Changing the Color of a Stamped Walkway

What once was gray is now red with the aid of a solid-color acrylic stain.

Question:
How can I change the color of a stamped slab after it has been poured? The customer was not happy with gray and now wants a reddish-brown color.

Answer:
You can change the color of stamped work once it has been placed by applying different types of stains, tints, or dyes. The type of coloring method you use will depend on the look desired and amount of color changed needed. Here’s a handy guide:

• -For minor color adjustment – Use an impregnating stain or diluted acid stain.
• -For medium color adjustment – Use a full-strength acid stain, dye, or tinted sealer.
• -For complete color change – Use an acrylic or solid-color stain.

When using any of these methods to change or adjust concrete color, be sure to profile the surface according to the product manufacturer’s recommendations to ensure proper penetration and adhesion. In addition, the surface should be completely dry and the temperature above 50 F and below 90 F.

Finally, no matter what method you use, always prepare a small sample for pre-approval by the client.

Release Powder Causes Streaking

Applying release powder with a heavy hand or on a slab that is too
wet can result in streaking.

Question:
Why did I get black streaks on the surface of this stamped concrete slab after removing the residual release powder, and how do I get rid of them?

Answer:
As the photo shows, the streaks of black release powder appear to be oriented in the direction in which the powder was thrown. This often occurs when too much material is applied at one time or on a slab that is too wet. Hard, clumpy release powder can also contribute to streaking. Always roll or “fluff” the powder in its container before application to break up any clumps. Also take care to cast the powder evenly across the slab rather than downward.

Another consideration is proper dispersion of the pigment in the release powder. If the material is not mixed properly at the manufacturing plant, streaking like this can occur. Always retain the batch number or a sample of the material so the manufacturer can run a test to verify if this was the cause of the problem.

Removing these types of blemishes can be difficult. You can try lifting the streaks of powder by scrubbing with a dilute acid solution. Another option is to disguise them by applying a stain or tint.

Sealing Concrete in Cold Weather

A white, milky haze can form when sealer is applied below its minimum
film-forming temperature – usually around 50 degrees F. The best
remedy is stripping and resealing.

Question:
Are there general guidelines I need to be aware of when sealing exterior concrete in colder temperatures?

Answer:
As a general rule, 50 degrees F is the minimum ambient air and surface temperature needed when applying most concrete sealers and coatings. All sealers and coatings have a minimum film-forming temperature, or MFT. When the temperature falls below the MFT, cross-linking does not occur and the sealer film does not come together properly. Typical signs that the sealer MFT was not met include white hazing, white dusting, and delaminating.

It’s easy to judge air temperature by looking at an accurate outdoor thermometer or getting a current local weather report. Obtaining surface temperature is a bit more challenging. I have found that using an infrared temperature gun is the best way to get accurate concrete surface temperature readings.

Another factor to consider when sealing concrete in temperatures at or close to 50 degrees F is the type of solvent the sealer is based on. Since temperature is a catalyst, the colder the temperature, the slower the sealer cures. When the weather gets colder, switching to a sealer that uses a faster solvent might help avoid problems. Water is the slowest of the solvents, so I would avoid use of a water-based sealer when outdoor temperatures drop. Acetone is probably the fastest solvent, so consider using a sealer with a high acetone content when sealing in cold temperatures. As an added note, spraying vs. rolling is always recommended when applying sealer in colder temperature because it leaves even, thin coats for the best possible cure.

I often hear from applicators who say they have been applying sealers successfully at air temperatures of 35 to 45 degrees F for years. Every project has different environmental factors to consider, thus the 50 degree rule is a suggested safe guideline. Those who choose to seal concrete at temperatures below 50 degrees increase their risk for failure substantially. Is it better to be safe than sorry? From the number of emails and calls I receive to help solve cold-weather sealer problems, I think safe. If a sealer fails because it was applied below the MFT, often the only remedy is labor-intensive and costly stripping and resealing.

Best Method for Coloring Step Faces

Applying a paste made with color hardener is an effective, easy way
to color step faces. Using the same hardener that was applied to the
flatwork portions of the project helps to ensure color uniformity.

Question:
When stamping and coloring a set of steps, how do I get the risers, or step faces, to match the treads and adjoining patio surfaces?

Answer:
This can be accomplished with dry-shake color hardener or integral color, but using color hardener often achieves the best results. When using integral color, timing is everything. No matter how smooth the surfaces of your forms, some toweling will need to be done to the faces once the forms are removed. This means removing the forms while the concrete is still wet enough to take a trowel and be imprinted, but not too wet so the stairs slump and lose shape. This can be a tricky proposition, depending on the number and size of the steps.

When using the color hardener method, you can pull forms the same day (with a much longer set time if desired) or after many days. When you’re ready to color the step faces, you mix the hardener with a concrete polymer glue – acrylic or polyvinyl acetate (PVA) work best – to create a paste that you apply with a trowel as if icing a cake. By roughening the step faces a little after removing the forms (a light broom finish works well) you provide a better surface for the color hardener paste to grab onto.

Note that with both methods, knowing the best time to imprint the concrete is important to achieving good results, so experience is critical. The water-cement ratio of the concrete is also important. Do not add water to the step faces when using integral color, as this will dramatically lighten the color. When using the color hardener method, strive to make the color hardener paste the same consistency and slump as the concrete was when the hardener was applied to the flat surfaces. This will ensure a more uniform color match to the flatwork areas. Be careful of using color hardener paste for step faces when integral color has been used on the rest of the project. There are many variables to control, so achieving a good color match becomes very difficult.

How to Achieve Color Consistency When Tinting Overlays

Question:
I have not had much success tinting overlays on site with tint packs. What is the best way to tint overlays to ensure color consistency?

Answer:
The same basic principles apply when integrally coloring an overlay as when integrally coloring concrete. (See The Concrete Network article Understanding Colored Concrete Common Problems, Why They Occur, and How to Avoid and Fix Them.) The big difference is that the margin of error is much smaller when coloring a few gallons of overlay vs. coloring a large batch of concrete. The biggest factors to consider are batch size and batch-to-batch consistency. The larger the batch, the lower the chance for color inconsistency.

For the best results, tint or color the liquid component of the overlay mixture (water or polymer) in one large container. If you do not have a large enough container, “box” the liquid by pouring it back and forth between smaller containers until you feel you have mixed it multiple times and have achieved a consistent color. You can then measure out the colored liquid with a graduated measuring container to ensure you get the same amount of colored liquid in each batch. Here’s a tip: Cut a small hole in the side of your measuring container at the top of the liquid level to be sure you get the same amount in each batch.

Another important consideration is to use a reputable tint manufacturer that uses a batch number system, with retained material samples kept from each batch, so you can go have the color checked if you have a color issue. Remember to write down those batch numbers just in case!

Applying Water-Based Sealers

Water-based sealers tend to foam and bubble after being applied with
a roller. Instead, use a micro-fiber applicator.

Question:
What is the best method for applying water-based sealers?

Answer:
Water-based sealers require a different method of application from traditional solvent-based sealers. Unlike solvent-based products, which are basically resins (acrylic, wax, polyurethane, etc.) dissolved in solvent, water-based sealers use a mixture of water and other chemicals to keep the dissolved resin in suspension. Because of this emulsion chemistry, these sealers tend to foam easily when applied by a sprayer or roller, resulting in lots of soap-like bubbles. Thus the best method of applying a water-based sealer is with a micro-fiber applicator or mop. Pour a puddle of the sealer directly onto the floor, and then use a clean, flat micro-fiber applicator to push and pull the sealer across the surface until you get a very thin film.

Be aware that the typical coverage rate for water-based sealers is 400 to 600 square feet per gallon, about 100 to 200 square feet more per gallon than with solvent-based sealers. That’s because water is slower to evaporate than a solvent, giving you more time to spread the sealer into thinner coats. As with any sealer, applying multiple thin coats is always recommended.

Fixing Cracks Before Overlay Application

An epoxy crack filler, feathered 2 inches on either side
of a crack.

Question:
What is the best method for fixing cracks in concrete prior to applying a decorative overlay?

Answer:
The term “fixing cracks” is a bit misleading since there is no way to “fix” cracks in concrete, only fill them. The proper term to use when treating cracks prior to placement of a decorative overlay is “bridging.” This is accomplished by filling or covering the crack with a rigid material so it becomes flush or slightly higher in elevation than the surrounding surface. (Avoid soft, flexible crack fillers.)

To prepare the crack for repair, use a crack-chaser diamond blade to remove any loose concrete and to straighten out the crack as much as possible. The repair material filling or covering the crack needs to bond tenaciously to the concrete surface and be applied so that it gradually feathers down to surface level. The crack filler also needs to bond well with the overlay, so it needs to have a rough profile. The three most common methods for bridging cracks prior to applying a decorative overlay are:

• -Rigid Epoxy Systems – These are usually two-part epoxies available in self-mixing cartridges or for mixing onsite. You apply them using an injection gun or putty knife to fill the crack to overflowing, and then you feather out the epoxy on either side of the crack anywhere from 2 to 5 inches (see photo). Placing mesh tape into the wet epoxy provides additional reinforcement. Once the epoxy begins to set, course sand is cast onto it to provide a rough bonding surface for the overlay.
• -Cement-Based Systems – Polymer-modified cement-based repair mortars are mixed onsite and applied with a trowel or putty knife (again, using the feathering technique). Multiple thin applications are usually required to avoid shrinkage cracking. To provide a good bonding surface for the overlay, roughen the surface by scratching it with a broom or trowel as it begins to set.
• -Crack Suppression Mats – These have been used successfully for years in the tile industry. Roll out the mat to cover the crack by 5 inches on either side (or cover the entire surface if cracking is extensive), and adhere the mat to the surface using a bonding primer or glue. This type of crack suppression method is also a great way to cover seams when applying decorative overlays over wood and other non-concrete surfaces.

No matter the type of crack treatment used, assuring that the crack is properly bridged is critical in eliminating shadowing and show-through of the original crack in your newly applied overlay.