Polished concrete is a concrete floor which has been mechanically ground, honed and polished with bonded abrasives in order to achieve a desired level of sheen or gloss. The Polished Concrete Process Concrete polishing begins with floor preparation. During this process, the floor is inspected and cleared to remove all unnecessary materials that can damage both the floor and the grinding machines. Floor inspection also includes hardness testing. Before a floor can be polished, it must have a hardness reading of 4,000 psi or higher for best results. A reading below this measure means the floor is soft and can crack under pressure. The next step is the initial grinding. In this step, low grit diamonds (e.g. 16- to 20-grit) are used to grind down all uneven surface until the floor is flat. The next step is patching. Normally, all concrete floors have cracks. Patching ensures that all cracks, pinholes, and micropits are covered with grout before the concrete polishing process continues. After patching comes floor hardening. This step is also called densifying. Densifiers make the concrete surface hard enough for grinding and polishing. Densifiers to achieve a minimum of 32 to 40 mpa (megapascal) before it can undergo grinding. Densifying solutions also seal the floor from problems like rebar expansion, mildew, and mold. Once the floor is hard enough, the next step is honing. This process is needed to refine the hardened surface using finer abrasives (e.g. 200- to 400-grit resin). After honing, the next step is polishing. Here, the floor is polished with much finer grit in order to achieve the desired sheen level, such as matte, satin, semi-gloss, and high-gloss. In a matte finish, the objects reflected on the floor have a hazy appearance. In a satin finish, the reflection of objects have a low-luster appearance. A semi-gloss sheen would have easily-identifiable reflections. Side and overhead lighting will also be visible. A high-gloss sheen will have highly identifiable reflections of images. When viewed from different angles, the floor will appear to be wet. Polished Concrete Densifiers A concrete densifier is a solution used to penetrate into the concrete and create a chemical reaction to harden and dust-proof the surface. During concrete polishing, the surface is processed through a series of steps (in general a minimum of four grinding steps of processing is considered polished concrete) utilizing progressively finer grits of industrial diamonds bonded in metal, hybrid or resin to make diamond polishing pads. Under certain circumstances polished concrete may be LEED approved. Historically, concrete was not considered fully polished before reaching a 1600 grit, with any grinding below 1600 grit being categorized as a honed floor , and final finishes normally targeting the 1600 or 3000+ grit level. However, modern industry standards, such as those established by the Concrete Polishing Council (CPC) of the American Society of Concrete Contractors (ASCC), classify finishes into four distinct levels of gloss based on the grit utilized. A Level 1 (flat or ground) finish stops below a 100-grit resin bond, resulting in a hazy appearance with minimal reflection. Level 2 (satin or honed) stops at a 400-grit resin bond, yielding a low-luster matte finish. True polishing begins at Level 3 (semi-polished), which uses an 800-grit or higher diamond abrasive to achieve sharp light reflectivity. Finally, a Level 4 (highly polished) finish utilizes 1500 to 3000-grit resin bonds or high-speed burnishers. Processing concrete up to the 1500 or 3000-grit level adds substantial depth and richness to the concrete's color, producing a mirror-like clarity and a luxurious appearance that can rival polished marble. Dyes designed for concrete polishing are often applied to add color to polished concrete, alongside other decorative options such as scoring, creating radial lines, grids, bands, borders, and custom designs.
History Polished lime mortar was discovered in the city of Jericho, in the valley of the River Jordan. According to the Bible, Jericho fell in 1200 BC. Researchers have since discovered several layers of concrete from earlier settlements. Among the layers was a polished screed floor.
New or retrofit In simple terms, the process of polishing concrete is similar to sanding wood. Heavy-duty polishing machines, for example concrete grinder, are used to gradually grind down surfaces to the desired degree of shine and smoothness. The closest equivalent example would be polished granite or marble.
New floors
The mix design of the concrete should be 3500 psi or higher. The concrete should be poured full depth to take advantage of the concrete's full structural strength, and also help to prevent cracks. The concrete should always be professionally finished with power trowels and cured with water for seven days. Polishing should not begin until the concrete is fully cured (generally 28 days). The concrete slab can contain decorative aggregates to create many different aesthetic appearances, see Terazzo. The aggregates are often sized 8 – 20 mm, but almost anything can be used. According to the Global Concrete Polishing Institute, a FF (floor flatness) level of 50 or greater is desired to facilitate the polishing process. During the finishing phase, any decorative aggregates such as seashells, glass chips or even metal car parts can be dropped into the surface of the concrete.
Retrofit floors Retrofit floors can be done in different ways depending on the conditions of the concrete. If the existing concrete is in good condition the surface can be polished to just about any standard. If the existing floor slab is in poor condition, it can be cut, or ground and the natural aggregate can be featured as the "exposure level". If the existing surface is in very poor condition a topping slab with a minimum thickness of 50 mm (2 inches) can be added on top of the existing slab.
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