Ambient Cure Conditions
Ambient conditions play the decisive role in determining the thoroughness of the overall cure process, and sealcoating performance. The following conditions are the recommendations of the industry and its research association, Pavement Coatings Technology Council.
Temperature (ambient & pavement). Sealer should not be applied unless the pavement temperature is at least 50°F and the air temperature is 50°F and rising. The fusion of the binder particles (in sealcoatings) to form a uniform and continuous film depends on their ability to soften under the ambient and pavement temperatures. The process of fusion is greatly enhanced at higher temperatures, say 75°F to 85°F. Conversely, the fusion process is significantly reduced at temperatures below 50°F.
Cold Temperature Application. When sealcoating is applied below 50°F, refined tar (or asphalt) particles do not soften and form a continuous film – so they leave clay and filler particles uncoated. The color of the sealcoating cured under such conditions usually turns out grey and blotchy and never returns to its characteristic dark slate/black color, even when pavement temperatures rise later; the pavement temperature does not reach high enough to re-melt the binder particles and force them to flow and form a continuous film. Even if the pavement temperatures reached high enough it still will not be sufficient to re-mobilize the binder particles to flow and envelope the clay and filler particles to form a continuous film. Needless to say sealcoating cured under cold weather conditions lacks the integrity and is liable to fail prematurely.
Hot Temperature Application. Sealcoatings should not be applied under the summer sun (over 90°F) without first cooling the surface with a fine mist of water, also called “fogging.” Water should be used to dampen the pavement — without leaving puddles.
If applied to a hot pavement without “fogging” it, the sealcoating film almost gets “baked” as soon as it hits the pavement. With the sudden loss of the film fluidity, the binder particles are immobilized and do not fuse properly. Devoid of the proper fusion process, binder particles do not effectively envelop the clay and filler particles in the sealer film to attain the proper hardness. They stay as thermoplastic entities in the film, which become sticky and soft under hot ambient and pavement temperature conditions, thus causing potential “tracking” problems.
Relative Humidity or Humidity
Relative Humidity (R.H.) also plays a significant role in the cure mechanism because it directly influences the rate of water loss from the sealcoating film. Relative humidity is the ratio of the actual moisture content of the air, at a specified temperature, to its total capacity. For example, 50% R.H. means that only half of the air’s total capacity to hold water has been used and it is capable of absorbing another 50% of moisture or vapor from the surroundings.
Conversely, 90% R.H. means the ambient air is loaded with moisture and has very little (only 10%) room left to hold additional water. Sealcoating applied under highly humid conditions takes a long time to cure because there is very little room for the air to absorb water released from the film – the film will release only the amount of water that can be accommodated by the atmosphere. (The atmosphere and the surrounding environment can be thought of as a sheet of paper towel: When dry it will soak up the spill but will not mop up if the towel is too wet.) So sealcoatings will cure faster at lower humidity than at higher humidity. Under highly humid conditions, sealcoatings must be allowed longer drying time before the application of subsequent coats and finally opening to traffic.
The interdependence of temperature and relative humidity on the water evaporation rate is depicted in Figure 3. You will notice that the water evaporates more than three times faster at 40% R.H. than at 80%, at a given temperature. Along the same lines, the capacity of the air to hold water to the point of saturation increases with the increase in temperature.