Concrete and cement products often use different additives to achieve different properties related to the strength, cure time, workability, or other characteristics of the end product. When considering a floor overlayment, portland cement is often modified with Calcium Aluminate Cement and other additives in order to achieve the desired characteristics for a thin, hard, workable layer.

Investigation into alternatives to portland cement, particularly for concrete overlayments, has yielded other formulations that do not require additives to achieve the desired characteristics. Calcium Sulfoaluminate (C₄A₃S) Cement in particular is often used as a replacement for portland cement in overlayments. Part of the reason is that portland cement has excess water after hydration. Evaporation of the excess water can lead to high shrinkage of the overlayment, causing cracking, gaps or other issues with the finish which can be reduced but not eliminated by using other additives. By contrast, C₄A₃S cement hydrates more fully leaving very little excess water for evaporation thus considerably reducing shrinkage. There are other characteristics that make it better suited to floor overlayments and finishing too. In terms of construction timing and scheduling, C₄A₃S cement can gain structural strength in one hour—much more quickly than portland cement which can take 3 – 7 days. Additives can be used in portland cement to help it gain strength faster but may lead to drawbacks in the finished floor appearance. From a sustainability perspective, it is worth pointing out that C₄A₃S cement has an environmental impact that is 60% less than that of the environmental impact of portland cement.

Because of our focus on concrete floor overlayments and the preferred use of C₄A₃S cement for this type of system, the basis for concrete discussed throughout this article will assume C₄A₃S cement is being used.

Design Considerations

When should you consider using a polished concrete floor overlayment system? Basically anywhere that concrete floors are used. Polished overlayments can be readily applied over the top of new concrete floors and they are also a preferred way to create “like-new” floors when renovating existing buildings too.

Just as concrete floors in general are appropriate for consideration in commercial and industrial buildings, so too are polished overlayments quite appropriate in these settings. They have become a popular choice for high foot-traffic areas that need to be kept clean since the finish is durable, slip resistant, easy to clean and maintain. These attributes plus the range of design appearance options has led to many installations in retail stores, hospitality facilities and restaurants where durability and great appearances are important. These interior design qualities also make polished overlayments a great choice for other locations such as commercial buildings, lobbies, and offices. Additionally, they have been used very successfully in institutional settings such as museums, schools, civic centers, sports arenas, and exhibit halls. Even industrial settings have benefited where foot traffic is combined with service vehicles and a clean, durable floor surface is needed such as in warehouses, manufacturing facilities, or airports.

Not to be overlooked is the use of polished overlayments for residential buildings that use concrete floors. This is particularly appropriate for residences that seek to incorporate passive solar heating in the design. Concrete floors acting as a thermal mass component of the space can be located to collect and store heat from the sun in the concrete during the day and allow it to radiate directly back to the space at night. The common challenge to this approach is the finished appearance of the floor. With a full range of choices in color, texture, and patterns, that problem is solved, meaning passive solar residences can perform well and look great too. Further, since the overlayments are typically installed at the very end of the project in any application, they are not subject to much other, if any, construction activity meaning they are less likely to get damaged by other trades.

Creating a Polished Concrete Floor System

Once the decision is made to use a polished concrete overlayment as a finished floor surface, then there are a number of steps that need to be accounted for. Before starting, it is important to recognize and plan out all aspects of the job including surface preparation, type of primer(s) used, mixing procedures, placement plan, and finishing method. In order to avoid any surprises, a mock-up of at least a small area is recommended prior to starting any project.

Surface Preparation

The first step involves preparing the concrete surface, whether new or existing, and is critically important since a high percentage of all flooring failures are due to some aspect of subfloor preparation. The most common flooring type where this has been an issue is when resilient flooring (vinyl tile, vinyl sheet flooring, linoleum, rubber flooring, etc.) is placed on top of concrete. Hence, ASTM F710 “Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring” has become the recognized standard for concrete substrate preparation. Although specifically written for resilient flooring, the substrate preparation methods are applicable for polished concrete overlayments too.

ASTM F710 covers the procedure for determining the acceptability of concrete floors for the installation of resilient flooring and includes suggestions for ensuring that the constructed concrete floor is acceptable for such installations. As a flooring standard, it does not cover tests for adequacy of the concrete floor to perform structural requirements. Of primary significance is the requirement for a permanent, effective moisture vapor retarder under all on- or below-grade concrete floors. Moisture can readily make its way up through concrete and cause the separation of any finish flooring and the concrete substrate. Therefore, all concrete slabs should be tested for moisture regardless of age or grade level. The standard goes on to indicate that concrete floors need to be permanently dry (i.e. fully cured with no excess water remaining), clean, smooth, structurally sound, and free of substances that may prevent adhesive bonding. Any surface cracks, grooves, depression, control joints or other non-moving joints, and other irregularities should be filled with the appropriate patching material. The surface of the concrete floor should be cleaned of any pre-existing or added coatings, materials or compounds by scraping, brushing, vacuuming, or any other effective method.

All of this preparation work is relevant to overlayments since they are designed to bond to profiled, sound substrates. For uneven floors or existing floors that have been damaged, a self-leveling underlayment or appropriate patching material can be placed to create a uniform substrate ready to receive the final overlayment. Note that a variety of patching and crack repair materials are available on the market and should be confirmed with manufacturers for compatibility with other components of the system. Generally speaking, the substrate is prefilled to 3/8 inch from the finish floor level. This leaves room for the overlayment to be placed, creating the final floor surface.