Densification for High Performance Floors

Transcription

1 Densification for High Performance Floors Colloidal Silica Densifiers Harden Concrete for Economical, Sustainable Flooring Solutions By Steven H. Miller In many cases, the best floor-covering is nothing. Leaving the concrete slab exposed and finishing it as the floor can be economical, aesthetically successful, and sustainable. It is so durable that it may well be the ultimate" floor the last floor that will ever need to be installed in the space. The key to optimizing the performance of concrete as a floor is densifiying. Densifiers make concrete floor more durable, and abuse-resistant. Densification eliminates wear-induced powdering of the surface known as dusting. It makes the surface less permeable to liquids, improving stain-resistance and making cleaning easier and more eco-friendly. The densified floor does not require sealants or waxes. Densifiers have been used for decades on industrial floors where appearance was not a paramount concern, and more recently as an integral part of finishing concrete floors to architectural appearance standards. Now, the most recent advance in densifier technology colloidal silica has made the process easier, faster, more economical, more sustainable, and more effective. Why Densify? New or existing concrete can be improved by densification. It transforms concrete into a more practical flooring solution, and is applicable to a wide variety of projects. Owners of retail, commercial, and institutional environments are opting to expose the concrete floor instead of covering it. Densification expands appearance finishing options, enabling the success of finishing techniques such as burnishing and polishing. A densified, diamondpolished concrete slab can achieve a high gloss that can rival the look of natural polished stone at a fraction of the cost. A slab burnished to a sheen after densifying can achieve almost as attractive a result, but faster and even more affordably. Combined with techniques like dyeing, staining, or grinding to expose aggregate, a broad palette of visual options are available for taking concrete beyond a featureless gray surface. The cost of densification and polishing is competitive with the least expensive floor coverings available. Even with the added expense of coloring, it is still more affordable than the lowest end carpet or stone floors. Densifiers are also used where appearance is not a concern because of the improved performance of the slab. As it is easy to keep clean and safe, the 6/2/09 page 1 of 10

2 densified floor is suitable for warehouses, factories, and maintenance facilities such as automobile service bays. Chemical densifiers (also called hardeners or densifier/hardeners) are a small investment that leads to long-term savings and improved performance for decades. Using the concrete as the floor is one of the most sustainable flooring solutions available. The concrete slab is the floor that's "already there." Forgoing floor covering reduces material consumption during construction, and may reduce it on an on-going basis for the life of the floor. The concrete and its surface finish will likely outlast many replacement cycles of materials like carpet or vinyl tile. Concrete has no negative impact on indoor air quality: neither the concrete nor the densifier emit volatile organic compounds (VOCs), a problem associated with many floor covering materials or the adhesives used to apply them. Easier cleaning reduces maintenance-related energy consumption, and eliminates the need for waxes and harsh chemical cleansers and strippers. Polished concrete may also reflect and distribute light differently, making it possible to save energy by reducing the use of artificial lighting. The denser floor is also more healthy for human habitation another important aspect of sustainability because there are fewer places to harbor microbes or allergens. How Densifiers Work The need for densifying is rooted in concrete s nature. A concrete slab, in its natural state, is softest at the top layer. Bleed water rising during concrete finishing brings the finest aggregate particles and laitance to the surface. These soft, fine-grained materials can be troweled to a smooth, even surface, but they are not as hard as the slab s interior parts. The bleed water also increases the water-to-cementitious material ratio (w/cm) at the concrete surface, reducing its strength. Concrete that tests to a serviceable 20,680 kpa (3000 psi) or greater compressive strength can have a thin surface layer that is considerably softer and less resistant to abrasion, wear, and dusting. This soft material is also very difficult to polish, and not hard enough to keep its polish under heavy traffic. Water evaporating from fresh concrete also creates a pore structure in the slab. These pores allow the surface to absorb liquid spills readily, making it easy for the concrete to stain and harder for it to be cleaned. They can also interfere with the scratch patterns of the polishing process. A chemical densifier can improve both these aspects of the slab. Densifiers are silica-based compounds that react with lime (calcium hydroxide) in concrete. During hydration, approximately 20 percent of a concrete mixture s portland cement is converted to lime, which has no structural value in concrete. This lime stays in the slab. Densifiers react with lime to form calcium silicate hydrates (CSH), the same type of mineral compound that acts as the binder in concrete and imparts strength. Additional CSH fills the pores in concrete to increase the hardness and stain resistance of the surface. 6/2/09 page 2 of 10

3 Products that react with lime in this manner "pozzolans" such as fly ash or silica fume are often added into a fresh concrete mixture before it is placed, to increase strength and reduce cement consumption. Densifiers, by contrast, are surface-applied after the concrete has set. They can be used on freshly placed concrete, or on slabs that have been in place for many years. Advanced Densifiers A new generation of densifiers based on colloidal silica is replacing older densifiers based on chemical compounds called silicates. Colloidal silica performs the same functions, but makes the densification process itself faster, easier, more affordable, and more sustainable. It is competitive in material cost, and reduces time and labor expenses for densification by as much as 75%. Colloidal silica has also been used to rescue slabs that have not responded to silicate treatments. (See Sidebar, Densifiers in Action: Two Case Studies) The first generation of concrete densifiers was generally made from silicate compounds such as sodium silicate and potassium silicate; this range of silicatebased compounds was later broadened to include lithium silicate. All these silicate compounds are highly caustic, with a ph of 11 to 12, similar to the alkalinity of lime itself. Sodium- or potassium-silicate application is also timeconsuming and labor-intensive the chemical has to be worked with a broom for an hour to help precipitate silica and scrub it into the slab surface. The process leaves behind a caustic, gelatinous slurry that has to be scrubbed off thoroughly and disposed of. (Sodium hydroxide, also known as lye, is a by-product of sodium silicate densifiers and is sometimes present in their residue.) Applicators often try to neutralize the slurry with other chemicals before disposal, costing additional time and money. Many silicate-based densifiers require an overnight curing period before polishing can begin. Inadequate removal of sodium- and potassium-silicates often leads to concrete discoloration by salt deposits a problem known in the industry as "whiting." The only way to remove whiting is to re-grind the slab, an expensive and timeconsuming process. Whiting can also be a danger with lithium silicates if the material is over-applied, tempting some applicators to under-apply the product and ultimately fail to adequately densify the floor. The high ph of silicate materials also makes them dangerous and unpleasant for applicators to handle, especially during the removal process. With increasing restrictions on disposal of caustic substances and construction waste, properly getting rid of the silicate slurry has become an issue for applicators to deal with. The new water-borne colloidal silica product, available from Lythic Solutions ( under the trade name Lythic TM Densifier, eliminates these problems. It is made from 5-nanometer diameter amorphous silica particles, mechanically suspended in water rather than chemically tied up in a compound. 6/2/09 page 3 of 10

4 This colloid has a much lower ph. It is shipped as a concentrate that has ph between 9.5 and 10.5; ph is measured on a logarithmic scale, so this represents a ten-fold to hundred-fold reduction in alkalinity compared to silicate-based densifiers. The concentrate is designed to be diluted 4:1 with water, and dilution of the concentrate to working solution may alter the ph slightly, depending on the acidity of local water supply, but this has not been found to impact performance. Lythic Solutions president David Loe says that he favors concentrate because he does not want to make his customers pay for shipping water. Colloidal silica is more immediately available for reaction in concrete than a silicate compound the molecule has more chemically reactive sites, and the greater ph difference between colloidal silica and lime makes the reaction begin quickly, within minutes after application. The particles consist of nearly pure silica, and the colloid s extremely low sodium content eliminates the danger of whiting. There is no danger of over-application. The simplified application process reduces risks and cost, and only minimal training is required. There is no removal step (as required with silicates) and consequently no caustic slurry to dispose of only a small amount of dry powder residue that can be broomed off, or will be vacuumed up during polishing. The time and steps saved by eliminating the scrub-on/scrub-off process lowers labor costs, potentially enabling applicators using colloidal silica to keep prices down. Applicators report savings of 10 percent or more of the entire project labor, plus savings from the elimination of scrubbing equipment. Eliminating overnight curing can also reduce transportation-related energy consumption by minimizing the number of return trips to the jobsite. As with any new construction product, the safety of the material is an important concern. Although the use of colloidal silica in construction is new, the material has been employed in other industries for decades with well-documented safety. The colloidal silica used in concrete densifiers is amorphous silica, which is not associated with silicosis (a disease linked to crystalline silica dust). A recent independent review of scientific literature concludes: Overall, amorphous silica is an inert substance and does not pose a high risk to human health when exposure occurs at low levels. For this reason, it seems unlikely that silica exposure during the pouring application of amorphous colloidal silica hydrosol onto concrete will pose significant harm to workers if applied in a wet state. 1 The Cutting Edge 1 See Exposure to Amorphous Colloidal Silica and Associated Health Effects, by Olivia Ellis, California State University, Northridge, CA, USA, Department of Environmental and Occupational Health (October 2008). 6/2/09 page 4 of 10

5 As more projects are done with colloidal silica, new properties are being discovered that suggest even higher performance potential. Colloidal silica bonds not only to concrete, but also to itself. This makes it possible to increase the surface density and hardness even after all available lime in the surface layer has been used up. Silica is hard it is the mineral component of quartz, a material renowned for its hardness and its addition to the surface increases abrasion resistance. This property has been put to work in recent projects involving two separate densifier applications. The first application acts as a foundation for the second, allowing additional particles to bond. The floors are then burnished to a very durable shine without the time and expense of diamond polishing. (See Sidebar, Densifiers in Action: Two Case Studies) Summary Concrete is one of the world's oldest construction products, but it is continually advancing to meet the needs of the times. Improved densifying technology is a classic example of this process: taking advantage of the basic nature of the material, densifiers make a concrete slab more durable, sustainable, maintainable, and aesthetically versatile. Densification makes the concrete slab an ideal flooring solution for the economic and environmental requirements of our age. The newest densifying technology, colloidal silica, carries those sustainability and cost advantages to the next level, and adds new performance levels to the densification process. SIDEBAR Densifiers in Action: Two Case Studies One measure of a product's performance is how it deals with less-than-ideal conditions. A supermarket in Kent, Washington, USA had a white-cement concrete floor that had been previously polished, including densification with a typical sodium silicate densifier. However, the slab had prematurely lost its shine and was difficult to clean, both indications that the surface was no longer adequately dense. "I think it originally was densified when the slab was less than two weeks old," explains Brad Sleeper of Sustainable Flooring Solutions, Vancouver, WA, the contractor that was called in to revive the floor. "At that stage of curing, concrete has less than 50% of its potential calcium hydroxide. It's possible that calcium hydroxide build-up was insufficient to react with a slow-working sodium silicate densifer. I believe that is the reason it failed to hold its polish very long." 6/2/09 page 5 of 10

6 The 6000m 2 store is open on a 24-hour basis. Although work was performed at night to coincide with the times of lowest customer traffic, there were some shoppers present during work activity. This meant using methods that minimize work byproducts and control mess. It required completing all the steps in each discreet area, from grind through polish, before moving to the next area. By using processes that eliminate wet slurry, cause no odor, and contain dust effectively, the crew was able to work without curtaining off sections of the store, leaving it continuously accessible. "We had shoppers walk right into our work area at times," Sleeper recalls. Wet grinding and polishing produces a messy slurry that has to be contained, removed and disposed of. Instead, Sustainable Flooring Solutions chose dry diamond grinding. The floor was ground smooth with 200- and 400-grit abrasives on the grinding machines with high-efficiency dust collection. Sleeper also made the clean choice in densifying, using a spray-applied colloidal silica product, Lythic Densifer, that does not need to be scrubbed in or scrubbed off, and produces no wet residue, helping minimize both hazards and mess. The densifier was applied to keep the floor wet for about 15 minutes, and then allowed to dry, about 1 hour. During that time, it reacts sufficiently to continue polishing immediately thereafter, 75 percent faster than the hour of scrubbing-in required for sodium silicate materials, and eliminating the overnight curing time associated with older densifiers. This allowed Sleeper to move through the process without delays and achieve a very high production rate. "After the colloidal silica treatment, the floor has been able to hold a polish," says Sleeper. It was polished with 1500-grit diamonds, a high polish. "I've seen it a year later, after a year's worth of 24-hour traffic, and it still shines beautifully." Burnishing: Fast and Affordable In Killingworth, CT, USA, a hardware store was being built in the winter, with a slab-on-grade floor. To finish the floor with epoxy or similar coatings would have required keeping the floor warm for hours epoxy cures, a difficult and expensive process. Instead, the owner chose to densify and burnish the floor. Elegant Concrete Polishing, New Haven CT, prepared the slab by first honing it with sanding screens on autoscrubber and then grinding with a series of 50-grit to 400-grit resin-bonded diamond pads on a polishing machine. The floor was then densified with Lythic colloidal silica densifier. Nathan Chistner, president of Elegant Concrete Polishing, recalls that this particular slab required an unusually large quantity of densifier. "No two parts of a slab are exactly the same," Christner explains. "We watched the slab very carefully, following the directions from Lythic Solutions, the manufacturer, to keep the slab saturated for 15 minutes. Some areas took more than others. We eventually had to use about a gallon of working solution for every 300 sf (equivalent to 1 6/2/09 page 6 of 10

7 litre/7.4m 2 ), higher saturation than we usually need with this product." The high saturation rate suggests that the slab was fairly soft, possibly due to extremely slow curing during the cold winter weather. By making sure that densifier saturated the entire slab, Christner was able to bring all portions of the floor up to an equal and consistent hardness. After an hour of curing, the floor was given a second application of densifier to build up the surface. "Then we applied Lythic Protector," explains Christner, "That product has colloidal silica and a thin polymer layer. We burnished the Protector application with a black pad. We used an infrared temperature gage to be sure the burnishing brought to floor temperature up to 90 F." The friction-generated heat of burnishing helps cure the polymer, helping improve the sheen of the floor." Christner was impressed with the gloss of the burnishing. "After we finished it, you couldn't tell the difference between that 400-grit burnish with Lythic Protector and a 3000-grit diamond polish." The gloss of a burnished finish will require more maintenance to retain its glossy look as compared to a true 3000-grit diamond polish. A good maintenance program will use the proper cleaner and cleaning pads. Diamond impregnated cleaning pads have been found to work best. "The floor looks beautiful, says Christner. "It maintains very well, with no visible scratches even after a lot of construction-worker traffic. It is very abrasion-- resistant. The customer is very happy." 6/2/09 page 7 of 10

8 aia-portland-2-crop.jpg Photo courtesy of Lythic Solutions, Inc. Using the concrete slab as the floor, instead of covering it up, is a flooring solution with economic, sustainable, and durability benefits. This polished concrete floor, at the Portland, OR office of the American Institute of Architects, also demonstrates the aesthetic potential of the material. UofOScience 1.jpg Photo courtesy of Lythic Solutions, Inc. Concrete floors are finding favor in a variety of settings including retail, commercial, and institutional facilities such as this science building at the University of Orgeon. 6/2/09 page 8 of 10

9 brewery-shot-hi-res.jpg Photo courtesy of Lythic Solutions, Inc. Densified concrete is a good flooring choice in environments where food is handled such as this micro-brewery because the dense surface offers little place to harbor microbes or allergens. It is easy to keep clean without solvents or other harsh chemicals. The gloss of a well-burnished or polished floor also gives a strong visual impression of cleanliness, an important psychological factor in retail environments. winco-1-corr.jpg Photo courtesy of Lythic Solutions, Inc. By choosing exceptionally clean polishing processes such as dry grinding and spray-applied colloidal silica densifier, the contractor was able to polish this supermarket floor while the store was in full operation. 6/2/09 page 9 of 10

10 AceHardware-0696-corr.jpg Photo by Steven H. Miller, used courtesy of Lythic Solutions, Inc. This burnished floor in a Connecticut, USA hardware store is almost as shiny and reflective as a diamond-polished slab. Ace-Hardware-Christner.jpg Photo by Steven H. Miller, used courtesy of Lythic Solutions, Inc. Nathan Christner, a concrete floor finishing contractor who is experienced at diamond polishing, finished this slab with a new, affordable burnishing procedure. He used two applications of Lythic colloidal silica densifier and a coat of Lythic Protector, a floor conditioner that combines colloidal silica and a thin polymer layer. According to Christner, "After we finished it, you couldn't tell the difference between that 400-grit burnish with Lythic Protector and a 3000-grit diamond polish." 6/2/09 page 10 of 10