The 21st Avenue undercrossing bridge, which carries Route 99 traffic over 21st Avenue in Sacramento, is a notable example of the benefits that precast concrete and accelerated bridge construction bring to a community.
The original bridge, built in 1958, consisted of two single span cast-in-place reinforced concrete cored slab bridges on strutted abutments, that were later linked by a cast-in-place prestressed cored slab. The bridge deck was showing severe signs of distress in the form of heavy pitting, abrasion, cracks, and spalls indicating it had to be replaced. However, the high volume of traffic on Route 99 meant the project design had to be completed quickly and with as little disruption as possible.
Caltrans proposed replacing the existing voided slab deck, with precast prestressed adjacent box girders using Ultra High Performance Concrete (UHPC) connections to provide a superior joint connection and to prevent longitudinal cracking. The design included 35 precast prestressed box girders, roughly 51 feet long, four feet wide and 2.25 feet in depth, with UHPC placed in the keyways between the girders to form the transverse connection.
The project followed an accelerated bridge construction method, which dramatically reduced traffic disruption.
100 hour project
Typically, a project of this scale would require a single lane close for six months, as crews worked incrementally to build one side of the bridge then the other. Instead, Caltrans opted to shut down Route 99 completely for four days, to demolish the existing bridge and construct the replacement.
It was difficult to convince internal and external stakeholders to shut down the busy highway, but the project team convinced them that the brief shut down was a better alternative. It was the first high-profile ABC bridge project in California that used precast elements to replace a bridge in such a short time.
With the route closed, the existing bridge superstructure was demolished, the top of abutments were leveled and resurfaced, a half-inch-thick elastomeric pad was placed on the abutment seats, then the girders were erected. Each girder was placed in under 10 minutes over the course of four hours. Then UHPC was placed in the key way joints and cured, a one-inch-thick polyester concrete overlay was placed, and the median barrier was constructed.
The bridge reopened to traffic on Tuesday June 15th 11:00pm – six hours ahead of schedule. The schedule for conventional cast-in-place construction would have taken 6 months to complete. The designers believe many more bridges will be replaced using precast concrete girders and accelerated bridge construction methods in the future.