Damage of Reinforced Concrete Structures due to Steel Corrosion

Abstract

Reinforced concrete (RC) became one of the most widely used modern building materials. In the last decades a great interest has been shown in studying reinforcement corrosion as it became one of the main factors of degradation and loss of structural integrity of RC structures. The degradation process is accelerated in the case of RC structures situated in aggressive environments like marine environments or subjected to de-icing salts. In this paper it is shown how steel corrosion of the embedded rebars occurs and how this affects the service life of reinforced concrete structures. Also, an experimental study regarding the combined effect of carbonation and chloride ingress was realized. Samples with and without rebars were drilled from a RC slab which was stored in the laboratory for two years. Non-steady state migration tests were realized in order to determine the chloride profile, while the carbonation depth was measured using the colorimetric method based on phenolphthalein spraying. It was concluded that carbonation has a significant effect on chloride ingress, increasing it.

Full text

Damage of Reinforced Concrete Structures Due to Steel Corrosion
SOSDEAN Corina1, a *, MARSAVINA Liviu1,b and DE SCHUTTER Geert2,c
1 Politehnica” University of Timisoara, Department Strength of Materials, Blvd. M. Viteazul, No. 1,
Timisoara 300222, Romania
2 Ghent University, Magnel Laboratory for Concrete Research, Department of Structural
Engineering,Technologiepark-Zwijnaarde 904, B-9052 Ghent, Belgium
acorina.sosdean@yahoo.com, bmsvina@mec.upt.ro, cGeert.DeSchutter@UGent.be
Keywords: corrosion, reinforced concrete, steel rebar, structural integrity, carbonation, chloride
ingress
Abstract. Reinforced concrete (RC) became one of the most widely used modern building
materials. In the last decades a great interest has been shown in studying reinforcement corrosion as
it became one of the main factors of degradation and loss of structural integrity of RC structures.
The degradation process is accelerated in the case of RC structures situated in aggressive
environments like marine environments or subjected to de-icing salts. In this paper it is shown how
steel corrosion of the embedded rebars occurs and how this affects the service life of reinforced
concrete structures. Also, an experimental study regarding the combined effect of carbonation and
chloride ingress was realized. Samples with and without rebars were drilled from a RC slab which
was stored in the laboratory for two years. Non-steady state migration tests were realized in order to
determine the chloride profile, while the carbonation depth was measured using the colorimetric
method based on phenolphthalein spraying. It was concluded that carbonation has a significant
effect on chloride ingress, increasing it.
Introduction
Due to its many advantages such as: low cost, convenient raw material source, wide applicability,
acceptable performance…reinforced concrete has become the most widely used construction
material in the world. As reported by Lippiat and Ahmad [1], about one ton of concrete is produced
each year for every human being in the world. Still, in the last decades a great interest in studying
the durability issue of concrete has been shown, as a consequence of the huge amount of money
spent on repair or reconstruction of RC structures. According to Knudsen et al. [2] in 1998, a large
number of reinforced commercial buildings, domestic dwellings, marine structures, bridges...
particularly those over 30 years of age started to show deterioration; the annual cost of repair work
on concrete structures was over US$ 5 billion in Western Europe alone. According to the study
completed by Koch et al. [3] in 2002, the annual cost of highway bridge repairs and maintenance is
over $8.3 billion in the United States. With regard to De Sitter’s Law of Fives [4], major repair cost
is five times more the cost of maintenance repairs if they would have been done on time. Also, all-
out replacement cost is five times more the major repairs cost if they would have been done on time.
In general, the durability of a RC is defined by its ability to serve its intended purpose for a
designed service life while resisting weathering action, chemical attack, abrasion…In normal
conditions, in sound concrete, the cement matrix forms a passive film along the surface of the
embedded rebars. Under this condition, the corrosion rate is negligible, even if the concrete is
permeated by oxygen and moisture [5]. During the service life of the structure, this protective layer
can be disrupted or destroyed, resulting in the corrosion of the rebar. The volume of rust products is
considered to be four- six times larger than the one of the steel, which causes cracking, delamination
and spalling. Carbonation and chloride ingress are considered to be the main causes of steel
corrosion. In aggressive environments such as the marine environment or when being exposed to

deicing salts, the high concentration of chloride ions causes local depassivation of the reinforcing
steel and pitting corrosion occurs. Carbonation-induced corrosion develops due to the neutralization
of the alkalinity of the concrete by the carbon dioxide in the atmosphere.
Corrosion mechanisms
Corrosion of reinforcing steel in concrete is an electrochemical process that requires the presence
of an anode, cathode and an electrolyte. Due to the fact that the passive layer is destroyed, the
electrochemical potential becomes more negative locally, which causes iron atoms to lose electrons
and this part of the rebar becomes the anode, while the rest of the rebar is the cathode. The released
electrons move through the reinforcement and are absorbed in the concrete pore solution
(electrolyte) and oxygen and water combine producing hydroxyl ions (OH-) [6]. The reactions that
take place are [6]:
- anodic reaction:
Fe→Fe2+ + 2e- (1)
- cathodic reaction:
O2 + 2H2O + 4e- (2)
The hydroxide ions migrate through the pore solution towards the anode where they combine
with the ferrous ions and ferrous hydroxide is created which is transformed to rust if it continues to
oxidize:
Fe2+ + 2OH-→Fe(OH)2 (3)
4Fe(OH)2 + 2H2O + O2→4Fe(OH)3 (4)
Chloride corrosion happens when the chlorides penetrate the passive layer and react with iron
(Fe2+ + 2Cl-→FeCl2). Ferrous chloride combines then with oxygen and water [6]:
4FeCl2 + O2 + 6H2O→FeOOH+ 8HCl (5)
The released chloride ions continue to react with ferrous ions, encouraging further oxidation of
the iron, so that the process is continuous and the chlorides act as catalyst, increasing the volume of
rust.
Even though carbonation and chloride ingress are the main causes of rebar corrosion, there are
very few publications regarding their combined effect on RC structures. According to [7]
carbonation pushes the chloride front forward by liberating chlorides that were bound in non-
carbonated concrete. Yoon [8] experimentally investigated the interaction between carbonation and
chloride penetration and their effects on concrete, under various boundary conditions and found that
carbonation of concrete could significantly accelerate chloride penetration. Other studies [9] show
that carbonation has a direct influence on chloride penetration, decreasing it.
Experimental program
Details of specimen
In this study, two types of samples are being used: type S (without crack and without rebar) and
type SR (without crack and with rebar), previously described in [10]. It is important to mention that
in the case of samples SR the concrete cover depth was 2.5 cm, which means that the rebar was
situated right in the middle of the sample. All samples were obtained by drilling from a RC slab

having a C30/37 concrete class with a maximum size of 14 mm of the coarse aggregate. This slab
was subjected to an artificial failure of the central support and successive vertical loading until
collapse. More details about the slab and the test set-up are described in detail in [11, 12]. The age
of the concrete was estimated to about 2 years; after collapsing the slab was stored in the laboratory,
but not in a controlled condition environment. Cylinders with 100 mm diameter and 50 mm
thickness were obtained after being cut from the initial drilled core having 140 mm thickness. It is
necessary to mention that the surface of the core exposed to the environmental conditions in the lab
is referred to as the top surface, while the cut one as the bottom surface.
Testing method
In order to determine the chloride ingress, two sets of accelerated migration test were conducted
according to NT Build 492 [13], with an external electrical potential being applied for 24 hours. The
main difference between them consists in the sample’s surface chosen to be in contact with the
NaCl solution (Figure 1).
Figure 1. Surfaces tested in the accelerated migration test
In total twelve type S samples and six type SR samples were used, in which half of them had the
bottom surface exposed and the other half the top one. After the test, samples type SR were cut in
half and sprayed with 0.01 N AgNO3 solution and the chloride penetration profile was determined
using the colorimetric method [14]. Samples type S were spilt in two and one half was used to
determine the chloride profile, while the other one was used to obtain the carbonation depth. The
same method previously presented was used and the chloride penetration depth was measured from
the visible white silver chloride precipitation (Figure 2 and Figure 3). The carbonation profile was
determined using the colorimetric method based on phenolphthalein spraying which assesses a
carbonation depth corresponding to a pH value roughly equal to 9 and it’s applied either on cores of
real structures or on specimens carbonated in laboratory conditions [15]. After it was applied on the
fresh split specimen, the solution remained colorless where carbonation occurred and it turned
purple where the pH was higher than 9 (Figure 2 and Figure 3).
Results and discussions
Carbonation depth and chloride profile
Based on the colorimetric methods presented above, both the carbonation depth and the chloride
depth were determined. In both Figure 2 and Figure 3, the carbonation profile (yellow line) and the
chloride profile (red line) can be observed. Also it can be observed that carbonation occurred only
on the surface exposed to the external environment (top surface). A mean carbonation depth of 8.3
mm with an average chloride penetration depth of 20.63 mm was determined for the six type S
samples which had the top surface exposed, while the others had a chloride penetration average
depth of 15.87 mm. According to Bertolini [5], the highest penetration rate of carbonation is
normally found on sheltered concrete exposed to 60% to 70% relative humidity (e.g. inside a

building). This seems to explain the high carbonation depth measured, even though the slab’s age is
only 2 years.
Figure 2. Carbonation depth and chloride profile for sample S III- top surface exposed
Figure 3. Carbonation depth and chloride profile for sample S IV- bottom surface exposed
Migration coefficient
After measuring the chloride penetration depth, the non-steady state migration coefficient was
determined according to [13]. Figure 4 shows the effect of carbonation on the diffusion coefficient.
It can be easily seen that the migration coefficient has a significantly higher value of 7.82x10-12 m2/s
for the samples type S which had the top surface exposed and which were carbonated, compared to
5.89x10-12 m2/s, the value obtained for type S samples which had the bottom surface exposed and
that were not carbonated. Also, it can be observed that for samples type SR (with rebars in them),
the value 8.4x10-12 m2/s of the migration coefficient obtained for samples having the top part
exposed is almost double than 4.45x10-12 m2/s, obtained for those that had the bottom surface
exposed. It is important to mention that in the case of the samples having rebars, at the end of the 24
h accelerated migration test, corrosion already occurred, for all type SR samples. According to Yuan
et al. [16], carbonation changes the nature of hydration products, which has a great effect of the
chloride binding of the cement pastes, by decreasing the chemical binding capacity of cement-based
materials. Thus, in the presence of carbonation, reinforced concrete is at higher corrosion risk. Also,
it is possible that the existence of the rebars and of corrosion products may act as mitigating
mechanisms, obstructing the further ingress of chlorides [10, 17]. This could be a possible
explanation of the fact that the diffusion coefficient for sample SR which had the bottom surface
exposed is lower than the one obtained for sample SR which also had the bottom surface exposed. It
must be mentioned that the values of diffusion coefficient used in this study are mean values.

Figure 4. Comparison of diffusion coefficients according to the exposed surface
Conclusions
Due to the fact that the main reasons of rebar corrosion are carbonation and chloride ingress, it is
very important to have a better understanding of their combined effect since corrosion initiation can
be considered the main parameter when determining the service life of a structure. It is well known
that corrosion may have a significant impact on the durability and integrity of RC structures due to
the fact that oxides produced at the steel surface can produce tensile stresses in the concrete cover
causing cracking, spalling and delamination, but also the reduction of the bond between the rebar
and concrete.
In this research, the effect of both carbonation and chloride ingress on RC structures has been
studied. Based on the presented experimental results it is clear that carbonation has a significant
effect on chloride ingress. The chloride migration coefficients obtained emphasize this idea as the
values obtained for carbonated samples are considerably higher than the ones obtained for non-
carbonated samples. Also, it is important to mention that for this study, carbonation was not induced
artificially using laboratory methods, the samples being obtained from a natural carbonated RC slab.
It is possible that the accuracy of these results would be higher if more experimental results would
be available.
Acknowledgement
This paper is supported by the Sectoral Operational Programme Human Resources Development
POSDRU/159/1.5/S/137516 financed from the European Social Fund and by the Romanian
Government. Also, the financial support of the BOF-Flanders is greatly acknowledged within the
framework of a Bilateral Scientific Agreement between UGent, Belgium and UPT, Romania.
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