Asphalt-Rubber Systems In Road
Rehabilitation
by Joe Cano, P.E., Technical Director, International Surfacing, Inc., Chandler, AZ

Joe Cano is a graduate of Arizona State University with a Bachelor of Science degree in Civil Engineering. He retired from the City of Phoenix in January, 1992 after 32 years of service. Prior to joining International Surfacing Systems in March of 1992, Mr. Cano served as the Engineering Supervisor of the Materials Laboratory for Phoenix Street Transportation Department, responsible for the testing and quality assurance of all construction materials used by the city. He developed the gap-graded Asphalt-Rubber hot mix used by the city to resurface badly cracked streets. Mr. Cano has authored several papers on the use of Asphalt-Rubber including the Twenty-Year Study of AsphaltRubber Pavements in the City of Phoenix, Arizona. He is a member of the RPA Board of Directors and its Technical Advisory Committee.

Asphalt-Rubber is a blend of asphalt cement, reclaimed tire rubber, and certain additives in which the rubber component is at least 15%, by weight, of the total blend. It is mixed at a temperature of 250'F to react the rubber and cause swelling of the rubber particles.

In the mid 1960's, the City of Phoenix pioneered the use of Asphalt-Rubber through the efforts of Charles McDonald, then Materials Engineer with the City of Phoenix. Asphalt-Rubber, for road rehabilitation, has been in use by cities and counties in Arizona and California since 1969. It is currently in use in many states across the country.

Asphalt-Rubber is utilized in two application processes:

(1) Chip Seals - an Asphalt-Rubber membrane spray is applied over the road surface and covered with one-size aggregate.

(2) Asphalt-Rubber Hot Mixes - Asphalt-Rubber material is substituted for conventional asphalt binder in the hot mix.

Asphalt-Rubber has superior engineering properties over conventional asphalt. It is more flexible and more resistant to cracking caused by wheel loading pressure, and it is very effective in controlling reflective cracking in overlays.

Asphalt-Rubber paving material is more resistant to rutting or pavement deformation because its viscosity is much less temperature dependent compared to that of asphalt cement. Asphalt-Rubber is cost effective as it will last longer than conventional asphalt. It has been demonstrated that the thickness of AsphaltRubber concrete pavement can be reduced to half the required overlay thickness of conventional asphalt concrete pavements.

Stress Absorbing Membrane

(SAM)

1. Description:

Surface treatment. Asphalt-Rubber is spray applied to pavement surfaces at 0.5 to 0.7 gallons per square yard (gpsy) and then covered with clean, nominal 3/8" single-sized chips. The A-R application rate is adjusted for the existing road surface characteristics, and the aggregate is sized according to the membrane thickness. A tack coat of emulsified asphalt, diluted 1:1 with water, may be used on oxidized existing pavement surface. A 0. 1 to 0. 15 gpsy fog seal application is recommended to maximize chip retention The fog seal should also consist of emulsified asphalt, diluted 1:1 with water.

2. Purpose:

Provides a lasting waterproof, skid resistant and durable surface which resists oxidation and cracking, and flexes to conform with movements of the underlying pavement surface.

3. Significance:

Asphalt-Rubber SAM improves and extends the serviceability of asphalt concrete pavements which are experiencing distresses, such as alligator and block cracking. The pavement life is extended by (a) decreasing the amount of surface water intrusion into the base and subgrade by providing a waterproof membrane to obtain maximum stability of the structure, (b) reducing the oxidation of the existing surface, (c) binding the existing surface together, and (d) reducing the spalling around reflective cracks.

4. Appropriate Uses:

There are a variety of situations where Asphalt-Rubber SAMs provide highly effective alternates to existing conventional strategies, methods, and/or materials.

The following list identifies some of the appropriate uses.


For rehabilitation / maintenance:

· over a pavement with fatigue cracking.

· over a pavement with oxidative or block cracking.

· over pavements with raveling.

· surface treatment for low volume roads.

· to extend the life of severely distressed pavements (basket cases) that require reconstruction for which funds are not yet available.

· to improve fractional characteristics (skid resistance).

Stress Absorbing Membrane Interlayer (SAMI)

1. Description:

A SAM followed by the placement of one or more courses of asphalt concrete or Asphalt-Rubber concrete (ARC).

2. Purpose:

The use of Asphalt-Rubber SAMIs extends the serviceable life of overlays by significantly retarding the rate of reflective crack formation. The SAMI also waterproofs the underlying pavement and significantly retards age hardening of underlying asphalt concrete.

3. Significance:

Many pavements that are in need of rehabilitation because of extensive cracking also require improvement of rideability and/or structure. A SAMI placed as an interlayer is more effective than as a surface course in reducing reflective cracking. It also provides significant reduction in tensile stresses transmitted to layers above.

4. Appropriate Uses:

Appropriate uses for SAMIs include most of those previously listed for SAMs. SAMIs can be effectively used in cases where it is necessary to improve ride quality or structural capacity by constructing an overlay. An additional function for which SAMIs are appropriate is to control reflective cracking over cement-treated and lime-stabilized bases.

Because the low-modules SAMI significantly improves resistance to reflective cracking, asphalt concrete overlay thickness can be generally reduced. Use of a SAMI also reduces the amount of surface preparation, particularly crack sealing, required prior to overlaying. These factors reduce the cost of the overlay.

Asphalt-Rubber Concrete

(open-graded)

1. Description:

A standard asphalt concrete opengraded friction course (OGFC), except that the binder used is a reacted AsphaltRubber material with substantially higher binder content.

2. Purpose:

To provide a more durable friction course which has increased stability and greater resistance to reflective cracking, oxidation, stripping, and chain wear. To significantly reduce pavement noise levels. To improve safety by reducing spray from the passage of trucks and improving visibility as well as reducing hydroplaning by preventing accumulation of surface water. To improve rideability and skid resistance.

3. Significance:

The use of Asphalt-Rubber as a binder for asphalt concrete open-graded friction courses solves many of the durability problems associated with opengraded mixtures. Use of A-R in OGFC permits higher binder contents, thus thicker binder films, due to the high viscosity of the A-R, without excessive drain-off.

This results in increased durability due to the greater oxidation resistance of the Asphalt-Rubber binder and thicker binder films. The antioxidants and carbon black in the tire rubber retard aging of the A-R material.

Asphalt-Rubber is more flexible at lower temperatures and stiffer at higher temperatures than the base asphalt cement. These characteristics, combined with greater film thickness, result in a mix which is highly resistant to reflective and thermal cracking. The combination of stiffer binder, at high temperatures, with the aggregate to aggregate contact of OGFC mixes results in a mix which is highly resistant to permanent deformation (rutting).

4. Appropriate Uses:

Open-graded pavements are typically free -draining. Therefore, if the pro posed pavement is to be bordered by curb-and-gutter, proper grade alignment for adequate drainage of surface water from the open-graded layer can be a critical factor in design.

Open-graded ARC overlays are highly resistant to reflective and thermal cracking. In moderate climate areas, thin (1 ") open-graded overlays placed directly over short-jointed (<20' spacing) plain PCC pavements have proven very ef. fective in resisting reflective cracking The high binder content of these mixes also provides increased flexibility and resilience to resist permanent deforma. tion, as well as reducing problems with aging and reveling. When an open. graded friction course is to be placed over pavements with moisture susceptibility, a SAMI should be considered in the design.

In areas with significant wet weather accident rates, open-graded pavements may reduce accident occurrence. The porous nature of such mixes prevents accumulation of water on the surface, reducing spray, nighttime glare, and hydroplaning. Thus, both visibility and frictional characteristics are improved.

Where traffic noise is a problem. thin lifts of open-graded ARC mixes car economically provide significant noise abatement. Documented noise reductions of 50 to 70% can be readily achieved.

Asphalt-Rubber Concrete

(gap-graded)

1. Description:

ARC gap-graded aggregate gradings are much cleaner, generally having a target of 20% passing the #8 and a maximum 7% passing the #200 sieve. Aggregate grading limits for gap gradations fall in between the respective limits for dense and open gradations. When compacted, the crushed coarse aggregate forms a structural skeleton (matrix) with minimum voids in the mineral aggregate (VMA) of up to 19%.

2. Purpose:

To provide a more durable and flexible asphalt concrete pavement which has increased resistance to reflective cracking, rutting, and oxidation. To reduce tire noise and provide excellent structure support.

3. Significance:

The use of Asphalt-Rubber binder in gap-graded mixes results in a flexible material which can be used as an overlay to extend pavement life, to provide structure support, improve ride quality and skid resistance, and decrease traffic noise level. Gap-graded mixes demonstrate an increased ability to accommodate deflections and provide superior structure support, thereby decreasing required overlay thickness when compared to conventional mixes. Higher film thickness and binder content reduces problems with raveling and chain wear as well. Air void contents are generally similar to those of standard densegraded asphalt concrete. These mixes are relatively impermeable, as interconnection of voids is limited. ARC gapgraded mixes are very similar, in fact virtually identical, to the "split- mastic" mix being used in Europe.

4. Appropriate Uses:

ARC gap-graded mixes may be used as overlays to extend pavement life or as the surface course of new pavement construction. As overlays, ARC pavements are highly resistant to reflective cracking and rutting. They reduce tire noise and improve ride quality and frictional characteristics. These mixes can be used to decrease required thickness of pavements. The mix has high flow (deformation) values as a result of the large amount of Asphalt-Rubber binder. This characteristic makes the mix very flexible and enables it to prevent reflective cracking.

Three Layer System

1. Description:

Construction of an asphalt concrete or ARC leveling course, followed by an application of an Asphalt-Rubber SAMI, which is then covered with a gap or opengraded ARC or asphalt concrete course.

2. Purpose:

To provide a means of restoring rise quality and smoothness to deteriorated concrete pavements as an alternative to grinding and grooving, thick overlay or reconstruction. To provide increased structural capacity for distressed PCC or asphalt concrete pavements exhibiting cracking, faulting and/or rutting. Also to extend pavement life by providing a durable and long-lived surface without reconstruction.

3. Significance:

Asphalt-Rubber three-layer systems have provided an effective means of rehabilitating deteriorated concrete pavements at up to half the cost of grinding and grooving. The stress absorbing characteristics of the sandwiched AsphaltRubber layer reduce reflection of cracks through the system. When an ARC surface course is used in conjunction with the SAMI, resistance to reflective cracking is further increased along with a significant reduction in tire noise. Noise reduction is an important added benefit, particularly in urban areas and residential areas.

Two Layer System

4. Appropriate Uses:

A number of combinations of types of asphalt and/or Asphalt-Rubber concrete pavements (such as dense, opengraded, and gap-graded) can be included in three layer systems. The table below presents the primary combinations recommended for design consideration. For the surface courses, use of Asphalt-Rubber concrete is strongly recommended to optimize the performance and serviceable life of the three layer system due to enhanced resistance to cracking.

Selection of the gradation of the surface mix depends heavily on anticipated traffic and site conditions. Appropriate uses of the respective types of ARC have already been described. All properly designed ARC mixes are highly resistant to permanent deformation (rutting) and reduce noise. Open- graded ARC mixes can provide superior performance and safety in many cases, but adequate drainage is necessary. Gap-graded mixes are relatively impermeable and can provide higher stability without sacrificing resistance to reflective and thermal cracking.

1. Description:

Application of an Asphalt-Rubber SAMI, which is then overlaid with an AR( surface course.

2. Purpose:

The SAMI provides a waterproof seal and minimizes and retards reflective cracking. The overlay restores ride quality , reduces tire noise, extends pavement life, and provides a durable pavemenat surface. The overlay may be designed to increase structural capacity as well.

3. Significance:

Prolongs the serviceable life of both the existing pavement structure and the new overlay. Provides all of the previously described benefits of a SAMI plus those of ARC overlays.

4. Appropriate Uses:

Open-graded ARC or gap-graded ARC are recommended to overlay to improve ride quality and where resistance to reflective cracking is considered critical to performance and serviceable life. They are effective over both asphalt and PCC pavements.

Disclaimer

The contents of this paper do not necessarily reflect the official views or policies of either the Federal Highway Administration or the State of California. They reflect the views of the author, who is responsible for the facts and accuracy of the data presented herein. Also, neither the State of California nor the United States Government endorse products or manufacturers. Trade and manufacturers names are presented herein because they ire considered essential to the objective of this paper.