Corrosion and Deterioration of RC Structure

Corrosion and Deterioration of RC structure 1
CORROSION AND DETERIORATION OF RC STRUCTURE
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Corrosion and Deterioration of RC structure 2
Corrosion and Deterioration of RC Structure
Summary
Structural concrete can deteriorate due to several factors including physical and chemical effects.
One of the major causes of concrete structures deterioration today is corrosion of reinforced
steel. Occurrence of corrosion results to structural weakening, which is caused by a decrease in
cross-section of the steel, concrete spalling, concrete cracking, surface staining, and concrete
delamination. The corrosion ends up reducing the reinforced concrete (RC) structure’s service
life. The main problem associated with the safety and structural soundness or integrity of RC
structures is the reduction of its load-bearing capacity. The main purpose of this project was to
investigate the extent of corrosion and deterioration of RC structure – a bridge girder, and to
establish potential factors that contributes to deterioration of bridges in general. This was
attained by carrying out several tests and investigations including non-destructive testing and
visual inspection of the bridge.
Corrosion of Steel Reinforcement in RC Structures
Corrosion of reinforcement has a significant effect on the durability of RC structures hence it is
worth being investigated. The damage of concrete caused by corrosion is among the leading
causes of decreased durability of RC structures. A study conducted by researchers worldwide
found that steel reinforcement rusting causes more than four-hundredth of structural failures
across the world.
Effect of Reinforcement Corrosion on Structural Soundness
There are two major damaging effects associated with corrosion of steel reinforcement of
concrete structures. These are:

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i) Corrosion produces rust that has volume equivalent to two to four times more than the
volume of steel. The increase in volume results to a corresponding increase in
concrete’s tensile stresses that causes cracking and spalling of concrete cover. The
loss of concrete cover reduces the load bearing capacity of the RC structure and also
further exposes the steel reinforcement to the harsh environmental agents.
ii) Corrosion causes reduction of steel’s cross sectional area. This reduction in cross
sectional area of steel makes the RC structure unable to support its design loads.
Therefore corrosion of steel reinforcement does not only affect the external appearance of RC
structures but also significantly affects their structural integrity, performance, functionality and
safety.
Corrosion Mechanism of Steel Reinforcement in Concrete
Corrosion Cell
Corrosion of steel reinforced in concrete is linked to electrochemical process. The process by
which the steel corrodes is apparently similar to the process that occurs in a flash battery. In this
scenario, the chemically corroded steel surface acts as a mixed conductor comprising of cathodes
and anodes that are electrically connected via the steel itself, resulting to various reactions. The
water pores that are present in concrete acts as aqueous medium. Al these create a corrosion cell
within the reinforcement, as shown in Figure 1 below

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Figure 1: Corrosion cell of steel reinforcement
Thermodynamics of Corrosion
Corrosion is understood to be an electrochemical process which takes place in the presence of
oxygen and water. Equation 1 and 2 below describes the key redox reactions of corrosion.
Equation 1 represents iron’s anodic oxidation whereas equation 2 represents oxygen’s cathodic
reduction. Equation 3 represents the general equation of corrosion, where Fe(OH)2 is just one of
the possible products that are produced during corrosion. The specific products produced depend
on the conditions present including oxygen, moisture, temperature and pH, among others.
Fe → Fe2+ + 2e- .................................................................... (1)
H2O + ½O2 + 2e- → 2OH .................................................... (2)
Fe + H2O + ½O2 → Fe(OH)2 ................................................ (3)
The state of steel fixed in untainted concrete is typically passive as a result of the pore solution’s
high alkalinity. This leads to formation of a passive film – an iron oxide layer that acts as a
protective layer. The thermodynamic fields of corrosion, passivity and immunity of iron and iron
oxide present in the solution are shown in Pourbaix chart shown in Figure 2 below. The dashed
lines in the chart shows the equilibrium potentials of iron. Hydrogen or oxygen reduction takes
place below the dashed lines.

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Figure 2: Pourbaix chart for iron at room temperature (25°C)
The diagram in Figure 2 above basically shows how passivation process of iron occurs in
aqueous solutions. The process starts with rapid increase of anodic current with increasing
potential with subsequent dissolution of the iron from the original free surface of iron oxide. The
current starts decreasing rapidly when the passive layer of iron starts growing. Within the passive
region, dissolution of iron can continue but at a low rate hence corrosion rate in this region is
negligible. Within the passive region, the iron is already under the cover of a very thin passive
layer (usually 1 to 5 nm). This film is usually composed of iron oxides (Fe3O4 and Υ-Fe2O3). The
reactions presented in equations 4 and 5 below show how the iron oxides are formed.
3Fe + 4H2O → Fe3O4 + 8H+ + 8e- ........................................................ (4)
2Fe + 3H2O → Fe2O3 + 6H+ + 6e- ......................................................... (5)
It is worth noting that since chemical composition of pore solution in concrete is more complex
than that of aqueous solutions, the exact microstructure and composition of the passive oxide
film created in the two is different.

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Pitting Corrosion
There are several researches that have been conducted on pitting corrosion but the manner in
which chloride ions contribute to pitting corrosion is yet to be understood fully. However
passivity breakdown is attributed to one of the following mechanisms: adsorption mechanism,
film breaking mechanism and penetration mechanism. The principle of penetration mechanism is
that the great potential difference in the passive layer causes penetration of the chloride ions
present in the electrolyte through the metal surface’s passive film. According to film breaking
mechanism, incoherence present in the passive layer allows chloride ions to reach the metal
surface directly. In adsorption mechanism, the chloride ions get adsorbed to the passive layer
resulting to progressive thinning until the end of dissolution. Details of establishing the exact
mechanism that result to breakdown of passive film is not covered in this thesis. Nevertheless,
the general process on how pitting corrosion occurs in concrete due to induced chloride ions is
represented by the schematic in Figure 3 below
Figure 3: Pitting corrosion induced by chloride
After passivity has been broken down, a pit gets created and dissolution of iron continues
(equation 1). The electrons move from the anode to cathode, where the process of oxygen
reduction occurs (equation 2). For the positive charges that are generated at the anode to be