Corrosion in Reinforced Concrete Structures

Corrosion in Reinforced Concrete Structures

The report demonstrates the corrosion process in reinforced concrete structures. Corrosion is a gradual destruction of concrete, metal and steel structure generally caused by the chemical reactions. The chemical process of corrosion involves the combination of anode, cathode and electrolyte, and the presence of water and oxygen are the major agents of corrosion. Typically, rusting is one of the major damages caused by corrosion due to the electrochemical oxidation. In the last few decades, the costs of corrosion have reached an alarming rate where the estimated cost burden yearly is more than $174 billion. The report recommends several processes to repair corrosion. The repairing process includes coating systems, cathodic protection, electrochemical techniques, and corrosion inhibitors.

Introduction

Corrosion is a gradual destruction of concrete and steel structure generally caused by chemical reactions. Corrosion is caused by the electrochemical oxidation when metals are in contact with an oxidant such as oxygen, and rusting caused by the formation of iron oxides is a typical example of electro chemical corrosion that damages metal through oxides and salt. Generally, corrosion deteriorates useful structural properties and materials leading to the degradation in their appearance, strength, and permeability to liquids. In the United States, the increase in the number of deteriorated bridges between 1970s and 1990s alarmed the State Highway Agencies because they pose serious risks and safety hazards to motorists.

Corrosion is an inevitable process that occurs when refined metals are converted into form of carbonates, oxides and sulphides. The corrosion process generally occurs when metals are exposed to reactive environment generally lead to cost burden on organizations. In some countries, the cost burden can be substantial reaching as much as 3.5% of a country's GNP. The deterioration of steel structure is a major problem to both business organizations and government because the costs of replacing and repairing the steel structure have become an economic burden to both the Highway agencies, and local authorities. In the United States, the costs of repairing and replacing steel and concrete structures are estimated to reach $20 billion per year and increasing by $500 million yearly. The primary cause of deterioration of steel structure is corrosion generally caused by chlorides, and the two main sources of chlorides are seawater and deicing chemicals.

Highway agencies often face challenges to maintain bridges and other heavy construction projects due to the impact of corrosion. Approximately 80,000 federal aided constructed bridges and 130,000 non-federal aided bridges in the United States are deficient in some way due to the effect of corrosion. (U.S. Department of Transportation, 2000).

Fundamental objective of this report is to investigate the corrosion process in reinforced concrete structures. The report provides the corrosion process to enhance greater understanding on the method corrosion happen to metal and concrete structure.

Corrosion Process

Corrosion process occurs when electrochemical process occurs with the reaction of electric current. Corrosion may occur through various mechanisms, which include:

Corrosion of reinforcement, due to:

Chloride ions

Carbonation

Impinging cracks due to change in the rebar environment

Sulphate attack of concrete.

Salt recrystallisation (exfoliation)

Soft water or acid attack of concrete.

AAR (Alkali Aggregate Reaction),

TICC (Thermal incompatibility of concrete components)

Shrinkage.

Frost Damage

Structural designers need to consider these factors during design and specification of steel and concrete structure.

On the other hand, corrosion process occurs on steel through three stages:

Anode

Cathode

Electrolyte .

At anode level, irons lose electrons and become iron ions (Fe+2) and caused the oxidation reaction. At the cathode level, the water reacts with electrons to form hydroxyl ions (OH). On the other hand, electrolyte facilitates the flow of electric current (electrons) between anode and cathode. Essentially, both anodic and cathodic reactions are critical to develop a corrosion process, which generally happens concurrently. In the steel corrosion process, anode and cathode could be next to each other or could be separated. The corrosion is at microscopic scale when the anode and cathode are next to each other; however, the corrosion is at macroscopic scale, when the anode and cathode are separated. On the other hand, the corrosion process could be the combination of microcell and macrocell.

Environmental factors also have a strong impact on the corrosion of concrete that surround the steel. The chloride could enter the concrete from the top surface because the top mats of the steel are generally exposed to higher concentration of chlorides. The corrosion process occurs on the concrete when the negative (anodic) top mat of concrete reacts with the positive cathodic of the top mat through electrolyte reaction. The result of the process leads to the galvanic type of corrosion leading to the macro cell corrosion process. The electric circuit is established through the electrolyte, wire ties, steel bars and metal supports of the concrete.

The corrosion also occurs when there is an interaction of oxygen, water, and aggressive ions such as chloride. Generally, the intrusion of chloride ions is the most important factors that lead to the corrosion of steel embedded concrete, and the chloride ions will be able to penetrate the concrete steel through crack. Theories support the impact of the chloride ions on steel. The absorption theory argues that chloride ions are absorbed into the steel directly and attack the surface of the steel. Transitory complex theory also argues that chloride ions act as catalyst because there is a combination of chloride ions with ferrous ion to form chloride ions complex leading to the diffusion of anode.

Carbon dioxide (CO2) could also lead to corrosion, and corrosion occurs when carbon dioxide penetrates into the concrete and dissolves in the pore solution thereby forming carbonic acid. The acidic reaction with alkali form carbonates and lowers the pH level of the steel concrete. Typically, when alkalinity of the structure reaches low level, the steel bar depassivated with the present of water and oxygen.

The process of corrosion on steel in the presence of oxygen without chlorides takes the following process:

1.

At anode level, iron is oxidized to the ferrous state, which releases electrons.

Fe ( Fe++ + 2e-

2.

These electrons then migrate into the cathode and combine with oxygen and water to form

Hydroxyl ions.

2e- + H2O + 1/2O2 ( 2OH-

3.

The hydroxyl ions then combine with the ferrous ions and form ferrous hydroxide.

Fe++ + 2OH-( Fe (OH) 2

4.

With the presence of oxygen and water, the ferrous hydroxide is further oxidized and forms Fe2O3.

Fe (OH)2 + O2 + H2O ( 4Fe (OH)3

Fe (OH)3 . Fe2O3 ( 2H2O

On the other hand, "The corrosion of steel in concrete in the presence of chlorides, but with no oxygen (at the anode), takes place in several steps": (U.S. Department of Transportation, 2000 P. 7).

1. At the anode level, iron reacts with chloride ions in order to form an intermediate soluble iron- chloride complex.

Fe + 2Cl-( (Fe++ + 2Cl-) + 2e-

2.

Moreover, when the iron-chloride diffuses from the higher pH and oxygen concentration, the chloride ions reacts with hydroxyl ions to form Fe (OH) 2, and the Fe (OH) 2 reacts with water to form ferrous hydroxide as being revealed in the equation below:

(Fe++ + 2Cl-) + 2H2O + 2e- Fe (OH) 2 + 2H+ + 2Cl-

More importantly, the deterioration of chloride-induced reinforced concrete occurs at four stages:

1.

Corrosion initiation and corrosion propagation

2.

Cracking - Occurs when the concrete tensile stresses exceeds the tensile strength.

3.

Delamination -- This occurs when there are cracks on the roadway surface leading to a fracture plane.

4.

Spalling -- Occurs when inclined cracks hit the roadway surface, the traffic loading normally cause the cracked delaminated portions to delineate from the structure.

Factors influencing the corrosion rate on steel reinforcing bars are as follows:

Presence of oxygen and water,

Ratio of the anode and cathode on a steel surface,

Amount of chloride ions present in the pore water,

Resistivity of the concrete,

Temperature,

Both internal and external relative humidity,

Concrete microstructure.

When concrete is dry, oxygen can freely move in the concrete through spores, which accelerates the corrosion process. Moreover, presence of water could accelerate the corrosion process because the concrete and steel that have been embedded in water generally has lower resistivity to corrosion process than a dry concrete. More importantly, the increase in the ratio area of cathode to the ratio area of anode could affect the corrosion process, and this leads to a current density in the amount of electrical current passing through the anode unit area. An increase in the current density leads to an increase in the corrosion rate. Pore water chemistry such as ionic strength, pH, Redox potential and cation composition could also influence the corrosion process.

The corrosion damage has led to the deterioration of many steel and concrete. The damage caused by the corrosion has the cost burden on the appropriate authorities.

3. Corrosion Damage

Corrosion damage is inevitable in the steel and concrete structure because there is no amount of "organic coating that can withstand the extreme combination of constant wetting and high temperature and high humidity that reinforcing steel is exposed to in some marine environments." (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

CATEGORY

INDUSTRY SECTORS

ESTIMATED DIRECT COST PER SECTOR

$ billion

Percentage

Infrastructure (16.4% of total)

Highway Bridges

8.3

37

Gas & Liquid Transmission Pipeline

7.0

27

Waterway and Ports

0.3

1

Hazardous Material Storage

7.0

31

Airport

Railroads

SUBTOTAL

$22.6

Utilities (34.7% of total)

Gas Distribution

5.0

10

Drinking Water & Sewer System

36.0

75

Electrical Utilities

6.9

14

Telecommunications

SUBTOTAL

$47.9

Transportation (21.5% of total)

Motor Vehicles

23.4

79

Ships

2.7

9

Aircraft

2.2

7

Railroad Cars

0.5

2

Hazardous Materials Transport

0.9

3

SUBTOTAL

$29.7

Production & Manufacturing (12.8% of total)

Oil & Gas Exploration and Production

1.4

8

Mining

0.1

1

Petroleum Refining

3.7

21

Chemical, Petrochemical & Pharmaceutical

1.7

10

Pulp and Paper

6.0

34

Agricultural

1.1

6

Food Processing

2.1

12

Electronics

Home Appliances

1.5

9

SUBTOTAL

$17.6

Government (16.6% of total)

Defense

20.0

99.5

Nuclear Waste Storage

0.1

0.5

SUBTOTAL

$20.1

TOTAL

$137.9

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.