Effect of the high temperatures on the microstructure and compressive strength of high strength fibre concretes

Construction and Building Materials 199 (2019) 717–736

Contents lists available at ScienceDirect

Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat

Effect of the high temperatures on the microstructure and compressive
strength of high strength fibre concretes
Hugo Caetano a, Gisleiva Ferreira b, João Paulo C. Rodrigues a,⇑, Pierre Pimienta c
a

LAETA, Department of Civil Engineering of University of Coimbra, Portugal
Department of Civil Engineering, Faculty of Civil Engineering, Architecture and Urbanism, State University of Campinas, Brazil
c
CSTB – Centre Scientifique et Technique du Bâtiment, France
b

h i g h l i g h t s
 Compressive strength at high temperatures of fibre concretes studied.
 Influence of the high temperatures on the physical and chemical changes of the concrete studied.
 Behavior of the new Dramix 5D steel fibre concretes in fire studied.
 Model for compression strength at high temperatures of fibre concrete proposed.

a r t i c l e

i n f o

Article history:
Received 11 August 2018
Received in revised form 6 December 2018

Accepted 13 December 2018

Keywords:
High strength concrete
Fibres
Steel
Polypropylene
High temperatures
Compressive strength
Thermal analyses

a b s t r a c t
The high temperatures of fires affect the physical and chemical properties of the concrete and thus influence its mechanical properties. This paper presents the results of an experimental investigation on the
compressive strength at high temperatures of high-strength fibre concretes. The influence of the high
temperatures on the physical and chemical changes of the concrete was also analysed by Thermo
Gravimetric Analysis/Differential Thermal Analysis (TGA/DTA), X-Ray Diffraction (XRD) and Scanning
Electron Microscopy with Energy Dispersive Spectrometry (SEM/EDS). Five concrete compositions with
different steel fibre contents and types have been tested: one without steel fibres (reference composition), two with Dramix 3D steel fibres and two with Dramix 5D steel fibres (45 and 75 kg/m3). This
new type of steel fibres, the Dramix 5D, presents a double curvature at its ends, allowing a more efficient
anchorage in the cementitious matrix. The behaviour at high temperatures of concretes made with these
5D fibres has been compared with the one of concretes made with the Dramix 3D steel fibres. Therefore,
the impact of the high temperatures on the compressive strength and morphology of the high-strength
fibre concretes made with the Dramix 3D and 5D steel fibres has been evaluated. The paper proposes also
models for the compressive strength at high temperatures of the studied high strength fibre concretes.
Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction
Fiber-reinforced concrete is a composite material widely used
in Civil Engineering, and the compressive strength is a very important parameter on the design of reinforced and pre-stressed concrete structures.
The concrete elements, when in fire, are subjected to the high

temperatures and this may result in significant losses of their
load-bearing capacity due to reduction of material’s strength and
stiffness [1]. The introduction of steel fibres in the normal and high
strength concretes, in suitable dosages, may cause improvements
⇑ Corresponding author.
E-mail address: (J.P.C. Rodrigues).
/>0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

on the mechanical properties of the concrete either at ambient
and high temperatures [2–7]. Researches described that
high-strength concrete begins to lose compressive strength for
temperatures lower than the ones of the normal concrete. The
high-strength concrete starts to reduce its compressive strength
for temperatures of nearly 150 °C (corresponding to a significant
loss of nearly 30% of the initial strength) while in the normal
strength concrete this reduction only occurs for temperatures of
nearly 350 °C. This performance results from the higher pore pressure effect caused by the lower permeability of the high-strength
concrete [8].
Steel fibre reinforced concrete (SFRC) is a composite material
that may mitigate the consequences of the high temperatures
exposition [3,9]. This Steel fibres improve the ductility, energy


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

Notation
CaCO3
CaO

Ca(OH)2
CH
C-S-H
DTA
EDS
SD
SEM
SiO2
TGA
XRD

Calcium carbonate
Calcium oxide (Free lime)
Portlandite
Calcium hydroxide
Calcium silicate hydrate
Differential thermal analysis
Energy dispersive spectrometer
Standard deviation
Scanning electronic microscopy
Quartz
Thermogravimetric analysis
X-ray diffraction

absorption capacity, cracking control and toughness of the concrete [4].
Barros et al. [10] concluded that the post-cracking residual
strength could be much higher in concrete reinforced with steel
fibres than in plain concrete with the same strength class, due to
the reinforcement mechanisms provided by the fibres on bridging
the cracks. In consequence, the reinforcement of concrete with

steel fibres allows a high level of stress redistribution, providing
greater deformation capacity of a structure between the crack
beginning to its failure, which increases structural safety.
Another problem associated with the high-strength concrete is
the spalling. This phenomenon occurs because of the low permeability and water-cement ratio of the concrete [8,11]. Eurocode 2
[12] and some researchers [11,13–16] have suggested that spalling
can be minimised by adding polypropylene fibres to the highstrength concrete.
The sublimation (160–170 °C) and vaporization (350 °C) of the
polypropylene fibres create new pores and microcracks in the concrete matrix that may increase the permeability of the concrete
[4,17–19]. These phenomena reduce the damages and resistance
of the concrete subjected to the high temperatures of the fire
[17]. Yermak et al. [4] also confirmed this positive effect of the
cocktails of steel and polypropylene fibres by porosity and permeability tests. Thus, the simultaneous use of steel and polypropylene
fibres reduces the brittle behaviour of the concrete depending
obviously on the content of fibres used [20].
In order, to prove the performance of the high-strength concrete
reinforced with steel and polypropylene fibres, it is necessary more
research because of several chemical and physical transformations
of the paste and aggregates, which results in changes in the concrete’s mechanical performance and durability [21–26]. Khoury
[22] mentions that these changes depend from parameters such
as the concrete’s composition, moisture content, load level, heating
and cooling rates, time of exposure to elevated temperatures, time
after cooling and number of thermal cycles with heating and cooling. Also, the RILEM TC 200 HTC [27] indicates the endogenous
parameters that most affect the compressive strength of the concrete at high temperatures are the type of aggregate, rate of dehydration and sealing of the specimens.
Well-hydrated Portland cement paste consists mainly of calcium silicate hydrate (C-S-H), calcium hydroxide (CH) and calcium
sulfoaluminate hydrate [28]. When cement paste is exposed to
high temperatures, the hydrated products gradually lose water,
which generates in water steam and increases the pore pressure
in the concrete [17].
This phenomenon starts at approximately 100 °C and continues

up to 500 °C, which corresponds to the vaporisation temperature of
the crystalline water in the concrete [29–31]. At about 300 °C, the

W/C
b-C2S
h
fcm,cube
fcm
fck,cube
fc, h
fcm, h

Water/binder ratio
Larnite
Temperature
Mean value of the cube compressive strength of the
concrete at ambient temperature
Mean value of the compressive strength of the concrete
at ambient temperature
Characteristic value of the compressive strength of the
concrete at ambient temperature
Compressive strength of the concrete at temperature h
Mean value of the compressive strength of the concrete
at temperature h

interlayer and chemically combined water of the C-S-H and sulfoaluminate hydrates would be lost, but under temperatures of
around 900 °C, the complete decomposition of C-S-H occurs. Further dehydration of the cement paste, due to decomposition of
the calcium hydroxide, begins at about 500 °C [32–33].
Calcium hydroxide (CH) loses water between 400 and 500 °C,
but if CO2 is available, above 400 °C, it may form calcium carbonate

(CaCO3). Also, the decomposition of the CH could be quickly
undone while it cools down to ambient temperature [30].
Some authors [34,35] studied the performance of concrete
structures exposed to fire to ascertain the effects of temperature
on their microstructure and the properties of the aggregates. Initially, the vaporisation of the free water between 100 and 140 °C,
increasing the pore pressure of the cementitious matrix. At
400 °C, the dehydration of calcium hydroxide and C-S-H gel begins,
which leads to shrinkage and reduction in the concrete’s strength
[36].
Some of the temperature effects are due to chemical changes
and moisture transport within the cement paste, and another is
due to damage (microcracks) resulting from temperature gradients
and deformational incompatibilities between the aggregates and
cement paste.
According to Lim [33], the development of microcracks on the
interface between dehydrated cement particles and cement paste
matrix and changes in C-S-H microstructure are considered as
main factors that cause the thermal degradation of the cement
paste. Different types of cracks may be found on the concrete after
its exposition to high temperatures.
According to Henry et al. [36] and Picandet et al. [37] in the first
phase of the concrete’s heating (500 °C), the pores and the preexisting microcracks close due to contraction of the concrete’s
overall volume. However, in the second heating phase (>500 °C),
bridge cracks occur because of the different performance between
aggregates and mortar matrix.
Concrete is a heterogeneous material composed of aggregates
embedded in the cement paste matrix. The heterogeneity of the
concrete’s constituents can result in severe thermal damages in
the interface, such as the cement paste-aggregate interface, due
to the different behaviour of the constituents at high temperatures

[38]. Siliceous aggregates are predominantly quartz, which
changes from trigonal a-quartz to hexagonal b-quartz at 575 °C,
causing a volume increasing of approximately 6%, and is decomposed at around 800 °C [38].
In the case of carbonate rocks, a similar disturbance can begin at
700 °C as a result of the decarbonisation. In addition to possible
phase transformations and thermal decomposition of the aggregates, its mineralogy determines the response of the concrete at
high temperatures. For instance, it determines the differential


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

normal and high strength concretes. They concluded that for temperatures between 200 and 400 °C the heating rate is a major factor for the decreasing on the weight loss, compression and tensile
strengths of both types of concrete. In this range of temperatures,
the modulus of elasticity is also very affected. For temperatures
above 600 °C, the compressive strength is almost negligible in
agreement with other similar experimental works. There is a
strong need to establish constitutive relationships and damage
microstructure for modelling the fire response of mix fibers (steel
and polypropylene) high-strength concretes.
The creation of theoretical and numerical models that accurately predict the mechanical behaviour of concrete at high temperatures is very complex. The classical isotropic theory of
nonlocal damage was adequately modified to take into account
both the mechanical damage and the deterioration of thermochemical material at high temperatures [41].
However, several theoretical models are currently available in
the scientific literature to simulate the failure processes of concrete

thermal elongation between the aggregate and the cement paste,
and the maximum strength of the interfacial transition zone [33].
The nature of the aggregates is closely linked to the concrete’s

thermal expansion and conductivity coefficients because while
siliceous concretes have a slight contraction when subjected to
temperatures between 300 and 900 °C, calcareous concrete has
an expansion which leads to the development of cracking. It happens because of the higher degree of porosity and the coefficient
of thermal expansion of the calcareous aggregates. In this sense,
the author states that the type of aggregate greatly affects the
mechanical strength of the concrete at high temperatures and after
fire.
According several authors [5–7], this behavioural difference
results from the dense microstructure of the high-strength concrete (because of the low W/C ratio) that gives to the highstrength concrete a low permeability hindering the water vapour
in the pores from being released when the temperature increases
and with this making concrete more prone to spalling.
However, in the temperature range of 400–800 °C both concrete
lose most of their original strength, especially at temperatures
above 600 °C due to the decomposition of the calcium silicate
hydrate gel (C-S-H) that is the responsible for the mechanical
strength of the cement. Above 800 °C, the loss of the original
strength for both concrete is almost complete.
The influence of the long-term loading on the compressive
strength and modulus of elasticity of the concrete at high temperatures was also studied by Jonaitis and Papinigis [39]. They concluded that the decrease of the compressive strength is less
when the concrete is heated first and then subjected to a longterm loading than when is heated after being subjected to a
preloading.
An experimental study conducted by Aidoud and Benouis [40]
analysed the effect of the high temperatures on the behaviour of

Fig. 2. Slabs representative of each concrete composition.

Table 1
Density (kg/m3) of the materials used in the manufacture of concrete compositions.
CEM

LF

S

LG1

LG2

LG3

PF

SF3D

SF5D

SP

W

3130

2700

2640

2680

2680

2680

910

7850

7850

1060

1000

Fig. 1. Fibres used in the concrete compositions: a) polypropylene; b) steel fibres 3D and b) steel fibres 5D.

Table 2
Concrete compositions (in kg/m3).
Concrete Composition

CEM

LF

S

LG1

LG2

LG3

PF

SF3D

SF5D

SP

W

RC
3D_45
3D_75
5D_45
5D_75

400
400
400
400
400

200
200
200
200
200

479

479
479
479
479

543
528
518
528
518

290
290
290
290
290

373
373
373
373
373

2
2
2
2
2

0

45
75
0
0

0
0
0
45
75

8
8
8
8
8

144
144
144
144
144


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

Fig. 3. Equipment used for: a) drilling the slabs; b) cut and c) rectify the top faces of the concrete specimens.

Fig. 4. Specimens for the compressive strength tests and location of the
thermocouples.

structures subjected at the same time to high temperatures and
mechanical loads. Recently models for simulating the behavior of
materials reinforced with fibres at high temperatures have
appeared [42–44].

In addition to the mechanical compression tests, thermogravimetry, X-ray diffraction and SEM-EDS tests were also carried
out. Thermogravimetry is a thermal analysis technique that stands
out now of the evaluation of morphological and chemical changes
of the compounds formed during the Portland cement hydration.
In this experimental test, the mass change of a specimen placed
in a crucible and controlled atmosphere, as a function of temperature or time, it is continuously recorded as the temperature
increases. The thermogravimetry with the differential thermal
analysis (DTA) are suitable techniques for the hydration study of
the concrete.
X-ray diffraction (XRD) is a technique used for the mineralogical
evaluation of concrete and its crystalline structure. Also, it allows
the qualitative and quantitative chemical identification of the crystalline phases found in the material at high temperatures. Acoustic
emission could be another non-destructive technique that could be
used to evaluate the actual state of damage of the concrete Acoustic Emission (AE) test as can be seen in other research in this area
[45].
The morphology of the concrete specimen was monitored by
scanning electron microscopy (SEM), and energy dispersive X-ray
spectroscopy (EDS) was used to identify and quantify the chemical
elements present in the compounds identified in the images.
In this context, an experimental work has been carried out at
Coimbra University to investigate the mineralogical and

Fig. 5. Specimens for the thermal analyses tests. a) Cylindrical slice of concrete of 3 mm thickness and 70 mm diameter and b) piece of concrete impregnated with epoxy
resin.


H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

Fig. 6. Experimental test set-up for the compressive strength tests.

microstructure changes of the high-strength concretes in study, as
well as compressive strength at high temperatures.

2. Experimental setup and program
2.1. Materials and compositions
In the mix design the following materials were used: Portland
cement CEM I 42.5 R (CEM), limestone gravel with 11–18 mm
(LG1), limestone gravel with 8–14 mm (LG2), limestone gravel

721

with 4–8 mm (LG3), quartz sand with 0–8 mm (S), limestone filler
(LF), polypropylene fibres (PF), steel fibres of Dramix 3D (SF3D),
steel fibres of Dramix 5D (SF5D), superplasticizer Sika ViscoCrete
3002 HE (SP) and water (W). Dramix is a trade mark of Bekaert,
Belgium. In Table 1 it is possible to observe the density of the
materials used in the manufacture of the concrete compositions.
The steel fibres Dramix 3D have 60 mm in length (l), 0.90 mm in
diameter (d), 65 length/diameter ratio (l/d), with 1160 MPa of tensile strength and 210 GPa of modulus of elasticity. The steel fibres
Dramix 5D have 60 mm in length (l), 0.90 mm in diameter (d), 65
length/diameter ratio (l/d), with 2300 MPa of tensile strength and
210 GPa of modulus of elasticity [46,47]. The major diferences

between Dramix 3D and 5D steel fibres are that the 3D have a single curvature at both ends and the 5D have a double curvature at
both ends. Fig. 1 shows all the used fibres.
In this research, five concrete compositions were studied
(Table 2). The compositions are referenced as RC, 3D_45, 3D_75,
5D_45 and 5D_75, where RC stands for the composition without
steel fibres, 3D_45 and 3D_75 for compositions with Dramix 3D
steel fibres with amounts of 45 and 75 kg/m3, and 5D_45 and
5D_75 for compositions with Dramix 5D steel fibres, also with
amounts of 45 and 75 kg/m3, respectively. All tested compositions
had a dosage of 2 kg/m3 of polypropylene fibres and 0.36 of water
to cement ratio.

2.2. Experimental program
The experimental program included five different compositions
of high-strength fibre concretes. The tests were conducted at the
Laboratory of Testing Materials and Structures (LEME) of Coimbra
University (UC), in Portugal. They were carried out mechanical
compression and thermal tests. The experimental program of the
compressive strength consisted of 60 tests at ambient and high
temperatures (300, 500 and 700 °C). In the tests at hightemperature an initial pre-load of 20% of the average value of the

Fig. 7. Schematic experimental test set-up representation for the compressive strength tests.


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

Fig. 8. Temperature evolution in the specimen as a function of time for the 300 °C test series.

Fig. 9. Temperature evolution in the specimen as a function of time for the 500 °C test series.

Fig. 10. Temperature evolution in the specimen as a function of time for the 700 °C test series.


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736
Table 3
Values of fcm,cube (in MPa).
Composition

Age of specimens (days)

RC
3D_45
3D_75
5D_45
5D_75

7

14

21

28

150 (first day of testing)

270 (last day of testing)

55.3
60.9
64.9
66.0
67.6

62.0
71.0
72.0
73.0
74.0

61
70.6
74.9
74.0
78.0

66.7
72.0
76.7
79.9
77.0

71.2
77.3
80.7
83.2

82.2

74.2
81.4
81.3
84.8
86.5

compressive strength obtained during the compression tests at
ambient temperature (0.2 fcm). For each test series, 3 specimens
were tested, to obtain a better correlation of results. To evaluate
the development of the temperatures inside the specimens it was
carried out more 15 simple heating tests.
The thermal analysis tests were carried out at the Laboratory of
Pedro Nunes Institute, in Coimbra, Portugal. Twenty-two specimens were used in this experimental program of thermal analyses.
They were carried out XRD and TGA-DTA tests for the reference
concrete composition and SEM/EDS tests for all the concrete compositions in study. For each type of test and concrete composition,
only one specimen was tested. For the TGA-DTA and XRD tests, 2

Table 4
Concrete classes.
Concrete compositions

fcm,cube (MPa)

fck,cube (MPa)

Resistance classes

RC

3D_45
3D_75
5D_45
5D_75

67
72
77
80
77

63
68
73
76
73

C50/60
C55/67
C55/67
C60/75
C55/67

specimens were used (temperature increased from ambient to
1000 °C), while in the SEM/EDS tests only one specimen of each
composition was used and for each temperature level (ambient,
200, 500, 800 °C).

2.3. Specimens
In this experimental work, the cylindrical specimens used in the

compressive strength tests were obtained by core drilling of slabs
representative of each concrete composition to obtain a better representation of the material and to avoid interfering in the orientation of the steel fibres during the casting process (Fig. 2).
Afterwards, the specimens were cut and the top faces rectified in
a way that they were parallel among them (Fig. 3).
The specimens had a height of 210 mm and a diameter of
70 mm. The dimensions of these specimens were mainly limited
by the size of the furnace’s internal chamber. However, although
they did not have a standard size, the specimen’s dimensions
respect the length/diameter ratio between 3 and 4 (slenderness)
and the specimen’s diameter is more than 4 times the size of bigger
aggregates, according to the RILEM recommendations [48].

Table 5
compressive strength of each concrete composition in function of the temperature.
Specimen

fc, h (MPa)

fcm, h (MPa)

SD (MPa)

20 °C
RC_3
RC_5
RC_10
3D_45_1
3D_45_2
3D_45_17
3D_75_2

3D_75_3
3D_75_17
5D_45_1
5D_45_3
5D_45_16
5D_75_1
5D_75_3
5D_75_5

79
78
79
86
81
84
82
85
84
80
83
85
93
92
95

79

0.47

84

2.05

84

1.25

83

2.05

93

1.25

500 °C
RC_11
RC_12
RC_13
3D_45_11
3D_45_12
3D_45_13
3D_75_11
3D_75_12
3D_75_13
5D_45_11
5D_45_12
5D_45_13
5D_75_11
5D_75_13

5D_75_21

45
44
45
51
52
48
54
51
48
51
53
48
55
51
53

44

0.47

50

1.70

51

2.45

51

2.05

53

1.63

Specimen

fc, h (MPa)

fcm, h (MPa)

SD (MPa)

300 °C
RC_6
RC_7
RC_15
3D_45_6
3D_45_7
3D_45_16
3D_75_7
3D_75_10
3D_75_16
5D_45_6
5D_45_7
5D_45_8
5D_75_6

5D_75_7
5D_75_8

78
75
75
90
83
84
87
90
93
83
85
83
90
85
80

76

1.41

86

3.09

90

2.45

84

0.94

85

4.08

700 °C
RC_17
RC_18
RC_24
3D_45_10
3D_45_24
3D_45_27
3D_75_5
3D_75_8
3D_75_24
5D_45_26
5D_45_27
5D_45_28
5D_75_26
5D_75_27
5D_75_28

18
20
18
28

24
26
26
24
21
25
24
27
31
28
31

19

0.94

26

1.63

24

2.05

25

1.25

30

1.41


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Fig. 11. Mean ultimate load and standard deviation for the different concrete compositions and temperature series.

Fig. 12. An example of stress-strain curves selected from each concrete compositions and temperature series.

For temperature measuring five type K chrome-alumel thermocouples, were placed in the surface and central axis of the specimen as represented in Fig. 4.
In thermal analyses tests cylinders similar to the ones of the
compressive strength tests were first cut in slices of 70 mm diameter and 3 mm thick (Fig. 5a).
In the TGA-DTA and XRD tests, these slices of concrete were
then split into smaller pieces and after crushed with a pestle up
to a fine powder of 100 mm fineness.
The specimens for the SEM/EDS observations were obtained by
splitting the slices of concrete into small pieces and then impreg-

Table 6
Relative values of compressive strength of each concrete composition in function of
the temperature.
h (°C)

20
300
500
700

Relative compressive strength
RC

3D_45

3D_75

5D_45

5D_75

1.00
0.97
0.56
0.24

1.00
1.02
0.60
0.31

1.00
1.08
0.61
0.29

1.00
1.01
0.61
0.31

1.00
0.91
0.57
0.32


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

nated with epoxy resin to minimise any damage during the grinding and polishing processes (Fig. 5b). These specimens were after
grounded with silicon carbide papers of decreasing grit size, and
after dried they were sputtered with a gold alloy film.
2.4. Experimental Set-ups
The compressive strength tests set-up (Fig. 6) consisted on a
‘‘SERVOSIS” tensile-compression machine of 600 kN (a) capacity, a
cylindrical oven with 90 mm of diameter and 300 mm of height,
internal dimensions, capable to reach the maximum temperature
of 1200 °C (b), a Datalogger TDS-601 (c) to data acquisition, the

universal testing machine controller (d), the furnace controller (e),
a laptop computer (f) and a load cell (g). The pull rods of the
tensile-compression machine were made of refractory steel. Fig. 7
shows a schematic representation of the experimental test set-up.
The TGA-DTA and XRD tests used a Setaram (Setsys Evolution)
analyser complemented with a Philips X’Pert diffractometer with
cobalt (ka1 = 1.78897 Å) radiation.
The SEM/EDS observations used a Field Emission Scanning Electron Microscope (FESEM) ZEISS MERLIN coupled with an OXFORD
energy dispersive spectrometer X-RAY. An EDWARDS EXC 120

sputter coater was used on the coating process with gold alloy film
of the polished surfaces of the concrete specimens.

Fig. 13. Relative compressive strength of the different concrete compositions as a function of the temperature.

Table 7
Tabulated relative compressive strength values for the proposed model of preloaded HSFC with calcareous aggregates.
h (°C)

20
300
500
700

Results from the experimental data

Results from the proposed model

RC composition (only PP fibres)

3D_45 and 5D_45 compositions

3D_75 and 5D_75 compositions

1
0.97
0.56
0.24

1

1.02
0.61
0.31

1
0.99
0.62
0.31

1
0.91
0.56
0.24

Fig. 14. Comparison between the proposed model for the prestressed compressive strength of HSFC with calcareous aggregates at high temperatures and test data from
others authors.


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

Fig. 15. Comparison between the proposed model and models from another’s authors for the stressed compressive strength of HSFC with calcareous aggregates at high
temperatures.

Fig. 16. Comparison between the proposed model, tests data and models from others authors for the unstressed compressive strength of HSFC with calcareous aggregate
concrete at high temperatures.

Fig. 17. Comparison between the proposed model, tests data and models from others authors for the compressive strength of HSFC at high temperatures.

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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

Fig. 18. Mean ultimate load of each concrete composition in function of the content and geometry of the fibres for different temperatures.

Table 8
Specimens after test.
h (°C)

RC

3D_45

3D_75

5D_45

5D_75

20

S03_RC

S02_3D_45

S03_3D_75

S01_5D_45

S01_5D_75

300

S07_RC

S06_3D_45

S07_3D_75

S07_5D_45

S06_5D_75

500

S12_RC

S12_3D_45

S12_3D_75

S11_5D_45

S11_5D_75

700

S24_RC

S10_3D_45

S8_3D_75

S27_5D_45

S28_5D_75


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

2.5. Test procedure
The adopted procedure for the compression tests followed the
recommendations of RILEM [48]. In the tests carried out at ambient
temperature, the pull rods of the tensile-compression machine
were aligned vertically with each other, and the concrete specimen
was centred in the compression rods. Then the load was applied at
a speed of 0.25 kN/s until the concrete specimen rupture.
In the tests carried out at 300, 500 and 700 °C, the pull rods of
the tensile-compression machine were aligned vertically with each
other, and the concrete specimen was centred in the compression
rods. Then, a compression load was applied at a speed of 0.25 kN/s
until the target load value 20% of the average value of the compressive strength obtained during the compression tests at ambient
temperature (0.2 fcm). Subsequently, the furnace was closed and
was insulated with ceramic wool to avoid thermal losses during
heating.
Afterwards, the specimens were heated up at a rate of 3 °C/min

until the desired level of temperature was reached (300, 500 or
700 °C). The target temperature was then maintained for 60 min
so that the temperature in the specimen could stable and uniform.
During this heating process, the load on the specimen was kept
constant. When the desired temperature level in the specimen
was uniform, the load was increased at a speed of 0.25 kN/s up
to the concrete rupture (if that had not already happened during
the heating process). The displacements and loading forces were
also measured and recorded continuously, with the goal of obtaining the curve force-displacement of the specimens from different
compositions. During the compression tests the displacement that
was recorded by the LVDT installed in the tensile-compression
machine Servosis, refers to the movement of the machine pull rods
that were in contact with the concrete specimen. The load level of
0.2 fcm was adopted, because tried to simulate the service loads on
concrete structural elements subjected to the compression
stresses.
In heating tests, the evolution of the temperature inside the
specimen was measured as well as inside the furnace. The evolution of the temperature in the specimens was measured over time
to determine the heating time required to uniform the temperature
in the specimens for each temperature level.
The TGA-DTA tests were used to provide information on the
chemical reactions of the concrete due to the heating and calculates the calcite and portlandite contents. The concrete powder
samples were heated from ambient temperature to 1000 °C, at a
heating rate of 10 °C/min, in an argon atmosphere (50 ml/min).

Fig. 20. XRD peak patterns at different temperatures.

The XRD tests were carried out to allow further insight into the
mineralogy of the binder and other constituents of the concrete
such as the aggregates. The X’Pert diffractometer with cobalt

(ka1 = 1.78897 Å) radiation, with steps of 0.025° and 1 s of time
per step, between 5° and 80° 2 theta, was used.
The specimens for the SEM/EDS observations were firstly
heated up to the desired temperature level (200, 500 and 800 °C)
at a heating rate of 1 °C/min. Once the temperature was reached,
it was maintained for 60 min. The specimens were then cooled
down inside the furnace up to ambient temperature. Meanwhile,
unexposed specimens (reference specimens) were left at ambient
temperature. The polished surfaces of the concrete specimens were
then coated with a gold alloy film in the sputter coater for following SEM observations.

3. Results
3.1. Thermal tests
In Figs. 8–10 all the thermocouples in the specimen reached
300, 500 and 700 °C at 200, 250 and 300 min, respectively
The temperature inside the furnace was kept uniform throughout the whole test, meaning an adequate thermal exposure of the
specimen. Furthermore, it was also verified a good thermal exposure of the specimen over its entire height, possible to generate a
very stable temperature inside the concrete mass, close to the recommended heating rate specified in the RILEM recommendations
[48].
3.2. Compressive strength

Fig. 19. TGA and DTA curves for an RC specimen.

3.2.1. Analysis of results
For each concrete composition, the compressive strength, at
ambient temperature and 7, 14, 21, 28, 150 and 270 days, has been
assessed (Table 3). The strength class of the concrete was established by the EN 206-1 (2016) [49] (Table 4).


H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

In Table 5, it is possible to observe the values of the compressive
strength of each concrete composition in function of the testing
temperature and Fig. 11 it is the graphical representation of these
values.
The values for the 3 tests carried out of each test series are presented. They are very close to each other meaning good representativeness of the results. It is also possible to verify that at the
ambient temperature and 300 °C the compressive strength is practically the same and with the increase of temperature (500 and
700 °C) a marked decrease in the compressive strength is observed.
In Fig. 12 they have presented an example of stress-strain
curves selected from each concrete composition and temperature
series. It is observed that at 20, 300 and 500 °C the curves tend
to exhibit a semi-elastic behaviour of the material up to the breaking point while at 700 °C, more properly for the RC compositions,
3D_45 and 5D_75, the curves appear to have a semi-plastic behaviour at the moment before the material rupture.
3.2.2. Temperature influence
In Table 6 it is possible to observe the mean values of the relative compressive strength of the different concrete compositions
tested as a function of the temperature. Fig. 13 is the graphical representation of these values.
For the 300 °C test series there was a slight increase on the relative compressive strength for some concrete compositions, in
which the 3D_45, 3D_75 and 5D_45 had an increase of 2%, 8%

729

and 1%, respectively. However, for the RC and 5D_75 concrete compositions, it was observed a decrease in the relative compressive
strength of 3% and 9%, respectively.
For the 500 °C test series it was observed a decrease in the relative compressive strength for all concrete compositions. This happens because at around 400 °C the calcium hydroxide starts
dehydrating, being more steam generated, leading to a significant
reduction on the material strength. Thus, the ultimate strength
was 56%, 60%, 61%, 61% and 57% of the value at ambient temperature, respectively for the RC, 3D_45, 3D_75, 5D_45 and 5D_75 concrete compositions.
For the 700 °C test series, the values of the relative compressive
strength decreased even more presenting a value of 24%, 31%, 29%,
31% and 32%, respectively for the RC, 3D_45, 3D_75, 5D_45 and

5D_75 concrete compositions. The reason for this further decreasing on the compressive strength with the temperature is due to the
fact that at more or less 600 °C the aggregates undergo a sharp
thermal elongation (resulting in internal stresses which cause disintegration of the concrete). Due to the high gel decomposition of
the calcium silicate hydrate (C-S-H) that is the component responsible for the mechanical strength of the cement.
3.2.3. Comparison of constitutive models
A few experimental tests exists in the literature about the
mechanical behaviour of high-strength fibre concretes at high
temperatues, specialy with these 3D and 5D fibres. In this sense

Fig. 21. SEM images at ambient temperature (a) and after exposure to high temperatures: 200 °C (b); 500 °C (c) and 800 °C (d).


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

Fig. 22. SEM images of concrete with the identification of portlandite (1) and C-S-H (2), and EDS spectrum analysis of the aggregates (1) and cementitious matrix (2) at
ambient temperature.

based on the compiled data, the paper proposes a new compressive
strength-temperature relationship for high-strength fibre
concretes (HSFC). Table 7 shows the values for the relative compressive strength of a HSFC with calcareous aggregates at ambient
and high temperatures. Also, based on the results of this experimental work, a constitutive model (Eq. (1)) is presented to predict
the evolution of stressed compressive strength of the HSFC with
calcareous aggregates at high temperatures.

(

f cm ; h ¼

À300 Â 10À6 h þ 1

20

h < 300

À1:67 Â 10À3 h þ 1:4125 300

h < 700

ð1Þ

The model proposed in Eq. (1) describes the variation of the relative compressive strength of the concrete subjected at high temperatures and can be implemented in a finite element software for
the analysis of HSFC structures in fire conditions. The proposed
model will be then compared with the experimental data collected
and the existing models proposed by others researches. In Fig. 14 it
is possible to observe the proposed model and the results obtained
by other authors for the compressive strength of HSFC with calcareous aggregates at ambient and high temperatures; Abrams
(1971) (reported in [50]), Castillo (1987) [51], Khoury and Algar
(1999) (reported in [52]), Phan and Carino (2003) [52] and Kim
et al. (2009) [53].
Up to 300 °C, the proposed model presents compressive
strength values slightly higher than those of other authors, and
for this temperature range, this increase in the strength is not
problematic since the structural elements still present a great
range of resistance until their collapse. Between 300 and 700 °C,
the compressive strength values of the proposed model are slightly
lower and therefore more conservative than the results obtained
by other authors. In this way, the safety of concrete structures is
not compromised. Under the preloaded condition, the specimens

could not sustain the preload beyond 700° C, the temperature at
which the specimens collapse.

Fig. 15 compares the proposed model with the models proposed
by other authors as Phan and Carino (2003) [52], Aslani (2013) [50]
and Hertz (2005) (reported in [50]) for the stressed compressive
strength of HSFC with calcareous aggregates at high temperatures.
Fig. 16 shows the comparison between the proposed model and
the ones of Phan and Carino (2003) [52], EC 2 calcareous aggregate
concrete (2004) [12], Aslani and Bastami (2011) [54] and Xiao and
Ezeliel (2013) [55] and the experimental data by Hager (2013) [32]
for unstressed compressive strength of HSFC with calcareous
aggregate concrete at elevated temperatures.
Fig. 17 compares the proposed model with EC 2 [12], the experimental data by Kim et al. (2009) [56] and Santos et al. (2009) [57]
and with two different patterns proposed by Aslani and Samali
(2014) [3]. The proposed model presents a reasonable adjustment
for the experimental data and the models of the other authors.
3.2.4. Influence of the fibre geometry and content
The influence of the temperature on the compressive strength is
essential for verifying the impact of the geometry and content of
the steel fibres on the compressive strength at high temperatures
of the concrete (Fig. 18).
In Fig. 18 it can be observed for all tested temperatures that the
addition of steel fibers to the concrete contributed to increasing the
compressive strength. At ambient temperature, it is found that for
the dosage of 45 kg/m3 the compressive strength presented by the
compositions with 3D and 5D steel fibers are very similar to each
other. However, for the dosage of 75 kg/m3 the 5D steel fibers
showed a higher compressive strength than the 3D steel fibers concrete composition.
At 300 °C, it is seen that the addition of 45 and 75 kg/m3 of steel

fibers to the concrete causes an enhancement on the compressive
strength, as compared to the compressive strength presented by
the composition without steel fibers. As far as 3D and 5D steel
fibers are concerned, the compositions with 45 and 75 kg/m3 of


H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

3D steel fibers obtained better results of compressive strength as
compared to the respective compositions with 5D steel fibers.
At 500 °C it is found that for both the 45 and 75 kg/m3, the concrete compositions using the 5D steel fibers showed higher values
of compressive strength than the concrete compositions with 3D
steel fibers. At 700 °C, it is observed that for the 45 kg/m3 content
of steel fibers, the compressive strength is practically the same for
the 3D and 5D steel fibers concretes. However, for the 75 kg/m3
content, the 5D steel fibers have slightly higher values of compressive strength when compared to the 3D fibers concrete.
3.2.5. Specimens after compressive tests
In Table 8 are illustrated the specimens after compression tests
at different temperature levels. It is possible to observe the colour
change in the test specimens due to their exposure to different
temperature levels and to observe that the fibres allowed to keep
the two parts of the specimen together after reaching the maximum peak of compressive strength.
3.3. TGA-DTA tests
The TGA-DTA curves for the RC concrete is shown in Fig. 19. The
different thermal events can be observed. Both the weight loss

731

between the 25 and 200 °C (2.20%) and the broad endothermic
peak are due to vaporisation of the free water and decomposition

of the C-S-H [32,58].
The weight loss between the 200 and 400 °C was 1.21%. This
slight variation in weight may result from the continuous dehydration of the C–S–H [59–61]. According to several authors [62,63] the
variation in weight between 200 and 400 °C is mainly due to the
loss of water as well as the first stage of dehydration and breakdown of the C–S–H.
Between 400 and 500 °C, 0.79% weight loss was still observed.
This weight loss is associated with an endothermic peak as a result
of the decomposition of the portlandite (CaOH2) into free lime
(dihydroxylation) [60–66]. The portlandite content was 1.60%.
The endothermic peak at 573 °C can be observed and is due to
the transformation of quartz-a into quartz-b, corresponding to
the transition phase a ? b. This transformation occurs with an
expansion (microcracks on siliceous aggregates).
Between 600 and 900 °C, there is the highest endothermic peak
linked with the highest weight loss as well as two phenomena: i)
direct result of the decarbonisation of the CaCO3 (mainly from
the aggregates) and ii) formation of C2S [60]. The last endothermic
peak at 850 °C was associated with the decomposition of the dolomite. The calcium carbonate content (calcite) was calculated by the

Fig. 23. SEM images of concrete with the identification of steel fibres (1), calcareous aggregate (2) and cementitious matrix (3) and EDS spectrum analysis, after exposure to
200 °C.


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

Fig. 24. SEM images of concrete with the identification of steel fibres (1), calcareous aggregate (2) and cementitious matrix (3) and EDS spectrum analysis, after exposure to
500 °C.

TGA and is 62%. Dollimore et al. [67] stated that the portlandite’s
dissociation starts at around 780 °C, causing a high variation of
mass at about this temperature. Subsequently, the breakdown of
the CaCO3 occurs near a temperature of 900 °C. The decomposition
of these two last compounds causes a drastic deterioration of the
cementitious matrix and the aggregates.
3.4. XRD tests
The XRD patterns obtained at ambient temperature and 200,
500 and 1000 °C for the X-ray diffraction of the RC concrete are
illustrated in Fig. 20.
The results obtained in the diffractogram at ambient temperature and of the sample heated up to 200 °C allowed identifying
the presence of different crystalline phases, such as portlandite
(CaOH), calcite (C) and quartz (Q). The last phase is present in
the siliceous aggregate (sand). Although calcium silicate hydrate
(C-S-H) is a common product in the hydration of the Portland
cement, it has an amorphous structure, which is why it is not
detected in the diffractogram. Also, the phases found at 20 and

200 °C are the same, so it is possible to conclude that up to
200 °C there are no significant changes in the structure of the
concrete.
It is no longer the case for temperatures of 500 °C since only
quartz and calcite can be found. These results confirm that portlandite (CaOH2) decomposes before 500 °C, more precisely
between 400 and 450 °C, as reported by other authors [60–65].
At 1000 °C, the results obtained by X-ray diffraction revealed
the presence of different crystalline phases such as quartz, cristobalite, calcium oxide and larnite (Ca2SiO4).
Larnite was obtained from the decomposition of the C-S-H, and
the calcium oxide was a direct result of the decarbonisation of the
CaCO3 [68]. Mineral cristobalite is a high-temperature polymorph
of silica. All XRD results agree with the TGA-DTA ones.

3.5. SEM-EDS tests
These tests allowed a quick visual determination of the groups
of different chemical compositions constituting a complex
material, as well as the identification of microcracks, partial dete-


H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

733

Fig. 25. SEM images of concrete with the identification of steel fibres (1), calcareous aggregate (2), and cementitious matrix (3) and EDS spectrum analysis, after exposure to
800 °C.

rioration of the CH and C-S-H and formation of calcium oxide
crystals.
Fig. 21 presents the SEM images obtained with scanning electronic microscopy at ambient and high temperatures with x100
magnification.
In the SEM images at ambient temperature and 200 °C (Fig. 21a,
b, 22 and 23), no visible cracking could be distinguished, and the
surface of the concrete did not exhibit deterioration.
Between 500 and 800 °C, physicochemical changes in the
cement paste and aggregates occurred (Figs. 21c and 24). Sand
quartz (SiO2) experiences allotropic transformations from quartza to quartz-b accompanied by an expansion of nearly 0.8% in volume at 573 °C. The calcareous aggregates become unstable at
600 °C due to transformation of the calcium carbonate (CaCO3) into
calcium oxide (lime – CaO) and carbon dioxide (CO2), and between
600 and 900 °C, calcium carbonate (C–S–H) decomposes into calcium oxide (lime - CaO) and forms b-C2S [69].
At 800 °C (Figs. 21d and 25) the cracks’ pattern was more pronounced, and the cement matrix was strongly deteriorated.
Figs. 21–24 present some micrographs of concrete specimens at

ambient temperature (a) and after exposure to high temperatures:

b) 200 °C; c) 500 °C and d) 800 °C (See Fig. 25).
Fig. 26 presents SEM images, with x30000 magnification, of
specimens at ambient temperature (a) and after exposure to high
temperatures: b) 200 °C; c) 500 °C and d) 800 °C.
The SEM images (Fig. 26a) and b)) allowed to observe the presence of portlandite (CaOH2) and calcium silicate hydrate (C-S-H),
and to verify that between 20 and 200 °C, from a microstructural
point of view, there was not any sensible degradation of the concrete. According to different authors, portlandite and C-S-H
remained intact until 300 °C. At 300 °C, the degradation of these
solids started [70–73] in addition to the appearance of nonhydrated cement particles [73]. Between 450 and 550 °C, portlandite starts decomposing into free lime (dihydroxylation)
[11,18], and in Fig. 26 c) it is no longer possible to observe portlandite crystals. Wang [71], Kim [73] and Lim [72] pointed out that
at 500 °C, the hexagonal portlandite crystals started to deform.
Between 500 and 800 °C, portlandite and the calcium silicate
hydrate (C-S-H) were completely decomposed, producing voids
and cracks, increasing the paste’s porosity. In Fig. 26 d) it is


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H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

Fig. 26. SEM images of concrete with the identification of portlandite (1), C-S-H (2) and calcium oxide (3) at ambient temperature (a) and after exposure to high
temperatures: 200 °C (b); 500 °C (c) and 800 °C (d).

possible to see calcium oxide crystals which are a direct result of
the decarbonisation of the CaCO3.
4. Conclusions and discussion
This paper presents the results of experimental work on the
behaviour in compression at high temperatures of five high
strength fibre concrete compositions (ambient, 300, 500 and
700 °C). The main objective was to evaluate the influence of various parameters that may interfere on the compressive strength

of the concrete, such the temperature, the effect of adding steel
fibres as well as the steel fibres geometry and amount. In addition
to compression tests, it was carried out complementary tests to
analyse the temperature effect on the concrete microstructure
transformation on testing and its implication in the compressive
strength at high temperatures. The following conclusions can be
drawn from the results of this research:
- The furnace and test procedure adopted in the tests at high
temperatures allowed proper distribution and thermal exposure of the concrete specimens during the tests.
- As regards the influence of temperature on the variation of the
compressive strength, it is verified that at 300 °C the compressive strength tends to be the same as that presented at ambient
temperature with a variation of ±10%. However, at 500 and
700 °C, a reduction in the value of the compressive strength is
observed when compared to the values obtained at ambient
temperature between 39 and 44% and 68 and 76%, respectively.

- In relation to the influence that the steel fibre contents on the
compressive strength, except for the temperature of 700 °C
and for the 3D steel fibre concrete, all the results showed that
the adoption of 75 kg/m3 of steel fibres equals or increases its
compressive strength when compared with the ones with
45 kg/m3 or without steel fibres.
- In relation to the 5D steel fibre concretes and its influence on
the compressive strength, it was concluded that at 45 kg/m3
the results of the compressive strength for the concretes with
5D steel fibres are equal or smaller than the results for the ones
with 3D steel fibres, except for the 500 °C. Regarding the content of 75 kg/m3, the results showed that there is a slight advantage in the adoption of 5D steel fibres compared to the 3D steel
fibres, except for the 500 °C.
- The proposed model for preloaded HSFC with calcareous aggregates at ambient and high temperatures fit well with the experimental results obtained and also show close agreement with
other existing models for HSFC.

- The proposed model has two crucial temperature zones which
reflect the practical design needs and main concrete behaviour
changes in fire, and it becomes simple for both manual design
calculations and finite element computational analysis.
- The proposed model can be implemented in a finite element
software to perform the analysis of structural elements subjected to high temperatures.
- Between 20 and 200 °C, it is not possible to microscopically
identify any significant change in the concrete structure since
the crystalline phases identified in the XRD tests are the same.


H. Caetano et al. / Construction and Building Materials 199 (2019) 717–736

However, there is a small loss of mass of the concrete, found in
the TGA-DTA tests due to the evaporation of water and the existence of an endothermic reaction linked to the decomposition of
the calcium silicate hydrate (C-S-H).
- Between 200 and 400 °C, the dehydration process of the calcium silicate hydrate (C-S-H) continued gradually and the loss
of mass decreased slightly, however, except for a significant
increase in cracking, there is no endothermic reaction in the
thermogravimetry graph, which means that significant
microstructural changes in the concrete’s morphology are not
identified.
- For temperatures between 400 and 700 °C, more precisely
between 420 and 500 °C, an endothermic peak is observed in
the TGA-DTA tests, accompanied by a reduction in mass, which
results in the dihydroxylation of the portlandite (CaCO3) leading
to calcium oxide. The absence of the crystalline phase of the
portlandite in the XRD tests and its absence in the SEM observations confirm the occurrence of this transformation.
- Between 700 and 900 °C, although the loss in mass increases
with the temperature, it is at 700 °C that the greatest loss in

mass occurs and in a more abrupt way. This phenomenon is
essential for the decarbonisation of the limestone aggregates,
generating more calcium oxide. Also, it is in this temperature
range that another endothermic reaction takes place, and new
crystalline phases originate as in the case of the cristobalite
and larnite. Since the decarbonisation of the limestone aggregates occurs, the microstructural strength of the concrete is
seriously compromised, and the main consequence of this
transformation is the inability of the concrete to provide
mechanical resistance to the pressure that is exerted on it.
Conflict of interest
None.
Acknowledgements
The authors gratefully acknowledge to SECIL S.A. (www.secil.
pt), BEKAERT (www.bekaert.com), SIKA (pt.sika.com) for their support in this investigation and Brazilian National Council for Scientific and Technological Development – CNPq ( for
the post-doc scholarship given to the second author.
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