Optimization of Concretes and Repair Materials for Corrosion Resistance

DTFH61-93-C-00028A major cause of this concrete deterioration (cracking, delamination, and spalling) is the corrosion of embedded steel reinforcement, initiated by chloride ions from deicing salts and salt water spray that have penetrated the concrete cover. The research was structured to address the three principal rate phenomena that control corrosion-induced deterioration of concrete bridge components: (1) chloride permeation rate, (2) corrosion rate of the steel bar, and (3) deterioration/damage rate. Low chloride permeability is critical to achieving the desired life of a concrete structure. A predictive model was developed to determine the effect of concrete properties on chloride permeability. The concrete properties that had a significant effect were in order of greatest effect: mineral admixture, coarse aggregate, water-cement ratio, cement chemistry, and fine aggregate. The mineral admixture had nearly twice the effect of other concrete variables on the chloride permeability. The addition of any of the mineral admixtures tested tended to decrease the chloride permeability. Silica fume was the most effective. Corrosion rate is dependent on the environmental variables temperature, relative humidity, and chloride concentration and, over the range tested, each variable is equally important in establishing the corrosion rate of steel. Concrete mix designs also have a significant influence on corrosion resistance of a concrete structure. A predictive model was developed to estimate the corrosion of steel as a function of concrete mix design. The concrete mix components that had a significant effect on corrosion behavior were, in order of greatest effect: mineral admixture, fine aggregate, water-cement ratio, cement chemistry, coarse aggregate, and air content. The mineral admixture had nearly twice the effect of other concrete mix variables on the corrosion behavior of steel. Only silica fume decreased the corrosion rate to lower values than having no mineral admixture. Based on a limited data sample, the rate of damage (cracking/spalling) is related to the amount of corrosion product that can diffuse into the concrete matrix (i.e., the less diffusion into the concrete matrix, the greater the rate of damage). A good correlation was shown between cumulative corrosion prior to cracking and modulus of elasticity of the concrete. A six-step optimization model was proposed for predicting the life of a concrete structure and permitting economic analysis and optimization of resources. The model is based on prediction models developed in this research and requires input of the following by the bridge designer: (1) concrete mix components/parameters, (2) climatic conditions, and (3) frequency of salt applications

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