The Flagstaff Interstate 40 concrete pavement to be rehabilitated is located between the Flagstaff Interchange (Mile Post Marker 195) and the Walnut Canyon Interchange (Mile Post Marker 205). The concrete pavement originally built in 1969 (1) consisted of two 3.65m (twelve foot) eastbound and westbound lanes of 200mm (8 inch) thickness with 25mm (3 inch) thick hot mix asphalt (HMA) shoulders. The total width of the pavement surface is 11.6m (38 feet). The concrete pavement is non-reinforced except for reinforcing steel that tie the two concrete lanes together longitudinally. The concrete pavement had sawed and skewed joints that were randomly spaced 4.6m (15 feet) apart. Joints originally were not sealed. Underneath the concrete pavement is a 175mm (7 inches) cement treated base (CTB), built out of a frost susceptible limestone aggregate. Beneath the CTB is an unbound subbase, 175mm (7 inches) thick composed of the same limestone. Underlying the subbase is a clayey cinder material representative of the natural ground. The supporting base layers are the same under the shoulders except that 100mm (four inches) of cinder base was placed between the asphaltic concrete and the CTB.

The concrete pavement began to fail in 1974, just five years after construction. The failure began as large corner cracks and progressed to transverse cracks and severe spalling at the transverse joints. Maintenance sealed the cracks and patched spalls as best as possible and in the process expended considerable funds averaging over $80,000 per year.

As the pavement deteriorated the cracking continued to increase. From 1980 to 1989 the percent cracking increased from one percent to nine percent (Figure 1). The ride quality also suffered, even though maintenance repaired the worst locations. In 1980 the ride was 1500mm/km (100 inches/mile) and by 1989 it had increased to 1980mm/km (132 inches/mile). A value of 2220mm/km (148 inches/mile) is considered objectionable. Twenty percent of the project miles exceeded the objectionable ride level before overlaying. In general the performance of the concrete pavement was very poor. In addition the traffic loadings over the course of time increased dramatically. In 1969 the annual 80kn (18 kip equivalent single axle loads (ESAL's)) was about 120,000. In 1990 it was 1,600,000 and by 1999 it is presently 2,500,000 which is about 21 times as great as 1969.

Design began in 1988 and reconstruction was very strongly considered. The adjacent 6.7km (four mile) section of I-40 (MP 191-195) had experienced the same type and degree of failure and had been reconstructed. The reconstruction (2) involved the building of detours and closing the interstate in one direction for one year. Thus two reconstruction projects were built over a two year period in the years of 1985 and 1986. The reconstructed sections were composed of 200-275mm (8-11 inches) of HMA on top of 150mm (6 inches) of permeable asphalt bound base and 100-175mm (5-7 inches) of drainable aggregate base. The pavement structural section was placed on top of a geotextile separation fabric to keep the wet clay from pumping into the aggregate base. A complete edge drain system with slotted pipe was also installed. The total cost of construction of the two projects was about $15 million. In order to reconstruct the Flagstaff I-40 (MP 195-205) project it would have been necessary to build the project in four phases, since detours of more that 8.3km (five miles) were not allowed. In addition the construction would have taken four years to complete, however it was strongly questioned whether maintenance could maintain the pavement for that long, given its very poor condition (Figure 1). The overall cost of reconstruction was estimated to be at least $30 million. It was finally concluded that the project could not be reconstructed. In addition, due to money and time constraints the project would have to be overlaid within a tight budget and work completed in one construction season of about six months. Various overlay strategies were considered including many different overlay thicknesses, use of a fabric interlayer, asphalt rubber interlayer, various mixes, edge drains and cracking and seating. Each alternative was discussed and reviewed at both the central office and the district office. In addition the project was also selected (3) as a Strategic Highway Research Project (SHRP) Specific Pavement Studies (SPS-6) to test various overlay and rehabilitation strategies on concrete pavement. With an asphalt rubber binder the selected project design strategy represented a test of whether a relatively thin pavement overlay could control reflective cracking. Although the design was for ten years virtually everyone involved in the project considered this to be at best a six year design given the thin overlay design section and the very poor condition of the concrete. After much internal discussion and debate the final pavement design section included edge drains, crack and seat of the concrete pavement, a 125mm (five inch) overlay composed of a 75mm (3 inch) conventional dense hot mix asphalt (HMA) and 50mm (2 inch gap graded) asphalt rubber mix (AR-AC). An
asphalt rubber open graded friction course (AR-OGFC) 12.5mm (one-half inch) thick was placed as the final wearing surface on the two travel lanes (Figure 2). The asphalt rubber used on the project was specified to be 80%, AC-10 asphalt binder, hot reacted with 20% ground tire rubber. No other additives or modifiers were used.

The overlay thickness and layer placement was discussed right up to the final days before the bid advertisement. The discussion centered around whether the AR-AC or the HMA should be placed directly on top of the broken concrete. Previous experience with asphalt rubber interlayers indicated that the AR-AC should be placed on top of the cracked surface before overlay. The other position of placing the AC on top of the broken concrete seemed more in keeping with its role as a leveling and structural layer which would probably crack very soon after construction (first winter). The top overlay of AR-AC and AR-OGFC would then perform not only as the leveling and structural layer but also as the final flexible layer capable of resisting reflection cracking. The project was designed in this manner, however, a test section was built with the 50mm (two inch) AR-AC on top of the broken concrete pavement and a 50mm (two inch) HMA placed on top as the final overlay.

The use of the AR-OGFC as the final wearing course had been previously tried on a concrete pavement in Tucson, Arizona. Its performance in Tucson on Interstate 19 has been very good and it was always considered as the most appropriate wearing course. Typically in 1990 OGFC's in Arizona were placed with conventional AC-20 (PG-64-16) at a six percent maximum binder content which is normally the maximum the rock can hold before an excess drains off.

Asphalt rubber is over ten times more viscous than AC-20 (PG-64-16) at hot mixing temperatures of 177°C (350°F) and thus can be applied to an OGFC rock gradation at a rate of nine to ten percent by weight of the mix. This extra coating thickness increases durability and slows down aging. In addition the thick rubbery coating helps to retard reflection cracking.

In addition to the material related design issues, constructability issues were addressed in the design by meeting with district construction personnel. It was agreed early on that the project construction phasing should be such that the project could be completed in one summer paving season. To do this the specifications required the contractor to begin with the edge drain. After sufficient edge drain was completed the crack and seat would start. Crack and seat operations had to be done at night to avoid interfering with the edge drain installation and to be in sync with the overlay operation. The crack and seat drop height and spacing was checked by deflection testing and cores to verify the quality of the work. In addition an incentive of $15,000 per day to finish paving the overlay ahead of schedule was included. The maximum incentive was set at $450,000 with a due date of October 1, 1990. With all the numerous design issues described in plans and special provision specifications the project was bid in April, 1990.