Concrete and Water
The construction industry’s rule of thumb is that concrete takes one month per inch of the slab’s thickness before non-breathable flooring can be applied. There are various other factors, however, that can affect moisture in concrete. All of these things must be considered in order to make a proper recommendation for resinous floor coatings.
Before applying a resinous flooring system, applicators are encouraged to check the moisture content and alkalinity of the concrete. Photos courtesy Tnemec Company Inc.
In the case of new concrete, the free water — sometimes called “water of convenience” — needs time to evaporate from the slab. Design mixes will dictate cure times and can be significantly different. The placement of a vapor barrier below the slab reduces the presence of moisture due to hydrostatic pressure. No matter the scenario, moisture detection must be performed to recommend a coating system with confidence.
The concrete mix design, especially the water/cement ratio (w/c), has a direct impact on the drying time of the concrete. If the ratio is high, more water will need to evacuate the slab. A high w/c mixture is also more porous, allowing moisture to travel through the slab easily. Conversely, a low w/c concrete dries faster, but may not be suitable for some projects.
LEED and other “green” considerations may also affect the moisture levels in concrete. Some recycled materials, such as fly ash, silica fume and slag cement, can contribute to credits for pre-consumer content. These materials have the tendency to make concrete more dense, which can extend the drying time. Lightweight concrete can be another concern because of its porosity; it absorbs and retains a lot of moisture and can take twice as long to dry.
Aside from the mix, surrounding environmental conditions — such as temperature, humidity, and sun and wind exposure — will also play a role in the cure of newly placed concrete. In a more humid environment, water will take more time to evaporate from the concrete. In a dry climate, water will evaporate in less time.
In fast-track projects, with everything on a tight schedule, it is not always possible to wait this long. Fast-track construction can be detrimental to the success of a floor coating without proper consideration. Furthermore, a rushed project schedule may not allow for proper moisture testing, which states that a building should be enclosed with its HVAC system running 48 hours before testing begins.
For slabs-on-grade, a vapor barrier is important to controlling moisture in the slab. Since concrete is inherently porous and full of capillaries, it will draw moisture through the slab if it has a source. Properly protecting concrete from the underlying soil greatly enhances the success of moisture-sensitive floor coatings.
Depending on the product or system, coatings manufacturers currently recommend three different methods for measuring moisture in concrete: ASTM D 4263, ASTM F 1869 or ASTM F 2170. These three methods are very different in terms of how they detect moisture and how their results should be interpreted. Reliability is a concern for moisture detection, as many variables can affect their outcome. It should be noted that a test result only represents the specific period in time in which the test was performed.
The concrete floor in this fire station in Elgren, Ill., was coated using Series 241 Ultra-Tread MVT from Tnemec to combat higher-than-usual relative humidity readings during application.
ASTM D 4263-05, or the Plastic Sheet Method, is widely known as an old test method, although it is still technically valid and referenced by many manufacturers. The Plastic Sheet Method detects whether or not there is moisture in the uppermost layer of the concrete. If the result is negative, the flooring applicator may be free to install the coating. If the result is positive, the question left unanswered is, “How much moisture is there?”
ASTM F 1869-11, or the Calcium Chloride Test, is similar to the Plastic Sheet Method because it only measures the moisture in the upper layer of the concrete slab; however, this test is quantitative and is widely used in the industry today. Three pounds per 1,000 square feet per 24 hours is generally the highest level of moisture allowed before a non-breathable coating may be installed. However, some products in the industry allow higher acceptable test results. As with every test, the results are meaningless if not conducted properly. For the Calcium Chloride Test, this means that the HVAC system must be operating under normal conditions for 48 hours before the test is conducted.
ASTM F 2170-11, or the Relative Humidity Probe Test, is a test that is gaining popularity. Unlike the Plastic Sheet and Calcium Chloride tests, this method measures relative humidity within the slab, at 40% of its total depth. At this depth, the measured RH is representative of the whole slab, so as moisture begins to move from bottom to top, the results will still be relevant. Many manufacturers who reference this test require RH levels below 75 to 80 percent before a coating system is installed. Again, the building must be conditioned for at least 48 hours prior to performing this ASTM.
Although the Calcium Chloride and the Relative Humidity tests are the most widely used methods for measuring moisture levels in floors, their results cannot always accurately predict whether or not a flooring application will fail. One or more test methods may need to be performed several times — in various locations on the floor — to ensure that the moisture conditions are stable or decreasing.
Moisture and Alkalinity
This laboratory in Farmington, Conn., utilized a coating applied a high-performance moisture control system directly to the concrete.
As the most important component of concrete, cement contains a number of highly alkaline substances that are water-soluble. These alkali salts are distributed uniformly throughout a concrete slab when installed. But as the slab dries, moisture moves upward as water molecules dissolve the alkalis and carry them with it.
Once concrete is hydrated, the drying process consists primarily of free moisture moving through the slab and exiting the top as water vapor in two different phases. The first phase begins immediately after a slab is installed as excess water makes its way out. The second phase starts when the building is climatized, and the dry air above pulls further moisture from the slab while attempting to find equilibrium with the surrounding environment.
When non-breathable flooring is installed on concrete, the deep-seated moisture will continue to redistribute itself, bringing even more moisture and alkalis to the surface. If the concrete has excessive moisture and/or is especially porous, a highly alkaline condition can be created at the concrete/coating interface, where the salts are deposited. This phenomenon can interfere with the adhesion of certain types of coatings and non-breathable flooring systems.
Osmosis versus Alkalinity
Osmotic blistering of coatings generally refers to the passage of water through the coating as it tries to find equilibrium with salts on the other side. In this case, the coating is the membrane separating the areas of different solute concentrations. In the case of concrete and flooring, osmosis can also play a role in coating failures, but in a different way.
In flooring situations, the concrete slab itself is the membrane. After the concrete slab has been placed in service and the coating has been installed, the slab can be divided into two distinct layers: the bottom, which contains a high level of moisture; and the top, which is drier, and contains a high concentration of alkali salts.
With a non-breathable coating installed, the moisture present in the concrete can no longer exit through the top. The difference in soluble salt concentrations between the two layers of the slab creates a pressure gradient, as the water moves to the surface to try to equalize the solution. Although this is just one theory of how moisture causes flooring failures, it’s easy to imagine how a sudden change in pH, and then osmotic pressure and excessive moisture, can cause bond failure in even a well-prepared floor.
Another theory associated with moisture-related coating failure is that of an alkali silica reaction (ASR) near the concrete/coating interface. This reaction is caused by the use of certain concrete aggregates that react with alkalis in the presence of moisture. When the reaction takes place, a gel-like substance is formed around the aggregate that causes pressure and often expansion of the concrete. If all of the ingredients are there, ASR can be very destructive to coating systems and nearly impossible to control.
Combating MVT and Alkalinity
In September 2014, a project began at AVX, Inc. in Fountain Inn, South Carolina, that would include nearly 200,000 square feet of interior concrete floors. Before application, the flooring applicators conducted calcium chloride tests in each area of the facility in accordance with ASTM D 1869. The results of the test showed that concrete slabs around the building’s perimeter registered far above 3.0 pounds — the desired moisture vapor pressure of the manufacturer’s epoxy primer. So the crew turned back to the manufacturer for a solution.
This facility in Fountain Inn, South Carolina, installed their resinous flooring system, featuring Series 208 Epoxoprime MVT, following several moisture tests on the previously existing concrete.
After considering the results of the test, the manufacturer’s coatings consultant recommended a new technology to help combat the effects of moister vapor transmission (MVT) and high alkalinity. The specified intermediate and finish coats could remain the same, but the primer would need to be changed.
The prime coat was changed to a unique, two-component clear polyamine epoxy coating formulated to reduce the potential for failure due to the high alkalinity associated with MVT. This low-viscosity, high-solids epoxy primer helps limit concerns with MVT in thin-film flooring systems because of its ability to penetrate the concrete surface, providing superior adhesion and ultra-low permeability. When applied in a single coat, this primer can provide protection from a sustained exposure to pH levels up to 14, MVT levels up to 15 pounds (per ASTM F 2170) and up to 95-percent RH (per ASTM F 2170-11).
Applicators prepared the concrete floor to provide a surface profile of ICRI-CSP 3. Following surface preparation, the unique polyamine epoxy primer was applied to the horizontal concrete at approximately 16.0 mils dry film thickness (DFT). This primer was topped with a high-performance floor coatings system, including a clear, two-component polyamine epoxy and an aliphatic moisture-cured urethane finish.
After the installation — even with the high humidity and MVT concerns — the resinous flooring system continues to perform to expectation. Although there are many theories about moisture-related coating failures, many manufacturers and applicators in the concrete industry believe that alkalinity is the main culprit and that MVT alone does not create enough pressure to disbond a coating from a concrete slab. With new technology like the epoxy primer used at the Shoal Creek WTP, the effects of MVT can be controlled and help eliminate the threat of disbondment and bubbling due to moisture and high alkalinity.
Planning for MVT
All concrete should be tested for pH level, moisture and relative humidity. If necessary, after assessments are made, various products exist that can be applied to concrete to combat the negative effects of MVT. These coating products are typically applied directly to the concrete floor in order to help prevent problems at the bond line. Check with the manufacturer of your specified coating system whenever these concerns arise and ask for a viable recommendation; down the line, this recommendation could save your floor.