Tải bản đầy đủ

High performance concrete under elevated temperatures

Construction and Building Materials 44 (2013) 317–328

Contents lists available at SciVerse ScienceDirect

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

High performance concrete under elevated temperatures
Abdullah Huzeyfe Akca, Nilüfer Özyurt Zihniog˘lu ⇑
_
Department of Civil Engineering, Bog˘aziçi University, Istanbul,
Turkey

h i g h l i g h t s
 Performance of HPCs under elevated temperatures.
 Use of PP fibers with air entraining admixture to decrease damage.
 Decreased spalling and increased residual strength.
 Complete disintegration of dense matrix under very high temperatures.
 Microstructural examination of cement paste–aggregate interface.

a r t i c l e


i n f o

Article history:
Received 4 February 2013
Received in revised form 28 February 2013
Accepted 2 March 2013
Available online 10 April 2013
Keywords:
High performance concrete
Elevated temperatures
Polypropylene fibers
Air entraining admixture
ESEM

a b s t r a c t
In this study, PP fibers and air entraining admixture (AEA) were used together in an high performance
concrete (HPC) mix so as to create interconnected reservoirs in concrete and to improve fire performance
of HPC. For this reason, nine mixes of HPC incorporating blast furnace slag with 0.24 water-to-binder
ratio and various PP and AEA contents were produced. Specimens were cast in two different sizes in order
to see the effect of size and 18 series of specimens were obtained. These series subjected to elevated temperatures (300 °C, 600 °C and 900 °C) with a heating rate of 10 °C/min and after air cooling, residual mass
and compressive strength of specimens were determined. The heated specimens were observed both at
macro and micro scales to investigate the color changes, cracking and spalling of HPC at various temperatures. Also, thermogravimetric analyses were performed on powder samples from each nine mixes.
Results showed that addition of AEA diminished the decrease in residual strength but this result was
found to be irregular after 300 °C for thick specimens. The collaboration of AEA and PP fibers decreased
the risk of spalling of HPC. Also, size of specimen was found to be important in deterioration of HPC.
Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction
Concrete with increased strength and durability has been primarily used in special constructions such as high rise buildings,
infrastructures and nuclear power plants since it became commercially available [1]. Thenceforth, some of these HPC structures exposed to severe fire conditions have exhibited poor performance.
The main reason of this insufficiency of HPC at high temperatures
is a result of the changes made in the composition of concrete
mixes. Decrease in water to cementitious ratio, use of supplementary cementitious materials and plasticizers lead to impressive
improvements such as strength, rheology of fresh concrete, impermeability and compactness. On the other hand, in most cases these
changes may lead to a decrease in fire performance of HPC [2].
_ ßaat Mühendislig˘i
⇑ Corresponding author. Address: Bog˘aziçi Üniversitesi, Ins
_
Bölümü, 34342 Bebek, Istanbul,


Turkey. Tel.: +90 212 359 70 39, Mobile: +90 533
690 22 44; fax: +90 212 287 24 57.
E-mail addresses: abdullah.akca@boun.edu.tr (A.H. Akca), Nilufer.ozyurt@boun.
edu.tr (N. Özyurt Zihniog˘lu).
0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.conbuildmat.2013.03.005

Lower water to cementitious materials ratio leads to lower
porosity and this decreases permeability of concrete. With the increase in temperature, water in the pores of concrete evaporates
and consequently pressure within the cement paste increases. Reduced permeability of HPC limits the diffusion of water vapor from
the concrete pores and therefore pore pressure continues to increase until the internal stresses reach the tensile strength of concrete and eventually causes spalling [3].
Free water and moisture gradients influence the behavior of
concrete at elevated temperatures and according to Hertz, they
must be regarded as main reasons of spalling [4]. Meyer-Ottens
treats that tensile stresses caused by steam in the closed pores of
normal concrete can reach the tensile strength of concrete with
more than 3% moisture by weight [5]. Hertz concluded traditional
concrete with less than 3% moisture by weight will not spall and in
the range of 3–4% moisture by weight has a risk of spalling, on the
other hand, concrete with a dense microstructure (most of the
HPC) may spall even when the moisture content is zero [4]. Due
to the increased impermeability, only the crystal water arisen from


A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

318
1000
900
800
700
600
500
400
300
200
100
0
0

60

120

180

240

300

360

420

480

Time (Min)
Fig. 1. Heating cycles.

dehydration of hydrates at high temperatures may cause the spalling of concrete.

Recently, many studies have focused on the contribution of different materials such as fibers and mineral admixtures to improve
fire resistance of HPC [2,6,7]. Addition of PP fibers into HPC was
found as an efficient way to avoid spalling of concrete. Because,
PP fibers melt in concrete above 170 °C and leave micro channels
in concrete and these channels form a network more permeable
than cement matrix which contributes to outward migration of
gases and water vapor and result in the reduction of pore pressure
[7–10]. As a mineral admixture, inclusion of silica fume caused
reduction in residual strength and spalling of concrete by densifiying microstructure [11,12]. On the contrary, addition of fly ash or
slag showed better performance and also in some studies, strength
gain observed at temperatures ranged from 200 °C to 300 °C because of tobermorite formation [13].
Furthermore, rapid heating of concrete is another factor which
causes a high temperature difference between the deeper zone
and the surface of a specimen and therefore explosive spalling
may occur during heating [14]. Anderberg stated that during his

Table 1
Properties of cement, slag and polypropylene fibers.
Cement and slag

Polypropylene fibers

Chemical composition

Cement (%)

Slag (%)

SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2O
K2O
Loss on ignition
Specific gravity (g/cm3)
Specific surface (cm2/g)

20.17
4.91
3.41
64.28
1.18
2.84
0.13
0.96
1.61
3.14
3910

38.37
11.89
1.05
37.25
8.13
0.38
0.28
1.28
0
2.93
4320

Length (mm)
Diameter (lm)
Specific gravity (g/cm3)
Specific surface (cm2/g)
Fiber number (fibers/kg)
Tensile strength (MPa)
Modulus of elasticity (GPa)
Melting point (°C)

12
32
0.91
1340
110 Million
250
3.5
165

Table 2
Mix proportions.

a
b
c

Series

W/B

Cement (kg/m3)

Slag (kg/m3)

Water (kg/m3)

Sand (kg/m3)

SPa (kg/m3)

PP (dm3/m3)

AEAb (g/BA)c

F0A0
F0A0.5
F0A1
F8A0
F8A0.5
F8A1
F16A0
F16A0.5
F16A1

0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24

515
515
515
515
515
515
515
515
515

435
435
435
435
435
435
435
435
435

227
227
227
227
227
227
227
227
227

1175
1175
1175
1175
1175
1175
1175
1175
1175

15
15
15
15
15
15
15
15
15

0
0
0
8
8
8
16
16
16

0
5
10
0
5
10
0
5
10

SP stands for superplasticizer.
AEA stands for air entraining admixture.
BA stands for binder amount of 1 m3 concrete mixture in kilogram.

Table 3
The number of specimens spalled at different temperatures (out of 3 for each series).
Series

Maximum temperature
300 °C

T5F0A0
T5F0A0.5
T5F0A1
T5F8A0
T5F8A0.5
T5F8A1
T5F16A0
T5F16A0.5
T5F16A1
a

0
0
0
0
0
0
0
0
0

600 °C
3
3
3
2
0
0
0
0
0

Series
900 °C
a

N.E.
N.E.
N.E.
2
0
0
0
0
0

T10F0A0
T10F0A0.5
T10F0A1
T10F8A0
T10F8A0.5
T10F8A1
T10F16A0
T10F16A0.5
T10F16A1

N.E. – these specimens were not exposed to 900 °C since equivalent specimens spalled at 600 °C.

Maximum temperature
300 °C

600 °C

900 °C

0
0
0
0
0
0
0
0
0

3
3
3
2
0
0
0
0
0

N.E.
N.E.
N.E.
1
0
0
0
0
0


A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

Fig. 2. (a) A PP fiber passing through an air void, (b) a micro-channel formed after
heating (a part of a melted PP fiber reaches to an entrained air void creating a
micro-channel).

319

experiments only rapid fires have given rise to spalling [15]. Related to this, size and shape of concrete can be considered as factors which directly affect the heating rate of concrete. When a
structural member subjected to high temperatures, the heat level
of thin parts increases rapidly and this may cause spalling due to
rapid heating [4].
On the other hand, although they are widely used especially in
HPC mixes, there are a limited number of studies on the effect of
chemical admixtures on concrete under fire conditions. For
example, as a chemical admixture, air entraining admixtures
(AEAs) are used for producing air bubbles in concrete to improve
resistance of concrete to damage caused by freezing and thawing
situations. Moreover, the entrained air enhances the workability
and may reduce bleeding and segregation of concrete mixtures at
fresh state and with the increase in air voids in the concrete thermal conductivity of the concrete decreases at the hardened state
[16]. As Riley stated the surface of concrete with low thermal conductivity which acted as a refractory material would effectively
produce an insulation layer to inner parts of concrete [17]. In a
study conducted by Seçer, AEA was used to entrain air into concrete in the ratios of 4%, 6% and 8% in volume and these concretes
subjected to temperatures of 300 °C, 500 °C and 700 °C. Results of
the study showed that as the air content of concrete increased,
reduction in the strengths of concretes subjected to high temperatures diminished.
In this study, PP fibers and AEA were used together in HPC so as
to form a more permeable network consisted of micro channels of
melted PP fibers and entrained air voids in concrete at elevated
temperatures. Thus, it was aimed to permit the evacuation of gases
and water vapor appeared due to heating. To the authors’ knowledge this is the first study to discuss the combined effects of polypropylene fibers and air entraining admixture on the properties of
high performance concrete under elevated temperatures. Considering the moisture and size factors which influence the effect of
temperature, moisture contents of HPC specimens were adjusted

Fig. 3. Outer surfaces of the specimens of T5F16A1 series. (a) specimen kept at room temperature (20 °C), specimens heated to (b) 300 °C, (c) 600 °C, (d) 900 °C (2X
magnification).


320

A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328
content becomes more significant since HPC is much denser than normal concrete.
In this study, moisture contents of HPC specimens were aimed to adjust approximately to 3% by weight to simulate the most negative condition. Therefore, moisture contents of one specimen from each series were determined in accordance
with BS1353 before exposure to elevated temperatures [20]. All the specimens were
found to have moisture content in the range of 2.7–4.4% which can be considered as
in the critical region.
2.2. Heating procedure
An electrical furnace that was capable to operate up to 1250 °C was used. After
the curing period, moisture content of specimens reached the desired value and
specimens of each series were exposed to 300 °C, 600 °C and 900 °C temperatures
for an hour in the furnace. The heating rate was set to 10 °C/min which can be considered the same as the average heating rate of standard fire curve (ISO-834) for the
first 90 min. It should be emphasized that this heating rate is detrimental because
of the thermal gradients between the outer part and the inner core of the specimens
which cause to additional internal stresses in concrete and initiates spalling [4]. At
the end of the set exposure time, the hot concrete specimens were not taken out
until the furnace cooled down to 100 °C with a cooling rate of 3 °C/min. Fig. 1 represents the heating cycles.
2.3. Test procedures
Mass measurements and compressive strength tests were performed on both
unheated and heated concrete specimens. Also, specimens were observed at both
macro and micro scales.
2.3.1. Macroscopic and microscopic observation
Assessment of fire-damaged concrete begins with visual observation of color
change, cracking and spalling of concrete. Changes on visual appearance of concrete
give information about the temperature which concrete has been exposed. In the
scope of this study, occurrence of spalling, crack patterns and color changes were
examined at macro scale. Additionally, changes on surface and interior part of the
heated specimens were observed microscopically.

Fig. 4. (a) Popouts (8X magnification) on sand particles heated to 900 °C (b)
aggregate cracking (6X magnification) on the specimen (T5F16A1) heated to 900 °C.

approximately to 3% by weight to simulate the most negative condition before heating and concrete specimens with two different
sizes were prepared to see the effect of size at high temperatures.
2. Experimental study
2.1. Materials and mix design
CEM I Type 42.5R Portland cement, ground granulated blast furnace slag (GGBS)
from Karabük, river sand, multifilament polypropylene (PP) fiber, a high-range
water reducing admixture based on chains of modified polycarboxylate ether and
an air entraining admixture based on oil alcohol and ammonium salt were used
in the production of concrete. It should be emphasized that no coarse aggregate
was used in the mixture. The maximum size of the sand used was 1 mm. The properties of cement, slag and PP fiber are presented in Table 1.
Nine mixes of HPC with 0.24 water-to-binder ratio and various PP and AEA contents were produced. Mix proportions are shown in Table 2. F0A0 is control group and
represents concrete with no PP fiber and AEA. F8, F16 specimens contain PP fiber at
8‰ and 16‰ of the volume of the concrete amount, respectively. A0.5 and A1 specimens have AEA to binder ratio of 0.5‰ and 1‰ respectively. Mixes were cast into
10 Â 10 Â 50 cm prisms and the specimens were kept in laboratory environment
for 24 h. After demoulding, the specimens were labeled and then they were cured
in a water tank at 20 °C for 10 days. After 10 days of curing period 10 Â 10 Â 50 cm
specimens were cut with a diamond blade in order to obtain 10 Â 10 Â 10 cm cubes
(represented by T10) and 10 Â 10 Â 5 cm prisms (represented by T5) and then these
specimens were kept in laboratory environment for 3 months.
At elevated temperatures, it is known that the extent of damage increases with
an increase in the moisture content and 3–4% moisture by weight was found to be
critical for normal concrete by various researchers [4,18,19]. For HPC moisture

2.3.2. Measuring mass loss
Mass of each specimen was measured before heating test and measured again
after the heated specimens cooled down to room temperature. Moreover, thermogravitmetric analyses were performed on powder samples obtained from each concrete type by using a TA Instruments Q50 thermal analyzer. The thermal analyzer
heated the sample to 863 °C (which is the maximum heating capacity of the used
testing machine) with a constant rate of 10 °C/min and simultaneously measured
the mass of the sample. Finally, thermogravimetric (TG) and differential thermogravimetric (DTG) curves of samples were drawn.
2.3.3. Residual strength measurement
After exposure to high temperatures, three specimens of each series were subjected to compression test in accordance with BS 12390 to measure the residual
compressive strength [21]. The specimens with dimensions of 10 Â 10 Â 10 cm
were tested as they were. On the other hand, all the 10 Â 10 Â 5 cm specimens were
cut using a diamond blade in order to obtain 5 Â 5 Â 5 cm cube specimens for compression test. These dimensions were considered representative of the material
since concrete produced in this study contained only fine aggregates with a maximum size of 1 mm and entrained air voids whose maximum and average diameter
were 500 lm and 100 lm, respectively. The fiber length (12 mm) was also smaller
than 1/3 of the smallest dimension of the 5 Â 5 Â 5 cm specimen. This test method
consisted of applying a compressive axial load to cube specimens at a constant rate
of 0.2 MPa/s until failure occurred.

3. Results and discussion
The experimental test results obtained from compression tests
and mass loss measurements and visual observations are discussed
in this section. Summary of the results are given in the form of tables and figures.
3.1. Spalling
Explosive spalling of some specimens were observed when the
specimens exposed to 600 °C and spalling began at approximately
500 °C (This statement is done based on the sound of explosive
spalling) [22]. Partial spalling such as corner spalling and surface
layer delamination was not observed in this study. Table 3 shows
IDs and number of specimens destroyed due to explosive spalling.


A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

321

Fig. 5. Inner surfaces of specimens of T5F16A1 series (a) the specimen kept at room temperature (20 °C), specimens heated to (b) 300 °C, (c) 600 °C, (d) 900 °C

As is seen in Table 3 none of the specimens spalled at 300 °C, all the
non-fibrous specimens were exploded at 600 °C and therefore
specimens from these series were not exposed to 900 °C.
As a similar result to the findings of Han, no explosive spalling
was observed when PP fibers were used except some specimens
with IDs T5F8A0 and T10F8A0 [23]. Explosive spalling was observed in these fibrous specimens and this phenomenon can be explained by dense microstructure of HPC. According to Peng (based
on experiments done using HPC with a compressive strength of
80 MPa), regardless of the interconnected channel system formed
by PP fiber melted above 170 °C, concrete at 0.24 water-to-binder
ratio was so dense that it could still keep the water pressure high
enough to result in explosive spalling [14]. It should be noted that
while the specimens with 8‰ fibers and no air entrainment
(T5F8A0 and T10F8A0) exploded, the air entrained specimens of
the same series (T5F8A0.5, T10F8A0.5, T5F8A1 and T10F8A1) were
not exploded as can be seen in Table 3. This result could be attributed to the effect of air entrainment.
It is hypothesized that, the contribution of air entrainment to
resist against explosive spalling began with PP fiber addition. Entrapped and/or entrained air voids in concrete are almost closed
and as Hertz stated; if water vapor cannot escape from these closed
pores it causes increase in pore pressure and increases the risk of
spalling [4]. In the air entrained PP fiber reinforced concrete, most
probably micro-channels were formed due to melting of PP fibers
at above 170 °C and some of the closed pores connected to each
other by these micro-channels. On the other hand, in non-fibrous
air entrained concrete, absence of fibers limited the ability of water
vapor to escape from the entrained air voids in HPC and thus spalling occurred.
To examine this effect, microstructures of concrete specimens
were examined by using an environmental scanning electron microscope (Philips XL30 ESEM-FEG/EDAX) and formed

micro-channels were observed. In Fig. 2a, PP fiber passes through
an air void and in Fig. 2b, a melted PP fiber creates a micro-channel
by reaching an entrained air void. Consequently, this result shows
that the existence of both PP fibers and entrained air voids in HPC
may reduce the risk of explosive spalling.
3.2. Color and cracking observation on the outer surfaces of the
specimens
In all cases red discoloration was observed at 300 °C, gray discoloration was observed at 600 °C on the outer surface of the specimens and at 900 °C the surface colors of the specimens were
changed to whitish gray. Ingham stated that red color change is a
result of hydrated iron oxides present mostly in siliceous aggregates, pink to red discoloration is very important and has a structural significance because it means that temperature around
300 °C where the reduction in concrete strength mostly began
was attained [24].
The deterioration of a structural member exposed to 300 °C can
be repairable. On the other hand, whitening of a structural member
indicates that temperature has exceeded 600 °C and it corresponds
to a serious loss in compressive strength. After this amount of
strength loss, concrete cannot be repairable anymore and it cannot
withstand service loads.
Outer surfaces of the specimens were examined visually and by
using a stereomicroscope (Nikon SMZ1500) (Fig. 3). Almost no
cracks were observed on the outer surfaces of the specimens that
were heated until 300 °C. On the other hand, cracks with an opening going up to 0.2 mm were observed on the surfaces of the specimens heated to 600 °C. Surface cracks formed on the specimens
exposed to 900 °C were larger and the crack widths were in the
range of 0.3–0.4 mm. Moreover, siliceous sand particles were separately heated to 900 °C and cracks and popouts on the surfaces


322

A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

Fig. 6. Decomposition of a specimen heated to 900 °C.

were observed due to the volume change of sand particles (Fig. 4a).
The effect of volume change of sand particles can also be seen in
Fig. 4b.

3.3. Color and cracking observation on the inner surfaces of the
specimens
On the other hand, color of the inner surface of specimens is less
influenced than that of outer surface at the same temperature level. Color of cement paste in inner surfaces is prominently darker
than in outer surfaces at 300 °C and 600 °C, Fig. 5b and c. Moreover,
reddened fine aggregates in the inner surface are lighter than outer
surface at these temperatures. This could be due to the fact that the
exact same temperature may not be reached in the inner sections
of concrete.
Furthermore, no cracking was observed on the inner surfaces of
the specimens which were heated up to 600 °C. Inner cracks both
in aggregates and cement paste were only visible in the specimens
which were heated to 900 °C as seen in the Fig. 5d. Moreover,
widths of the cracks in the inner zone of the concrete were smaller
than that of the outer zone and were around 0.04 mm.
The specimens heated to 900 °C decomposed and decomposition did not take place immediately. One or two large cracks
(0.3–0.4 mm) were observed in the first day (following removal
of the specimens from the furnace, Fig. 6a), then, these cracks
turned into spider web-like cracks in the 2nd day (Fig. 6b) and finally complete disintegration of specimens were observed on the

Fig. 7. Average mass losses of specimens.

3rd day (Fig. 6c). In literature this phenomenon is explained by
the decomposition of hydrates which starts at 400 °C and almost
ends at 900 °C. Therefore, most of the dehydration takes place
and concrete loses almost all of its initial strength and stability
at 900 °C due to loss of all chemical water [25]. Culfik and Ozturan


A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

TGA

323

Instrument: TGA Q50 V6.7 Build 203
0.05

100
0.04
Deriv. Weight (%/°C)

98
Weight (%)
96

0.03

94

0.02

92
0.01
90
88

0

200

400
600
Temperature (°C)

0.00
1000

800

Universal V4.7A TA Instruments

Fig. 8. TGA and DTG curves (F0A0).

Table 4
Average initial and residual compressive strength values of T5 series (the results given are the average compressive strength values calculated by using the data obtained from 3
specimens). The residual strength values given for the specimens exposed to 900 °C should be carefully evaluated based on the explanation given in Section 3.5
Mixes

T5F0A0
T5F0A0.5
T5F0A1
T5F8A0
T5F8A0.5
T5F8A1
T5F16A0
T5F16A0.5
T5F16A1
a
b

Control strength

Residual strength

20 °C

300 °C

(MPa)

(MPa)

(%)

(MPa)

(%)

(MPa)

(%)

121.10
93.60
91.37
114.70
86.23
92.63
109.47
92.13
81.03

101.40
112.07
104.99
71.27
75.28
78.33
69.03
71.18
72.04

83.7
119.7
114.9
62.1
87.3
84.6
63.1
77.3
88.9

E.S.a
E.S.
E.S.
57.41
48.53
55.69
52.81
48.22
46.97




50.1
56.3
60.1
48.2
52.3
58.0

E.S.
E.S.
E.S.
C.D.b
C.D.
18.05
C.D.
18.59
C.D.






19.5

20.2


600 °C

900 °C

ES stands for explosive spalling.
CD stands for complete disintegration.

Table 5
Average initial and residual compressive strength values of T10 series (the results given are the average compressive strength values calculated by using the data obtained from 3
specimens). The residual strength values given for the specimens exposed to 900 °C should be carefully evaluated based on the explanation given in Section 3.5.
Mixes

T10F0A0
T10F0A0.5
T10F0A1
T10F8A0
T10F8A0.5
T10F8A1
T10F16A0
T10F16A0.5
T10F16A1
a

Control strength

Residual strength

20 °C

300 °C

(MPa)

(MPa)

(%)

(MPa)

(%)

(MPa)

(%)

133.97
98.33
80.76
125.17
97.70
95.74
124.27
99.65
94.34

121.57
94.54
77.72
110.33
85.51
91.16
120.13
78.81
80.58

90.7
96.1
96.2
88.1
87.5
95.2
96.7
79.1
85.4

E.S.a
E.S.
E.S.
85.09
55.29
59.55
75.71
58.76
51.89




68.0
56.6
62.2
60.9
59.0
55.0

E.S.
E.S.
E.S.
42.08
22.61
24.81
37.49
21.27
23.48




33.6
23.1
25.9
30.2
21.3
24.9

600 °C

900 °C

ES stands for explosive spalling.

exposed their specimens to 900 °C. They also reported hair-like
cracks in first day and then complete disintegration of concrete
specimens 1 day after cooling period [26]. Only six specimens of
two series (T5F8A1 and T5F16A0.5) did not decompose. These
specimens were exposed to water during the cutting process and
most probably were healed when came contact with water
[27–29]. This recovery can be attributed to regeneration of
some of C–S–H bonds on rehydration [28,29]. Detailed information
about disintegration of these specimens will be given in
Section 3.5.

3.4. Mass losses
After exposure to high temperatures mass losses of the specimens, resulting mainly from water and carbon dioxide transport
and loss, were recorded. Residual masses of each series can be seen
in Fig. 7 except the spalled series. Average mass losses of specimens exposed to 300 °C, 600 °C and 900 °C were 5.2%, 9.8% and
12.9%, respectively.
Results of thermogravimetric analyses were similar. Average
mass losses of powder samples at 300 °C, 600 °C and 863 °C were


324

A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

Fig. 9. Residual strengths of the specimens with increasing air entrainment.

Fig. 11. Residual strengths of the specimens with increasing specimen size.

est and appears between approximately 550 °C and 700 °C and
corresponds to the loss of carbon dioxide ensuing from the carbonation products and the loss of water ensuing from the decomposition
of calcium silicate hydrates (C–S–H) [24,25,30–32]. During the last
peak mass of the samples reduced approximately 4%. This explains
the serious loss of strength after 600 °C. TGA and DTG curves obtained for the other samples were not published here since they
were very similar to the one given below for F0A0 sample.

3.5. Residual strength

Fig. 10. Residual strengths of the specimens with increasing PP fiber content.

4.6%, 7.9% and 11.6%, respectively. These results were smaller than
the residual mass values found by weighing the full-size specimens. This is expected since the full-size specimens included
closed pores which may entrap extra moisture when compared
to powder samples. Moreover, the specimens heated in the furnace
were held at their maximum temperatures for an hour before they
left for cooling, while the powder samples exposed to TGA test
were cooled down immediately.
Fig. 8 represents the TGA and Differential Thermo Gravimetry
(DTG) curves. As shown in DTG curve, there are three peaks on the
graph and these peaks represent instantaneous mass losses with
temperature. The first peak is wider than others and starts with
the beginning of the heating (approximately 20 °C) and continues
until the complete evaporation of moisture of powder samples
(approximately 150 °C). Average mass loss of the samples is 3.6%
around this region. The second peak appears between approximately 400 °C and 450 °C and corresponds to the loss of water from
portlandite [24,25,30–33]. The instantaneous mass loss of the samples during second peak was 0.7%. Finally, the third peak is the high-

Compressive strength tests were conducted on three specimens
of the control series before beginning heating cycles. Compressive
strength values ranged from 80 MPa to 130 MPa at ambient temperature for different mixes.
After exposure to high temperatures, the residual compressive
strengths of specimens were measured. Residual strength measurements could not be conducted for non-fibrous specimens after
exposure to 600 °C, since all the specimens were ruined due to
explosive spalling. The overall results of the residual compression
test show that the concrete specimens exposed to lower temperatures are stronger than the specimens exposed to higher temperatures as expected. Tables 4 and 5 show the original and residual
strength of all series.
An important note should be given here. Tables 4 and 5 show
some residual strength values for the specimens heated up to
900 °C. However, these values may be misleading since all of these
specimens were found to disintegrate couple of days after testing.
The testing program was prepared such that specimens of T10 series were tested before T5 series. All the specimens of T10 series
were tested 1 day after heating. The specimens showed some
residual strength as is seen in Table 5. Unfortunately, after couple
of days the tested specimens were found completely disintegrated.
Having that in mind, it was decided to test 2 series of the 5 cm
thick specimens immediately, while keeping the rest of them in
the laboratory environment for 3 days to check if the same phenomenon will occur. The specimens that were spared to be tested
immediately were first cut to obtain 5 Â 5 Â 5 cm cubes and then
tested. The specimens of these 2 series (T5F8A1 and T5F16A0.5)
showed some residual strength as can be seen in Table 4, while
the specimens kept in the laboratory environment were
completely disintegrated in 3 days. The specimens that were not


A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

(a)

325

(b)

(c)
Fig. 12. ESEM pictures of the specimens kept at room temperature (20 °C). (a–b) matrix aggregate interface, (c) hydration products.

disintegrated during and after the test were exposed to water during the cutting process and most probably were healed when came
contact with water [27–29] as explained before in the end of Section 3.3. This recovery can be attributed to regeneration of some of
C–S–H bonds on rehydration [28,29]. The parts of these specimens
(T5F8A1 and T5F16A0.5) further observed for couple of months
after the test and no disintegration was observed. This is an important information and may be the subject of another project.
Table 4 shows that residual strength increases when air entraining admixture was used for all the specimens with a thickness of
5 cm when the specimens were heated to 300 °C and 600 °C,
respectively. A similar comment cannot be made for the specimens
heated to 900 °C since most of them were completely disintegrated. This result is also valid for some of the 10 cm thick specimens. Residual compressive strengths (%) of specimens at various
temperatures with different air entrainment are shown in Fig. 9.
According to the results, air entrainment affects the residual compressive strength of concrete. However, air entrained non-fibrous
specimens exploded above 600 °C. With the absence of microchannels formed by melted PP fibers, this situation can be explained by the increased pore pressure in the closed air voids of
concrete. Air entrained specimens lost strength less than others
(except T10F16 series) at 300 °C for all specimens and residual
strengths percentages of T5 series increased with air entrainment
at 600 °C. However, the positive effect of air entrainment was not
clear on the specimens of T10 series after 300 °C. Also, there is
not a prominent difference between the residual strength percentages of A0.5 and A1 series.
All the fibrous specimens withstood heating cycles and they
were subjected to compressive strength test. Residual compressive
strengths (%) of specimens at various temperatures with different
PP fiber ratios are shown in Fig. 10. Although PP fibers prevented
spalling of specimens, their existence in material adversely affected the residual compressive strength of HPC. Adding more PP

fiber into HPC mixes had negative effect on the residual strength
of the specimens and these results confirm the findings given in literature [10,34].
Residual compressive strengths (%) of specimens at various temperatures with different specimen sizes are shown in Fig. 11. Size of
the specimens affected the compressive strength of HPC. The specimens with 10 cm height showed better performance at elevated
temperatures and this effect is clearer when temperature was increased up to 600 °C (The specimens heated up to 900 °C were assumed to represent no residual strength since they were
disintegrated 3 days after the test). Specimens with 5 cm height retained 77% and 54% and 20% of their ambient strength at 300 °C,
600 °C and 900 °C, respectively. On the other hand, specimens with
10 cm height retained 89% and 60% of their original strength for
300 °C, 600 °C. The residual strength results measured are in agreement with the values given in Eurocode 2 for concrete with siliceous aggregates which retains 85%, 45% and 8% of its initial
strength at 300 °C, 600 °C and 900 °C, respectively [35]. The difference in residual strengths between the specimens with 5 cm height
and the specimens with 10 cm heights can be a result of rapid heating of specimens with smaller size as mentioned by Hertz [4].
3.6. ESEM observations
Selected specimens were examined by using an environmental
scanning electron microscope for better evaluating the effects of
high temperatures on the microstructures of the specimens. The
regions which are considered as matrix–aggregate interface were
especially chosen to be examined since hydration products are
supposed to develop in these regions [36]. Energy dispersive Xray spectroscopy (EDX) analyses were carried out to detect CSH
phases. Phase regions with a Ca/Si ratio between 0.8 and 2.1 are
considered to be CSH regions [36,37]. The regions examined in
the scope of this study had a Ca/Si ratio around 1, 7. This result


326

A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

is similar to the average Ca/Si atomic ratio reported by Djaknoun
[36]. Fig. 12 represents ESEM pictures taken on a specimen kept
at room temperature (20 °C). Fig. 12a and b shows aggregate–matrix interface. Aggregate–matrix interface of the specimens kept at
the room temperature represents a continuous structure with no
pores and cracks. Cement hydration products have well defined
crystal structure, with portlandite and CSH (Fig. 12c).
Fig. 13a and b represent ESEM pictures of a specimen heated to
300 °C. Fig. 13a shows matrix aggregate interface. Fig. 13b shows a
magnified view of the cement matrix given in Fig. 13a. Cement
paste still has crystal structure characteristics, however is not as
distinctive as it was for unheated specimen.
Fig. 14 shows pictures taken on the specimens heated up to
600 °C. Fig. 14a again shows aggregate–matrix interface. Structure
of the hydration products is amorphous. Fig. 14b shows the cracks
on the surface. As can be seen on the figure, cracks are spread all
over the surface of the specimen due to the important amount of
water loss from the structure.
Fig. 15 shows pictures of a specimen heated up until 900 °C. As
mentioned before, almost all of these specimens disintegrated couple of days after being removed from the electrical furnace (the
specimen shown below is one of the specimens which was not disintegrated). Fig. 15a shows porous cement–aggregate interface.
Fig. 15b represent, the cement structure which has amorphous
structure, Fig. 15c shows a cracked sand particle and finally
Fig. 15d shows the cracks spread all over the specimen.

(a)

(b)
(a)

Fig. 14. ESEM pictures of the specimens heated to 600 °C. (a) matrix aggregate
interface, (b) distributed cracks on the surface of the specimen.

ESEM pictures show the effect of high temperatures on the
microstructure of HPC. Concrete microstructure is highly damaged
with an increased temperature leading to failure of the specimens.
4. Conclusions
Behavior of HPC under high temperatures is different than normal concrete due to very dense microstructure. Precautions should
be taken to decrease the damage occur when HPC exposed to high
temperature. In this study air entraining admixture was used together with polypropylene fibers to create channels for evacuating
water vapor. Following conclusions were drawn as the result of
this study.

(b)
Fig. 13. ESEM pictures of the specimens heated to 300 °C. (a) matrix aggregate interface,
(b) cement paste.

 Spalling of HPC seems to be dependent on presence of PP fiber
in concrete. Explosive spalling was observed especially in nonfibrous specimens and began after 500 °C. For other HPC


A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

(a)

(b)

(c)

(d)

327

Fig. 15. ESEM pictures of the specimens heated to 900 °C. (a–b) matrix aggregate interface, (c) cracks on an aggregate, (d) distributed cracks on the surface of the specimen.













(almost all fibrous specimens) spalling was not observed. Only
two specimens with 8‰ fiber addition and no air entrainment
(T5F8A0 and T10F8A0) were exploded, and the air entrained
specimens of the same series were not exploded. This shows
that air entrainment is effective in reducing the risk of spalling.
PP fibers have a remarkable effect on the risk of spalling as mentioned above and with air entrainment this positive effect can
be improved. However, the strength loss of HPC increased at
elevated temperatures with the addition of PP fibers.
Addition of AEA increased noticeably the residual strength of
HPC when small section specimens were used. However, this
positive effect was found irregular after 300 °C when 10 cm
thick specimens were under consideration.
Different percentages of air entraining admixture (0.5‰ and
1‰) were used for this study to see the effect of dosage on
residual strength. The positive effect created by air entraining
admixture was found to become irregular when the dosage
was increased.
Change in specimen dimensions also influenced the residual
compressive strength. As the size of the specimens decreased,
the loss in strength increased at high temperatures, probably
due to the fact that the small size specimens experienced high
temperatures at a greater rate.
The density of cracks decreased from high temperature exposed
surface to inside of the specimens, probably due to the fact that
relatively lower temperatures were experienced inside the
specimens when compared to outer surfaces.
The specimens heated to 900 °C were completely disintegrated
after 3 days. Only the specimens exposed to water one day after
heating to 900 °C were not disintegrated.

Based on the results of this study it can be said that using PP fibers and air entraining admixture together to decrease the risk of
spalling may be an effective method. Further research is needed
to comprehensively evaluate the subject.

Acknowledgements
The authors gratefully acknowledge the financial support of
Bog˘aziçi University Research Fund (Project Code 11A04P2). The
support of AKÇANSA Cement and BASF-YKS Construction Chemicals is also acknowledged. The authors also would like to thank
Ümit Melep, Yener Aydin and Bilge Uluocak for their support during experimental measurements. The first author is grateful for the
financial support given by The Scientific and Technical Research
_
Council of Turkey (TÜBITAK).

References
[1] Naus DJ. Effects of elevated temperature on concrete materials and structures
– a literature review. Washington: US Nuclear Regulatory Commission Office
of Nuclear Regulatory Research; 2005.
[2] Kalifa P, Menneteau FD, Quenard D. Spalling and pore pressure in HPC at high
temperature. Cem Concr Res 2000;30(3):1915–27.
[3] Phan LT, Carino NJ. Fire performance of high strength concrete: research
needs. Philadelphia: NIST Special Publications; 2000.
[4] Hertz KD. Limits of spalling of fire exposed concrete. Fire Saf J
2003;38(1):103–16.
[5] Meyer-otetens C. Zur Frage der Abplatzungen an Bauteilen aus Beton bei
Brandbeanspruchungen. Berlin: Deutsher Ausschuss für Stahlbeton; 1975.
[6] Colombo M, di Prisco, Felicetti R. Mechanical properties of steel fibre
reinforeced concrete exposed at high temperatures. Mater Struct
2010;43(4):475–91.
[7] Kalifa P, Chene G, Galle C. High temperature behavior of HPC with
polypropylene fibers from spalling to microstructure. Cem Concr Res
2001;31(2):1487–99.
[8] Han CG, Hwang YS, Yang SH, Gowripalan N. Performance of spalling resistance
of high performance concrete with polypropylene fiber contents and lateral
confinement. Cem Concr Res 2005;35(4):1747–53.
[9] Komonen J, Penttala V. Effects of high temperature on the pore structure and
strength of plain and polypropylene fiber reinforced cement pastes. Fire
Technol 2003;39(1):23–34.
[10] Poon CS, Shui ZH, Lam L. Compressive behavior of fiber reinforced highperformance concrete subjected to elevated temperature. Cem Concr Res
2004;34(12):2215–22.
[11] Aydın S. Development of a high-temperature-resistant mortar by using slag
and pumice. Fire Saf J 2008;43(8):610–7.


328

A.H. Akca, N. Özyurt Zihniog˘lu / Construction and Building Materials 44 (2013) 317–328

[12] Sarshar R, Khoury GA. Material and environmental factors influencing the
compressive strength of unsealed cement paste and concrete at high
temperatures. Mag Concr Res 1993;45(162):51–61.
[13] Poon CS, Azhar S, Anson M, Wong YK. Comparison of the strength and
durability performance of normal and high strength pozzolanic concretes at
elevated temperatures. Cem Concr Res 2001;31(2):1291–300.
[14] Peng G, Yang W, Zhao J, Liu Y, Bian S, Zhao L. Explosive spalling and residual
mechanical properties of fiber-toughened high-performance concrete
subjected to high temperatures. Cem Concr Res 2006;36(4):723–7.
[15] Anderberg Y. Fire exposed hyperstatic concrete structures. Division of
structural mechanics and concrete construction. Lund Institute of
Technology, Bulletin vol. 32; 1973.
[16] CIP 15. Chemical admixtures for concrete. Maryland: National Ready Mixed
Concrete Association; 2001.
[17] Riley MA. Possible new methods for the assessment of fire – damaged
concrete. Mag Concr Res 1991;43(4):87–92.
[18] Waubke NV. Transportph.anomene in Betonporen. Disertation 6/6, Technishen
Hochschule Braunschweig; 1966.
[19] Connolly RJ. The spalling of concrete in fires. PhD thesis, Aston University;
1995.
[20] Series BS 1353. Determination of moisture content of autoclaved aerated
concrete. London: British Standards Institution; 1997.
[21] Series BS 12390. Testing Hardened Concrete. London: British Standards
Institution: 2009.
[22] S
ß ahmaran M, Özbay E, Yücel HE, Lachemi M, Li VC. Effect of fly ash and PVA
fiber on microstructural damage and residual properties of engineered
cementitious composites exposed to high temperatures. J Mater Civ Eng
2011;23(12):1735–45.
[23] Schneider U, Herbst H. Permeability and porosity of concrete at high
temperature, technical report 403. Berlin: Deutscher Ausschuss für
Stahlbeton; 1989.
[24] Ingham JP. Application of petrographic examination techniques to the
assessment of fire-damaged concrete and masonry structures. Mater Charact
2009;60(7):700–9.

[25] Lin WM, Lin TD, Powers-Couche LJ. Microstructures of fire-damaged concrete.
ACI Mater J 1996;93(3):199–205.
[26] Cülfik MS, Özturan T. Effect of elevated temperatures on the residual
mechanical properties of high-performance mortar. Cem Concr Res 2002;
32(5):809–16.
[27] Carette GG, Painter KE, Malhotra VM. Sustained high temperature effect on
concretes made with normal Portland cement, normal Portland cement and
slag, or normal Portland cement and fly ash. Concr Int 1982;4(7):41–51.
[28] Poon CS, Azhar S, Anson M, Wong Y. Strength and durability recovery of firedamaged concrete after post-fire-curing. Cem Concr Res 2001;31(9):1307–18.
[29] Crook DN, Murray MJ. Regain of strength and firing of concrete. Mag Concr Res
1970;22(6):149–54.
[30] Villain G, Thiery M, Platret G. Measurement methods of carbonation profiles in
concrete: thermogravimetry, chemical analysis and gammadensimetry. Cem
Concr Res 2007;37(8):1182–92.
[31] Morsy MS, Galal AF, Abo-El-Enein SA. Effect of temperature on phase
composition and microstructure of artificial pozzolana-cement pastes
containing burnt kaolinite clay. Cem Concr Res 1998;28(8):1157–63.
[32] Zhi Xing Z, Beaucour A, Hebert R, Noumowe A, Ledesert B. Influence of the
nature of aggregates on the behaviour of concrete subjected to elevated
temperature. Cem Concr Res 2011;41(4):392–402.
[33] ASTM Standard C856. Standard practice for petrographic examination of
hardened concrete. ASTM International, West Conshohocken; 2011.
[34] Yaprak H, Karacı A. Polipropilen Lifli Betonların Yüksek Sıcaklık Sonrası Basınç
Dayanımlarının Yapay Sinir Ag˘ları ile Tahmini. Int J Eng Res Develop 2009;
1(2):23–8.
[35] Series BS EN 1992-1-2. Eurocode 2: design of concrete structures – Part 1–2:
general rules – structural fire design. London: British Standards Institution;
2010.
[36] Djaknoun S, Quedraogo E, Ahmed Benyahia A. Characterization of the behavior
of high performance mortar subjected to high temperatures. Constr Build
Mater 2012;28:176–86.
[37] Gartner EM, Kurtis KE, Monteiro PJM. Propesed mechanism of C–S–H growth
tested by soft X-ray microscopy. Cem Concr Res 2000;30:817–22.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Tải bản đầy đủ ngay

×