By Jigar Desai, PhD, P.E., ECS Mid Atlantic
Have you ever heard of words used for flooring over concrete slabs like vapor barrier, sweating, blistering, musty odors, peeling, moisture, and high internal relative humidity? If you have, I can empathize with you since you may have dealt with a flooring failure situation that may have cost you a dear penny. Thoughtful planning is critical to any project’s design to avoid flooring failures. Do your homework and ask the right questions.
Consider this scenario for a 20-year-old mid-rise building with concrete floor slabs on the ground and elevated composite concrete slabs on a metal deck. A renovation for a new tenant includes a new interior build-out and a variety of floor finishes with a combination of luxury vinyl tile (LVT), carpet, ceramic tiles, and some polished concrete floor. Each of these floor finishes has a different surface preparation requirement. Still, one thing in common is that the concrete floor slab must meet a particular moisture requirement to install any new floor covering over it, i.e., the concrete must be dry enough when tested under specific standard ASTM test methods. When proper surface preparation, including assessment of the existing substrate, is not performed, the failure of flooring systems becomes inevitable. To prevent flooring over concrete surfaces from failing due to moisture-related issues, below are some guidelines:
1. Hire a Consultant to Evaluate Existing Floor Conditions: A flooring consultant is not a salesperson from the flooring materials vendor or a flooring installer, but rather an independent consultant with experience in materials science, knowledge of how flooring systems work (or fail), experience in construction and sequence of construction activities, understanding of how ambient conditions contribute to the performance of flooring systems and focused on ensuring that the new floor covering is compatible to the existing floor while being able to evaluate the flooring installation requirements.
2. Testing of Existing Concrete Floor and Soil Subgrade: Conduct moisture (vapor emission rate testing per ASTM F1869 and internal relative humidity of concrete testing F2170) testing before installing the concrete flooring is required for almost all flooring systems. In addition, for old slabs used for applications that led to exposure to oily and greasy substances, sampling of the concrete floor for chemical analysis is recommended. Since moisture transmits from underneath the floor slab, it is a good practice to sample and test soils of the floor slab subgrade for moisture content. For old concrete slabs that show signs of cracks, discoloration, or a white powdery substance along the cracks, a laboratory petrographic examination of concrete core samples from the floor slab to look for any harmful reactions within the concrete is highly recommended.
3. Vapor barriers: Check for an effective vapor barrier’s presence (or absence) beneath the concrete slab. Older buildings may not have a vapor barrier under the slab, or if they have one, it may be so thin and disintegrated that it would be ineffective to serve as a barrier. Consider a moisture mitigation system before installing the new floor covering if a vapor barrier doesn’t exist. For new construction, ensure a vapor barrier is part of the design and guidelines of ACI 360R-10 are followed for floor slab design.
4. Proper drainage: Ensure proper site grading and drainage to direct water away from the concrete slab. Poor drainage can lead to water accumulation and moisture-related problems. Implement appropriate slopes and install drains if necessary. Ensure that roof downspouts do not dump water near the exterior walls and foundation of the building.
5. Water-reducing admixtures: Consider using water-reducing admixtures in the concrete mix. These additives help reduce the need for water (and subsequent moisture), and some additives also provide additional water resistance capabilities. Consult with a professional to determine the most suitable admixture for your project.
6. Moisture mitigation systems: In cases with high moisture (moisture vapor emission rate or high concrete internal relative humidity) levels, consider installing moisture mitigation systems such as moisture vapor barriers over the slab or epoxy-based coatings. These systems provide an additional layer of protection against moisture-related problems. A mitigation system works only when the underlying substrate (concrete floor slab) is sound and has no issues due to internal deleterious reactions.
7. Regular maintenance: Perform routine inspections of the concrete flooring to identify any signs of moisture issues, such as discoloration, efflorescence, or mold growth. Promptly address leaks, plumbing issues, or water intrusions to prevent further damage.
Implementing these preventive measures and working with experienced professionals can significantly reduce the risk of concrete flooring failure due to moisture-related problems.