Douglas L. Edwards, Federal Highway Administration
The era of high performance concrete (HPC) in Florida bridges actually began following a violent storm in Tampa Bay in 1980. A freighter crashed into one of the main piers of the Sunshine Skyway Bridge, causing collapse of a truss span and the deaths of 35 people. The replacement bridge, opened in 1987 and built in the corrosive waters of Tampa Bay, required more than 221,000 cu yd (169,000 cu m) of concrete. This project marked a turning point with respect to the use of high-quality concrete by the Florida Department of Transportation (FDOT).
During the 1970s, the FDOT became increasingly aware of structural concrete deterioration, especially along Florida’s 1200 miles (1930 km) of coastline and intra-coastal waterways. In response, FDOT undertook to define areas with environments of similar corrosive aggressiveness within the State. In 1981, this effort resulted in the publication of “Corrosion Maps” showing three levels of environmental aggressiveness based upon criteria for pH value, resistivity, sulfate concentration, and chloride concentration.
Much concrete research was conducted by the FDOT during the 1970s. This research indicated that the addition of fly ash benefited a concrete structure in three ways:
• Improved corrosion protection
• Improved sulfate resistance
• Reduced heat of hydration
When the new Skyway Bridge was being planned, an expert board of concrete technology consultants was assembled to study the concrete durability problems in Tampa Bay. This group advised the State that all concrete used in the structure should contain fly ash. When the new bridge construction began in 1982, fly ash was officially required. This initiated an intensive effort in Florida to incorporate fly ash in structural concrete.
Following the Skyway project, FDOT began development of a standard concrete specification to incorporate many of the durability features utilized on this unique structure. During this period, corrosion was detected in the substructures of several relatively new bridges in the Florida Keys. These bridges were built with conventional concrete and epoxycoated reinforcement. This created an urgent need for an alternative means of corrosion protection. A wide range of HPC mixtures was produced and tested to establish optimum mix designs for durability. This effort led to progressive refinements in the FDOT concrete construction specifications, corrosion classification parameters, and corrosion protection design procedures and requirements.
Since 1985, all proposed FDOT bridge sites have been required to have soil and/or water testing performed. One of three corrosion environments is then assigned to each bridge component. These environmental classifications then establish steel reinforcement cover, and in conjunction with strength requirements, the FDOT concrete class to be used. Moderate and extremely aggressive environments currently require the use of fly ash or ground granulated blast furnace slag with specified minimum concrete compressive strengths ranging from 5500 psi (38 MPa) to 8500 psi (59 MPa). Type II cement is specified for extremely aggressive environments. When this classification is due to chlorides in the water, calcium nitrite and silica fume are specified for specific structural elements. When silica fume is specified, the rapid chloride permeability is limited to a maximum value of 1000 coulombs. The FDOT corrosion specialists predict that these mixes will provide a minimum design life of 75 years in Florida’s severe marine environments.
To date, the FDOT has focused its research on the durability aspects of HPC since this has a far greater economic impact on their program than increased strength. Research is providing a better understanding of the effects of cold joints and cracks on reinforcement corrosion. As greater knowledge is gained in the fields of concrete materials and admixtures, corrosion monitoring, and alternative corrosion protection methods, it is expected that Florida’s HPC criteria will continue to evolve.