Slab Moisture Mitigation:
Moisture mitigation is risk mitigation.
Within any facility there are multiple activities taking place; people are hard at work, and most of them are standing, sitting, walking, or working on a floor. While we may not always consider the importance of the floor, imagine trying to operate your business without one.
Most flooring will be applied to a concrete slab, whether carpet, resilient, ceramic, wood, or a resinous system. Concrete must have moisture in order to develop and maintain its strength, but excess moisture can create a host of expensive problems in your building, not the least of which is floor covering bond failure.
Floor covering bond failures due to slab moisture vapor emissions are estimated to cost over a billion dollars each year. Even if the floor doesn't fail, there is still an increased risk of mold and mildew growth. So what can we do to avoid having to replace failed systems and shut down businesses in the process? We need to test for moisture and make a business decision.
Where does moisture come from?
Moisture comes from two principal sources; free water from the concrete mix (about a 25% ratio, by weight, of water to cement is required to hydrate the cement and cause the reaction that turns cement, aggregate and water into concrete) and from the earth beneath the slab.
Why does it come to the surface?
Before a covering is installed on the floor, the slab is trying to achieve equilibrium with the ambient air (if the ambient relative humidity is low then the slab will dry faster than if it is very humid.) After a low or impermeable flooring system is installed, the moisture in the slab will attempt to again achieve equilibrium within itself. Moisture that has settled lower in the slab will migrate towards the surface. As this moisture migrates to the surface of the slab it brings salts from the slab and creates a moist, highly alkaline environment at the interface of the flooring and the slab. This alkaline environment creates a new imbalance with the moisture in the slab and increases the level of migration in order to bring more neutral water from the slab to the area of higher alkalinity causing bond failures.
Vapor Retarder, Critical Design Element?
According to the American Concrete Institute interior slabs that will have floorcoverings installed should receive a vapor retarder meeting maximum 0.1 perms (ASTM E1745) directly beneath the slab. A blotter or cushion layer of aggregate over the retarder should not be used as this creates a reservoir between the slab and vapor retarder for moisture, which can later migrate through the slab. All seams of the vapor retarder should be overlapped 6" and taped. All penetrations should be sealed watertight according to manufacturer's recommendations. Particular care should be exercised so that the membrane is not punctured during placement accidentally or intentionally (people have been known to cut holes in the membrane to accelerate the removal of bleed water and allow for faster finishing.)
What can be done to minimize the problem?
- Proper water/cement ratios should be a maximum of 0.42-0.5 w/c. Water in excess of 0.25-0.28 is considered "water of convenience" and only assists in placement.
- The pour should be monitored to prevent additional water from being added during placement.
- Porous aggregates should be avoided. If lightweight aggregates are needed above grade they should have a low porosity.
- Hard trowelling should be avoided. This can create a thin densified layer on the surface that inhibits adequate drying.
- Curing should be performed using blankets or sheeting for a minimum of 7 days (curing compounds can inhibit adhesive bond to concrete, slow drying, and may need to be mechanically removed prior to installing floorcoverings or resinous systems adding cost.)
- Curing compounds do not "wear off" as the concrete cures despite claims of some manufacturers.
- The slab should be under roof and conditioned as soon as possible.
What about admixtures that "eliminate moisture problems"?
While there are a few manufacturers who claim that Lithium or Potassium Silicate admixtures can reduce moisture vapor emissions, there is no evidence to support these claims. These silicates are actually used as densifiers in polished concrete. One of the benefits of polished concrete is that it permits moisture vapor transmission.
- These products can help reduce the size of capillaries within the slab and may reduce the transmission of liquid water, but will not stop vapor, which is the problem.
- When evaluating products ask to review the ASTM E96 test results for vapor permeability.
- Warranties and installation limitations should be reviewed on any mitigation system.
So if you have done everything right in your mix design and placement then there shouldn't be any moisture issues, right?
The following table demonstrates the amount of water that goes into a 4" slab at the corresponding water/cement ratio and how much needs to come out in order to "dry".
| Water/Cement Ratio | Gallons/yd | Gallons/1000 Sqft | Water of Convenience/1000 Sqft |
| 0.4 | 23 | 283 | 111 |
| 0.5 | 28 | 344 | 172 |
| 0.6 | 34 | 418 | 246 |
| 0.7 | 39 | 480 | 308 |
| 0.8 | 45 | 554 | 382 |
| 0.9 | 51 | 627 | 455 |
| 1.0 | 56 | 689 | 517 |
After you have taken the .25 water mix required for hydration into consideration, a .6 water/cement ratio a 1000 sqft slab would need to shed a percentage of the remaining 246 gallons of water to "dry" to the point where it meets the RH requirements for floor installation.
Moisture Testing
Overview
Despite our best efforts, we cannot control everything that can affect the drying of our slab so how do we know that when we install our floor that the slab is adequately dry? We test it. Testing does not allow for a warranty against moisture related failures, it simply minimizes the risk.
The GC has to get the job done on time, and the owner wants the job done right, early and under budget, and the flooring contractor wants to give you the best installation. Who should be responsible for moisture testing? Testing should be performed by someone who does not have a vested interest in the outcome, a third party who is an ICRI (International Concrete Repair Institute) Certified Tester. This should be outlined in the specifications for the project.
Testing Methods
Situ Probe Relative Humidity Test (ASTM F-2170)
This is the preferred method. This test uses probes placed in the slab (40% depth for slab on grade or pan, 20% depth for suspended slabs) to give a quantitative reading of the relative humidity (RH) of the slab. This method gives consistent and reliable readings of the relative humidity within the slab which gives you a better picture of what will happen once a floorcovering/resinous system is installed and the slab moisture equalizes. ASTM requires this test to be performed once the slab is in a conditioned environment. Although it will not provide an ASTM certified result, some may choose to place the probes and perform initial testing prior to conditioning to get an idea of slab moisture.
Calcium Chloride (ASTM F-1869)
This method is still specified by many manufacturers, but waning. This test will give a quantitative reading of the moisture vapor emissions at the time that the test is performed. It cannot tell you what the slab moisture content is, so it cannot predict what will happen once a system is installed. There are also frequent false readings with this test; it can be dramatically affected by ambient conditions; and it can be inconvenient in occupied spaces. This test may be used in conjunction with In Situ RH testing. This test must be performed with the slab in a conditioned environment for 48 hours prior to and during the testing process, failure to do so results in a meaningless result.
Moisture Meter (ASTM F-2659)
This method is specified by very few manufacturers. This test uses conductivity to determine moisture content near the surface of the slab. This test does not test RH at depth in the slab and like CaCl, cannot predict what will happen once flooring is installed. Conductive metals and foreign objects near the surface of the slab can affect readings.
Plastic Sheet Method (ASTM D-4263)
This method is a qualitative test and should never be used to determine whether or not the moisture content is sufficiently low to install a floorcovering or resinous system. A test resulting in a positive moisture presence will require further testing, but a negative test does not mean that the slab is at a sufficient level for installation.
If it is determined that you have too much moisture, what are your options?
- You can wait until the slab dries to an appropriate level.(If construction schedules permit, you may be able to wait a period of time and retest, though there is no guarantee of the amount of time it may take to reach the desired level.)
- You can install a moisture mitigation system.(There are several quality epoxy based systems for mitigating moisture vapor emissions that can be installed to allow you to stay on schedule. There are also many more systems that are untested and use questionable methods and chemistries.)
- You can roll the dice.(You may decide that the cost of mitigating is greater than the risk, inconvenience and cost of having to replace the flooring if there is an issue down the road.)
- Non moisture sensitive flooring options - Polished concrete, ceramic tile and certain resinous systems may be suitable for slabs with moisture higher levels of moisture vapor transmission.
Selecting the Right Moisture Control System
Overview
There are a variety of mitigation systems on the market utilizing various prep methods and chemistries. When selecting a system it is important to consider why you are considering that system: solve your immediate installation problem and insure against a potential floorcovering failure. Moisture mitigation is risk mitigation and we all want the best chance of success. Epoxy type mitigation systems have consistently demonstrated superior success rates across the board and are recognized as the "right" way to get it done.
Check that the manufacturer of the mitigation system can provide documented evidence of a proven track record of successful performance over an extended period of time.
Epoxy systems are typically installed over a properly prepared (shot blasted) floor that gives the epoxy an opportunity to penetrate and achieve maximum bonding characteristics. Epoxies also cross link as they cure which increases performance. They typically have a perm rating (ASTM E96) around .2 or less. A quality system should also have a dry film thickness around 15 mils.
What does the manufacturer say about how much moisture the system can handle AFTER installation?
Moisture levels at the surface can rise once a system is installed (this is one reason why it is important to test RH and not rely on CaCl alone.) If testing was only done using CaCl you may not have a good indication of what lies beneath the surface so you want a system that will handle those increases. If there is no effective moisture vapor retarder below the slab you likewise need a system that can handle whatever comes its way.
Make certain that the system can handle elevated pH levels.
As moisture migrates up through the slab it will increase the surface alkalinity. There are many instances where an adhesive has been placed and through moisture migration of the slab, the alkalinity increased enough to effectively destroy the adhesive. A pH tolerant system is ideal.
Ensure that the system has sufficient thickness to completely cover the area.
Proper preparation of the substrate should include creating an adequate surface profile so the system can achieve a strong mechanical bond. Typically proper prep is achieved through shot blasting which can create a profile as deep as 10 mils so a system that is 10 mils or less could potentially not provide full coverage over the substrate.
Does the system priced include patching and leveling the floor to accept floorcovering?
Some mitigation systems are simply that, an epoxy moisture mitigation system. This is probably adequate if you are planning on a resinous finish, but you want to make sure that the resinous system is compatible with the mitigation system. If you are planning on installing floorcovering over the mitigation system, you will need to consider patching and leveling the floor to meet the flooring manufacturer's installation guidelines.
What kind of warranty does the system come with? Who is involved in the warranty process? What steps need to be taken to ensure that the warranty covers your project? What is included in the warranty (some systems only warranty product replacement on the mitigation system leaving you responsible for the labor and floorcovering cost)?
Selecting The Right Contractor to Install your Mitigation System
Overview
Like selecting the right product, selecting the right contractor is vital to the success of the system. Remember, this is risk mitigation. You will probably never see the mitigation system, it will be buried beneath layers of floor patch and floor covering. There is no way to know that it was installed incorrectly until it fails.
Images of Flooring Fails from Other Contractors:
Ask around. Ask for references. Ask the manufacturers.
If you know that this is a fast track job, or that there is a high probability of moisture vapor related issues, budget for and specify for mitigation. Many medical facilities now specify mitigation as standard protocol. They cannot risk the cost of a failure.
If this is an existing facility, test in advance. Know what you are working with and plan for it.
DMA Floors is a certified installer for several moisture mitigation systems and is certified by the International Concrete Repair Institute (ICRI) for slab moisture testing. We also conduct several AIA training seminars on moisture mitigation each year (contact to set up a seminar.) We have been installing floors for over 30 years and understand what can go wrong and what you can do to mitigate your risk. Even if we are not installing your flooring, we are interested in helping you find peace of mind that it will last.
When is the Low Bid Not Really?
- Floor Prep - The key to installing any epoxy system is proper floor prep. Failing to shot blast or scarify to achieve the proper profile will cause the mitigation system and the floorcovering it is supposed to protect to fail. You won't know this was done improperly until it fails!
- Inadequate System - There are many systems on the market that make many claims, but few are proven and provide the warranty and support to back it up.
- Patching/Leveling - Does the bid include patching and leveling the floor after the system is installed in order to prepare it to receive floorcovering?
- Patch Under Epoxy - If the floor needs to be patched prior to application of the system, is the patching material moisture tolerant? Most are not and will cause a failure.
- Spread Rate (Thickness) - Failure to apply the epoxy at the specified spread rate eliminates any warranty and effectively nullifies any protection that the system is supposed to provide. You won't know this was done improperly until it fails!
Not to say that anyone would intentionally use substandard procedures or products, but with the competitiveness of the bid process it is vital that scopes be reviewed properly and the quality of the installer be taken into consideration.
