Low temperature and high humidity are poor cure conditions. A 20º F drop in temperature will double the drying time while a 35º F drop will triple the drying time.
Sealcoating professionals perennially encounter a “good problem” every fall when they are under the gun to finish the projects on their books before the snowflakes start flying. This article looks at factors that must be monitored to achieve optimum sealer performance. Rushing through the jobs in the zeal to satisfy the pent up demands may adversely affect your reputation and business.
Most sealcoatings are water-based coatings, either based on refined tar (RTS) or asphalt emulsion (AE). Commonly sealcoatings are supplied as concentrates, containing, typically 60% water (by volume). For application, additional water is added along with sand and additives, according to the mix designs suggested by the manufacturer. The final mix typically contains 70% water by volume. Even if you don’t care to remember these numbers, just remember that water has the largest proportion in the sealer mix.
Sealcoatings dry and cure by releasing all the water to the air, from top to bottom in successive layers. The escape of water becomes progressively difficult from the bottom layers as the film dries. Sealcoating appearing dry on the surface may still hold moisture in the bottom layers. For optimum performance it is essential that all water and volatiles be released from the applied sealer film. Even a trace amount of water left in the film may freeze and damage the film (possibly causing disbonding and peeling) if the temperatures dip below freezing in the night. And, sealcoatings are likely to fail prematurely if not allowed to cure sufficiently.
Critical factors in dry and full cure of sealcoatings are:
Temperature. Ambient and pavement; high temperature favors evaporation from the film, low temperature impedes it.
Relative Humidity (R.H.) or Humidity. The capacity of the ambient atmosphere to hold water; 50% R.H. means 50% of the capacity is still available to hold water, coming out of the sealer film. Conversely 90% R.H. means that only 10% of the capacity is available, therefore the drying of the sealer will be retarded. Low humidity facilitates the water removal whereas high humidity significantly retards it.
Wind Velocity is another factor that helps remove the water vapors from the immediate ambient vicinity.
All the factors, especially temperature and humidity work in tandem. High temperature and low humidity accelerate water evaporation; low temperature and high humidity are poor cure conditions, under which sealer films take significantly longer to cure. The differences are staggering: a 20°F drop in temperature will double drying time while a 35°F drop will triple drying time.
Similarly, a sealer film will take three times longer to dry if the humidity jumps from 40% to 80%. So it is understandable why sealer takes so much longer to dry and cure under adverse drying conditions. In late fall and early spring we encounter unfavorable conditions, which must be taken into consideration. Here are some helpful suggestions.
- Observe the sun pattern and the sun cycle. Sealer will take much longer to dry in the shade than in the direct sunlight. The temperature difference between the shaded and sunny areas could be easily 20-30 degrees. Northern exposure shall be considered because such areas do not get much sunlight. Avoid heavy coats of sealer; thick applications should be spread out evenly.
- Wait for any morning fog (nearly 100% humidity) to lift and pavement to warm before the start of sealcoating. Both fog and cold temperatures in the morning work against the drying of the sealer.
- In the mix design use 5% less water.
- Allow extra drying time (4-6 hours. between the coats.
- Days will get shorter so plan to get all sealcoating done by early afternoon, giving the sealer 3-4 hours of sunlight.
- For multiple coats, you may have to schedule the job over two days. Don’t rush the job. Barricade the sealcoated jobs if not sufficiently dry to take foot traffic by the time you leave.
- Make absolutely sure that the temperature does not dip below freezing overnight. Sealcoating must have 8 hours of drying before any threat of freezing.
- Make sure that the pavement is dry. Damp pavements really inhibit the drying of the sealcoating.
- Remove falling leaves from freshly sealcoated lots, even touch up if they drop in the wet sealer coat.
Girish C. Dubey is president STAR Inc., Columbus, OH (www.starseal.com). He sincerely acknowledges the help of colleagues Greg Lucas, general manager, McConnell and Associates, and Ed Miller, president of Star Seal Of Pennsylvania, for their invaluable suggestions.