MICROSTRUCTURE ANALYSIS AND RESIDUAL STRENGTH OF … · ABSTRACT This study investigates the...

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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

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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


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


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.



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".




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



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].




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



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




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



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


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




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


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).



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.




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.





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.




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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