What is The Cause of Hydrostatic Pressure?
  • HPS Flooring

What is The Cause of Hydrostatic Pressure?

Updated: Jul 2

Hydrostatic pressure (also referred to as capillarity or vapor pressure) is an resinous flooring installer's worst nightmare. It can cause blisters, bubbles, and many other undesirable effects. So what is hydrostatic pressure, how can you prevent it, and how can you fix it?

What is Hydrostatic Pressure?

Hydrostatic pressure is one of the most misunderstood terms in resinous flooring and is actually a very rare problem, but the term is often used, inadvertently, to describe all water related problems. Hydrostatic pressure (also referred to as capillarity or vapor pressure) is caused by a moisture source being present underneath the concrete slab and rising through its pores.

As moisture vapor rises, it dissolves salts in the concrete and becomes alkaline, thereby damaging the floor coating. The moisture migration is typically not a problem when an effective moisture barrier is used and only becomes a major problem when either the barrier is punctures or the moisture gets trapped beneath an impermeable top coating.

Many hydrostatic problems can be attributed to less experienced resinous flooring installers. These mistakes can result in excess concrete slab moisture caused by a vapor retarder with poor permeability, insufficient site evaluation or geo-technical survey to identify natural water sources, excessive troweling that prematurely seals the slab surface, or surface membranes applied before the slab was adequately dry.

This one major reason it is important to have an experienced, knowledgeable resinous flooring expert.

Other causes of high moisture could be from poor water drainage, from sprinkler use, broken pipes, production runoff, or increased rainfall. This excess water can collect against a concrete foundation but generally it will not build up the volume to cause problems attributable to hydrostatic pressure.

The general rule of thumb is for every foot below grade you have about 7 pounds per linear feet of pressure. Armed with this rough calculation you should be able to consult with the manufacturer’s data sheet to see if the flooring system will withstand the hydrostatic pressure.

On the other hand, if the high pressure is caused by a burst pipe then you are dealing with a totally different issue, hydraulic pressure, that requires its own unique fixes.

How Do You Identify Hydrostatic Pressure?

The best way to prevent hydrostatic pressure related issues is to first diagnose if the problem exists in the first place. The easiest way to test for signs of hydrostatic pressure is to grind a spot, wait a day, and see if the concrete gets darker.

You can also do nearly the same test with a plastic sheet, held down with duct tape, and see if water droplets accumulate on top. You can also accurately measure moisture content using concrete moisture meter.  

If your moisture content is above 5% then you have a high danger of hydrostatic issues occurring. A very common test is a calcium chloride (CaCl) test, which is a surface-based assessment of concrete moisture, that measures moisture vapor emission rate (MVER) from 1,000 square feet over a 24 hour period.

The issue with a CACl test is it is sensitive to ambient conditions and does not necessarily provide an indication of the moisture condition deeper within the slab. Although ASTM F1869 recognizes CaCl testing, ASTM International has disallowed this test for lightweight concrete.

A significantly better assessment method is a  relative humidity (RH) test because it measures the amount of moisture deep in the concrete slab using a probe.

The probes’ measurements provide a fast quantitative assessment of the overall moisture condition of the slab. In 2002, ASTM International acknowledged the integrity of RH testing by developing the ASTM F2170 standard.

The optimal hydro-static pressure test is to use a combination of these testing methods to make a full assessment.

How Do You Fix Hydrostatic Pressure and The Problems It Causes?

If you have hydro-static pressure than water migrates through concrete pores, cracks, or other imperfections. The force of the water can cause bubbles to form in the coating and when popped they will emit pressurized water/chemical mixtures.  

When the hydro-static pressure of water in the concrete exceeds the ability of the coating to adhere to the substrate, the bubbles spread laterally then combine, and eventually the coating will delaminate.

Different coating fare better or worse under hydro-static pressure. For instance, epoxy coatings are more permeable than urethanes which relieves stress.

Enamels perform even better but have the negative side effect of delaminating due to reactions with cement alkali (saponification). Overall, urethane cements prevent moisture transmission more effectively than most other flooring systems on the market.

While some coatings deal with hydro-static pressure better than others it is important to note that no coating adheres to concrete so well that it is able to resist it forever.

So what should you do if you do detect hydro-static pressure? Well, not all hope is lost. The only control for hydro-static pressure is to stop the flow of water through the concrete.

This can be done with a waterproof barrier between the concrete and the source of water called a positive-side water barrier. Also, water-based epoxy systems can be used as moisture barriers to deal with hydro-static pressure.

There are also breathable water-based epoxy systems that allow water vapor to escape the concrete substrate, preventing the flooring from bubbling and cracking. Click here to learn more about water-based epoxies and different epoxy systems.