Imagine that you've just spent thousands of dollars laying down a new concrete floor for your office or plant. You've coated it with the latest decorative epoxy finish or some attractive tiles. But to your horror, just days after the application, you start seeing blisters appear, or the tiles start lifting. Your floor is ruined -- and let's not start to think about how much money has gone down the drain. Whether you like it or not, you've joined the thousands who have learned firsthand of the effect of osmotic blistering.

You may remember osmosis from your high school chemistry class. It's the phenomenon in physical chemistry by which solutions try to reach equilibrium across a semi-permeable membrane. Water will flow from a more dilute solution to a more concentrated one when separated by a semi-permeable membrane. Such membranes allow for the passage of water but not of a dissolved substance. When osmosis occurs in construction, the results are usually disastrous.

"We see a lot of cases of osmotic blistering in new construction because the bond interface of the newly coated concrete slab acts as a semi-permeable membrane, one of the key elements needed for osmosis to occur," says Marc Schroeder, president of Liquid Plastics Inc, a company that specializes in the formulation of protective coatings and membranes. "The osmosis process continues until either equilibrium is met or until the hydrostatic pressure generated by the increase in volume of the more concentrated solution equals that of the osmotic pressure. In the case of a coated concrete floor, the pressure builds up at the interface between the coating and the concrete and causes blistering and progressive disbondment. The pressure generated by the osmosis can be as high as 3000 psi, which is much greater than the bond of epoxies and other flooring systems to concrete substrates."

What Causes Osmotic Blistering?

In order for osmotic blistering to occur, three things are needed. First is the previously mentioned semi-permeable membrane, which, in most construction cases, is the bond interface or the extreme upper layer of the concrete. The second is a concentration of water-soluble material (organic or inorganic), which can be anything from the resin ingredients in the epoxy coating to the material that can form at the surface of concrete when it is acid etched or when workability or air entraining admixtures are added to the concrete. Adding to the problem is the fact that Portland cement has a naturally occurring soluble salt content that can also act as a catalyst for osmotic blistering.

"The third item that is needed to form osmotic blisters is the presence of water," says Schroeder. "Even when we think concrete is dry, it still contains up to 5% of free water by weight. This is enough moisture to start the osmotic blistering process. Adding to the problem are other outside moisture sources like groundwater, drains, water left over from cleaning processes, or condensation. Once you have a combination of water, semi-permeable membrane, and water soluble material, you have the perfect environment for osmotic blistering."

The worst part about osmotic blistering is that there really is no litmus test to determine whether or not it will occur. Schroeder says he has seen it occur in projects that were six months old and projects that were six days old. An anhydrous calcium chloride test kit can be used to determine moisture content in the concrete and its potential driving force. But, even when less than the usually acceptable upper limit of 3 pounds of moisture vapor emanating out of 1000 sq ft in 24 hours is experienced from testing, failures occur.

"When osmotic blistering does occur, it's not necessarily because of a defective product by the epoxy manufacturers," he says. "Coating manufacturers usually expect their products to work with a moisture content of up to the 3 pound limit, but the variable and indeterminable factors of solubility of entrained salts and driving forces (relative humidity and temperature differentials) can exacerbate the situation beyond anticipation."

Prevention Techniques

So what can companies do to prevent osmotic blistering? Steps can be taken to minimize the chances of it happening, though there is really no guarantee that it won't happen. Some suggestions from Schroeder include:

• If possible, minimize the amount of soluble salts in the concrete design mix.


• Let the concrete dry for at least 2 months and/or ensure a fully functional water vapor barrier is underneath the slab when placed.


• Avoid acid etching as surface preparation for a coating application.


• Don't wash the concrete with detergent before applying coating.


• Ensure that the epoxy coating resin constituents are accurately proportioned and mixed.

Obviously some of those options aren't viable when it comes to timely construction. How many contractors have the option of letting their slabs dry for at least two months? What if the specification calls for acid etched concrete? Worse yet, most of the extreme cases of osmotic blistering will occur even if these suggestions are followed.

For example, when the floor lies below grade, it is extremely susceptible to osmotic blistering because of higher moisture contents. Schroeder's company worked on such a project at the Hong Kong International Airport where the 600,000 sq ft baggage handling area lies 14 feet below sea level and is fully saturated with salt water.

"We were called in to consult on the Hong Kong problem five years ago when they were getting ready to open the new baggage handling facility," says Schroeder. "They were close to finishing the facility when they realized that a conventional epoxy coating system would not function over such a fully saturated slab."

Schroeder and his sister company developed a product called Cemprotec E-Floor, and the Hong Kong airport was one of the first sites for the product. Based on an epoxy, metakaolin, and Portland cement-modified polymer coating, the E-Floor creates a dense matrix that can resists hydrostatic pressures up to 142 psi, yet can still allow the passage of water vapor. This allows the damp substrates to breathe by dissipating the water vapor into the coating without the buildup of pressure and subsequent surface blistering. The water vapor migrating through the E-Floor to the epoxy topcoat is stopped by the epoxy film, but the vapor pressures generated are not sufficient to blister the epoxy topcoat. Since application in Hong Kong, the coating system has been performing perfectly, without blistering.

"The amazing thing about osmotic blistering is most people in the construction industry know very little about it," says Schroeder. "This spring I consulted on a 160,000 sq ft project in Haverhill, MA, that had put down an epoxy coating system with an osmotic mitigating saturating primer underneath. Unbelievably, the coating started to fail after only six days. The engineers were baffled. Calcium chloride tests indicated a range of 3-6 pounds of water per 1000 sq ft per 24 hours. The contractor had to strip the 60,000 sq ft of the floor already coated and failing to recover the project. Cemprotec E-floor was then installed first and then topcoated with the originally specified epoxy coating. Because of the success of this project, our relationship developed with the epoxy manufacturer to the point where they now recommend E-Floor where they find possible moisture problem areas for their coatings. We've done projects with them in places like Sea World where the high moisture content would make it nearly impossible for any typical coating to work."

There is no question in Schroeder's mind that osmotic blistering is one of the least understood problems in the construction industry. He says many times engineers can take steps to reduce chances of it happening by eliminating water sourcing into the slabs and minimizing the presence of soluble salts. But sometimes designers think that saturating the concrete surface with epoxy primer will close the pore structure, but that doesn't always work.

"For some projects, it's just not possible to prevent osmotic blistering," says Schroeder. "Places like the Hong Kong Airport, Sea World, and other areas where water is a constant menace will always present a challenge. The E-Floor has proven itself to be a solution in the fight against osmotic blistering, but whatever an engineer chooses to do, it's clear that they will always have to be on the lookout for the conditions that can cause osmotic blistering and take proactive steps to prevent it from happening, or risk the consequences."