The weather pattern has been changing in recent years and especially this year it seems that it transitioned from winter to summer, skipping the spring totally. Sealcoating in hot summer months poses its own challenges that need to be recognized and handled adequately to assure both sealer performance and worker safety. Instead of just recommending adding more water to the mix or fogging the pavement, my approach in discussing this topic will be to drill deeper in the mechanism of proper film formation and describe the factors that profoundly influence initial drying, the final cure and the overall performance. This topic shall be approached from three relevant aspects; pavement preparation, mix design and worker safety.
Sealcoatings are intimate dispersions of Refined Tar (RTS), Asphalt Emulsions (AE), or other specialty resins, clay/fillers, and specialty chemicals in water. Being water-based, such coatings are safe to handle and store. The major components of a sealer are:
- Refined Tar, asphalt and specialty resins are called binder or the glue that holds the inorganic fillers (clay, sand/aggregate etc.) tightly bound in the cured sealer film. Binder is the backbone of sealcoating, the component that protects the asphalt, in the pavement. Binders are commonly “thermoplastic” materials – which means they soften and melt at higher temperatures and become harder at cooler temperatures. In a sealer film the binder forms a continuous film, enveloping the filler particles, thus holding them tightly in the fully cured film.
- Clay, fillers and aggregates impart proper toughness and dimensional stability to the sealcoating film and help reduce the tackiness (thus tracking) of the binder at elevated surface temperatures.
- Specialty chemicals serve many crucial functions such as the emulsion stability, water repellency of the cured film, adhesion, color stability, sand suspension, ease in application etc.
- Water is the fluid medium in which the above components are dispersed. Water is the largest component. In most concentrated sealcoatings, water constitutes about 60% by volume. This proportion increases as water is added in mix designs, for example at a 30% dilution rate, the proportion increases to nearly 70%, by volume.
Sealcoatings are commonly supplied as concentrates, which have to be mixed with water, sand/aggregate, and additives (optional), prior to application. Mix designs are the suggested guidelines for the proportions in which these components are to be mixed, according to the usage requirements of the pavement.
- Water (a) gives fluidity to the sealer, (b) satisfies the water demand of sand and fillers, which absorb water. In addition, sand and fillers also absorb binder, which is called the binder demand. (c) Water also wets out the pavement for proper flow, leveling and adhesion of the sealer to the pavement.
- Sand /Aggregate (a) imparts slip resistance and traction (b) provides uniform textured, non-streaky appearance with reduced sun glare (c) improves wearability and (d) hides minor surface defects.
- Additives are used to boost the overall sealer performance, speed up drying or sand suspension or all of the above. There are many additives that are recommended to perform specific function in sealer mix designs.
The Cure & Film Formation Mechanism
Sealcoatings, being water-based, attain full cure through the loss of all the water from the wet film. As the water leaves, the volume of the wet film shrinks in proportion to the amount of water (by volume) in the mix. For example, if the mix design has 70% water by volume, the wet film will shrink by 70%, i.e. down to 30% of the original volume.
As the water evaporates, it creates a steady turbulence in the sealer film, which forces the suspended particles (binder, filler) into a closer proximity. The film becomes progressively denser, thus forcing the binder particles to touch each other and fuse into a continuous film, encapsulating the filler particles in the process. In addition, binder being in excess (about 50% more than filler volume), simultaneously bonds to the pavement.
The Cure Conditions
The ideal cure conditions are low humidity, moderate temperature and moderate wind velocity. Conversely, high temperatures and high humidity in hot summer months commonly require adjustments in the mix designs for achieving adequate drying and cure for optimum performance. By hot temperatures and high humidity we mean ambient temperatures in excess of 90° F and 85% Relative Humidity. Needless to say that at such ambient temperatures (higher than 90° F), the pavement is a lot hotter. If a standard sealcoating mix (with 30% water and 2-3 lb. of sand) is applied, the wet film of the sealer may not have sufficient fluidity for good coalescence (fusion) of the binder, thus resulting in poor, uneven film formation. Such film is liable to be full of voids, thus having poor performance properties.
Applying sealer on a hot pavement may not as bad as frying eggs on a hot pavement but it comes close. As soon as the sealer is applied the water “flashes off” from the film and the components in the film (binder and filler) become immobile and cure to a film that’s akin to having a “Swiss cheese” -- full of holes.
The Suggested Approaches
- Treat the pavement. Cool down the temperature of the pavement by "fogging” with a fine mist of water, making sure that water does not puddle on the surface. All puddles should be squeegeed out, otherwise the sealer will just slide off during the application in such areas. A slightly damp surface is preferable. Granted that "fogging" adds another step in application, but it sure helps the sealer in “wetting” the pavement, getting pulled in the profile (wicking effect), and uniformly spreading out on the surface for uniform bonding. The advantage with "fogging" is that you do not have to change the mix design, thus avoiding the risk of thinner cured films.
- Change the mix design. Adding 5% more water to the mix design is a commonly used practice, recommended by sealer suppliers. In order to maintain the same dry film thickness as in the standard mix design, you may consider applying 3.5% more wet sealer per square yard. Although you may find this step cumbersome, my intent is raise the awareness by pointing out the finer point in sealer mix designs and application.
- Use a performance-boosting additive. A common problem encountered with sealcoating cured under hot climatic condition is the appearance of aggressive tire markings (although they “heal” with time), which may become an issue with some customers. The use of an additive based on rubber/polymers has been found helpful in reducing tire markings. Such additives alter the viscoelastic profile of the binders -- thus making them less sticky under hot conditions -- while improving flexibility at cold temperature -- thus diminishing early surface damage of sealcoatings.
Worker Safety and Care
As with any successful business your employees are your invaluable assets. Your sealcoating crew has to be protected from hot weather sealcoating hazards. Make sure that they all use Personal Protective Equipment (PPE) and clothing. In addition, pay special attention to the following precautions;
- Avoid dehydration. Frequently drink fluids to stay hydrated. Please make sure that the crew takes breaks to rehydrate and rest. Immediate medical help should be sought for any signs of dizziness or dehydration.
- Wear proper clothing. Full-sleeved shirts and long pants are required and should be worn regardless of temperature. There are some ways to keep your crew cooler on those hot days while keeping them safe. Disposable spray suits, pants, and long sleeve tops are available. These items are very light and breezy, but keep unwanted sealer away from the skin. By providing these items to your team they can wear lighter clothing underneath such as shorts or short sleeve shirts.
- Wear appropriate gloves and boots. These can be used when made of lighter material can as long as they remain water resistant. Enforce the use of eyewear as well as a face shield.
- Apply sunscreen. Enforce the use of an SPF30 or above, for skin protection.
Sealcoating in summer under hot climatic conditions may not only be uncomfortable for the crew but can result in poor sealer performance if proper precautions and adjustments are not made to the sealer and its application. In addition, special attention must be paid to safeguard the health of the crew and their safety.
Girish C. Dubey, is president of STAR, Inc., Columbus, OH (www.starseal.com). STAR has affiliate sealer producing operations throughout the United States.