In California, as elsewhere in the U.S. and overseas, aging pavement systems, together with increased traffic volumes that are considerably greater than those for which the systems were originally designed, have required new approaches to pavement design, rehabilitation and construction practices.
In 1998, in response to this situation, the California Department of Transportation (Caltrans) embarked on a Long-Life Pavement Rehabilitation Strategies (LLPRS) program to rebuild approximately 2,800 lane-km of deteriorated freeways in the 78,000 lane-km system of highways under Caltrans jurisdiction. The goals of this program include providing pavements with design lives of 30 or more years with minimal maintenance and utilizing fast-track construction to minimize inconvenience to the traveling public.
The purpose of this article is to briefly summarize the asphalt mix and structural designs as well as construction experience for the recently completed I-710 Freeway Rehabilitation Project in Long Beach, Calif. This project is one of the first major freeway rehabilitation projects in the U.S. incorporating 55-hour weekend closures for the construction of long-life asphalt pavements. It includes both full-depth asphalt concrete sections that replace the existing portland cement concrete (PCC) pavement as well as asphalt concrete overlays on cracked and seated PCC. The project was a cooperative effort among Caltrans, industry and academia (LLPRS Flexible Pavement Task Group) in terms of both design development and construction evaluations. It utilized asphalt mix and structural pavement section designs based on technologies developed through the Strategic Highway Research Program (SHRP) as well as innovations in construction specifications and requirements.
I-710, located in Southern California in Los Angeles County, is a heavily trafficked route carrying traffic in and out of the Port of Long Beach. It was selected as an LLPRS project with, prior to construction, 155,000 ADT (average daily traffic) during weekdays, of which 13% are trucks bound to and from the port. The specific section of I-710 selected by Caltrans District 7 for this project lies between the Pacific Coast Highway (SR1) and I-405. The freeway has three lanes in each direction and includes four overpasses, a total of about 4.4 km (26.3 lane-km).
Having been constructed in 1952 and not overlaid prior to reconstruction, the PCC pavement was in poor condition. Two rehabilitation strategies were selected:
• Where the clearance was acceptable, the existing PCC was cracked and seated and overlaid with asphalt concrete (2.8 km total length); and
• Under the structures where minimum clearance requirements did not allow an overlay, full-depth asphalt concrete sections were utilized with the freeway grade reconstructed and lowered to provide the required clearance (1.6 km total length).
Fighting fatigue
SHRP mix evaluation technology, enhanced by research from the Caltrans Partnered Pavement Research Center Program, was used in the development of both mix and structural pavement section designs. The pavement section was designed to accommodate 200 million ESALs, the traffic estimated for a 30-year period by Caltrans District 7 staff.
For the mix design and validation process, representative materials used for paving in the Los Angeles basin were shipped to the University of California at Berkeley. The asphalt binders were supplied by Valero Marketing & Supply, formerly Huntway Refining, and the aggregates were supplied by Vulcan Materials Co., formerly CAL/MAT. This effort was coordinated through the Asphalt Pavement Association located in Southern California (APACA).
Two asphalt binders were utilized, an AR-8000 paving asphalt (AASHTO MPI designation PG64-16) and a polymer-modified asphalt—PBA-6a* (AASHTO MPI designation PG64-40). (The * refers to the fact that this material contains additional elastomeric components).
Repeated load shear (RSST-CH) and flexural fatigue tests were performed in the laboratory for both mix design and analysis purposes. To further evaluate the mix design prior to construction, an overlay utilizing materials shipped in from Southern California was constructed on an existing jointed PCC pavement at the Richmond Field Station of UC Berkeley. The pavement material was mixed at a plant operated by Dumbarton Quarry Associates and placed by O.C. Jones Engineering and Construction.
Underneath bridges and other structures where it was not possible to place an overlay and maintain the vertical clearance, a full-depth asphalt concrete section was designed to replace the existing PCC structural section.
A number of combinations of materials were evaluated. The pavement section selected consists of the rut-resistant mix (PBA-6a* binder), the AR-8000 mix with 4.7% binder content, and a “rich-bottom” layer constructed with the AR-8000 binder at 5.2% binder content. This layer is placed in the bottom portion of the full-depth section to improve the fatigue resistance of the pavement and should not affect the rutting resistance of the mix near the surface.
The design includes an asphalt rubber open-graded friction course placed on the surface of the PBA-6a* mix. This layer, in addition to reducing hydroplaning, tire spray and noise, also will serve to reduce wear on the PBA-6a* mix. It is intended that this mix will be periodically removed and replaced during the design life of the structure.
Design of the overlay pavement structure required a different approach than that used for the full-depth AC replacement structure. In this case, the primary concern was to select an adequate thickness to mitigate the loss in pavement serviceability resulting from reflection cracking.
The resulting design consists of: 1.2-in. leveling course; asphalt-saturated geotextile fabric as an interlayer; 3.8-in. asphalt concrete course (same mix as leveling course); 3-in. asphalt concrete course; and a 1-in. open-graded friction course with asphalt rubber binder.
For successful performance of these pavement structures, strict attention to pavement construction was required, necessitating careful control of the mix components, mix compaction and layer thickness by the contractor, Excel Paving Co. of Long Beach.
Molding traffic
Construction of the project was accomplished in six stages. In the first stage, the median was widened and the old guardrails were replaced with concrete barriers. The second stage included excavating, widening and paving the outside shoulders up to the existing pavement surface elevation. The remaining four stages involved the main work of rehabilitating the four full-depth asphalt concrete (FDAC) sections under the overpasses and the two AC overlays of the cracked and seated PCC sections. In the Caltrans plan, a total of 10 consecutive weekend closures was scheduled for completion of these last four stages.
The contractor revised the Caltrans staging plan by splitting the freeway into eight segments in order to complete those segments in eight consecutive weekend closures.
During the main rehabilitation work, the contractor was required to adopt a “counterflow traffic” closure strategy. With this strategy, one direction of the freeway is closed for the construction work zone. Traffic is then rerouted through crossover areas located at both ends of the work zone to two lanes in each direction (three traffic lanes plus the shoulder) on the other half of the freeway. A moveable concrete barrier separates the two directions of traffic.
During the first four weekend closures, the contractor shut down the southbound side for construction while maintaining two lanes of traffic in each direction on the northbound side. The closure was reversed during the latter four weekend closures. A short (eight-hour) full closure of the entire freeway was used for mobilization and demobilization in each 55-hour weekend closure. Because of extreme time, space and resource constraints, many activities were performed concurrently.
At each weekend closure, median and outside shoulders were completely overlaid and/or replaced with AC, together with three main traffic lanes, in four strips (pulls), each approximately 4.3 m wide. An alternating strip sequence was utilized to avoid potential paving suspension due to AC cooling time. It enabled hot-mix asphalt delivery trucks to reach the discharging location without driving on the hot AC, eliminating material pickup by truck tires. The 1-in. asphalt rubber friction course was placed during weekday nighttime closures, after completion of all weekend closures.
