By Dwight Walker, P.E.
Asphalt emulsions are widely associated with more routine applications like tack coats and seal treatments. While these uses of emulsions are important and successful, modern emulsions are also suited for more high performance pavement preservation applications.
The two most common high performance applications include engineered chip seals and micro-surfacing. Other uses include ultra-thin bonded wearing courses, cape seals, etc.
The remainder of this article provides more information on chip seals and micro-surfacing.
High performance chip seals
Chip seals are surface treatments that protect and extend the performance life of pavements. They are constructed by spraying an asphalt emulsion onto the pavement with a distributor then immediately covering the emulsion with a layer of uniform-sized aggregate chips. The resulting surface treatment is then rolled to seat the aggregate and broomed to remove any excess chips.
High performance chip seals (sometimes known as engineered chip seals) incorporate a modified emulsion to give improved performance and service life. The modifier can be a polymer or crumb rubber or some other additive. These modifiers improve the durability, flexibility, and strength of the residual asphalt from the emulsion. Modifiers also may increase chip retention.
Recent developments in equipment and the application process of some high performance chip seals make it possible to apply the modified emulsion along with the aggregate in a single pass. This eliminates the need for a distributor in some applications.
San Antonio chip seal
Like many highway agencies, the San Antonio district of the Texas Department of Transportation (TxDOT) has switched from an emphasis on construction to focusing on preservation of their pavements. These treatments, locally referred to as seal coats, use a specific emulsion, a CHFRS-2P, which is a cationic, high float, rapid set emulsion with a polymer additive.
The district uses this particular emulsion because of good adhesion and durability properties. The rapid set characteristic is appreciated because it breaks and sets quickly, allowing a quicker return to traffic. The high float property lessens long-term bleeding and the polymer provides early strength. The seal work is done by district personnel using emulsion made by Ergon Asphalt & Emulsions with a polymer additive furnished by BASF.
The San Antonio district has 10,900 lane-miles of pavement, including metro San Antonio. The rural pavements are on a 7-year sealing cycle.
Micro-surfacing is a treatment designed to seal and restore pavement surfaces and to provide good skid resistance. It can be used to restore the cross-section of rutted pavements. Micro-surfacing is similar to slurry seal but is generally considered to be stronger and more durable. It is a combination of polymer-modified emulsion, high quality aggregate, mineral filler, water and other additives.
By using a quick-setting emulsion, micro-surfacing can be opened to traffic in about an hour in most cases. Like most emulsion applications, micro-surfacing offers environmental advantages. Notably, the treatment requires no heating and drying of aggregate and has minimal hydrocarbon emissions.
Two gradations have been standardized for micro-surfacing, Type II and Type III. Type II is more commonly used and is used to fill surface voids and correct moderate surface conditions. It is often used as a medium-textured pavement surface to correct weathering and raveling. It provides an adequate wearing surface for medium to heavy traffic applications such as arterial roads, highways and airports.
Type III is coarser and provides maximum pavement friction and improved durability due to the depth of the application. It is best suited for higher traffic pavements such as expressways, major highways and arterials. Type III, with its increased stability, is well-suited for rut filling and re-establishing the profile of pavements with minor surface irregularities.
Both Type II and III microsurfacing mixtures can be placed in multiple layers.
The Federal Lands Highway (FLH) Division of the Federal Highway Administration (FHWA) initiated a study to develop a guide for the use of polymer-modified asphalt emulsions in surface treatments. The study was quite comprehensive and included:
- Review of more than 100 publications
- Holding meetings with industry, academia, federal and local agencies
- Developing a performance-related draft specification
- Placing field trials on several FLH projects.
The study’s documentation of the performance benefits of modified emulsions led to the conclusion that all emulsions for chip seals, slurry seals and micro-surfacing moving forward should be modified.
The study reported these emulsions prevent early failures of chip seals by promoting quicker and stronger aggregate chip adhesion. Other advantages include quicker opening to traffic, less bleeding and flushing and better overall performance of high-volume pavements.
For micro-surfacing, the FLH study found that the chemical break of modified emulsions (rather than breaking by evaporation) gains strength more quickly than unmodified emulsions, so lane closures and delays are minimized. Long-term durability was also improved.
When high performance asphalt emulsions are properly formulated and the treatment applications are correctly designed and constructed you can expect improved pavement performance.