How to Test Sealers for Adhesion

Question:
How do I know if the sealer on my stamped concrete patio is still working, or if it’s beginning to lose adhesion and needs to be stripped and replaced?

Answer:
There are two common ASTM field tests for determining sealer adhesion in the field, known as “tape tests.” The specific application and performance criteria for these tests are described in ASTM D3359, “Standard Test Method for Testing Adhesion by Tape Test,” available at ASTM International.

The first of the two tests is the X-cut test and is primarily intended for use with coatings on metal substrates. The test better suited for the thin-build sealers and coatings often used on decorative concrete is the cross-hatch test. It provides the best results on coatings less than 5 mils (125 microns) thick. The test consists of a cross hatch, or “#” pattern, cut into the sealer or coating with a sharp razor blade or scalpel. A cutting guide or special cross-hatch cutter should be used if testing per ASTM specifications. For a simple field test to determine adhesion, hand cutting will suffice. After making the cross-hatch incision, apply pressure-sensitive tape or clear plastic packing tape over it and then rub over the tape until it is well adhered to the sealer or coating. Next, remove the tape and assess the results per ATSM D3359 guidelines. If the sealer is well adhered, the cross-hatch pattern should be clean with very little or no sealer removed by the tape. If the sealer is exhibiting adhesion failure, the cross-hatch cuts will have rough edges and sealer may be present on the tape. If the coating can be easily removed at the cut marks, this is another indication that the sealer is compromised and may no longer be well adhered to the substrate.

Cross-hatch test for sealer adhesion being performed on stamped concrete.

Beware of Applying Acrylic Sealer Too Thickly

Question:
Here’s a photo of a self-leveling overlay that I stained and sealed with a 30%-solids acrylic sealer. The slab is in a screened porch under a deck. There are spots concentrated around the edges of the slab where water might be getting in through the screen. I thought the spots were left from the water, but I tried scrubbing them and they won't come off. They do disappear when they are wet.

Have you ever seen anything like this? Normally when my sealer has moisture issues, it appears cloudy. Some of these spots almost appear as if the sealer has cracks in it. It’s weird!

Answer:
The acrylic sealer is actually fracturing. Typically this occurs when the sealer is applied too heavily, as it appears to be in this case. Acrylic sealers are designed to go down very thin. Even with two coats, the thickness should be only about 1 to 2 mils. To give you some reference, a credit card is about 120 mils thick.

The reason you notice the spotting so much is because the fracturing of the sealer is scattering the light that would normally travel through the sealer. When light can travel through a sealer without interference, you’ll see a nice, clean reflection. Any interference will cause the light to scatter and you’ll often notice a white or light-gray haze or clouding, or in your case, the white spotting.

I am not a fan of acrylic sealers with a solids content above 24%. At higher solids, you need to be very careful to apply the sealer thinly. Acrylics are tough stuff, but they are prone to cracking once they exceed 1 to 2 mils in thickness. The substrate they are applied to also plays a part. Self-leveling overlays are pretty dense and usually have a polymer-rich paste on the surface. Unless this paste is sanded off prior to staining or sealing, you run the risk of limited sealer penetration and compromised adhesion. Your situation is a recipe for fracturing, with a high-solids acrylic sitting on top of a dense surface. Opening the surface by lightly sanding is usually enough to allow penetration.

If you like the high-gloss look of an acrylic but want to avoid this problem in the future, consider using a two-part polyurethane, which is designed to go down at 3 to 5 mils. Or spread out the acrylic more thinly to achieve a 1- to 2-mil thickness and then apply multiple coats of a wax-modified finish coat or sacrificial topcoat designed for concrete floors.


Acrylic sealers applied too heavily may fracture.

Repairing Shrinkage Cracks in Stamped Concrete

Question:
How do I deal with small cracks and fractures that occur along the edges and corners of the stone imprints in some of my stamped concrete work? They are random and do not occur on all my projects. They seem to happen with deeper, more aggressive stone patterns. How do I fix these cracks, and how do I keep them from happening in the future?

Answer:
These are shrinkage cracks caused by pushing imprinting tools into the concrete after the surface has become too hard. This surface hardening, known as “crusting,” can be caused by exposure to sun and wind, overfinishing, the concrete mix design, and even the concrete color (darker colors more readily absorb heat from the sun).

A good analogy is what happens when you cook pudding. When the pudding comes off the stove, it’s a viscous liquid, similar to concrete when it comes out of the truck. The pudding then goes into the fridge to cool. If it cools too quickly, the rapid temperature drop causes a skin to form on the surface. The pudding then thickens and hardens from the top down. The same thing occurs when concrete dries too fast from the top down. Remedies for reducing surface crusting include using admixtures to slow the concrete set time, using surface evaporation retarders, avoiding darker colors, and scheduling the concrete pour for the coolest time of the day. Using stamping tools with deep grout lines, such as random stone and large slate patterns, can aggravate the cracking problem.

One way to repair these small cracks is to use a colored cement paste, or color patch. Some color hardener manufacturers offer these patches in their standard color palette for use in filling and repairing these types of cracks and small popouts. Some contractors will sift the sand out of the color hardener they are using on the project and use this to make a color patching material. I recommend wetting color patch materials with a 50:50 blend of concrete bonding polymer and water. Use a wooden tongue depressor or gloved finger to smear the color patch into the cracks. Soften the edges with a damp rag or sponge to blend the patch and color into the slab. Let these repairs cure for 24 hours.

Advice on Using Chemical Strippers

Question:
What is the best way to chemically strip sealers from concrete.

Answer:
The issue of stripping sealers is never at the forefront of what I write about, but in some cases it is required in order to repair or complete a specific decorative project.

The first thing that needs to be clarified is what a chemical stripper is and how these strippers work. Too often I get asked which solvent I recommend for stripping a sealer. First and foremost, solvents are not strippers! A solvent may loosen a sealer or coating, but left alone after spraying, the solvent will have little to no long-term effect on the sealers commonly used in the decorative industry. Using a solvent to eliminate trapped moisture or rewet the surface is a whole different process, and is covered in other blog entries (read Blisters, Bubbles on Sealer Surface and Preventing Moisture Problems). A chemical stripper is a chemical compound that actually destroys the coating, usually turning it into sludge. Another common misconception is that an acid will remove a sealer. Most common acids (such as hydrochloric and phosphoric) will have no effect on cured acrylics, polyurethanes or epoxies.

Strippers come in many forms, but they all have one thing in common. They need time to work and have to stay wet to stay active. Depending on the type and thickness of the sealer being removed, multiple stripper applications and hours of dwell time may be needed. Once strippers dry out they stop working. This is why most strippers are in gel form, because they hold better to most surfaces and they hold in moisture, allowing them to stay active longer. Today, you have a choice of environmentally friendly strippers, such as natural soy and citrus-based products, or the good old-fashioned methylene-chloride-based strippers that will take the skin off your fingers given the chance. While the environmentally friendly strippers are safer to use, they require more time to work. No matter what type of stripper you use, read and follow the manufacturer’s instructions. Be sure to protect anything you don’t want to strip, and be prepared for a pretty miserable job.

Here are a couple of tricks I have learned over the years that can make stripping easier:

• Use a walk-behind scrubber or rotary floor swing machine with a soft- to medium-bristle brush to help remove all the stripper and sealer sludge. Protect the machine with plastic, and use brushes that are resistant to the chemical strippers.


• Another method that I heard about from a contractor, tried myself, and found to be quite clever is to create a chemical stripper “cooker” using wet cotton sheets. First, soak the sheets in water and ring them out until they are damp. Apply a uniform layer of chemical stripper over the concrete surface and then cover the surface with the damp sheets. Make sure to get as much contact as you can between the sheet and the surface, pushing the sheet down into the grout lines and deep textures. Cover the damp sheets with a sheet of plastic, and seal the ends as best as possible to keep the moisture in. Let this sit for hours, and if all goes well, the active stripper will liquefy the sealer, and via osmosis, the sealer sludge will soak into the sheet, greatly reducing the cleanup required.

Excess Antiquing Color Causes Problems

Question:
I have a pool deck in which the sealer seems to be flaking off and coming up in some areas. This is not an issue that we have dealt with before, so I am a little concerned. Can you let us know what is occurring and how we can remedy this issue?

Answer:
This is actually one of the most common issues we face with stamped concrete. It is in fact not a sealer issue, but rather the antiquing color that is causing the sealer to fail. The sealer failure can occur within weeks of application, but more often shows up 6 to 12 months down the road.

Imprinted concrete looks pretty bland and unrealistic without highlights or antiquing, which give the pattern definition and color variation. These highlights make the concrete look like stone, tile or whatever natural material the installer is trying to mimic. The highlights can be accomplished in a multitude of ways, with release powder being the most common. Other popular methods include stains, tints, dyes and colored sealers. Virtually any means of getting some contrasting color to stick in the depressions and textured areas of the surface will work. The problem occurs when too much secondary color is present. The depth and type of texture on the imprinting tool will determine the amount of secondary color to use. More aggressive textures with deep grout lines, lots of deep veins, and rough slate or stone surfaces will accommodate more secondary or antiquing color. The opposite holds true for light textures with smoother surfaces and non-aggressive patterns. A good rule to work by is that secondary color should make up 5% to 30% of the final color. In your case, however, the secondary color makes up almost 100% of the surface color.

The real curve ball is that stamped work with such high ratios of secondary color can look amazing and beautiful. Once sealed, the work looks great, the applicator gets paid and everyone is happy. The problem is that you have ticking time bomb, and it is just a matter of time before it explodes. Have you ever considered what secondary color is made of and how it works? No matter whether you use release powder, stains, washes or tints, you are filling the surface pores of the concrete with solid material. Those solids are filling the voids that the sealer needs to fill in order to “bite” or adhere to the concrete. The more secondary color present, the bigger the problem. The sealer will encapsulate the solid color in an attempt to do its job, but if there are no pores to fill, adhesion is compromised, which leads to failure when external forces exceed the ability for the sealer to hold. This is usually why we see these types of failures in the spring, after a winter assault of freeze-thaw cycles, deicing salts and snow shovels. As a result, the sealer lifts up in small circular areas and takes the secondary color with it, since the color is all it had to hold onto. You are then left with a stamped slab that has round, discolored spots. The “discoloration” is actually the base color that should have been visible in the first place, but was covered with too much secondary color.

The repair is fairly simple in theory, but more difficult in practice. The sealer has to be chemically stripped, but the process will usually remove most of the secondary color as well. Once the stripping is complete, you can remove any residual secondary color, give the surface a good cleaning, and then allow it to dry before resealing. The hard part is selling the client on the new “correct” color combination of their patio. What was mostly dark brown with hints of tan is now mostly tan with hints of dark brown.

A good rule to work by is that secondary color should make up 5% to 30%
of the final color. In this case, the secondary color makes up almost 100%
of the surface color, causing sealer failure.

Is it OK to Restain Acid-Etched Concrete?

Question:
I applied a concrete stain to a large 8-year-old concrete patio in Reno, Nevada—an area subject to somewhat extreme temperature variations. I initially washed the patio with a mix of muriatic acid and water to clean it. I hosed it off, waited a day or so, and applied the stain with a sprayer, following the manufacturer’s directions. Two years later, the patio now has areas of peeling. I would like to restain the whole patio again to make the color more uniform. I have heard that if you have washed a patio with muriatic acid, you cannot acid stain it. I am trying to find an expert who can tell me how to properly prep the surface, apply the stain and protect it. The stain I initially used was an acrylic.

Answer:
The product you used—a tinted acrylic stain—is not an acid-based concrete stain. It is a translucent concrete paint and is topical, which means it forms a colored coating on the surface of the concrete that will wear over time if not maintained. The surface preparation for these types of stains usually requires acid etching to open the surface to allow the acrylic to adhere to the concrete. But once concrete has been acid etched, the ability of a true acid stain to take is diminished. You will need to do a test to see how the acid stain takes to get a true indication of color and effect.

As far as your particular situation, you should strip off the remaining acrylic stain and clean the surface with soap and water and a clean water rinse. After the area dries, apply the stain, following the directions provided by the stain manufacturer. If you’re concerned an acid stain won’t take, you may want to reapply the same acrylic stain after cleaning the surface. No matter the stain you use, protecting it with a sealer and proper sealer maintenance will be key to keeping the stain looking good. In your area, plan on resealing every 12 to 16 months.

Humidity Can Cause Moisture Problems

Question:
Chris, I am doing a stained concrete project using an acetone-based stain. I plan on sealing the stained concrete with a water-based urethane. This is a large 8,000-square-foot floor in the cafeteria of a local manufacturer, and it receives heavy traffic. I just completed a moisture test (using a calcium-chloride test). I did three tests, with results that ranged from 6.13 to 6.58 pounds. But I understand the recommendation is no more than 4 pounds. This floor is on grade, and the concrete is over two months old with a vapor barrier under the slab. The interior humidity has been high (around 70%) due, I think, to rainy weather and wet drywall mud.

Here is my question: Because of the high moisture readings, should I do this job? Is 4 pounds a strict guide, or are there some tolerances? For example, is 6.58 pounds acceptable but something higher, like 12 pounds, too high? This is the first time I have done a test like this.

Answer:
According to the ASTM standard for conducting a calcium-chloride test (ASTM F 1869), a reading above 4 pounds—the amount of moisture flow in pounds per 1,000 square feet over a 24-hour period—is considered a no-go for sealing. This is especially true for nonbreathable coatings, like the polyurethane you plan to use. That being said, you have a slab that should eventually drop below 4 pounds because of the vapor barrier.

You hit the nail on the head in regard to humidity. This is a very real environmental factor often missed when contractors prepare to seal, especially interior floors. Concrete is a sponge, and if conditions are right it will absorb and hold moisture, both in liquid and gas forms. The process of osmosis now comes into play—the movement of a liquid or gas from an area of high concentration to low concentration until balance is achieved. If the air above your floor has a high concentration of water vapor, or humidity, then the concrete will absorb moisture until it contains water equal to what is in the air. The practice of closing doors and windows to “dry-in” a room or building can actually trap moisture and create a steam room of sorts, making the problem worse or creating a problem where one didn’t exist. Drywall mud, paint and rainy or hot, humid weather can cause high moisture contents indoors, which will cause concrete to absorb moisture. This moisture will stay in the concrete until the moisture content of the air above the slab abates. Whether the humidity is manmade or natural, it can cause short-term high moisture readings, especially in the spring and fall, when days are warm and nights are cool.

Wait for a dry spell, open as many windows and doors as possible, use a fan to move air across the floor, and take another test after 24 to 36 hours of drying. If the HVAC system in the building is operational, run the air conditioner to help remove moisture. When the moisture readings drop low enough for you to seal the concrete (at or below 4 pounds), apply the sealer in the afternoon or evening, when humidity is lowest.