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Stripping Concrete: Mechanical or Chemical?

Cuviello Concrete achieved the clarity of reflection in this floor by fully refining the surface. “We do not skip any grits in our eight-step grit sequence, and we always drop back one grit size with a separation of at least 50 when we transition from metal to resin abrasive,” says owner Jim Cuviello.
Cuviello Concrete achieved the clarity of reflection in this floor by fully refining the surface. “We do not skip any grits in our eight-step grit sequence, and we always drop back one grit size with a separation of at least 50 when we transition from metal to resin abrasive,” says owner Jim Cuviello.

While a declining economy is closing the door on a lot of new construction, it is opening a window for renovation. Refurbishing and reusing is an attractive option for homeowners and commercial enterprises that might have built from scratch in more extravagant times.

Grinding is the most common way to mechanically strip a concrete floor before a new sealer or finish is applied. It is also the first step in polishing.
Grinding is the most common way to mechanically strip a concrete floor before a new sealer or finish is applied. It is also the first step in polishing.

This is a window of opportunity for decorative concrete contractors who can help customers make an old floor look like new again. And since most buildings have a concrete floor somewhere, from a home basement to a commercial warehouse, the potential market is sizeable.

Of course, the first step in making an old concrete floor look new is getting rid of everything that makes it look old. In this case, past is prologue. The original floor covering or finish not only determines what is required to strip and prep the floor but may impact the look of the finished project as well.

Materials that may need to be removed or repaired include grout, sealers, polymeric coatings, and adhesives and mastics for carpet or tile. There are two general methods for stripping concrete floors: mechanical and chemical. The choice between the two depends on several factors, including what material must be removed, job-site limitations, and what equipment and expertise are available.

A chemical stripper was applied to remove chalk lines and the clear enamel spray coat that had been used to protect them.
A chemical stripper was applied to remove chalk lines and the clear enamel spray coat that had been used to protect them.
Even though the chalk and enamel are gone, a faint dark line where the chemical stripper was applied “ghosts” through this stain finish. To avoid this, stripper should have been applied to the whole floor.
Even though the chalk and enamel are gone, a faint dark line where the chemical stripper was applied “ghosts” through this stain finish. To avoid this, stripper should have been applied to the whole floor.

Mechanical Concrete Stripping
Mechanical techniques for abrading a floor include shot-blasting, sandblasting, grinding and scarifying. Of these, shot-blasting is the most aggressive. Tom Sylcox, production manager for Floorgen, based in Newburgh, N.Y., says shot-blasting is typically used to remove acrylic coatings. "The challenge is making sure not to go too deep," he says. "Removing 'craters' is not an enjoyable process. It takes time and increases the cost to the customer."The uneven surface left by shot-blasting can cause other problems as well, explains Jim Cuviello, director of the Concrete Processing and Polishing Technical Institute and owner of Cuviello Concrete, based in Stevensville, Md. Shot-blasting can be used for prepping concrete, but it's best used by an experienced operator and under certain circumstances, he says. "In most cases, an inexperienced operator will leave deep cornrows that are problematic if the floor is going to be polished. This is not the best way to prep a floor for polishing. Cornrows can even telegraph through a thin overlay."

The light epoxy coating on this floor was initially ground with a 70-grit metal abrasive in an aggressive pattern with lower surface contact and higher psi. This was followed by 70-grit metal with more surface contact.
The light epoxy coating on this floor was initially ground with a 70-grit metal abrasive in an aggressive pattern with lower surface contact and higher psi. This was followed by 70-grit metal with more surface contact.

Sandblasting is a similarly aggressive technique for removing tough coatings, and it's best suited for outdoor projects or large, unoccupied indoor spaces.

Another aggressive mechanical stripping technique is scarifying. This can be thought of as a compromise between blasting and grinding. A scarifying machine looks like a grinder, but its "star cutters" operate in three dimensions, breaking up the surface below. This makes scarifying well suited for removing tough materials such as mastics, epoxies and nonskid coatings. It also profiles the concrete for better adhesion of overlays or for thick coatings.

The most widely used mechanical technique for stripping is grinding. As grinding is also the first step required for a polished concrete floor, it often serves a dual purpose. "We strip mechanically whenever we want to profile the concrete surface for an overlay, epoxy or polished concrete project," says Thomas A. Jagger, owner and president of Jagger Scored/Stained Concrete Inc. in Texas.

Shawn Halverson of Surfacing Solutions Inc. in Temecula, Calif., adds: "For an overlay, grinding or scarifying is great. It cleans the sealer off and profiles the floor in the same step."

"We go through eight levels of grit to grind and polish a floor," says Cuviello. "The one we start with depends on the condition of the floor. Removing coatings or glues requires lower (coarser) grit. The lowest available is 6, but we usually start at 30 to prep. The coarser the grit you have to use to prep the floor, the longer it takes for the next grit to refine the concrete and the longer the overall process takes. The higher the starting grit, the less work it is for the contractor."

Sylcox recommends grinding to give old concrete a fresh, clean look. "The cream layer takes so much abuse," he says. "Start with a 30 grit to take the cream off and get to the nice exposed aggregate that you can now polish or seal."

Halverson agrees. "If you're going to stain, grinding can add to the look," he says. "For example, you may need to grind to get carpet glue off, but then do a nice clean grind to expose a little aggregate and you get a beautiful effect."

The epoxy on this lighting factory floor was removed with a scarifier.
The epoxy on this lighting factory floor was removed with a scarifier.
The factory floor was then ground and polished to a high gloss, finishing with a 3,000-grit polish.
The factory floor was then ground and polished to a high gloss, finishing with a 3,000-grit polish.

Chemical Concrete Stripping
Despite its rather fierce-sounding name, chemical stripping can be safe for the environment and leave less of a mark than mechanical means on finished concrete. "We strip chemically whenever we don't want to change the concrete surface, say for an acid-stain project," Jagger says.

Chemical stripping is by far the most common method of removing a sealer to prepare concrete for resealing or refinishing, according to Chris Sullivan, national sales manager for QC Construction Products. He explains that there are three families of chemical strippers: solvent-based, caustic and biochemical.

The most commonly used chemical strippers are solvent-based and typically contain methylene chloride. These are also the most aggressive. "This is a very reactive, flammable, strong oxidizer, and needs to be treated with respect," Sullivan says. "Contractors must wear rubber gloves, long sleeves, and respirators, as this will burn skin and lungs."

While such an aggressive stripper is not required in every case, methylene chloride does have its uses. "If you've got four layers of 20-year-old epoxy and you can handle the fumes and removal, this is your choice to get it stripped with just one application," Sullivan says. These solvent strippers work from underneath a coating by breaking down the bond between the coating and the substrate.

Caustic chemical strippers are water-based alkaline materials that are very reactive. The alkaline active ingredients react with esters and oils in certain sealers, breaking down the materials and turning them into soap, a chemical reaction known as saponification. "While caustic strippers are readily available, we don't see them much in decorative concrete because we don't see many oil-based sealers these days," Sullivan says.

Biochemical strippers, the newest chemical strippers, are growing in popularity. They are based on oils or esters derived from natural raw materials such as soybeans, pinesap and citrus trees. These take off most coatings, according to Sullivan, but might take longer or require multiple applications to work on high-performance coatings such as thick epoxies and polyurethanes. These materials clean up with water and often have a citrus smell. While they are not hazardous, Sullivan still recommends the use of gloves when handling them, as they do contain chemicals as active ingredients. These create a reactive stripper that Sullivan claims is more environmentally friendly than solvent-based and caustic strippers. They are made from renewable resources and do not produce hazardous byproducts. They are similar to caustic strippers in working time and application.

Aside from protective clothing as recommended on the product label, no special equipment is required for chemical stripping. "The process of applying the stripper is simple," Halverson says. "Just pour it out, squeegee or broom it evenly and wait. The harder the surface is you're trying to remove, the longer it will take to break down." He recommends removing the stripper after an hour and applying a fresh batch if there are multiple layers of coating on the floor.

Most chemical strippers are supplied as gels. Gels stay in place and dry out more slowly then liquids, keeping the reaction working longer. "Gel is also easier to clean up," Sullivan says. "If it is a smooth surface, just scrape it off with a large blade scraper. On a textured surface, use a brush. Citrus- and soy-based gels can go directly into the regular dry waste. Methylene chloride stripper waste should be allowed to dry to a solid state before disposal. Never put any chemical stripper down the drain."

While the choice between mechanical and chemical stripping depends on several factors, it is certain that a floor must be thoroughly stripped and cleaned of contaminants such as adhesives and old sealers for a concrete renovation project to succeed.

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