Standards for Concrete Floor and Slab Construction

Usually, the primary issue in the field related to flooring over concrete is the coordination between the trades doing different parts of the work. Flooring contractors do not usually do concrete work; that is the purview of the general contractor or specialty concrete subcontractors. Similarly, those trades do not commonly do flooring installations, which is typically the realm of a specialized flooring trade, often with training and certification in the types of flooring they install. In recognition of this division of labor, communication and cooperation between the trades is clearly important. Equally so is the specification-writing process, which can directly influence that cooperation.

The most recognized source for reliable information related to good concrete design and construction is the American Concrete Institute (ACI). Founded in 1904 and headquartered in Farmington Hills, Michigan, ACI today has a worldwide reach in the “development, dissemination, and adoption of its consensus-based standards, technical resources, educational programs, and proven expertise for individuals and organizations involved in concrete design, construction, and materials.” As a trade organization, it engages those who “share a commitment to pursuing the best use of concrete.”

ACI 302.1

When it comes to concrete floors, the most widely recognized standard that is typically part of most concrete floor specifications is ACI 302.1: Guide to Concrete Floor and Slab Construction. This long-standing document is continuously reviewed by a large and diverse committee of professionals from across the industry. As such, it has gone through a number of updates and revisions over time, but the core purpose remains to help concrete installers (i.e., “concreters”) achieve “a hard and durable surface that is flat, relatively free of cracks, and at the proper grade and elevation.” Recognizing that achieving this quality is dependent on both the mixture proportions and the quality of the concreting and jointing operations, this document addresses both materials and labor processes. In particular, it points out that the timing of concreting operations is critical, especially pertaining to concrete finishing, jointing, and curing. Lack of attention to any of these aspects of concrete work contributes to undesirable characteristics in the concrete floor surface. Therefore, the first quality standard for any concrete floor, including those that will receive resilient flooring, is to follow the guidelines and specifications of ACI 302.1.

Using this standard as a basis, high-quality concrete floors for different classes of service are meant to be achieved, whether slabs on ground or suspended floors. The standard focuses on different aspects of construction, beginning with site preparation all the way through final finishing and curing. It also outlines ways to measure and account for the flatness or levelness of the concrete floor.

One key aspect of this standard that is focused on a successful outcome is based on the need for coordination between trades, specifiers, and construction managers. ACI 302.1 is very clear in stating “a thorough preconstruction meeting is critical to facilitate communication among key participants and clearly establish expectations and procedures that will be employed during construction to achieve the floor qualities required by the project specifications.” Similarly, it calls for adequate supervision and inspection of the concreting work, particularly when it comes to finishing. If resilient flooring is planned to go over the concrete floor, then it would be logical that a representative of the flooring trade be present at the preconstruction meeting to voice expectations and needs. Similarly, a flooring trade representative should be able to inspect the concrete floor as it is being finished to provide feedback and input for further coordination between the trades.

Moisture Protection

Chapter 3 of ACI 302.1 addresses design considerations such as the soil support system, concrete characteristics, and tolerances. It also, quite significantly, addresses moisture protection for “any slab on ground where the floor will be covered by moisture-sensitive flooring materials such as vinyl, linoleum, wood, carpet, rubber, rubber-backed carpet tile [etc.],…” Clearly, there is a recognition by the concrete industry that resilient flooring (including those specifically listed) requires the proper moisture protection for a successful floor assembly condition.

Moisture protection in ACI 302.1 is based on other recognized standards such as ASTM E1745: Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs. This standard covers “flexible, preformed sheet membrane materials to be used as vapor retarders in contact with soil or granular fill under concrete slabs. The materials shall be subject to tests for water-vapor permeance, tensile strength, and puncture resistance.” With this as a basis, ACI 302.1 states that an appropriate vapor retarder should have a permeance (water-vapor transmission rate) of less than 0.3 perms. The recognized test that they reference for determining such permeance is ASTM E 96: Standard Test Methods for Water Vapor Transmission of Materials.

There is also very clear language in ACI 302.1 regarding the advice given for proper moisture protection, stating, “The selection of a vapor retarder or barrier material should be made on the basis of protective requirements and the moisture-related sensitivity of the materials to be applied to the floor surface.” For resilient flooring, this means the sensitivity to moisture is high and should be addressed accordingly. The standard also addresses the common—although not always sufficient—construction practices, stating, “Although conventional polyethylene film with a thickness of as little as 6 mils (0.15 millimeters) has been used (as a vapor retarder), the committee strongly recommends that the material be in compliance with ASTM E 1745 and that the thickness be no less than 10 mils (0.25 millimeters). The increased thickness offers increased resistance to moisture transmission while providing greater durability during and after installation.” Hence, the thicker vapor retarder and the compliance with ASTM E 1745 should be a critical part of any specification that calls for resilient flooring being placed over a concrete slab on grade. The difference in cost is negligible in the overall construction budget but can save a lot of time and money by avoiding problems later.

ACI 302.1 points out an important consideration for designers, stating, “A number of vapor-retarder materials have been incorrectly referred to and used by designers as vapor barriers. True vapor barriers are products that have a permeance (water-vapor transmission rating) of 0.00 perms when tested in accordance with ASTM E 96.” This might seem like a minor point, but in actuality, for consistency, the term vapor retarder should be used in specifications particularly since 0.3 perms is the standard, not 0.0. It goes on to point out, “The laps or seams in either a vapor retarder or barrier should be overlapped 6 inches (150 millimeters) (ASTM E 1643) or as instructed by the manufacturer.” This is a very critical point since there is great risk of moisture penetration if the edges of the sheets of a vapor barrier do not overlap and seal properly. Placing sheets adjacent to each other without this overlap simply creates gaps where moisture from the ground will migrate up and make the vapor retarder quite compromised rather than truly continuous.

There is one other crucial aspect to vapor retarders covered by ACI 302.1, namely their placement. Most designers assume that the vapor retarder should be placed directly under the concrete slab and over the granular fill that is the typical base of any concrete slab on ground. For most conditions, this is correct and in fact is the placement preferred by resilient-flooring manufacturers. This location will typically limit the flooring to moisture exposure from the slab only, not from the gravel base. However, in some cases, placing the vapor retarder directly below the slab can affect the drying and curing of the concrete, such that the conditions on the top and bottom are sufficiently different to cause problems. Hence, ACI 302.1 shows an option to install the vapor retarder below the granular surface, such that ground moisture is still restricted but the impacts on the concrete are lessened during curing. Based on all of this and the understandable variation in design and field conditions, the committee that wrote the standard recommends that “each proposed installation be independently evaluated as to the moisture sensitivity of subsequent floor finishes, anticipated project conditions, and the potential effects of slab curling, crusting, and cracking.” These last three conditions are the types of problems that can be anticipated if the concrete cures improperly due, in part, to the incorrect placement of a vapor retarder. Hence the recommendation is made that “the anticipated benefits and risks associated with the specified location of the vapor retarder should be reviewed with all appropriate parties before construction.” This would include a resilient-flooring representative to weigh in on the sensitivity of the flooring to moisture.

Overcoming Problems

Most of the other chapters of ACI 302.1 address the proper means and methods to achieve acceptable quality of concrete floor under resilient flooring in a variety of conditions. Chapter 11 focuses on providing the readers with information on the “causes of floor and slab surface imperfections.” The intent of this section is to help inform everyone involved of potential problems and provide some reasonable means to avoid them based on the long-standing experience of the ACI. Some of the relevant issues related to resilient flooring include the following:

• Cracking: Concrete cures by incorporating water, which engages in a chemical reaction with portland cement and other ingredients of concrete. In so doing, the volume of the concrete reduces slightly, causing shrinking. If that is not planned for, cracking results due to this shrinking. It should be noted that a perfectly crack-free concrete floor is never achieved since all concrete will shrink, with some cracking occurring. ACI302 points out that the goal is to control that cracking so that the end result is an acceptable amount that does not diminish the integrity of the floor. The goal is not perfection since it is not attainable.