Artistry meets practicality on a wet slab of concrete

Photo © iStock. Photo courtesy Wagner Meters

by Jason Spangler
Creating an enduring structure requires careful planning for physical and aesthetic integrity. While this symbiosis of artistry and structural stability is not assured until the project is fully complete, getting there requires laying the right groundwork at the very beginning—during the planning phase.

Shortchange the planning process and a lot can go wrong. This is as true with the finished flooring installed over a concrete slab as with any other aspect of a construction project. If the expectations are not clearly specified and either the general contractor or the flooring installer moves ahead before the concrete is sufficiently dry, the outcome may well be catastrophic. Flooring adhesives may not stick properly, hardwood floors may buckle, cup, or warp, and floorcoverings may attract mold or mildew over time.

To avoid risking a flooring failure, critical details must be in place upfront—while creating the design and documenting the specifications. This is the first and sturdiest line of defense against experiencing nasty errors and omissions that have the potential to unravel one’s project and reputation.

It is important to examine many of the details that will need to be locked down, including the specified method of measuring moisture in concrete slabs. This makes it much more likely the project happens the way it should to ensure beautiful, lasting flooring.

Designing for success
Planning for a successful finished floor requires good teamwork. When architects and specifiers closely collaborate, their partnership makes certain the most appropriate methods and materials are selected for the project.

Several critical decisions should be made relating to the concrete drying process and getting the slab ready for the flooring installation. Each of these choices can impact drying and curing time. They can also minimize the risk of moisture-related flooring failure.

The Swedish Cement and Concrete Research Institute (CBI) has developed a formula to estimate minimum drying times for concrete slabs. (For more, see Howard Kanare’s second edition of Concrete Floors and Moisture, published by the Portland Cement Association [PCA] in 2008.) This formula relies on many of the variables explored in this article. It includes tables of numerical values for the following correction factors, which can be multiplied together to calculate an estimated number of days needed to dry the slab:

  • water-to-cement (w/c) ratio;
  • slab thickness;
  • one- or two-side drying;
  • ambient conditions (e.g. temperature and relative humidity [RH]); and
  • curing conditions.

This information can be used to create an appropriate timeline for the project from the very beginning. An accurate estimate of reasonable drying times, based on data someone on the project team has taken time to work out, helps to avoid undue delays, budget overruns, and pressures to move ahead when the slab is not yet ready for the finished floor.

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