Tack coat is a light asphalt emulsion applied to pavement in order to increase bonding ability, think of it like a glue used to hold the existing surface with new asphalt layers. Tack coat should be applied under every layer of asphalt plant mix.
Tack coat is made of asphaltic emulsion and is applied after a prime coat to existing pavement surfaces, as well as in between layers of hot mix asphalt. Tack coat helps to prevent slippage as well as the development of moisture pockets and bacteria. When it comes to priming asphalt the tack coat layer remains one of the most important components of the process.
Tack coat provides a bond…
- Between existing asphalt surfaces as well as new hot mix asphalt overlay.
- In between each layer of hot mix asphalt.
- On vertical surfaces (including gutters, construction joints and sidewalk curbs) where the new hot mix asphalt will match up to.
What Is Asphaltic Emulsion?
Asphaltic emulsion is made up of three key ingredients:
- Asphalt binder
- An emulsifying agent
Polymers may also be added to some mixes. Polymers may be pre-blended with the asphalt binder, or added as latex.
All asphalt emulsions must be in compliance with Standard Specifications, specifically, Section 94, “Asphaltic Emulsion.”
There are various types of asphaltic emulsions, but one of the key components that set emulsions apart is whether or not the asphalt particles are positively (cationic) or negatively charged (anionic).
Cationic Vs. Anionic Tack Coat
The emulsifying agent used in a particular asphaltic emulsion will determine if the emulsion is cationic or anionic.
Anionic is made up of emulsified asphalt particles that are negatively charged (anionic) and made for use with aggregates possessing a positive charge.
Cationic is made up of emulsified asphalt particles carrying a positive charge (cationic) and should be used with aggregates possessing a negative charge. This type of emulsion evaporates water quicker.
If there is no C at the beginning of an emulsion grade you know that the emulsified asphalt particles contain a negative charge (anionic). If there is a C at the start of the emulsion grade the charge is positive (cationic).
Other relevant terminology to know includes:
Residual Asphalt: This relates to how much asphalt binder remains on the pavement surface once water has fully evaporated from the emulsion.
Tack Coat Break: This is the process in which water separates from the emulsion, causing the tack coat color to change from brown to black.
Tack Coat Set: This is described when water fully evaporates from the emulsion and leaves behind a slim layer of asphalt binder on the pavement surface.
Different Grades Of Emulsion
Aside from cationic or anionic, there are other classifications that set apart different emulsions.
- SS- Slow Setting
- RS- Rapid Setting
- QS- Quick Setting
- LM- Latex Modified
- HF- High Float
1- Low Viscosity
2- High Viscosity
h- Hard grade asphalt with low penetration
Emulsions are largely classified based upon how long it takes for them to set after application. There are slow-setting, rapid-setting and quick-setting grades.
Slow-Setting Grades Of Emulsion: SS1, SS1h, CSS1 and CSS1h are all slow-setting grades of emulsion. Slow-setting emulsions never contain more than 43% water. Additional additives are often used to dilute water further.
Slow-Setting Grade Emulsions Are Best Used For: Contractors use slow-setting emulsions for tack coat. As the name suggests, they do take longer to set than other forms of emulsions. They are best for projects with lenient deadlines or when temperatures are warm.
Rapid-Setting Grades: RS1, RS2, CRS1, CRS2, PMRS2, PMRS2h, PMCRS2, and PMCRS2h are classified as rapid-setting emulsion grades. These types of emulsions contain no more than 45% water and additional additives. They should never be further diluted with water.
Rapid-Setting Grade Emulsions Are Best Used For: Commonly used in cooler weather or overnight thanks to the fact the break time is faster than slow-setting emulsions. While they set faster, they are more challenging to evenly apply and require a keen eye for detail to get the job done just right. Worth noting, emulsion PMCRS2 is known to develop “spider-webbing” when trying to achieve a low application rate by using a high-pressure number 1 nozzle.
Quick-Setting Grades: QS1, QS1h, CQS1 and CQS1h are all classified as quick-setting emulsion grades. These types of emulsions are made with the addition of a specially devised additive that actively decreases setting time. These emulsions should contain no more than 43% water and additives. It is possible to dilute with additional water.
Quick-Setting Grade Emulsions Are Best Used For: Commonly used at night or when temperatures are cooler, as well as when there are tight construction deadlines. Originally designed for use in slurry seals and micro-surfacing, quick-grade emulsions provide uniform tack coat coverage due to their lower viscosities and the fact they can be diluted with additional water. Depending on location, quick-setting emulsions may or may not be available and manufactures generally do not accept returns of excess tack coat.
Best Asphalt Tack Coat Practices
Experience, knowledge and an in-depth understanding of the functions associated with every element of tack coat are vital to creating a long-lasting high quality finished product that lasts for many years to come.
Kleenco Construction provides top-notch application of the appropriate tack coat for commercial asphalt. Contact us today to get a competitive quote!