Z. PAVLÍK et al.: PROPERTIES OF HIGH-PERFORMANCE FIBER-REINFORCED CONCRETE ...
481–487
PROPERTIES OF HIGH-PERFORMANCE FIBER-REINFORCED
CONCRETE AFTER THERMAL TREATMENT AT HIGH
TEMPERATURES
LASTNOSTI VISOKOKAKOVOSTNEGA, Z VLAKNI OJA^ANEGA
BETONA PO TOPLOTNI OBDELAVI PRI VISOKIH
TEMPERATURAH
Zby{ek Pavlík
1*
,JanFo øt
1
, Milena Pavlíková
1
, Lucie Zemanová
1
,
Jaroslav Pokorný
1
, Anton Trník
1
, David ^ítek
2
, Robert ^erný
1
1
Department of Materials Engineering and Chemistry, Faculty of Civil Engineering, Czech Technical University in Prague, Thákurova 7,
166 29 Prague, Czech Republic
2
Department of Experimental Methods, Klokner Institute, Czech Technical University in Prague, [olínova 7, 166 08 Prague, Czech Republic
Prejem rokopisa – received: 2018-03-20; sprejem za objavo – accepted for publication: 2019-01-17
doi:10.17222/mit.2018.050
A complex analysis of the effect of a high-temperature load on the parameters of a high-performance fiber-reinforced concrete
(HPFRC) is presented. The HPFRC containing steel fibers was exposed to temperatures of up to 1000 °C. In order to reveal the
effect of the elevated temperatures on the concrete performance, its basic physical, mechanical, hygric and thermal properties
were determined after the thermal treatment. To observe physical and chemical processes in the HPFRC as a function of tem-
perature, a simultaneous thermal analysis (STA) and a thermodilatometry analysis (TDA) were used. The high-temperature
exposure led to an increase in the porosity and pore size and to a significant decrease in the mechanical strength. The values of
hygric and thermal properties correspond to the changes in the material structure after the thermal treatment. STA and TDA data
showed structural changes of the studied material and allowed identification of critical temperatures for its damage.
Keywords: fiber-reinforced concrete, high-temperature exposure, thermal strain, structural changes
Avtorji predstavljajo kompleksno analizo vpliva termi~nih obremenitev na parametre visokokakovostnega, z vlakni oja~anega
betona (HPFRC). Preiskovani beton, ki je vseboval jeklena vlakna, so izpostavili visokim temperaturam do 1000 °C. Po toplotni
obdelavi betona so ugotavljali vpliv visokih temperatur na njegove fizikalne, mehanske in hidro-termalne lastnosti. Fizikalne in
kemijske procese HPFRC v odvisnosti od temperature so ugotavljali s pomo~jo simultane termi~ne analize (STA) in termo-
dilatometrije (TDA). Termi~na obremenitev preiskovanega betona je povzro~ila pove~anje velikosti por in posledi~no pove~anje
celokupne poroznosti ter znatno zmanj{anje njegove mehanske trdnosti. Vrednosti hidro-termalnih lastnosti po termi~ni
obremenitvi betona so se spremenile skladno s spremembami v strukturi materiala. Rezultati STA in TDA analiz ka`ejo na
strukturne spremembe preiskovanega materiala in omogo~ajo identifikacijo kriti~nih temperatur za~etka njegovih po{kodb.
Klju~ne besede: z jeklenimi vlakni oja~an beton, visoko temperaturna obremenitev, termi~ne deformacije, strukturne spre-
membe
1 INTRODUCTION
High-performance fiber-reinforced concrete (HPFRC)
became a popular and modern structural material for
versatile use. Under conventional environmental con-
ditions, concrete structures serve without any bigger
durability problems, especially in the case of applying
high-performance materials.
1
However, there are import-
ant cases where these structures may be exposed to much
higher temperatures (e.g., building fires and explosions,
chemical and metallurgical industrial applications, and
some nuclear-power-related postulated accident con-
ditions). Here, a concrete exposure to high-temperature
loading significantly reduces its excellent properties,
such as compactness, mechanical resistance, durability,
etc.
2
Fire represents one of the most severe risks to
buildings and structures. The Channel Tunnel fires in
1996 and 2008, and the St Gotthard Tunnel fire in 2001
are typical examples of a serious HPC damage due to a
high-temperature load. The fire-induced damage of HPC
structures is associated with a release and evaporation of
a significant amount of water. Due to this reason, internal
stresses are built up in the concrete, which can exceed its
tensile strength. Since HPC has a significantly finer pore
structure compared to the normal-strength concrete
(NSC), it is much more difficult for this internal pressure
to be released. In both progressive spalling and explosive
spalling, the load-bearing capacity of the concrete struc-
ture is reduced and a complete failure may ultimately
occur.
3
However, of the two previously assumed reasons,
evaporation is nowadays believed to be only reason for
internal stresses. The second reason is the gradient
between the temperature of the heated external layers of
a concrete element and its cooler inside where the latter
Materiali in tehnologije / Materials and technology 53 (2019) 4, 481–487 481
UDK 620.1:621.78:666 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(4)481(2019)
*Corresponding author's e-mail:
pavlikz@fsv.cvut.cz

hinders the expansion of the former and causes high
stresses to evolve in the heated layer.
4,5
This is why
different spalling types and patterns are observed on con-
crete elements of different dimensions and shapes (e.g.
concrete beams, columns, plates or walls; elements with
a rectangular or circular cross-section, etc.) as well as on
the same elements heated by different heating rates
(faster heating regimes most often result in more
aggressive spalling).
The surface of a concrete element can be heated by
both a convective heat flux (from the surrounding air of a
higher temperature) and a radiation heat flux (direct, e.g.,
from flames, and/or mutual, e.g., from other heated sur-
faces), resulting in a gradual increase of the element
temperature, starting from the surface zone. Increasing
temperature causes material dilatation, which is in part
due to concrete dehydration (products of chemical reac-
tions have a greater volume than the initial volume of
concrete), in part due to the material cracking and
progressive crack opening,
6
and finally due to "normal"
thermal dilatation of the material skeleton. A thermal
dilatation of the external layers of a heated element is
constrained by the core material, which has a lower
temperature. This causes considerable macro-stresses in
the external layers of the element, compression in the
direction parallel to the surface, and traction in the direc-
tion perpendicular to it, as well as an accumulation of the
elastic strain energy. The tensile stresses may cause
further development of cracks and fractures, parallel to
the element surface, resulting in a subsequent degrada-
tion of the material strength properties in the surface
zone.
4
An intensive research was carried out to propose
solutions that would provide active or passive protection
against spalling. In order to minimize HPC spalling,
different types of fibres based on steel, basalt, jute, poly-
vinyl alcohol (PVA), polypropylene (PP), polyacryl-
nitrile (PAN), etc., were tested separately or as a hybrid-
fibre reinforcement, where combinations of different
fibre materials and sizes were used.
Steel fiber reinforced concrete (SFRC) exhibits
several excellent properties, such as flexural, tensile and
shear strengths, toughness, impact resistance, crack
resistance and resistance to frost damage. Although steel
fibers may not offer any obvious advantage from the
fire-endurance point of view, previous works have shown
that steel fibers can affect the spreading of cracking,
hence, potentially improving the performance of
concrete after its exposure to high temperatures.
7
Sideris
et al.
8
reported on steel-fiber-bridge cracks caused by an
increased vapor pressure, which underwent a more
gradual degradation. Lau and Anson
7
found that incor-
porated steel fibers are beneficial to the concrete exposed
to high temperatures of up to 1200 °C. Additionally, the
steel fibers may cause a short time delay of spalling due
to the increase in the tensile strength of the cement
matrix. However, the time of the spalling delay lies in the
order of minutes. Nevertheless, the steel fibers can shift
threshold temperatures to higher levels as introduced by
Sideris et al.
8
Since SFRC has already found application
in the building industry, for example, in tunnel lining and
precast tunnel segments, its behavior during high-tempe-
rature loading and its residual parameters are of particu-
lar importance. On this account, the HPFRC reinforced
by steel fibers in an amount of 120 kg/m
3
was studied
and the results are reported in this article.
2 EXPERIMENTAL PART
2.1 Materials and specimens
The HPFRC mix, developed in our laboratory with
special attention paid to its high mechanical resistance
and durability, was examined. The tested material
consisted of Portland slag cement CEM II/A-S 42.5 R
(the ^í`kovice cement factory, Czech Republic, member
of the LafargeHolcim group), silica sand with the maxi-
mum particle size of 2 mm, mineral microfillers, steel
fibers and batch water. Steel fibers MasterFiber 482
(BASF) with a tensile strength of 2200 MPa were
straight, 13 mm long, 0.2 mm thick (with an aspect ratio
of 65) and brass coated (Figure 1).
The chosen thickness and length are proper for
application in the HPC and UHPC where the aspect ratio
in a range of 65–100 is recommended. For the optimal
workability of a fresh concrete mixture, a superplasti-
cizer based on polycarboxylate ether was used. A
detailed composition of the studied HPFRC is given in
Table 1.
The specimens prepared were cubes with a side
length of 100 mm and rectangular prisms having dimen-
sions of 40 mm × 40 mm × 160 mm. The cast samples
were stored for 24 h in a highly humid environment of
approximately 98 % RH and after demolding, they were
cured for 27 d in water with a temperature of (23±2) °C.
After this time, they were stored in the laboratory
Z. PAVLÍK et al.: PROPERTIES OF HIGH-PERFORMANCE FIBER-REINFORCED CONCRETE ...
482 Materiali in tehnologije / Materials and technology 53 (2019) 4, 481–487
Figure 1: Steel fibers

(RH = (50±5) %, T = (23±2) °C). The storage of spe-
cimens in a laboratory environment was necessary for
their free drying to a low equilibrium moisture content in
order to prevent their damage by the accelerated water
evaporation. At the age of 120 d, all of the HPFRC
specimens were first dried at 105 °C until their mass was
constant, i.e., the mass change was lower than 0.1 % of
the specimen mass. After this age, some of the prepared
specimens were placed into an electrical furnace with a
heating rate of 5 K/min. The maximum temperature was
(200, 400, 600, 800 and 1000) °C, respectively. The
applied heating rate was in our case less than that of the
ISO 834 standard curve, which was the limitation of the
equipment available. The holding time at the maximum
temperature was two hours. After two hours, the tempe-
rature of whole sample reached equilibrium (no tempe-
rature gradients occurred). Before the particular tests of
HPFRC residual properties, the specimens were cooled
down under laboratory conditions.
Table 1: HPFRC mix amounts
Material Dosage (kg/m
3
)
Silica sand 0.18–2 mm 824
Silica sand 0.063–1 mm 411
Master Fiber 482 (BASF) 0.2/13 mm 120
CEM II 42.5 R 690
Silica fume 100
Slag 80
Superplasticizer 40
Water 160
2.2 Measurement methods
The experimental testing of the HPFRC properties
included determination of the pore-size distribution,
basic physical, mechanical, hygric and thermal proper-
ties. For each test, three parallel samples were used.
Among the basic physical properties, bulk density,
matrix density and total open porosity were measured.
The bulk density  b
(kg/m
3
) of the cubic specimens was
determined using the standard gravimetric principle
according to EN 12390-7.
9
The specimen volume was
obtained by measuring the specimen length and weigh-
ing its dry mass. The expanded combined uncertainty of
the bulk density was 2.4 %. The matrix density  mat
(kg/m
3
) was obtained with a Pycnomatic ATC helium
pycnometer (Thermo Scientific). Using the measurement
results from the bulk-density and matrix-density tests,
the total open porosity  (%) was calculated.
10
The
expanded combined uncertainty of the total open poro-
sity was 3 %.
The effect of high temperatures on the porous
structure of the studied HPFRC specimens was examined
using mercury intrusion porosimetry (MIP). For the
measurement, a combination of Pascal 140 and Pascal
440 (Thermo Scientific) porosimeters was used. These
apparatuses are able to identify the pores having
diameters in ranges of 116–3.8 μm and 15–0.0036 μm,
respectively. The sample mass was about 1–2 g, de-
pending on the rate of material damage and porosity
increase. The samples were taken from the inner part of
the cubic specimens used for testing residual thermal
properties. For each elevated temperature, three parallel
samples were tested to ensure the reproducibility of our
results.
The mechanical properties including flexural
strength, compressive strength and dynamic Young’s
modulus were measured. The flexural strength f
f
(MPa)
was determined using the three-point-bending test
arrangement described in standard EN 1015-11.
11
The
span length between supports was 100 mm. The com-
pressive strength f
c
(MPa) was measured in accordance
with the same standard using the fragments of the
specimens from the flexural-strength tests. The loading
area was 40 mm × 40 mm. The expanded combined un-
certainty of both strength tests was 1.4 %. The dynamic
Young’s modulus E (GPa) was measured with a DIO 562
pulse ultrasonic device (Starmans Electronics) with a
frequency of 50 kHz.
Based on the water-sorptivity concept, the water
absorption coefficient A (kg/(m
2
·s
1/2
)) was measured
following EN 1015-18.
12
From the measured water
absorption coefficient and saturated moisture content
value w
sat
(kg/m
3
) of the analyzed material, the apparent
moisture diffusivity  (m
2
/s) was calculated.
The water vapor transmission has a clear effect on the
evaporation during the concrete heating. The associated
material properties were measured using the cup method
in a dry-cup arrangement of the test, i.e., the cup con-
tained silica gel and was placed in a controlled climatic
chamber where a constant relative humidity of 50 % and
constant temperature of 21 °C were maintained. The
water vapor diffusion permeability (s), the water vapor
diffusion coefficient D (m
2
/s) and the water vapor resist-
ance factor μ (–) were determined using the procedure
introduced by Foøtetal.
13
An ISOMET 2114 transient-impulse device (Applied
Precision) was used for the measurement of heat trans-
port and storage properties including thermal conducti-
vity  (W/(m·K)), thermal diffusivity a (m
2
/s) and volu-
metric heat capacity c
v
(J/(m
3
·K)) at room temperature.
For the characterization of the chemical and physical
processes in the HPFRC during its exposure to high
temperatures, a simultaneous thermal analysis (STA) and
thermodilatometry (TDA) were applied. The STA was
performed using a Labsys Evo apparatus (Setaram) in a
temperature range of 25–1200 °C, with a heating rate of
5 K/min. The measurement was done in an argon
atmosphere with a flow rate of 40 ml/min. The powdered
samples with a mass of about 50 mg and without any
fibers were placed in an alumina crucible with a lid. The
length changes of the studied material induced by a
high-temperature load were measured using a horizontal
thermodilatometer produced by Clasic CZ, Ltd. Rec-
tangular prisms with dimensions of 20 mm × 20 mm ×
Z. PAVLÍK et al.: PROPERTIES OF HIGH-PERFORMANCE FIBER-REINFORCED CONCRETE ...
Materiali in tehnologije / Materials and technology 53 (2019) 4, 481–487 483

160 mm were used as specimens. The measurement was
done in a static air atmosphere in a temperature interval
from 25 °C to 1000 °C. The applied heating rate was 1
K/min to minimize any thermal gradients in the heated
specimens. From the measured temperature-induced
length changes, the linear thermal expansion coefficient
was obtained. The STA and TDA experiments were
repeated three times to ensure the reproducibility of our
results.
3 RESULTS AND DISCUSSION
The values for the matrix and bulk density and po-
rosity of the studied HPFRC are given in Table 2. The
HPFRC was found to have a low porosity and a dense
internal structure. From the quantitative point of view,
the porosity values measured for the reference samples
and samples exposed to a heating temperature of 200 °C
are almost similar, corresponding to the porosity values
measured, e.g., by Yu et al.
14
The temperature exposure
starting at 400 °C yielded a higher porosity and a de-
crease in the bulk density, clearly demonstrating the
decay effect of a thermal treatment on the concrete struc-
ture.
Table 2: Bulk and matrix density and porosity of HPFRC at room
temperature
Maximum load
temperature
 b
(kg/m
3
)  mat
(kg/m
3
)  (%)
Reference 2363 2561 7.8
200 °C 2349 2554 8.0
400 °C 2260 2666 15.2
600 °C 2250 2670 15.7
800 °C 2220 2930 24.2
1000 °C 2110 2920 27.7
The coarsening of the pore structure due to the
chemical and physical decomposition of the HPFRC
examined with the MIP analysis is depicted in Figure 2.
For the reference specimen and the specimens exposed to
the temperature of 200 °C, the maximum pore diameter
is about 10 μm, whereas most of the pores have a size
between 0.01 μm and 0.50 μm. This means that the
amount of large capillary pores (classification according
to Taylor
15
) having a negative effect on the strength of
the material and water permeability is limited, yielding a
low porosity and a high mechanical resistance. On the
other hand, the MIP data clearly revealed the influence
of a high-temperature load on the pore structure of the
concrete. With the increasing maximum temperature of
the material, the pore volume and the pore size systema-
tically grew, clearly supporting the relevance of the
obtained material parameters listed above.
The mechanical properties of the studied concrete
specimens are summarized in Table 3.
Table 3: Residual and reference mechanical properties (flexural
strength f
f
, compressive strength f
c
, and Young’s modulus E)o f
HPFRC measured at room temperature
Maximum load
temperature
f
f
(MPa) f
c
(MPa) E (GPa)
Reference 31.9 170.3 52.1
200 °C 31.0 168.9 47.9
400 °C 23.4 128.4 31.5
600 °C 19.1 118.1 15.4
800 °C 10.5 64.4 9.6
1000 °C 1.3 16.1 4.6
It can be seen that the influence of thermal treatment
on the mechanical properties of the tested HPFRC can be
divided into three main stages. The mechanical pro-
perties of the specimens exposed to the maximum tem-
perature of 200 °C are more or less comparable to the
properties of the reference specimen. Due to the thermal
treatment at temperatures in the interval from 400 °C to
800 °C, the mechanical properties dramatically decrease
and at temperatures above 800 °C, most of the mecha-
nical strength is lost. The measured mechanical proper-
ties are comparable to the data published, e.g., by Ma et
al.
16
who observed similar retained mechanical properties
after high-temperature tests.
The structural changes that the HPFRC underwent at
high temperatures are clearly indicated by the STA re-
sults given in Figure 3. In the temperature interval from
25 °C to 300 °C, the liberation of physically bound water
from the pores and the dehydration reaction of C-S-H
phases took place. Additionally, AFt and AFm phases
dehydrated at 110–156 °C as described by Liu at al.
17
The endothermic peak at 445 °C corresponds to the Port-
landite decomposition.
15
The next relevant temperature
for the studied material is 572 °C when the transforma-
tion of the quartz aggregate took place.
18
This reaction is
accompanied by a sharp endothermic heat-flow peak and
a volume expansion. There is no change of mass during
the quartz transformation as seen from the TG curve. In
the temperature interval from 560 °C to 850 °C, the
calcite and C-S-H phases’ decomposition occurred. The
carbonation of C–S–H leads to the formation of vaterite,
which seems to be an unstable type of calcium carbo-
nate.
16
The temperature range of the corresponding
Z. PAVLÍK et al.: PROPERTIES OF HIGH-PERFORMANCE FIBER-REINFORCED CONCRETE ...
484 Materiali in tehnologije / Materials and technology 53 (2019) 4, 481–487
Figure 2: Cumulative pore-size distribution

decompositions is difficult to determine because the
peaks overlap. The significant exothermal peak corres-
ponds to the crystallization of wollastonite.
19
TDA data is shown in Figure 4. The thermal-strain
data clearly refers to the material-length changes due to
the linear heating of up to 1000 °C and indicates several
structural changes. Looking at the linear thermal expan-
sion coefficient plotted as a function of temperature, at a
temperature of about 445 °C, one can observe the effect
of the decomposition of portlandite and at a temperature
of 572 °C, the transformation of quartz that both
led to a significant thermal expansion. Another distinct
expansion was observed in the temperature interval from
about 790 °C to 900 °C. Here, the length changes were
induced by the decomposition of calcite, carbonized cal-
cium silicate hydrates and crystallization of wollastonite
as recorded during the STA. In the temperature interval
from 590 °C to about 680 °C, no significant expansion
was observed and the studied material even underwent a
limited thermal shrinkage. This was a result of the strain
incompatibilities between the cement paste and the
aggregates. While the aggregates and mineral fillers
dilate with the increasing temperature until they are
chemically degraded, the cement paste shrinks as soon as
it loses water due to drying and dehydration. Apparently,
in this temperature interval the rate of the aggregate’s
chemical degradation was lower compared to the shrink-
age of the concrete matrix. The shrinkage recorded in the
temperature interval from about 720 °C to 758 °C was
found by Estevez
20
and can be assigned to the formation
of a new silica compound.
The thermal properties of the investigated HPFRC
specimens are given in Table 4. This data corresponds to
the measured porosity values and characterizes the ma-
terial behavior from the point of view of heat transport
and storage after the exposure to elevated temperatures.
In the literature, the thermal and hygric properties of the
HPC are studied rarely. Vejmelková et al.
21
investigated
the thermal and hygric properties of the HPC with
metakaoline. For the HPC, they obtained a porosity of
13.0 %, a thermal conductivity of about 1.8 W/(mK) and
a thermal diffusivity of about 1.04 × 10
–6
m
2
/s. We
obtained similar values for our specimens exposed to
400 °C, having a porosity of 15.2 %. It is obvious that a
high-temperature load leads to a deceleration of heat
transport and thus to a slower water evaporation from the
hydrated concrete structure. This phenomenon is favor-
able for possible evaporation of free water and water of
hydrated products because a dense HPC structure pre-
vents free water from escaping, causing a considerable
internal vapor pressure, which often results in spalling.
In this way, the high-temperature-induced changes of a
pore structure retard partially the concrete damage.
Additionally, the higher porosity of high-temperature-
exposed specimens allows the evaporation of water from
concrete hydrates. This leads to a lower internal vapor
pressure compared to undamaged concrete.
Table 4: Thermal properties (thermal conductivity  , volumetric heat
capacity c
v
and thermal diffusivity a) of HPFRC measured at room
temperature
Maximum load
temperature
 (W/(m·K))
c
v
(×10
6
J/(m
3
·K))
a
(×10
–6
m
2
/s)
Reference 2.20 1.72 1.27
200 °C 2.17 1.71 1.26
400 °C 1.70 1.69 1.01
600 °C 1.41 1.63 0.87
800 °C 1.22 1.56 0.78
1000 °C 1.06 1.40 0.76
The hygric properties of the studied HPFRC speci-
mens are summarized in Table 5.
Table 5: Hygric properties of HPFRC tested at room temperature
Maximum
load tem-
perature
A (kg/
(m
2
·s
1/2
))
 (m
2
/s)  (s) D (m
2
/s) (–)
Reference 0.0001 1.72×10
–11
6.1×10
–12
8.3×10
–7
27.7
200 °C 0.0003 1.84×10
–11
7.4×10
–12
1.0×10
–6
22.7
400 °C 0.0306 5.93×10
–8
1.1×10
–11
1.5×10
–6
15.0
600 °C 0.0803 3.06×10
–7
1.4×10
–11
1.9×10
–6
12.0
800 °C 0.1491 5.67×10
–7
1.8×10
–11
2.4×10
–6
9.4
1000 °C 0.2124 9.45×10
–7
2.4×10
–11
3.3×10
–6
7.1
Z. PAVLÍK et al.: PROPERTIES OF HIGH-PERFORMANCE FIBER-REINFORCED CONCRETE ...
Materiali in tehnologije / Materials and technology 53 (2019) 4, 481–487 485
Figure 4: Thermal strain and linear thermal expansion coefficient of
the studied HPFRC
Figure 3: DSC and TG curves of the studied HPFRC

For both, the reference specimen and the specimen
exposed to 200 °C, the liquid water-transport properties
were almost undetectable. With the increasing load tem-
perature, all the studied hygric properties increased due
to the structural changes described above. From the
practical point of view, the increase of the pore volume
and changes in the pore-size distribution accelerate the
water evaporation. This is partially in conflict with the
above stated effect of the increased porosity with a
higher temperature load and thus certain retardation of
concrete overheating. Nevertheless, the deceleration of
the overheating of the concrete is only limited because
the thermal-conductivity values are high for all the stu-
died specimens. In the case of underground tunnel lin-
ings and other structures exposed to excessive moisture,
an opening of the concrete pore structure and thus an
increase in the water ingress into the material can induce
or/and accelerate its damage.
In order to support the data above and visualize the
damage of the HPFRC, optical microscopy was used to
identify the crack formation and steel-fiber deterioration.
The used optics consisted of an objective revolver
(0.58–7×, Navitar), lens attachment 2× and a color 3.15
Mpx CMOS sensor camera (ProgRes CT3). In Figure 5,
optical scans of the reference specimen and specimens
exposed to 600 °C and 1000 °C are shown. One can ob-
serve significant changes in the material structure fol-
lowed by color changes, steel-fiber damage and cracks
formed due to the joined thermo-hygric-mechanical
effects described above.
4 CONCLUSIONS
The experimental analyses and tests made in this
work revealed significant structural changes in the stu-
died HPFRC at temperatures higher than 200 °C. Such a
complex analysis of the changes in the material parame-
ters of hybrid-fiber-reinforced high-performance
cement-based composites subjected to high temperatures
has not been presented yet and will contribute to an
increase in the level of scientific knowledge in the
studied field. An increase of the total open porosity and
pore size due to the exposure to elevated temperatures
led to a significant reduction of the mechanical strength.
The changes in the properties of the investigated HPFRC
were explained with STA and TDA data that allowed an
identification of the transformation and decomposition of
hydrated products in the HPC matrix.
A unique complete set of thermal and hygric proper-
ties of the HPFRC as a function of thermal treatment
represents valuable information for practical use of the
HPFRC. It can be applied as in-put data for the mo-
delling of concrete damage and coupled mechanical and
hygrothermal performance at a high-temperature expo-
sure. As many numerical models have been deve-
loped,
22,23
attempting to simulate and predict the spalling
phenomenon for specific concrete mixes, structural
forms and fire-loading scenarios, various material para-
meters are applied as input data for a numerical analysis
of concrete damage at high temperatures. For example,
Gawin at al.
4
presented a fully generalized, coupled,
multi-phase, hygro-thermo-mechanical model for heated
concrete where the dehydration degree obtained from the
TG data, porosity, density, mechanical properties,
effective thermal conductivity, specific heat capacity and
thermal-strain values obtained with this research can find
use. Similarly, Zhang et al.
24
applied, in their model of
concrete spalling, other parameters including water-satu-
ration degree, thermal-expansion coefficient, thermal
conductivity, specific heat capacity, mechanical para-
meters, water-vapor diffusion coefficient, etc. The ob-
tained hygric and thermal properties of the tested
HPFRC can find use also in the design of engineering
structures and buildings. Based on a literature analysis,
we can say that this data is of a particular importance for
future research focused on the development of the
HPFRC resistant to elevated temperatures. Our future
research activities will comprise design and complex
testing of a new advanced type of HPFRC with hybrid
fiber reinforcement, while the experimental tests will be
aimed at both the residual properties after a temperature
load and the concrete properties measured directly at
high temperatures.
Acknowledgment
The authors greatly acknowledge the financial
support of the Czech Science Foundation within project
Z. PAVLÍK et al.: PROPERTIES OF HIGH-PERFORMANCE FIBER-REINFORCED CONCRETE ...
486 Materiali in tehnologije / Materials and technology 53 (2019) 4, 481–487
Figure 5: Optical-microscopy images of HPFRC: a) reference sample, b) sample exposed to 600 °C, c) sample exposed to 1000 °C

P105/12/G059 and of the CTU in Prague within project
No SGS17/166/OHK1/3T/11.
5 REFERENCES
1
A. Dufka, T. Melichar, J. Byd`ovský, J. Vanìrek, The development
of new types of secondary protection for concrete structures exposed
to extreme conditions, Mater. Technol., 51 (2017), 533–540,
doi:10.17222/mit.2015.249
2
A. Caverzan, M. Colombo, M. di Prisco, B. Rivolta, High perform-
ance steel fibre reinforced concrete: residual behaviour at high
temperature, Mater. Struct., 48 (2015), 3317–3329, doi:10.1617/
s11527-014-0401-9
3
M. Bene{, R. [tefan, Hygro-thermo-mechanical analysis of spalling
in concrete walls at high temperatures as a moving boundary
problem, Int. J. Heat Mass Transf., 85 (2015), 110–134,
doi:10.1016/j.ijheatmasstransfer.2015.01.050
4
D. Gawin, F. Pesavento, B. A. Schrefler, Towards prediction of the
thermal spalling risk though a multi-phase porous media model of
concrete, Comput. Methods Appl. Mech. Eng., 195 (2006),
5707–5729, doi:10.1016/j.cma.2005.10.021
5
J. Jansson, Fire Spalling of Concrete: Theoretical and Experimental
Studies, PhD Thesis, KTH Royal Institute of Technology, Division of
Concrete Structures, Stockholm 2013
6
U. Schneider, Concrete at high temperatures – A general review, Fire
Safety J., 13 (1988), 55–68, doi:10.1016/0379-7112(88)90033-1
7
A. Lau, M. Anson, Effect of high temperatures on high performance
steel fibre reinforced concrete, Cement Concrete Res., 36 (2006),
1698–1707, doi:10.1016/j.cemconres.2006.03.024
8
K. K. Sideris, P. Manita, E. Chaniotakis, Performance of thermally
damaged fibre reinforced concretes, Constr. Build. Mater., 23 (2009),
1232–1239, doi:10.1016/j.conbuildmat.2008.08.009
9
EN 12390-7:2000 Testing hardened concrete – Part 7: Density of
hardened concrete, CEN, Brussel
10
M. Záleská, M. Pavlíková, Z. Pavlík, O. Jankovský, J. Pokorný, V.
Tydlitát, P. Svora, R. ^erný, Physical and chemical characterization
of technogenic pozzolans for the application in blended cements,
Constr. Build. Mater., 160 (2018), 106–116, doi:10.1016/j.conbuild-
mat.2017.11.021
11
EN 1015-11:1999 Methods of test for mortar for masonry – Part 11:
Determination of flexural and compressive strength of hardened
mortar, CEN, Brussel
12
EN 1015-18:2002 Methods of test for mortar for masonry – Part 18:
Determination of water-absorption coefficient due to capillary action
of hardened mortar, CEN, Brussel
13
J. Foøt, Z. Pavlík, J. @umár, M. Pavlíková, R. ^erný, Effect of
temperature on water vapor transport properties, J. Build. Phys., 38
(2014), 156–169, doi:10.1177/1744259114532612
14
R. Yu, P. Spiesz, H. J. H. Brouwers, Mix design and properties
assessment of ultra-high performance fibre reinforced concrete,
Cement Concrete Res., 56 (2014), 29–39, doi:10.1016/j.cemconres.
2013.11.002
15
H. F. W. Taylor, Cement Chemistry, 2
nd
ed., Thomas Telford, London
1997
16
Q. Ma, R. Guo, Z. Zhao, Z. Lin, K. He, Mechanical properties of
concrete at high temperature—Areview ,Constr. Build. Mater., 93
(2015), 371–383, doi:10.1016/j.conbuildmat.2015.05.131
17
S. Liu, L. Wang, Y. Gao, B. Yu, Y. Bai, Comparing study on hyd-
ration properties of various cementitious systems, J. Therm. Anal.
Calorimetry, 118 (2014), 1483–1492, doi:10.1007/s10973-014-
4052-4
18
P. J. Heaney, D. R. Veblen, Observations of the  - phase transition
in quartz: A review of imaging and diffraction studies and some new
results, Am. Mineralogist, 76 (1991), 1018–1032
19
A. Yazdani, H. R. Rezaie, H. Ghassai, Investigation of. hydrothermal
synthesis of wollastonite using silica and nano silica at different
pressures, J. Ceram. Process. Res., 11 (2010), 348–353
20
L. P. Estevez, On the hydration of water-entrained cement–silica sys-
tems: Combined SEM, XRD and thermal analysis in cement pastes,
Thermochim. Acta, 518 (2011), 27–35, doi:10.1016/j.tca.2011.
02.003
21
E. Vejmelková, M. Pavlíková, M. Keppert, Z. Ker{ner, P. Rovna-
níková, M. Ondráøek, M. Sedlmajer, R. ^erný, High performance
concrete with Czech metakaolin: Experimental analysis of strength,
toughness and durability characteristics, Constr. Build. Mater., 24
(2010), 1404–1411, doi:10.1016/j.conbuildmat.2010.01.017
22
C. Davie, C. H. J. Pearce, N. Bi}ani}, A fully generalised, coupled,
multi-phase, hygro-thermo-mechanical model for concrete, Mater.
Struct., 43 (2010), 13–33, doi:10.1617/s11527-010-9591-y
23
D. Ru`i}, J. Kol{ek, I. Planinc, M. Saje, T. Hozjan, Non-linear fire
analysis of restrained curved RC beams, Eng. Struct., 84 (2015),
130–139, doi:10.1016/j.engstruct.2014.11.012
24
Y. Zhang, M. Zeiml, Ch. Pichler, R. Lackner, Model-based risk
assessment of concrete spalling in tunnel linings under fire loading,
Eng. Struct., 24 (2014), 207–215, doi:10.1016/j.engstruct.2014.
02.033
Z. PAVLÍK et al.: PROPERTIES OF HIGH-PERFORMANCE FIBER-REINFORCED CONCRETE ...
Materiali in tehnologije / Materials and technology 53 (2019) 4, 481–487 487