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Three different high performance concretes were used in the bridge.
HPC for the Angeles Crest Bridge 1
Jose Higareda, California Department of Transportation
The Angeles Crest Bridge 1 is located along scenic Route 2 northwest of the city of Los Angeles within the Angeles National Forest, California. The 208-ft (63.4-m) long single span, precast, prestressed concrete, spliced bulb-tee girder bridge was used to span over an area that was washed out during the spring thaws of 2006 and 2007.
The cross section of the bridge consists of six 96-in. (2.45-m) deep girders spaced at 77 in. (1.96 m) centers and a 7.7-in. (195-mm) thick cast-in-place concrete deck for a total width of about 42 ft (13 m). The girders were shipped to the site in lengths of 56, 92, and 56 ft (17, 28, and 17 m). Two 2-ft (0.7-m) long closures produced a total girder length of 208 ft (63.4 m). The girders were spliced together on the ground in a staging area near the bridge location and then moved onto the abutments. The individual girder segments were pretensioned for transportation and then post-tensioned in two stages. The first post-tensioning stage occurred in the staging area after the closures had achieved the required strengths. The second stage took place after the girders had been moved onto the abutments. Intermediate and end diaphragms were then cast, followed by placement of the concrete deck.
Precast Girder Concrete
The specified concrete compressive strength for the precast bulb-tee girders was 8500 psi (59 MPa) at 56 days. Because the bridge is located at an elevation of 6500 ft (1981 m) above sea level in a freeze-thaw environment, there was an additional requirement for 4.5 to 7.5% air entrainment. Air entrainment can reduce the strength of concrete by as much as 5% for each 1% increase in air content. This meant working with a concrete mix that, without air entrainment, would achieve a strength of about 12,000 psi (83 MPa). The measured compressive strength of the air-entrained concrete was 8750 psi (60.3 MPa) at 28 days. The concrete mix proportions are provided at the end of this article.
The concrete strength requirement for the closures was high as well. The design required a compressive strength of 8000 psi (55 MPa) at 56 days with 4.5 to 7.5% air entrainment. The strength requirement was lower at the splice locations because they are located away from the midspan of the girder and have lower flexural stresses. The strength for the splice concrete could have been reduced further if the splices had been moved even further away from midspan. However, the middle segment length of 92 ft (28 m) was already considered a difficult length to ship given the very sharp turns in the highways leading into the Angeles National Forest.
Batching, placing, and curing the splice concrete in the field, away from a controlled environment such as the precasting yard, proved to be challenging. Caltrans worked with the general contractor to establish a special batching process at the jobsite. The contractor used a consultant to develop the mix, prepare mixing procedures, and perform trial batches. The mix was designed to have a water-cementitious materials ratio of 0.32. The volume of concrete required for the splice placements was relatively small. For this concrete only, it was economically feasible to import special, high quality coarse and fine aggregates. Batch-mixer trucks properly equipped for performing volumetric proportioning were used to produce the required concrete on site. The measured compressive strength of the closure concrete was 8080 psi (55.7 MPa) at 7 days. The concrete mix proportions are provided at the end of this article.
The 7.7-in. (195-mm) thick, cast-in-place concrete deck had a specified compressive strength of 5000 psi (34 MPa), an air entrainment of 4.5 to 7.5%, and a drying shrinkage not to exceed 0.035% after 7 days of moist curing and 56 days of drying. The mix for this concrete was produced at a batch plant where a hydration stabilizer was added to accommodate a 75 minute haul time. Though a shrinkage-reducing admixture was used, the drying shrinkage requirement proved to be too stringent for use with local aggregates. The mix, however, did meet the Structural Engineers Association of California (SEAOC) specification limit for Class M concrete of 0.036% after 21 days of drying and was accepted. The concrete deck was cast in winter and the contractor placed insulated blankets on the deck and wrapped the deck and girders with polyethylene sheets. Heaters were placed between the girders to keep the deck concrete warm during the initial days of curing. The measured compressive strength of the concrete was 5190 psi (35.8 MPa) at 42 days. The Area Bridge Maintenance Engineer reported during a summer inspection: "You appear to have been successful in mitigating the cracking as the only cracks I could find were some hairline cracks at the westerly end."
Concrete Mix Proportions
For further information about this bridge, see ASPIRE™ Spring 2010.
HPC Bridge Views, Issue 62, July/Aug 2010