Where does moisture come from?

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?

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?

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 about admixtures that "eliminate moisture problems"?

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, you have done everything right?

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”.

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.

Testing Methods

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.

Too much moisture? What are your options?

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.

Documented evidence

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.

How much moisture AFTER installation?

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.

Check elevated pH levels.

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.

Check thickness.

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.

Patching and leveling

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)?

Ask

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.

Low bid, not really?

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.