Rodger D. Rochelle, North Carolina Department of Transportation
HPC is rapidly gaining prominence in highway bridge construction because of the advantages of higher strength and greater durability. Unfortunately, the concept of designing for durability is more elusive than the quest for high strength. Bridge designs often include the 100-year flood, a 475-year seismic event return-period, or perhaps a Method II vessel impact analysis, all of which target a probabilistic service life. Similarly, the design should satisfy a 100-year service life when concrete is exposed to a chloride environment.
This approach has broadened the bridge corrosion protection policy in North Carolina. Unfortunately, due to the heterogeneity of concrete, arduous numerical analyses are required to predict the rate of chloride ingress within a concrete structure. In practice, such analyses are not feasible. Instead, comparative studies serve to evaluate the array of corrosion mitigation measures available with HPC. Such an investigation is conducted for North Carolina’s major coastal structures, targeting a service life of 100 years. Fick’s Second Law of Diffusion is modeled to optimize the durability design by examining each structural element independently. Various applications of this law are used to predict the service life provided by different protection measures.
Protection measures may be categorized in three ways. Physical systems enhance durability with tangible, physical barriers to chloride penetration. They commonly include increased concrete cover and epoxy-coated reinforcing steel. Passive systems act to slow down chloride ingress by decreasing the concrete’s permeability and typically include the use of fly ash, microsilica, or ground granulated blast furnace slag (GGBFS). Finally, active systems, in the form of corrosion inhibitors, strive to chemically elevate the corrosion threshold of the reinforcing steel. Fick’s Law encompasses each of these systems, albeit with varying degrees of convenience. The model also differentiates the chloride load and loading rate among structural elements. For instance, the splash zone piles may be subjected to a direct, immediate 20 lb/cu yd (12 kg/cu m) chloride load whereas a bridge deck may experience a maximum chloride load of 5 lb/cu yd (3 kg/cu m) deposited over several years.
Chloride loads are first determined by generating chloride profiles from neighboring structures. In coastal regions of North Carolina, surface chloride concentrations range from 5 to 23 lb/cu yd (3 to 14 kg/cu m). A preliminary durability model is then created incorporating concrete cover, water-cementitious materials ratio, and epoxy-coated reinforcing steel. Next, the model is expanded to include passive and active corrosion mitigation systems as necessary. The threshold chloride concentration is incrementally adjusted to reflect the presence of corrosion inhibitors while the concrete permeability is reduced according to the presence of mineral admixtures. A reduction in concrete permeability is based on a combination of results from AASHTO T277* testing, existing chloride profiles, and literature review.
The first structure designed using this procedure was the 5-mile (8-km), $94 million bridge to the Outer Banks over the Croatan Sound, estimated to be completed in December 2001. The highly corrosive environment of the Sound has a variable chloride content in the water ranging up to 13,000 ppm. The structure contains approximately 190,000 cu yd (145,000 cu m) of concrete, the vast majority of which includes three levels of corrosion protection. Each type of structural element was analyzed independently such that, theoretically, all members begin to corrode simultaneously. Numerous levels of calcium nitrite, chloride load, and levels of concrete permeability were considered. Hundreds of possible options were pared down to the most cost-effective treatment schemes for each element. Among these options, constructibility requirements were addressed to further refine the schedule for corrosion mitigation techniques, resulting in the prescription for corrosion mitigation measures summarized below.
Calcium nitrite is used throughout the structure to elevate the corrosion threshold of all members. Microsilica is mandated in elements in which low permeability is required at an early age. Class F fly ash is used to reduce permeability in both the substructure and superstructure. Higher amounts of fly ash are incorporated into the pile caps to reduce the heat of hydration in these mass concrete elements. GGBFS is allowed as an alternate to fly ash in all precast members. Epoxy-coated reinforcing steel is used throughout the structure and concrete cover is greater in all substructure elements. The water-cementitious materials ratio is limited to 0.40 and 0.43 for precast, prestressed concrete and cast-in-place concrete, respectively, and all precast, prestressed concrete members are designed for zero tensile stress under full service loads.
*Standard Method of Test for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.
The durability design procedure culminates in a simple and direct two page prescriptive specification. This specification is a notable departure from the original intent for specifying HPC in North Carolina bridges. In fact, AASHTO T277 research was initiated to generate criteria for a performance-based specification. However, the research was remarkably conclusive, rendering the performance specification obsolete. Furthermore, the prescriptive specification arguably avoids surcharges buried within the bid to cover the unknown cost of developing and testing mixes to satisfy a strict performance criterion, and eliminates concerns about the variability of the AASHTO T277 test method.
Further Information
For further information, contact the author at [email protected] or 919-250-4048. For a discussion of Fick’s Law, see Amey, S. L., Johnson, D. A., Miltenberger, M. A., and Farzam, H., “Predicting the Service Life of Concrete Marine Structures: An Environmental Methodology,” ACI Structural Journal, Vol. 95, No. 2, March-April 1998, pp. 205 – 214.