CORROSION FORECASTING FOR 75-YEAR DURABILITY DESIGN OF REINFORCED CONCRETE

This investigation addressed the prognosis for 75-year durability of the substructure of Florida Department of Transportation (FDOT) marine bridges constructed with promising concrete formulations. Thirteen bridges, most built with improved concrete formulations, were investigated to determine rate of chloride ion penetration and how it may be affected by preexisting stress cracks. Sound concrete made per recent FDOT specifications for high cement factor, low water cement ratio (w/c) and pozzolanic cement replacement exhibited very slow chloride penetration in aggressive marine bridge substructure service. The best performing concrete, having >752 lb/cu yd (446 kg/cu m) cementitious content, 20% fly ash cement replacement, and w/c ~0.32 showed an average chloride diffusivity ~0.01 sq in./y (~2 x 10 to the -9 power sq cm/sec) at age 11 years in the tidal and low elevation region. Thin (typical ~0.15 mm) stress cracks were found in many of the substructures examined. Many of these cracks in footers or piles reached down to the waterline and extended to at least the rebar depth. Crack incidences in the order of one crack every several meters of waterline perimeter were not uncommon. Even though the cracks were thin, there was substantial preferential chloride penetration immediately around the crack compared with the surrounding sound concrete in the splash evaporation zone. However, no clear indications of corrosion were observed in any of the crack locations examined. Numerical modeling indicates that even very thin preexisting cracks could substantially increase chloride penetration in the immediately surrounding concrete, and that corrosion if initiated could be locally severe. Experiments revealed that the amount of critical corrosion penetration needed to cause damage was greater when corrosion was localized than when corrosion was more uniform. An integrated corrosion initiation and propagation model for sound concrete was created that takes into account the concrete mixture proportions, rebar cover and size, and system geometry. Additional modeling revealed that rebar itself can act as an obstruction to the diffusional chloride flow, causing a local increase in concentration and considerable relative reduction in the projected time to corrosion initiation when the rebar cover is low or the critical chloride concentration high. Derating factors to account for this effect were computed and proposed for use. Further modeling indicated that the region immediately above high tide may be amenable to cathodic prevention of the passive steel with sacrificial anodes.