Concrete moisture and water-based adhesives

January 3, 2018

Photo courtesy Paul Potts

By Paul Potts
Government regulations requiring co-ordinated changes to manufacturing, design, and construction practices with a single regulation are difficult for the industry to assimilate successfully. The banning of solvent-based adhesives was one such example.

In 2009, along with similar regulations in the United States, Environment Canada issued new rules to reduce ground-level ozone—a significant source of smog. The rule committed both countries to banning products emitting volatile organic compounds (VOCs) in solvent-based adhesives (as well as hairspray propellants), as this contributed to ground-level ozone.

Floorcovering adhesive manufacturers responded by developing water-based adhesives that did not produce as many VOCs. Unlike the traditional solvent-formulated products that were unaffected by moisture, these new adhesives could be degraded by surplus moisture in concrete, causing delamination of the floorcovering. This anomaly caught many in the industry off guard, resulting in numerous floorcovering failures before awareness of the problem began to flow through the industry from manufacturers to designers, specification experts, and construction managers.

More than a decade after it became known water-based adhesives were susceptible to deterioration when brought into contact with surplus moisture in concrete, manufacturers, architects, engineers, construction managers, and contractors have still not made a thorough effort to minimize the risks of adhesive degradation through moisture contact. What is needed is an acceptance of responsibility by each party for their part of the solution.

Floorcovering manufacturers deserve some of the blame for this slow response. Few made an effective use of their marketing networks to communicate the dimensions of the problem to architects, engineers, and interior designers. Further, to this day, not all manufacturers have adopted clear and effective installation instructions to determine when the moisture conditions in the slab are safe for applying their products.

Construction schedule and the critical path
The most significant ramifications for the owner and the construction manager are the changes in the critical path and an increase in the duration before floorcovering can get started. The critical path must be organized to provide an enclosure for the concrete to prevent rainwater from rewetting the slab and continuously setting back the start date for floorcoverings.

Further, the duration in the schedule—waiting for the concrete to dry to a safe level—must be planned in terms of months, not weeks, regardless of any special techniques employed for shortening the wait-time. The traditional practice of scheduling floorcovering 30 days after concrete has cured is no longer advisable.

Water-based adhesives can be susceptible to deterioration when brought into contact with surplus moisture in concrete. This often results in a floorcovering failure.
Photo courtesy Chris Maskell, NFCA

Moisture and wait-time
There are four sources of moisture in slab-on-grade thatcan be harmful to water-based adhesives and delay the start of floorcoverings:

Surplus water
Excessive surplus water can extend the wait-time for concrete to dry by several months. Surplus water is the water that is not hydrated with cement and must evaporate before concrete is safe for moisture-sensitive adhesives. It is the specifier’s loosely written water-cement ratio (w/c) design that is responsible for most excess surplus water. A correct w/c ratio specification would provide enough water to bring about complete hydration of cement, leave enough surplus water to allow for placement, and account for the rapid evaporation of moisture in the first few hours before curing is started. Thus, while a certain amount of surplus water is needed, this surplus must be thoughtfully calculated. The w/c ratio should not be viewed solely as a tool for easing placement—which is a function of water-reducing admixtures (WRAs)—but should be written to minimize surplus mixing water.

Specifiers can have a direct effect on shortening the wait-time by specifying the lowest practical w/c ratio (0.45 w/c or lower) and reducing the cement paste requirement with an optimal selection of aggregates. ( For more on aggregate selection, see this author’s article, “Critical Threshold for Concrete Moisture Content,” published in the December 2017 issue of The Construction Specifier, the official magazine of CSI. Visit[3].) Reducing the cement paste reduces total water in the mixture.

Concrete does not harden by drying, but becomes a solid mass when the water in the mixture hydrates with cement and encloses and adheres to the aggregate. As the concrete hardens, surplus water that has not hydrated with cement or evaporated is encapsulated in the gel system of the concrete, becoming a source of internal humidity—a long-term hazard to moisture-sensitive adhesives and floorcovering.

This concrete likely has excess water to ease placement.
Photo ©

Many architects and structural engineers design concrete as though strength and slump were the only important factors, even though a lower strength requirement is directly related to excessive surplus water—the lower the strength, the higher the water content. From a structural standpoint, slab-on-grade concrete with 24 MPa (3500 psi) compressive strength is adequate for the expected loads in commercial buildings and eases placement; however, it will have a w/c ratio of 0.6, with a lot of surplus water. The extra water increases the shrinkage potential of concrete, making it prone to cracking and curling, and increases the wait-time for drying concrete by months.

An optimal water-cement ratio of 0.45 will result in an average compressive strength of 31 MPa (4500 psi), with enough water for complete hydration of cement and adequate surplus water to compensate for moisture lost to evaporation before curing is started. With this w/c ratio, there will be minimal surplus water left behind to evaporate before scheduling the application of floorcovering. Under these conditions, curing should be started as soon as possible because there is a minimal amount of moisture to lose. This optimal w/c ratio also results in concrete that is stiff and difficult to place. The architect or engineer should include a minimum requirement for a mid-range water reducer in the mix design to discourage workers from adding water at the site.

Ambient humidity and extra means of drying concrete
Concrete containers will hold water, but water vapour readily passes through concrete. Regardless of its appearance, bare concrete is constantly exchanging moisture with the atmosphere. When the relative humidity (RH) of the ambient air is greater than the humidity of the concrete, the latter absorbs moisture. (When the humidity conditions are reversed, the concrete absorbs moisture.) It may be useful to cover the slab with a temporary vapour barrier if high ambient humidity conditions are slowing the drying process.

The building HVAC equipment or separate portable dehumidification equipment is quite useful in any program to dry concrete and reduce the wait time. Even circulating the air is helpful, but running the air-conditioning system is much more effective. Unfortunately, lowering the air temperature also reduces the moisture carrying capacity of the air, reducing the efficiency of the system. Some building HVAC systems can be operated with both cooling and heating coils active at the same time; thus, cooling the air to wring out the moisture and reheating it to make it as efficient as possible. Portable dehumidifiers are also very effective at reducing the ambient air relative humidity and accelerating drying.

This photo shows a portable dehumidifier, which can be very effective at reducing ambient relative air humidity and accelerate drying.
Photos courtesy Paul Potts

Soil moisture and groundwater
Soil moisture and groundwater move through concrete to the adhesive layer by capillary action—that is, the ability of a liquid to flow in narrow spaces (capillary pores) without the assistance of, and in opposition to, external forces like gravity. (A good working definition can be found at[6].) Moisture moves readily to the surface of concrete at a rate that depends on that material’s porosity and permeability. (It is worth noting these properties are improved using a low w/c ratio mix design.)

A functioning vapour barrier directly under the slab will cut off most soil moisture and groundwater from entering the slab. The American Concrete Institute (ACI) once recommended to place a blotter layer of aggregate between the vapour barrier and concrete, but this is no longer suggested except in specific circumstances. (For more on where to place the vapour barrier, see American Concrete Institute [ACI] Report 302, Section [“Vapour retarder/barrier location”].)

A correctly placed vapour retarder or barrier will be continuously lapped 150 mm (6 in.), with taped seams, turned up at the edges, and provided with special fittings to waterproof mechanical and electrical pipes and equipment poking through the plastic sheet. ACI 302.1, Guide to Concrete Floor and Slab Construction, differentiates between vapour retarders and vapour barriers. Retarders are sheets up to 0.25 mm (10 mil) thick with a perm rating not greater than 0.2085 metric perms (0.3 U.S. perms). Vapour barriers are defined as sheets 0.38 mm (15 mil) thick with perm ratings of 0.00695 metric perms (0.01 U.S. perms). The thicker sheet is less likely to be punctured during construction operations. A single 13-mm (½-in.) hole in the vapour barrier will allow gallons of water to accumulate between the sheet and concrete.

There is an industry discussion taking place that for a vapour barrier to be defined as such, the material should have a rating of 0.0 metric perms (0.0 U.S. perms); all other products should be referred to as ‘vapour retarders,’ with an efficiency rating on a sliding scale from the most porous of 0.2085 metric perms (0.3 U.S. perms) down to 0.0. Plastic sheet used for a vapour barrier must meet the standards described in ASTM E1745, Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs. More detailed information on the selection and application of vapour barriers can be found in CSA A23.1, Concrete Materials and Methods of Concrete Construction, and ACI 302.2R, Guide for Concrete Slabs that Receive Moisture-sensitive Flooring Materials (i.e. requirements for vapour barrier materials that meet ASTM E1745 standards installed in accordance with ASTM E1643, Standard Practice for Selection, Design, Installation, and Inspection of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs).

Allowing rainwater or water from construction operations to accumulate on the concrete will increase drying time.

Rewetting concrete
Providing an enclosure to protect concrete from rainwater has already been discussed. The contractor must also take precautions against rewetting the concrete with construction operations. Using a power water-sprayer to remove membrane-forming curing compounds will introduce moisture into the slab and sets back the start of moisture-sensitive adhesives and floorcoverings.

Curing materials
Curing is the process of holding mixing water in the concrete long enough for near-complete hydration of the cement with water. However, to restart evaporation, the curing materials must be completely removed. Plastic sheet has the advantage in that once it is removed, evaporation begins immediately. Membrane-forming curing compounds may be more appropriate, depending on circumstances, but removal of  the compound requires some mechanical means if the floor is completely sheltered from sunlight.

Moisture testing
Excess water gathers in the pores of concrete, first as a liquid (when the concrete is wet), then as moisture vapour (during the curing process). Surplus moisture vapour not hydrated with cement is locked in the gel and capillary pore structure of the hardened concrete, taking months to evaporate to levels safe to apply adhesives. Moisture vapour produces a measurable level of relative humidity in the concrete that can serve as a good barometer of when the concrete is safe for the application of moisture-sensitive adhesives.

The anhydrous calcium chloride test to establish the moisture vapour emission rate (MVER) was once considered the industry standard for determining the moisture in concrete, but its failure to predict the incidence of water-based adhesive failure has become well-known. Manufacturers have been slow to recognize the limitations of this test and have not universally revised their installation instructions to direct the use of the more reliable RH tests. Today, the MVER test is considered a superficial methodology, measuring, at best, the moisture in the top 25 mm (1 in.) of the concrete while overlooking the relative humidity deeper within the slab that will eventually come to
the surface. (A Concrete Construction article entitled, “Slabs on Grade Concrete Floor Moisture Tests[8],” has more. )

The quantitative RH probe test (which is described in ASTM F2170-11, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In-situ Probes) measures the internal RH of the slab. Today, more manufacturers require RH testing; but if the RH test is not included in the instructions, a meeting becomes necessary to resolve the issue.

The RH test equipment is expensive and requires training to be done properly. This is better done by an independent agency with the calibrated equipment and skills to perform the test. Such agencies should be contracted by the owner and not the contractor; further, they should not be given ‘go or no-go’ authority—that judgement must remain the responsibility of the floorcovering installer so as not to undermine the risk relationship between the parties.

All parties bear responsibility for the incidence of moisture-related floorcovering failures following the introduction of water-based adhesives. Most manufacturers were slow to connect the dots and did not use their network of sales organizations to get architects and engineers involved in solving the problem and few manufacturers stayed on top of the connection between water-based adhesive failures and the limits of the MVER test to prevent them. At the same time, many architects and engineers did not recognize that standards for designing concrete that made structural sense were not necessarily effective at preventing floorcovering failures.

Further, few owners and construction managers faced up to the adjustments in the sequence of construction and the duration required to get concrete dry enough for applying the floorcovering. The wait-time duration may not always fit well into the owner’s occupancy expectations, but it is necessary if the owner is to avoid inheriting legal responsibility for floorcovering failures. Concrete contractors must exercise care installing the vapour barrier and resist the temptation to add water to concrete at the site. Finally, floorcovering contractors must insist on waiting until the RH tests prove it is safe to install the floorcovering or request a written directive from the owner’s representative to proceed under adverse conditions.

Paul Potts is a technical writer, owner’s representative, and construction administrator. He has worked in the construction industry as an independent contractor and administrator for architects, engineers, and owners in Michigan. Potts can be contacted via e-mail at[9].


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  8. Slabs on Grade Concrete Floor Moisture Tests:

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