Kevin Pruski, Texas Department of Transportation
In June 2003, the Texas Department of Transportation (TxDOT) awarded a contract to replace two parallel structures connecting Galveston Island with the Texas mainland. The northbound bridge built in 1938 and the southbound bridge built in 1959 are being replaced because of deterioration from chloride-induced corrosion.
The new bridges include precast, prestressed concrete beams in the approach spans and a cast-in-place concrete segmental box girder with a main span of 350 ft (107 m). The designers used generous structural proportioning, increased concrete cover to the reinforcement, and high performance concrete (HPC) as primary tools to achieve the goal of a 100-year service life for the new causeway. TxDOT specified the use of supplementary cementitious materials (SCM) to create durable concrete for the project. TxDOT also specified lowering the water-cementitious materials ratio (w/cm) for the concrete and the use of a calcium nitrite corrosion inhibitor for the precast, prestressed concrete beams.
Structural Considerations
The durability of concrete structures can be enhanced considerably by reducing the probability of significant crack development. With this in mind, the designers of the structural components appropriately sacrificed efficiency by using a low design stress in the reinforcing steel to achieve reduced crack widths. A maximum working stress limit of 22 ksi (152 MPa) was used for design purposes.
The thickness of concrete cover protecting the reinforcing steel was specifically addressed. For the members near the splash zone, the concrete cover was 4 in. (100 mm), double what is typically used. Practical issues with formwork also had an impact on cover. The standard configuration of strands in the precast, prestressed concrete beams was modified to obtain a minimum cover of 2.75 in. (70 mm) to the steel.
Concrete Requirements
TxDOT used prescriptive concrete mix requirements to provide specific characteristics and properties considering the environment, locations in the structure, and design of the structural element. Class F fly ash up to 30 percent of the cementitious materials was the primary SCM required in the concrete. Class F fly ash was chosen because of its availability and to improve resistance to sulfate attack and alkali-silica reactivity, to lower the concrete permeability, and to lower the heat of hydration.
For the columns, pier caps, and segmental superstructure, a requirement to include either silica fume or an ultra-fine Class F fly ash up to 7 percent of the cementitious materials was added. The contractor chose to use the ultra-fine fly ash.
The concrete for the precast, prestressed concrete beams was required to include either Class C or Class F fly ash in the mix. Class C fly ash was allowed because the concern for sulfate attack on the elevated beams was not a factor. In addition, using Class C fly ash typically does not have the same negative impact on strength gain as occurs with a Class F fly ash. A calcium nitrite corrosion inhibitor at a rate of 3 gal/cu yd (15 L/cu m) was required in the precast, prestressed concrete beams.
To get this project completed rapidly, TxDOT added a $20,000 per day incentive for early completion or penalty for late finish. Concerns were expressed that requiring the use of Class F fly ash at prescribed rates could delay progress of the work to allow time for strength gain if very tight controls were not used at the batch plant. Consequently, contractors would increase bid prices to account for this uncertainty. Therefore, TxDOT provided a performance-based option to address concrete permeability. The contractor chose the prescriptive requirements.
Construction Provisions
Many of the bridge elements are designated as mass concrete. This designation puts a maximum placement temperature of 75°F (24°C) on fresh concrete and a maximum temperature differential of 35°F (19°C) on in-place concrete. Additionally, TxDOT placed a general note in the plans requiring the contractor to keep all forms in place for 4 days before removal. This is believed to reduce the risk of thermal shock and subsequent cracking that can occur when the forms are removed while the concrete is at a high temperature. The contractor is required to submit a heat generation and dissipation plan.
Observations
Execution of the durability plan has brought about new challenges that are being addressed as they arise. The first large footing placement revealed that the mass concrete provisions were not being met. A compromise was reached on how to handle the situation, which resulted in the concrete mix design being changed. The allowable amount of Class F fly ash was increased to 40 percent of the cementitious materials, the maximum temperature differential within the member was increased to 50°F (28°C), and the maximum temperature in the concrete limited to 160°F (71°C).
The contractor chose to develop the mix designs using a low w/cm ratio. It was thought that the slow strength gain, resulting from the requirement to use SCM, would be reversed by using a low w/cm. A low w/cm is good for low permeability but there is concern that autogenous shrinkage may cause cracking. Some cracking has occurred, which prompted the requirement that all noticed cracks be sealed by epoxy injection. Perhaps the incentive to finish early negatively affected the plan to get the most durable structure.
Lessons Learned
The main lessons learned from this project are that a high performance concrete structure requires continuous attention throughout the project and the project team should be prepared to make adjustments as the need arises.