Investigation on Improving the Residual Mechanical Properties of Reinforcement steel and Bond Strength of Concrete Exposed to Elevated Temperature

Abstract

Concrete and reinforcement steel are essential building materials widely used in composite construction due to their advantages, such as strength, durability and ease of availability. Fire is one of the critical hazards that causes severe damage to the structure and leads to progressive collapse. Due to the intensity of fire exposure, the concrete and reinforcing steel significantly losses their inherent mechanical properties and service life. Minimizing the fire-induced damage and failure are the primary objectives in the design of concrete structures. Therefore, an extensive experimental attempt was undertaken to evaluate the pull out behaviour and also to improve the bond performance of concrete and reinforcement steel when exposed to elevated temperature. A cement based perlite coating was developed as a protective material to safeguard the concrete in the study. All the pull out bond test specimens were heated following the ISO 834 standard fire curve, and subsequently cooled either by air or water. Residual mechanical properties of steel rebar such as yield strength, ultimate strength, elastic modulus, shear and bending capacity were evaluated after the exposure to elevated temperature. Investigations were conducted on pull out specimens to evaluate the bond stress slip behaviour and bond strength of concrete. A detailed physical observation was made on the failed concrete specimens to examine the damage. A drastic reduction in bond strength of concrete and tensile strength of the rebar were observed while increasing the duration of heating. Also, it was observed that the coated specimens exhibited better performance by retaining the bond strength and yield strength of the rebar.

Full text

Case Studies in Construction Materials 16 (2022) e01128
Available online 6 May 2022
2214-5095/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Investigation on improving the residual mechanical properties of
reinforcement steel and bond strength of concrete exposed to
elevated temperature
Tattukolla Kiran
a
,
1
, N. Anand
a
,
*
,
2
, Mervin Ealiyas Mathews
d
,
3
,
Balamurali Kanagaraj
a
,
4
, A. Diana Andrushia
b
,
5
, Eva Lubloy
c
,
*
,
6
, Jayakumar G
a
,
7
a
Department of Civil Engineering, Karunya Institute of Technology and Sciences, Coimbatore, India
b
Department of Electronics and Communication, Karunya Institute of Technology and Sciences, Coimbatore, India
c
Department of Construction Materials and Technologies, Faculty of Civil Engineering, Budapest University of Technology and Economics, Budapest
1521, Hungary
d
L&T Edutech, Larsen and Toubro Limited, Chennai, India
ARTICLE INFO
Keywords:
Elevated temperature
Reinforcement steel
Yield strength
Bond strength
Perlite coating
Pull-out behaviour
ABSTRACT
Concrete and reinforcement steel are essential building materials widely used in composite
construction due to their advantages, such as strength, durability and ease of availability. Fire is
one of the critical hazards that causes severe damage to the structure and leads to progressive
collapse. Due to the intensity of re exposure, the concrete and reinforcing steel signicantly
losses their inherent mechanical properties and service life. Minimizing the re-induced damage
and failure are the primary objectives in the design of concrete structures. Therefore, an extensive
experimental attempt was undertaken to evaluate the pull out behaviour and also to improve the
bond performance of concrete and reinforcement steel when exposed to elevated temperature. A
cement based perlite coating was developed as a protective material to safeguard the concrete in
the study. All the pull out bond test specimens were heated following the ISO 834 standard re
curve, and subsequently cooled either by air or water. Residual mechanical properties of steel
rebar such as yield strength, ultimate strength, elastic modulus, shear and bending capacity were
evaluated after the exposure to elevated temperature. Investigations were conducted on pull out
specimens to evaluate the bond stress slip behaviour and bond strength of concrete. A detailed
physical observation was made on the failed concrete specimens to examine the damage. A drastic
reduction in bond strength of concrete and tensile strength of the rebar were observed while
* Corresponding authors.
E-mail addresses: tattukollakiran@gmail.com (T. Kiran), nanand@karunya.edu (N. Anand), mervi.567@gmail.com (M.E. Mathews),
balakmurali31@gmail.com (B. Kanagaraj), andrushia@gmail.com (A.D. Andrushia), lubeva@web.de (E. Lubloy), jayakumar.civil@gmail.com
(J. G).
1
0000-0001-7445-5989
2
0000-0001-7643-9747
3
0000-0003-0170-2138
4
0000-0003-3006-370X
5
0000-0003-0001-2733
6
0000–0001-9628–1318
7
0000–0001-5348–6452
Contents lists available at ScienceDirect
Case Studies in Construction Materials
journal homepage: www.elsevier.com/locate/cscm
https://doi.org/10.1016/j.cscm.2022.e01128
Received 30 December 2021; Received in revised form 26 April 2022; Accepted 29 April 2022

Case Studies in Construction Materials 16 (2022) e01128
2
increasing the duration of heating. Also, it was observed that the coated specimens exhibited
better performance by retaining the bond strength and yield strength of the rebar.
1. Introduction
Reinforced concrete is primarily building material used in the construction industry. Concrete is a composite material and its
thermal behaviour depends on its constituents. Structural concrete looses its mechanical properties above 500 ◦C, which is being
considered as a critical temperature for the strength loss [1]. However, the mechanical properties such as compressive strength, tensile
strength, exural strength and bond strength of concrete are entirely different under the effect of elevated temperatures. All these
properties are mainly dependents on the design compressive strength considered for design application. As there is an inter relation
between the compressive strength and other mechanical properties, design critical temperature may affects the strength properties of
concrete [2]. The temperature variation may signicantly affect the strength properties by increasing the demand in safety factor.
Appropriate protection methods can retain the mechanical properties of concrete from the effect of critical temperature.
Generally, the cover of RC structural elements is designed to shield the reinforcement bar from the hazard of aggressive envi-
ronments or to protect from the re exposure [3]. The sulfate deterioration or temperature exposure may signicantly affect the
strength of cement-based materials. Therefore it has become a severe durability problem in onshore and offshore structures [4,5]. The
concrete cover plays a prominent role in safeguarding the reinforcement steel against elevated temperatures. With the failure of
concrete cover, the re exposure can directly reach the reinforcement bar, and it may reduce the mechanical properties of the rein-
forcement steel. Occasionally, due to prolonged exposure, the structure may experience progressive collapse [6].
The Reinforcing steel retains its strength only up to 500 ◦C beyond which a drastic reduction occurs [7]. Reinforcing bars showed a
reduction in strength of 10–15% at a temperature of 600 ◦C [8]. At temperatures between 700 ◦C and 950 ◦C, the mechanical properties
of rebar had reduced by 50–65% [9]. The modulus of elasticity of the rebar dropped almost to its initial value, and the rebar with a
higher diameter showed higher performance at elevated temperatures [10]. Specimens exposed to elevated temperatures between
500 ◦Cand 700 ◦C were observed with a change in microstructure. A decrease in pearlite, cementite and an increase in ferrite were also
observed due to the changes in microstructure. A signicant strength reduction was noticed in the reinforcement bars after the re
exposure [11]. The yielding of steel taking place in a reinforced concrete bending member, when it is subjected to transverse loading.
Under bending the embedded steel undergoes bending tension. However, the tension capacity of steel is generally represented by
conducting direct tension test. The tensile stresses are taken by steel bars in the bending members made of RC. Therefore, other than
yield strength, the bending strength and its deformability of steel are to be evaluated to ensure the safety. An attempt has been made in
the present study to investigate the bending ability of steel bar in terms of its strength and deformation.
Due to the re exposure, spalling occurs in the cover concrete portion beyond which the temperature reaches the reinforcement bar
and affects the yield strength and elastic modulus. Due to different types of cooling, which are adopted to quench the re in practice the
mechanical properties are affected signicantly [7]. Air cooling is the natural process; however water cooling is also preferred to cool
the structural elements after the re exposure. The post re performance of reinforcing steel varies based on several parameters such as
type of steel (cold-formed, hot rolled, high strength steel or mild steel etc.), nature of cooling, rate of heating, chemical compositions,
the diameter of the rebar and nature of loading. These parameters strongly affect the residual mechanical properties such as yield
strength and elastic modulus of the reinforcement bar [10].
Bond between the concrete and reinforcing steel is considered to be an important strength property, as it is mainly contributing to
the load transformation in the RC structure. Failure of the re exposed reinforced concrete members are mainly due to its reduced bond
strength between the concrete and embedded reinforcement steel [12]. Therefore RC structures should be designed to satisfy the re
safety requirements specied in building codes [13–16]. After exposure to the elevated temperatures of 200–600 ◦C the bond strength
of recycled aggregate concrete decreased between 15% and 66% as compared to original unheated specimen [17,18].
Reduction in bond strength of concrete is mainly depends on the intensity of heating, exposure duration, diameter and embedded
length of steel reinforcement and strength grade of concrete. With an increase in temperature, the bond strength decreases by
increasing its slip values [12]. As a result of heating, the bond strength deteriorates due to the formation of micro cracks developed in
the concrete. It was reported that the temperature exposed specimens exhibited combined splitting–pull-out failure [19,20]. At
temperature (500 ◦C- 700 ◦C), the bond stiffness of concrete reduces drastically with the increase in temperature exposure. The
degradation of bond strength was found to be similar to that of compressive and tensile strength of concrete [21]. Giovanni and Andrea
(2021), evaluated the local bond properties between ordinary concrete and carbon steel subjected to elevated temperatures. It was
observed from the results that a reduction in residual bond strength of concrete lower than 50% is associated to temperatures higher
than 50 ◦C [22].
The reduction in material properties of materials due to the effect of elevated temperature under extreme re conditions may affect
the performance of structural members. Protecting the material properties by appropriate techniques may safeguard the structural
members by postponing the failure during re accidents. This is helpful to ensure the occupants serviceability by improving the
evacuation time during the event of building re. Therefore, it is the interest of authors to investigate the inuence of lightweight
protective coating on improving the re resistance of steel. Chartek 7 intumescent coating and the intergard 251 primer coating were
used as a coating material to protect the steel reinforcement from the re exposure. A good adhesion was observed and the mechanical
properties were retained up to 672 ◦C for the duration of 2 h [23]. At 175 ◦C, the intumescent coating reinforced with ceramic wool
ber show the better re performance and reduces the temperature penetration around 34.7% as compared to the unreinforced coating
T. Kiran et al.

Case Studies in Construction Materials 16 (2022) e01128
3
[24]. At higher temperature exposure, the composition of cementitious perlite binder with micro zinc pigment showed the better
protection as a sacrice material by retaining higher residual mechanical properties of structural steel [25].
Therefore, in the present investigation an attempt has been made to assess the post re performance of concrete and steel by
conduction appropriate strength tests. Effect of heating, inuence of cooling and contribution of perlite coating protection are some of
the parameters that were considered in the investigation for the analysis of test results. Many researchers conducted extensive in-
vestigations on mechanical properties of reinforcement steel subjected to different elevated temperatures [7–11]. There is a lack of
information on improving the post re performance of reinforcement bar after being exposed to higher temperatures. Currently, in-
formation is not available on the re resistance of reinforcement bars protected with cementitious perlite coating under re exposure.
Shear and bending capacity of reinforcement steel after the re exposure has not been studied yet. A detailed visual observation has
been made to analyse the damage of failed concrete and steel specimens in this investigation, which are not mostly covered in the
earlier studies.
1.1. Research signicance
The present investigation aims to evaluate and improve the performance of reinforcement bars after being subjected to elevated
temperature. The high yield strength deformed reinforced bars were used in the investigation. A cementitious perlite coating was
applied to the reinforcement specimens. In order to evaluate the performance of reinforcement bars, elevated temperature test con-
ducted for 30, 60, 90, and 120 min duration of heating. Temperature was maintained at a steady state condition for up-to 1 h duration
as per ISO 834 standard re curve. After heat exposure, two cooling methods were adopted to study the inuence of the rate of cooling,
air or ambient cooling and water cooling respectively. The residual mechanical properties of yield strength, ultimate strength, elastic
modulus, bend and shear capacity were evaluated. Cube compressive strength and bond strength of concrete were also examined for
the heated specimens. Failure patterns were examined for reinforced bars and concrete bond specimens after the exposure to un-
derstand the extent of damage.
2. Materials
2.1. Details of mix
Ordinary Portland cement (OPC) 53 grade conforming to IS 12269(2013) was used in the experiments [26]. The M-sand (man-
ufactured sand) was used as ne aggregate according to the guidelines of IS 383(2016) [27]. Crushed coarse aggregate was used with a
maximumsize of 20 mm. Potable water was used during the mixing and curing process. The properties of the materials used in the mix
proportions are mentioned in Table 1.
The concrete mix proportion was designed to achieve the M30 grade concrete. Moderate exposure condition with the target slump
ranging between 100 mm and 125 mm was considered in the mix design as per IS 10262 (2019)[28]. The details of the mix proportion
used in the investigation are illustrated in Table 2. The cube specimen of size150×150×150 mm
3
and bond specimen of size
100×100×100 mm
3
were cast and cured for 28 days. After 28 days of curing, the specimens were kept at room temperature to suf-
ciently remove the moisture prior to the heating test.
2.2. Reinforcement steel
High yield strength deformed (HYSD) bars Fe 500, Ø12mm dia and 400 mm length were obtained from a local manufacturer and
used in the experiments, as per the specications of IS 1608:2005 [29]. Fig. 1(a) shows the view of steel bars used in the investigation.
The chemical and mechanical properties of the reinforcement bar are given in Table 3.
2.3. Cementitious perlite coating (CPP)
Perlite is a porous mineral with low density and it is extracted from mined volcanic glass, as shown in Fig. 1(b). It is used as an
insulation material to resist higher temperatures. Moreover, the resistance offered by perlite during the exposure to re is in the range
between 1260 ⁰C and 1340 ⁰C [30]. The physical and chemical properties of perlite are summarized in Table 4. The Coating of
specimens was made with the composition of synthetic alkyd binder, micro zinc pigment, perlite ne powder and cement. The pre-
pared coating was applied on the reinforcement bars and allowed to dry in the air for 48 h at room temperature (27 ⁰C). Fig. 1(c)
represents the prepared cementitious coating which is applied on rebar.
Table 1
Material properties.
Material Density (kg/m
3
)Specic gravity Fine modulus Water absorption (%)
Cement 1438 3.15 – –
Fine aggregate 1624 2.61 2.58 –
Coarse aggregate 1797 2.94 – 115
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3. Experimental program
3.1. Testing of specimens under elevated temperature
A computerized digital temperature-controlled electric furnace is used to heat the concrete cube and bond specimens. Its sides and
the top are lined with electrical heating coils embedded in refractory bricks. The dimensions of the furnace are 700×500×500 mm, as
shown in Fig. 2(a). The maximum temperature capacity of the furnace is 1200 ◦C. All the reinforcement, concrete cube and bond
specimens are heated for the duration of 30 min (821 ◦C), 60 min (925 ◦C), 90 min (986 ◦C) and 120 min (1029 ◦C) by following the
ISO 834 standard re curve [31]. In the case of coated specimens, they are heated for 30 min (821 ◦C) and 60 min (925 ◦C). During the
heating, once the target temperature is reached for the desirable duration, the steady state of heating is followed for one hour duration.
After the targetable heat exposure, the furnace switches off automatically. Then the specimens are cooled under two conditions, either
by air cooling or water cooling. In realistic scenarios, both cooling methods are common after a re accident. The rate of reduction in
temperature for air and water cooling are recorded by control panel and infrared thermometer respectively. The process of air-cooled
specimen is shown in Fig. 2(b). The specimens belonging to water cooling are cooled by water jets kit as shown in Fig. 2(c). Details of
Table 2
Details of mix proportion.
Mix
ID
Cement (kg/
m
3
)
Fine aggregate (kg/
m
3
)
Coarse Aggregate (kg/
m
3
)
w/c
ratio
Super plasticizer
(%)
Slump
(mm)
Compressive strength
(MPa)
CC 385 815 1095 0.5 – 115–134 34.5
Fig. 1. View of (a) reinforcement bars (b) lightweight expanded perlite (c) Improvement coating on reinforcement steel bars (I).
Table 3
Chemical and mechanical properties of the reinforcement.
Chemical properties (%)
Carbon (C) Manganese (Mn) Chromium (Cr) Copper (Cu) Sulfur (S) & Phosphorus (P) Molybdenum (Mo)
0.25 1.15 0.091 0.15 0.08 0.0102
Mechanical properties
Yield strength (MPa) Ultimate strength (MPa) Elongation (%) Reduction in rebar (%) Elastic modulus(MPa) F
u
/f
y
ratio
510 600 18 78.5 2.1 ×10
5
1.107
Table 4
Physical and chemical properties of expanded perlite.
Physical properties Chemical composition
Color White Silicon dioxide(SiO2) 71–75%
Bulk density (kg/m
3
) 70 Alumina Oxide(Al2O3)9–16%
Specic gravity 2.21–2.40 Potassium Oxide(K2O)2–5%
Thermal conductivity (w m
−1
◦C
−1
) 0.031–0.035 Sodium Oxide(Na2O) 1–4%
Specic heat (kJ kg
−1
◦C
−1
) 0.22 Ferric Oxide(Fe2O3)0.5–1.4%
Size 3.0–0.6 mm Calcium Oxide (CaO) 0.7–1.4%
Free moisture 0.5% max Magnesium oxide (MgO) 0.55–1.11%
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the test specimens are illustrated in Table 5.
Type-K thermocouples were used to measure the temperatures as a function of time in the concrete cube and bond specimens at
different locations (surface and core). An integrated microprocessor-based data acquisition system was used for processing the time-
temperature response.Fig. 3 shows the heating-cooling cycle regime reinforcement and concrete specimens.
3.2. Test on mechanical properties of concrete and reinforcement specimens
3.2.1. Tension test
After the temperature exposure, the air and water cooled specimens were tested immediately. The tension test was carried out as
per the guidelines of IS 1608 (2005) [29]. A computerized Universal Testing Machine (UTM) of 1000 kN capacity was used to carry out
the experiments shown in Fig. 4(a). The rebar was xed at the top and bottom jaws of the UTM. The tension test was performed based
on the load control at a rate of 5 mm/min till the failure. The applied tensile load and the extension reinforcement bar values were
recorded in the UTM at corresponding intervals with the use of a data acquisition system, respectively.
3.2.2. Double shear test
The shear test is performed to determine the ultimate shear strength of the reinforcement bar. A load controlled Universal Testing
Machine (UTM) is used to carry out this test. The reinforcement bar is placed in the shear test set up which is shown in Fig. 4(b). The
vertical load is applied on the specimen in such a way that to create the shearing, while increasing the loading till level the rein-
forcement bar is cut into three pieces and maximum load is recorded corresponding to the specimen breaks. Therefore, the shear
strength is calculated by maximum load applied at which the specimen breaks divided by the cross sectional area of the specimen.
3.2.3. Bend test
Reinforcement bars are subjected to bend during the fabrication of structural elements in various forms. Also some times during
placing and transporting the rebar may bend and it leads to reduction in mechanical properties. Considering this, the present study is
conducted to evaluate the bend strength and deformability of rebar. The test arrangement and procedure was followed as per ASTM E-
290 [32]. Fig. 4(c) shows the experimental set up of a bend test with reinforcement steel. The bar to be tested is supported by two
supports with a distance of three times the bar diameter and the plunger. The load is applied on the middle of the reinforcement
specimen through a plunger between the two supports. The load is applied till the bend of almost 180 ⁰C to ensure the material
ductility. All the heated specimens are tested carefully and results are reported.
3.2.4. Compressive strength
Compressive strength of concrete is evaluated for the specimens before and after the exposure to elevated temperature. Cube
specimens of size 150×150×150 mm were used in the study. The specimens were tested after the 28th day as per the guidelines of IS
Fig. 2. View of (a) furnace (b) heated specimens (c) water cooling process of reinforcement and concrete specimens.
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516 (2004) [33]. The specimens were tested using a Compression Testing Machine (CTM) of 2000 kN capacity at a loading rate of
180 kg/min as shown in Fig. 4(d). Compressive strength values were reported based on the average value of three specimens.
3.2.5. Pull out test
The bond friction between concrete and rebar is considered as an important factor that ensures the load transfers in the structural
elements. The dimension of the bond specimen is 100×100×100 mm. A thermo mechanical treated (TMT) rebar of 400 mm length
with a 12 mm diameter is used to embed with in the concrete.The experiment was conducted as per the guidelines of IS, 2770 (2007) .
The pullout test was performed using a universal testing machine with a maximum capacity of 1000 kN. The pull out test setup was
done and it is shown in Fig. 4(e). The concrete bond specimen was positioned in the UTM in such a way that the bar could be drawn
axially from the bond specimen. The dial gauge was xed at the bottom tip of the rebar, the load and corresponding slip readings of the
reinforcement were noted until the failure of specimens. Bond strength of concrete was calculated by dividing applied load by the
surface area of the specimen using the following Eq. (1).
τ
bu =pmax
π
DL (MPa)(1)
Where p
max
=Ultimate load; D=dia of the rebar; L =Length of the rebar embedded in the concrete.
4. Results and discussion
4.1. Time temperature measurement
Table 6 shows the details of temperature measured in the concrete at different stages. Fig. 5 shows the location of thermocouple in
concrete cube. Due to the effect of soaking period, the target temperature penetrates and well distributed throughout the concrete
cube. After the soaking period, the concrete specimens were tested and the results of the residual compressive strength were presented.
The time–temperature responses of concrete cube specimens are shown Fig. 6.
Table 5
Details of the test specimens.
Mechanical properties Dimension of the specimen Duration of heating
(min)
Type of
cooling
Stress strain behavior, Yield strength, Ultimate strength,
Elastic modulus.
0,30,60, 90,120 Air/Water
Shear strength 0,30,60, 90,120 Air/Water
Bend strength 0,30,60, 90,120 Air/Water
Compressive strength 0,30,60, 90,120 Air/Water
Bond strength 150×150×150mm 0,30,60, 90,120 Air/Water
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Fig. 3. Heating cooling regime of heated specimens.
Fig. 4. View of test setup (a) tension test (b) shear test (c) bend test on rebar (d) compressive strength test (f) pull out test.
Table 6
Details of temperature measurement.
Duration of
heating
&Temperature
Temperature (⁰C) at surface
(T1)
Temperature at core after reaching the target
temperature (T2)
Temperature at core after the soaking period
(T3)
30(821 ◦C) 821 565 821
60(925 ◦C) 925 750 925
90(986 ◦C) 986 825 986
120(1029 ◦C) 1029 896 1029
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4.2. Physical observations
4.2.1. Steel reinforcement after exposed to elevated temperature
After the heating and cooling process, the damage level of reinforcing steel bars was examined. The changes thatoccurred in surface
texture and color changes of reinforcing steel were observed for air and water-cooled specimens, as shown in Fig. 7. It is noted that in
Fig. 5. Location of thermocouples in the concrete cube.
Fig. 6. Time temperature response of concrete cube specimens after soaking period. Note: I-Measured temperature @target level, II-Measured
temperature after the soaking period.
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the case of 30 and 60 min duration of heating, the surface texture become black with scratches on the ribs that are slightly peeled for
both air and water-cooled specimens. But in the case of specimens with perlite coating exposed to 30 min duration of heating, the
surface damage was not signicant for both air and water-cooled specimens. However, due to high-temperature exposure, the color of
the coating changed from dark yellow to dark brown and for water-cooled specimens, white layers were visible on the surface due to
the reaction of dry hydroxide.
Further increasing the duration of heating between 90 and 120 min, color of the specimens turned from buff gray to bluish dark
with minor silver-white patches on the surface of steel. This is due to the increase in temperature beyond 700 ◦C, the metal on the
surface of the steel rebar reacts with the oxygen in the air to form a metal oxide [35]. This leads to an increase in the weight and a
change in color. As the temperature increases, the steel layer peels off with a color change was observed. In coated specimens at 60 min
of heating, signicant damage was observed in the coating layer for both cooling modes. Immediately after the temperature exposure,
the coating was removed to examine the surface defects in the reinforcing steel. Color changes and surface damage were not observed
for the specimens coated with perlite layer.
4.2.2. Concrete specimens after exposed to elevated temperature
After the cooling process, surface cracks and spalling was observed on specimens as shown in Fig. 8(a) and 8(b). The crack widths
were evaluated by using elcometer. The elcometer measurement is in the range of 0.0–1.8 mm with a least count of 0.02 mm. In the
case of air-cooled specimens, at 30 min exposure the visible surface damages were not identied [36]. But the minor cracks were noted
and the crack width was found to be in the range of 0.03–0.05 mm. It was noticed that, the crack width on the surface of the specimens
Fig. 7. Surface damage and color changes of reinforcement specimens for different heating and cooling conditions.
Fig. 8. Surface view and crack width images of concrete specimens exposed to elevated temperature (a) air cooling (b) water cooling.
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increased with a rise in duration of heating 60–90 min. For the specimens heated up to 120 min, wider thermal cracks were observed
with higher crack propagation. Specimens exposed to water cooling exhibited minor cracks for 30 min heating duration. Wider cracks
were observed at the heating duration of 60 and 90 min. It is due to evaporation of absorbed water present in the concrete micro-
structure and excessive drying on sudden cooling which leads to the formation of thermal cracks on the surface concrete after being
exposed to elevated temperature [37].
Fig. 9 shows the crack width values of concrete specimens exposed to elevated temperature. The result was conducted by recording
more number of crack widths in the concrete surface. The measured average values are considered for the result analysis. In case of
30 min duration of heating, the crack widths are found to be 0.05 mm and 0.10 mm for air and water cooling specimens. For 120 min
heated specimens of 0.41 mm and 0.52 mm for air and water cooled respectively. From the crack width measurements, it can be
concluded that as the duration of heating increases the crack density of thermal cracks increases signicantly [38]. Moreover, larger
crack widths are noticed at 120 min duration of heating and more cracks are noticed for water cooled specimens than for natural air
cooled specimens. It is because the decomposition of Ca(OH)
2
into CaO (lime) and H
2
O (water) during the cooling process results in a
severe damage to the concrete surface. The surface of water cooled specimens changed to whitish color and the similar observations
were reported by [36].
4.3. Stress- strain behaviour of reinforcement steel after temperature exposure
A well dened stress and strain behavior of reinforcement steel was observed for both heated and unheated specimens. Fig. 10(a)
and (b) shows the stress strain behaviour of reinforcement steel specimens exposed to different elevated temperatures. From the
plotted Fig. 10, it is observed that peak stress of the rebar decreases with increasing duration of heating. A stress reduction was initiated
at 30 min duration of heating due to the softening of steel at higher temperature. Unprotected specimens exposed to 60 min, 90 min
and 120 min of heating, exhibited higher strength reduction for both air and water cooled conditions. However, in the case of coated
specimens,the strength reduction was found to be less than that of the unprotected specimens.
On the other hand, the ultimate strain increases with increase in temperature. In fact, it is noted that at higher temperature
exposure, the surface damage and micro scratches were identied on reinforcement steel. Further, water cooling on the specimens
results in lesser yield strength and higher strain values. This condition is more pronounced to higher durations of heating (90 and
120 min). A descending curve was noticed; once the steel rebar reaches its ultimate strain which indicates the failure of specimen.
Similar observations are reported by [39].
4.3.1. Residual yield strength, ultimate strength and elastic modulus
From the experiment of stress-strain behaviour of reinforcement steel a valid data was demonstrated such as yield strength, ulti-
mate strength and elastic modulus. As these material properties of reinforcement steel contributing in deciding the design strength of
reinforced concrete elements, this investigation is conducted for both reference and heated specimens. After the temperature exposure,
the material properties were evaluated for both air and water-cooled reinforcement steel specimens.
The yield and ultimate strength values of rebar for both air and water cooled specimens are illustrated in Fig. 11(a) and 11(b). From
the experimental results it is found that as the temperature increases the yield plateau decreases and the same decline trend is observed
up to 120 min duration of heating. The reduction in yield and ultimate strength was observed as 30%−22% for air and water cooled
specimens at 30 min (821 ◦C) duration. Reduction in ultimate strength was found to be 24%−14%. Loss in yield strength of the
specimen exposed to 60 min (925 ◦C) was observed as 43%−38% and for ultimate strength the reductions were 29.5%−22.7% of its
initial strength. In the case of 90 min (986 ◦C) duration, the loss in yield strength was found to be higher in the rage of 67.5%−65.5%,
Fig. 9. Thermal crack width of concrete specimens.
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and for the ultimate strength it was 68.5%−58.6%.
At 120 min (1029 ◦C) exposure, the observed loss in yield and ultimate strength loss was 87.2%−81.5% and 72.5%−71.8%
respectively. However in the case of coated specimens exposed to 30 min duration, the strength loss was noticed as 2.25%−3.5% and
for 60 min duration the specimen losses its yield strength in the range of 12.2% −9.5%. At higher durations of heating, the mechanical
properties of steel reduce drastically.
From the test results it is observed that at 60, 90 and 120 min duration of heating the mechanical properties of the reinforcement
steel decreases gradually. The loss in strength is mainly attributed due to the grain size and chemical composition of steel. Beyond the
60 min duration of heating, the outer layer of martensite vanishes completely and the microstructure transforms into the grains of
austensite and ferrite throughout the cross-section. Further the austensite transforms into pearlite when a highly heated specimen
cooled to room temperature results in the formation of ferrite and pearlite throughout the reinforcement steel [40]. Percentage in-
crease in ferrite proportion declines the tensile strength, and this may be due to the transformation of pearlite and ferrite as reported by
[10]. After exposure to higher temperature exposure, the reinforcing steel losses its mechanical properties, it is due to the changes in
chemical properties of steel as reported by [35].
At 30 min duration of heating, the coated specimen retains the residual strength about 15–20% when compared to the heated
specimen. But in the case of 60 min duration of heating the performance retention capacity is low between (10–12%) strength loss is
decreased as similar to 60 min heated specimens. For all the cases, the air cooled specimens exhibited poor performance when
compared to water cooled specimens. It was evident that the coated specimen showed excellent protection at 30 min duration heating.
Based on the past experimental studies, it can be inferred that the range of 500 ◦C to 800 ◦C was critical to the strength loss of
reinforcement steel [8]. In the present study, it is remarkable that up to 821 ◦C the reinforcement steel offers good performance due to
its protection.As stated in the literature, 500 ◦C is critical to strength loss; it is esteemed to protect the steel reinforcement to safeguard
Fig. 10. Stress strain behavior rebar after re exposure (a) air cool (b) water cool.
Fig. 11. Residual strength of reinforcement steel after elevated temperature (a) yield strength (b) ultimate strength.
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the structural elements from the effect of re.
Elastic modulus of reinforcement steel for both heated and unheated specimens were calculated from the elastic range of the stress
strain graph [40]. The similar results were observed for elastic modulus as like yield and ultimate strength of steel. The residual elastic
modulus values were found to reduce drastically beyond 60 min duration of heating, and the strength loss was more pronounced at
90 min and 120 min durations of heating. The reductions in elastic modulus were found to be in the range of 60.1%−72.5%
respectively. While increasing the duration of heating the high strength steel bar loses its yield strength and shows the poor ductility.
However in the case of coated specimens, better performance was observed up to 30 min duration of heating and beyond that the
performance was not remarkable. In the case of cooling type, the specimens of water cooling exhibit lower strength loss than air cooled
specimens for both coated and uncoated type of specimens. Furthermore, while increasing the duration of the heating the water cooled
specimens showed minor strength gain between 2.1%−5.2% at the duration of 60 min and similar observations are reported by [10].
4.3.2. Reduction factor
Based on the results, the reduction factors of yield strength, ultimate strength and elastic modulus were calculated by the ratio of
strength of heated specimens to the strength of reference specimens which are shown in Fig. 12(a), 12(b) and 12(c). The comparison
was made to understand the post re behavior of reinforcement steel with the test results of past investigations. The comparison was
done in the similar type of specimens used in the various studies by the researchers. Very few researchers have addressed the effect of
water cooling on the residual strength of reinforcement steel.
From the results of reduction factors data it was noticed that the temperature between 200 ◦C −500 ◦C the strength gain was
observed from the past studies. For the temperature above 500 ◦C-800 ◦C, the strength loss was remarkable, but above 900 ◦C the
strength loss was more prominent due to the higher temperature exposure. Similar observations of reduction in strength were observed
in the present study because at higher temperature exposure the physical, chemical and microstructure changes may lead to a sig-
nicant strength loss of reinforcement steel.
A numerical equation was developed using the experimental dataset to predict the reduction factors of yield strength, ultimate
strength and elastic modulus of reinforcement bars. Results of the developed relation (2a to 2 f) has been evaluated with experimental
Fig. 12. Reduction factor of (a) yield strength(b) ultimate strength(c) elastic modulus.
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values of yield strength, ultimate strength and elastic modulus of steel at ambient and elevated temperatures. The predicted reduction
factor values are found to be in good agreement with experimental values.
Reduction factor for Yield strength.
Air cooling:
28 ≤t≤1029◦C;fyt
fy
= − 6.51 ×X10−9(T)3+1×X10−5(T)2−0.0043(T) + 1.1115 (2a)
Water cooling:
28 ≤t≤1029◦C;fyt
fy
= − 4.32 ×X10−9(T)3+5×X10−6(T)2−0.0014(T) + 1.0355 (2b)
Reduction factor for Ultimate strength.
Air cooling:
28 ≤t≤1029◦C;fut
fu
= − 4.56 ×X10−9(T)3+6×X10−6(T)2−0.0022(T) + 1.0577 (2c)
Water cooling:
28 ≤t≤1029◦C;fut
fu = − 4.66 ×X10−9(T)3+6×X10−6(T)2−0.0022(T) + 1.0577 (2d)
Reduction factor for Elastic modulus.
Air cooling:
28 ≤t≤1029◦C;Et
E= − 6.48 ×X10−9(T)3+1×X10−5(T)2−0.0044(T) + 1.1138 (2e)
Water cooling:
28 ≤t≤1029◦C;Et
E= − 4.76 ×X10−9(T)3+6×X10−6(T)2−0.002(T) + 1.0513 (2 f)
Where,fyt,fut,Et are the yield, ultimate and elastic modulus values after the temperature exposure,fy, fu, E are the yield, ultimate and
elastic modulus at ambient temperature, T is the temperature exposure.
4.4. Shear strength of reinforcement steel
Fig. 13 shows the shear strength of reinforcement steel exposed to elevated temperature and cooled by air or water. From the
experimental results it was observed that all the heated specimens exhibited signicant reduction in the shear strength of rein-
forcement steel, it is evident from the results shown in the (Fig. 13). At 30 min duration of heating the loss in residual strength observed
as 26.1% (AC) and 23.2% (WC) when compared with reference specimens. In the case of 60 and 90 min duration of heating, a similar
trend of loss in shear strength was observed and it was about 33.4%−40.5% and 30.1%−35.8% for air and water cooled specimens
respectively. Further at 120 min duration of heating, a signicant strength loss is observed as 60.2%−56.3% (AC&WC). In the case of
coated specimens at 30 min duration of heating the strength loss is found to be 3.25%−5.06%, however at 60 min duration heating the
Fig. 13. Shear strength of reinforcement steel exposed to elevated temperature.
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strength loss was observed as like uncoated heated specimen for both cooling conditions. The current investigation conforms that
coated reinforcement exposed to 30 min retains the residual strength as compared to heated specimens. Also, the results indicate that
as the duration of heating increases the shear strength of reinforcement steel decreases drastically. The air cooled specimens were
found to have higher strength loss than the water cooled specimens.
4.5. Bend test on reinforcement steel
The bend strength result of rebar is presented in Fig. 14. From the results, while increasing the duration of heating the reduction in
bend strength of reinforcement was observed for both air and water cooled specimens. At 60–120 min of heating, the higher strength
reduction was observed as 53.25–67.29% when compared to the reference specimen. In the case of coated specimens, up to 30 min
duration of heating a minor strength loss was observed, however at beyond 30 min of heating the strength reduction observed was
similar to that of uncoated specimens. The strength loss was higher for the specimens with prolonged durations of heating due to the
inuence of thermal exposure. Major patches and loss of ribs were observed in the reinforcement steel which is shown in Fig. 5. Among
air and water cooling, the air cooled specimens exhibit higher strength loss than water cooled specimens. The coated specimens could
retain the strength properties for up to 30 min duration of heating, like other mechanical properties. As the strength of steel is one of
the key design parameters, the residual bend strength of rebar is gaining more importance in the investigation.
It is due to the prolonged steady state temperature exposure for a period of one hour. Therefore the coating is effective in protecting
the materials up to 30 min (821 ◦C), beyond which the yield strength, bend strength and shear strength reduce considerably.
4.5.1. Load-deformation behaviour of re damaged reinforcement steel
The ultimate load and deformation values recorded the damage to determine the bend strength of steel. Figs. 15a and (b) shows the
load deformation behaviour of reinforcement steel specimens that were exposed to elevated temperature after the cooling process. It is
seen from the plotted curve that, the load-deformation curve of the reference specimen is linear in the initial stage of loading with a
lesser deformation than the exposed specimens. For the temperature exposure specimens, the load values are found to decreases with
an increase in the deformation values.
It is observed that, the higher reduction in ultimate load was found in the range of 53.2–67.2% at 120 (1029⁰C) minutes duration
for air and water cooled specimens as compared to reference specimen. The deformation of unexposed specimen is noticed as 110 mm.
In the case of 30 (821 ◦C) and 60 (925 ◦C) minutes of exposure an increase in the deformation was observed between 12.2% and
16.7%. For 90 (986 ◦C) and 120 (1029 ◦C) minutes of exposure the higher deformation values were observed between 19.7% and
21.9% as compared to the reference specimen. Similar trend was observed in the deformation behaviour of water cooled specimens.
The specimens cooled by air have shown in higher deformation i.e., between 7.5% and 10.2% than that of the specimens cooled by
water spraying. At prolonged duration of heating (1029 ◦C), once the specimen reaches the ultimate load, the deformation values were
found toincreases drastically.
4.6. Compressive strength of concrete
The compressive strength of concrete after the exposure was evaluated by experimental results and it is illustrated in Fig. 16. The
characteristic compressive strength of concrete was achieved as 34.5 MPa, for the specimen which was not exposed to elevated
temperature. In the case of heated specimens, at 30 min duration of heating a sharp strength gain was observed as 3.8%. This is due to
the rehydration of unreacted cement paste which is inuenced to evaporate the free water in the concrete internal pores during
temperature exposure.
Fig. 14. Residual bend ultimate load of Reinforcement steel exposed to Elevated Temperature.
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Fig. 15. Load-deformation behaviour of reinforcement steel specimens (a) air cool, (b) water cool.
Fig. 16. Compressive strength of concrete specimens exposed to elevated temperature.
Fig. 17. Residual compressive strength results exposure specimens from literature.
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However specimens exposed at 60 min and 90 min duration of heating exhibited a higher strength loss of about 22.4% and 35.4%
(AC&WC) and 36.4% and 48.2% (AC&WC) respectively when compared to reference specimens. At 120 min duration of heating, both
air and water cooled specimens showed worst performance when compared with reference specimens and the strength loss was found
to be between 51.2% and 65.5%. Due to the decomposition CSH gel, the cement paste loses its binding properties and calcium hy-
droxide turns into calcium oxide, which results in a loss in compressive strength of concrete. After testing, the internal concrete
surfaces were observed through the naked eye, it was noted that the bond between aggregates and cement paste was poor and also had
voids and minor cracks which were observed, due to the prolonged re exposure. The color of aggregate turns from blue to reddish due
to the effect of temperature exposure.
For the entire duration of heating, the time temperature response was measured at the core position of the concrete specimen. At
120 min duration of heating the strength reduction was observed to be almost half the value of the original unheated specimen. This is
because of penetration of temperature in the core of concrete specimens of 1029 ◦C. Whereas, at 60 min and 90 min duration of
heating the temperature penetrations were measured as 925 ◦C and 986 ◦C respectively. It is conrmed from the temperature inside
the core of concrete specimens that the increase in temperature is directly proportional to the reduction in strength of concrete [41].
Fig. 17 shows the comparison of the percentage residual compressive strength of concrete exposed to elevated temperature based
on the results of past studies conducted by researchers [2,12,20,36,38,42,43]. It is noted that, the experiments are conducted by
following different re rating curve. Time temperature relation, duration of heating, rate of heating, type of cooling, rate of cooling are
some of the key inuencing factors that effects the test results of specimens. Material related parameters such as type of concrete,
strength grade, porosity, density and moisture content also affect the residual strength results of concrete. Therefore considering these
aspects, the percentage residual strength results of concrete were compared with the results of studies conducted based on the similar
target temperature. It can be seen that, the results are comparable with marginal deviation and it may be due to the changes in
experimental parameters. Similar rate of reduction in strength was observed in the present study and variation was observed between
7.52% and 9.22%.
4.7. Bond strength of concrete
After the cooling process, the pull out test was conducted following the guidelines of IS, 2770:2007. Bond strength concrete after air
and water cool is shown in Fig. 18. From the test results, it can be seen that the original specimens (uncoated and unheated) have
shown the bond strength in the range of 9.15–9.25 MPa. Specimens with coated rebar (unheated) exhibited the similar bond strength
without any signicant strength loss. At 30 min (821 ◦C) duration of the heating, a minor strength loss was observed compared to the
reference specimen. In the case of 60 min and 90 min duration of heating the bond strength loss was 35.8% and 52.3% respectively for
both air and water cooled specimens. After the duration of 120 min, air and water cooled specimens exhibited higher strength loss of
69.5% and 72.8% respectively as compared to reference specimens. The increase in the duration and temperature decreased the bond
strength of the concrete signicantly [12].
The bond specimen with coated reinforcement retains the bond strength of about 15–20% at 30 min duration of heating for both air
water cooled specimens. In the case of coated bond specimens the concrete fails rst similar to reference specimens. However, the
adhesion nature of concrete surface and reinforcing steel is found to be excellent as in the reference specimen. At prolonged duration
(120 min) with higher temperatures, the water molecules present in the concrete evaporate due to the generation of heat waves
resulting in the development of pores [3]. Due to this, the adhesion nature of concrete with reinforcement steel gets affected. But the
perlite coating protects the reinforcement bar from the effect of elevated temperature, due to this reason the bond strength of the
coated specimen is retained at elevated temperature.
When concrete is exposed to high temperature the physical, chemical and mechanical properties are severely affected. However, at
higher duration of heating i.e., between 60 and 120 min the gel weakens due to the decomposition of CSH gel and disintegration of
Fig. 18. Bond strength of concrete after exposed to elevated temperature.
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matrix bonding. It leads to the deformation of aggregate, and also the loss between cement paste and aggregate results in development
of minor cracks in the microstructure of concrete. Moreover these thermal cracks deteriorate the bond between concrete steel rein-
forcement, while testing the tensile cracks were developed at a faster rate because of the mechanical loading [20].
It is noted that the concrete material gets affected before initiating the damage of reinforcement steel at elevated temperature. Due
to the damage of the concrete matrix the adhesion between concrete and steel bar is reduced, and loss of ribs in rebar leads to reduce
the friction between concrete and reinforcing steel. Loss on the mechanical interlocking occurs between concrete and steel ribs [44].
The difference in the strength between these two samples is marginal. It is prominent that, the additional layer of perlite coating in the
rebar does not affect the bond strength of concrete. Also, the failure modes of these two specimens are almost same.
The bond specimen with coated reinforcement retains the bond strength of about 15–20% at 30 min duration of heating for both air
and water cooled specimens. However, in the case of coated bond specimens the concrete fails rst similar to other specimens,
nevertheless the adhesion nature of concrete surface and reinforcing steel is found to be excellent as in the reference specimen. At
prolonged duration with higher temperatures, the water molecules which are present in the concrete evaporate and due to the gen-
eration of heat waves a number of pores and voids develop. It is evident from the surface of damaged samples. Also these thermal
pressures reach the reinforcement bar and affect the adhesion nature of concrete with reinforcement steel. But the perlite coating
protects the reinforcement bar from the effect of elevated temperature, due to this reason the bond strength of the coated specimen is
retained at elevated temperature.The minimum protection offered by the specimen heated up to 30 min duration was noted in the
range of 15–20% as compared to the uncoated specimens. But in the case of higher temperature exposure i.e., more than 30 min of
exposure, the protection offered by the material is not effective.
During the experiment, the slip values were noted. Specimens with higher bond strength show lower slip values and it was between
0.62 mm and 0.92 mm (Ref and 30 min heated specimens). In the case of heated specimens (60, 90 and 120 min) the bond strength of
concrete decreases with increase in slip values i.e., 1.25 mm, 1.89 mm and 2.1 mm and 1.34 mm, 1.92 mm and 2.5 mm for air and
water cooled specimens respectively. Effect of temperature had a signicant inuence on residual bond strength and slip values. Lower
bond strength and sudden slip failure were observed. Fig. 19 shows the bond strength results of temperature exposed specimens and
their corresponding slip values. A linear increase of bond stress with a short range of slip was observed for reference specimens.In the
case of specimens exposed to elevated temperature, the rate of reduction in bond strength was found to be higher. It is due to the poor
concrete matrix around the friction of reinforcement steel reported by [12].
The water cooled specimens exhibited higher bond strength loss when compared with air cooling bond specimens. Because of
sudden forced cooling, the water cooling specimens are subjected to a thermal shock with the steep temperature gradient, and
therefore the surface damage is also higher for ange specimens and the similar observations are reported by [45].
Therefore, the critical temperature (925 ◦C −1029 ◦C) causes the reduction in bond strength of concrete. As per the experimental
study, it is the temperature that is required to cause the failure. However it depends on the temperature and duration considered in the
investigation.
To conrm the critical temperature on core of concrete bond specimens, the time temperature response was measured for 30 min,
60 min, 90 min and 120 min duration of heating. Therefore, the critical temperature to cause the decrease of bond strength in the
concrete was identied.For all the specimens, the temperature difference is measured between the surface and the core portion. The
measurement of time temperature readings of concrete bond specimens for different durations of heating is illustrated in Fig. 20. From
the gure it is clearly identied that while increasing the duration of heating the penetration of temperature is increased. Temper-
atures of specimens measured at the core of 30 min and 60 min duration of heating were 821 ◦C and 925 ◦C respectively. For 90 min
and 120 min the temperature values were 986 ◦C and 1029 ◦C respectively. It is evident that, due to this increase in temperature the
bond strength of concrete decreases while increasing the duration of heating.
Figs. 21(a) and (b) show the correlation between bond strength of concrete and yield strength of reinforcement for different
duration of heating and cooling. Reductions in bond and yield strength of reinforcement is directly proportional for both heated and
unheated specimens.An excellent correlation was found to exist between bond strength of concrete and yield strength of reinforcement
steel.
4.8. Inuence of heating and cooling
4.8.1. Inuence of rate of heating
The details of measured rate of heating and rate of cooling are given in Table 7. It is observed that at 30 min (821 ◦C) the observed
rate of heating is 6.86 ◦C/ minutes. In the case of 60 min (925 ◦C) and 90 min (986 ◦C) exposures, the rate of heating decreases to 3.46
and 0.67 ◦C/ minute respectively. For the target duration of 120 min (1029 ◦C), the rate of heating is measured as 0.47 ◦C/minutes.
Even through the rate of heating decreases, the reduction in strength properties of reinforcement steel and concrete increases due to the
temperature level and prolonged duration of heating.
4.8.2. Effect of soaking time
Soaking period is dened as the duration for which the specimen is kept at the peak temperature. The duration of steady state
(soaking period) increases the effect of elevated temperature on the strength properties of concrete. It is seen from the Fig. 6 that the
process of steady state enhances the temperature across the cross section. The target temperature almost reaches the core of specimen.
Therefore, the one hour duration maintained in the heating process affects the strength properties critically than other type of heating
cooling regime. It is evident that, the increase in core temperature that is equivalent to target temperature achieved bythe steady state,
is responsible for higher rate of strength reduction.
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4.8.3. Effect of temperature on residual strength of steel reinforcement
Fig. 11(a) and (b) shows that, at temperature above 60 min (925 ◦C), the residual yield and ultimate strength of reinforcement steel
decreases after 1 hr soaking time. At 90 min (986 ◦C), the strength reduction is observed as 17–38% in yield strength and 10–27% in
ultimate strength as compared to the unheated specimen. At prolonged exposure of 120 min (1029 ◦C), the yield and ultimate strength
values were found to be decreased drastically. It can be seen that, at higher temperatures the reduction in strength properties of steel
Fig. 19. Bond strength of concrete specimens exposed to elevated temperature (a) air cooled (b) water cooled.
Fig. 20. Time-temperature response of concrete bond specimens.
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decreases drastically after 1 hr soaking period. Therefore, temperature exposure, duration of heating, and soaking period are the key
factors that affect the strength properties of materials. It is proved from the earlier studies that, the prolonged duration signicantly
reduces the strength of concrete [20]. Experimental results showing the residual strength properties are given in Table 8.
4.8.4. Inuence of rate of cooling
In order to simulate the effect of heating cooling regime, two types of cooling were considered in the investigation. Air cooling is
considered to be a gradual type of cooling in which the exposed specimens are allowed to cool by natural air. This process is allowed till
the specimen reaches the ambient temperature level. In this case, the rate of cooling is gradual (Table 7) as it takes considerable
duration of time to bring the temperature down to reach the room temperature. In the case of water cooling, it is a rapid process as it is
done in a forced manner by a sudden application of water on the heated specimens. In this case, the recovery of specimens to reach the
room temperature is lesser than the air cooled specimens. However, in both the cases the target temperature achieved in the core
portion of specimens are same. But the type of cooling and rate of cooling had an inuence on residual strength properties of concrete.
It was proved from the earlier studies that sudden thermal shock and volume change of paste and aggregate are responsible for the
Fig. 21. Correlation between bond strength and yield strength of specimens (a) air cooled (b) water cooled.
Table 7
Details of rate of heating and rate of cooling of specimens.
Duration of heating
(min)
Exposure Temperature
(◦C)
Temperature variation
(◦C)
Rate of
heating
(⁰C/min)
Temperature after thesoaking period
(◦C)
Rate of
cooling
(◦C/min)
5,10 and 15 556,659 and 718 – – – – –
30 821 793 6.86 821 20 10
60 925 104 3.46 925 23 12
90 986 61 0.67 986 38 15.2
120 1029 43 0.47 1029 42 16
Table 8
Reduction factor for the residual strength of mechanical properties.
Exposure temperature (◦C) Yield strength (MPa) Compressive strength (MPa) Bond strength
(MPa)
Reduction factors
y
t
/y
o
u
t
/u
o
b
t
/b
o
AC WC AC WC AC WC AC WC AC WC AC WC
28 510 33.4 9.2 1.0 1.0 1.0 1.0 1.0 1.0
821 372.8 410.9 34.6 24.2 6.8 6.3 0.73 0.80 0.75 0.85 0.73 0.68
925 285.8 315.5 25.91 21.2 4.4 4.09 0.56 0.62 0.70 0.77 0.47 0.44
986 165.6 175.6 21.5 17.2 3.19 3.1 0.32 0.34 0.39 0.41 0.34 0.33
1029 95.86 98.2 16.2 11.5 2.78 2.5 0.18 0.19 0.27 0.28 0.30 0.27
I-821 455 468 NA NA 8.19 7.5 0.89 0.91 0.80 0.87 0.89 0.81
I-925 355.5 379.1 NA NA 5.8 5.4 0.69 0.74 0.75 0.81 0.63 0.58
y
t
=yield strength at elevated temperature, yo=yield strength at ambient temperature and b
t
=bond strength at elevated temperature and b
o
=bond
strength at ambient temperature.
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signicant strength reduction in concrete [10].
4.9. Failure modes of test specimens
4.9.1. Failure modes of reinforcement specimens
Fig. 22 shows the failure of reinforcement specimens. It is the observation that all the damaged rebars appear with necking failure.
Specimens of 30–60 min duration of heating, the shear slip fracturesare observed and the core area is found to be at.In the case of 90
and 120 min duration of heating the cup and cone failure mode is observed due to the increase of shear slip and the core area is coarse
granular for both air and water cooled specimens.
4.9.2. Failure modes of concrete bond specimens
Fig. 23 shows the typical failure modes observed for unheated and heated specimens. The crack pattern assessment was done
through visual observations. Two types of failure modes were observed across the test specimens, i.e., concrete splitting (splitting
failure) and combined (splitting-pull out) failure [46].
During the pull-out test on unheated reference specimens, the stress concentration is usually higher near the rebar. The ultimate
stress concentration can be observed at the interface between rebar and concrete at the top surface of test specimens [47]. It was
observed that cracks had started to develop from the interface of the rebar-concrete and propagated through the lateral sides, and then
reached towards the bottom side of the specimen. During the loading phase, the concrete resisted the stress development, resulting in
failure of splitting, and thus the ultimate bond strength capacity was not achieved in the case of unheated specimens. In the case of all
heated specimens (30, 60, 90, and 120 min), splitting-pull out failure was observed as shown in (Fig. 18). However, it is evident from
the Figure that the intensity of failure increases as the duration of heating increases.
The variation in failure intensity is mainly due to reduced bond strength at high temperatures. The development of higher thermal
stress due to the effect of increasing in heating duration resulting in the development of micro-cracks further turned into wider cracks.
As the exposure temperature increases, the microstructure of concrete gets weaker, and the accumulation of wider cracks disintegrates
the rebar-concrete mechanical interlocking (bond strength) [20]. Thus, the slippage of rebar is higher at lower stress levels. Therefore,
heated specimens undergo combined failure as the ultimate bond strength capacity equally dominates with the stress development in
exposed concrete.
5. Conclusion
An extensive investigation was undertaken to evaluate and improve the performance of reinforcement steel after the temperature
exposure with different heating and cooling phases. Effect of elevated temperature affects the mechanical properties such as yield
strength, ultimate strength, elastic modulus, bend strength and shear strength of rebar. Compressive strength and bond strength of
concrete also degraded due to the inuence of higher temperature exposure. Physical changes and failure patterns of reinforcement
steel and concrete specimens were examined. Following are the major ndings of the study experimental investigation:
1. Reinforcement steel specimens have shown changes in the color with respect to type of cooling. Air cooled specimens turned into
quite dark blue, and the water-cooled specimens were turned from buff gray to bluish dark with silver white patches.
2. Perlite coating in the steel reinforcement improves the residual strength properties such as yield and ultimate strength about
15–20% for 30 min duration of heating as compared to uncoated heated specimen. Furthermore, at 90 and 120 min the higher loss
in yield and ultimate strength was noted i.e., 87.2–81.5% and 72.5–71.8% for both air and water cooled specimens.
3. At elevated temperature, the perlite coated specimens have shown excellent passive protection to retain the mechanical properties
of reinforcement steel (yield strength, bend strength and shear strength). It is because of the outer layer of perlite retards the
temperature exposure without affecting the chemical composition of steel up to 30 min duration of heating.
4. A gain in compressive strength was observed as 3.82% (30 min), it is due to the rehydration of unreacted cement paste in concrete.
While increasing the duration of heating (60, 90 and 120 min) the strength reduction of concrete was noticed as 22.4%, 35.4% and
65.2% respectively.
5. In the case of reinforcement steel the performance of water cooled specimens was found to be better than that of air cooled
specimens. However, the concrete specimens (cube and bond) exhibited poor performance under water cooled conditions.
Fig. 22. Failure mode of reinforcement steel at different duration of heating.
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6. The bond strength results indicates that, while increasing the duration of heating the bond strength of concrete decreases with an
increase in slip values compared to reference specimens. It is due to the penetration of thermal stress in to a concrete and it reduces
the bond strength between concrete and reinforcement steel.
7. In the case of 30 min exposure, splitting failure mode was observed in the bond specimens similar to that of reference specimen.
However, while increasing the duration of heating splitting pullout failure mode was observed.
Declaration of Competing Interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to
inuence the work reported in this paper.
Acknowledgments
The authors wish to acknowledge the Science and Engineering Research Board, Department of Science and Technology of the
Indian Government for the nancial support (YSS/2015/001196) provided for carrying out this research.
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