Effect of transverse reinforcement corrosion on compressive strengthreduction of stirrup-confined concrete: an experimental study
ALI GOHARROKHI1, JAMAL AHMADI1,*, MOHSEN ALI SHAYANFAR2,
MOHAMMAD GHANOONI-BAGHA3 and KIARASH NASSERASADI1
1Department of Civil Engineering, Faculty of Engineering, University of Zanjan, Zanjan, Iran2School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran3Department of Civil Engineering, East Tehran Branch, Islamic Azad University, Tehran, Iran
e-mail: [email protected]; [email protected]; [email protected]; [email protected];
MS received 24 May 2019; revised 28 October 2019; accepted 15 December 2019
Abstract. Stirrups of reinforced concrete members are very prone to corrosion compared with longitudinal
reinforcements, resulting from their small concrete covers, which lead to concrete cracking and spalling. Due to
the adverse effects of corrosion, this article aims to investigate the amount of reduction in the capacity of
reinforced concrete specimens in different corrosion degrees. For this purpose, an experimental investigation is
carried out on 22 reinforced and non-reinforced rectangular prism specimens, of which 12 reinforced specimens
are corroded. The test variables contain the corrosion percentage, and the stirrup diameter and spacing.
Eventually, all specimens are tested for compressive strength for 90 days. The experimental results show that the
reduction of compressive strength depends on the corrosion percentage and stirrup diameter. According to this
conclusion, a new formulation is proposed to express the relationship between compressive strength reduction
and its effects.
Keywords. Concrete; stirrup; confinement; corrosion; compressive strength.
1. Introduction
Corrosion in reinforced concrete structures constructed in
areas with high corrosive factors is an inevitable fact that
seriously affects the serviceability, safety and performance
of such structures and reduces their lifetime [1–5]. In the
reinforced concrete structures, the occurrence of corrosion
in the transverse reinforcements is earlier than the longi-
tudinal reinforcements. This is due to the fact that the
transverse bars are closer to the concrete surface than the
longitudinal reinforcements. On the other hand, those that
have smaller diameters than the longitudinal bars lead to
high vulnerability to corrosion and more serious damage to
structures [6, 7].
Several experimental studies on corroded reinforced
concrete specimens demonstrated that the corrosion of
transverse reinforcements plays a vital role in the behaviour
of reinforced concrete members [8–12]. In this regard, Xia
et al [11] investigated the behaviour of corroded reinforced
concrete beams under a concentrated load. They demon-
strated that there is a considerable discrepancy between the
corroded and non-corroded beams as well as an increase in
the beam displacement when the amount of the concentrated
load is more than 20–30% of the ultimate load. In
another study, Ou and Chen [12] applied a cyclic loading to
corroded reinforced concrete beams. Based on their results,
one can realize that corrosion up to 6% would not have a
significant effect on the beam behaviour. Despite an
increase in the corrosion rate, they concluded that the mode
of failure changes from flexural to flexural-shear, the
amount of ultimate load does not change significantly and
the ultimate displacement and ductility decrease. Shayanfar
et al [13] conducted an experimental investigation into
reinforced concrete specimens with different water/cement
ratios under accelerated corrosion and proposed an equation
for reduced compressive strength caused by corrosion.
Ghanooni-Bagha et al [3] evaluated the reduction of com-
pressive strength attributable to corrosion in self-compact-
ing concrete with mineral admixtures. They concluded that
the compressive strength approximately decreases by 20%
when the crack width roughly increases by 1 mm, indi-
cating 7–12% corrosion in the reinforcements.
Despite various research studies along with several
analytical models concerning the strength of confined
concrete, there is no attempt at considering the effects of
the corrosion on reinforcements. Due to the limited*For correspondence
Sådhanå (2020) 45:49 � Indian Academy of Sciences
https://doi.org/10.1007/s12046-020-1280-0Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)
experimental investigations regarding the corroded transverse
reinforcements, this article experimentally assesses the
reduction of confinement strength of reinforced concrete
specimens caused by the corrosion of stirrups. The
experimental study includes (i) the construction of 22
reinforced and non-reinforced rectangular prism specimens,
in which 12 reinforced specimens are corroded, (ii)
implementation of the test program for 90 days and (iii)
consideration of the corrosion percentage, stirrup diameter
and stirrup spacing as the main variables of the experi-
mental study. Results indicate that the reduction of com-
pressive strength depends on the corrosion percentage and
stirrup diameter. Accordingly, a new formulation is pro-
posed to express the relationship between the compressive
strength reduction and its consequences on the rectangular
cube concrete specimens. The main novelty of the proposed
formulation is to utilize it in order to estimate the residual
strength of reinforced concrete with corroded stirrups for
the rehabilitation and retrofitting of reinforced concrete
structures.
2. Confinement mechanism by stirrups
The confinement of concrete increases the concrete strength
and plastic strain [14–17] and limits the crack growing in
concrete [18]. When the transverse reinforcements have
low stresses, it is reasonable to expect a non-confined
concrete behaviour. On increasing stresses in the transverse
reinforcements the concrete approaches its ultimate
strength, leading to the expansion of internal cracks and
large lateral strains. In such a case, the confinement of
concrete by the transverse reinforcements helps increase the
concrete strength and provide an appropriate confinement
behaviour of concrete [14, 19].
Because the pressure of confinement attributable to
stirrups is applied only at edges where the longitudinal
reinforcements are present, a part of concrete core behaves
as a non-confined area. This phenomenon is depicted in
figure 1. Under a compressive load applied to the concrete,
the lateral strain increases. In such a case the stirrup forces
at the areas of longitudinal reinforcements reach the failure
strength and remain constant, whereas they decrease at the
areas between the longitudinal reinforcements.
Figure 2a shows the confinement of a square concrete
column. In the case of using cross-ties or internal stirrups
for holding the longitudinal reinforcements the other areas
reach the maximum forces, as illustrated in figure 2b for
different reinforcement arrangements.
The distribution of pressure caused by the confinement
depends on the arrangement of reinforcements. Figure 3
depicts the cross-section of a concrete column with four
stirrup legs in the direction y. The confinement stresses in
the directions x and y for calculating the compressive
strength of concrete are given as
flx ¼P
Asfyt
dcy:sð1Þ
fly ¼P
Asfyt
dcx:sð2Þ
where dc, fyt, As and s denote the width of concrete core
perpendicular to the confinement stress, the failure stress of
stirrup, the cross-section of the stirrup leg and the stirrup
spacing, respectively.
There are several analytical models for the stirrup-con-
fined concrete [17]. One of the simplest and most useful
approaches is to use the confinement effectiveness coeffi-
cient (ke). This coefficient is based on the configuration and
longitudinal space of stirrups, and the percentage and
spacing of the longitudinal reinforcements. Table 1 lists
two well-known analytical models of the confinement
effectiveness coefficient. In this table, wi, qcc and nl are the
ith clear transverse spacing between adjacent longitudinal
bars, the ratio of the area of the longitudinal steel to the area
of section core and the number of longitudinal reinforce-
ments at the edge of stirrup or the number of cross-ties
around the column perimeter. By determining the effective
confinement stress in any direction and using figure 4, it is
possible to obtain the strength of confined concrete in the
following forms:
Figure 1. The concrete confinement in columns with rectangular
stirrups [20].
49 Page 2 of 9 Sådhanå (2020) 45:49
flxe ¼ keflx; ð3Þ
flye ¼ kefly: ð4Þ
3. Corrosion mechanism
The corrosion mechanism begins with destroying the pro-
tective layer on the steel bars. In general, the steel rein-
forcements are corroded by moisture and oxygen when
sufficient chlorine ions reach their passive layers. Under
such circumstances the environment serves as an anode,
and the remaining passive steel behaves as a cathode [23].
From figure 5, it can be observed that an increase of rust
volume in the confined concrete leads to an increase in the
internal stress and cracking of the concrete cover [24, 25].
Depending upon the ratio of steel bar diameter to the
concrete cover, the research works in [26, 27] demonstrated
that the reduction of 20–300 lm in the diameter of the steel
bar causes cracking in the cover. On this basis, it is
expected that the cracking will lead to a more severe pen-
etration of destructive factors and considerable damages to
the structure [24, 28].
4. Experimental work
In order to investigate the effect of corrosion of stirrups on
the ultimate strength of confined concrete under uniaxial
compression, the experimental work began with preparing
22 rectangular prism specimens, including the cross-section
of 200 9 200 mm2 and 320 mm height. They were cllasi-
fied into two non-reinforced and 20 reinforced concrete
specimens, of which 12 reinforced specimens were cor-
roded. Furthermore, four standard cylindrical concrete
specimens were constructed to determine the compressive
strength for 28 and 90 days. The variables of experimental
investigation included the percentage of corrosion, and the
diameter and spacing of stirrups. Finally, all specimens
were tested for the compressive strength for 90 days.
On the other hand, the concrete was designed based on
the 28-day compressive strength of 25 MPa. The concrete
mix design is listed in table 2. The cement for the concrete
specimens was ordinary Portland cement (Type II). The
fine aggregate was river sand; the coarse and fine aggre-
gates were crushed, and the maximum size of coarse
aggregate was 19 mm (coarse and fine aggregate density
was 2600 and 2550 kg/m3, respectively, and fineness
modulus of fine aggregate was 2.65). Also, in the concrete
mixes, a superplasticizer was used to reach
suitable workability.
Moreover, the curing of the corroded and non-corroded
specimens was performed under the same condition. The
longitudinal reinforcements of the diameter of 12 mm (d12)
and the stirrups of the diameters of 4, 6 and 8 mm (d4, d6
Figure 2. (a) The lateral pressure in square columns. (b) The lateral pressures for different reinforcement arrangements [21].
Figure 3. The confinement stress caused by the rectangular
stirrups [19].
Sådhanå (2020) 45:49 Page 3 of 9 49
and d8) were applied to construct the reinforced specimens
as shown in figure 6. The yielding stresses of d12 and d8 are
identical at 391 MPa; yielding stress of d6 is 262.4 MPa,
and d4 is equal to 240.92 MPa.
4.1 Specimens preparation
The reinforced specimens were made from 4 longitudinal
reinforcements of the diameter of 12 mm as well as the
stirrups of the diameters of 4 and 6 mm, which were placed
at 70 and 140 mm spaces. At the end of the reinforced
specimens, as shown in figure 7, the stirrups of the diameter
of 8 mm were used to prevent a splitting fracture of con-
crete at the loading zones [29, 30]. The specifications of the
reinforced concrete specimens are available in table 3.
According to ACI 318-14, a 90-degree hook was used for
stirrups by providing a sufficient straight extension [31]. In
order to utilize the accelerated corrosion technique, a
coated copper wire for the connection of middle stirrups
was exploited to provide an electric current. Before
applying corrosion, specimens were cured for 28 days. Due
to corroded specimens exposed to moisture, 90 days spec-
imens are preserved in a water reservoir.
4.2 Applying corrosion to stirrups
The corrosion of steel bars in reinforced concrete members
is an electrochemical process. When an electric direct cur-
rent (DC) is manually applied to steel, one can expect that
the electrochemical process expedites the corrosion of steel.
An effective way of applying damage stemming from the
corrosion of reinforcements in experimental investigations
is the accelerated corrosion technique [13]. On this basis, the
specimens are placed in a plastic tank containing 5% saline
(NaCl) solution. Subsequently, a power supply is used to
apply an external DC [32]. From figure 8, one can discern
that the positive and negative sides of the power supply are
connected to the steel (the transverse reinforcements) and a
copper sheet, respectively. It is worth remarking that the
positive side acts as an anode, while the negative side
behaves as a cathode. In order to facilitate the compressive
strength test, the upper part of the specimen is placed out of
the water surface. It needs to be mentioned that a practical
approach to protecting the reinforcements against corrosion
is to use zinc-rich [33]. Therefore, the longitudinal and
transverse bars of the diameters of 8 mm were covered by
zinc-rich before the process of reinforcement.
4.3 Loading
Before the loading process, the specimens were brought out
from the reservoir containing 5% saline solution and dried
for 1 week in the laboratory environment. Subsequently, all
specimens were subjected to an axial compressive loading
Figure 4. The strength of confined concrete as a function of
effective confinement stresses [22].
Figure 5. The concrete cracking caused by corrosion.
Table 1. Analytical models of the confinement effectiveness
coefficient.
Author Confinement effectiveness coefficient
Mander et al [22]
ke ¼1�
Pw2i
6dcxdcy
� �
1� s2dcx
� �1� s
2dcy
� �
1� qcc
Paultre and Legeron
[17] ke ¼ nl�2nl
� � 1� s2dcx
ð Þ 1� s2dcy
� �
1�qcc
49 Page 4 of 9 Sådhanå (2020) 45:49
test using a 2000 kN concrete compression testing machine.
Eventually, the test was terminated at the failure or fracture
times.
5. Experimental results
This section presents the results of an experimental inves-
tigation into the effect of corrosion on the reduction in the
compressive strength of corroded and non-corroded stirrup-
confined concrete specimens. As discussed earlier, the
specimens were tested at the same age (for 90 days), in
which case one can consider that the increase in the men-
tioned age does not have any influence on the increase of
the concrete strength. Additionally, similar loading rates
were incorporated into all specimens.
In order to evaluate the effect of corrosion of rein-
forcements on the confinement the amounts of corrosion
and confined force were defined as the variables, and the
other parameters for all specimens remain invariant. Based
on Eqs. (1) and (2), the variables of the confined force
included the diameter of the stirrup, the width of the con-
crete core and the spacing of stirrups. In this regard, fig-
ure 9 shows the manner of fracture of the non-corroded
specimens. The ratios of the increase in the compressive
strength of the confined concrete to the non-confined con-
crete (fcc/fc0), as well as the comparison of this ratio with
the analytical models of Mander et al [22] and Paultre and
Legeron [17], are listed in table 4. Also, this table presents
the mean and standard deviation associated with the
Table 2. Concrete mix design in the experimental program.
w/c W
C
(kg/m3)
CA
(kg/m3)
FA
(kg/m3)
fc (28 days)
(MPa)
fc (90 days)
(MPa)
0.48 180 375 896 879 23.4 29.8
c: water to cement; CA: coarse aggregate; FA: fine aggregates.
Figure 6. The geometry and details of specimens [29, 30].
Figure 7. The details of reinforcements.
Table 3. Specifications of the reinforced concrete specimens.
Name Stirrup diameter (mm) Spacing of stirrups (mm)
A 6 (d6) 140
B 6 (d6) 70
C 4 (d4) 140
D 4 (d4) 70
Figure 8. The process of applying corrosion.
Sådhanå (2020) 45:49 Page 5 of 9 49
experimental results and analytical models. The results
have the smallest difference in Mander’s analytical model.
After the corrosion of specimens and the implementation
of their compressive strength test, the amount of reduction
in the confined strength of the corroded specimens con-
cerning the non-corroded ones is determined. According to
the ASTM-G102 standard, the percentage of corrosion is
then obtained using the reduced weight of corroded stirrups
in the specimens. Table 5 gives the values of corrosion
percentages and the parameter of confinement strength
reduction (k) obtained from Eq. (5):
fcc�corr ¼ 1� kð Þfcc ð5Þ
The changes in the reduction of compressive strength
against different corrosion levels and bar diameters are
illustrated in figure 11. As can be seen, the specimens with
small stirrup spacing (i.e., large confinement) indicate
minor effect of corrosion. More precisely, the specimens
prepared from the stirrup d4 demonstrate high levels of
corrosion due to the high corrosion degrees, which lead to
cracking (figure 10). In contrast, the specimens prepared
from the stirrup d6 have minor corrosion percentages. This
is because of the occurrence of cracking (the corrosion
consequence) in minor corrosion degrees. The other
important observation in figure 11 is concerned with the
Figure 9. Process of cracking: (a) the non-reinforced cylindrical
specimen and (b) the non-corroded reinforced specimen after the
separation of the concrete cover.
Table 4. Ratio of increase in the strength of confined specimens and comparison with analytical models.
Specimen fcc/fc0* (MPa) fl (MPa)
Ke fcc/fc0 Experimental/model
[22] [17] [22] [17] [22] [17]
A0 1.14 0.829 0.289 0.145 1.12 1.18 1.02 0.97
B0 1.36 1.659 0.597 0.300 1.35 1.56 1.01 0.87
C0 1.03 0.288 0.287 0.145 1.07 1.11 0.96 0.93
D0 1.1 0.577 0.594 0.300 1.15 1.22 0.96 0.90
Average = 0.99 0.92
1 – average = 0.01 0.08
Standard deviation = 0.03 0.04
Average results of two specimens at the age of 90 days.
Table 5. Relationship between the changes in confined strength
and corrosion.
d6 d4
Specimen Cw (%) fcc/fc k Specimen Cw (%) fcc/fc k
S = 140 mm
A0 0.00 1.14 0.00 C0 0.00 1.03 0.00
A1 2.10 1.11 0.03 C1 7.14 0.95 0.08
A2 3.40 0.99 0.15 C2 8.50 0.94 0.09
A3 5.56 0.91 0.23 C3 10.71 0.95 0.08
S = 70 mm
B0 0.00 1.36 0.00 D0 0.00 1.10 0.00
B1 2.30 1.34 0.02 D1 5.10 1.05 0.05
B2 3.70 1.26 0.10 D2 10.10 1.02 0.08
B3 4.63 1.13 0.23 D3 12.05 1.03 0.07
Figure 10. Damage of the corroded reinforced specimens:
(a) cracking and (b) crack expansion.
49 Page 6 of 9 Sådhanå (2020) 45:49
low slope of the diagram in the low corrosion degrees (i.e.
approximately 2%). When the degree of corrosion increa-
ses, however, one can discern that the diagram slope
increases and the compressive strength decreases signifi-
cantly. Moreover, it is observed from figure 11 that up to
3% corrosion, the slope of the diagram for the stirrups with
larger diameters is small. For corrosion larger than 3%, the
compressive strength is highly sensitive to corrosion.
Based on the results obtained from figure 11, the fol-
lowing formulation expresses the effect of the stirrup cor-
rosion on the reduction of the confined strength for the
preparation of the specimens from the stirrup d6 using the
least-square regression method:
k ¼ 0:05Cw � 0:04 ð6Þ
where Cw denotes the corrosion level. Note that the corre-
lation coefficient is given by 0.83. Figure 12 depicts the
experimental data and the fitting line. After calculating k,the reduced compressive strength can be determined using
Eq. (2).
By eliminating the specimens C3 and D3, the formula-
tion regarding effect of stirrup corrosion on reduction of
confined strength for the preparation of specimens from d4is expressed by Eq. (7):
k ¼ 0:009Cw þ 0:005 ð7Þ
Figure 11. Changes in the reduction of the compressive strength against different corrosion levels and stirrup diameters.
Figure 12. Fitting line for the stirrup d6.
Figure 13. Fitting line for the stirrup d4: (a) all specimens except for C3 and D3 and (b) all specimens.
Sådhanå (2020) 45:49 Page 7 of 9 49
For this equation, the correlation coefficient is 0.91. It is
essential to clarify that the main reason for eliminating the
specimens C3 and D3 is related to their invalid descending
diagram forms. Having considered all specimens, one can
propose Eq. (8) with the correlation coefficient equal to
0.76:
k ¼ 0:007Cw þ 0:015 ð8Þ
Finally, figure 13 shows the experimental data and fitting
line for the stirrup d4.
6. Conclusions
This work experimentally investigated the reduction in the
compressive strength of concrete caused by the corrosion of
stirrups. Accordingly, several rectangular prism specimens
reinforced by constant longitudinal reinforcements of the
diameter of 12 mm and transverse bars of the diameters of
4 and 6 mm placed at 70 and 140 mm spaces were con-
structed and tested under different corrosion degrees in
order to determine the ultimate compressive strength. Since
limited investigations have assessed the reduction of con-
finement caused by the corrosion of stirrups, a limited
number of specimens have been considered for initial
evaluation. However, the results are limited to the condi-
tions and the specimens studied in this article. Further
studies and consideration of various conditions are needed
to obtain a more comprehensive relationship. Based on the
experimental results, a formulation was proposed to cal-
culate the reduction of the compressive strength concerning
the corrosion degree of the bars with different diameters.
The main reason for choosing the bar diameter is related to
the discrepancy in the sensitivity to corrosion.
The experimental results demonstrated that the stirrups
with large spacing and minor confinement were more sensi-
tive to corrosion. Moreover, it was observed that the prepa-
ration of specimens from the stirrups d4 further reduced the
compressive strength in the lower corrosion degrees (ap-
proximately 3%). With increasing degree of corrosion, the
specimens prepared from the stirrups d6 were more sensitive
to corrosion. This conclusionmay stem from the fact that low
corrosion degrees leads to more reductions in the bond of the
stirrups of smaller diameter. To summarize, it can be con-
cluded that the corrosion of transverse reinforcements
decreases the confined strength. However, this reduction
depends on the stirrup diameter and corrosion degree. In this
regard, the stirrups of the diameters of 4 and 6 mm are sen-
sitive to low and high corrosion degrees, respectively. Thus,
it is recommended to use stirrups with small diameters in
areas exposed to serve environmental conditions.
List of symbolsAs cross-section of the stirrup leg
Cw corrosion level
dc width of concrete core perpendicular to the
confinement stress
fc0 compressive strength of non-confined concrete
fcc compressive strength of the confined concrete
flx confinement stresses in the x direction
fly confinement stresses in the y direction
fyt failure stress of stirrup
ke confinement effectiveness coefficient
nl number of longitudinal reinforcements at the edge of
stirrup
s stirrup spacing
wi ith clear transverse spacing between adjacent
longitudinal bars
k parameter of confinement strength reduction
qcc ratio of the area of the longitudinal steel to the area of
section core
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