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International Journal of Civil Engineering and Technology (IJCIET)
Volume 9, Issue 4, April 2018, pp. 15–31, Article ID: IJCIET_09_04_003
Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=4
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication Scopus Indexed
MICROSTRUCTURE ANALYSIS AND
RESIDUAL STRENGTH OF FIBER
REINFORCED ECO-FRIENDLY SELF-
CONSOLIDATING CONCRETE SUBJECTED TO
ELEVATED TEMPERATURE
S. A. Salih
Department of Building and Construction Engineering,
University of Technology, Baghdad, Iraq
M. R. Aldikheeli
Department of Structures and Water Resources,
College of Engineering, Kufa University, Najaf, Iraq
F. M. Al-Zwainy
Department of Civil Engineering, College of Engineering,
Al-Nahrain University, Baghdad, Iraq
ABSTRACT
This study investigates the influence of elevated temperature on the sustainable
fiber reinforced Self-Consolidating Concrete (FSCC). In addition to the determination
of residual mechanical properties (compressive strength, splitting tensile strength and
modulus of elasticity) of (FSCC), the microstructure of these mixes was also studied.
The results indicate that FSCC with high volume Class F fly ash showed best
mechanical properties when subjected to elevated temperature compared to Reference
and Cement Kiln Dust mixes. At 400 °C, the maximum relative residual compressive
strength, splitting tensile strength and modulus of elasticity was for 60% fly ash mix
and these were (93%, 69% and 64%) respectively. The residual strength dropped
sharply for all FSCC mixes after exposure to 400 °C. The microstructural
observations are congruent with the residual mechanical properties of the studied
FSCC mixes. At 400 °C, the microstructure of FAF SCC mixes and 50BF mix seems to
be stable with only minimal visible crack while for REFF and CKDF SCC mixes a
slight damage to the microstructure was occurred as the cracks appeared to be
elongated and the pores become coarser.
Keywords: sustainable, fiber reinforced Self-Consolidating Concrete (FSCC), elevated
temperature, microstructure, Class F fly ash, cement kiln dust, weight loss.
S. A. Salih, M. R. Aldikheeli and F. M. Al-Zwainy
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Cite this Article: S. A. Salih, M. R. Aldikheeli and F. M. Al-Zwainy, Microstructure
Analysis and Residual Strength of Fiber Reinforced Eco-Friendly Self-Consolidating
Concrete Subjected to Elevated Temperature, International Journal of Civil
Engineering and Technology, 9(4), 2018, pp. 15–31.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=4
1. INTRODUCTION
Civil engineering meeting one fundamental challenge that is to perform projects in amity with
nature using the impression of sustainable development. The concrete industry may be
considered to be unsustainable due to the huge production and consumption cycles of concrete
have substantial environmental influences [1]. So the study of "green", "sustainable" or "eco-
efficient" concrete has advanced rising attention among the major contemporary publications
about concrete because the affairs concerning the industrial wastes recycling, durability of
concrete, environment and the cost will place a pressure on the employment of waste
materials [2]. Self-Consolidating Concrete (SCC) is a significant advance in the concrete
technology and it is widely used in the world and among the most important users; the power,
nuclear, gas and oil industries. Because their greater structural function, ecological kindliness,
and energy-conserving effects, the uses of such concretes are rising day by day [3]. Fire
considers one of the most real severe risks that SCC may exposed to and can cause the
collapse of the structures and lead to loss of life, homes, and livelihoods and regrettably,
though there are noteworthy developments in science and innovations, dangers to structures
because of elevated temperature during fire events are expanding as opposed to diminishing
[4]. Due to its high specific heat and low thermal conductivity, concrete is quite famous for its
capability to withstand high temperatures and fires. Otherwise, it does not signify that
elevated temperatures does not impact the concrete. Properties such as compressive strength,
tensile strength, elasticity and others are greatly influenced by high temperature and the
permanent damage may shorten the expected service life of the structures due to loss of
structural integrity [5]. Many studies [6-8] have been carried out to identify the deterioration
in mechanical properties of concrete during fire exposure, which is mainly due to three
’material’ factors: (i) physicochemical changes in the cement paste; (ii) physicochemical
changes in the aggregate; (iii) thermal incompatibility between the aggregate and the cement
paste. The deterioration is also influenced by ‘environmental’ factors, such as temperature
level, heating rate, load level and so on. The mix design, namely the type of aggregate and
cement and the interaction between them has also a major influence on the way concrete
degrades with temperature [9]. At room temperature, the use of fibers enhances concrete
possibilities since fibers arrest cracks and retard their propagation. At high temperature, the
toughness of concrete can be improved clearly by using Steel fiber (SF) and polypropylene
(PP) fiber can reduce the spalling [10]. There are several studies on the impact of elevated
temperature on mechanical and different characteristics of concrete like [11-17]. Some studies
such as Phan and Carino [11] and Khoury [12] concluded that in the range between 20 °C and
150 °C there was a decrease in compressive strength for normal strength concrete. Other
researchers like Ghandehari et al. [13] and Han et al. [14] assessed the residual mechanical
properties of high strength concretes after exposure to elevated temperatures and stated when
compared to strength at 100 °C, there was a little enhancement in concrete strength after
exposing to 200 °C. Self-compacting cement paste was investigated by Ye et al. [15] after
subjected to high temperature. They studied microstructural alterations in addition to the
phase allotment. They concluded that when compared to high performance cement paste, a
greater variation of total porosity was occurred to self- compacting cement paste. The above
studies revealed that in spite of the significant work that has been done to investigate the
Microstructure Analysis and Residual Strength of Fiber Reinforced Eco-Friendly Self-Consolidating
Concrete Subjected to Elevated Temperature
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influence of different temperature levels on different types of concrete, there are only a few
works in which the impact of elevated temperatures on the SCC have been investigated.
2. RESEARCH SIGNIFICANCE
Recently, Self-Consolidating Concrete (SCC) and especially sustainable one (because the
construction industry is moving fast towards sustainability) is broadly used in situations that
are mandatorily subject to elevated temperature such as in petrochemical industries, furnaces
walls, industrial chimneys, nuclear applications, etc. or in situations that are accidentally
exposed to elevated temperature such as in buildings or tunnels due to human mistakes or
terrorist attacks. So there is a need to recognize its behavior when subjected to elevated
temperatures particularly that Self-Consolidating Concrete comprises different types of filler
materials so different performances are expected. There is a slightly little studies existing on
the performance of Self-Consolidating Concrete at raised temperature especially that contains
high volume level of replacement materials. So in this work an investigational program has
been prepared to take into account the influences of Portland Limestone Cement, high volume
class (F) fly ash and locally available cement kiln dust on the fire performance of sustainable
fiber reinforced SCC.
3. MATERIALS & EXPERIMENTAL PROGRAM
3.1. Materials Characteristics
3.1.1. Cement
In the present study the cement used was local Portland-lime stone cement (PLC) available in
the markets, Karasta CEM II/A-L 42.5 R. It complies with European Standard EN 197-1 [18]
and Iraqi industrial license No: 3868. The physical and chemical characteristics of cement
used in this study are presented in Table 1.
3.1.2. Aggregates
As fine aggregate natural sand was used in this work. It has a fineness modulus of 2.5 and
within the grading zone 3. As a coarse aggregate crushed gravel of 20mm maximum size was
used. Both kinds of aggregate were agreed to the Iraqi specification No.45 / 1984[19].
3.1.3. Chemical Admixture
A high performance superplasticizer based on modified polycarboxylic ether which is
commercially famous (GLENIUM 54) was used, for the liquefaction of the concrete mixtures
to achieve the desired workability, throughout this study as a "high range water reducing
admixture" (HRWRA). It complies with ASTM C494 [20].
3.1.4. Fly Ash
Fly ash used in present study was obtained from Turkey. The physical and chemical
properties of fly ash are put in Table 2. As per ASTM C618 standard [21], the fly ash utilized
is regarded as type F fly ash.
3.1.5. Cement kiln dust
Cement kiln dust (CKD) is a by-product of cement production. Table 2 indicates the chemical
composition and the (SEM) of the CKD used are shown in Figure 1.
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Table 1 Chemical and physical characteristics of Portland limestone cement (PLC) used a
Oxides or Property PLC test
results
Requirement of
EN 197-1 [18]
Requirement of Iraqi industrial
license No: 3868 b
SiO2 18.8 - -
Al2O3 4.8 - -
Fe2O3 2.7 - -
CaO 61.9 - -
MgO 2.5 - ≤ 5.0%
SO3 2.6 ≤ 4.0% ≤ 2.5% if C3A less than 5%
≤ 2.8% if C3A more than 5%
Na2O 0.2 - -
K2O 1.1 - -
(Na2O)eq.c 0.92 - -
L.O.I 4.5 - -
Fineness (m2/Kg) 390 - -
Initial setting time (min.) 128 ≥ 60.0 ≥ 45.0
Final setting time (hr.) 3.3 - -
2 days compressive
strength (MPa) 23 ≥ 20.0 ≥ 20.0
28days compressive
strength (MPa) 49 ≥ 42.5 ≥ 42.5
a. Chemical analysis and physical properties were carried out in the laboratory of Al –
Kufa cement mill.
b. Limit by ICOSQC (Iraqi Central Organization for Standardization & Quality Control).
c. (Na2O) eq. = Na2O+0.658 K2O.
Table 2 Chemical analysis and physical properties of the fly ash and cement kiln dust.
Oxides or property Fly ash Cement kiln dust ASTM C618-05[21]
Class F requirement
SiO2 50.5 16.7
SiO2 + Al2O3 + Fe2O3 ˃ 70 Al2O3 22.7 4.5
Fe2O3 9.3 2.0
CaO 10.8 44.5
MgO 1.2 1.3 -
Na2O 1.0 0.3 -
K2O 0.8 3.7 -
TiO2 0.7 - -
SO3 1.5 5.5 5.0 max.
Loss on Ignition 1.2 20.0 6.0 max.
Specific gravity 2.12 - -
Specific surface area (m2/kg) 420 565 -
3.1.6. Polypropylene fiber
Monofilament polypropylene fibers were used in this work. It was provided from market and
it is commercially known "RHEOFIBRE".
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Concrete Subjected to Elevated Temperature
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3.2. Experimental Program
3.2.1. Mix proportions
Because of SCC mixes highly reliant on the properties and the composition of its ingredients,
it can be considered a delicate mix. Two disagreeing properties should be found in each SCC
mix, and these are the high flow-ability and the high segregation resistance. In the present
work the reference FSCC mix (REFF) was designed according to Okamura and Ouchi [22]
taking into account the recommendations of the EFNARC [23] and ACI 237 [24]. Table 3
shows the mixture proportions of the mixes. Since the amount of polypropylene fiber greatly
affected the fresh properties of self-consolidating concrete, many trials were conducted to
select the best volume fraction of fiber (Vf) and it was 0.15%. This volume fraction is within
the recommended quantity by El-Dieb and Taha [25].
Figure 1 SEM for CKD used
Table 3 Mix proportions of the concrete mixes.
Mix ID Mixture proportions (kg/m
3)
Cement Fly ash Cement kiln dust Water Sand Gravel W/P a SP
b
REFF 500 - - 180 800 800 0.36 0.8
40FAF 300 200 - 180 800 800 0.36 0.7
50FAF 250 250 - 180 800 800 0.36 0.6
60FAF 200 300 - 180 800 800 0.36 0.55
20CKDF 400 - 100 180 800 800 0.36 0.9
30CKDF 350 - 150 180 800 800 0.36 1.1
50BF 250 150 100 180 800 800 0.36 0.9
a. W/P: water / powder : water / (cement + FA +CKD)
b. Sp: superplasticizer : (Lit/100Kg cementitious material)
3.2.2. Mixing Sequence and Samples Preparation
In this study drum type mixer of 0.1 m3 capacity was used to mix the concrete ingredients.
The dry constituents of concrete mixes were placed in the mixer such that the cement or
(cement plus powder materials) is placed between two layers of sand followed by two layers
of gravel, this prevents spillage of cement in air, the dry materials were well mixed for about
3 minutes to attain uniform mix. Then, about 80 % of the required quantity of tap water was
added and mixed thoroughly for another 3 minutes. Finally, The HRWR diluted with the
residual mixing water was then presented through 30 second, and the concrete was mixed for
2.5 minutes [26]. In the end the fibers were distributed by hand in the mixture to reach a
regular scattering throughout the concrete, then mixture was mixed for two minutes. The
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concrete remained at rest in the mixer for one minute to enable any large air bubbles
entrapped during mixing to rise to the surface, the concrete was then remixed for one minute
[27]. After the end of mixing the concrete was cast in the moulds without any vibration and
immediately covered with wet burlap and plastic wrap and remained undisturbed for 24 hrs. in
laboratory conditions. After 24 hrs. , specimens were removed from the moulds and placed in
curing tank up to 28 days, then they removed form curing tank and cured in air in the lab
conditions until 91 days.
3.2.3. Heating and Cooling Procedure
At the period of 91 day curing, samples were put in the manufactured electrical kiln which has
a capacity of 1200 ˚C (the temperature inside the furnace was at the room temperature at the
time of putting the specimens) then heat was applied at a rate of 5 ˚C/min until the desired
temperature was reached. In addition to room temperature four temperature degrees were
investigated (200 ˚C, 400 ˚C, 600 ˚C and 800 ˚C). After reaching the target temperature, the
specimens were remained at this temperature for two hours as shown in Figure 2. To ensure
that the specimens were reached to the maximum temperature two type "K" thermocouples
were placed at the surface of the specimens and the temperature was read by using a digital
"ELE" thermometer as shown in Figure 3. After that the kiln was turned off and the samples
were slowly cooled inside the furnace.
Figure 2 Heating cycles imposed.
3.2.4. Test Procedure
3.2.4.1. Tests on Fresh FSCC
To calculate and evaluate the fresh features of SCC there are several test methods that had
been developed around the world. Among these test methods there is no single test that can be
used alone to assess all of the main parameters, so a combination of tests is necessary to
totally describe a SCC mix. In this work the three main characteristics of SCC which named
(Filling ability, Passing ability and Resistance to segregation) were performed according to
the methods mentioned in EFNARC [23] and/or ACI 237 [24].
Microstructure Analysis and Residual Strength of Fiber Reinforced Eco-Friendly Self-Consolidating
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Figure 3 Measuring the specimen temperature by using ELE thermometer.
3.2.4.2 Tests on Hardened FSCC
3.2.4.2.1 Compressive Strength Test:
Concrete compressive strength test was performed matching to the BS EN 12390-3 [28] on
100 mm cubes, by using ELE digital compression machine of 2000 KN.
3.2.4.2.2 Splitting Tensile Strength Test:
Splitting tensile strength of the concrete was carried out in according to ASTM C496-04 [29]
on cylinders of (100 mm × 200 mm) by using the same machine used for testing the
compressive strength. The specimen was placed horizontally between the plates of testing
machine and the load was increased at a rate of (0.94 KN/s.) until failure by splitting along the
vertical axis takes place.
3.2.4.2.3 Static Modulus of Elasticity Test:
Based on ASTM C469-02 [30], the elastic modulus was calculated using (d=150 mm, h=300
mm) cylindrical specimens and mechanical strain gauges (ELE) of effective length equal to
150 mm. The chord modulus was used in this study and in this modulus, the slope of a line
drawn from a point representing 50µЄ to the point corresponding to 40% of the ultimate
stress and it is calculated as follow:
where:
Ec= chord "Young" modulus of elasticity,(MPa)
S2= stress corresponding to 40% of ultimate load,(MPa)
S1= stress corresponding to a longitudinal strain (0.00005),(MPa
= longitudinal strain produced by stress S2
4. RESULTS & DISCUSSION
4.1. Fresh Properties
Table 4 presents the fresh properties namely (filling ability, passing ability and segregation
resistance) of the studied mixes accompanied by the acceptable criteria proposed by EFNARC
[23] and ACI 237 [24]. The fresh properties of CKDF mixes were lower than that of FAF and
REFF mixes and it reduced as CKD replacement increase. This reduction in fresh properties
may be imputed to absorbing mixing water by CKD due to the free lime that found in CKD
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which rapidly reacts with water in addition to the higher fineness of CKD [31]. The binary
mix (50BF) which contain CKD together with FA showed better fresh properties than mixes
with CKD alone. This may be due to spherical shape of FA particles which induces a ball
bearing effect so the poor filling and passing abilities of CKD can be overcome by the
incorporation of FA together with CKD to make binary system. The presence of
Polypropylene Fiber cause a reduction in fresh properties and this may be due to the increase
of friction between aggregates and the fibers throughout the matrix and due to fiber tangling
that made its dispersion is difficult in addition to that fibers tend to cause fiber-aggregate
interlock that resisted the aggregate movement which reduces the filling ability.
4.2. Hardened Properties at Elevated Temperature
4.2.1. Residual Compressive Strength
The results of residual compressive strength after exposed to different temperature levels were
shown in Table 5 and Figure 4. It is detected that the global impact of exposing FSCC
specimens to high temperatures mostly results in reduction in compressive strength. For all
FSCC mixes, at 200 °C there were an improvement in residual compressive strength as shown
in Figure 4 and it was (109%, 111.5%, 116%, 124%, 108%, 107% and 111%) for REFF,
40FAF, 50FAF, 60FAF, 20CKDF, 30CKDF and 50BF mixes respectively. This improvement
in strength at 200 °C is imputed to the pathways formed by the melting of the PP fibres at
(165-170) °C so the water vapour will escape freely through the pores and getting out the
surface. Another reason for this strength gain for mixes may be due to gradual movement of
moisture from mortar at early stage of heating leads to remain some moisture in it that will
permit for the hydration of the unhydrated cement particles (especially with the high amount
used) to be accelerated so additional hydration products will be formed. For FAF mixes, in
addition to the unhydrated cement particles unhydrated fly ash grains may react with
(Ca(OH)2) and generate C–S–H like gel. At 400 °C, it can be seen that the FAF mixes loss
lower strength compared to the REFF mix. This may be ascribed to the following; at high
temperature and pressure a reaction takes place between lime and unhydrated fly ash and as a
result for this reaction the tobermorite gel will be created and this gel is a three times stronger
than the CSH gel. For 60% FA mix the highest relative remaining compressive strength was
and it (93%). The potential explanation is the larger percentage of unhydrated fly ash in this
mix. The residual compressive strength dropped for all FSCC mixes after exposure to 600 °C.
For REFF mix the relative residual compressive strength was (68%) and this high residual
strength may be due to the limestone (CaCO3) blended with cement required a high
temperatures (above 750 °C) for complete analyzing to (CaO) and (CO2) therefore, it is a heat
absorbing material [32]. The important merit of the results at 800 °C is the higher relative
residual strength for all FAF mixes compared to REFF and CKDF mixes and it was (62%,
65% and 71%) for 40FAF, 50FAF and 60FAF mixes respectively.
Table 4 Fresh properties of FSCC mixes.
Mix ID
Filling ability Passing ability (J-ring test) Segregation
resistance %
Slump flow
(ds) mm
Spread time
(T50) S.
Differences in
heights (mm) Flow (dj) (ds-dj)
REFF 620 4.0 6.1 600 20 5.1
40FAF 655 3.5 5.5 638 17 3.4
50FAF 671 3.0 5.0 655 16 3.0
60FAF 682 3.0 4.7 668 14 2.5
20CKDF 596 4.4 7.0 572 24 2.0
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30CKDF 583 5.0 7.7 548 35 1.8
50BF 625 4.0 6.0 603 22 2.7
Acceptance
criteria of
SCC
suggested by
ACI [24] 450 – 760 2 – 5 - 0 – 25 0 - 10
EFNARC
[23] 550 – 850 2 – 5 0 – 10 - -
This result was in agreement with Aydin and Baradan [33], where they used X-ray
analyses to examine the microstructure of cement paste incorporating fly ash, they discovered
that at 800 °C, gehlenite creation was detected alongside quartz, feldspar, and calcite and this
shows that when the temperature has been raised up to 800 °C, glassy phases that are molten
seem. After the mortar become cold, mortar compressive strength rises because this molten
phase fills in the pores.
Table 5 Residual compressive strength of FSCC mixes.
Mix ID
Residual compressive strength (MPa) a
Max. temperature °C
27 200 400 600 800
REFF 68.7 (100) 75.1 (109.3) 56.0 (81.5) 46.7 (68.1) 33.9 (49.4)
40FAF 64.2 (100) 71.6 (111.5) 54.5 (85) 46.9 (73.1) 39.8 (62)
50FAF 58.8 (100) 68.3 (116.1) 52.4 (89.2) 45.8 (78.0) 38.4 (65.3)
60FAF 52.3 (100) 64.8 (124) 48.5 (92.7) 42.1 (80.5) 37.0 (70.8)
20CKDF 39.7 (100) 43.0 (108.3) 31.3 (78.8) 26.6 (67) 19.5 (49.1)
30CKDF 36.0 (100) 38.5 (107.0) 26.5 (73.6) 23.3 (64.8) 16.9 (47.0)
50BF 67.4 (100) 75.2 (111.5) 56.4 (83.7) 48.6 (72.1) 37.7 (56.0)
a. The values in brackets indicate the relative increase or decrease in residual
compressive strength as compared to room temperature (27 °C).
Figure 4 Residual compressive strength of FSCC mixes.
4.2.2. Residual Splitting Tensile Strength
From the results indicated in Table 6 and Figure 5 it is obvious that at high temperature the
tensile strength declined more rapidly than the compressive strength because the features of
the interfacial tranzition zone (ITZ). At 200 °C the relative residual splitting tensile strength
was (73%, 75%, 77%, 80%, 74%, 72% and 75%) for REFF, 40FAF, 50FAF, 60FAF,
20CKDF, 30CKDF and 50BF mixes respectively. This reduction in splitting tensile strength
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on heating to this temperature is due to dilation that initially occurred to the cement paste
because the normal thermal expansion of it giving rise regional breakdowns in bond between
the aggregate and the cement paste. This expansion is opposed by a contraction as water is
driven off. These two contrasting movements gradually weaken and produce cracks leads to
reduce the splitting tensile strength. At 400 °C and 600 °C further reduction in splitting
strength was happened and the relative remaining splitting strength was (63%, 64%, 67%,
69%, 63%, 62% and 64%) and (48%, 51%, 53%, 54%, 46%, 45% and 48%) for REFF,
40FAF, 50FAF, 60FAF, 20CKDF, 30CKDF and 50BF mixes respectively. This reduction in
splitting tensile strength occurs due to the development of cracks and microcracks that result
from both dissociation of (Ca(OH)2) at about 530 °C and the chemical changes in the
aggregates.
Table 6 Residual splitting tensile strength of FSCC mixes.
Mix ID
Residual splitting tensile strength (MPa) a
Max. temperature °C
27 200 400 600 800
REFF 6.75 (100) 4.93 (73.0) 4.25 (63.0) 3.24 (48.0) 2.41 (35.8)
40FAF 6.10 (100) 4.60 (75.4) 3.94 (64.5) 3.11 (51.0) 2.37 (38.8)
50FAF 5.34 (100) 4.09 (76.6) 3.60 (67.4) 2.83 (53.0) 2.13 (39.8)
60FAF 5.05 (100) 4.02 (79.7) 3.46 (68.5) 2.72 (54.0) 2.07 (41.0)
20CKDF 3.95 (100) 2.93 (74.1) 2.50 (63.3) 1.83 (46.3) 1.42 (36.0)
30CKDF 3.66 (100) 2.63 (72.0) 2.28 (62.3) 1.64 (45.0) 1.31 (35.8)
50BF 6.50 (100) 4.87 (75.0) 4.16 (64.0) 3.14 (48.3) 2.47 (38.0)
* The magnitudes in parentheses represent the relative decrease in residual splitting tensile
strength as compared to room temperature (27 °C).
Figure 5 Residual splitting tensile strength of FSCC mixes.
4.2.3. Residual Modulus of Elasticity
Although the knowledge of elastic modulus is significant in design especially for pre-stressed
concrete members, there are fewer researches on modulus of elasticity compared to
compressive strength after exposure to elevated temperature. At all exposure temperature
levels, the general trend for the modulus of elasticity is approximately the same as in other
mechanical properties as shown in Table (7) and Figure (6). High drop occurred in elastic
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modulus particularly at 400 °C and this because the rather higher lessening in compressive
strength and developed deformation (higher strains), and the relative residual modulus of
elasticity was (55%, 59%, 60%, 64%, 55%, 52% and 56%) for REFF, 40FAF, 50FAF,
60FAF, 20CKDF, 30CKDF and 50BF mixes respectively. After that (600-800 °C) gradual
decay for modulus of elasticity was happened and the relative residual modulus of elasticity at
800 °C was (28%, 31%, 32%, 33%, 22%, 19% and 29%) for above mentioned mixes. This
reduction may be due to the development of high vapour pressure that can widen the linked
network of micro-cracks and alter them into macro-cracks and as a result, the modulus of
elasticity drops. For (50BF) mix the relative residual modulus of elasticity was (44%) at 600
°C so this mix maintains higher modulus of elasticity than REFF and CKDF mixes. The
pozzolanic activity of the fly ash (the pozzolanic influence could be even increased in the
tested elevated temperature and in existence of water vapour, which could generate an internal
autoclaved status), the influence of activation of CKD for FA and the accelerated hydration of
cement at high temperature can be the reasons of this result.
5. MICROSCOPIC OBSERVATIONS (SCANNING ELECTRON
MICROSCOPY) (SEM)
At 200 °C, it can be noticed from Figure 7, that the microstructure of FSCC mixes that
contain FA (Figure 7 A) converted into a matrix containing more reacted materials as shown
in Figures (7 B, C) accompanied with improved densification. This is due to the gradual
movement of water from cement paste (capillary water evaporates first then gel water moved
from gel pores to capillary pores and then evaporates) permits the remaining water for
accelerated hydration (due to high temperature) for the unreacted fly ash and cement particles
(particularly with the high amounts used). So this will generate more hydration products
(CSH and CH) and this interprets the increment in compressive strength for FSCC mixes at
this temperature especially for FAF mixes and 50BF mix as reported previously in the current
study. For CKDF mixes (Figure 7 D) it can be observed some microcracks in cement paste
(CP) due to shrinkage.
Table 7 Residual modulus of elasticity of FSCC mixes.
Mix ID
Residual modulus of elasticity (GPa) a
Max. temperature °C
27 200 400 600 800
REFF 42.79 (100) 36.8 (86.0) 23.53 (55.0) 18.18 (42.5) 12.19 (28.5)
40FAF 37.4 (100) 33.02 (88.3) 22.10 (59.1) 16.94 (45.3) 11.59 (31.0)
50FAF 36.58 (100) 32.81 (89.7) 22.02 (60.2) 17.26 (47.2) 11.70 (32.0)
60FAF 35.61 (100) 32.08 (90.1) 22.96 (64.5) 17.8 (50.0) 11.85 (33.3)
20CKDF 32.83 (100) 27.41 (83.5) 18.05 (55.0) 12.14 (37.0) 7.22 (22.0)
30CKDF 31.37 (100) 25.72 (82.0) 16.31 (52.0) 10.47 (33.4) 5.96 (19.0)
50BF 41.44 (100) 35.63 (86.0) 23.28 (56.2) 18.03 (43.5) 11.93 (28.8)
* The magnitudes in parentheses represent the relative decrease in modulus of elasticity as
compared to room temperature (27 °C).
S. A. Salih, M. R. Aldikheeli and F. M. Al-Zwainy
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Figure 6 Residual modulus of elasticity of FSCC mixes.
At 400 °C, the microstructure of FAF SCC mixes and 50BF mix seems to be stable with
only minimal visible crack as shown in Figure (8 A and B) while for REFF and CKDF SCC
mixes a slight damage to the microstructure was occurred as the cracks appeared to be
elongated and the pores become coarser as shown in Figure (8 C and D).
At 600 °C, it is obviously that the pore structure changed significantly where it had
increased porosity and the main two reasons for this increment were the decomposition of
hydration products and the magnification of the pores or the creation of more cracks due to
increased pore pressure as shown in Figure 9.
Figure 7 SEM images of A) FA mix at room temperature. B) FA mix at 200 °C. C) 50B mix at 200
°C. D) CKD mix at 200 °C.
For FAF mixes (Figure 9 A) the formation of cracks is less than REFF and CKDF mixes
(Figure 9 C and D) because FAF mixes contain less (Ca(OH2)) (because pozzolanic reaction
consumed it) and as (Ca(OH2)) decompose at about 530 °C so REFF and CKDF mixes
exhibited more cracks. Due to thermal incompatibility between aggregate (Agg.) and cement
paste the thermal cracks may occurred in the "ITZ" zone as shown in Figure (9 C and D)
because the cracks followed the weakest zone.
Microstructure Analysis and Residual Strength of Fiber Reinforced Eco-Friendly Self-Consolidating
Concrete Subjected to Elevated Temperature
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Figure 8 SEM images at 400 °C of A) FA mix. B) 50B mix. C) REF mix. D) CKD mix.
At 800 °C, the microstructure of REFF and CKDF mixes (Figure 10 C and D) appears in
alveolate form, where a high number of pores presents and no crystals found due to
decomposition of all hydration products, this reflected the sever cracks observed at the surface
of the specimens and pointedly falling of the strength at this temperature degree. In general, at
800 °C, the loss of microstructural integrity (weak structure) was mainly due to disruption of
the main hydration products, decomposition of (CaCO3), predominance of cracks (high
number and width), increased porosity and pores coarsening. All the aforesaid reasons made
the degradation in the strength occurred at this temperature is logical.
Figure 9 SEM images at 600 °C of A) FA mix. B) 50B mix. C) REF mix. D) CKD mix.
S. A. Salih, M. R. Aldikheeli and F. M. Al-Zwainy
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Figure 10 SEM images at 800 °C of A) FA mix. B) 50B mix. C) REF mix. D) CKD mix.
With respect to polypropylene fibers addition (Figure 11), it can be seen that before
exposed to elevated temperature a tight bonding between polypropylene fibers and the
surrounding paste, where a good connection with the products of hydration, no apparent
interfacial cracks found between PP fibers and matrix as shown in Figure 11 A. At 200 °C the
polypropylene fibers were melt and the empty channels can be clearly recognized in Figure 11
B.
Figure 11 Polypropylene fibers at (A) ambient temperature and (B) 200 °C.
6. CONCLUSIONS
1. Sustainable Fiber reinforced Self-consolidating concrete mixtures can be produced
with Portland limestone cement, high-volume class F fly ash, cement kiln dust and a
low dosage of super plasticizers without the use of any viscosity modifying
admixtures and with satisfactory fresh properties.
Microstructure Analysis and Residual Strength of Fiber Reinforced Eco-Friendly Self-Consolidating
Concrete Subjected to Elevated Temperature
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2. Because of the unavailability of fly ash in Iraq the substitution of less-expensive CKD
as a partial cement replacement improves the sustainability of SCC with reasonable
strength. Where it reduces cost and environmental pollution from the disposal of
CKD.
3. At ambient temperature, the presence of Polypropylene Fibers leads to a reduction in
flow capability and passing capability of Self consolidating concrete mixes but they
still meet the requirements of SCC.
4. The impact of elevated temperature on compressive strength can be parted into notable
ranges. Where at 200 °C, an increment in strength was detected in FSCC mixes. At
400 °C most mixes lost insignificant percentage of their original strength. At 600 °C
and beyond, FSCC mixes lost their strength rapidly.
5. Fly ash FSCC mixes exhibit the best performance among the mixes where the relative
residual compressive and splitting strengths at 800 °C were (62%, 65%, 71%) and
(39%, 40%, 41%)
6. 6- High drop happens in modulus of elasticity particularly after 600 °C and the relative
residual modulus of elasticity at 800 °C was (28%, 31%, 32%, 33%, 22%, 19% and
29%) for REFF, 40FAF, 50FAF, 60FAF, 20CKDF, 30CKDF and 50BF mixes
respectively.
7. From SEM, the microstructural observations at elevated temperature are congruent
with the residual mechanical properties and the visual inspection of the studied FSCC
mixes.
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