As more and more contractors begin to diversify their business by including concrete work into their repertoire, even more that are solely dedicated to decorative concrete have begun to flood the industry. Although most have had previous concrete experience, others have found themselves immersed in decorative concrete with no formal training or experience.
Fortunately for the latter, their work generally does not include the forming and placing of concrete. Often this new breed of decorative contractor starts their work after the concrete is hard. But in order for them to truly understand the medium, decorative contractors should understand concrete basics, as well as placing and finishing, and the related issues with each step.
Placing and finishing concrete
Although some decorative concrete contractor's work doesn't involve forming and placing concrete, they should still know the basics.
It is crucial that the concrete not be deposited faster than it can be spread, struck off, consolidated, and bullfloated. More important, the concrete should be continuously deposited as close as possible to its final position. Begin placing along the perimeter at one end of the work, placing in a manner than can easily be struck-off. Concrete should be placed so that minimal horizontal movement is required.
Striking or screeding operations provide flat surfaces contoured to the right elevation. It can be achieved using a sawing motion or in a straight pulling motion. Decorative finishes typically start after strike-off and bull floating operations are complete.
Water, moisture, and water-vapor transmission
Water movement through slabs and the moisture content of concrete can have a profound impact on when a concrete finish or sealer can be applied for a decorative concrete application. When the concrete is mixed there is more water than needed for the hydration of the cement. The additional water is sometimes referred to as “water of convenience” to help finishers during placement. Additional water also helps ensure that there is enough for hydration when water-cement ratios are low. Eventually, slabs dry out and the excess water evaporates.
Measuring the relative humidity (RH) of slabs tells you when they are dry enough for application of finishers and sealers. If you use an acrylic sealer, you don't have to worry too much about the RH because they are breathable. But for other coatings and sealers, the moisture content should be measured. Manufactures of these products now have published RH numbers for application purposes.
Efflorescence's impact on decorative finishes
Efflorescence is a white calcium carbonate deposit that alters the appearance of colored concrete. They are caused when calcium hydroxide dissolved in water moves to the surface where it reacts with CO2 in the air. The process starts at hydration and can happen at any time during the life of a slab. It's harmless, often unsightly, and difficult to remove. Any decorative concrete application containing portland cement can produce efflorescence.
Low temperatures, moist conditions, and the water used to assist in troweling an application also can induce efflorescence. According to the Portland Cement Association, when water with dissolved calcium hydroxide reach the surface of a slab, the water dissolves leaving the salt deposit that then combines with carbon dioxide from the air to produce calcium carbonate deposits. Efflorescence can occur shortly after initial set, or it can occur gradually over time. Calcium salts in subgrade soils also can cause efflorescence.
The easiest time to remove efflorescence is before it combines with carbon dioxide. At this point, it still will dissolve in water. Pressure washing or wet scrubbing combines the solution with water so it can be rinsed away. Fresh water must be used so that no residue is left to dry on the concrete. An air jet or wet vacuum should be used to remove any standing water as it will cause new efflorescence to appear.
To remove efflorescence try dry brushing, a combination of water and a stiff brush. If that doesn't work use a combination of mild acids (such as vinegar, citric acid, or with a 10% solution of muriatic acid) and scrubbing. The muriatic acid is dangerous, so wear the proper safety gear and wet the concrete surface thoroughly before and after the muriatic acid solution is applied. After rinsing, the concrete should be neutralized with baking soda or an equivalent.
Learning more about concrete basics
All contractors new to the decorative concrete industry are encouraged to participate in technical courses on the basics of decorative concrete to further enhance their knowledge and skill of concrete. Specifically geared toward contractors involved in decorative concrete, the American Society of Concrete Contractors' Decorative Concrete Expo is scheduled for April 3–5, 2008 in Phoenix. The expo will feature seminars and demonstrations of decorative concrete techniques, including the latest tips, materials, and opportunities in the decorative concrete industry. For more information, visit www.ascconline.org.
In the end, creativity is the foundation of a successful decorative application. By learning and understanding what your peers are doing in terms of decorative applications, you will find yourself limited only by your own capacity to create.