Carbonation-induced Corrosion of Blended-cement Concrete Mix Designs for Highway Structures

Corrosion of concrete reinforcing steel in highway structures has traditionally been related to chloride ions. However, another cause of reinforcing steel corrosion is concrete carbonation which decreases the pH of the pore solution, Carbonation-induced corrosion tends to develop later and proceeds at slower rates than chloride-induced corrosion. Since very long service life goals are currently proposed for new highway structures, concrete carbonation is a likely concern when projecting durability. The objective of this investigation was to establish the prognosis for development of carbonation-induced corrosion of concrete mix designs proposed for new construction projects. Accelerated laboratory carbonation exposures to 60% relative humidity with 0.5% and 4% COZ were conducted with concrete mix designs containing from 20% to 50% pozzolanic cement replacement, 2 different water-tocementitious ratios, and with small levels (-2 kg/m”) of Clcontamination (non-contaminated specimens were also included). Both the carbonation and the electrochemical behavior were monitored as a function of time. Carbonation rates were found to be directly dependent on the pozzolanic content. Polarization Resistance measurements showed that steel corrosion rates at 60% RH were small where chloride contamination was not present but increased to -0.2 @Jcm* with moderate Clcontamination. Corrosion rates increased by over one order of magnitude for all mixes (except a low water-to-cementitious ratio, 20% pozzolanic cement replacement mix) during a post-carbonation high humidity exposure.