On The Job
3. PARTICLE SHAPE We want to limit flat or elongated particle shapes as they are susceptible to bleeding or excessive chip loss. Aggregates that are too round should also be avoided as they are susceptible to rolling and displacement by traffic. Angular aggregates on the other hand are not as susceptible to rolling displacement by traffic and they also lock together better. 4. CLEANLINESS You want to make sure you have as clean of a rock as you can get with 1% or less passing the #200 sieve. "Dusty aggregates will prevent bonding with the binder," Tomkins says. "Any time you can limit the amount of dust you have in your aggregates, the better. If you can't limit the amount of dust, then you need to take that in to consideration when selecting your emulsion." 5. AGGREGATE TOUGHNESS & SOUNDNESS The aggregates you select for your chip seal should be abrasion, degradation and polishing resistant. They should also be selected to weather freeze-thaw cycles. "You want to consider having a Los Angeles abrasion, British Wheel or other requirement in your specification that helps you make sure you have a nice tough aggregate," Tomkins says.
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The binder or emulsion that you select will need to have good coverage, adhere to the aggregate and stay in place, while also sealing minor cracks.
Emulsified Asphalt (Cationics/Anionics): • Lower application temperatures • Not as sensitive to moisture in aggregates • Pre-coated aggregates are not needed • Longer curing period required
6. AGGREGATE ABSORPTION The final consideration when choosing an aggregate is absorption. The more absorptive the aggregate you have, the more binder that will need to be applied. Tomkins says that pre-coated aggregates may also be used to address the absorption of the aggregate. "If aggregates are absorptive, like limestone, and I place them on top of the emulsion and it sucks up some of the binder into the aggregate, I might not have enough to keep the aggregate in place and I want to take that in to account in my design," Tomkins says. "Hot-applied chip seals usually do have pre-coated aggregates which adds a little more cost however it definitely helps out."
No matter what type of binder you choose, you need to make sure you are putting the material down in ideal conditions. Before beginning the sealing work, prepare and clean the existing surface. Fill any potholes, level the ruts and seal large cracks. Weather conditions during construction also have a direct influence on performance so the weather should be warm and dry to promote proper binder setting and curing. Make sure the surface temperature is also warm enough to accept the material. Monitor the weather forecast to avoid potential weather problems and avoid possible chip loss.
Choose the Right Binder The binder or emulsion that you select will need to have good coverage, adhere to the aggregate and stay in place, and seals minor cracks. The material should be fluid yet viscous, develop adhesion quickly and not bleed when applied. "You do not want the binder to run off the roadway when applied and you want to be able to get traffic back on it quickly," Tomkins says. "There are a few different types available and you want to select the one that will work best for your application based on these advantages and disadvantages." Hot Applied Asphalt Cement: • Little to no curing period • Quick return to traffic • Better aesthetics by utilizing precoated aggregates • High application temperatures • Sensitive to moisture in aggregate • Increase cost to pre-coat
Chip Seal Limitations While these pavement preservation techniques are great for certain pavements, it’s important to note where chip seals are not recommended. Chip seals are not intended to add any structural life to an asphalt pavement as they do strengthen the surface or increase the load-bearing capacity. Chip seals are also not designed to smooth rough pavement, bridge cracks winder that 1/4-in. or address ruts greater than 1/2-in.
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March/April 2021 • Pavement Maintenance & Reconstruction • www.ForConstructionPros.com/Pavement
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