What Every Flooring Installer Needs to Know about Hydrostatic Pressure

Previously published by The Flooring Contractor magazine

Whether it is excessive concrete moisture or hydrostatic pressure, either can ruin a hardwood floor. But which one is more damaging to floors?

According to the World Floor Covering Association (WFCA) and the Floor Covering Installation Contractors Association (FCICA), concrete moisture is a major cause of flooring failures.

As a result, it creates significantly greater economic damage than hydrostatic pressure. In fact, many industry experts believe excessive concrete moisture cost North American commercial property owners more than $1 billion annually. This doesn’t even count residential problems or the cost of downtime while a damaged floor is re-installed.

But What about Hydrostatic Pressure?

Certainly, hydrostatic pressure is a powerful natural force that can move rocks, buckle walls, and cause havoc with concrete, but it does not explain every instance of water intrusion in concrete.

Yet, when discussing flooring failures, many contractors and others in the industry often attribute these failures to hydrostatic pressure. They use the term loosely to explain any moisture problem that occurs in a concrete slab.

So as to not confuse concrete moisture with hydrostatic pressure, let’s understand what hydrostatic pressure is. Simply put, it’s the constant pressure exerted by standing (“static”) water (“hydro”).

It can be caused by floods, excess rainfall, foundations or basement walls built fully or partially below the water table, or water running down a hillside from above. Whenever there’s excess water, it causes the soils around a home to be saturated. With nowhere for the water to go, it causes tremendous pressure against a concrete slab or foundation wall.

hydrostatic pressure

Hydrostatic pressure is exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity. Water will travel any route it can to maintain equilibrium.

To illustrate, imagine a basement installed beneath the ground’s water level. With water weighing slightly more than 60 lbs. per cubic foot, the pressure from the water can easily build against the walls of the basement. If the soil around the basement is saturated with water, there could be tens of thousands of pounds of hydrostatic pressure against the foundation.

Over time, as the water pressure increases, so does the potential for leaks into the basement. In rare instances, hydrostatic pressure can even break through a concrete wall.

Basement foundations are especially vulnerable to hydrostatic pressure that sits beneath the soil in moist soil conditions. In fact, the deeper the foundation and the wetter the soil conditions, the greater the risk for structural damage.

Note: Hydrostatic pressure cannot develop naturally in a slab on grade, but it can exist if a slab is fully or partially below grade level.

However – and this is also important to note – hydrostatic pressure does not always occur in every slab below the soil line. Hydrostatic pressure impacts a below-grade concrete slab when it is below the water table or when the slab intrudes into a natural water pathway.

At times, poor drainage may cause water to collect against a concrete foundation. But in most cases, the water will not build up the volume to cause problems from hydrostatic pressure.

Concrete + Moisture = Flooring Failures

Concrete is a hygroscopic material which absorbs and loses moisture. In other words, regardless of its composition, it acts like a sponge, absorbing moisture from the air or exposure to water. And if the humidity on one side of the slab is different from that on the other, it will transmit moisture through the slab.

Moisture vapor emission occurs when water migrates from an area of high vapor pressure – such as damp concrete or wet soil – to an area of low vapor pressure – like a dry building interior. Since concrete is highly alkaline in nature, as moisture naturally moves upward through the concrete, it carries the alkalinity with it, eventually collecting at the bond line between the concrete and flooring. This causes a breakdown of the adhesives, and consequently, the flooring.

Every concrete slab has moisture and will always have moisture. What you want is a slab that has an acceptable level of moisture to accept the flooring you want to apply.

Concrete moisture can come from both internal and external sources. Water mixed with cement is an internal source. Rainwater, a broken water pipe, poor drainage, flooding, and even humidity in the air are external sources.

Prevent Flooring Failures Quickly, Easily

Unless you properly measure the moisture level in a slab before the flooring installation, you can’t possibly know whether the slab is ready. Two test methods are commonly used in the U.S. to determine a slab’s readiness.

Since the 1940s, the flooring industry has used the calcium chloride (CaCl) test to determine the moisture conditions of a concrete slab. But the CaCl test only measures moisture at the surface of the slab. Since we know that moisture levels within a slab vary by depth, any moisture test that only measures moisture at the surface cannot give you consistently reliable, usable results. In fact, many CaCl test results have been proven to be either false or misleading.

Because the CaCl test is often unreliable, the ASTM F2170 in-situ Relative Humidity (RH) test has replaced it as the preferred industry test – a preference highly warranted.

For instance, the F2170 RH test uses probes that take readings deep within the concrete, so they’re not susceptible to ambient temperature and humidity conditions above the slab. In fact, scientific evidence consistently proves the F2170 in-situ concrete RH test is the most accurate and reliable method for determining when a slab is dry enough for flooring installation.

Rapid RH L6 concrete moisture testing system

Secure digital transmission of data is revolutionizing concrete RH testing procedures by saving more time and adding to greater data integrity.

It’s why more professionals in the flooring industry today are switching to the in-situ RH test. And why many flooring manufacturers use the RH test as a condition of their warranty.

In order to meet the ASTM F2170 standard, many flooring professionals use Wagner Meters’ Rapid RH® L6 system for concrete RH testing. The Rapid RH test system affords contractors and flooring professionals a scientifically-proven, cost-effective, and accurate means of strictly adhering to ASTM F2170.

This system comes with a NIST-traceable factory calibration certificate and NIST-certified Smart Sensor probes, providing reliable accuracy and completely eliminating the need for periodic calibrations typically required by other sensing technologies.

Installers place these single-use, factory-calibrated probes inside the slab at 40% depth. Then, with a simple hand-held device called the Total Reader®, RH measurements can be regularly taken at probe level to give instant feedback to the installer of how close (or far) the slab is to being properly dried.

If this reading is taken at close-to-mid slab depth, it will be representative of the level of moisture that the flooring will be exposed to after the slab is covered and a new equilibrium reached. Acceptable levels of RH within the slab range from 75-90% depending on the flooring installed, but it is important that you confirm the correct RH conditions from the manufacturer of the flooring and/or adhesive you intend to install.

With the Rapid RH in-situ concrete moisture testing system, installers can quickly and accurately track the RH in the slab’s multiple test locations. After an initial ASTM-required 24-hour equilibration period, the Rapid RH L6 in-situ test takes only minutes to give you a reading as to whether or not the concrete is ready for the flooring application or needs more drying time.

Changes to ASTM F2170 now provide even faster results. Official, documentable results from an RH moisture test once took 72 hours, but after ASTM International updated the Standard in early 2018, installers are now allowed to document and proceed with RH test results within 24 hours. Given that other test methods typically require up to 72 hours, the in-situ RH test is both faster and more reliable – allowing installers to accelerate their construction and remodeling projects.

Hydrostatic Pressure – The Major Cause of Flooring Failures?

The major cause of both commercial and residential flooring failures is concrete moisture – not hydrostatic pressure.

Give the realities of moisture in concrete and the necessity of measuring that moisture accurately, only one test method excels above all others: ASTM F2170 in-situ concrete RH testing.

In-situ RH testing alone gives flooring professionals the best information for managing their flooring project successfully. Validated by scientific research and field experience, it ensures quality flooring, installed on time and on budget.

For more information about moisture testing with the Rapid RH L6 in-situ concrete moisture testing system, call (541) 291-5123.

Last updated on July 28th, 2020

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