by Joe Nasvik

 

Imagine a concrete floor as shiny as glass–this is a look that diamond polishing can achieve. Two years ago very few people in the United States had even heard of polished concrete other than terrazzo floors. Terrazzo floors use carborundum stones to grind and polish floors to a 120- to 150-grit finish (grit refers to the micron particle size of the diamonds or other grinding abrasive material). Diamond-polishing technology now makes it possible to grind and polish much faster on concrete surfaces at up to 3000-grit finishes.

Today, most decorative concrete contractors are aware of the creative possibilities that polishing techniques can add to their finishes. But the reasons for polishing concrete go far beyond decorative. Warehouse owners, for example, are willing to pay for diamond polishing of their floors in order to reduce maintenance costs and solve problems involving sealers or water vapor transmission. John Abrahamson, National Sales Manager for Vic International, Knoxville, Tenn., notes that an added benefit for warehouse applications is that forklift tire marks don’t adhere to the surface.

But the new technology for polishing concrete comes from the stone-polishing industry, not the terrazzo industry. According to Abrahamson, it has been available for 4 years in the United States with significant improvements in the past year–especially in dry, dust-free grinding. Although there have been polished concrete floors in Europe for about 8 years, advertised as “superfloors,” developments in diamond technology are the result of U.S. efforts.

Advantages of polishing

Saving money on maintenance, solving existing concrete problems, providing commercial floors that don’t show traffic wear (store aisles, for example), and providing new, artistic decorative concrete surfaces are the most common advantages of polished concrete.

 

Polished concrete floors are essentially no-maintenance surfaces. Owners of commercial and warehouse floors typically find it expensive to maintain their floors with sealers and waxes. And if a floor surface is sealed so that water vapor can’t get through, there can be additional problems down the road. Removing all previous treatments and then polishing the floor can be a one-time fix that saves money. After diamond polishing, the only maintenance required is removing dirt, oils, and other materials that could stain. Even chewing gum won’t stick to a polished floor, according to Bill Kulibert, president of American Polished Concrete, Shreveport, La., a company that specializes in warehouse flooring and large commercial projects.

Diamond-polished floors also have good water vapor transmission. Some of Kulibert’s work has come from others’ failures, for example removing epoxy sealers that have caused trapped water problems or that haven’t bonded properly.

Decorative effects

There are two ways to polish concrete:

* Polish only the cement paste and fine aggregate brought to the surface of the concrete during final troweling.

* Grind and polish into the matrix of a slab to expose large aggregates.

To further enhance the look, special aggregates like broken glass, seashells, bits of plastic, or pieces of metal–and anything else creative minds can imagine–can be seeded into the surface of freshly placed concrete.

Doug Demmert, Demmert & Associates, Glendale, Calif., was recently asked to grind a poorly done 3500-square-foot colored residential floor. Removing the sealer and polishing the floor solved the problem and provided a new decorative effect.

Decorative concrete contractors like polished concrete because of the range of artistic possibilities that can be achieved. Demmert, whose work is mostly interior residential, often grinds floors deeply enough to reveal the aggregate, resulting in a look that many customers find appealing. Recently, he chemically stained a floor when the grinding process was half complete. “The resulting finish and color look more natural than typical chemical staining,” he states.

Lance Boyer, Trademark Concrete, Los Angeles, who subcontracts polishing, recently completed a project with Type III cement, which he says provides a softer color. He seeded aggregate onto the surface and floated in zinc strips, then exposed them by grinding and polishing. His company also recently polished an integrally colored slab, adding chemical stain highlights just before final polishing.

Scott Thome, a sales representative in Minnesota for Los Angeles-based L.M. Scofield, is aggressively pursuing the polished, colored overlay market. Taking a lesson from the terrazzo industry, he will set zinc strips on a large upcoming project to form complicated patterns and will place several colors of overlay cement between the strips. The overlay will be 3/8 inch thick, making it economical to use the more expensive green and blue concrete colors. Because there are no large aggregates, the polished appearance is more even and controllable. The architect on this job, Sam Sisic, Antunovich and Associates, Chicago, said that this installation will have very heavy foot traffic and acknowledges that the owner is taking a chance using a new product like polished concrete, but he is confident that the results will justify the decision.

Machines and equipment

Machines and equipment for polishing concrete come from the stone-polishing industry and are manufactured primarily in Germany, Sweden, and Australia. They fall into two categories: floor machines and handheld tools.

The most important feature of floor machines is a planetary drive system–a large primary disk between 19 and 32 inches in diameter traveling in one direction and holding smaller satellite heads that rotate in the opposite direction. Diamond polishing heads are attached to the satellite heads, which are 8 1/2 to 10 1/2 inches in diameter. The benefit of a planetary drive is that the diamonds travel randomly over a large diameter, resulting in flatter floors and no linear tracking marks. Traditional machines have individual grinding heads that travel in their own radius, leaving scratches or permanent marks.

A polishing machine should have at least three primary features:

* Variable speed adjustments

* Capability of wet or dry grinding

* Weight–typically between 300 and 1500 pounds (heavy machines grind more aggressively).

Sinclair Equipment, Diamond Springs, Calif., includes two motors on its machine, one for the planetary head and one for the satellites. Each motor has variable-speed control and can be reversed. Frank Sinclair, the owner, says that cutting is more aggressive in reverse.

Floor machines have large power requirements–from 220 to 460 volts–often 3-phase, and up to 30 amps. European machines, which operate on electrical standards different from those of U.S. machines, require transformers to step voltages up or down. Floor machine prices range from about $7900 to $30,000, and many can also be used to remove coatings on floors.

 

The smaller hand-held tools are used for polishing concrete countertops and treating the edges and tight places on floors where large grinders can’t reach. The Leitch Company, San Leandro, Calif., makes a 12-inch planetary-head, hand-held machine with three satellite disks. It is the choice of many contractors for small polishing operations. Several companies make angle grinders equipped with either water or dust pickup systems for even smaller polishing tasks. Prices for these tools run from $500 to $1700.

Wet or dry?

Most equipment on the market can be used either wet or dry, but manufacturers and contractors don’t agree on whether grinding and polishing are best with or without water. Some claim the finish looks better dry; others think that the polish looks more natural wet. Special dust-collection systems are needed to create a safe work environment for dry grinding. Abrahamson encourages contractors to use the dry system, saying that the grinding process is faster, the operator can see his work better, and the dry dust is more environmentally acceptable than the slurry from wet grinding. His company sells Swedish-made dust-collection equipment that removes 99.99% of the dust, making jobsites safe and clean. Dry cutting heads travel at higher rpm and require more power.

The problem with wet grinding is collecting and disposing of the slurry. Today that involves extracting most of the water from the slurry, treating it to get a neutral pH, then legally disposing of both the water and the remaining solids.

Opinions about the level of polish that can be attained by wet or dry systems also differ. Final dry polishing produces significant heat between the diamond and the concrete surface. The heat burnishes the surface, creating a look different from a wet polish.

Manufacturing diamonds and choosing the right one

Diamond-grinding and polishing-head technology are complicated, according to Ernesto Dossena, with GE Superabrasives, Italy, the largest manufacturer of U.S. diamonds. GE develops diamonds for maximum finishing results and productivity when grinding and polishing concrete, terrazzo, and stone. These diamonds have excellent heat-transfer capabilities for dry grinding and improved lubrication qualities for wet grinding. They are also coated to ensure good contact with matrix materials. Titanium-coated diamonds provide the best possible bond. “Rough polishing demands blocky, high-impact-resistant crystals; midstage polishing [demands] a high percentage of cubo-octahedral crystals with high fracture strength; final stage polishing [demands] irregularly shaped crystals,” he adds.

It’s also very important to choose the right matrix for both productivity and diamond life.

 

The choice of polishing heads depends on the hardness or softness of the material being polished. Use the hardness of the aggregates in the concrete, and regularly consult your supplier, to guide your decision about the right diamond product to use.

Abrahamson agrees that diamond technology is the key to the polishing process and adds that the progress of U.S. manufacturers makes polishing concrete more productive here than in Europe. Some polishing machine manufacturers have also developed their own proprietary diamond mixes. Diamonds for coarse grinding are usually embedded in a metal matrix that wears away just fast enough to expose new diamonds. According to Elna Beck, a technical salesperson at Leitch, polishing hard concrete requires diamonds embedded in a soft metal matrix. The reverse is true of soft concrete (soft concrete results in the shortest diamond life).

According to Dr. Kris Kumar, GE Superabrasives, Worthington, Ohio, resin-bonded diamonds (used for final polishing) are made by adding diamonds to either thermoplastic resins or thermosetting resins, then molding and hardening. For smaller applications, diamonds are mounted in a polymer matrix and secured to a cloth backing, bonded to a “hook and loop” pad for easy mounting to the polishing tool.

How the process of concrete polishing works

The first step in polishing concrete is to remove any existing sealers and coatings by shotblasting or with coarse-diamond cutting heads. Next, any defects are patched. Cracks, control joints, and isolation joints are filled with flexible epoxies so that the grinding bits can easily pass over them.

Jason Root, a sales and technical representative for Blastrac, Phoenix, recommends using 25- or 30-grit diamond cutting heads to remove concrete faster if an owner wants to reveal the aggregate in the concrete. Abrahamson recommends starting with 40-grit diamond cutting heads if polishing is to be confined to the surface layer of cement paste and fine aggregate. The purpose for this first grind is to remove the lumps and bumps left by the finishing process and also to remove the curling that typically is found at control joints. Most of the removal occurs in this first step.

Soft concrete consumes more diamond than does hard concrete, and hard concrete typically produces a better polish. Tom Graf, owner of Graf Construction/Concrete Arts, Hudson, Wis., thinks it best to perform all coarse grinding steps first and chemically harden the concrete before polishing begins. Abrahamson and others recommend two applications of a chemical hardener such as sodium silicate (sold under several different trade names) after the first grind. Lithium silicate or fluorosilicate products will also work. These products react with calcium hydroxide in the concrete to produce additional cementitious material. The process closes some of the pores in the concrete, adding stain resistance. Silane-based sealers can increase stain resistance even more.

Graf prefers dry grinding with machines that weigh up to 600 pounds. While the main part of the floor is being ground, other workers use hand grinders and a specialized walk-behind grinder to treat edges, following the same steps as on the rest of the floor. The entire process removes approximately 1/16 inch of concrete.

Kulibert states that foot traffic and forklift traffic will produce a 400-grit polish on concrete, so he typically polishes to this level or a little higher, which is enough to produce a low-maintenance surface.

Exposed-aggregate finishes

Approximately 3/16 to 1/4 inch of surface removal is necessary to expose the large aggregate. This can involve extensive grinding time unless very coarse grinding heads or scarifying equipment is used first. Installing exposed-aggregate polished floor surfaces can be risky business–especially if you didn’t place the concrete. It’s common to discover locations where too little aggregate shows on the surface because workers walked through the fresh concrete, pushing down the large aggregate. Concrete for exposed-aggregate polished floors should be placed with the same precautions as used for regular exposed-aggregate surfaces.

Here are some tips for placing concrete for polished exposed-aggregate finishes:

* Increase the weight of large aggregates in the concrete mix by 300 to 400 pounds (as much as 2300 pounds total).

* Use a well-graded aggregate mix that provides more stone-to-stone contact.

 

* Place low-slump concrete so that segregation in the mix is minimized.

* When placing concrete, have workers use shovels to remix the concrete wherever they step.

* Broadcast seeded aggregate on the surface and carefully float it into the surface.

* Be careful not to leave bull float marks. “Cream” left at the edges of the bull float can leave streaks, too.

* Install the flattest possible surface, which will greatly enhance the final appearance of the job and save time and money during polishing.

* To save grinding time, after initial set try washing off just enough cement paste to reveal the tops of the aggregate.

Concrete countertops

Fu-Tung Cheng, Cheng Design, Berkeley, Calif., designs kitchens and precasts concrete countertops. In his book Concrete Countertops he details his process for grinding and polishing concrete to remove surface blemishes, efflorescence, color irregularities, patches, and blemishes left by the mold. Integrally colored concrete, special aggregates, and embedded objects greatly add to the appearance. By casting in molds, face down, surfaces to be ground are very flat, resulting in less grinding time and a more uniform result. Cheng performs the coarse grinding before the concrete is 10 days old because it’s faster and easier. Fine polishing is completed after 10 days and before the cure cycle is complete. He typically polishes up to 600 grit, seals his countertops, and recommends maintenance with wax.

 

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