Frequently Asked Questions

What can I do about moisture problems in a new concrete building?

A: Moisture problems can occur in tight concrete buildings without adequate ventilation that haven’t gone through one or two winter heating seasons to allow water to evaporate from the concrete. Unfortunately, air conditioning isn’t a very efficient way to remove the moisture because cool air won’t hold as much moisture as warmer air. Thus turning down the thermostat to cool the building even more isn’t likely to help. It can even make the problem worse by increasing condensation on the cooler surfaces. The air conditioner may also be oversized which prevents it from cycling on frequently enough to remove the moisture.

Two steps will help to solve the problem. Use dehumidifiers with air conditioning set at moderate levels (75°F or so) during the first two summers of operation to drop the indoor relative humidity below 80 percent. Dehumidifiers are a more efficient way of removing moisture than reducing air temperature through air conditioning.

Secondly, ventilate the building as much as possible when heating and air conditioning aren’t being used. The tighter the building, the harder it is to get rid of excess moisture. Install an air exchanger to provide fresh air when the heating and cooling systems are not operating. Also, use bath and kitchen exhaust fans to remove moisture produced by occupants. Many builders now install ventilation ducts in walk in closets to increase ventilation and decrease moisture problems. If not, keep the closets open while you are experiencing problems.

Another solution is to turn on the heating system after the building has been completed for two weeks or so, regardless of the season. During this period ventilation should be also be kept high. In a house with forced air, the fan should be on continuously. While it’s true that external heat drives some moisture further into the concrete (toward the cold side), the heat also reduces relative humidity in the building and allows water near the interior concrete surfaces to evaporate. The obvious downside to this approach is an uncomfortably high temperature in the building while finish work is being done, or delayed use of the building if heating is turned on after completion but before occupancy.

Condensation problems are often made worse when vinyl or other impermeable wall coverings are placed over the concrete at an early age. Evidence of moisture problems will be visible under the wall covering. Always discourage the use of impermeable wall coverings in hot and humid climates (such as on the Gulf coast). To help prevent this problem in moderate climates, delay applying the wall covering until the building has gone through two winters with the heating on and two summers with indoor relative humidity less than 80 percent.

What causes concrete to crack?

A: Unexpected cracking of concrete is a frequent cause of complaints. Cracking can be the result of one or a combination of factors such as drying shrinkage, thermal contraction, restraint (external or internal) to shortening, subgrade settlement, and applied loads. Cracking can be significantly reduced when the causes are taken into account and preventative steps are utilized.

How can I remove graffiti from concrete?

A: Many products are suitable for removing spray paint and felt tip markings from concrete surfaces. These products are generally effective also for removing crayon, chalk, and lipstick. The manufacturer’s directions should always be followed. If satisfactory results are not obtained with the first remover tested, a second or third attempt with other products should be made. A single product may not remove multiple types of stains.

Several proprietary chemical strippers are available, many of which contain a citrus-based solvent, methylene chloride, or potassium hydroxide. Citrus-based solvents are the least aggressive and may not work on certain paints, but they are the safest to use and often have less stringent disposal requirements. For best results, allow products based on potassium hydroxide to soak into the concrete surface for several hours before rinsing. These products also require a subsequent application of an acid neutralizer.

Abrasive cleaning can remove graffiti, but it will also remove the outer layer of concrete, making it more vulnerable to weathering. The cleaned area may also look different from the rest of the surface. After the graffiti is removed, or preferably before a structure is placed in service, a graffiti barrier coating or sealer should be applied. The surface treatment should keep graffiti out of the concrete pores and on the surface for easy removal.

What could have caused dark spots in a driveway?

A: Discoloration of this type can be caused by a number of factors that include nonuniform distribution of calcium chloride in the concrete, nonuniform hard steel troweling, or localized differences in water-cement ratio.

In this case, the craze cracking provides an additional clue. When concrete isn’t struck off and floated correctly, low spots or birdbaths may be present. Puddles of bleedwater collect in these low spots. Then inexperienced finishers sometimes dust the bleedwater surface with dry cement so they can finish the concrete faster. This has two possible effects:

• It may lower the water cement ratio, thus darkening the concrete.
• It may increase the shrinkage of the high-cement-content layer, thus creating craze cracking.