The Complete Guide to Moisture Problems on Concrete Floors and How to Prevent Them

What is a Moisture Problem on a Concrete Floor?

Moisture problems can be identified by an accumulation of water and wet spots on the floor, or visible mold.
Concrete floor moisture can be a reason for concern in any building or home. It is an issue that should not be ignored or brushed off. If unchecked, mold can lead to the following problems: rotting hardwood, damping carpet & de-lamination.

Moisture issues on concrete floors could be the result of many different factors such as, but are not limited to:

  • Water left over from concrete mixing which is also known as bleed water.
  • Naturally-occurring sources like precipitation, groundwater, condensation or infiltration, and inflow.
  • Water escaping from nearby unnatural sources such as broken plumbing pipes.
  • The most concerning are a wrong concrete mixing ratio or an issue with the foundation.

Concrete is naturally porous. The moisture on the surface might not be a concern, but moisture traveling from below or within the concrete will tend to migrate up and through it, finding its way out through any available path.

Moisture is by far the most common issue that can compromise a concrete asset and the coating that protects it.
Concrete is more likely to be affected by moisture because of a channel of capillaries that form during the hydration phase. The number of channels can be dramatically increased or decreased depending on the water-cement ratio used in the mix. The permeability of concrete can be reduced by using mixed designs with lower water-cement ratios, adding various kinds of admixtures, or applying a protective layer on the surface. Moisture-related issues are common when applying protective coatings to concrete.

Signs of Moisture on Concrete Floors

You will be able to notice moisture on the concrete floor right away if you pay attention. However, if you’re not yet sure, there are certain signs to look out for including:

  • Condensed moisture vapor can deposit alkali/salt, which looks like white residue, on the floor. This indicates a moisture problem.
  • Stains on the floor usually black in color
  • Damp spots or areas of darker discoloration on the floor
  • Signs of de-lamination on flooring that uses adhesive and/or grout during installation
  • Cracking, lifting, bubbling, or peeling of the floor surface

Causes of Excess Moisture in a Concrete Slab

Even though existing water sources could pose a problem, they’re not usually the real culprits. The problem could come from the concrete slab itself. Slabs should be installed on a well-drained sub-floor that is adequately protected against water and moisture intrusion. These problems can be solved by using the right vapor retarder for your specific situation.

Always make sure you follow the construction code’s specifications which recommend vapor retarders with a perm rating of 0.3 perms. Installing a vapor retarder directly on the ground/ soil during construction affects its performance. Separating the vapor retarder from the ground/soil with a granular fill will create an elevation to increase separation and provide better results. The concrete may not have had enough time to dry, which could cause cracks. This is a very common problem when there’s not enough lead time.

Dangers of Moisture Vapor Transmission

Some of the potential flooring issues that can result from excessive moisture vapor transmission are:

  • De-lamination or bonding failure of a floor covering due to the highly alkaline liquids that can condensate underneath
  • Cracking or bubbling of a floor covering or resinous floor coating system
  • Development of uneven walking surfaces, creating slip-and-fall hazards
  • Reduced lifespan of the flooring material; damaged aesthetics
  • Invalidation of floor covering warranty, depending on MVT limits
  • Eventual deterioration and crumbling of the concrete substrate
  • Development of mold or pathogen growth either on top of the concrete slab or beneath the flooring material

Concrete Slab moisture testing methods and why it matters

Many tests can be done to check for moisture in concrete slabs. This section will provide information on how to do these tests and what they mean when they are done correctly.

The most basic test for measuring concrete readiness is ASTM D4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method. The Plastic Sheet Method involves taping a 457 x 457mm (18 x 18 in.) square plastic sheet to the concrete floor and waiting at least 24 hours before removing it. Placing an incandescent lamp close to the plastic sheet can help promote moisture to migrate.

Once the plastic sheet is removed, the concrete is inspected for darkening or other signs of moisture. Although this test has existed for decades and is still used as a surface-level test, it has several flaws and sheds little light on the severity of the moisture problem within the concrete slab. First, it does not quantify the amount of moisture; it simply indicates whether or not moisture is present. Second, it only detects moisture in the upper portion of the slab, not in the middle or lower areas where moisture is more apt to reside.

Calcium Chloride Test: Measuring Moisture Vapor Emission Rate of Concrete Sub-floor, per ASTM F1869-11, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Sub-floor Using Anhydrous Calcium Chloride. The calcium chloride test uses the weight differential of calcium chloride salt placed on the surface of the slab for up to 72 hours to identify a moisture problem. To perform this test, the calcium chloride is placed in a sealed dish and the rate of evaporation is calculated based on the weight differential.

After 72 hours, the disk is retrieved and weighed, and compared to the disk’s pre-test weight. The dish is weighed to record pounds of water emitted per 93square meters (1,000 square feet). This weight difference indicates how much moisture vapor has emerged from the slab during the 72 hours. Failure is identified if the calcium chloride tablet is above 3 pounds in weight. Note that this test can give false readings because 90% of moisture measured comes from the top ½” of concrete. This test does not detect moisture below ¾”.

Relative Humidity Testing: Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes as per ASTM F2170 is a more accurate testing method. This method requires holes to be drilled into the concrete and a sleeve with a humidity probe inserted. This test is more precise because it can show humidity readings at various depths in concrete. Probes can also be left in concrete to deliver readings over time. The quantitative result and the measurement within the middle of the slab make this test a more accurate and reliable way to measure concrete slab moisture.

Electronic Moisture Meter: Standard Guide for Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement, and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter as per ASTM F2659-10(2015). This test focuses on obtaining the comparative moisture condition within the upper 1.0 in. (25.4 mm) stratum in concrete, gypsum, anhydrite floor slabs, and screeds for field tests. A concrete moisture meter measures concrete water percentage in a non-destructive manner. Due to the wide variation of material mixtures and additives used in floor slabs and screeds, this methodology may not be appropriate for all applications.

Mold is a type of fungus that feeds off of water and, in some cases, food. These grow on surfaces where there is moisture. Moisture problems are most commonly found in the basement or a crawlspace. Mold can cause health problems such as allergies, asthma, breathing problems, and even death when they are inhaled while being cleaned up.

Mitigating concrete moisture issues

Once you have your concrete moisture testing completed, there are a lot of methods available to solve any troubles you might have. There are a few different things you can try when it comes to mitigating the damage from water damage.

  • Waiting – For starters, coating manufacturers usually recommend you allow at least 28 days for curing before coatings are applied (although some new “green” coating formulations can be applied after 14 days). Waiting for concrete to dry out on its own is the easiest and cheapest mitigation option. It only requires patience, and it’s the preferred method provided it doesn’t delay or disrupt other ongoing work.
  • Dehumidifying – Equipment supplementing a building’s HVAC system is commonly used to help lower the relative humidity or increase the temperature of new concrete to accelerate drying. This technique can also add to project timelines and requires care and constant monitoring of moisture levels.
  • Employing vapor barriers or retarders – These systems either slow or completely stop the migration of moisture from the ground to a slab on grade. Choose a material that isn’t easily damaged during installation, as any advantage gained by using a barrier is lost if it tears.
  • Moisture mitigation systems – These systems are applied as topical treatments to concrete and have been successful in reducing moisture vapor emission rates. They include liquid membranes, reactive penetrants, modified cementitious overlays, dispersive membranes, assembly systems, and combination systems.
  • Design against moisture Some naturally-occurring moisture is unavoidable, but other problems can be eliminated through design. For instance, flat expanses, dimensional changes or voids can all harbor water. During design, these should be avoided if possible. If these features are critical to the function of a facility, designs should include channels designed to divert water away from problem areas.
  • Use green concrete coating systems These systems start with the application of a high-solids, no-VOC epoxy primer applied to a new concrete surface well before concrete’s 28-day cure window has closed. These formulations penetrate and then form strong bonds with the concrete surface. It significantly reduces the impact of vapor transmission and allows for mid-and top coating of the surface sooner, leading to shortened project timelines. Floor coatings will have varying tolerance to moisture vapor levels, and this should be reflected on the product data-sheet. As a general rule, moisture vapor transmission rates should not exceed 1.3 kg (3 lb)/24 hrs or 75 to 80 percent relative humidity (RH) for most conventional floor primers.

Issues pertaining to moisture during construction must be handled with urgency and professionally though curbing them before the construction project is complete is m ore advisable. Emphasis should be put on correct concrete mixing ratios and testing for moisture as construction is ongoing and signs of it handled with immediate effect.

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