Recycling full width in a single pass
The recycling train — consisting of WR 4200 and WM 1000 slurry mixer — recycled the Interstate across the full lane width in a single pass, at the average recycling depth of 4 inches. Generally, the hard shoulders were recycled during the day and the driving lanes always at night. The recycled lanes were immediately opened to traffic the following morning. After the completion of project sections, the recycled base layer was to be overlaid with 75 mm (3 inches) of dense-graded asphalt, and later, 70 mm (2.75 inches) of open-graded friction course, resulting in a completely new driving section.
The recycled base material makes a very high quality base course, and the mixing results can be compared to those of stationary mixing plants. That’s due to the WR 4200’s integrated twin-shaft pugmill mixer, which produces a homogeneous construction material mix from the crushed pavement material and binding agents. The mixing capacity is approximately 400 tons per hour.
The recycling train had an average working speed of 15 to 20 feet per minute. An average of 1.2 miles of CIP pavement was completed each night shift.
Because there were no intrusions into adjacent traffic lanes, disruption to traffic was kept to a minimum. And the cutting width of the recycler is variable from 9-foot 8-inches to 13-foot 9-inches, so the entire lane width - including overlaps — could be recycled in one pass, eliminating longitudinal joints in the traffic wheel paths.
“This feature was especially important when recycling the shoulders, where the width was not constant,” Wirtgen’s Marshall says. “The ability to vary the recycling width on the move allowed changes in pavement width to be accommodated on the near side, while maintaining a constant and straight joint with the offside traffic lane. This feature meant that the recycling equipment remained in one lane, minimizing traffic disruption.”
On I-80, the WR 4200 cut the existing pavement to the required depth and width, and the milled material was fed into its pugmill, where the binding agents, foamed asphalt and cement slurry were injected, along with a water spray, to bring the recycled material optimum condition for compaction.
On exit from the pugmill, the homogeneous material was then fed into a tamping, vibrating screed unit directly attached to the WR 4200, which placed the recycled material as a new traffic lane. Each of the three spray systems — foamed asphalt, cement slurry and water — were regulated by a microprocessor on board the WR 4200 to ensure that the correct values were delivered to the mixing chamber, in accordance with the mix design, relative to forward speed.
Following the CIP recycling, the reclaimed, stabilized base course material was compacted with a combination of a steel drum vibratory roller and a pneumatic roller. Once the required compaction was achieved, the recycled lane was opened to traffic immediately following the construction shift.
Meeting environmental, construction needs
The I-80 CIP project clearly met the initial project objectives of Caltrans and contractor Teichert.
Caltrans’ foamed asphalt design strengths and required densities were based on what it had experienced with full-depth projects. “We’re very, very happy that our preliminary falling weight deflectometer testing showed the in-place material being stronger than our design material,” Peterson says. “We will continue to test every six months for the next three to five years.”
There was a substantial reduction in traffic congestion on this heavily traveled route, which connects San Francisco with Reno, and follows the Donner Pass over the crest of the Sierra Nevada range. Restriction of the recycling equipment to one lane, with only two deliveries of materials per shift (liquid asphalt and water), meant the adjacent traffic lanes were not affected by the recycling process. The ability to open the recycled lane to traffic at the end of the production shift greatly reduced lane closures and resulting traffic congestion.