How Does Moisture Move Through Concrete?

Previously published by Concrete Decor Magazine.
moisture movement through concreteUnderstanding how moisture moves through concrete is critical to successful flooring. It helps you reduce the risk for future moisture-related flooring failure during installation. It also helps you identify and assess the cause of moisture-related issues that have already occurred.

The Dynamics of Moisture Movement Through Concrete

Despite its outward appearance, concrete isn’t solid. It’s filled with air pockets and veins. When poured, the concrete slab contains the batch water from the mix.

As it sets, water begins to evaporate, and the air pockets and veins begin to form within the slab. These veins are called “capillaries.” As concrete hardens, batch water and vapor get pulled into these air pockets. This process is called “capillary action.”

Moisture also enters the concrete slab through external sources. Most slabs are poured over a vapor retarder because moisture from the ground below will seep into concrete. Indeed, sub-slab vapor is a primary cause of moisture-related distress in concrete floors that don’t have a moisture vapor barrier.

Concrete will also pull moisture from the air and release moisture into it. It depends on whether the air is holding more or less moisture than the slab. This is why the relative humidity (RH) in the air and within the slab is so critical.

The RH indicates how much moisture is already present relative to what can be absorbed. If the air has an RH of 30%, it’s holding 30% of the moisture it can. If the RH of the slab is higher, moisture will tend to move from the concrete into the air. The reverse is true if the RH in the slab is lower than the RH in the air.

Yet, if the air has reached its dew point, vapor can tend to concentrate on the surface of the slab. The dew point is the air temperature at which it can hold no more moisture. The dew point is determined by temperatures of the air and slab, and the RH in the air.

If the temperature of the slab is below the dew point, condensation will appear on the slab’s surface. Knowing the dew point of the environment around the concrete is vital to assessing drying times and moisture condition of the slab.

The Many Ways in Which Excess Moisture Movement and Moisture Content in Concrete Can Manifest Itself

excess moisture in concrete slabExcess moisture manifests itself in various ways that result in unsightly or possibly hazardous floor conditions. These are some of the ways moisture-related distress shows up on concrete flooring:

  • Osmotic blisters: Moisture pulled from the slab through an adhesive or synthetic resin gets trapped below a non-permeable floor covering. The resulting swelling is “osmotic blisters.”
  • Microbial growth occurs when there’s excessive moisture in the concrete and high levels of humidity in the air.
  • Microbial growth can corrode and weaken concrete. Signs of microbiologically induced deterioration (MID) include cracking and flaking on the surface.
  • When moisture rises to the surface, it brings salts within the concrete with it. The moisture will eventually evaporate away, but the salts remain. They can show up as efflorescence, those white streaks and stains that appear on concrete.
  • The salts also cause adhesive degradation, which can result in floor coverings separating from the slab. Flooring separation can also occur due to coating debonding (also called delamination). Rising moisture trapped underneath impermeable flooring will bring very basic chemicals with it to breakdown the bonding agent or laminate agent.

Testing a Concrete Slab’s Moisture Before Flooring Goes On

Rapid RH L6 and Datamaster concrete moisture testEnsuring the concrete slab has released enough excess moisture and has reached levels stipulated by the flooring or adhesive manufacturer before installation is your best opportunity to prevent moisture-related failure.

Measuring the RH of the air is straightforward. Measuring it in the concrete slab requires using RH sensors embedded into the slab. ASTM F2170 (Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes) sets out the requirements for conducting an RH test properly.

Based on decades of scientific research, the RH in situ probe is the most reliable and accurate method of measuring the RH within the slab. It’s the only concrete moisture test that measures moisture below the surface.

Knowing how moisture moves through concrete, it’s clear why tests that measure the surface are unreliable.

Since the ASTM F2170 standard was first published in 2002, the science on RH testing has evolved, so has F2170. Most recently, the standard reduced the waiting period needed between inserting the RH probe and taking a reading from 3 days down to 24 hours.

Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry

The RH in situ test kits have also evolved as technology improves. For example, Wagner Meters’ most recent RH test kit, the Rapid RH® L6 Smart Sensors, use digital technology to simplify reporting and improve data integrity.

The L6 Total Reader® a pen-sized electronic device, captures readings from each RH probe and sends them, via Bluetooth® to the DataMaster™ L6 app that logs the data for analysis and produces emailable F2170 reports. There’s no space for human recording errors.

Another distinguishing feature about the L6 Smart Sensor is the greater responsiveness, especially at RH ranges between 90% and 100%. This improvement in the L6 Smart Sensor means that the user can install the sensor very early during the concrete slab drying process and collect periodic readings for a more accurate estimation of drying time. More accurate estimations allow for tighter scheduling and less downtime.

Regardless of which kit you use, RH in situ testing is always the most accurate method for determining a concrete slab’s moisture condition. You can learn more about moisture in concrete and the hardening process at the Wagner Meters library of resources. While you’re exploring the resource library, sign up for a free webinar that takes a deep dive into the science of moisture and concrete.

Last updated on April 6th, 2023

1 Comment

  1. Rick Stooksberry says:

    We have a sewer line leak below a commercial lease space. Multiple In-situ test RH at 96-99. Soil samples from under slab show presence of Enterococci – bacteria commonly found in human waste. Building owner doesn’t think it is a big deal as it is under slab. My question is can this bacteria migrate through the 4” concrete slab on grade and affect health of space occupants

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.