PAINTSQUARE BLOG

Coating new concrete is a simple matter, correct? All you need to do is use a manufacturer’s alkaline-resistant primer and a good topcoat and, voilà, it’s done. And, I’ll be darned if it doesn’t look pretty good to boot.

Or how about if it’s a repaint project, and the substrate is cracked and crazed, and maybe there is even some rusted rebar showing through (i.e., spalling)? You caulk and patch those areas and recoat it the same manner as above. Or, you install two coats of elastomeric to “waterproof” it. Or, if it’s been previously coated with an elastomeric, you simply apply more. What could be easier?

Well, not so fast, Bucky. While most of the time, these installations go without a hitch, they sometimes don’t, because there are many things that can go wrong with something so “simple.”

Comprehending Concrete

When painting new concrete, often problems occur because folks new to the trade (and many old dogs as well) don’t really have a full understanding of the physical nature of concrete.

As defined by the American Concrete Institute (ACI), concrete is a composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregate, usually a combination of fine aggregate and coarse aggregate; in Portland-cement concrete, the binder is a mixture of Portland cement and water. In short, this means that concrete is, more or less, a man-made sponge.

Concrete was first used by the Romans to build the Coliseum, the Pantheon, aqueducts, baths, and a wide variety of other structures. It is not new to mankind.

But the biggest difference between today’s concrete and that used by the Romans is that modern concrete can be poured. Roman materials were drier, stiffer, and required hand-layups. Modern materials can be, and often are, modified with the addition of fibers, epoxy and other resins, as well as reinforcing steel.

Also, once formed into a shape (i.e., a wall, floor slab, or other building element), the modern concrete mixture is then typically handed over to a painter who has been asked to coat it with XYZ materials.

So What Now, Bucky? The Ball’s in Your Court

The most prevalent idiom about coating new concrete is that it has to be “cured” for 28 days before it can be painted. What is not generally known about this fact is that the 28-day-cure truly has little to do with whether or not concrete can be painted. Rather, this is when concrete reaches its maximum compressive strength. And while it certainly can be argued that waiting 28 days to paint is a good practice, it is not cast in stone.

The biggest difference between today’s concrete and that used during the Roman age is that today’s is pourable and Roman materials were drier and stiffer.

The fact is you can very successfully paint concrete and other cementitious surfaces, such as plaster and stucco, much sooner than that.

But what parameters are necessary to successfully paint concrete? First and foremost is that the moisture content and pH of the concrete must be at or below the range specified by the coating manufacturer. 

Testing Types and Procedures

Testing pH is performed in a number of ways, ranging from using strips similar to those used to test home pool/spa water, to pencils (used carefully as they contain wax), to digital gages.

One proprietary product I like to use for large projects is the Rainbow Indicator. I like this product for pH testing because it is simply sprayed on the surface. Once dry, the product will indicate the concrete’s pH by reacting with the concrete paste to turn a specific color, which can then be matched to a chart. The specifics can be viewed at www.germann.org/Brochures/Carbonation.pdf.  

Testing for moisture content on concrete floors can be as simple as taping a plastic sheet to the concrete per ASTM D4253. For both floors and walls, a moisture meter can be used. Generally speaking, concrete moisture content should be less than five percent prior to coating application.

Sometimes testing for moisture content can be as simple as taping a plastic sheet onto a concrete floor (per ASTM D4253).

There are a number of meters that can be used to determine the moisture content of concrete. However, these meters must be used in strict accordance with the manufacturer’s instructions as each scale used is often specific to that meter. They are not interchangeable.
 
No matter which method you use, you need to be sure that enough tests are performed so that your sampling is representative of the whole. In my view, this means that at least one test for every 1,000 square feet of surface area to be painted is done at various heights on a wall.

For example, if charged with coating 10,000 square feet of surface area, at least 10 tests are necessary. If the building is 24 feet high, half of these readings should be taken at randomly selected areas in the 12-foot- to 20-foot mark, with the balance done from the ground to the six-foot level.

These individual readings must then be averaged and compared to the known materials in the technical or product data sheet (TDS or PDS). For example, we find that an average pH of 12.5 exists. Turning to the primer’s TDS, it states that this material can be applied to a substrate with a pH up to 13.

Recalling that our 12.5 is an average and close to this limit, we again turn back to the list and see that only one area was over 13 at 13.5. Knowing this, we can wait, retest, and then coat that specific area later when its pH comes down naturally.

Some pH tests react with the concrete paste, turning colors which can then be matched with a corresponding test strip.

Or, if production is king, as it most often is, a dilute muriatic acid solution can be sprayed onto the substrate that should lower the pH to within the acceptable range. Of course, additional testing needs to be performed to verify the measurements.

What to Do about Low pH or High Moisture Content?
 
The next logical question is, “What happens if the moisture content is too high or the pH is low, which for concrete would be at 9 or less? And what about repaints? What happens then?”

Taking the first question and paraphrasing it a bit, what does it mean when the pH is 9 or less? It typically means that the concrete surface is “carbonated,” which is one way concrete corrodes.

However, the extent of this damage (or potential damage) depends entirely on how deep into the concrete this carbonation extends. If it’s an inch or more, then the reinforcing steel is in jeopardy because the protection afforded by the high pH (alkalinity) is gone, as the thin layer of oxides formed by the high pH that protected the steel are removed.

Once this occurs, the concrete’s salts and water will quickly go to work to corrode the steel and, in so doing, will cause it to swell and spall.

In this situation, my recommendation is to advise the owner of the facts and clearly state that you cannot provide any warranty on the work. At this point, the owner may be best served by bringing in a concrete or coating expert.

If the moisture content is too high, you can, of course, wait for it to decrease naturally. Alternatively, the concrete can be caused to dry using dehumidification or fans. The latter is often all that is needed as air exchanges are the most critical component needed for drying.

Other Issues

Other concrete-related issues include:

• When repainting previously coated concrete surfaces, examine the pre-existing dry film thicknesses (DFT) of sound coatings. This is best done by using a micrometer to remove some of the coating at various areas so that the coating’s DFTs can be measured.

• If 20 mils or more of elastomeric coating are present and it is intact (other than being faded and dirty), rather than recoating it with elastomeric, owners are best served by recoating with a 100% acrylic material. This allows the pre-existing elastomeric to remain permeable instead of being blocked, which can cause vapor build-up and surface bubbling.

• Laitance, which is a weakly bonded layer of concrete, fine particles and aggregate brought to the surface by an excess amount of water in the mix, must be removed prior to coating.

• Efflorescence, a white crystalline or powdery deposit that is essentially concrete salts that have leached out due to a leak or other water flow, must be removed prior to coating. The source must also be stopped or it will continue to occur.

When it comes to painting concrete, looks are deceiving and “simple” applications require skill and attention to detail. After all, concrete is a man-made sponge.

But when charged with coating a new concrete surface, taking a few simple precautions, such as determining the pH and moisture content, can pay off in success and consequent profits.

ABOUT THE THE BLOGGER Burt Olhiser Contractor-turned-consultant Burt Olhiser has seen it all—or a good portion of it—as a painting contractor, consultant, and teacher/instructor. He calls it “Like it Is!,” based on his extensive experience in various facets of the trade. But, he adds, he’d certainly like to hear other views and accounts of “How it Is.” SEE ALL CONTENT FROM THIS CONTRIBUTOR