Michael J. Abrahams, Parsons Brinckerhoff Quade & Douglas, Inc.
The design of the bridge was challenging because of the need for a cost-competitive design, a ductile and relatively lightweight structure to satisfy the relatively high seismic demands, a structure strong enough to withstand ship collision and high wind forces, and a 100-year service life. For example, the towers of the cable-stayed bridge were designed with enough reinforcement to withstand hurricane-generated wind loads. Yet, the reinforcing steel in the towers was limited so that ductile hinges could form at the tower bases without requiring excessive amounts of reinforcing steel or generating excessive forces on the drilled shafts.
The 100-year service life was an important challenge. As this was a design-build project, it was necessary to develop the most economical plan that would be responsive to the project criteria. There were many options available, including the use of solid stainless steel reinforcement, but cost considerations in a competitive design-build environment did not favor this approach.
In addition, there was the need to demonstrate that the design would meet a 100-year service life. There was no code to follow, nor was there much literature on the subject. Available analytical models for service life were overly simplistic. There was little guidance available on how to quantify environmental effects at a particular site. For example, literature on airborne chlorides was limited to data on balconies. Thus, the design team was tasked with developing an appropriate analytical model and the appropriate environmental conditions to be used at the bridge site e.g. water salinity, annual amounts of chlorides applied to the deck, level of airborne chlorides, etc. Considerable judgment was needed. Particularly helpful in developing these data were measurements of chloride levels that had been collected by SCDOT on the adjacent 1929 Grace Memorial and 1956 Pearman Bridges.
The approach adopted was to utilize uncoated reinforcing steel, and to specify the required permeability, which, in combination with the assumed rate of chloride application and concrete cover specified in the design criteria would provide the 100-year service life.* In the splash zone, two alternatives were developed. One utilized concrete with a maximum permeability of 500 coulombs and a minimum concrete cover over the reinforcement of 4 in. (100 mm). The second used concrete with a maximum permeability value of 1400 coulombs and minimum a cover of 6 in. (150 mm).
The contractor then solicited quotes from local concrete suppliers for concrete that would meet the typical material and strength requirements as well as the project-specific permeability values. Two local suppliers used different approaches. One supplier used slag cement while the other supplier used Class F fly ash to achieve the required permeability.
In developing this approach, it was recognized that fly ash concrete does not achieve its full permeability for approximately one year and that conducting permeability tests at 28 days would not result in an economical mix with fly ash. Thus, the specification allowed the use of the accelerated curing method used by the Virginia DOT.
There were no design limitations due to the use of low permeability concrete. However, the more economical mix was the one using fly ash. This may not be the case in other locations where local material prices may dictate a different approach. A concrete sealer was applied to the elements in the splash zone to improve the concrete’s durability for the first year.
The use of performance-based specifications for determining concrete mix proportions is a departure from traditional bridge construction projects in the United States, where the tendency has been for each state highway department to prescribe the mix to be used for each portion of the structure. A performance-based approach may be a more objective and cost-effective way of developing long-lasting economical structures. If this approach is to be used, there is a need for both better analytical models that have been peer reviewed as well as site-specific environmental criteria. It is suggested that the latter may be developed by agencies on a regional or statewide basis, just as seismic and wind criteria have been developed based on local site conditions.
*See HPC Bridge Views, Issue No. 29.