Seven asphalt pavement overlays
By Mike Aurilio, MASc
As part of their Every Day Counts initiative, the Federal Highway Administration (FHWA) looks at innovative solutions with the potential to save time, money and resources that they believe are underutilized by state agencies.
Targeted Overlay Pavement Solutions (TOPS) were one of the focuses during round six of the program. These pavement treatments are targeted for high-priority or high-maintenance locations where traditional solutions have not performed well. There are seven types of TOPS which are valuable to the asphalt industry:
1. Asphalt Rubber Gap-Graded
2. Crack Attenuating Mixture
3. Highly Modified Asphalt
4. High-Performance Thin Overlay
5. Open-Graded Friction Course
6. Stone Matrix Asphalt
7. Ultra-Thin Bonded Wearing Course
1. Asphalt rubber gap-graded
Asphalt rubber gap-graded mixtures are highly durable mixtures that have been used in places like Arizona and California to improve resistance to reflective cracking, rutting, thermal cracking and oxidation. NCAT has shown the mix can be used as surface, intermediate and base layer. These mixtures are produced using asphalt binder containing approximately 20 percent ground tire rubber and a gap-graded aggregate gradation to allow for higher binder content. Typically, the largest aggregate used is 3/8 inch or 1/2 inch and the mixture can be compacted from 1.25 to 2.25 inches.
2. Crack attenuating mixture
Crack attenuating mixtures are fine-graded mixtures with high binder content that can be used as an interlayer between the existing pavement and a new asphalt layer to reduce reflective cracking. TxDOT developed this mixture by adopting the principles of rich bottom layer mixtures, however, the layer is also rut resistant and can be used as a surface course as well (1/2- to 1-inch-thick layers). The use of high-quality aggregate (good durability and friction) is advised as well as using 7 percent of a polymer-modified binder. The ability to use this mixture in thin layers compensates for the cost use of premium materials and high binder contents.
3. Highly modified asphalt
Highly modified asphalt mixtures are produced using binders containing six to eight percent polymer and are typically modified using styrene-butadiene-styrene. The high polymer content creates a phase inversion where the binder acts as though it is dispersed in the polymer matrix. This greatly improves the cracking and rutting resistance of the mixtures. These mixtures have been used in many applications including full depth and thin overlays. Concerns with workability or compatibility can be addressed with specialty, low-viscosity polymers.
4. High-performance thin overlay
High-performance thin overlays are fine-graded mixtures with a maximum aggregate size of 3/8 inch and held together with polymer-modified binders. These mixtures are designed with Superpave volumetrics and performance testing but require limitations on the use of RAP and sands. They are placed with a thickness of 1 inch in maintenance applications, but they can also be used as a leveling course. TxDOT and NJDOT have used these mixes quite frequently with great success.
5. Open-Graded Friction Course
Open-graded friction course mixtures are produced with a high percentage of coarse aggregated resulting in 15 to 25 percent air voids. This design allows for the drainage of water which greatly reduces the potential for hydroplaning and spray making these pavements much safer. Tire noise can also be decreased by 3 to 6 decibels due to the porous nature of the surface. Despite these advantages, these pavements may be more susceptible to moisture damage which could decrease the service life. Open-graded friction course is also known as porous asphalt or popcorn mix because of its high air void content and appearance.
6. Stone Matrix Asphalt
Stone matrix asphalt or stone mastic asphalt is a high-performance gap-graded mixture that uses coarse aggregate skeleton to promote a higher degree of interaction between the stones with a high asphalt binder content which contains a stabilizing agent. These mixtures are highly rut resistant and generally perform better than dense-graded mixtures in heavy traffic applications. Stone matrix asphalt is more expensive than dense-graded mixtures and may be more cost-effective when used in the right application according to life cycle cost analysis.
7. Ultra-Thin Bonded Wearing Course
Ultra-thin bonded wearing course is 3/8 to 3/4 of an inch thick open-graded layer paved on top of a polymer-modified emulsified asphalt membrane. It can be used as a maintenance application for the surface course to correct distresses such as raveling, some cracking, loss of friction and smoothness. Like open-graded friction courses, this treatment can help reduce water spray and noise levels. Texas and Alabama pioneered the use of ultra-thin bonded wearing course, but Minnesota has used it extensively since 1999.
Each of these treatments has its list of benefits so it’s possible that one of these approaches can be used by an owner to address a concern they have that they cannot adequately address with their current approaches. Promotion of these techniques can certainly lead to better quality pavements across North America.
Aurilio is Terminal Manager at Yellowline Asphalt in Ontario, Canada.