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Understanding Polished Concrete

Brad Burns - HTC, Inc. Technical Director

(10-15-2015)

Understanding Polished Concrete

Polished concrete, discovered roughly 14 years ago, has etched its way into history as a sustainable, low maintenance flooring option in just about every market segment. We have all walked across a polished concrete surface at some point within the past few years. It is such a popular and fast growing industry, that in many cases it is outpacing consumers and contractors alike with new technology and methods. This fast growth has put the industry in a somewhat volatile state leaving the consumers confused as to what is and what isn’t polished concrete and what performance aspects to expect. Polished concrete can be a low maintenance flooring option with an extremely low cost life cycle compared to other floor coverings. However, to get the best performance, you must be aware of the processes required to produce a quality product.

Highway Screed 
 Bump cut or Hwy Screed used to increase floor flatness (FF)

 

The Concrete Polishing Association of America (CPAA) was formed in November 2009 to create standards for processing concrete to a polished finish and to help define these processes so that architects, designers, and consumers could better understand the selection process. The CPAA’s leadership consist of both manufacturers and contractors to offer continuity when creating these standards and procedures. This is very beneficial to the architectural, design and consumer community to help eliminate confusion and double standards in such a fast paced industry. These standards and process have permeated the entire market, even reaching other countries such as Asia, UK, Russia, South America, and many others.

 

What is “polished concrete” and what are the realistic expectations regarding performance? The CPAA has defined polished concrete as; “ The act of changing a concrete floor surface, with or without aggregate exposure, until the desired level of a glossy finish is achieved.” What does this mean? How is this achieved? How does this relate to performance? I will answer these questions and more as we explore the many different variables that contractors face when processing concrete to a polished finish.

First, let’s continue to further define “polished concrete”. There are three basic versions of polished concrete according to the CPAA. They are bonded abrasive polished concrete, burnished polished concrete, and topical polished concrete.

Polished concrete definitions

Bonded abrasive polished concrete is defined as the processing of a concrete floor surface with bonded abrasives until the desired class of aggregate exposure and the desired level of glossy finish is achieved. Processing is further defined as the act of changing a concrete floor surface by means of a multi-step mechanical process that involves cutting and/or refining the surface to maximum potential with a bonded abrasive medium where each step cuts progressively finer microscopic peaks and valleys.

 Burnished polished concrete is defined as the frictionrubbing of a concrete floor surface with abrasive pads until the desired level of glossy finish is achieved.

 Topical polished concrete is the application of a liquid applied coating to a concrete floor surface to achieve a glossy finish.

As you can see, the “glossy finish” is the common denominator with all three versions. This is also the culprit causing architects, designers and consumers confusion between the different versions because in most cases they all look alike. However, they perform quite differently, leaving the consumer questioning their choice for flooring. When comparing the performance of the different versions, it is imperative that the processes be evaluated and understood.

Bonded abrasive polished concrete is the most popular and sought after version of polished concrete. It is also the most complicated of the processes and requires the installer to have a very in depth understanding of the concrete as well as the diamond tooling and equipment used in the process. There are many variables to be considered when providing bonded abrasive polished concrete.

Understanding the process

First, the concrete substrate must be hard and flat. The ideal mix design would be a 4000 lbs mix, minimizing any admixtures, and finished to at least a 50-floor flatness (FF). This is relatively easy to achieve with a “bump cut” or “highway screed” – a 12-inch screed used to locate and remove the high and low spots of the concrete substrate during the finishing process.

Before the grinding is started, it is very important that the polishing technician, architect, general contractor and customer have the same expectation of the design and finished product. If the customer is expecting a polished floor without aggregate showing in the finished floor then the polishing technician shouldn’t start with a very aggressive diamond tool. A mock-up will usually solve this issue and offer the polishing technician valuable information about the technical process required to deliver a quality product to the customer based on the set of variables surrounding this particular project. Once the design intent or specification is agreed upon and the mock up has been approved (and the concrete has gained sufficient strength), then the polishing technician can begin his grinding process.

The “polishing process” is divided into three sections; grinding, honing, and polishing. Each phase has certain grit sequences and is designed to allow the polishing technician a gauge to evaluate performance of the overall process. This helps to eliminate confusion, frustration and disappointment in the final steps of the process when the overall goal has not been achieved. The last thing a polishing technician wants is to have completed the entire process and find that the clarity, sheen or aggregate levels are not what the customer expected and have to start all over.

Understanding abrasive diamond tools

Diamond abrasives are the most versatile, most important and by far the most misunderstood tool at the polishing technicians disposal. When the polishing technician has reached a level of craftsmanship that he fully understands the result that a particular diamond tool is capable of producing based on its design, grit and matrix, then he is capable of achieving dynamic results on a consistent basis. This understanding comes either with experience or intense training (and for some individuals it requires both!).

 

 
 Scratch pattern from metal bond diamond tooling using an HTC 650 Grinder

Diamond tools are available in a range of grits from #6 to #3000 with the lower numbers being a more aggressive tool. They also come in two different matrixes – metal bond and resin bond. Metal bond tools are used primarily for the grinding and early honing stages of the polishing process. Resin bond tools complete the remainder of the honing steps and further refine the substrate throughout the polishing phase. To get the best results from the diamond tools, I must break this down even further and explain the differences between metal and resin bond tools.

 

Metal bond diamond tooling is available in several matrixes, otherwise known as hardness, and designs in order for the polishing technician to receive the intended result. Each diamond tool will leave its own unique imprint on the concrete surface, commonly referred to as a scratch pattern. Metal bond diamond tools vary in design; single segment, double segment, 3 segment, 5 segment, or 10 segments allowing more surface contact with the abrasive tool to better refine the concrete substrate.

 

The polishing technician must correctly match the diamond abrasive tool’s matrix, design and grit with the hardness of the concrete substrate and the architect’s design to achieve the proper end result. This is where the breakdown in the process usually begins since most polishing technicians are unaware of the intended result with a diamond tool’s design or grit. Most polishing technicians are taught to follow a specified grit sequence and not inspect the flooring to ensure that the diamond tool is actually working. As a result, they go through all the correct steps but fail to reach the final expectation of the customer.

 

When the polishing technician has determined that the diamond tool is working and is properly matched to the concrete substrate, then they must achieve maximum refinement of the concrete surface before moving on to the next grit in the polishing process. This particular step of the process is crucial to achieving consistent results on each project. This step is also the most overlooked and misunderstood throughout the entire process, so let’s take a close look at it for better clarification.

 

Below are two images of 150-grit metal bond scratch patterns. The one on the left was created by the technician moving quickly over the substrate. The one on the right was created by the technician moving slowly and methodically over the substrate, taking the time to ensure that the diamond tool was refining ALL of the concrete substrate with each pass. As you can see, there is a huge difference between the two and the difference can also be seen in the end results. The photo below illustrates an area that was processed using the same polishing technician, equipment, and polishing process grit sequences.

 

However, the area on the left shows that the polishing technician failed to achieve maximum refinement with the diamond tooling compared to the area on the right. The results are astonishing when the polishing technician is taught to properly use the diamond tooling to achieve the best potential outcome for the project. As you can see, the floor has better clarity and sheen. In addition, because the substrate has a more refined surface, the performance of the flooring will be much greater offering better slip coefficient, easier maintenance, and substantial longer lifecycle. The grinding and honing phases should constitute about 60 percent of the project’s time frame. When the polishing technician has taken the time to reach maximum refinement in the earlier stages, then the later stages (polishing) seems to be relatively quick and simple.

 

 Scratch Pattern Comparison

 The scratch patterns on the left were created by the technician moving
quickly over the substrate, while the one on the right was created by the
technician moving slowly and methodically over the substrate. The scratches
are still there, however, they appear invisible because they are concentrated
in a tight area, which has refined the concrete.

Understanding the chemistry

Chemicals used during the polishing process are also an area of mass confusion. What is a densifier? When should it be applied? Should polished concrete get a “sealer”?

These are just a few of the questions I hear.

Densifiers are used to harden the concrete substrate. This is beneficial in several aspects. First, harder concrete is easier to polish and create a high gloss finish. Secondly, a harder concrete substrate performs better as a flooring option. Densifiers are complicated to understand but relatively easy to use. Densifiers usually consist of either lithium, potassium, or sodium silicate. The lithium, potassium or sodium portion of the chemical make up is used to transfer the silicate into the concrete. When silicate makes contact with calcium hydroxide, which is commonly referred to as free lime, located within the concrete substrate, it is transformed and creates a crystalline structure. This crystalline structure helps to add strength and abrasion resistance to the concrete surface. When the densifier is added to the concrete within the polishing process is best dictated by the concrete itself. If the concrete is soft and porous then using a lot of densifier throughout the entire process would be very beneficial. This would help the polishing technician achieve maximum refinement with the diamond tooling. However, if the concrete substrate has a high structural strength, was finished to a hard, dense surface, then little additional densifier would be required. Most densifiers react with concrete in the same fashion creating a harder, denser surface. So why would a polishing technician choose one brand over another? Usually this decision is based on a relationship with the manufacturer.

There are also some labor savings between the different brands/types of densifiers that are a consideration for the polishing technician.

 

Sealers should be a major concern for both the consumer and polishing technician. Sealers are widely being misused throughout the industry to create a false shine, costing the consumer valuable savings in both maintenance and performance of the flooring. Uneducated, and in some cases unethical, contractors are using sealers to eliminate some of the processing steps and covering up the lack of refinement previously described. In the process, they are lowering the price and expectation of quality with each project. Stain protectors, commonly referred to as “guards” within the industry, provide a level of protection to the concrete surface against etching and staining. Manufacturers are working hard to improve these products to offer the customer a level of stain protection along with the added benefit of less maintenance.

 

So with all of these complex issues, why should you choose polished concrete as a flooring option or a vocation? From a consumer’s standpoint, polished concrete offers the best lifecycle, with low cost maintenance, of any flooring choice available in the market place. And as a vocation or career, there is nothing like seeing an old, worn concrete slab turned into a remarkable, dazzling, functional work of art. Consumers throughout the world are reaping the benefits of polished concrete flooring: low maintenance, low installation costs, long life cycle, and the satisfaction of utilizing a reusable/recyclable resource. Polished concrete has penetrated every market from small residential projects to extremely large manufacturing facilities. As with anything else in today’s society, to reap the rewards and benefits of a product one must take the time and invest in educating themselves on the product and/or service. With the proper education you will be able to make the right choice in design, function, aesthetics and level of craftsmanship that is right for you and your project.

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