Producing quality polished concrete floor slabs

by Katie Daniel | April 21, 2016 12:14 pm

By Paul Jeffs
Producing a good quality polished concrete floor may appear to be a relatively simple task. (This article only discusses polishing of newly constructed floor slabs. Existing slabs can be polished, but the esthetic quality will be primarily dependent on the already existing near surface conditions with regard to the presence and extent of defects and, of course, the skills and techniques of the polished concrete contractor. When the term polished concrete is used it refers to the practices of grinding, honing, and polishing). Certainly, when conditions and circumstances are favourable during the various phases of construction, highly durable, attractive surfaces and finishes can be achieved that require very little maintenance. Unfortunately, the achievement of these features when conditions and circumstances are not favourable can be extremely complex and sometimes considerably more costly.

To understand the many complexities, one should examine the members of the project team, how they can influence the quality of a polished concrete floor surface, and their typical roles, responsibilities, objectives, and expectations. Then, one should examine the factors adversely or beneficially influencing the achievement of each member’s objectives, as well as how these factors can vary considerably from project to project. (Unfortunately, it has been this author’s experience that, prior to a project start-up, each team member has varying levels of awareness of the factors that can inadvertently cause problems and sometimes result in conflict. Team members are therefore often unprepared for the consequences when conditions and circumstances turn out to be unfavourable.)

One should also examine  the various problems that can sometimes result when conditions and circumstances are not conducive to the achievement of an acceptable finish. However, the term ‘polished concrete’ should first be defined and explained.

What is polished concrete?
Canada’s Concrete Floor Contractors Association (CFCA) defines polished concrete as:

a sustainable exposed concrete surface finish obtained through intensive diamond grinding incorporating a densifying liquid hardener. During the polishing process, cement paste is removed from the surface of the concrete which may expose underlying concrete fine and coarse aggregates.

However, polished concrete practices have evolved over the last decade and there are now numerous proprietary systems available offering similar esthetic effects, but using different techniques and/or materials to achieve the surface finish. In recognition of this evolution, the Concrete Polishing Association of America (CPAA) has published a more comprehensive definition. It states polished concrete is:

The act of changing a concrete floor surface, with or without aggregate exposure, until the desired level of finished gloss is achieved by using one of the listed classifications; Bonded Abrasive Polished Concrete, Burnished Concrete, or Hybrid Polished Concrete.

CPAA then goes on to define bonded abrasive polished concrete as being:

The multi-step operation of mechanically grinding, honing, and polishing a concrete floor surface with bonded abrasives to cut a concrete floor surface and to refine each cut to the maximum potential to achieve a specified level of finished gloss.

CPAA concludes its definition with the statement—this system yields the most durable finish and requires the least maintenance. Interestingly, unlike CFCA, the U.S. group does not refer to the use of a densifier within its definitions.

The final surface finish may be a natural colour, or it may be enhanced by application of dyes, tints, or stains during multi-step operations. This information is missing from both organizations’ definitions. Alternatively, an integrally mixed pigment may be included during the concrete batching operations, or a pigmented dry-shake hardener can be applied during the finishing operations.

There is rarely consensus within the polished concrete industry regarding the best materials and techniques. However, there is a general agreement a multi-step, mechanical operation involving cutting and refining the surface is essential for the achievement of a high-quality, durable finish.
Each step cuts progressively finer microscopic peaks and valleys and should continue until the abrasive under use can no longer refine the appearance. The number of steps and the specific abrasive used during each step will be determined by the desired finish.

In the past, a major issue has arisen regarding accurate communication with the polished concrete contractor as to the desired finish, and expectation of esthetic quality and uniformity of appearance. In an attempt to provide owners and specifiers with standardized terminology, both CFCA and CPAA have published classifications for the degree of aggregate exposure and finished gloss. Unfortunately, although similar, each classification is different (Figure 1).

Project team members and responsibilities
Projects including polished concrete floor construction will generally, but not necessarily, involve a combination of some or all of the following project team members.

Owner
Owners, who may employ a project manager to administer the project, should be responsible for the selection of the aggregate exposure classifications and finished gloss. They should also establish their level of expectation regarding acceptable minimum esthetic quality and uniformity within a given financial budget. However, owners should be aware there is a higher price to pay for the achievement of uniformity and high esthetic surface quality.

Owner’s representative
The owner’s representative, typically an architectural or engineering firm acting as the prime consultant, is primarily responsible for advising the owner on factors affecting the achievement of the selected classifications. This entity should also communicate the client’s expectations to the project team and state the importance of considering the factors affecting the classifications. For example, installation of building interior partition walls before the polished concrete operations could reduce the ability to achieve an optimal level of flatness essential for uniform results when the concrete is polished.

It is also important the prime consultant provide information to the concrete supplier to ensure compliance with CSA A23.1-09 (R2014), Concrete Materials and Methods of Concrete Construction  and the achievment of special performance requirements of the concrete. (CSA A23.1-09 (R2014), Concrete Materials and Methods of Concrete Construction requires the owner—or by default the prime consultant—to specify the owner’s performance criteria for architectural and other property requirements).

Specifier
The specifier, generally retained or employed by the owner’s representative, will typically require a particular type of polished concrete system be applied by an approved polished concrete contractor—although, sometimes a polished concrete contractor will be specified. It should be the specifier’s responsibility to communicate key requirements within the project specifications to all bidders, and especially those substrades whose operations can influence achievement of an acceptable level of performance. It is essential the specifier is aware of what those requirements are—including the practices influencing flatness and uniformity of the constructed slab’s near-surface properties.

In this author’s opinion, the specifier should have the concrete supplied in accordance with the performance method of specification, but with some qualifications such as stipulating that supplementary cementing materials should not be used.

General contractor
General contractors (GCs) or construction managers (CMs) will often purchase the concrete. They will also typically sub-contract or issue trade packages for subgrade construction and concrete placement and finishing. In addition to managing the project, they may be responsible for some other specific requirements, such as curing procedures and the provision of ancillary protection requirements. Under CSA A23.1-09 (R2014) Concrete Materials and Methods of Concrete Construction / Test Methods and Standard Practices for Concrete for performance specifications, the contractor is responsible for working with the concrete supplier to meet performance criteria for both the plastic and hardened concrete.

To ensure the true costs of producing a polished concrete slab are understood during the bidding processes, it is important the GC or CM realistically understands any implications the specifications can have on mix design parameters. Bidders should also be informed of any additional procedures not typically needed for conventional concrete slab placement and finishing. The GC or CM should administer the various sub-trades using the floor before and after it is polished so permanent defects are not caused during work. Further, it is important the GC or CM include for the cost of any procedures and materials required should rapid drying conditions prevail at the time of concrete placement and finishing—unless this task has specifically been given to the concrete floor contractor.

Supplier
Concrete suppliers are responsible for producing and supplying concrete using a mix design formulated to satisfy the requirements of CSA A23.1-09 (R2014). They are also required to certify their concrete will comply with the specified performance criteria. In this author’s opinion, prior to providing a quotation during the bidding process, the concrete supplier needs to be informed supplementary materials should not be used. The reasoning behind this decision should also be communicated.

Concrete floor contractor
The concrete floor contractor (CFC), who generally sub-contracts to the GC or works under a CM trade package, may also provide polished concrete applications, although this is rare. The CFC’s responsibilities should include identifying additional procedures and processes required by the specifications for placing and finishing the concrete. This is important so an acceptable esthetic and uniform quality surface finish can be achieved by the polished concrete contractor. To avoid potential conflict, these additional procedures and processes need to be costed and included in the CFC’s bid for the work.

Polished concrete contractor
The polished concrete contractor (PCC), who may also be the CFC, but is typically a specialist flooring company that applies toppings or coatings, should be responsible for producing the specified aggregate exposure and finished gloss requirements. The PCC should ensure all critical factors influencing the specific work, but outside of its control, are understood by all project team members.

Manufacturer or supplier
The polished concrete system manufacturer or supplier, who provide the densifiers, dyes, stains, and sealers comprising a particular system, should be responsible for educating team members on the important factors considered for optimal use of its products.

Optimum esthetic quality
There are many factors to be carefully evaluated to lessen or avoid potential problems and achieve adequate results.

Near-surface compressive strength
Achieving adequate compressive strength of test cylinders cast from concrete used for slabs and toppings is rarely a problem. However, achievement of optimal compressive strength of the near-surface concrete is critical for adequate wear-resistance performance of polished concrete—particularly when dyes or stains are included as part of the process, and this is not necessarily guaranteed by high compressive strength results.

Although concrete mix design parameters have a major influence on achievement of optimal near-surface concrete strength, even more important factors include whether ambient conditions are conducive to rapid drying, and how effective protection measures are during placement, finishing, and curing. Under rapid drying conditions, even high-strength concrete may not provide the required wear and abrasion resistance properties without adequate precautions. Therefore, planning for appropriate execution of protection measures can be critical.

Rapid drying conditions
In cases when rapid drying conditions prevail during  the placement and finishing operations, and the rate of evaporation becomes faster than the concrete can replenish surface moisture with bleed water, the concrete finishers can face difficulties timing the commencement and duration of the various operations—particularly the floating and trowelling procedures.

Rapid drying of the surface before commencement of curing procedures can then lead to several concerns. For example, after concrete placement desiccation of the near-surface zones could be sufficient to adversely influence the natural cement hydration reactions, leading to lower strength and reduced wear and abrasion resistance. This phenomenon could result in non-uniform wear patterns and varying degrees of finished gloss within the polished concrete finish. Further, difficult finishing conditions can result in non-uniform distribution of fine and coarse aggregates within the upper thickness of plastic concrete—especially if the concrete is over-worked. This could lead to non-uniform exposure of aggregate. Under extreme conditions, rapid drying of the surface can result in plastic shrinkage, cracking, or crazing—especially if protection is not provided during and after the finishing operation, and curing does not commence immediately after. (When drying conditions are severe during placement and finishing operations, CSA A23.1-09 (R2014) requires one or more precautions be taken, including dampening the subgrade prior to placement, covering the concrete surface with polyethylene sheeting between the various finishing operations, and applying fog spray immediately after placement and before finishing, or as a continuous operation. The use of spray-on mono-molecular materials (i.e. evaporation retardants) is also permitted, although certain precautions are advised).

Chemical densification
Contrary to some claims, grinding and honing the surface of concrete does not ‘close up’ or toughen the near surface thickness. In fact, these operations ‘open up’ the surface and increase porosity and the potential for the development of wear-patterns. This is why liquid chemical densifiers after the initial grinding operations are used. If stains or dyes are to be employed, they should be in advance of the chemical densification.

The most commonly applied densifiers are based on potassium, sodium, or lithium silicate. These materials chemically react with calcium hydroxide compounds formed by portland cement concrete’s naturally occurring hydration reactions. In very simple terms, the result of the reactions is the formation of additional calcium silicate hydrate within the cement matrix—a much more inert hydrated compound. These products of the reactions fill the capillary cores of the cement paste matrix, resulting in a denser, tougher near-surface zone.

However, use of SCMs, such as fly ash or ground-granulated blast-surface slag, have similar reactions and can therefore ‘interfere’ with the near-surface zone densification effectiveness by varying degrees. Concrete suppliers will routinely include CSMs within their mix design, and are permitted to do so by CSA A23.1-09 (R2014). However, the type of CSM and the amount used will vary from plant to plant and from region to region, potentially leading to variations in the results of polished concrete applications from project to project.

Floor flatness criteria
Uniformity of aggregate exposure is also influenced by the techniques used by the concrete floor contractor to achieve a given floor flatness (FF) and smoothness. (For more, see “Concrete Floors: Flatness vs. Smoothness[1]” by Keith Robinson in the September 2015 issue of Construction Canada). CPAA recommends use of a highway straightedge and the achievement of an overall flatness of floor flatness F_F50, with a minimum local value of F_F35. However, to achieve this degree of flatness, CSA A23.1-09 (R2014) recommends special narrow-strip placement with mechanical screeds and highway straightedges. These are unlikely to be utilized by the CFC unless required by the specifications. However, this author believes a more realistic tolerance for overall flatness would be a F_F25, achieved by a procedure typically including mechanical screeding and use of a highway straightedge before steel trowel finishing. Although they enhance the achievement of optimal flatness, great care must be taken when using a highway straightedge to avoid over-working the surface.

Other criteria
Other conventional practices that can adversely influence the achievement of acceptable esthetic quality include the use of below-slab moisture-vapour retarders (MVRs), construction of toppings, and curing procedures. Under rapid drying conditions, MVRs can provide similar effects to toppings cast over precast suspended slabs. Both can accentuate the problems associated with timing and duration of the various finishing operations, due to the creation of differential moisture conditions between the upper and lower zones of the slab or topping. This phenomenon can increase the potential for problems associated with curling and warping, as well as drying, shrinkage, and cracking. Topping delaminations can also occur under severe rapid drying conditions.

The absence of curing, or poor curing practices, can also result in similar defects. After termination of curing, it is important the surfaces of cured concrete slabs be kept free of any objects until polished concrete operations can begin. (Ideally, storage on a polished concrete floor should be prevented in any event.) This is especially important when dyes, tints, or stains are used. It can also be a particular concern if rapid drying conditions were experienced during concrete placement and finishing, or if curing was delayed. The undesirable result can be the creation of ‘footprints’ of the objects, which may become evident because of their influence on the uniformity of hydration reactions.

CPAA recommends evaporation control and wet curing, rather than topically applied curing compounds. However, for optimal results, this author recommends the concrete surface be well-wetted and covered with polyethylene sheeting immediately after final finishing. The sheeting should be positioned so contraction joints can be cut during the curing process, as soon as the concrete has sufficiently stiffened. The surface should then remain continuously wet for seven days.

Most product manufacturers recommend a minimum delay of 56 days be provided so the effects of drying shrinkage can be mitigated and undesirable stress can be avoided at joints—both of which can be a complication regarding timing for joint-filling. However, this requirement is due to the extended hydration reactions created by use of suppplementary cementing materials, which are not recommended to be used for concrete required to be polished. Therefore, in theory, joints could be filled much earlier.

Standards
CSA A23.1-09 (R2014) does not specifically cover polished concrete. However, the standard does establish some practices, techniques, and materials for conventional concrete that, if followed, could adversely impact the achievement of a uniform finish. Some of these practices have been previously discussed, but the standard also allows the lowest flatness and levelness tolerances to be used for residential, institutional, and commercial floors—the major marketplace for polished concrete.

Polished concrete contractor influence
The skills, abilities, and diligence of the PCC’s operators and supervisors will influence the esthetic quality of the final polished concrete floor. Therefore, the uniformity of the finish could vary depending on the individual’s skillset. A commitment to employee training will improve these characteristics and traits, while neglect in training can cause poor performance.

The quality and maintenance of the equipment used during the grinding, honing, and polishing stages can give variable results, as can the quality of the abrasives, how often they are replaced, uniformity of motion, RPMs, and equipment speed of operation. Further, the point at which the type of abrasive is switched can have an influence, as can how well the floors is cleaned between various stages.

Pre-construction meetings
The Ontario General Contractors Association (OGCA) and the Ready Mixed Concrete Association of Ontario (RMCAO) have published, Best Practices Guidelines for Concrete Construction. (This document can be downloaded from betonabq.org/pdf/aBest_Practices_Guide_.pdf[2]). Armed with an awareness of the issues discussed within this article, following the recommendations provided within the document chapters covering  “Quotation Submittal and Review Process”, and “Pre-Construction and Pre-Placement Meetings” should result in the best resolution of any challenges and produce the desired esthetic and durability properties for polished concrete floor slabs.

Conclusion
The concrete industry seems to be fragmented with regard to polished concrete. There are many companies and individuals who may have a high level of competence within their own area of expertise, but nonetheless have different levels of awareness and knowledge of best practices for polished concrete applications.

Since skill levels, materials, and techniques can vary between established PCCs, it is not surprising there continues to be occasions when expectations are not met. It is difficult enough for all project team members to have a reasonable chance of success—when weather conditions are adverse at the time of concrete placement and finishing, challenges can sometimes become insurmountable without adequate planning and execution.

The polished concrete industry is still relatively new. Unfortunately, learning experiences within the concrete industry have not always been shared well in the past—particularly with regard to problems. Therefore, unless already planned, perhaps now is the time for companies specializing in polished concrete to create or promote the formation of a Canadian Polished Concrete Association and provide peer-reviewed knowledge and expertise providing the needed education and thereby avoiding problems or conflict in the future.

The clause dealing with “Concrete with special performance or material requirements” in CSA A23.1-09 (R2014) welcomes proposals for the inclusion of new technology within future editions, provided performance criteria superseding conventional concrete technology is covered and substantive supporting data is included.

Paul Jeffs has more than 45 years of experience in the construction industry around the world. He is principal of PJ Materials Consultants Ltd., a Guelph, Ont.-based company that provides consulting and sub-consulting services across Canada for the investigation, construction, and restoration of masonry and concrete structures. He can be reached at pjeffs@pjmc.net[3].

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