solvent-based products that are applied to the polished concrete surface and penetrate the surface to provide lasting color without affecting the texture of the concrete. Offering a multitude of bright colors, concrete dyes are particularly popular for applications that require bold colors or dramatic patterns.4 Dyed concrete is a popular choice for commercial buildings (e.g. medical and educational), where color is used to direct traffic, distinguish spaces, and create welcoming atmospheres.

Stenciling

Stenciling concrete uses templates or patterns to apply stains, dyes, or paints to the concrete surface and create decorative patterns or intricate graphics. This technique allows for a high degree of customization and can be used to add logos, wayfinding elements, or artistic designs to a polished concrete floor. Stenciling is often combined with staining or dying to create multi-colored or textured effects, so the contractor must possess expertise in product application, including surface preparation and testing.5

One crucial consideration (among many) when applying a stain, dye, or stencil to a polished concrete floor is the contractor’s ability to color match in instances where the concrete has been damaged or repaired. Even with newly poured concrete, there may be imperfections like hairline cracks, minor spalling, and damage from spills of acidic products like coffee or soda. These require extra attention to correct. Specified control joints must also be properly filled and colored to match the slab.

Wet polishing or dry polishing

While a contractor’s staining, dying, and stenciling skills are important to the art of concrete polishing, their experience with polishing tools, equipment, and techniques is vital to the science that brings about the desired results. When planning a polishing project with a qualified contractor, part of the decision-making will be whether to wet polish or dry polish. This decision will impact both the project environment during the polishing process and the work processes leading up to the finished floor. Both wet and dry polishing use a sequence of grits to achieve the level of surface refinement required to meet design specifications.

For a new poured-in-place concrete floor, a qualified concrete polishing contractor will meet with the structural engineer in advance to gather concrete specifications and select the polishing technique and materials accordingly. Collaboration at this stage ensures the contractor can produce the correct polish level and aggregate exposure to meet the client’s expectations.

Here is an overview of the differences between the two techniques, plus a look at the role of densifiers:

Wet Polishing

When wet polishing concrete, contractors use water during the grinding phase to keep the slab surface lubricated. This technique controls silica dust in the work area by trapping the dust in water. Using water also lowers the amount of heat produced by grinding friction. Thus, applying the correct amount of water is vital for keeping the work area dust-free and allowing the grinding process to be most effective. Wet grinding when polishing requires an experienced hand to precisely control the water and the polishing equipment for optimal results. The contractor must also properly capture and safely dispose of the resulting dust-and-water slurry generated during the grinding process and keep the surface clean between each step. Proper handling of the slurry is necessary for keeping the work area safe and readying the surface for subsequent processes. It is also an observable quality of a safe and orderly worksite to clients.

Some important considerations for specifiers to know about wet polishing include:

• Wait time between steps. Wet grinding may take more time than dry grinding because contractors must manage the slurry and disposal process.

• Access to water and water usage. The project site will need to have access to a water source, and there are safety considerations when using the high voltage machines in the presence of water.

• Certain Distinction of Image (DOI) and gloss levels may not be achievable. Higher-gloss finishes may benefit from dry polishing.

Dry polishing

Without water to capture dust, dry grinding equipment must be outfitted with a dust collection system. Dust collection, which typically includes a high-efficiency particulate air (HEPA) filter, protects the contractor and other site personnel from breathing hazardous silica dust particles. Dust collection systems also keep the work area clean as the contractor works, differentiating it from wet processes that require more time for cleanup. Dry polishing also provides the contractor, designer, and client with the benefit of more readily seeing the surface as work progresses, instead of having to wait for a wet surface to dry. Like wet grinding, the contractor must possess expertise in product application and equipment control and carefully collect and dispose of the dust to keep the work area safe and create exemplary results.

Considerations for specifiers when considering a dry polishing technique include:

• Dust control. The importance of controlling dust during dry polishing cannot be overstated. Specifying the use of a qualified contractor with proven experience in dry polishing techniques and equipment will help protect the health and safety of site personnel by ensuring the contractor’s tools and machines are properly equipped to capture hazardous dust.

• Risk of glazing. Inexperienced contractors performing dry polishing are at greater risk of overworking the concrete surface, which can cause tools or equipment to overheat and, in worst cases, melt.

Densifiers

Concrete densifiers, sometimes referred to as hardeners, are products applied during the polishing process to improve a floor’s surface strength and long-term performance. Using a combination of silicate and water-based compounds, densifiers react on the surface of the concrete to produce additional calcium silicate hydrate (CSH). Silicates are combined with carrier compounds, most commonly sodium-, potassium-, or lithium-based, to aid the process. By bonding loose and weak materials more tightly together, the surface density and abrasion resistance of the concrete surface is greatly increased. Chemical densifiers are used on polished as well as non-polished concrete to reduce dusting and to make the surface less permeable to liquid spills.

Properly trained contractors understand the different characteristics and requirements of densifier products, and they accurately assess the readiness of concrete for application based on its porosity and surface condition. Trained contractors are also equipped to use the correct application techniques and product quantities based on product indications and concrete hardness to achieve optimal penetration and reaction.

Regardless of which polishing method a contractor employs, proper training and proven hands-on experience with equipment and materials are essential. Specifying contractors that use employees with documented training in concrete polishing helps protect the project from end to end, ensuring worksite cleanliness and safety during the process and professional results at the conclusion.

Inexperience is written all over the floor

Since the basic principles of concrete polishing are similar to sanding wood and polishing other materials such as stone or metal, novice contractors are sometimes overly confident and overstate their understanding of equipment and techniques. This presumption could not be more wrong.

To master the skills of professional concrete polishing, there is no substitute for formal instruction and hands-on training in real-world applications. Concrete polishing is a complex process that demands precision, patience, and deep material knowledge, and even the most experienced contractors can encounter conditions that adversely affect the quality of the finished floor.

To explain how challenging concrete polishing is and why specifying a qualified contractor is imperative, here are some common mistakes that undertrained or inexperienced contractors make and the impacts they can have on flooring results.

Insufficient surface preparation

Issue: Failing to properly prepare the concrete surface before beginning the polishing process. This includes not repairing cracks and not removing debris.
Impact: Leads to a poor quality finish with visible imperfections and a high potential for rework.

Incorrect grit progression

Issue: Skipping grits or not using the correct sequence of abrasive pads during polishing. Untrained contractors may try to speed up the process by jumping from a coarse grit to a fine grit without the necessary intermediate steps.
Impact: Produces a floor that is improperly refined, shows scratches, or does not achieve the client’s desired level of polish.

Inadequate water use for wet grinding stage

Issue: Not using enough water during wet grinding or applying an uneven application of water across the slab surface.
Impact: Can lead to uneven polishing, swirl marks, and machine burn marks on the concrete surface due to insufficient lubrication and cooling.

Overworking the concrete

Issue: Spending too much time polishing one area or using too much pressure. This is often due to inexperience with equipment or trying to correct an imperfection.
Impact: Causes dips and waves in the floor, making the surface uneven and negatively affecting gloss and reflectivity.

Improper care of equipment

Issue: Using equipment that has not been properly maintained, has dull grinding or polishing pads, or is performing poorly.
Impact: Reduces the efficiency of the polishing process, which can lengthen project time and lead to poor quality finishes or damage to the concrete surface.

Ignoring environmental conditions

Issue: Not considering the temperature and humidity of the environment where the polishing is taking place.
Impact: Can affect how densifiers and sealants react and lead to a less durable finish.

Improper use of densifiers

Issue: Applying densifiers too early in the process or not allowing adequate time for the densifier to penetrate and react with the concrete.
Impact: Prevents the concrete from reaching its full hardness and durability and affects the longevity and performance of the final polished surface.

Rushing the process

Issue: Trying to complete the polishing process too quickly by rushing through the necessary polishing passes.
Impact: Compromises the quality and durability of the polish, leading to a finish that may not meet client expectations or performance standards.

Every step of the polishing process is an opportunity for mistakes when performed by an undertrained or inexperienced contractor, and each can have the costly consequences of rework and disappointed clients. Specifying a qualified contractor minimizes these risks and significantly reduces the incidence of callbacks.

A need of a qualified contractor

A highly qualified concrete polishing contractor can support the specification process in several ways that will benefit the safety, sustainability, and long-term performance of a concrete floor. Key responsibilities of the specifier that can be aided by a qualified contractor include:

Developing specifications

The first step in preparing specifications for polished concrete floors is for the specifier to understand and articulate the designer’s intent in terms of the desired sheen, appearance of aggregate, color, durability and need for maintenance. Once these have been determined, it is important to work with the structural engineer to ensure the concrete mixes are appropriate for the slabs that will be polished.

Performance specifications are an ideal way to inform the contractor of design goals while allowing the contractor to use their expertise to suggest the materials and methods that will achieve those goals. However, if prescriptive specifications are preferred, consultation with a qualified contractor will be valuable for preparing effective specifications. As an expert in concrete materials, equipment, and techniques, a qualified contractor can accurately inform decisions about the quality and types of materials to be used and the best polishing techniques for the environment and design. They will also ensure their work complies with all relevant codes, safety standards, and best practices. Either way, samples and mockups should be specified as a way of making sure that the proposed materials, equipment, and methods will achieve the desired results.

Material and product selection

When researching and selecting appropriate materials and products that meet the project’s design requirements, budget constraints, and sustainability goals, a qualified contractor can help ensure the materials and products specified align with project goals while still being able to produce the desired result. They can also assist with evaluating alternative materials and products for performance, cost-effectiveness, and environmental impact.

Project coordination

Working closely with architects, engineers, and other design professionals to understand and adapt to project requirements is not a skill that comes naturally to all contractors. A qualified contractor will have real-world training and experience communicating with project stakeholders and will help the work progress more smoothly.

Compliance and standards

A critical part of formal training for concrete polishing contractors is ensuring all the specified materials and worksite practices comply with local, state, and federal building codes and regulations. This means specifiers can count on qualified contractors being up-to-date on industry best practices, new techniques, safety requirements, and material handling and disposal. Industry standards can also be helpful in articulating performance criteria such as sheen and aggregate exposure.

Environmental responsibility

Incorporating sustainable building practices and materials into specifications to meet environmental certification requirements such as LEED is increasingly required of specifiers. Expert flooring contractors who stay up-to-date with the newest, most sustainable products and practices can inform these choices to help limit embodied carbon in the polishing process and safely dispose of waste.

Cost control

Balancing high-quality standards with cost constraints is a delicate act for specifiers. A qualified concrete polisher can work with the specifier to review materials and alternatives that may offer cost savings without compromising quality or design.

Risk management

Mitigating risk is one of the most vital responsibilities of a specifier. To ensure liability concerns, warranty requirements, and performance criteria are adequately addressed in the specifications, specifiers can include using a warranty contractor. Choosing a warranty contractor is its own insurance policy for concrete flooring work. It provides clients with peace of mind, and it also guarantees the flooring is completed correctly the first time.

Concrete advice

Specifiers play an important role in the quality of concrete polishing, acting as a bridge between the intentions of the architect or designer and the skills of the contractor. When polished concrete is part of the design, it is up to the specifier to determine how the design requirements will be met, while tending to budget constraints and sustainability goals.

Specifying a qualified concrete polishing contractor ensures the partner chosen is also up to this task—equipped with the specialized concrete material knowledge, hands-on product and equipment experience, and communication skills to produce high-quality work on time and on budget.

Specifiers can also obtain guidance from established industry organizations. The American Concrete Institute6 and the American Society of Concrete Contractors7, for example, supply standards for qualities such as concrete specular gloss/sheen8, distinction of image, and aggregate exposure. By applying these standards and requiring trained, responsible concrete polishing contractors, specifiers help ensure consistency, quality, and safety across all concrete finishing projects.