Improved Concretes for Corrosion Resistance

DTFH61-93-C-0028A major cause of concrete deterioration on bridge structures is the corrosion of the embedded steel reinforcement. In response to the continued problem of corrosion, the Federal Highway Administration (FHWA) initiated this research aimed at (1) quantifying the corrosive conditions fostering concrete bridge deterioration and (2) identifying concrete materials which consistently provide superior performance when used in bridge applications. The experimental phase of this research project was divided into three tasks: Task A - Corrosive Environment Studies, Task B - Concrete Chemical and Physical Properties, and Task C - Long-Term Corrosion Performance. This Interim Report reviews the results of Tasks A and B and provides recommendations for performing Task C. In Task A, laboratory experiments were conducted to characterize the corrosive environment and to establish boundary conditions for environmental variables of moisture content, chloride concentration, and temperature. Special test specimen design and test procedures were developed to permit uniform chloride diffusion to the steel surface. A full factorial matrix of experiments was performed for three levels each of chloride concentration, relative humidity, and temperature. A regression model was developed to predict corrosion rate and corrosion potential as a function of environment for two different concretes. In Task B, experiments were performed to identify the chemical components of concretes and to determine how they effect corrosion induced deterioration of concrete structures. The dependent variables of interest in examining corrosion induced deterioration of concrete are corrosion rate, corrosion potential, chloride permeability, electrical resistivity, and physical properties. The independent concrete design mix variables examined included: water-cement ratio, air content, coarse aggregate type, fine aggregate type, mineral admixture, and cement type. Because of the large number of independent variables and the number of levels of interest for the variables, an optimized experimental design was developed to permit the estimate of the main-effect terms for each independent variable. Models were developed to predict the effect of the independent variables on corrosion rate and corrosion potential in each of two environments, chloride permeability, resistivity, and compressive strength. The data developed were used to make recommendations for the concretes to be tested in the Task C long-term experiments

Neutralization of Chloride in Concrete

DOT-FH-11-8133This study was concerned with one solution to the problem of deterioration of bridge decks due to corrosion of reinforcing steel caused by chloride from deicing salts. Flushing of salts by application of water to the underside of the concrete and electrochemical removal of the chlorides were investigated. Flushing was found to be ineffective and work on it was abandoned early. However, the electrochemical removal technique showed promise of success first in the laboratory and later in a small scale field trial. In the latter chloride present at the level of the top rebar mat was reduced to below the concentration that is known to cause corrosion in 24 hours of treatment, and active corrosion of the steel that was going on was completely stopped