Studies indicate that many construction problems involve floor finishes, and most of those problems involve moisture. Since most non-residential projects have a finish floor over concrete, this first issue of Technical Scribe focuses on moisture requirements for a successful finish floor over a concrete substrate. A successful floor finish begins with the hard stuff — the concrete substrate. Moisture is involved in three of the essential steps of concrete delivery — Mix Design, Placement, and Surface Preparation. Any one of these steps can affect moisture requirements and determine drying time and performance.
With accelerated construction schedules, reducing drying time is essential to allow on-time installation of floor finishes. One of the ways that concrete drying time is determined by controlling moisture in the concrete. However, proper water amounts are essential to control concrete’s heat of hydration — the chemical reaction that occurs when water is added to dry cement materials to make the concrete hard. Without the hydration process, we would be trying to bond flooring to powder and rocks.
While concrete mix design is somewhat of a gray area and involves many components, water:cement ratios and supplemental additives, such as fly ash, are some are the most pertinent to reducing drying times and achieving successful floor finishes. Concrete drying time and adhesion of bonding materials can be affected by the density of the concrete. The surface density is affected by the water/cement ratio, additives, and curing methods. Lower water:cement ratio usually means a denser and higher strength concrete, and a higher water:cement ratio usually means the opposite. Low density usually means higher porosity, lower strength, faster drying, and better adhesion. Water:cement ratios between .43 and .47 can produce strong concrete that will dry reasonably fast.
Additives, such as fly ash, that often replace Portland cement in the mix can produce a denser, higher strength concrete. LEED projects often use higher fly ash proportions than usual for the recycled content points. Also, aggregate used in lightweight concrete is more porous and holds more water than normal weight concrete aggregate. Therefore, it can take longer to dry than normal weight concrete. Just as the ingredients and their proportions can affect a cake’s outcome, the ingredients and their proportions can affect concrete’s performance.
Figure 1 (below) shows the relationship of water:cement ratio and drying time, and it seems that the often quoted adage, “Concrete takes 28 days to dry,” does not always hold water. Perhaps “Concrete will not be finished before its time” is more applicable.
Figure 1 – Correlation of Water: Cement Ratio to Drying Time
Placement includes the vapor barrier, concrete finishing, and curing.
Vapor Barrier: A vapor barrier should be used under concrete slabs on grade even if a finished floor will not be applied. If a floor finish is applied in the future, installing a vapor retarder now will pay dividends later. The vapor barrier should be at least 15 mils thick, comply with ASTM E1745 Class A, and have a maximum perm rating of 0.010. Joints should be overlapped approximately 6 inches and taped, penetrations booted, and ends turned up at terminations. If the slab is below grade or soil borings show that the water table approaches the slab, a waterproofing membrane is recommended in lieu of a vapor barrier. The importance of a vapor barrier and its being concealed after installation suggest that project contract documents require photographs of the installation for record.
When concrete is placed on grade without a vapor retarder or a waterproofing membrane, sub-grade moisture, including moisture from irrigation, can migrate up through the concrete by capillary action. As previously mentioned, porous concrete has more capillaries to transport moisture.
Concrete Finishing: Concrete finishing is not the same as a finished floor. Instead, it includes procedures to achieve the desired concrete surface, such as troweled, broom, exposed aggregate, etc. Hard steel troweling produces a smooth, dense surface that reduces drying time.
During mixing, concrete finishers sometimes get stir crazy and add water at the site to improve pumping and fluidity. This may be good for the finisher, but it is obviously bad for the concrete. When schedules get tight, contractors have been known to pour concrete in the rain which is like adding water.
After finishing, concrete surfaces will continue to dry until a floor finish is applied. When the floor finish is applied, moisture that was able to escape is now trapped between the concrete and floor finish. This can cause the moisture level below the floor finish to rise above the required level causing resilient finishes to blister, wood floors to buckle, bonded finishes to delaminate, and carpet finishes to mold.
Curing: Proper curing is critical because it promotes hydration. After the concrete has the desired finish, it is cured by covering it with ponding or sprayed water or a moisture-retaining cloth, such as burlap. These methods tend to add more moisture to the concrete which increases drying time. Curing is also done by covering the surface with liquid curing membranes that control the release of moisture during curing by self-dissipation or by covering with moisture-retaining covers, such as polyethylene. Curing membranes often leave residual traces of contamination that must be removed by cleaning with detergent and water that add more moisture to the concrete we’re trying to dry. However, the preferred method of removal is mechanical abrading.
When concrete moisture requirements or surface profile (flatness) conditions do not meet the floor manufacturer’s requirements, additional floor preparation steps must be performed. These steps include applying a topical moisture-reducing membrane over the concrete to reduce moisture to an acceptable level, and by mechanical abrading to achieve the desired profile. These procedures are seldom included in a bid and can be very expensive. Project contract documents should have a provision requiring the contractor to provide a concrete substrate that meets these requirements, including surface profile and moisture content. This simple provision usually eliminates surprises and finger pointing.
Levelness and Flatness: Except for concrete flatness causing water to form birdbaths and levelness causing water to collect at one end, flatness and levelness are not normally associated with moisture. Therefore, they will be discussed another time.
Cleaning and Surface Profile: At minimum, concrete substrate should be free of dirt, grime, grease, spalling, or other contaminants that may interfere with adhesion. All traces of sealers and curing compound/membranes should be mechanically removed because using chemicals and water only adds moisture to the concrete. Usually, beading water on concrete suggests that a sealer or a curing compound is present. Surface profile allows the adhesive to mechanically adhere to the concrete substrate.
Testing: Moisture testing should be performed after cleaning to prevent surface contamination from affecting test results. Most manufacturers consider a vapor emission rate of 3 pounds/24 hours/1000 square feet (calcium chloride test) and 75% concrete internal relative humidity to be acceptable. Since some adhesives can also be affected by high alkaline levels, and concrete is alkaline, pH should be range from 7-9, but it is not normally measured.
Achieving and maintaining acceptable moisture levels in concrete substrate at the time floor finishes are applied and thereafter begins with proper mix designs. However, if proper placement and surface preparation procedures are not followed, and if moisture levels are not verified by testing, your finish floor system could be a washout.
About the Author: Cris Crissinger, CSI, CCS, CCCA has over 30 years of experience preparing construction specifications. Previous to retirement, Cris was Director of Specifications for McMillan Pazdan Smith Architecture where he was responsible for evaluating new products, preparing project specifications, assisting in facility assessment, performing field investigations, and coordinating internal training programs. Crissinger is a member of the Construction Specifications Institute (CSI), the Building Performance Committee of ASTM International, and the Design and Construction Division of the American Society for Quality (ASQ), and serves in the community of the Construction Board of Appeals for Spartanburg, South Carolina. He has twice won the Richard M. Horowitz Award from Roof Consultants Institute (RCI) for his writing.