Asphalt emulsions can be used for almost any type of pavement application. They can be used for preventive and corrective maintenance on both asphalt and concrete pavements, stabilizing and reclaiming bases, building structural pavements and recycling worn out pavements. Recycling with asphalt emulsions is an environmentally attractive way to reduce costs and reuse paving materials.
Asphalt emulsions were created in the early 1900s, mainly for dust control and other spray applications. Today they are used for a wide variety of road construction and pavement preservation projects.
Tack Coats
A tack or bond coat is a spray application of asphalt emulsion that provides a bond between the existing pavement and a new overlay. Tack coats significantly increase the strength and fatigue life of pavements at a low cost.
A good tack coat provides a uniform thin, tacky, adhesive film without running off the road or causing slippage between the old and new pavement surface. The tack coat is sprayed at a uniform rate from an asphalt distributor. The sprayed emulsion is allowed to break before the new surface is applied.
The benefits of a tack coat include:
- Bond strength between pavement layers
- Increased pavement structural strength
- Reduced pushing and shoving in the new asphalt surface.
Chip Seals
Chip seals are surface treatments designed to prolong and protect the life of a pavement. Long-term pavement performance studies show that chip seals are one of the most cost-effective methods of pavement preservation.
In a single chip seal, an asphalt emulsion is sprayed on the pavement with a distributor, then immediately covered by a single layer of uniformly sized chips from a chip spreader. A double chip seal repeats the procedure using lower emulsion and aggregate application rates. The chips in the second application of aggregate are smaller than those used in the first application. After spreading the aggregate, the chip seal or double chip seal is rolled to seat the aggregate, and broomed to remove any loose chips.
Because chip sealing does not significantly increase structural capacity, the existing pavement must be structurally sound. Moderately severe cracks or other distresses should be sealed or repaired and the surface should be cleaned before treatment. Chip sealing is ideal for pavements with loss of surface texture and as a preventive maintenance treatment on aging pavements in good condition with minimal surface distress.
The benefits of chip seals include:
- Waterproofing and sealing small cracks
- Enriching hardened and oxidized asphalt
- Protecting underlying pavement from oxidation and aging
- Providing a low cost preventive maintenance technique.
Slurry Seals
A slurry seal is a mix of aggregate, emulsion and mineral fillers mixed and placed by a single machine. A slurry seal is designed to seal the pavement, restore uniform texture and appearance, and provide good skid resistance.
Slurry seals are available in three aggregate sizes. Type I uses a fine aggregate and is designed for parking lots and taxiways. Type II, the most commonly used type, uses a coarser aggregate and can be used on arterial roads, residential areas and highways. Type III uses the coarsest aggregate and can be used on freeways, high speed roads and runways.
The benefits of slurry seals include:
- Providing a weather-resistant surface
- Protecting the existing pavement from UV damage, oxidation and mechanical wear
- Excellent skid resistance
- Readiness for use in a few hours after application
- Cost-effectiveness.
Micro Surfacing
A micro surface is composed of dense-graded aggregate, polymer modified asphalt emulsion, water, mineral fillers and other additives. A micro surface is stronger and provides more durability than a slurry seal. The quick setting micro surface emulsion allows traffic to begin using the new surface in about an hour.
Micro surfacing is usually available in two aggregate sizes. Type II, the most common, uses a coarse aggregate. It is used for all applications, including arterial roads, residential areas, highways and airports. Type III, with a coarser aggregate, is used on high traffic pavements, high speed roads and airport runways.
The benefits of micro surfacing include:
- Reduced user delays
- Surface improvement
- Increased skid resistance
- Cost-effective, long-lasting pavement preservation.
Full Depth Reclamation
Full depth reclamation is a cost effective technique for reclaiming distressed pavements and providing structurally sound bases for existing roads.
In this process, a reclaimer pulverizes the existing pavement and its base 4 to 10 inches deep and mixes in asphalt emulsion. The mixed material is compacted with a padfoot compactor, then bladed to level the surface. After the level surface is compacted, a wearing course is added.
Full depth reclamation is an ideal treatment for thin bituminous pavements that have severe distresses such as ruts, cracks, potholes and base problems. Full depth reclamation is also a good choice for strengthening shoulders.
Full depth reclamation reclaims pavements with severe distresses. It can also be a first step in stage construction—adding more structure to meet increasing traffic or widening roads.
Hot In-Place Recycling
Hot in-place recycling (HIPR) rehabilitates asphalt pavements on site. The process typically removes 1 to 1-½ inches of pavement, mixes it with an emulsion, and places the recycled mix on the existing pavement in a continuous operation.
Before milling, the pavement is heated to approximately 300°F (150ºC) to soften the surface. The top 1 to 1-½ inches of the scarified pavement is then milled and mixed with an asphalt emulsion. The rejuvenated mix is then placed by a paver with a vibratory screed.
HIPR is typically used on pavements displaying advanced surface distress and oxidation. Because the process typically rehabilitates less than the top 3 inches, the pavement structure and its base must be in good condition.
The benefits of HIPR include interrupting and filling cracks; leveling ruts, bumps and potholes; rejuvenating the old pavement; and re-establishing drainage and pavement cross-sections.
Cold In-Place Recycling
Cold in-place recycling (CIPR) involves reconstruction of asphalt pavements experiencing severe distress, including transverse cracking, wheel rutting, deformation, oxidation and potholes. The CIPR process removes, crushes, and rejuvenates the old pavement materials into a new pavement structure.
A cold recycling train, containing several pieces of equipment performing specialized tasks, performs the work. The recycling train is often up to 100 yards long. Its first operation is milling the existing pavement to within 5 or 10 percent of its original structure. After milling, the material is crushed and screened. Depending on the mix design, virgin aggregates or recycled asphalt pavement (RAP) may be added, along with other additives, and mixed in a pug mill. The rejuvenated mix is then placed and compacted, and within a few days covered with a new surface course.
The benefits of CIPR include rehabilitating severely distressed pavements, and leveling potholes, ruts, bumps and shoved pavement. Benefits also include minimizing hauling and use of virgin materials, re-establishing drainage and crowns, rejuvenating the old pavement and improving skid resistance.
Dust Palliatives
Dust palliatives are diluted asphalt emulsions that reduce dust on unpaved roads. Most dust palliatives are sprayed by an asphalt distributor onto low volume unpaved roads. Application rates vary from 0.1 to 0.35 gallons per square yard.
Benefits from dust palliatives include:
- Reduction of maintenance grading costs
- Reduction of gravel loss
- Reduction of sediment runoff
- Reduction of vehicle damage
- Reduction of vehicle accidents.
Ultrathin Bonded Wearing Course
This surface treatment can be used for maintenance on an asphalt or concrete pavement. It can also be used as a surface course on new asphalt pavement construction.
The treatment is a one-pass paving process using a special machine that sprays a polymer modified asphalt emulsion, then places the ultrathin hot mix with a special screed. The ultrathin mat, usually ¾ inches thick, uses high quality aggregates while maintaining overhead clearances and curb heights. The ultrathin material is then compacted with a static 8- to 10-ton roller. The treatment provides a durable, drainable wearing surface.
Driveway Sealers
Asphalt emulsion driveway sealers are composed of a polymer-modified emulsion with fillers and polymers added. They are environmentally friendly with no coal tar. They rejuvenate aged driveway surfaces and seal out water. Asphalt emulsion sealers effectively preserve driveways.
Polymer-Modified Emulsions
Polymer modified emulsions provide increased service life and cost savings, and allow the use of emulsions in applications where conventional asphalt emulsions have not worked effectively.
Polymer-modified emulsions provide the same environmental benefits as traditional emulsions, with the addition of increased durability. Polymer modified emulsions also provide overall cost savings. Highways and other pavements can now be treated with polymer-modified seal coats instead of more expensive techniques.
A polymer can be elastic—like rubber or hard like a bowling ball. Its elasticity becomes evident during curing and throughout the life of the pavement. Adhesion to the aggregate occurs faster and allows the use of local aggregates that may not have performed well with unmodified emulsions.
The use of asphalt emulsions is steadily increasing throughout the U.S. Their adaptability and flexibility allow engineers to use them in a variety of ways.
The Basic Asphalt Emulsion Manual (MS-19), a joint publication of the Asphalt Institute and the Asphalt Emulsion Manufacturers Association (AEMA), is recommended for anyone needing more information on asphalt emulsions. Much of the material in this article was taken from MS-19 and from AEMA’s booklet, Asphalt Emulsions.