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Mechanical properties of concrete composites subject to elevated temperature

Fire Safety Journal 95 (2018) 66–76

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Fire Safety Journal
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fi r e s a f

Mechanical properties of concrete composites subject to
elevated temperature
Josef Novak *, Alena Kohoutkova
Czech Technical University in Prague, Faculty of Civil Engineering, Department of Concrete and Masonry Structures, Thakurova 7, 166 29, Prague 6, Dejvice, Czechia

A R T I C L E I N F O

A B S T R A C T

Keywords:
Fibre reinforced concrete
Heat transport test
Heat treatment
Mechanical properties

Elevated temperature
Structural behaviour

Fire resistance represents an important parameter which is necessary to consider during the structural design of
buildings. It is defined as an ability of building components to perform their intended load-bearing functions
under fire exposure. In terms of fire resistance, the right choice of a construction material plays a key role and can
reduce structural damage or even save human lives. The building industry offers a wide range of materials whose
structural behaviour is more or less affected by temperature. Recently, concrete has become one of the most
utilized materials used for a various kind of buildings. While the knowledge and experience with concrete
behaviour under ambient temperature are well-known, the behaviour under elevated temperature has to be
deeply investigated.
The paper deals with observing the behaviour of concrete composites with addition of fibres under ambient and
elevated temperature with the aim to determine the mechanical properties of materials. The experimental tests
were conducted on three selected concrete composites which differ in a type and content of fibrous reinforcement
used. The experimental work carried out was divided into several phases. First of all it was necessary to leave the
produced specimens aging and drying in order to minimize the risk of unexpected damage caused by concrete
spalling during heating. Time to time, the specimens were weighted with the aim to determine the loss of weight
imposed by drying. Then, a heat transport test was performed on a few reference specimens in order to determine
the time required for uniform heating the specimens up to 200  C, 400  C and 600  C. In the last phase, conventional testing methods were undertaken to determine the mechanical properties of concrete composites at
ambient and elevated temperature. A compression test and a splitting tensile test were conducted on 150 mm
cubes. Based on the results, the peak and residual strength of the materials were determined for various temperature levels. The obtained findings contribute to improving the knowledge in the field of both concrete
structures exposed to high temperature and structural behaviour of fibre reinforced concrete. The findings can be
also utilized in case of the structural design of concrete structures with the high risk of fire loading.

1. Introduction
Today, a structural design represents a very complex task which includes analyzing many parameters such as ultimate bearing capacity,
stability, deflection, rigidity of structure etc. The essential part, which
has to be paid attention to, is also fire resistance defined as an ability of
building components to perform their intended load-bearing functions
under fire exposure. Although the issue of fire safety seems to be not so
important, the inadequate fire design or a construction material choice
can cause structural damage or even loss of human lives. For instance, in
2008 an extensive fire occurred in the 13-story Faculty of Architecture
Building at the Delft University of Technology (TUD) in Netherlands [1].

Although all building occupants evacuated safely, the fire burned uncontrolled for several hours and caused the structural collapse of the
major portion of the building with a reinforced concrete structural system. The key cause of the collapse is assumed to be the occurrence of
spalling which rapidly and dramatically reduced reinforced concrete
member capacities. A massive fire also hit the Windsor Tower in Madrid,
Spain, in February 2015 [2]. The structural system of the tower was


constituted of a reinforced concrete core, interior columns, waffle slab
and steel exterior columns. The part of building was exposed to the fire
for 20 h resulting in the extensive structural damage. As lately analyzed, a
large portion of the floor slabs collapsed during the fire due to the failure
of reinforced concrete elements. However, according to other sources [3]

* Corresponding author.
E-mail addresses: josef.novak.1@fsv.cvut.cz (J. Novak), akohout@fsv.cvut.cz (A. Kohoutkova).
https://doi.org/10.1016/j.firesaf.2017.10.010
Received 4 January 2017; Received in revised form 5 September 2017; Accepted 29 October 2017
0379-7112/© 2017 Elsevier Ltd. All rights reserved.


J. Novak, A. Kohoutkova

Fire Safety Journal 95 (2018) 66–76

1.1. Steel fibre reinforced concrete

the collapse was induced by buckling of steel columns during the fire.
Today, there are many regulations and standards which deal with the
demands for the durability and fire resistance of concrete structures [4].
The European standard EN 1992-1-2 [5] is a widely-used document
which serves for designing concrete structures exposed to high temperature. Recommendations and principles stated not only in this standard
are usually based on the extensive experience and experimental investigations in the field of concrete composites subject to high temperature. However, the available data does not have to accurately describe
the structural behaviour of relatively new materials including fibre
reinforced concrete (FRC). While the knowledge and experience with
FRC behaviour under ambient temperature are well-known, the behaviour under elevated temperature has to be deeply investigated.
FRC is a building material whose utilization in the concrete industry
has been rapidly increasing. This development is motivated by its physical and mechanical properties which contribute to traditional concrete
elements and structures various economical benefits such as structure
subtlety, part or full elimination of conventional reinforcement,
enhanced impact resistance, resistance to mechanical loads and environmental loads. Recently, many comprehensive studies have been undertaken with the aim to observe the mechanical behaviour of FRC
exposed to elevated temperature. Although concrete is well-known for a
high degree of fire resistance, high temperature seriously damages
microstructure and mesostructure which results in generalised mechanical decay of a concrete composite [6]. As a consequence, the extensive
knowledge of mechanical properties of FRC exposed to elevated temperature seems to be decisive for a wider utilization of the material.
There are two fundamental types of methodological procedure used
for observing mechanical properties of concrete at elevated temperature.
Most of experimental investigations [7–33] are conducted on test specimens at ambient temperature after high temperature exposure and only
a few is performed on hot test specimens [34–36]. Such approach of
experimental testing is preferred mainly due to a simple way of testing as
tests are easier to conduct on test specimens at ambient temperature.
However, if results obtained from the tests on specimens after high
temperature exposure correspond enough to the mechanical properties of
a tested material at a certain temperature level has not been still fully
understood. Bamonte and Gambarova belong to authors which very
intensively deals with such issue and states in their publications [37,38]
that the hot and residual (after high temperature exposure) behaviour in
compression are very close; the only difference was observed in case of
the peak strain in compression which is larger on heated specimens in
comparison with specimens cooled down after temperature exposure.
The fire response of concrete composites is closely associated with
concrete composition, particularly with a type and content of concrete
components used. Generally speaking, concrete made of siliceous aggregates, such as granite, shows unfavourable mechanical properties at
high temperature compared to concrete composed of calcareous aggregates such as dolomite and limestone [34,39]. Recently, a lot of interest is
being paid on the possible use of metakaolin, fly ash and silica fume as
partial cement replacement in concrete subject to high temperature
[7,12,13]. Owing to silica fume and fly ash fineness, concrete composites
with such additions have denser microstructure and as a consequence
their explosive spalling tendency increases [13]. The amount and type of
fibres also have an influence on the structural performance of a composite exposed to high temperature.
A number of experimental investigations have been conducted up to
date with the aim to observe the fire response of FRC with a various type and
amount of fibres. Particularly, the studies are focused on the effect of a type,
shape and content of fibres on the mechanical properties of concrete composites, mostly compressive and tensile strength including elastic modulus.
Namely, it concerns steel fibres [7,10,11,13–16,19,21,24–27,29,32–35],
synthetic fibres [7,9,11–14,16,17,20,22,24,27,32,34,35] and mix of steel
and polypropylene fibres [7,11,16,18,24,27,32,34,35] which are widely
used in the concrete industry. There also a few investigations which deals
with carbon fibres [8,17] and glass fibres [17].

As the melting point of steel is relatively high in comparison with
other materials, the use of steel fibres seems to be beneficial for concrete
composites exposed to high temperature. Incorporating steel fibres into
concrete composites remains advantageous even when the concrete
composites are exposed to high temperature up to temperature up to
1200  C, particularly 1% content has no deleterious effect on heated
concrete. In fact, the inclusion of steel fibres in a concrete mix leads to an
improvement in both mechanical properties and resistance to heating
effects in comparison with unreinforced concrete [10,21,27,28,32].
Some experimental investigations [15,19,26] even demonstrate the
compressive strength of steel fibre reinforced reactive powder concrete
and geopolymer concrete gradually increases when the material is heated
up to 200–300  C, but starts to decrease as temperatures further increase.
The compressive strength of reactive powder concrete with 1% steel
content is higher between 200  C and 400  C than at room temperature
and subsides when temperature exceed 500  C. Furthermore, 2% and 3%
steel fibre content significantly increase compressive strength from
200  C to 300  C which then gradually decreases as the temperatures
reach 400  C and beyond. However, a higher content of steel fibres
cannot improve the compressive strength of concrete composites at
elevated temperatures [29]. On the contrary, it has been also demonstrated that steel fibres have negligible effect on high temperature
compressive strength and only improve tensile strength when temperature up to 400  C is considered [34]. The tensile behaviour of SFRC
subjected to elevated temperature is more sensitive to the volume fraction and the aspect ratio of the fibre than to its type [25].
SFRC also has the higher toughness after the high-temperature exposures when compared to the initial values of unheated concrete [7]. As
temperature increases, SFRC weakens and shows reduced stiffness with
the degradation depending on a type, aspect ratio, and volume fraction of
the fibre [25] and the reduction in modulus of elasticity is more pronounced than the reduction in compressive strength for the same heat
treatment [29]. Steel fibre reinforced recycled aggregate concrete exhibits the identical behaviour and loses stiffness much faster than
strength after exposure to elevated temperature [21]. As a consequence,
peak strains gradually increases together with temperature. The increase
in peak strains along with steel fibre content does not significantly differ
between ambient temperature and 200  C. When temperatures exceed
200  C, a higher steel fibre content generally is associated with a higher
peak strain [15]. Moreover, the addition of steel fibres does not eliminate
the spalling tendency of concrete mixtures [13].
1.2. Synthetic fibre reinforced concrete
In the concrete industry, synthetic fibres, regardless of a type, shape
and length, are mostly utilized with the aim to increase concrete spalling
resistance when concrete structures exposed to elevated temperature
[20,35]. As the melting point of synthetic fibres is relatively low, the
presence of fibres in a concrete composite subjected to elevated temperature affects the mechanical properties of the concrete composite,
particularly residual compressive strength, modulus of elasticity and
splitting tensile strength [9]. While the presence of polypropylene fibres
slightly increase ductility and the specific toughness, defined as the ratio
of the area under the stress–strain curve, and the compressive strength of
unheated concrete, after heating all the enhanced characteristics are lost
[7,27]. Many contributions even demonstrate that polypropylene fibres
have negative effect on the residual mechanical properties of polypropylene fibre reinforced concrete (PPFRC) after high-temperature
exposure as they significantly decrease the residual compressive
strength, elastic modulus and tensile strength as well as they increase
peak strain [13,20,34]. On the other hand, some experimental investigations show that polypropylene fibres can improve the relative
residual compressive strength of a concrete composite after the exposure
to fire [32]. While the presence of polypropylene fibres at different
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Fire Safety Journal 95 (2018) 66–76

dosages does not affect the residual compressive strength at 200  C and
400  C, it considerably increases the residual compressive strength of
concretes after exposure to 600  C [22]. Moreover, some even shows that
compressive strength of concrete with polypropylene fibre additions is
greater than unreinforced concrete without additions, when subjected to
temperature up to 400  C [28]. In comparison with SFRC, polyvinyl
alcohol fibres reduce the uniaxial compressive strength but cause no
appreciable change in elastic modulus in reactive powder concrete [35].

2. Experimental investigation
Based on the gap in the field of FRC performance at high temperature,
the presented experimental investigation was developed. The whole
work was carried out by the team of research workers at Czech Technical
University in Prague for one year. As the research was very extensive, the
investigation was divided into several steps in the following order - test
specimens production, heat treatment, compression test and splitting
tensile test.

1.3. Hybrid (steel þ synthetic) fibre reinforced concrete
2.1. Test specimens
The combination of steel and synthetic fibres represents a promising
alternative how to ensure good toughness of a concrete composite before
heating and improve its residual mechanical behaviour and spalling
resistance as well as the ductility after heating [16]. Although a few
contributions declare that a fibre cocktail does not have much effect on
high temperature compressive strength [34], the most of presented work
affirm that the incorporation of steel fibres can effectively improve the
compressive properties of a concrete composite when exposed to
elevated temperatures while polypropylene fibres enhance concrete
spalling resistance [18,32]. Specifically, the combination of steel fibres
and polypropylene fibres shows positive synergy effect on the post-peak
behaviour of concrete composites before and after exposure to high
temperature [16]. However, as synthetic fibres have a low melting point
and ignition point, only steel fibres provide the stability and enhanced
mechanical behaviour to a concrete composite after exposing to elevated
temperatures [27]. Considering steel to synthetic fibre content ratio, a
concrete composite containing 1% synthetic fibres and 1% steel fibres by
volume seems to produce the best results, balancing performance at high
temperature with consideration of initial mechanical properties [35]. As
a consequence, using hybrid fibre reinforced concrete (HFRC) (steel and
synthetic fibres) might provide necessary safe guarantee for the rescue
work and structure repair during and after a fire disaster.

All experimental tests were carried out on 150 mm cubes which are
made of three concrete composites with different composition (Table 1).
The investigated concrete composites are composed of easy available and
widely used components which include Portland cement 42,5 R characterized by high early strength in accordance with CSN EN 197–1 [40],
water and siliceous aggregate. The workability of fresh concrete mass
was maintained by using plasticizer Sika Visco Crete 1035 that also reduces the content of used mixing water. The investigated FRCs also
contain two types of fibres. Single hook end steel fibres Dramix.
RC-80/60-BN have tensile strength 1225 MPa and served as main reinforcement in the concrete composites. Whereas polypropylene fibres
were added in concrete with the aim to reduce the risk of explosive
concrete spalling which occurs at higher temperature [41,42].
Totally, it was produced about one hundred specimens which were
left aging and drying in standard conditions in a laboratory for one year.
During this process the specimens were irregularly weighted in order to
determine the loss of weight in time caused mainly by drying. However,
after one year the decrease in weight was negligible in a range between
0.5% and 1% of the initial weight (Fig. 1). As expected, the highest rate of
weight loss was observed immediately after the production when mixing
water is consumed for concrete hardening. Long-term aging in normal
conditions did not significantly affect the weight of specimens as well as
the water content.

1.4. Motivation

2.2. Heat transport test

The state of the art review highlights that most of the experimental
investigations have been undertaken on specimens cooled down after a
fire exposure to obtain material properties. Such methodological procedure likely considers further concrete deterioration in a cooling phase
but is not capable to describe material properties at elevated temperature. Since the main objective of the experimental investigation was to
obtain the properties of a material right at time of a fire which will be
subsequently used for a numerical simulation of steel-FRC composite
columns at a fire, the proposed experiments were conducted on heated specimens.
The review also indicates several issues associated with the heat
treatment of test specimens. In most cases, test specimens are heated in
an electric furnace with an automatic control system which only
monitors the temperature in the furnace. Heat treatment procedure,
specifically time required for uniform heating, is based only on previous experience and no details are usually provided about uniform
temperature distribution all over the specimen [7–25,27,28,32–35].
However, such approach might be inappropriate when various heating
rate and specimen shape are considered because the results obtained
from experimental tests on non-uniform heated specimens might
be vague.
The other issue is connected with limited database of results. While
there is extensive experience with conventional plain concrete at high
temperature [6] the effect of fibres on the behaviour of FRC has not been
fully understood. Specifically, very limited data are available when
pre-cracking and post-cracking tensile strength are considered. Therefore, it is desired to enhance the database and subsequently validate or
derive existing or entirely new stress-strain relations for a particular type
of FRC, respectively.

As the compression test and splitting tensile test were conducted on
hot specimens, first of all it was necessary to determine a time period
required for the uniform heating of the specimens as well as to determine
the rate of cooling in order to avoid excessive temperature loss during the
tests. The heat transport test arranged particularly for this investigation
was performed on nine 150 mm cubes; three specimens of each composite were gradually used for the three temperature levels - 200  C,
Table 1
Concrete composition.
Concrete component

Portland cement
42,5 R
Water
Fine aggregate
0–4 mm
Coarse aggregate
4–8 mm
Coarse aggregate
8–16 mm
Plasticizer Sika Visco
Crete 1035
Steel fibres Dramix
RC-80/60-BN
Polypropylene fibres
Forta Ferro 54 mm

68

Content [kg/m3]
Plain
concrete
(PC)

Steel fibre
reinforced concrete
(SFRC)

Hybrid fibre
reinforced concrete
(HFRC)

490

490

490

153
890

153
890

153
890

100

100

100

745

745

745

4.9

4.9

4.9

0

40

40

0

0

3


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Fire Safety Journal 95 (2018) 66–76

Fig. 1. Percentage loss of weight in time during specimens aging.

400  C and 600  C. The heat treatment was conducted by using a special
system (Fig. 2), which consists of a control machine Mannings HTC
70 kW, ceramic pads and K-type thermocouples measuring temperature
in a range from À200  C to 1000  C.
First of all, three holes serving for the thermocouples installation were
drilled in each cube. Two 75 mm deep holes were positioned 10 mm from
the edge, the last one at the centre with the aim to measure temperature
in the specimen core (Fig. 3). To ensure correct functioning of the thermocouples it was necessary to prepare the holes of the same diameter as
the thermocouples have, otherwise any air void in the holes could affect
the test results. Afterwards, two ceramic pads were attached directly to
four of six specimen sides. The position of ceramic pads was secured by
high temperature resistant glass wool insulation which was wrapped
tightly around the pads together with a specimen. The insulation used
simulates so called thermo-box which also ensures heat accumulation

during the heat transport test. Subsequently, totally five thermocouples
were installed in their position when three of them were inserted through
the glass wool into the prepared holes drilled in the specimen and the
other two were placed between the ceramic pads and a specimen to
measure the surface temperature of the ceramic pads. The temperature of
an unheated side was measured time to time by an infrared thermometer
and compared with the temperature distribution at the depth of
the specimen.
Although the temperature development in a fire defined by the
standard temperature-time curve (1) in accordance with CSN EN 1991-12 [5] is significantly faster, due to the control machine performance the

θg ¼ 20 þ 345: log10 ð8:t þ 1Þ

(1)

initial heat transport tests were conducted with the maximum heating
rate 1000  C/h in order to simulate as much as possible the real

Fig. 2. System used for heat transport test (a) control machine Mannings HTC 70 kW (b) K-type thermocouples (c) ceramic pads.

Fig. 3. Test specimen preparation for heat transport test (a) specimen with ceramic pads (b) specimen together with ceramic pads placed into thermo-box (c) specimen during heat
transport test.
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removed from the thermo-box by using fireproof gloves, wrapped in new
glass wool insulation and placed on steel plates. The intention of this
process was to simulate the steps required for performing the compression test and splitting tensile test on heated specimens and consequently
to precisely validate the rate of heat loss. While the temperature in the
core was stabilized, the temperature on the sides of specimens decreased
rapidly due to the low temperature of surrounding environment. The rate
of heat loss is more significant at specimens heated up to higher temperature due to a higher temperature gradient. Based on the obtained
data from the heat transport test, it was determined the maximum
allowable time period for the compression test and splitting tensile test
equal to 10 min.
The test specimens used for the heat transport test also served for
determining the volumetric mass density of materials at ambient and
elevated temperature. Before the heat transport tests, the prepared
specimens were weighed and measured in order to determine the material density at ambient temperature. The density ranged from 2350 kg/
m3 to 2400 kg/m3 which correspond to conventional concrete. After the
heat transport tests, the whole process was repeated. With the aim to
obtain as precise results as possible the holes for thermocouples were also
considered during the volume calculation. With increasing temperature
the volumetric mass density gradually decreased down to a range
2300 kg/m3 - 2350 kg/m3, 2250 kg/m3 - 2300 kg/m3 and 2200 kg/m3 2250 kg/m3 corresponding to the temperatures 200  C, 400  C and
600  C, respectively. The main cause is dehydration of the ingredients of
cement paste, dissociation of calcium hydroxide Ca(OH)2 into CaO and
water and free water evaporation [44]. The influence of melted polypropylene fibres was negligible as their content was low. Although the
composition of all three concrete composites differed, the effect of temperature on the volumetric mass density of the materials was almost
identical with no significant deviations.

conditions of a fire (Fig. 4). While the specimens of HFRC did not exhibit
any defects in response to the high temperature gradient, concrete
spalling occurred at the specimens made of SFRC and PC in the 26th
minute of the heat treatment. Concrete spalling was imposed by both
evaporable and nonevaporable water chemically bound with cement
paste which causes the pore pressure buildup and subsequently spalling
in concrete [43]. At the time of concrete spalling, the temperature on the
heated sides of specimens and in a core reached roughly 450  C and
60  C, respectively. While a massive explosion and extensive spalling of a
whole PC specimen happened, in case of a SFRC specimen only small
pieces of the size in millimetres were released owing to the presence of
steel fibres in the concrete composite. The presence of polypropylene
fibres in HFRC lowered the risk of concrete spalling as the fibres melted
with increasing temperature due to low melting point. The melted
polypropylene fibres increased the concrete composite permeability
witch it positively affected the pore pressure and subsequently decreased
spalling in concrete. As a consequence, the specimens were eventually
heated up gradually step by step when temperature was increased
depending on the amount of vapour visibly bursting out of the specimens
with the aim to avoid any damage caused by extensive vapour expansion
in SFRC and PC specimens. When no or little vapour was spotted, the
temperature was repeatedly increased by 50  C until a particular temperature level was reached. Such approach was used for the first PC cube
heated up to 600  C and as no concrete spalling occurred the identical
heat treatment pattern was applied for the other specimens. On average,
such heat treatment corresponds roughly to the heating rate 200  C/h.
The heat transport tests were performed to determine the time period
for the uniform heating up to 200 , 400  C and 600  C at all three concrete composites (Fig. 5). The temperature at the interface between the
ceramic pads and a concrete sample was increased step by step following
always the same pattern until 250  C, 450  C or 650 , respectively, were
reached. As the test continued, the interface temperature was kept at the
same level until the temperature in the specimen core reached 200  C,
400  C and 600  C respectively. The increase in temperature on the
heated sides of the specimen (at the 75 mm depth) was more rapid in
comparison with the specimen core which is placed further from the
source of heat. The temperature distribution at zero depth on the top
unheated side observed using the infrared thermometer differed from the
one obtained at the 75 mm depth. However, as the temperature gradient
decreased during the heat transport test, the difference in the temperature distribution at both depths ranged from 0  C to 8  C and consequently was negligible.
The uniform heating of the specimens up to 200  C, 400  C and 600  C
took approximately two, four and 6 h respectively regardless of the
concrete composition. When the intended temperature levels were
reached, the heat transport system was turned off in order to observe
specimen cooling in detail. First, the specimens were immediately

2.3. Compression test
The concrete compression test was conducted on the 150 mm cubes
under 20  C, 200  C, 400  C and 600  C in accordance with CSN EN
12390–3 [45]. First, the whole set of specimens was divided into four
groups according to the intended temperature level during testing. The
specimens tested under elevated temperature were heated up to the
intended temperature following the procedure used during the heat
transport test. The specimens together with two ceramic pads were
inserted into the thermo-box of glass wool (Fig. 6) and then two
thermo-couples were placed between the specimen and ceramic pads
with the aim to control and monitor the temperature of the ceramic pads.
The time and heat treatment were identical to those obtained from the
heat transport test. Heating up to 200  C, 400  C and 600  C took
113 min, 235 min and 370 min, respectively.

Fig. 4. Nominal fire curve and heating rates used for the heat transport test.
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Fig. 5. Heat transport diagrams (a) legend of thermocouples, (b) heating up to 600  C, (c) cooling of specimens heated up to 600  C, (d) heating up to 400  C, (e) cooling of specimens
heated up to 400  C, (f) heating up to 200  C, (g) cooling of specimens heated up to 200  C.

Fig. 6. Test specimens during heating.

deformation 0.02 mm/s in the range of 0–5 mm and 0.1 mm/s for
deformation greater than 5 mm. The obtained data from the test were
used for generating compressive strength-displacement diagrams which
describe the material behaviour at various temperature levels.

After the intended temperature in the specimen core was reached the
compression test was performed in a testing machine Inova 200F. The
time of compression test of each sample had to be kept under 10 min in
order to avoid excessive temperature loss. The test was driven by

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The presented results do no exhibit any significant deviations caused
by either incorrect conducting the experimental tests or poor-quality
manufacturing technology. The diagrams of the investigated concrete
composites regardless of the amount and type of used fibres correspond

The compressive strength-displacement diagrams (Fig. 7) show the
structural behaviour of tested specimens at different temperature
levels, mainly the peak and residual (post-peak) strength of the investigated materials.

Fig. 7. Compressive strength-displacement diagram of the concrete composites.
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200  C, 400  C and 600  C in accordance with CSN EN 12390–6 [47]. The
preparation phase was identical to that used for the compression test. The
specimens were divided into four groups according to the intended
temperature during testing, the specimens tested under elevated temperature were inserted into thermo-boxes together with two ceramic
pads and heated up for the specified time period to the intended temperature. After the intended temperature in the specimen core was
reached, the splitting tensile test was performed in a testing machine
Inova 200 F (Fig. 9). The time of the splitting tensile test of each sample
had to be kept under 10 min in order to avoid excessive temperature loss.
The test was driven by deformation 0.02 mm/s in the range of 0–5 mm
and 0.1 mm/s for deformation greater than 5 mm. Based on the obtained
data from the tests, splitting tensile strength-displacement diagrams
describing the material behaviour at various temperature levels
were generated.
The presented diagrams (Fig. 10) show the tensile behaviour of the
tested concrete composites at various temperature levels, mainly the
peak and residual (post-peak) splitting tensile strength. The obtained
results are with no significant deviations imposed by mistakes made
during testing or producing experimental specimens. The mechanical
behaviour described by the presented diagrams corresponds to the
typical structural behaviour of either plain concrete or FRC. In the
opening phase the splitting tensile strength raises almost linearly until
the peak strength is reached. While a sudden fall in strength occurs at PC
at this point, HFRC and SFRC exhibit residual strength. Such phenomenon results from the absence of fibres in PC which is then weak in tension. As consequence, a brittle failure occurs at specimens made of PC in
comparison with other two concrete composites. HFRC and SFRC are
characterized as FRC with a tension softening curve [48] as their residual
strength decrease after the first macro-crack occurs. The regions with
short-term increase in residual strength result usually from fibres which
are gradually activated with increasing loading until they are fully activated. Comparing to the composition of SFRC, HFRC contains extra
3 kg/m3 polypropylene fibres Forta Ferro 54 mm (Table 1) and as a
consequence its residual tensile strength is higher at ambient temperature when the polypropylene fibres act in the concrete composite
together with the steel fibres. Subsequently, the polypropylene fibres in
HFRC burn out with increasing temperature and their influence on the
concrete composite strength is negligible and as a consequence the peak
and residual strength of HFRC and SFRC are almost identical.
The most interesting finding relates to the peak splitting tensile
strength (when the first macro-crack occurs) of the investigated concrete
composites. The dependence of peak splitting tensile strength on
ascending temperature at both PC and SFRC with no polypropylene fibres
significantly differs in comparison with HFRC (Fig. 11). The peak
strength of HFRC gradually decreases with increasing temperature,
whereas the ultimate strength of both SFRC and PC at 200  C is lower
than the strength at 400  C. This phenomenon is caused by the testing
methodology when several types of stresses the hot specimens had to
withstand during the splitting tensile test to. Namely, it concerns
the stresses from static loading and pore pressure tending to tear the
specimens. Moreover, the material properties worsen with increasing

to the typical behaviour of concrete in compression. In the opening
phase, the compressive strength of the materials increases linearly until
the concrete composites start yielding and ultimate compressive strength
is reached. The modulus of elasticity defined by the slope of the linear
part of the curves significantly decreases at all concrete composites with
increasing temperature. This phenomenon is caused by the specimen
structure significantly damaged by cracks resulting from both the high
temperatures and pore pressure. In case of the HFRC, the material
structure at elevated temperature is also more porous as the polypropylene fibres melts due to the low melting point of the material [46].
Consequently, the presence of polypropylene fibres in a concrete composite also affects the peak compressive strength of material (Fig. 8).
Moreover, the specimens, regardless of the type of a concrete composite,
exhibit enhanced ductility with increasing temperature.
While the effect of temperature on the compressive strength of PC and
SFRC with no polypropylene fibres is noticeable at the temperature
higher than 400  C, the compressive strength of HFRC significantly starts
decreasing already at lower temperature due to the porous structure of
specimens caused by burned polypropylene fibres. This phenomenon is
particularly visible at SFRC whose compressive strength at 200  C,
400  C and 600  C corresponds to 99%, 99% and 55%, respectively, of its
initial strength at ambient temperature and HFRC whose compressive
strength at the same temperature levels represents 98%, 88% and 45%,
respectively, of its initial strength. The compressive strength of PC at
200  C, 400  C and 600  C is 94%, 94% and 51%, respectively, of the
initial strength at ambient temperature. In comparison with CSN EN
1992-1-2 [5] recommending to use for plain concrete with siliceous aggregates 95%, 75% and 45% of the initial compressive strength,
respectively, the obtained results from the experimental tests differ
significantly. The effect of steel fibres in SFRC and HFRC on the behaviour and compressive strength of the materials at ambient and elevated
temperature is negligible.
2.4. Splitting tensile test
Splitting tensile test was performed on 150 mm cubes under 20  C,

Fig. 8. Peak compressive strength of the concrete composites at ambient and elevated
temperature.

Fig. 9. Testing machine Inova 200 F (left) and split tensile test of heated specimen (right).
73


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Fire Safety Journal 95 (2018) 66–76

Fig. 10. Splitting tensile strength-displacement diagram of the concrete composites.

the porosity of HFRC at elevated temperature is higher than the porosity
of SFRC and PC due to the presence of the polypropylene fibres. The
polypropylene fibres melt shortly with increasing temperature and
consequently the material porosity increases. As a consequence, the
failure mode of HFRC specimens at 200  C was typical because most of
steam was released during the 2 h heat treatment. Whereas the SFRC and
PC specimens exhibited a explosive failure mode with the visible steam

temperature. Considering such statement, the detailed analysis of such
results can be carried out.
While the specimens of the investigated concrete composites were
being tested, there had been already pore pressure inside them resulted
from the heat treatment. The level of pore pressure mainly depends on
temperature gradient, time of the heat treatment and material porosity.
While the first two parameters are identical for all concrete composites,
74


J. Novak, A. Kohoutkova

Fire Safety Journal 95 (2018) 66–76

200  C, 400  C and 600  C. Based on the obtained data the following
conclusions are made:
▪ The long-term aging and drying of FRC specimens, regardless of
the type of fibre used, in normal conditions do not significantly
affect the weight of specimens as well as the water content.
▪ The presence of polypropylene fibres in a concrete composite
lowers the risk of explosive concrete spalling as the fibres melt
with increasing temperature due to the low melting point.
Consequently the fibres increase the concrete composite
permeability and decrease pore pressure. Considering the
heating rate 1000  C/h, HFRC, SFRC and PC exhibited no concrete spalling, low-level concrete spalling (in millimetres) and
extensive concrete spalling, respectively.
▪ With increasing temperature the volumetric mass density of
concrete gradually decreases approximately down to 2375 kg/
m3, 2325 kg/m3, 2275 kg/m3 and 2225 kg/m3 corresponding to
the temperatures 20  C, 200  C, 400  C and 600  C, respectively.
▪ The presence of polypropylene fibres in a concrete composite
affects the peak compressive strength of the material at elevated
temperature. In comparison with PC and SFRC, the peak
compressive strength of HFRC decreased more rapidly with
increasing temperature due to the higher porosity of specimens
resulted from melted polymer fibres. This phenomenon is
particularly noticeable at SFRC whose compressive strength at
200  C, 400  C and 600  C corresponds to 99%, 99% and 55%,
respectively, of its initial strength at ambient temperature and
HFRC whose compressive strength at the same temperature levels
represents 98%, 88% and 45%, respectively, of its initial strength.
▪ When conducting splitting tensile test at 200  C, the SFRC and
PC specimens exhibited explosive failure with visible steam
release. The main cause of such behaviour is pore pressure
which together with applied load tend to tear the test specimens.
As a consequence, the level of pore pressure inside concrete
composites affects the material capability to withstand tensile
stress. The presence of polypropylene fibres in HFRC increased
the porosity of the material, subsequently decreased pore pressure in the specimens and consequently increased the capability
to withstand applied load at 200  C. While the peak splitting
tensile strength of SFRC at 200  C, 400  C and 600  C correspond to 85%, 103% a 58% of its initial strength at ambient
temperature, the peak strength of HFRC represents 97%, 88%
and 41%, respectively, of its initial strength.
▪ The comparison between tensile strength recommended for
plain concrete with silica aggregates stated in CSN EN 1992-1-2
[5] and obtained results from the experimental investigation
shows a significant difference. While the standard recommends
to use 80%, 40% and 0% of the initial tensile strength of concrete for the fire design of structures exposed to 200  C, 400  C
and 600  C, respectively, the peak strength of PC obtained from
the experimental investigation corresponds to 57%, 81% and
52% of its initial strength. Higher strength of PC at 400  C than
at 200  C is caused by the explosive failure at 200  C described
in a detail above.

Fig. 11. Observed splitting tensile strength of concrete composites at ambient and
elevated temperature.

release while being tested. The main cause of such failure is that the low
material porosity enabled only little steam to be released within 2 h heat
treatment. Since the SFRC and PC specimens were exposed to high pore
pressure right at the time of testing, they failed at a lower level of applied
load. From my point of view, such phenomenon has not been investigated
yet as the most experimental tests are conducted on specimens cooled
down after fire exposure and thus with no pore pressure inside them.
The reason why such failure mode was not such distinctive during the
splitting tensile test of PC and SFRC specimens at 400  C and 600  C is
associated with the time of the heat treatment. To heat the specimens up
to 400  C and 600  C lasted twice and three times longer, respectively.
Such time periods enabled most of steam to be gradually released
through micro pores in the composites and consequently to reduce the
pore pressure right at the time of testing. No explosive failure mode was
also observed while the SFRC and PC specimens being tested at 400  C
and 600  C which it only proves the statement. The pore pressure inside
the SFRC and PC specimens gradually fell down with increasing temperature to the value the HFRC specimens had. Therefore, the peak
splitting tensile strength of the investigated concrete composites at
600  C is almost identical. While the peak splitting tensile strength of
SFRC at 200  C, 400  C and 600  C correspond to 85%, 103% a 58% of its
initial strength at ambient temperature, the peak strength of HFRC represents 97%, 88% and 41%, respectively, of its initial strength. The increase in strength of SFRC up to 103% at 400  C might result from
uncertainties and variance in the production and testing of the specimens. Nevertheless, increasing strength in a range from ambient temperature to 400  C is rather usual for reactive powder concrete
composites [15,18,26] but was also observed within the experimental
investigation of concrete composites composed of conventional concrete
components [14]. The comparison between figures stated in CSN EN
1992-1-2 [5] for plain concrete with silica aggregates and obtained results from experimental tests shows a significant difference. While the
standard recommends to use 80%, 40% and 0% of the initial strength for
the fire design of concrete structures exposed to 200  C, 400  C and
600  C, respectively, the peak strength of PC obtained from the tests
correspond to 57%, 81% and 52% of its initial strength.

Based on the obtained findings, HFRC with addition of steel and
polypropylene fibres is suggested to use for structures with the high risk
of fire loading. Despite the HFRC exhibited lower compressive strength at
elevated temperature in comparison with the other concrete composites,
owing to the presence of polypropylene fibres no explosive failure
occurred during the splitting tensile test. Moreover, polypropylene fibres
reduce the risk of concrete spalling which is usually the main cause of
concrete structure collapse. The experimental investigation also shows
that a tensile failure mode of hot specimens and specimens cooled down
after fire exposure might be different. Further within the ongoing project,
the obtained results will be validated on real-size concrete elements.

3. Conclusion
The paper presents the case study of the behaviour of the concrete
composites under ambient and elevated temperature with the aim to
determine the mechanical properties of the materials. The experimental
tests were conducted on three investigated concrete composites with a
various type and content of fibrous reinforcement - plain concrete (PC),
steel fibre reinforced concrete (SFRC) and hybrid fibre reinforced concrete (HFRC) with addition of steel and polypropylene fibres. Within the
scope of work the heat transport test, compressive and splitting tensile
tests were conducted at ambient temperature and elevated temperatures
75


J. Novak, A. Kohoutkova

Fire Safety Journal 95 (2018) 66–76

Acknowledgements
[23]

This work was supported by the grant Models of steel and fiber
composite columns exposed to fire, no. GACR 15-19073S, from the Grant
Agency of the Czech Republic (GACR).

[24]

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