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Mechanical vs Chemical Polishing

Does it matter?

Scott Hatmaker By Scott Hatmaker

The concrete polishing industry has taken the construction world by storm in recent years, particularly within the United States. This rapidly achieved status is owed in large part to the wide range of choices and possibilities available, and the wealth of applications in which the finish can serve as a suitable floor solution. Due to the large scope of services and products that currently fall under the very broad category of polished concrete, it is very important for the architect, as well as the client and contractor, to understand what finish they are actually specifying when selecting a polished concrete floor system. It is therefore one of the pivotal goals of the International Polished Concrete Institute to establish a standard under which to define the processes that fall under the umbrella of polished concrete, in order to ensure quality control and promote product knowledge within the construction industry.

There are many varying methods, processes, and products referred to as a polished concrete floor system, and while they are all suitable for one application or another, it is essential to understand the differences so that the proper process is chosen for a specific project. Differences in these processes greatly affect cost as well as finish, and in many budget-driven applications an inexpensive solution can work quite well. Projects in which aesthetics and ease of maintenance are primary concerns, however, may greatly benefit from a more costly yet longer-lasting slab that presents a much smoother and more brilliant sheen. Both of these scenarios as well as almost anything between can be achieved using polished concrete.

Ideally, every concrete floor system, particularly those in which the base slab is not covered over with tile or other flooring materials, is poured properly and begins as a smooth and level surface with few imperfections. Unfortunately, this is not always the case. Often there are hills and valleys in a slab, which are generally more prominent the larger the slab, and pitting can occur due to improper curing. Trowel marks are sometimes noticeable, and even the most perfect bare concrete floors will eventually need to be coated with a sealer, topcoat, or densifier in order to prevent surface dusting and micro-pitting over time and to protect the slab from staining or abrasion. A sealer or topcoat such as an acrylic, elastomeric waterproof, or epoxy-based coating may suit many applications and are fairly inexpensive, but generally do not fall under the scope of polished concrete.

A concrete densifier is an alternative to topcoats that is commonly used to strengthen and harden the surface of a slab, and is often defined as a concrete polishing solution. Densifiers are chemicals heavy in silica that react with calcium hydroxide in the concrete to fill pores and harden the surface wear layer of the slab. Several different catalysts are added to concrete densifiers and hardeners in an attempt to achieve greater penetration, but timing of the application and the application process are also important factors in allowing the product to penetrate the pores of the slab. While the effect of hardeners may outlast that of topcoats and is generally less maintenance intensive, some chemicals are sprayed on and allowed to soak or applied with floor scrubbers and provide a similar end result. While this process is not truly mechanically polished concrete, it is generally a very low-cost option and can provide an adequate solution for projects such as distribution centers as well as cost conscious clients. Many densifiers also have no noticeable VOC levels, and can aid in achieving LEED status or a green floor system.

Projects in which presentation is a factor may require a more honed and longer lasting floor system than a chemical application can achieve. Terrazzo floor toppings have been used successfully in the industry, and also present decorative options. Mechanically polished concrete, in which metal and resin bonded diamonds are used to grind and polish the actual concrete itself, presents a recent and viable alternative in these applications as well as many others. Mechanically polished concrete may be performed wet or dry, but the dry process is often the preferred method, as the dust created is more easily contained and disposed of than the slurry created by wet polishing. Dry mechanical concrete polishing hones the slab in much the same way the stone industry polishes marble or granite. Diamond polishing machines are used to grind the surface of the slab itself, initially with a coarse grit, in order to level the slab and remove imperfections. This makes it suitable for retrofitting applications as well as new construction. A concrete densifier is generally used in conjunction with the dry mechanical polishing process and should be applied after the first grinding step, which can potentially open the pores and aid­­ in allowing the chemical to penetrate. Subsequent grinds of the slab are generally done with finer-grit diamonds until the desired effect is achieved for the project. Mechanical polishing is usually a green process, and also a very low-maintenance option, as densifiers and pigments become integral to the slab, leaving no layer on the surface. The finished product requires only a mild detergent for cleaning and no waxing or buffing. Mechanically polished concrete finishes often far outlast topcoats and chemical polishes, and future touch-up polishes are a fraction of the initial cost.

It is important to note that within this process, both aesthetic and cost options are nearly limitless. Mechanical polishing is suitable for warehouse, industrial, and food processing applications and a two step dry mechanical polish using a densifier can be performed at a low cost while offering a much more honed surface than a troweled slab with only a topcoat or chemical densifier applied. Greater abrasion resistance, added reflectivity and light levels, and an improved coefficient of friction can all be achieved with mechanically polished concrete, and can be specified depending on the client’s budget and needs. In higher end projects where terrazzo, patterned tile, or even marble or granite may be considered, mechanically polished concrete can often provide an equally appealing and more versatile and cost-effective solution. Integral pigment can be added to the slab to create color, patterns, or logos, and various materials can be seeded into a slab or concrete topping before curing and ground smooth with the slab for a wide array of effects. The mechanical polishing process can include as many as seven or more grinding steps with densifiers often worked into the slab between passes. Higher end processes involving a greater number of steps and even denser grit levels can achieve a beautiful, almost mirror-like sheen comparable to stone flooring.

When specifying a densifier for a distribution center, a two-step polished concrete process for a warehouse, or a seven-step process for a museum, one should note that the appearance, reflectivity, abrasion resistance, maintenance level, and duration of the slab are being specified as well. It is therefore essential that the architect, client, and contractor are all aware of the different products and levels of service available in the realm of polished concrete, and the cost and benefits associated with each, in order to find the best solution for both the program and budget. I believe that the IPCI will provide a valuable resource in defining standards and specifications for levels of service and quality control within the polished concrete industry, and offer a clear explanation to industry professionals and clients alike as to which service best suits their particular needs.