" (U.S. Department of Transportation, 2000 P. 3). In the United States, the average bridge deck in a snow-belt state generally shows the spalling shortly after 7 to 10 years of construction, and requires rehabilitation after 20 years of construction. Typically, the repair of the damaged caused by corrosion is invariably expensive. A crack is one of the major damage caused by corrosion. Although structural cracks could be attributed to pure tension, torsion, pure bending, bond failure, and concentrated load, however, non-structural crack is attributed to the chemical process that occurs within the concrete or steel structure, and the damage include shrinkage, expansion process, and thermal movement.
Mackechnie & Alexande, (2001) argue that corrosion could lead to a distress in a concrete capable of causing spanning and cracking to the surrounding concrete. The expansion associated to hydrated oxides is that the steel may swell ten times of its original position leading to the red or brown rust. Typically, rusting is major corrosion damage on steel, and the red or brown rust is caused due to the oxygen concentration. On the other hand, black rust is formed under the low oxygen concentration and form a hard and dense layer, and this may be difficult to remove from the steel.
Metal dusting is another major cause of corrosion, which occurs when steel structures are exposed to environment coupled with carbon activities such as carbon dioxide and other synthesis gas. The corrosion is manifested through this reaction, which leads to a break up of metal to metal powder.
Zuo, Ornek, Syrett, et al. (2004) argue that high-temperature corrosion could lead to deterioration of metallic material, and the non-galvanic form corrosion can occur when a steel or metal is subjected to hot temperature that contain sulfur, oxygen or other compound capable of oxidizing metallic material. MIC (microbiologically influenced corrosion) could cause a sulfide stress cracking on both metallic and non-metallic material. Typically, some bacteria could oxidize sulfur leading to the production of sulfuric acid and the chemical process leads to biogenic sulfide corrosion. ALWC (Accelerated Low Water Corrosion) is a particularly aggressive form of corrosion that can damage the steel piles in the low water tide. The corrosion rate of this type is very high and could lead to premature failure of steel pile. (McGraw-Hill, 2012).
Roberge (2008) reveals the estimated cost of corrosion to the U.S. economy. The total direct costs due to the impact of corrosion is estimated to reach $137.9 billion yearly. The table 1 provides the overall direct costs of corrosion on different sectors in the United States.
Table 1: Estimated Direct Cost of Corrosion
ESTIMATED DIRECT COST PER SECTOR
Infrastructure (16.4% of total)
Gas & Liquid Transmission Pipeline
Waterway and Ports
Hazardous Material Storage
Utilities (34.7% of total)
Drinking Water & Sewer System
Transportation (21.5% of total)
Hazardous Materials Transport
Production & Manufacturing (12.8% of total)
Oil & Gas Exploration and Production
Chemical, Petrochemical & Pharmaceutical
Pulp and Paper
Government (16.6% of total)
Nuclear Waste Storage
However, "by estimating the percentage of U.S. gross national product (GNP) for the sectors for which corrosion costs were determined and by extrapolating the figures to the entire U.S. economy, a total cost of corrosion of $276 billion was estimated. This value shows that the impact of corrosion is approximately 3.1% of United States' GNP. This cost is considered a conservative estimate since only well-documented costs were used in the study. The indirect cost of corrosion was conservatively estimated to be equal to the direct cost, giving a total direct plus indirect cost of $552 billion or 6% of the GNP." (Koch, Brongers, Thompson, et al. 2001 P. 5).
Significant damages caused by corrosion necessitate the report to measure and monitor the strategies to address the impact of corrosion. The next section discuses corrosion monitoring and measures.
4. Corrosion Monitoring
"The term monitoring covers a range of options. In its simplest form it involves turning up on site and looking at the structure." (Atkins, Brueckner, & MacDonald, 2013 P. 2). It is essential to realize that corrosion-induced deterioration occurs when the loading of the structure is greater than the structure ability to resist the loading. An effective method to monitor this type of corrosion is to increase the resistance, decrease the loading, or implement both techniques.
Moreover, corrosion is also likely to occur due to the deterioration process such as expansive reactions, fatigue, excessive deflections, and freeze-thaw cycles. This process can make the concrete to crack, which allows chloride and water to get access to the interior of the steel and structure. "The factors that influence the corrosion of steel reinforcing bars embedded in concrete are the amount of chloride ions at the steel level, the resistivity of the concrete, temperature, relative humidity (both internal and external), and the concrete microstructure." (Lambert, & MacDonald, 2013 P. 14).
Mechanical or electrochemical process is a strategy to control this type of corrosion. Mechanical method is to install physical barriers to delay or prevent the ingress of moisture, chlorides and oxygen from entering reinforcing steel. Mechanical method to prevent corrosion on steel reinforcing bars includes sealers, admixtures, overlays, membranes, and coatings. Moreover, membranes and sealers produced with materials such as epoxies, resins, and emulsions are used to reduce or prevent the ingress of deleterious species.
Additionally, the durability and effectiveness of bridge decks or traffic-bearing surface could be achieved by using Portland cement concrete, latex-modified concrete, silica fume-modified concrete, polymer concrete overlays are and low-slump dense concrete. Organic or metallic coating could be used to protect reinforcing bars from corrosion. "Organic coatings that include a non-metallic fusion-bonded epoxy coating." (Lambert, & MacDonald, 2013 P. 14).
Metallic coating includes material such as stainless steel, zinc, and nickel to be used to prevent corrosion. The stainless steel and nickel also protect steel from corrosion. Corrosion-resistant materials, which include FRP (fiber-reinforced polymer), and austenitic stainless steels are other mechanical methods to protect steel from corrosion.
Electrochemical method includes the use of cathodic protection and chloride ion extraction could be used for rehabilitation of reinforced concrete. Material variables to prevent corrosion are to use a durable concrete, which include cement type, gradation, aggregate type, and the water-cement ratio. Design variables could also be used to monitor the corrosion, and the design variable to monitor the corrosion includes:
depth of concrete cover,
"physical properties of the hardened concrete," the spacing and the size of the steel. (Lambert, & MacDonald, 2013 P. 14).
Atkins, et al. (2013) argue that it is possible to detect corrosion by taking sample of the concrete using hammer drill and 12.16 mm diameter hole. The dust coming out from the concrete are collected and analyze to detect the presence of the chloride.
5. Repair Strategies (Source #7)
Mackechnie & Alexande (2001) argue that numerous repairing options are used for the corrosive concrete. Patch repairs is to repair damage done by the carbonation-induced corrosion. Localized low cover is the most successful form of repairs on the corroded reinforcement. Patch repair is as follows:
removal of delaminated and cracked concrete to expose the corroded reinforcement, clean the corroded reinforcement and apply protective coating such as zincrich primer coat or anti-corrosion epoxy coating to the steel surface, applying micro-concrete or repair mortar to replace the damaged concrete, possible application of sealant or coating to the entire concrete surface in order to reduce moisture levels within the concrete.
Coating systems are also beneficial on concrete structure. Application of hydrophobic coating could be used to reduce the moisture content in the concrete. Typically, hydrophobic coating repels water molecules from the concrete to facilitate drying.
"Hydrophobic coatings using silanes and siloxanes are generally most effective on uncontaminated concrete, free from cracks and surface defects. The feasibility of such an approach is questionable for marine structures where high ambient humidity, capillary suction effects and presence of high salt concentrations all interfere with drying." (Mackechnie & Alexande, 2001 P. 20).
Corrosion inhibitors are also used to reduce corrosion of metal. Corrosion inhibitors assist in reducing cathodic or anodic reactions and thereby suppress the overall corrosion rate on structure.
Electrochemical techniques are also used to reduce alkalinity present in the structure. Typically, realkalization and electrochemical chloride removal are used to restore the concrete region. Using ECR (Electrochemical chloride removal), the chlorides are removed by applying direct current between an electrode and the reinforcement.