Airport project falls into extremely important public infrastructure projects where durability and safety are of most important. Airport areas are prone to heavy localized corrosion such as pitting and crevice corrosion, the factors other than regular corrodants present everywhere excess corrosion due to exhaust emissions from the aircraft jet engines and high sulfur compounds present in such environment. Concrete structure in such areas suffer from severe rebar corrosion leading to issues regarding durability, public safety , loss of productive time and recurring maintenance cost. Looking to size of public infrastructures in general the amount of money needed to correct this problem is staggering especially considering the current state of economy.
the deterioration of a material or its properties due to a reaction of that material with its chemical environment – has been with us forever. People have recognized, accepted, coped with and, occasionally, battled corrosion for millennia. In the 19th century, we began taking steps to understand, prevent, and treat corrosion, and we have gradually expanded these efforts ever since. But recently, corrosion has become a major concern, partly because our demands for more complex and sophisticated systems and products have been satisfied by materials that are more susceptible to corrosion. The insidious and pervasive effects of corrosion have now reached the point where it is a major cost for our economy and quality of life – in fact recent studies estimate the direct cost of corrosion in the United States to be nearly $300 billion dollars per year.
The problem is caused primarily by inorganic-salt induced corrosion of steel in concrete. The salt, primarily chloride, penetrates the concrete from sources such as sea exposure. It can also be built in through the use of salt-contaminated aggregate, seawater in the concrete, or chloride-based admixtures.
The chloride ion initiates and catalyzes the corrosion reaction. The iron corrosion products resulting from the reaction occupy a much greater volume than iron and cause tremendous pressure on the concrete. The pressure causes the concrete to crack and spall, allowing even greater access of corrodents to the steel and accelerated deterioration of the structure.
Mechanism of Reinforcing Steel Corrosion in Concrete
The traditional view of the reinforced concrete structure is that the concrete is protective to the reinforcing steel bars through the combined effects of the chemical reactions between the steel and the cement hydration products and the environmental barrier provided by the concrete cover. If these conditions are maintained within the concrete mass, the steel bars do not corrode and the structure should have the expected trouble-free life span
Poor quality reinforced concrete structure contributes to a faster deterioration of the steel reinforcing bars. Low degree of compaction, excess water in the concrete mix, and the hydration process are considered the main factors to create voids within the concrete and make the concrete structure porous.
Porosity of concrete allows penetration and ingress of aggressive elements (e.g., chloride, oxygen, carbon dioxide, and other materials that vary from one location to another) to the embedded steel rebar and to initiate corrosion.
The primary factors controlling the initiation of the steel corrosion and its mechanism in concrete are summarized in the following points:
- The rate of steel depassivation
- The initiation of the macrocells due to the differential aeration and chloride absorption
- The low resistivity attributed by the concrete pore water
- The presence of oxygen to accelerate the corrosion process
The corrosion of steel in concrete is an electrochemical process, which results in the formation of a corrosion cell. The following corrosion mechanism is the most likely for steel rebar embedded in the concrete when significant variations exist in the surface characteristics of the steel. The steel surface initiates cathodes and anodes electrically connected through the body of the steel bar. The “half cell reaction” takes place, by inducing an electromotive force known as standard redox potential when the metal is connected to a hydrogen electrode – see Equation 1.
For iron: Fe –à Fe+2 + 2 e – (Anode)
The electrons liberated at the anode migrate to the cathode and react in various ways dependant upon the pH value and the availability of oxygen. See Equation 2, Equation 3, and Equation 4.
2e + 2H + ½ O2 ——à H2O
2e + H2O + ½ O2 —–à H2O
2e + 2H —————- à H2
The anodic and cathodic reactions are autocatalytic and result in the transformation of metallic iron (Fe) to rust. The rust formation is accompanied by a significant increase in the volume, suggested as large as seven times that of the original Fe volume. The volume increase causes concrete cracking and spalling.
Effect of Chloride Ions
When the steel is placed in a highly alkaline solution (pH >11.5), even in the presence of oxygen, corrosion will not be initiated. In fact, slightly rusted bars will be dissipated when placed in strong alkali. That is the reason why, during construction, slightly rusted steel bars do not create a concern.
The chloride ions ingress does not lower the pH in the concrete. However, it destroys the passive layer on the steel bars. The depassivated steel bars do not corrode in the presence of the chloride ions only. The corrosion occurs after the presence of the carbon dioxide lowers the pH below 11, thus contributing to corrosion initiation.
Sources of chloride are either in the concrete mix, mainly from the sand, aggregates, or the water used, or as chloride ingress from the environment, such as in the marine atmospheric environment.
Effect of Carbonation
Carbonation is the alkalinity loss in the concrete mass. The product of the reaction between carbon dioxide in the normal outside air and the alkaline products, mainly the calcium hydroxides, is calcium carbonate. In case of high water/concrete ratio, carbonation continues to the depth where the reinforcing steel bar is embedded.
When carbon dioxide penetrates through the concrete cover in the presence of water in the pores, it drives the pH to lower values which depassivates the steel
Other hydration products in the cement can go through the same reaction with carbon dioxide causing a significant quality loss of the cement and faster deterioration of the concrete mix.
Effect of other Elements
Sulfide can be found in the cement as a contaminant (more than 0.2%). The sulfide ion has been found more destructive to the steel rebar embedded in the concrete if it goes higher than the regulated percentage shown. Regardless of the sulfide ion source, it has been the cause of several cases of hydrogen embrittlement – particularly in pre-stressed rebar.
Mechanism of FBE coated steel corrosion in concrete
Corrosion control of the FBE coating is a function of the coating’s ability to provide a barrier against water, oxygen, chloride, and other aggressive elements that prevents permeation through the coating film to attack the metal substrate. There are critical properties required for corrosion protection FBE coatings that include adhesion and wetting ability to the rebar.
Epoxy coatings significantly reduce the corrosion rates of reinforcing steel. Epoxy-coated reinforcing steel maintains low initial and life-cycle costs over a 75-year life-cycle and use of epoxy-coated
reinforcing steel was found to be substantially more cost-effective than either using uncoated reinforcing steel in concrete containing corrosion inhibitors or stainless-steel reinforcing.
Epoxy coatings are the workhorses of the protective coatings industry. They have excellent chemical and
corrosion resistance, high mechanical strength, good adhesion to a variety of substrates and a
combination of other properties that have made them a material of choice for providing cost effective, long term protection on industrial, marine and offshore structures.
Epoxy Bar Use
2nd most common strategy to prevent reinforcement corrosion
–Following increased concrete cover
- 850,000,000 ft2 of decks
–>70,000 bridges in the US alone
–>600,000 ton/yr or 10 – 15% of all rebar in NA
- USA, Canada, Middle East, Japan, and India
Apart from properties listed above advantages to airport projects are as follows:
- Enhanced durability & life span of concrete structure at low Life cycle cost.
- Reduction in recurring cost of maintenance.
- Enhance public safety an availability of productive assets.
- AS FBEC is perfect barrier film it provides one more advantages from EMI- interferences in operation.