Arizona project review confirms long term performance of A-R

ARIZONA DELEGATION - Gene Morris retired engineer with the Arizona Department of Transportation and a member of the RPA Technical Advisory Committee, left, George Way, pavement services engineer for the ADOT, and Doug Carlson, RPA deputy director, right, during the Washington TRB conference in January. They reported on the A-R long-term performance review.

Last year the Asphalt-Rubber Team, a partnering effort of the Arizona Department of Transportation and the Rubber Pavements Association, agreed that an extensive field performance review of all Asphalt-Rubber projects over 10 years old should be conducted.
RPA provided the funding to hire an engineer to conduct the review and the services of Doug Carlson to assist in the review and compile the report. ADOT provided support services in the form of records and other department resources.

Gene Morris, a retired ADOT engineer and Research Director, was retained to direct the review.

In the last 10 years ADOT has placed over 2,500 miles of pavement overlays using Asphalt-Rubber hot mixes. Asphalt-Rubber is defined by American Society for Testing Materials (ASTM) as a mixture of 85% hot paving grade asphalt and a minimum of 15% ground tire rubber. However, the A-R binder commonly used by ADOT is 80% hot paving grade asphalt and 20% ground tire rubber. The asphalt and rubber are combined at a high temperature and reacted until elastomeric properties are obtained.

Two different mix designs are used, dependent on pavement type, field conditions and climate. One mix, used solely as a surface course, is an open graded mixture with design air voids of a minimum of 15%. The mix generally has a 9% binder content and has high friction characteristics. The second mix is essentially a dense graded mixture designed with approximately 5% air voids. Portions of the smaller aggregate and fines are reduced to provide a gap grading. The product is very similar to what is known as a stone-mastic (SMA) mix with the elastic reacted rubber used in place of inert fillers. Binder content for this mix is generally between 7.5 and 8.5%. ADOT calls its open graded mix Asphalt-Rubber Asphalt Concrete Friction Course (AR-ACFC). The gap graded mix is call Asphalt-Rubber, Asphalt Concrete (AR-AC).

The AR-ACFC has had extensive use on both Portland Cement Concrete Pavements (PCCP) and asphalt concrete (AC) pavements. Where used directly on PCCP the design thickness is one inch. When used in with an overlay, recycle, or new pavement, the thickness of the AR-ACFC is one half inch. The AR-AC has also been used over both PCCP and AC pavements. In cases where structural improvement is required it may be used in conjunction with a half inch AR-ACFC. This mixture is generally less than 2 inches, the most common application is one to one and one half inches (1-1.5") thick.

ADOT has also used Asphalt-Rubber Stress Absorbing Membrane Interlayers (SAMI's) on several occasions.

A total of eighteen full scale projects, fourteen now over ten years old, were evaluated for long term field performance the first phase of the Arizona Project Review.

Visual inspection of the eighteen Asphalt-Rubber projects that are ten or more years old indicate the pavements are performing extremely well. Examination of available PMS indicate very low cracking rates and maintenance costs.

Highlights of the conclusions:

• Used over PCCP, the AR-ACFCs have performed far beyond the expectation of any known alternative rehabilitation strategies or overlay materials that are currently available and economically competitive.
  
• The A-R overlay strategies used by ADOT have performed well in a diverse range of climates from cold and wet to hot and dry.
  
• The overlay design strategies used by ADOT for A-R materials performed extremely well under diverse loading conditions from light to very heavy traffic. The loading ranged from 110,000 18 kip ESALs per year for rural two lane highways to 2,100,000 18 kip ESALS per year on multi- lane urban interstates.
  
• The AR-ACFCs with binder contents of an average 9% demonstrated greater resistance to crack reflection than corresponding ARAC mixes with lower binder content. The appearance of oxidation was much less in the mixes with higher binder content. No raveling or spalling was observed where cracking had occurred.
  
• Reflective cracking observed was virtually all tranverse cracking and primarily associated with environmental forces.
  
• In areas of high traffic, reflective cracking was reduced.

The photo above shows a section of Arizona I-19 after the concrete cement was crack sealed prior to the 1988 overlay. Below is a view of the freeway 12 years later, when the overlay exhibited a cracking rate of only one percent.

Arizona I-19 Performance Data (Tucson south: MP 58 - 60)  Performance indicators Ride Skid Rutting Cracking Annual maintenance/lane mile  Values before overlay 172 38 n/a 6% $857  Values after 12 years 70 64 1% 1% $50