Sherman Peterson, New Mexico State Highway and Transportation Department
In the fall of 2000, the State of New Mexico constructed its first high performance concrete (HPC) bridge. The Rio Puerco Bridge is located on Old Route 66 west of Albuquerque. The purpose of the project was to establish the viability of HPC in New Mexico. The project required the introduction of new construction methods and quality control requirements.
HPC was used throughout the superstructure of the Rio Puerco Bridge. The bridge has three spans of 96.1, 101.1, and 96.1 ft (29.3, 30.8, and 29.3 m), which are made continuous for live loads. Each span consists of four 63-in. (1.6-m) deep bulbtee beams spaced at 12.6 ft (3.8 m) centers and supporting a 47.6-ft (14.5-m) wide, 8.7-in. (220-mm) thick cast-in-place concrete deck.
Precast, Prestressed Concrete Beams
The beams had specified concrete compressive strengths of 7000 psi (48 MPa) at release and 10,000 psi (69 MPa) at 56 days. Cementitious materials consisted of cement, silica fume, and Class F fly ash. A 3-day steam curing period was required to achieve the concrete strengths. Actual average strengths at release and 56 days were 7500 and 10,340 psi (51.7 and 71.3 MPa), respectively.
The beams were also instrumented and monitored by New Mexico State University for prestress losses using fiber-optic sensors. The average measured prestress loss at one year was 13 percent compared with final values predicted by the AASHTO LRFD lump sum method and AASHTO LRFD refined method of 22 and 27 percent, respectively.
Cast-In-Place Concrete Deck
The specified strength for the deck concrete was 6000 psi (41 MPa) at 28 days with a mix requirement of 7500 psi (52 MPa) at 56 days. Acceptance was based on the 28-day strength. In addition, the fresh concrete was required to have a higher slump than usual. Thus a maximum slump of 9 in. (230 mm) was established provided segregation checks were made. The concrete mix included Class F fly ash to mitigate the potential for alkali-silica reactivity, and a combination of water reducers to achieve the desired workability.
Placement of the HPC deck required a number of special procedures. First, a fogging system was developed to maintain a high localized relative humidity for the finished concrete in the otherwise arid New Mexico climate. The system consisted of a series of thin tubes suspended above the finished deck with small misting nozzles arranged in a grid pattern. Movable wind breaks surrounded the sides and rear of the fogging system. The entire apparatus was mounted on the finishing machine rails and followed the finishing operations across the bridge. Static wind breaks were also utilized. After finishing the deck surface, a curing compound was applied, and the deck was covered with wet burlap and polyethylene sheeting for a minimum of 14 days.
Another special requirement for this project was the placement of a test slab. A 44.5 x 30.2 ft (13.6 x 9.2 m) slab was placed using the proposed concrete mix, fogging system, and finishing machine. In the test, the measured slump was 8-3/4 in. (220 mm) and the air content was 5.2 percent. The concrete finishing machine produced a good surface finish with little hand finishing required.
The positive experiences from the test slab placement assured the Department and the contractor that the deck placement would be a smooth operation. These assurances proved to be valid, as the deck placement went just as smoothly as the test placement. The concrete for the bridge deck had a slump of 6-1/4 in. (160 mm) and an air content of 6.2 percent. Inspection of the deck after curing revealed only a few plastic shrinkage cracks.
Conclusions
The effects of New Mexico’s initial experience with HPC at Rio Puerco Bridge have been significant and lasting. HPC has since been used on the I-40/I-25 interchange in Albuquerque and more HPC projects are planned. In addition, the success of the HPC precast, prestressed concrete beams has resulted in an increased confidence level with prestressed concrete construction in general, and conventional designs are now produced with design concrete strengths of 7000 psi (48 MPa) compared to 6000 psi (41 MPa) used previously.
The material costs for HPC were 20 percent higher than conventional concrete on the Rio Puerco Bridge resulting in an overall increase of about 10 percent in the bridge cost. However, it is anticipated that, as more HPC projects are built, material costs will decrease to those of conventional concrete. In addition, the enhanced workability that can be achieved with HPC has been demonstrated to result in lower labor costs. In short, high performance concrete has proven to be a viable and effective alternative for bridge construction in New Mexico.