Design of concrete for moisture-sensitive floorcoverings

May 16, 2019

by Paul Potts

Photo © Paul Potts[1]
Photos © Paul Potts

It takes three to six months for concrete to dry to 70 to75 per cent relative humidity (RH), a safe level to place moisture-sensitive floorcoverings and adhesives. The drying duration, a critical factor in the overall schedule, is directly affected by the concrete mix design, the capillary break and vapour barrier, and curing methods. There are several parties, such as architect/engineer, construction manager, concrete contractor, batch-plant operator, soils/materials testing agency, and floorcovering contractor, each with contractual duties that will affect the successful installation of the floorcovering. While each party has separate duties, no single entity has overall responsibility for the entire process (The term construction manager in this article refers to both construction manager and general contractor (GC). The word architect or architect-engineer refers to both architect and engineer individually. Floorcovering refers to moisture-sensitive floor coverings and adhesives.).

Preconstruction meeting

Floorcovering failure due to excess relative humidity (RH) in concrete. Photo courtesy Chris Maskell [2]
Floorcovering failure due to excess relative humidity (RH) in concrete.
Photo courtesy Chris Maskell 

The preconstruction meeting is the opportunity to establish a collaborative working group. The leaders of the meeting (architect/engineer and construction manager) should describe the special challenges of placing concrete for moisture-sensitive floorcovering and reinforce the necessity of each party performing their duties with care. The architect/engineer must play a central role at the meeting because they have the greatest authority to discuss the design and construction documents.

Requirements to be discussed at the meeting include:

The batch-plant operator must also be at the meeting. They must be informed that allowing a driver to add water to the concrete at the site, unless approved by the owner or architect/engineer, could result in the material being sent back to the plant. They should also be reminded to make sure trucks returning from the site are emptied of leftover mix or rinse water.

Design and construction objectives

Long-term shrink cracking can telegraph through bonded floorcoverings like thin-set terrazzo and epoxy. Photos © Paul Potts[3]
Long-term shrink cracking can telegraph through bonded floorcoverings like thin-set terrazzo and epoxy.
Photos © Paul Potts

The principle objectives in planning concrete to receive moisture-sensitive floorcovering are (see the American Concrete Institute’s [ACI’s] 302.1R-06, Guide for Concrete Floor and Slab Construction):

 Mix design

According to the Portland Cement Association (PCA), “all desirable properties of concrete are improved by reducing the total water content to the practical minimum.” A low w/c ratio mix design reduces the delay required to bring the RH of the slab to a safe level to apply the floorcovering and also result in flatter concrete surfaces with less curling and long-term shrink cracking.

The w/c ratio, size, and quantity of coarse aggregate and cement content could affect the total water content. If properly selected, each requirement will result in minimizing the total water content. The w/c ratio is the most important factor when determining the drying rate of a concrete slab. The lower the ratio the faster concrete dries. A plasticizer may be needed to reduce placement difficulties.

 Preparation for placement

The vapour barrier in this photo has not been carefully placed and will cause weakened planes in the concrete.[4]
The vapour barrier in this photo has not been carefully placed and will cause weakened planes in the concrete.

When preparing for placement, several factors must be taken into consideration.

Welded wire reinforcement

While not connected with the drying time, it is worth mentioning WWR does not prevent random cracking, which is a function of jointing. However, WWR keeps jointing and random cracks tightly together and buttresses the tensile strength of concrete by distributing tensile stresses over a wider area. Rigid WWR should have 305-mm (12-in.) wire spacing in order to give workers a place to walk without pushing the reinforcement to the bottom and should be placed on chairs in the upper half of the slab to control cracking.

 Subsoil preparation, capillary break, and subterranean flowing water

Groundwater rises to the surface of the concrete slab by capillary action, which is the ability of a liquid to flow in narrow spaces (concrete pores) without the assistance of, and in opposition to, external forces like gravity. A capillary break, a 100-mm (4-in.) layer of 12-mm (½-in.) rounded aggregate, should be placed between the subsoil and the underside of the concrete with a vapour barrier on top. This prevents groundwater from reaching the underside of the slab, wicking to the surface and becoming a continuous source of moisture attacking the floorcovering adhesive. Flowing water must be directed away from the building with an engineered drainage system.

Supporting soils and the aggregate in the capillary break must be evenly graded to avoid creating a drag on the shrinking concrete that increases uncontrolled shrink cracking. To prevent weakened planes from developing in the concrete, conduits and pipes must be in the capillary break or subsoils and not on top of the vapour barrier.

 Vapour barriers

According to ACI 302.1R-06, vapour barriers, as opposed to vapour retarders, are defined as plastic sheets 0.38 mm (15 mil) thick with perm ratings of .00695 metric perms (.01 U.S. perms). Thicker vapour barriers with lower perm ratings reduce punctures and minimize the amount of groundwater penetrating the barrier. Thus, thinner and more porous vapour retarders are not recommended in this application. The contractor must pay careful attention to overlapping sheets, taping seams, and repairing holes that have been made by infrastructure penetrations. Careful attention must be paid to the maintenance of plastic sheet vapour barriers over the duration of the drying time. ACI 302’s most recent update recommends all vapour barriers be placed directly under the concrete.

 Building enclosure

The building should be enclosed to protect the concrete from the sun and prevent rain from rewetting the surface. The enclosure will also allow the use of portable dehumidification equipment and heat, if they are needed. As mentioned, the normal wait time before moisture-sensitive floorcoverings can be safely installed is three to six months. During this time, the building must be enclosed and construction operations that would rewet the concrete should be done elsewhere. Using heat and dehumidifiers can accelerate the drying process. High ambient RH will delay the drying time.

 Curing and drying concrete

Concrete hardens when cement particles are exposed to heat and water in a process called hydration—the chemical reaction continues as long as there are cement particles in the presence of free water. Curing is the process of covering the slab with a material that prevents moisture from evaporating prematurely.

Workers in this photo are walking down welded wire reinforcement (WWR) because the wire spacings are too small.[5]
Workers in this photo are walking down welded wire reinforcement (WWR) because the wire spacings are too small.

Plastic sheets are recommended in this discussion because evaporation restarts as soon as it is removed. Moisture curing processes introduce additional water into the concrete and membrane-forming curing compounds do not dissipate unless exposed to sunlight. It is advisable to place the plastic sheets over a wetted concrete surface. It is also recommended curing materials be kept in place for seven days, but with proper planning the curing time can be shortened. Once the curing covers are removed, moisture in the concrete begins to evaporate.

 RH and moisture emission testing

While other moisture tests are available, the industry has moved in the direction of RH testing using in-situ RH probes drilled into the slab (ASTM F-2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes) because RH is higher at the middle of the slab than near the top. RH probes and documentation should be managed by a trained technician employed by an independent firm with the ability to certify the results.

Once moisture evaporates from the coarse pores of the concrete, the RH falls to 80 per cent. Some manufacturers recommend installing their product once the RH is below 80 per cent, but this level is higher than the 75 per cent recommended in ASTM F710, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring. These are general statements and the contractor should carefully read the floorcovering and adhesive product literature and consult with the manufacturer’s representative before placing floorcovering on new concrete.

 Testing agencies

Even though MasterFormat places moisture testing in the ‘floorcovering’ section of the specifications, testing firms should be contracted to the owner to assure their independence. They must be instructed to immediately report to the architect and construction manager any pattern of nonconforming test results.


The owner has the highest stakes in a successful project but may not have the expertise or time to perform inspection of the work. Neither the construction manager nor the architect is required to make exhaustive or continuous onsite inspections to check the quality or quantity of the work, and according to their contracts do not have financial risk for the failure of a contractor to perform the work in accordance with construction documents (Neither the construction manager nor the architect will have control over or charge of or be responsible for acts or omissions of the contractor, as per American Concrete Institute [ACI]–A232 –2009, General Conditions of the Contract for Construction, Construction Manager as Advisor Edition.). Given these circumscribed responsibilities, no single person is responsible for achieving the overall document intent. What is required is a collaborative effort with the architect and construction manager and giving their attention to key stages in the design, placement, curing, and RH testing of concrete for moisture-sensitive floorcoverings and adhesives.

[6]Paul Potts is a technical writer, owner’s representative, and construction administrator. He has worked in the construction industry as both an independent contractor and administrator for architects, engineers, and owners in Michigan. Potts can be contacted via e-mail at[7].

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