Joseph L. Hartmann, Federal Highway Administration and Denis Dubois, Maine Department of Transportation
Over the last decade, the Maine Department of Transportation (MDOT) has been an aggressive pursuer of emerging and advanced concrete technologies. MDOT has incorporated the use of pozzolans and admixtures into mix designs in an effort to utilize the elevated durability characteristics of high-performance concrete (HPC) in their bridge inventory. In late 2002, MDOT and a local precast concrete producer discovered a mutual interest in using self-consolidating concrete (SCC) on a bridge project.
SCC is engineered to flow readily into place without segregation of the constituent materials; thereby, alleviating the difficulty of placing concrete in complex formwork or around congested patterns of reinforcing steel and prestressing strands. The result of using SCC can be a significant reduction in the vibration and finishing demands.
After reviewing their inventory of products typically used for bridge construction, MDOT decided to use SCC in the fabrication of some precast, prestressed concrete adjacent box beams. Normally, the beams are constructed in stages; cast the bottom slab first, install the void material and remaining reinforcing steel, and cast the remaining concrete. The delay between casting the bottom slab and the remaining concrete has a potential to cause cold joints in the beams. The complexity of the beam shape, amount of reinforcing steel, top strand layout, and required inserts can make this technique impractical. Therefore, the producer elected to place the voids and tie all reinforcing steel prior to casting the concrete. With this technique, concrete must flow under the void and consolidate around the prestressing strands and reinforcement in the bottom flange of the box with the aid of internal vibrators. Since the vibrators can only access the bottom flange through the thin web section on the sides of the void, there is a possibility of entrapping air voids. Ensuring proper placement of the bottom flange concrete is difficult and time consuming. For this reason, box beams were a perfect application for SCC.
MDOT chose the Ogunquit Beach Bridge project to showcase the use of SCC. This project consisted of a two-stage replacement and widening of an existing steel beam superstructure bridge. The replacement structure is comprised of thirty-three 32-in. (813-mm) deep by 48-in. (1219-mm) wide precast, prestressed concrete adjacent box beams. The bridge has three-spans of approximately 70, 72, and 70 ft (21.3, 21.9, and 21.3 m), 11 beam lines, and a 28-degree skew. At the time this project was identified for implementation of SCC, production was already underway on the box beams for use in the first stage of construction. Therefore, only the second stage beams were cast with SCC.
The concrete requirements for this project were a compressive strength of 6000 psi, (41 MPa) at 28 days, maximum watercementitious materials ratio of 0.40, and an air content of 5.5 to 7.5 percent. The SCC needed to meet the same requirements. Other details of the SCC mix, such as slump spread limits, visual consistency, and maximum mortar halo around the spread were agreed upon prior to production. The SCC used for the casting of the girders had a spread of 18 to 24 in. (460 to 610 mm) and an approximate unit weight of 143 lb/cu ft (2290 kg/cu m).
As with the introduction of all new technologies, there was an associated learning curve with the use of SCC. The involvement of the admixture supplier during the girder production significantly minimized the number of lessons learned resulting in the successful fabrication of 14 out of 15 box beams without defects.
After one beam was removed from the casting bed, a light sandblast revealed a line across the end of the beam, similar to a cold joint. Further investigation discovered a lack of bond across the line for the full width of the beam for a depth of 4 to 10 in. (100 to 250 mm). Many of the other beams had similar lines but not as pronounced as the one beam. Twelve cores taken from eight other beams revealed no lack of bond across the line. The extent of the problem was determined to be isolated to a single beam.
Further research in the use of SCC revealed that it is common to have lines that appear at the interface between successive lifts or layers of concrete placed into the forms. The lines and lack of cohesion, also called “folds” are a problematic characteristic of SCC. It is the result of the thixotropic behavior of SCC and is generally the result of improper placement techniques, material or formwork temperature differences, or time between consecutive casts. To avoid “folds,” it is important to proceed with the casting of SCC as continuously as possible. The single beam was determined to be repairable and a sealant was applied on the exposed face of the beam to prevent water and chloride intrusion.
The Ogunquit Beach Bridge is now open to traffic and is considered another successful implementation of advancing technology by the state.
Further Information
For further information, contact the second author at [email protected] or 207-624-3406.