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Diagnosing problems with decorative concrete: problem clinic answers your questions

by Joe Nasvik

Often decorative concrete has the same problems as "plain concrete". Adding color, chemical stain, or other treatments is like putting a filter over the lens of a camera — the results appear different bur any problems remain. In decorative concrete, real problems are usually under the surface, consequently more difficult to diagnose. Sometimes damage in the decorative process suggests underlying causes. Even consultants familiar with problems of plain concrete can be confused by decorative problems.

Problems with plain concrete

Some of the common concrete problems are caused by:

* Air entrainment--Most air problems arise when there isn't enough entrainment to prevent freeze-thaw damage. Air entrainment prevents concrete from scaling and spalling. In southern climates it helps to control excessive bleed water from coming off the surface and causing finishing problems. Too much air can cause scaling during machine troweling.

* High water-cement ratios--A water-cement ratio that is too high is characterized by lower than specified design strength, porous and weak surfaces, carbonation, excessive efflorescence, and increased shrinkage.

* Lack of curing--Too little curing can result in low strength, excessive amounts of unhydrated portland at the surface, carbonation, dusting, and excessive surface wear of high traffic areas.

* Low strength--Problems can stem from poor mix design, excessive water in the mix, or lack of curing. Low-strength mixes can increase the possibility of cracking, excessive surface wear, and porous concrete. When concrete is below 4000 psi strength in freeze-thaw climates, scaling can result. Surface strength is reduced when finished with bleed water present.

* High moisture levels in concrete--Waterproof coatings applied before moisture levels are low enough can result in blemishes, blistering, and spalling.

* Cold weather conditions--During cooler weather, the time lag between placement of the concrete and initial set is longer, causing extended bleeding times. Using unvented construction heaters can cause rapid carbonation of freshly placed concrete.

* Hot weather conditions--During hot weather, much less time is available between placement and initial setting. In addition, there can be rapid moisture loss from surfaces.

Problems with decorative concrete

Plain concrete problems can destroy decorative finishes, Here's how decorative concrete reacts:

* Air entrainment--Low air entrainment that results in spalling and scaling of the surface destroys decorative finishes. When dry-shake color is used, scales typically have color on one side and plain concrete bonded to the colored layer on the back side of the scale. This is because dry shake color hardeners provide a densified layer that effectively protects the colored layer from freeze-thaw damage. When air entrainment is excessively high, strength goes down, and there is virtually no bleed. It can also be difficult to properly "wet out" dry shake color hardeners.

* High water-cement ratios--Because of the more porous surfaces that result from too much water, colored finishes, including chemical stains, diffract more light, giving the impression of weaker coloration. Due to the weaker surface, traffic wear patterns can develop. In the case of chemical staining, wear call remove the colored layer.

* Lack of curing--These symptoms can be similar to high w/c ratio conditions. Unhydrated cement does not develop calcium hydroxide, so there is less of it for some decorative products to react with. Colored surfaces will appear less intense than well-cured concrete with the same amount of color. Stained surfaces also appear less intense. Dusting and traffic wearing patterns problems can also result. However, most decorative finishes can't tolerate the same curing methods used for plain concrete.

* Low strength--When there isn't enough cement paste in a mix, integral color isn't properly restrained in the paste, and color can be lost from the surface. Chemical stains may not have enough calcium hydroxide to react with, resulting in less coloration. Low strength in decorative finishes can cause traffic to wear into the finishes.

* High moisture levels in concrete--Chemical stains react differently in areas of a slab that have higher relative humidity. Decorative treatments, which do not have good moisture vapor transmission properties, can peel off the surface, blemish, turn cloudy white, or cause blisters to develop.

* Cold weather conditions--Long initial set times and excessive bleed water mean that more calcium hydroxide comes to the surface where it reacts with carbon dioxide from the air to form efflorescence. More laitance also comes to the surface from silica in the aggregates, causing hard white silicates to form. Also, slab finishes are often wet in appearance. In cold weather, concrete is usually covered with curing blankets or plastic, causing unsightly efflorescence markings.

* Hot weather conditions--When conditions are really hot, there is less time to perform all the added steps needed for some decorative finishes. In the case of stamped concrete it is more likely that impressions will be "mushy" at the beginning of the stamping process and too light, with little texture, at the end.

Decorative Problem Clinic

Hard sealers

Q. I recently poured a floor and treated the surface with acid-etch stains. I would like to seal the floor with something harder than acrylic sealers. What can I use?

A. There are many things to consider in the choice of a sealer. An important consideration is the rate of moisture vapor transmission (MVT). To that end, acrylic sealers are the safest. They are inexpensive and will bond to smooth surfaces (such as the acid-etch-stained finish you mentioned). They are also easy to reapply. Applying at least one coat of solvent-based acrylic and a top coating of water-based acrylic makes a nice finish. Water-based acrylics are harder than solvent-based and are more resistant to greases and oils. The floor can also be maintained easily with the same water-based material.

But if you feel it is necessary to use another type of sealer, epoxies and urethanes are the alternative. Water-based epoxies and urethanes are gaining in popularity, but be careful with placing them over green or blue acid-etch stain--they can seriously distort these colors because of their high pH.

Urethanes form the hardest surfaces and are often placed over a primer coating of epoxy, since epoxy bonds well to concrete, and urethane bonds well to epoxy. Re-coating involves more intensive prep work than fur acrylics. Also consider that most epoxies and urethanes form nonbreathable membranes, so MVT will be a serious issue if the subgrade under the concrete is damp. Do nothing until the floor has cured for a full 30 days, then do a moisture test to assure that work can safely proceed. Good water drainage away from the building and the subgrade will help to prevent problems with your floor over the long term.

Another option is to use epoxy sealers that have good MVT properties. Application is easier than with impermeable products, and much safer in the long run.

Removing release agent

Q. How soon can I clean the release agent off the surface of stamped concrete and seal it?

A. Under hot weather summer conditions you may be able to do this on the day following concrete placement. Watch for any signs of damage to the surface-. Stop immediately if you see damage, and wait another day or two. If you sawed the control joints the morning after placement, most of the release powder and sawdust can be removed with a fine bristle broom. The balance of the dust, which can blow around or track on shoes, can be lightly washed away with a garden hose.

The most popular way to perform final cleanup is with a pressure washer. The trick is to leave enough colored release agent to highlight and provide the desired appearance. Some worker skill is necessary to accomplish this. Another way is to scrub the surface with soap and water using a buffing machine with strip pads. Pads come in white, green, and black. Buy 1-inch-thick pads. White pads are the least abrasive and perhaps the best to use for this purpose. Rinse the residue off immediately after scrubbing. This method of cleaning removes the release powder from the top of the texture and leaves color in the joints and lower portions of the texture. Highlighting looks natural, and the color has a three-dimensional appearance.

As with all colored concrete surfaces, it's safest in terms of efflorescence problems, to use sealer as soon as possible after concrete placement. Solvent-based acrylic sealers are the best. Look for products that can be applied the same day. Be careful about the use of sealers rated as "cure and seal" products as they form coatings with limited breathability. For outdoor applications your slab must have good moisture vapor transmission properties.

--Joe Nasvik is senior editor of CONCRETE CONSTRUCTION.