Corrosion and Deterioration of RC Structure

This article discusses the corrosion and deterioration of reinforced concrete structures, specifically the effects of corrosion on structural soundness, the corrosion mechanism of steel reinforcement in concrete, and the factors that affect concrete steel corrosion. The article also includes information on non-destructive testing and visual inspection of a bridge girder to investigate the extent of corrosion and deterioration.

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:

Corrosion and Deterioration of RC structure 3
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

Corrosion and Deterioration of RC structure 4
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.

Corrosion and Deterioration of RC structure 5
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.

Corrosion and Deterioration of RC structure 6
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

Corrosion and Deterioration of RC structure 7
balanced and electro neutrality maintained, OH- and Cl- (anions) and Na+, Ca2+ and K+ (cations)
move away from and towards the cathode respectively. Within the formed pit, the dissolved iron
ions get hydrolyzed as described by equations 6, 7, 8 and 9. This hydrolysis results to
acidification.
Additionally, a porous cap comprising of passive film remnants and products of iron rust can be
formed. This significantly minimizes transport to and from the pit. When the environment is
alkaline, hydroxyl ions will move inside the pit for the formation of H+ to be balance (equations
6, 7 and 8). When migration of chloride ions creates an acid environment in the pit, hydrochloric
acid may be formed (equation 9). Considering the low pH in the pit, occurrence of cathodic
hydrogen evolution is possible. This means that the pit will act as a cathode and anode resulting
to occurrence of hydrogen embrittlement of reinforced steel.
Fe2+ + 2H2O → Fe(OH)2 + 2H+ ………………………………………………. (6)
2Fe2+ + 3H2O → Fe2O3 + 6H+ + 2e- ………………………………………… (7)
3Fe2+ + 4H2O → Fe3O4 + 8H+ + 2e- ………………………………………… (8)
3Fe2+ + 4H2O → Fe3O4 + 8H+ + 2e- ……………………………………….. (9)
A number of studies have also revealed formation of green rust (soluble iron chloride complexes)
represented as FeCl2. Green rusts are basically a group of FeCl2 compounds where Cl-, Fe2+ and
Fe3+ are present and only stable in environments that are deprived of oxygen. It is believed that
the mechanism results to production of these compounds inside the pit, which then diffuses to
oxygen and hydroxide richer regions and creation of solid rust products. This also produces
chloride ions that attacks the passive film even more. This catalytic effect also produces acid (as
shown in equation 10) that is aggressive to steel.

Corrosion and Deterioration of RC structure 8
4FeCl2 + O2 + 6H2O → 4FeOOH + 8HCl …………………………………………….. (10)
Factors Affecting Concrete Steel Corrosion
Factors that affect corrosion of steel fixed in concrete can be categorized into two main classes:
internal factors and external factors.
External factors
External factors are mainly environmental conditions and they include the following:
i) Moisture and oxygen
Corrosion is facilitated by the presence of oxygen and moisture. Presence of moisture meets the
requirement of electrolytic process of the corrosion cell, and both oxygen and moisture facilitates
formation of hydroxide (OH) that produces Fe(OH)2 – a component of rust. The presence of
oxygen facilitates cathodic reactions but corrosion cannot progress in the absence of adequate
oxygen because of cathodic polarization.
ii) Temperature and relative humidity
Carbonation of concrete is significantly affected by relative humidly. An increase in relative
humidity results to a decrease in concrete carbonation when the relative humidity in within the
range of 50-100%. A study revealed that when relative humidity is within 30-50%, a decrease in
relative humidity does not result to a corresponding decrease in concrete carbonation especially
when CO2 concentration is normal even when the steel is exposed for a longer period of time. On
the other hand, an increase in temperature can result to two effects: first is an increase in the rates
of electrode reaction, and second is a decrease in solubility of oxygen, which reduces the rate of
corrosion. If there is a conducive environment for the occurrence of corrosion, high humidity and
high temperature results to an increase in the corrosion rate.

Corrosion and Deterioration of RC structure 9
iii) Carbonation and acidic gaseous pollutants
Carbonation and acidic gases like NO2 and SO2 affects corrosion because of the ability to
decrease concrete’s pH. When the pH level reduces to a certain range, reinforcement corrosion
may start, in addition to loss of concrete passivity and catastrophic corrosion of the
reinforcement, as shown in Table 1 below.
Table 1: Reinforcement corrosion state at different pH levels
iv) Aggressive anions
The main aggressive anions are chloride ions that can reach the reinforcement level from the
external environment or via the ingredients of concrete. There are three forms in which chloride
can be present in concrete: acid soluble chloride, bound chloride and water-soluble or free
chloride. The corrosion process is only influenced by the free chloride ions. Generally, an
increase in chloride content results to a decrease in resistivity and an increase in the rate of
corrosion. Nevertheless, a change in pH is deemed to have an insignificant effect on the
corrosion rate because of a change in the concrete’s chloride content. Table 2 below shows the
risk of reinforcement in association with chloride content levels in carbonated and uncarbonated
concrete.
Table 2: Risk of corrosion in concrete with chloride

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Several countries have also come up with codes recommending limits of total chloride ion
content that should be contained in concrete, as shown in Table 3 below.
Table 3: Recommended concrete chloride content limits
v) Bacteria
There are three different ways in which bacteria action is effective in facilitating corrosion.
These are:
a) Bacteria reduces the amount of concrete cover through fragmentation of cementitious
materials.
b) In oxygen deprived environment, anaerobic bacteria generates iron sulfides that
facilitates corrosion reaction.

Corrosion and Deterioration of RC structure 11
c) Aerobic bacteria also facilitate creation of differential aeration cells than may lead to
corrosion.
Internal factors
The internal factors mainly comprise of factors associated with the quality of steel and concrete.
These factors include the following:
i) Cement composition
There are two main ways in which the cement present in concrete protects the reinforcing steel
from corrosion. First is by sustaining a high pH ranging between 12.5 and 13. This is due to the
presence of alkaline materials such as Ca(OH)2 that are produced when cement get hydrated.
Second is by binding a substantial quantity of total chlorides due to chemical reaction between
C4AF and C3A content present in cement inside the concrete. As a result, an increase in the
content of C3A in the cement causes the value of chloride to move to the higher side. Blended
cement, like micro-silica, which has high content of C3A, significantly increases resistance to
chloride corrosion and sulfate attack on reinforcement.
ii) Aggregate impurities
When the aggregates used contain chloride salts, serious corrosion problems arise. This is very
common in areas close to the sea and those sites whose groundwater has high chloride ions
concentration.
iii) Mixing and curing water impurities
When water used for mixing or curing concrete is contaminated with high chloride content or
significantly acidified, serious corrosion problems arise.

Corrosion and Deterioration of RC structure 12
iv) Admixtures
Calcium chloride is a very common admixture that is usually added to concrete to speed up
hydration of cement. However, this chloride content is detrimental to RC structures because it
facilitates corrosion of steel reinforcement.
v) Water-cement (w/c) ratio
W/c ratio principals affects the strength, workability, impermeability and durability of concrete.
This parameter does not directly affect corrosion rate in reinforcement. However, when a RC
structure comes in contact with an aggressive solution, the concrete’s permeability, which is
affected by the w/c ratio, affects the rebar’s corrosion. The w/c ratio is directly proportional to
the penetration depth of a certain value of chloride threshold. It has been found that carbonation
depth increases linearly with the increase in w/c ratio. Also, it has been found that an increase in
the w/c ratio results to a corresponding increase in the coefficient of oxygen diffusion, and vice
versa.
vi) Cement content
Concrete’s cement content affects various parameters including the strength and durability of
concrete. When the amount of cement used is inadequate, surface defects such as honeycombs
get formed in the concrete. These defects allow corrosion causing agents like O2, CO2, H2O and
Cl- to penetrate and diffuse into the concrete. These agents facilitate creation of differential cells
that initiate corrosion of reinforcement. Additionally, concrete that has low cement content lacks
plastic consistency hence it is not able to create a uniform passive film to cover and protect the
rebars, which creates loopholes for corrosion.
vii) Concrete cover

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The depth of concrete cover significantly affects corrosion of steel rebars because it determines
the ease with which carbonation or chlorides can penetrate through the concrete. However, the
concrete cover effect is restricted to the time when concrete is cast to the onset of corrosion.
Once corrosion starts, its rate will be determined by the thickness of concrete cover.
viii) Reinforcing steel structure and chemical composition
The structure and chemical composition of steel rebars determines the ability of the rebars to
withstand the impact of external factors and other internal factors affecting corrosion. These
parameters also affect creation of differential cells that are very critical to corrosion.
Macro-cell corrosion
In localized (also known as macro-cell) corrosion, there is a space separating the cathodic and
anodic parts of corrosion. For steel reinforcement fixed in concrete, macro-cell corrosion takes
place instead of localized corrosion because of the action caused by the significant quantities of
chlorides that are present in the concrete. The corrosion happens in the form of concentrated
anodic areas, corrosion pits that are bounded by large parts of passive rebars, which perform the
function of cathode.
Chloride attack
The basic approach in which chloride ions can find themselves in the concrete is through
concrete ingredients (aggregate, water or admixtures) during mixing. However, this is not the
case as most chlorides tend to penetrate from outside sources due to use of deicing salts or the
RC structure being located in marine environments. Once chlorides penetrate into concrete they
start disrupting the passive film of steel going into localized attack or pits. In completely
saturated concrete or submerged parts, penetration of chlorides is by diffusion. But in aerial areas

Corrosion and Deterioration of RC structure 14
or when the structure is submitted to cycles, such as the case of using deicing salts, penetration is
accelerated by capillary absorption. In these two cases, the rate of penetration depends on square
root of time. This phenomenon can be simulated in a similar way as that of carbonation and
expressed in a simplified form as: x=k (Cl) √ t. The ingress of chloride is usually modelled using
error function equation. This equation is a particular solution of the second law of Fick, given as:
Cx=Cs [ 1−erf ( x
2 √ Dt ) ]; where Cx = concentration of chloride at depth x, Cs = concentration of
surface chloride, D = coefficient of chloride diffusion, an t = time.
Subject to how localized or extended the corrosion is, the concrete may show cracks or fail to
show them. Sometimes it so happens in submerged areas that the steel corrodes without showing
any external signs of cracks or cracking of concrete cover. There are also numerous other factors
that influence the quantity of chlorides that are required to initiate corrosion. Some of these
factors include the following:
 Cement type – blending materials, quantity of gypsum in the cement, fineness of the
cement, etc.
 Moisture content and moisture content variation
 Water/cement ratio
 Compaction of concrete
 Curing of concrete
 Availability of oxygen
 Type of steel reinforcement
 Surface roughness of steel

Corrosion and Deterioration of RC structure 15
Each structure of steel reinforcement has a unique chloride content threshold. It is possible to
verify the particular chloride threshold of a structure that has already started corroding by
suitable testing. However, predicting threshold in a structure is quite difficult but it becomes
possible if some parts of the reinforcement have depassivated. But regardless of this difficulty,
all building codes have limits of chloride content that should be contained in water used for
mixing concrete. The common value used as a reference, in case there is no particular value
stated, is 0.4% of the weight of cement.
Chloride attack determination
It is very important to determine the amount of chloride resent in water or concrete. There are
two main concrete sampling techniques that are used for analyzing chloride in concrete. These
are: core drilling and collection of dust from borehole through dry drilling. Core drilling involves
grinding concrete cores at a depth or cutting them into slices and then crushing them into powder
form for further chloride investigation. Grinding methods usually allow penetration to a depth of
up to 0.5mm but the spatial resolution of chloride profile typically range to a few millimeters
hence care should be taken when using grinding/slicing technique to ensure that the sample taken
is a representative of the reinforcement depth. However, large variations have been found in
investigation of concrete powder samples collected using borehole dry drilling. The two main
sources of error when using this method are: precision of the depth at which the concrete
specimens are taken, and sample contamination from the transportation of powder that can result
to overestimation. In both techniques, it is important to carefully select the diameter of the core
or bore based on maximum aggregate size in order to minimize the influence it has on chloride
content measurement.

Corrosion and Deterioration of RC structure 16
It is possible to measure the chloride content present in concrete as a % of the weight of
concrete, in accordance with the guidelines of ASTM C114. In this method, the concrete sample
obtained is mixed with 10ml of extraction liquid followed by shaking the mixture for 5 minutes.
This extraction liquid works by eliminating disturbing ions like chloride ion extracts and sulfide
ions from the sample. The concrete sample is then tested using electrode to determine the content
of chloride as a % of concrete weight. Table 4 below presents different NDT methods that can be
used to evaluate chloride content in concrete
Table 4: Methods used for testing chloride content
Method Procedure or principle Restrictions or Limitations
Potentiometric titration Uses water soluble methods or
acid, where chloride content is
indicated by the final volume
Requires skilled personnel
Quantab test Chloride ions react with silver
dichromate to produce white
marks.
Suitable for low thickness, it
is hazardous and expensive.
Rapid chloride test Involves comparing unknown
solution’s potential difference
with those of solutions whose
chloride concentration is known
Presence of particular
materials affects the results
Rapid chloride permeability
test
Involves measuring the total
charge passed and using it to
assess movement of ions.
The current passed shows
movement of all ions present
in the concrete instead of
showing movement of
chloride ions only.
Salt ponding test Involves ponding a solution of
3% NaCl on the surface of dried
concrete sample after curing for
28 days, exposing the bottom
surface to the environment, and
taking a slice of 0.5 inch thick
to measure chloride
concentration
Its testing conditions are
complicated, it is time-
consuming, and only provides
average value and not the
actual value.
Electrical resistance test Involves connecting two cooper
plate electrodes on opposite
sides of the sample using thin
wet sponges followed by
connecting the electros to
Temperature variation affects
the results.

Corrosion and Deterioration of RC structure 17
resistivity meter.
Bulk diffusion test Similar to salt ponding test but
it starts with evaluating the
initial moisture content of the
sample and also the sample is
saturated in limewater instead of
being tested while dry.
It is time-consuming
Transportation of chloride in concrete
The pore structure and cement hydration makes concrete to be a porous material, in addition to
other factors like curing temperature or w/c ratio. There are three main kinds of pores present in
concrete: gel pores that range between 1 and 10 nm, capillary pores that range between 10nm
and 10μm, and air voids and macro pores that range between 10μm and a few mm. The
contribution of gel pores to transport processes of chloride is insignificant due to their small size
hence they are not considered when investigating the durability of steel rebars. Air voids and
macro pores are also said to have a small effect on the ingress of chloride based on sorption
properties. Nevertheless, they can increase the rate of transportation by gravitation. It has been
found that if air voids are present at the interface between steel and concrete, chloride-induced
corrosion resistance is strongly influenced. Therefore the quantity of capillary pores together
with interconnectivity level between them (that is permeability) are the ones that strongly
influences fixation of moisture, gases and ions and transport of chloride ions responsible for
corrosion process.
The major chloride transport mechanisms in concrete include: capillary suction, migration and
diffusion. Capillary suction mechanism takes place in partially or fully dried concrete as a result
of surface tension when concrete mixes with water or other dissolved ions. This mechanism is
very crucial for concrete in tidal & splash areas and also for concrete in road applications

Corrosion and Deterioration of RC structure 18
especially where use of de-icing salts is common. Diffusion is the relevant chloride transport
mechanism where concrete is saturated, like in submerged conditions. Occurrence of diffusion is
facilitated by concentration gradients. In non-steady-state conditions, where there is a change in
concentration gradient with time, the second law of diffusion of Fick (equation 11) can describe
the flux.
∂ c
∂ t =D ∂ ² c
∂ x ² …………………………………………………….. (11)
Where c = chloride concentration, D = coefficient of diffusion, t = time and x = depth.
If D is assumed to be constant, the mass balance can be solved using the solution of error
function solution, as shown in equation 12
c ( x , t )=ci+ ( cs−ci ) [1−erf ( x
2 √ Dt ) ] …………………………….. (12)
Where ci = initial chloride content present in concrete and cs = concentration of surface chloride.
Diffusion is a fairly slow process in comparison to capillary suction. But in practice, most
models for investigating chloride ingress still uses second law of diffusion of Fick even though
capillary suction mechanism is essential for assessing how chloride is transported within
concrete cover’s first few mm. To account for diffusion coefficient’s time-dependency, such as
model type, the D in equation 12 is described using the relationship in equation 13 below
D ( t ) =Do ( ¿
t )n
…………………………………………….. (13)
Relative humidity also significantly influences the transport processes of chloride in concrete.
Though diffusion of chloride ions requires the pore system to have a continuous liquid phase,

Corrosion and Deterioration of RC structure 19
transportation of oxygen is greater when relative humidity is low. Therefore concrete’s chloride
ions diffusion rate is greatest at saturation or near saturation and insignificant when RH is below
50%. On the other hand, diffusivity of oxygen decreases strongly when RH is greater than 70%
because diffusion of oxygen has to take place through the water that is present in the pores,
which is a very slower process compared to diffusion taking place in empty pores filled with air.
The diagram in Figure 4 below illustrates how relative humidity influences diffusion of oxygen
and chloride.
Figure 4: How RH influences diffusion coefficient of ions and gases
Besides moisture conditions (or relative humidity), the interaction between cement paste and
chloride also affects the ingress rate of chloride in concrete. Chloride can chemically react with
cement’s aluminate phases to produce Friedel’s salt, and can be adsorbed physically on pore
walls. This chemical and physical binding can significantly interrupt transportation of chloride
through the concrete and onset of corrosion because only free chlorides get a chance of
interacting with the steel rebars. Nevertheless, they can also be an additional source of chloride

Corrosion and Deterioration of RC structure 20
in case there is variations in temperature or a decrease in the pore solution’s pH caused by
carbonation.
Lastly, it is important to note that concrete mostly has cracks that can affect transportation of
chloride either largely or marginally. Presence of cracks is a phenomenon that has been in
existence for as long as humans started existing and are caused by different factors such as
plastic shrinkage. These cracks can be propagated or induced by different factors in the course of
the structure’s service life, such as mechanical loading. Based on the crack’s width, they can
provide paths along which oxygen and chloride can penetrate through the concrete to reach
rebars. Although it is generally assumed that diffusion coefficient is increased by presence of
even micro cracks, there is still no evidence showing the correlation between corrosion risk and
width of concrete cracks. Besides cracks, the interfacial transition region between aggregates and
cement paste is predicted to give higher diffusivity in comparison to the one of bulk cement
paste.
Carbonation-induced corrosion
Cement paste has high alkalinity of about 13. This alkalinity passivates the surface of steel
reinforcement thus protecting it against oxidation process that could otherwise corrode the
reinforcement. However, this pH can be reduced in the presence of acid attack, carbonation,
chlorides or a combination of two or all these substances. When this happens, the reinforcing
steel gets exposed to corrosion.
The occurrence of steel corrosion can be divided into three zones based on pH scale, as shown in
Figure 5 below

Corrosion and Deterioration of RC structure 21
Figure 5: Occurrence of steel corrosion on a pH scale
The cement paste has high alkalinity that is as a result of the high lime (calcium hydroxide
content). This lime is produced from hydration process of cement, as shown in equation 14
below.
CS + H → CSH + lime (calcium hydroxide) ………………………………….. (14)
When water mixes with calcium silicate compounds (CS) present in Portland cement, the two
react to produce lime (calcium hydroxide) and hydrated calcium silicates (CSH). Lime (plus
other alkaline earth elements oxides such as potassium (K) and sodium (Na)) produce an
environment that is highly alkaline in young or fresh concrete.
As concrete ages, reaction between lime and carbon dioxide in the atmosphere takes place to
produce calcium carbonate and water as shown in equation 15 below
Ca(OH)2 + CO2 → CaCO3 + H2O ………………………………………… (15)
The reaction in equation 15 neutralized concrete, resulting to a decrease in pH of concrete.
Corrosion of steel starts when pH of the concrete is below 10 (usually about 9.5 to 9.6), because

Corrosion and Deterioration of RC structure 22
this is the level where steel surface’s passivation protection disappears as a result of cement
paste’s alkalinity.
Half-cell potential or open circuit potential measurements
It is important to measure half-cell potential especially of RC structures that are directly exposed
to the atmosphere. The method used to perform these measurements is not affected by the size of
rebars, detailing of rebars and concrete cover depth. The measurements of half-cell potential will
show rebars that are corroding even to greater depths. The method is applicable at any stage
during the structure’s service life and in all kinds of climatic conditions. It is recommended to
take half-cell potential measurements on a free concrete surface only. This is because the
presence of layers such as paints, organic coatings and asphalt can make the measurements
impossible or erroneous. There are four main purposes for which half-cell potential
measurements are recommended for. These are:
i) Locate rebars that are already corroding and assess the condition of corrosion. This
can be done by inspecting and assessing the condition of a RC structure.
ii) Determine the need to carry out further destructive analysis so as to establish the
actual state of corrosion and potential risks of corrosion.
iii) Evaluate the state of corrosion of rebars after they have been repaired following
previous recommendations. This helps in evaluating the efficiency, effectiveness and
durability of the repair work done.
iv) Design cathodic protection systems’ anode layout or electrochemical repair
approaches.
All the same, it is important to note that quantitative information about the rebars’ actual
corrosion rate is not provided by measurements of half-cell potential.

Corrosion and Deterioration of RC structure 23
Half-cell potential is also referred to as open-circuit potential, corrosion potential or rest
potential. It is measured at different points along the section being investigated. Results obtained
from half-cell potential test can be interpreted in accordance with the guidelines of ASTM-C876
presented in Table 5 below
Table 5: Interpretation guidelines of half-cell potential measurements
Half-Cell Potential (mV) comparative to reference
electrode (Cu/CuSO4)
% probability of active
corrosion
<-350 90
-200 to -350 50
>-200 10
Concrete Resistivity (ρ)
Concrete’s electrical resistance plays a key role in determining the corrosion magnitude at any
particular location of RC structure. Concrete resistivity is measured in terms of concrete’s
electrolytic resistivity. The main units of ρ is ohm-centimeters. The likelihood of corrosion or its
actual occurrence can be classified using the values provided in Table 6 below. This table is best
suited for use when measurements from half-cell potential cell have indicated the possibility of
corrosion.
Table 6: Interpretation of reinforcement’s concrete resistivity
Concrete resistivity (ohm-
cm)
Likelihood of significant corrosion
<5000 Very High
5000— 10,000 High
10,000 — 20,000 Low / Moderate
>20,000 Low

Corrosion and Deterioration of RC structure 24
Concrete resistivity is related to content of charging and moisture together with their mobility.
Therefore measurements of concrete resistivity provide vital information that can be used to
interpret results of electrode potential obtained under similar conditions. Concrete resistivity
measurements can be affected by the aggregate size distribution in case of a significantly larger
concrete cover. The resistivity measurement is also largely influenced by the condition of
concrete’s top layer than the concrete layer’s condition close to the improvement level.
Generally, the rate of corrosion increases with increasing temperature because when the
concrete’s temperature changes, other parameters like oxygen diffusion and concrete resistance
also changes. The exact effect that temperature has on concrete’s corrosion rates is very complex
and is also influenced by the interaction and reactions among numerous other factors. It is rather
obvious that for steel in concrete to corrode, there should be sufficient oxygen supply for the
cathodic reaction to occur, and also moisture that is needed to act as a low resistance electrolyte.
The amount of oxygen available during manufacturing and processing of steel is usually greater
than the amount of oxygen required for corrosion of steel exposed to normal outdoor conditions.
Therefore the rate of corrosion in steel embedded in RC structures increases when there is a
decrease in concrete resistance and when the steel is exposed to normal outdoor conditions.
Sight mapping
Since raw data of bridges investigated in this case study is not available, the researcher has
processed a defect mapping on site during project initiation stage. Considering the significance
of defect indexing, it was important to ensure that the methodology selected to do the mapping
was the most appropriate. It is important to ensure that deterioration assessment of a concrete
structure starts right from onsite redaction. Relevant information should be filled in inspection
forms and available defect catalogues. For each defect to be identified properly, the research

Corrosion and Deterioration of RC structure 25
team identified various structural elements (including piers, bearings, decks, abutments and
beams) from the sight work they did. The bridge in this case study was a multi span bridge and
therefore each span was progressively numbered. The research team has filled the inspection
forms and used a suitable methodology to evaluate identified defects.
Some of the defects that are common in RC structures are: cracks, delamination, exposure of
steel rebars, and spalling. The research team recorded these defects by use of measurement and
sketches. Defects have been collected and classified as cracks, delamination, spalling, etc. The
defects have also been located in each section of the bridge so as to have a general view of the
bridge’s condition for further analysis.
The severity of defects was quantified by giving each defect a score on a scale of 1 to 5. The
score was dependent on several factors such as growth and development of the defect over time,
the urgency of repair intervention, and the defect’s influence or effect on the structure’s safety
coefficients. The defects were then categorized on a scale based on their deterioration level.
Finding the pulse velocity using surface or indirect transmission
It is recommended to use indirect transmission to determine pulse velocity under the following
conditions: when only a single face of the concrete structure or specimen is accessible, when
surface crack’s depth has to be determined, or if there is an intersection between the surfaces
concrete’s quality relative to the concrete’s overall quality. This is the least sensitive method and
the receiving transducer only produces 2-3% of the results obtained from direct transmission. In
addition, the pulse velocity measurements obtained from this method are affected by the concrete
that is close to the surface. It is worth noting that the concrete close to the surface is usually
different in quality from that deep inside the concrete structure. This means that results obtained
from indirect transmission method may not be the actual representative of the concrete.

Corrosion and Deterioration of RC structure 26
Generally, when concrete with same elements is measured, pulse velocity from indirect
transmission tend to be invariably less than the one obtained from direct transmission. The
difference can vary between 5 to 20% depending mainly on how the test is performed and the
quality of concrete. Where practicable, the difference should be determined through site
measurement. Indirect transmission approach also has some uncertainty associated with the
actual length of transmission due to the substantial size of contact areas between the concrete and
transducers. In this regard, it is recommended to take several measurements using transducers
located at varied intervals so as to prevent these uncertainties. For this to be achieved,
transmitting transducers have to be placed in contact with the surface of concrete at a fixed point
(x) whereas receiving transducers have to be posited at fixed increments (xo) along the selected
path on the concrete surface, as shown in Figure 6 below. The record of transmission time then
has to be plotted on a graph to show the relation between transmission time and transducers
interval. The slope of best-line of fit through the points represents the main pulse velocity of the
concrete surface.
Figure 6: Schematic of surface/direct transmission method
In this case study, surface and pulse velocity were measured in varied beams on the two bridges’
girders. The measurements were also taken at different times. The time interval was four months,

Corrosion and Deterioration of RC structure 27
which was enough to assess the pulse velocity of the concrete surfaces. The pulse velocity
measurements were done such that the pulse travelled at a time of b equivalent to 0.15m and a
maximum distance of 0.3m.
Transducer coupling
As a strategy of ensuring that the ultrasonic pulse that is produced by the transmitting
transducers is able to pass into the concrete and for the receiving transducers to direct it
carefully, there should be sufficient acoustic coupling between the face of every transducer and
the concrete. Most concrete have satisfactorily smooth surfaces that are appropriate to achieve
good acoustical contact when a coupling medium is used and by fixing transducers on the
concrete surface. It is important to ensure that the coupling link medium separating the
contacting transducers and the concrete is made of a very thin layer. Use of materials such as
grease, petroleum jelly, glycerol/kaolin paste and soft soap should be avoided. It is also
recommended to take readings of transit time repeatedly until obtaining the minimum value so as
to allow more time for the coupling layer to spread to a very thin film.
Energy Dispersive X-ray (EDX) Analysis
EDX analysis is also referred to as EDAX or EDS analysis. It is basically a technique that is used
for identifying composition of an element specimen or the specific area of interest of the
specimen. The system works by being integrated to a Scanning Electron Microscope (SEM),
hence it cannot work without the SEM. The EDX analysis is performed by bombarding the
sample with an element inside the SEM. During the process, electrons of the specimen’s atoms
collide with the bombarding electrons, causing some of the electrons to be knocked off. When an
inner shell electrons gets ejected, it creates a vacant position. A higher-energy electron that
originates from the outer shell ultimately occupies the vacant position. But for this to happen,

Corrosion and Deterioration of RC structure 28
some of the energy of the transferring outer electron must be given up through emission of an X-
ray. The quantity of energy that is released depends on the transferring electron’s shell where it
is transferring from and also the destination shell. Also during the transferring process, the
amount of X-ray energy released from each atom is unique. The amount of X-ray energy
released by a sample in the process of electron beam bombardment can be used to determine the
identity of the origin atom where the X-ray was released.
Temperature
The presence of high amount of CO2 in the air can be an indicator of higher rates of carbonation
in concrete. Atmospheric carbonation rate is highest when relative humidity is ranging from 50%
to 75%. The basic damage mechanism that carbonation can cause to a RC structure is corrosion
of steel rebars. When relative humidity is 100% it means that there is no space in the air to
accommodate additional water vapor at that particular temperature. On the other hand, when the
relative humidity is 50%, it means that the air is only holding half of the water vapor it can
accommodate at that particular temperature. The air temperature at which water vapor saturates
air is called dew point. Temperature is inversely proportional to the quantity of water vapor that
can accumulate at that particular time. This means that more water can accumulate at a lower
temperature, and vice versa.
Quantitative results for EDX test 3322
The EDX test performed showed that the alternative voltage was 30kV while take-off angle was
35°. The bridge in this case study was found in a non-aggressive environment but the results
obtained from EDX analysis indicated that the composition of cement is comparable to the
mineral compound in the typical Ordinary Portland Cement (OPC) that is used in Malaysia.

Corrosion and Deterioration of RC structure 29
Sample ‘a’ was found to have chloride content that is within the range that is deemed to initiate
depassivation layer. However, this content was not found in sample ‘b’. The concrete sample
should be analyzed further using XRD analysis technique considering the uniformity of the
grinded concrete specimen hence XRD test can give more accurate and accurate results than
EDX test.