Hydrostatic Pressure: What It Is and What It Isn’t

Hydrostatic pressure is a term that is often used when excess moisture has created problems with concrete slabs and connected flooring systems. However, it is not as generic a term as people in the industry often think.

What Is Hydrostatic Pressure?

At its simplest, hydrostatic pressure is the pressure created by standing or resting (“static”) water (“hydro”). We’ve all heard the story of the little Dutch boy who saved a town from the force of standing water when a dam sprang a leak. That same relentless pressure can impact your concrete walls and floors too.

While concrete is a relatively solid material when dry, it isn’t technically solid in the same way that stainless steel or marble is. It isn’t impervious to moisture.

As concrete dries, water vapor from the original concrete mixture exits the slab, creating small capillary networks. These pathways remain open until properly sealed, and can be the path of least resistance when water pressure builds up against a concrete contact point. While newer high-strength concretes can resist higher levels of pressure than older mixtures, they still can be susceptible if cracks form or hydrostatic pressure builds high enough.

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

What Causes Hydrostatic Pressure?

What Is Hydrostatic Pressure?
Normally this happens in basement floors or floors below water tables. The more water, the more pressure you’ll have at the bottom of the water table. Think of it like swimming; when you dive deeper, you feel more pressure on your ears. Same as hydrostatic pressure, the deeper your foundation and the wetter soil conditions, the higher the chance of moisture-related problems.

What Hydrostatic Pressure Isn’t

Hydrostatic pressure is a term often used rather generically to explain any moisture problem that occurs in a concrete slab, but more often than not, it isn’t the culprit. Several other sources of moisture could equally be responsible for water intrusion or moisture-related flooring issues.

Hydrostatic pressure, by nature, does not occur in slabs above grade. It does not even occur in every slab below the soil line. For a concrete slab to be impacted by hydrostatic pressure, it must be below the water table on the site or intrude into a natural water pathway. Water, underground as well as above, moves downhill under the pull of gravity, and so sites cut into a hillside stand a greater risk of having the “hillside side” be affected by hydrostatic pressure if adequate steps to redirect the water (and the subsequent pressure that might build up as it accumulates) are not taken. Poor drainage may cause water to collect against a concrete foundation but generally will not build up the volume to cause problems attributable to hydrostatic pressure.

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Only identifying the correct source of excess moisture will make proper remediation possible.

Other possibilities include:

  • Water Supply Sources
    Sprinklers, plumbing, city mains, and other water supply lines may be a source of moisture if they break or if a joint fails. If this occurs in a location with poor drainage or very dense soil, the water may end up in extended contact with the concrete and increase its internal moisture content, or relative humidity (RH). The pressure formed by a burst pipe is technically a type of hydraulic (or a mechanical force) pressure.
  • Inadequate Installation
    Educated WorkersObviously, we want to believe that every concrete and flooring professional is fully educated in his or her trade and uses the strictest standards to be sure each job is completed correctly. Unfortunately, the high level of flooring failure costs annually suggests that there is more to be learned. Some installation culprits that can result in excess concrete slab moisture may include a vapor retarder with insufficient or poor “perm” (permeability) rating, insufficient site evaluation or geotechnical survey to identify natural water sources, excessive troweling that prematurely seals the slab surface, or surface membranes applied before the slab was adequately dry.
  • Improperly Dried Concrete
    In cases where flooring failure is attributed to excess moisture, it is important to be sure that the slab was adequately dry before flooring was installed. Installers intent on meeting a construction deadline, or those who use surface-only test methods like calcium chloride testing, may not have had accurate moisture content data to base their adhesive or flooring material choices on. Only RH testing can adequately determine the true moisture conditions of a concrete slab and inform both schedule and flooring decisions.

Ultimately, if hydrostatic pressure is the culprit, the only way to correct it is to eliminate the pressure of standing water, a significant undertaking in any situation. However, accurate and comprehensive moisture testing and site evaluation can indicate the true source of concrete moisture intrusion to ensure proper and lasting remedies.

Last updated on June 8th, 2021