A laboratory study is performed to evaluate the influence of elevated temperature on the strength and microstructural properties of high strength concretes (HSCs) containing ground pumice (GP), and blend of ground pumice and metakaolin (MK) mixture. Twelve different mixtures of HSCs containing GP and MK were produced, water-to-binder ratio was kept constant as 0.20. Hardened concrete specimens were exposed to 250 C, 500 C and 750 C elevated temperatures increased with a heating rate of 5 C/min. Ultrasound pulse velocity (Upv), compressive strength (fc), flexural strength (ffs) and splitting tensile strength (fsts) values of concrete samples were measured on unheated control concrete and after air-cooling period of heated concrete. The crack formation and alterations in the matrix, interface and aggregate of HSCs were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and polarized light microscope (PLM) analyses. XRD, SEM and PLM analyses have shown that, increasing target temperature result with decrease in mechanical properties i.e. Upv, fc, ffs and fsts values. Elevated temperature also results with crack formation, and increasing target temperature caused more cracks. Alterations in the matrix, interface and aggregate were, also observed by these analyses. The experimental results indicate that concrete made with MK + GP blend together as a replacement of cement in mass basis behaved better than control concrete made with cement only, and concrete containing only GP as a cement replacement.
Trang 1The influence of elevated temperature on strength and microstructure
of high strength concrete containing ground pumice and metakaolin
M Saridemira, M.H Severcana, M Cifliklib, S Celiktena, F Ozcana, C.D Atisc,⇑
a
Department of Civil Engineering, Nigde University, 51240 Nigde, Turkey
b
Department of Geology Engineering, Nigde University, 51240 Nigde, Turkey
c
Department of Civil Engineering, Erciyes University, 38039 Kayseri, Turkey
h i g h l i g h t s
Influence of elevated temperature on mechanical properties of HSC is examined
The changes in microstructure of concrete were examined by XRD, SEM and PLM
Increase in temperature result with decrease in mechanical properties of concrete
High temperature caused cracks and alterations in microstructures of materials
Under elevated temperature concrete containing GP and MK blend behaved better
Article history:
Received 22 July 2015
Received in revised form 18 July 2016
Accepted 22 July 2016
Keywords:
High strength concrete
Elevated temperature
Microstructure
Interface
a b s t r a c t
A laboratory study is performed to evaluate the influence of elevated temperature on the strength and microstructural properties of high strength concretes (HSCs) containing ground pumice (GP), and blend
of ground pumice and metakaolin (MK) mixture Twelve different mixtures of HSCs containing GP and
MK were produced, water-to-binder ratio was kept constant as 0.20 Hardened concrete specimens were exposed to 250°C, 500 °C and 750 °C elevated temperatures increased with a heating rate of 5 °C/min Ultrasound pulse velocity (Upv), compressive strength (fc), flexural strength (ffs) and splitting tensile strength (fsts) values of concrete samples were measured on unheated control concrete and after air-cooling period of heated concrete The crack formation and alterations in the matrix, interface and aggregate of HSCs were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and polarized light microscope (PLM) analyses XRD, SEM and PLM analyses have shown that, increasing target temperature result with decrease in mechanical properties i.e Upv, fc, ffsand fstsvalues Elevated temperature also results with crack formation, and increasing target temperature caused more cracks Alterations in the matrix, interface and aggregate were, also observed by these analyses The experimen-tal results indicate that concrete made with MK + GP blend together as a replacement of cement in mass basis behaved better than control concrete made with cement only, and concrete containing only GP as a cement replacement
Ó 2016 Elsevier Ltd All rights reserved
1 Introduction
In recent years, high strength concretes (HSCs) containing
natural pozzolanas, which are either in raw or calcined condition,
such as metakaolin, zeolite, volcanic tuff and diatomite, are used
widely in the world The columns, shear walls, foundations,
bridges, skyscrapers, nuclear and power structures are among
the major areas for high strength concrete applications The
application fields of high strength concretes are expanding in time, since they show extraordinary structural performance, protect the environment, save energy by using pozzolanas[1,2] Moreover, the natural pozzolanas provide more advantage i.e the reduction of cost, reduction of heat leak, decrease of permeability and increase of chemical resistance, since they intensify the microstructure of concrete when used as cement replacement material [3] They also provide extra strength in the concrete by reacting with the cement hydration product Ca(OH)2 to form extra calcium-silicate-hydrate (C-S-H) gels particularly in transition zone[4,5]
http://dx.doi.org/10.1016/j.conbuildmat.2016.07.109
0950-0618/Ó 2016 Elsevier Ltd All rights reserved.
⇑Corresponding author.
E-mail address: cdatis@erciyes.edu.tr (C.D Atis).
Contents lists available atScienceDirect
Construction and Building Materials
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c o n b u i l d m a t
Trang 2Over the past several decades, due to its high pozzolanic
properties, the influence of using MK as supplementary
cement-ing materials in concrete on the mechanical and durability
related properties of concrete was studied by numerous
researchers[6–10] MK is an ultrafine pozzolanas, produced by
calcination of purified kaolin clay, heating in the temperature
range of 600–900°C [5,10] The main ingredients of MK are
amorphous SiO2 and Al2O3 MK has high pozzolanic activity
due to its glassy components Apart from the filling effect in
concrete, MK reacts with Ca(OH)2 to produce C-S-H gels in the
main bonding phase of concrete [11–13] It is known that Ca
(OH)2is the main element which causes the weakness of
inter-face between the aggregate particles-cementitious materials
Thus, it influences the strength, porosity, permeability and
dura-bility related properties of concrete[13,14] The replacement of
natural puzzolanas with cement, such as MK, consumes Ca
(OH)2 and improves the above-mentioned properties of
interfa-cial zone of concrete[13]
Concrete, which is one of the most widely used as building
materials in the world, has higher resistance to elevated
tempera-ture, when compared to other building materials i.e steel and
wood Nevertheless, this resistance is valid up to a certain
temper-ature level and exposure duration[15,16] When a certain time is
exceeded under elevated temperature, it brings about important
physical and chemical changes and resulting in deterioration of
concrete such as forming cracks, causing large pores, spalls and
reduction of the adherence between the aggregate
particles-cementitious materials in the concrete [16,17] Therefore, the
mechanical properties of concrete are decreased due to these
changes The reduction in these properties due to elevated
temper-atures was also associated with the heating rate of specimens
When the specimens are heated up to approximately 250°C
tem-perature, free water present in the specimens evaporates slowly,
and no structural damage occurs in the specimens Nevertheless,
rapid heating rate results in higher vapor pressure and causes
cracks in concrete [18] When the temperature of specimens
reaches approximately at 300°C, the water in the interface of
C-S-H gels is evaporated Micro-cracks occur approximately at
300°C temperature in the cement matrix and the bond between
the aggregate particles-cementitious materials[16,19] Therefore,
the mechanical properties of the specimens, exposed to higher
than 300°C temperature, gradually decrease due to the crack
growth and deterioration of C-S-H gels, when compared to
non-heated specimens
Previous papers studied on the influence of MK were, in general,
on the properties of concrete as a cement replacement material
Using MK as a cement replacement in concrete improved
mechan-ical and durability related properties at optimal replacement ratio,
which depends on the fineness and properties of MK used The
effect of ternary blend of MK and silica fume or fly ash on the
prop-erties of HSCs was also studied by many researchers Nevertheless,
there are no study investigating the effect of ternary blend of MK
and GP on the mechanical and microstructural properties of HSCs
exposed to elevated temperatures The aim of this paper was to
investigate the effect of 5%GP, 10%GP, 15%GP, 20%GP, 2.5%MK
+ 2.5%GP, 5%MK + 5%GP, 5%MK + 10%GP, 5%MK + 15%GP, 10%MK
+ 5%GP, 10%MK + 10%GP and 15%MK + 5%GP (5GP, 10GP, 15GP,
20GP, 2.5MK + 2.5GP, 5MK + 5GP, 5MK + 10GP, 5MK + 15GP,
10MK + 5GP, 10MK + 10GP and 15MK + 5GP) blend, as cement
replacement in concrete, on the ultrasound pulse velocity (Upv),
compressive strength (fc), flexural strength (ffs) and splitting
ten-sile strength (fsts) values of concrete studied In addition,
investi-gating the influence of elevated temperature on residual Upv, fc,
ffs, and fstsvalues of HSCs containing GP and MK + GP blend was
another aim of this work Crack formation, alterations in
microstructural properties of cementitious matrix, interfacial zone
between the aggregate particles and cementitious materials were also to be investigated by XRD, SEM and PLM analyses
2 Experimental study 2.1 Materials
The cementitious materials used in the mixtures were ordinary Portland cement (CEM I 42.5 R) complying with relevant TS EN 197-1 [20], GP and MK complying with relevant ASTM C-618
[21] Portland cement, GP and MK were procured from Nigde cement plant of CIMSA, Nevsehir Mikromin Company and BASF-The Chemical Company in Turkey, respectively BASF-The chemical com-positions, physical and mechanical properties of the cementitious materials are presented inTable 1
The fine aggregates used in the mixtures were natural sand-I (NS-I) and natural sand-II (NS-II) The coarse aggregates used in the mixtures were crushed limestone-I (CL-I) and crushed limestone-II (CL-II) The aggregates used were compatible with the requirements of TS 706 EN 12620+A1[22] The particle size, mixing ratio and specific gravity of aggregates used in mixtures are given inTable 2 In addition, the gradations of aggregates used are provided inTable 3, with the standard limits
2.2 Mix proportions Twelve HSC mixtures with 0.2 water binder ratio and 500 kg cementitious materials for a cubic meter were prepared These mixtures include one control concrete (C), four concretes contain-ing up to 20% GP and seven concretes containcontain-ing up to 20% MK + GP blend The details of mixture proportions of concretes con-taining GP and MK + GP are given inTable 4 The modified polycar-boxylic ether polymers based high range water reducing admixture
Table 1 Properties of cement, GP and MK admixtures.
Physical properties Initial-final setting time (min) 125–215
Specific surface area (m 2
Mechanical properties Compressive Strength (MPa)
GP = Ground pumice, MK = Metakaolin.
Table 2 The particle size, mixing ratio and specific gravity of aggregates.
Fine aggregate Coarse aggregate
Trang 3called as Glenium 51 was used in the concrete mixtures to
main-tain desired slump of 80 ± 20 mm
2.3 Mixing, casting, curing, heating and cooling details
The mixing, casting and compacting of concretes containing GP
and MK + GP blend were performed complying with relevant
stan-dard ASTM C192/C192M-14 [23] A power driven rotating pan
mixer was used for mixing, and a vibrating table was used in
cast-ing and compactcast-ing the samples After castcast-ing the fresh concrete
mixture samples into the molds, they were covered with wet
bur-laps for 24 h in the laboratory condition Afterwards, hardened
specimens were removed from the molds after a day, and were
placed in water tank with 24 ± 1°C temperature, until testing
The heating of concrete specimens carried out by exposing
them to 250°C, 500 °C and 750 °C each target temperatures Before
elevated temperature testing, specimens were removed from
water tank, and conditioned in laboratory condition for a week,
then dried for 24 h in an oven at 105°C The specimens were put
in a furnace at room temperature, and temperature was elevated
at a rate of 5°C/min up to target temperatures The specimens
were exposed to target temperature for 2 h in steady-state
condi-tion Then the power button on the furnace was shut off At the
end of heating process, the door of furnace was opened, and the
specimens were exposed to slow cooling in the air for 24 h
2.4 Testing procedure and methods
The Upv, fc, ffsand fsts values were determined on the control
concrete and concretes containing 5GP, 10GP, 15GP, 20GP,
2.5MK + 2.5GP, 5MK + 5GP, 5MK + 10GP, 5MK + 15GP, 10MK
+ 5GP, 10MK + 10GP and 15MK + 5GP The Upvand fctests were
performed, in accordance with ASTM C 597-09 [24] and TS EN 12390-3[25], on cubic specimens with a 10 cm side, at the ages
of 7, 28 and 56 days In addition, the Upv, fc, ffsand fststests were also performed on the same size cubic specimens after exposing them to 250°C, 500 °C and 750 °C temperatures at 56 days The
fcvalues on the concrete specimens were measured by compres-sion load applied with a rate of 0.10 MPa/s by using a 3000 kN capacity compression machine The ffs and fsts values were obtained by using flexural tensile testing and split tensile strength apparatus The ffsand fststests were carried out in accordance with
TS EN 12390-5[26]and TS EN 12390-6[27], respectively Flexural (ffs) and split tensile strengths (fsts) were measured at 56 days, by using prism specimens with dimension of 10 10 40 cm, and cubic specimens with a 15 cm side, respectively
In this study, microscopic analyses of HSCs containing GP and
MK + GP exposed to elevated temperature were performed by using a Philips Panalytical EMPYREAN type XRD, Zeiss EVO 40XVP type SEM, and Nikon ECLIPSE E400 Pol type PLM
XRD and SEM analyses were used to investigate the changes in the chemical component, mineralogical structure, microstructure and interface between the aggregate particles and cementitious materials of the control concrete and concretes containing 5GP and 5MK + 5GP exposed to 25°C, 500 °C and 750 °C temperatures These analyses were performed on the small pieces taken from the specimens used for the PLM analysis For the SEM analyses, the small pieces were mounted on the brass stubs using carbon tapes and, were covered with gold
PLM analyses were used to investigate the cracks and alter-ations in the cementitious matrix, interface between the aggregate particles and cementitious materials, and aggregate microstruc-tures on the thin segments taken from cubic sample with a
10 cm side The analyses were carried out on the control concrete and concretes containing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP after exposing them to 25°C, 500 °C and 750 °C temperatures After exposing the concrete specimens to the target temperatures and cooling, they were cut in four equal parts using a rotary saw and 5 5 10 cm prism specimens were prepared as shown
part was divided into two equal parts using a rotary saw and one of the parts was overlaid in the acetone to clean the free particles and pores This cleaned part was embedded in resin to absorb resin in a vacuum desiccator until there is no micro-air bubble in the part The part embedded in resin for the PLM analyses were left to harden in the laboratory condition as seenFig 1d The hardened parts were adhered to the 5 5 0.5 cm size glass to obtain thin-ner section The adhered parts were cut very thin by a cutting machine to make thin parts 5 5 0.03 cm in size by using a sen-sitive diamond saw as seenFig 1c and e Then, thin sections were eroded for use in the PLM analysis size The surfaces of thin
Table 4
Mixture proportions of concretes containing GP and MK + GP (kg/m 3 ).
Mixtures No Meaning Cement
kg/m 3
GP
% MK
% Water kg/m 3
NS-I (0–1 mm)
NS-II (0–5 mm)
CL-I (5–12 mm)
CL-II (12–22 mm)
SP kg/m 3
Table 3
Total used aggregate grading with standard limits.
Sieve size (mm) Passing from sieve (%)
A limit B limit C limit Total used aggregate
Trang 4sections were moistened to increase the quality of images using
the microscope camera In this way, the microstructure images of
thin sections between the lines seen inFig 1f were obtained to
investigate the cracks and alterations in the cementitious matrix,
interface between the aggregate particles and cementitious
mate-rials, and aggregate microstructures
3 Test results and discussion
3.1 Ultrasound pulse velocity and compressive strength
The normalized Upvand fcvalues of HSCs containing GP and MK
+ GP at the ages of 7, 28 and 56 days are given inTable 5 Besides,
the effects of GP on the Upvand fc values of HSC are shown in Figs.2a and 3a in 3D graphs, and also the effects of MK + GP on the Upvand fcvalues of HSC are shown in Figs.2b and3b in 3D graphs at the ages of 7, 28 and 56 days As shown in Figs 2a and3a, the Upvand fc values of concrete containing 5%
GP increases at all ages, while these values for concrete containing 10%, 15% and 20% GP decrease These figures shows that, the Upv
and fc values of the control concretes varied between 5.38–5.44 km/s and 75.50–81.35 MPa, while these values for concretes containing GP ranged between 5.28–5.45 km/s and 65.13–84.19 MPa, respectively, depending on the curing time and replacement level of GP The effect of MK + GP on the Upv
and f values of concrete can obviously be observed from
Fig 1 Preparing of thin part for microstructure analyses.
Trang 5Figs.2b and3b The concretes containing MK + GP had higher Upv
and fcvalues than the control concretes at the same ages, except
for 20% replacement level of MK + GP The Upvand fcvalues of
con-cretes containing MK + GP ranged between 5.39–5.50 km/s and
75.62–87.84 MPa as seen in these figures, respectively The highest
Upv and fc values were obtained between 5.44–5.50 km/s and
82.77–87.84 MPa for the concretes containing 5MK + 5GP In the
Upvand fcvalues of HSCs containing GP and MK + GP at the ages
of 7, 28 and 56 days are evaluated, it was concluded that the
con-crete containing MK + GP shown better performance than that of
concrete containing only GP Rashiddadash et al.[28]investigated
the fcvalues of the polypropylene fiber and steel fiber reinforced
concretes containing MK and GP They prepared concrete
contain-ing 10%GP and 15% GP, 10%MK and 15% MK, 7.5% MK + 7.5% GP,
and a control concrete They reported that the fc values varied
between 18–44 MPa and 16.7–37.6 MPa for control concretes and
concretes containing GP, respectively They observed that the early
and long-term fc values of concretes containing GP were lower
than that of the control concretes, depending on the replacement
level of GP However, they reported the fcvalues varied between 18.7 and 46.5 MPa for concrete containing MK were higher than that of the control concretes They concluded that, due to high fine-ness and reactivity of MK, the concretes containing MK had rela-tively higher fc development than that of the control concrete and concrete containing GP
A high temperature furnace used in this study to heat, cubic concrete specimens with a 10 cm a side, up to 750°C temperature
is shown inFig 4b The Upvand fcvalues of concretes containing GP and MK + GP exposed to 250°C, 500 °C and 750 °C temperatures are normalized according to the Upvand fcvalues obtained from unheated (25°C) specimens at the age of 56 days, and these values are presented inTable 6 In addition, the changes in the Upvand fc values of concretes containing GP and MK + GP exposed to 250°C,
500°C and 750 °C temperatures, compared with unheated coun-terpart control concrete samples which is shown inFigs 5 and 6
as 3D graphs
Table 5
The normalized f c , f sts and f fs values of HSCs.
f c = Compressive strength, f sts = Splitting tensile strength, f fs = Flexural strength.
7 28 56
5.26
5.30
5.32
5.36
5.40
5.44
U pv
GP, %
5.44-5.46
5.40-5.42
5.36-5.38
5.32-5.34
5.28-5.30
7 28 56
5.36
5.38
5.40
5.42
5.44
5.46
5.48
5.50
0
2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5
U pv
MK+GP, %
5.48-5.50 5.46-5.48 5.44-5.46 5.42-5.44 5.40-5.42 5.38-5.40 5.36-5.38
Time, days
Time, days
(a)
(b)
Fig 2 The U pv values of HSCs: a) containing GP and b) containing MK + GP.
7 28 56
64 66 68 70 72 74 76 78 80 82 84
f c
GP, %
84-85 82-84
78-80
74-76
70-72 68-70
64-66
7 28 56
74 76 78 80 82 84 86 88
0 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5
f c
MK+GP, %
86-88 84-86 82-84 80-82 78-80 76-78 74-76
Time, days
Time, days
(a)
(b)
Fig 3 The f c values of HSCs: a) containing GP and b) containing MK + GP.
Trang 6Fig 4 a) Broken specimen in the normal temperature, b) electric furnace for high temperature, and c) broken specimen after high temperature.
Table 6
The normalized U pv and f c values of HSCs exposed to elevated temperatures.
U pv = Ultrasound pulse velocity, f c = Compressive strength.
25 250 500 750 1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
4.4
4.8
5.2
5.6
U pv
GP, %
5.2-5.6 4.4-4.8 4.0-4.4 3.2-3.6 2.4-2.8 2.0-2.4 1.2-1.6
25 250 500 750 1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
4.4
4.8
5.2
5.6
0 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5
U pv
MK+GP, %
5.2-5.6 4.8-5.2 4.4-4.8 3.6-4.0 2.8-3.2 2.0-2.4 1.6-2.0
(a)
(b)
Temperature, ºC
Temperature, ºC
Fig 5 The effect of high temperature on the U pv values of HSCs: a) containing GP
25 250 500 750 25
30 35 40 45 50 55 60 65 70 75 80 85
f c
GP, %
80-85 75-80 70-75
60-65 55-60 50-55
40-45 35-40 30-35 25-30
25 250 500 750 30
35 40 45 50 55 60 65 70 75 80 85 90
0 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5
f c
MK+GP, %
85-90
75-80
65-70
55-60
45-50
35-40
Temperature, ºC
Temperature, ºC
(a)
(b)
Fig 6 The effect of high temperature on the f c values of HSCs: a) containing GP and
Trang 7The Upvvalues of heated concretes containing GP and MK + GP
reduce gradually as the temperature increase, as shown in
con-tents of concretes have no significant effect on the decrease of Upv
values when compared to strength of control Portland cement
con-crete as shown in these figures
As the temperature increases, the Upv values of concretes
decrease gradually together with the growth and increase of the
amount of pores and space within the concrete As shown in
while the average fc values of concretes containing GP show no
change, the average fc values of concretes containing MK + GP
reduce about 3% It is reported that, near such temperatures, the
fc values of concretes decrease due to the occurrence of micro
cracks caused by evaporation of water and the growth of pore
structure inside the concrete[18]
When the temperature reached to 500°C, while the average fc
values of concretes containing GP decrease about 11%, the average
fcvalues of concretes containing MK + GP reduce about 12% Some
researchers stated that loss in fc is generally associated with the
dehydration of C-S-H gel and the volumetric expansion due to
changing shape of the chemical compound Ca(OH)2 to CaO that
is known to happen between 500°C and 600 °C[18,29,30]
More-over, in these temperatures, the adherence between the aggregate
particles and cementitious materials is impaired, due to the
cementitious materials shrinkage resulting from the loss of water
together with the expansion of the aggregates[30]
The fc loss in the concrete increased substantially when the
temperature was reached to 750°C and over temperatures due to
the disintegration of C-S-H gel and increase of the macro cracks
[19,31,32] In this study, as shown inFig 6a and b the greatest fc
loss was observed at 750°C temperature At this temperature,
while the average loss in the fcvalues of concrete containing GP
was about 58%, the average loss in the fcvalues of concrete
con-taining MK + GP was 59% compared to fcbefore heating It can be
seen fromFig 6a and b that the replacements of GP and MK + GP
with cement have no significant influence on the fcloss occurring
due to elevated temperatures
3.2 Flexural and splitting tensile strengths
The ffsand fststests on the concretes containing GP and MK + GP
at the age of 56 days were performed on prism specimens with dimension of 10 10 40 cm, and cubic specimens with a 15 cm side
The ffsand fststest results of concretes containing GP and MK + GP are shown inFigs 7 and 8, each result being average of three concrete specimens Besides, the normalized ffsand fstsvalues of these concretes are also provided inTables 5 and 7 It can be seen from the results that GP has not enhanced the ffsand fstsvalues compared to the results of the control concrete, except for 5% GP content, and it could be concluded from the results that the higher amount of GP in the mixture leads to decrease in the ffsand fsts val-ues However, it can be seen from the results that MK + GP blend has enhanced the ffsand fsts values up to 20% MK + GP content compared to the results of the control concrete, and concretes con-taining GP The highest ffs and fsts values were obtained as 10.31 MPa and 5.70 MPa from concrete containing 5MK + 5GP mix-ture, while the lowest ffsand fstsvalues were obtained as 8.91 MPa and 5.29 MPa from a concrete containing 5%MK and 15%GP blend together
On the other hand, as the temperature increase, the ffsand fsts values of concretes containing GP and MK + GP exposed to
250°C, 500 °C and 750 °C temperatures decrease gradually as shown inFigs 7 and 8, in 3D graphs Normalized ffsand fstsvalues
of these concretes are presented inTable 7, with respect to the ffs
and fsts values of unheated concrete at the age of 56 days It is observed that GP and MK + GP contents of concretes have no important effect in a decrease of ffsand fsts values as shown in these figures and table When the temperature reached to 250°C,
500°C and 750 °C, while the average ffsvalue of concrete contain-ing GP decrease about 6%, 20%, and 60%; the average ffsvalue of concrete containing MK + GP decrease about 9%, 23%, and 66% compared to none heated concrete ffsvalue, respectively Similarly,
in these temperatures, while the average fsts values of concrete containing GP decrease about 9%, 22%, and 67%; the average fsts
25 250 500 750 1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
f sts
GP, %
5.5-6.0 5.0-5.5 4.0-4.5 3.5-4.0 2.5-3.0 1.5-2.0 1.0-1.5
25 250 500 750 1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
0 2.5+2.55+5 5+10 5+15 10+510+1015+5
f sts
MK+GP, %
5.5-6.0 5.0-5.5 4.5-5.0 3.5-4.0 3.0-3.5 2.5-3.0 2.0-2.5 1.5-2.0 1.0-1.5
Temperature, ºC
Temperature, ºC
(a)
(b)
Fig 7 The effect of high temperature on the f sts values of HSCs: a) containing GP
25 250 500 750 2.0
3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
f fs
GP, %
9.0-10.0 8.0-9.0 7.0-8.0 6.0-7.0 5.0-6.0 4.0-5.0 3.0-4.0 2.0-3.0
25 250 500 750 2.0
3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0
0 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5
f fs
MK+GP, %
10.0-11.0 9.0-10.0 8.0-9.0 7.0-8.0 6.0-7.0 5.0-6.0
Temperature, ºC
Temperature, ºC
(a)
(b)
Fig 8 The effect of high temperature on the f fs values of HSCs: a) containing GP and
Trang 8value of concrete containing MK + GP decrease about 11%, 22%, and
67% As shownFigs 7 and 8, the greatest ffsand fsts losses were
observed at 750°C temperature The explanation for this decrease
is the disintegration of C-S-H gels and macro cracks as stated in
compressive strength section
3.3 XRD analyses after elevated temperatures
X-ray diffraction (XRD) analyses were performed on powdered
samples obtained from control concrete and concretes made with
5GP and 5MK + 5GP after exposing them to 25°C and 750 °C
tem-peratures as shown inFig 9a, b and c Besides, the chemical
com-ponent and mineralogical structures of these concretes were
determined by X-ray fluorescence and XRD semi-quantitative
anal-ysis as seen inTables 8 and 9, respectively The results of analyses
indicated that the major mineralogical structures were calcite and
quartz from the aggregates as the main impurity in the concretes,
as well as some traces of feldspar and dolomite Moreover, the
analyses indicated that other mineralogical structures were Ca
(OH)2, C2S, C3S and C-S-H from the cementitious materials The
reduction in the XRD peak for mineralogical structures of concretes
was observed when the temperature was 750°C The reduction in
the calcite and quartz may be due to transformation of amorphous
phase of SiO2and lime of calcite[33,34] The dehydration caused
by the decomposition of Ca(OH)2, C2S, C3S and C-S-H gel plays a
dominant role in the reduction [35] The reduction in the XRD
peaks for the control concrete was higher than the concretes
con-taining GP and MK + GP at 750°C temperature as shown inFig 9
These situations are supported by the chemical components and
mineralogical structures of concretes as seen inTables 7 and 8
3.4 SEM analyses after elevated temperatures
Microstructural analyses of concrete specimens were carried
out by scanning electron microscope (SEM) The microstructure
and interface between the aggregate particles and cementitious
materials of the control concrete and concretes containing 5GP
and 5MK + 5GP exposed to 25°C, 500 °C and 750 °C temperatures
were examined on the crushed sample surfaces As seen in
interface due to the increase of temperature At 25°C temperature,
the internal structure of concretes is compact, and the C-S-H gels
are as the shape of block When exposed to 500°C temperature,
internal structure of concretes still compact, but the pores in the
Ca(OH)2and C-S-H gels start to increase When the elevated
tem-perature was 750°C, the deterioration in the Ca(OH)2and C-S-H
gels emerges in the internal structure of concretes Particularly,
when compared to 500°C temperature, the cracks and pores in
the matrix and interface were doubled with increasing tempera-ture The increases of cracks and pores are caused to decrease in the fcvalue It is concluded that the reduction in the fcvalue is a natural results of deterioration, large pores and crack formation
in the concrete specimens due to elevated temperature Similar conclusions and reports are revealed in the literature[31,36,37]
3.5 PLM analyses after elevated temperatures Microstructures of matrix, interface and aggregate on the thin sections were examined by using polarized light microscope (PLM)
3.5.1 The effect of elevated temperature on the matrix structure The matrix structures of the control concrete and concretes con-taining 5GP, 20GP, 5MK + 5GP and 10MK + 10GP when exposed to
25°C, 500 °C and 750 °C temperatures were examined by using PLM image analyses as seen inFig 11 The changes of the crack for-mation and deterioration on the matrix of the specimens were evaluated by these image analyses As seen inFig 4c, the discol-oration in the specimens was observed when the specimens exposed to 500°C and 750 °C temperatures This discoloration was the cause of altered and oxidized zones in the matrix Nearly
no crack was observed on the matrix of the specimens when exposed to 500°C temperature However, the cracks were observed on the matrix of the specimens when exposed to 750°C temperature These cracks caused considerable reduction in the mechanical properties of the specimens As shown in Fig 11, the GP and MK + GP contents have not an important effect on the cracks and deterioration occurred on the matrix of these specimens due to elevated temperature Similar observations have been reported by Akca and Zihniog˘lu [37], Akçaözog˘lu [38] and Ingham[39]
3.5.2 The effect of elevated temperature on the interface structure The interfacial zones are the weakest bond of the concrete and the cracks development commonly reveal in the interfacial zones between the aggregate particles and cementitious matrix [38] The weakening of bonding strength in these zones due to the elevated temperature causes major decrease in the mechanical properties of concrete[38,40] Hence, the bonding strength in the zones has a significant effect on these properties at elevated temperature [38] Because of this, in this part, the interfacial structure between aggregate particles and cementitious matrix
is examined
The interfacial structure between aggregate particles and cementitious matrix of the control concrete and concretes contain-ing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP when exposed to
25°C, 500 °C and 750 °C temperatures were examined by using
Table 7
The normalized f sts and f fs values of HSCs exposed to elevated temperatures.
f sts = Splitting tensile strength, f fs = Flexural strength.
Trang 9PLM image analyses as seen in Fig 12 The changes of cracks,
spaces and deterioration on the interfacial structure of these
concretes were evaluated in these image analyses As seen from the image analyses, no cracks, spaces and deterioration were
Fig 9 XRD analyses of HSCs exposed to 25 °C and 750 °C temperatures: a) control concrete, b) concrete containing 5GP and b) concrete containing 5MK + 5GP.
Trang 10observed between the aggregate particles and cementitious matrix
in the concretes not exposed to elevated temperature Besides, as
seen from the image analyses of concretes subjected to normal
temperature (25°C) inFig 12, the bonding strength between the
aggregate particles and cementitious matrix was very strong
How-ever, the increase at the cracks, pores and deterioration and the
decrease at the bonding strength between the aggregate particles
and cementitious matrix were observed depending on the increase
in the temperature The micro-cracks, micro-spaces, a little
deterioration and bonding strength are revealed in the interfacial zones of the concretes when exposed to 500°C temperature, while the macro-cracks, macro-spaces, significant deterioration and impaired bonding strength are shown in the interface zones of the concretes when exposed to 750°C temperature The effect of
GP and MK + GP contents on the interface structure between aggregate particles and the cementitious matrix decreased as the temperature increased Akca and Zihnioglu [37]investigated the colour changes, cracks and spalls of HSC exposed to elevated
Table 8
The chemical components of concretes by X-ray fluorescence.
Component
(%)
C 750 °C C 5GP 750 °C
5GP 5MK + 5GP 750 °C
5MK + 5GP SiO 2 30.43 27.96 29.65 28.84 29.42 28.22
CaO 40.54 42.62 40.45 42.49 38.84 42.05
Al 2 O 3 4.94 4.78 4.85 3.83 5.57 4.85
Fe 2 O 3 2.13 2.05 2.03 1.57 2.14 1.52
Na 2 O 0.63 0.49 0.62 0.43 0.75 0.42
LOI a
18.13 19.03 18.67 19.54 20.03 20.48
Total 99.87 99.85 99.87 99.90 99.84 99.88
a
LOI = Loss on ignition, C = Control concrete, GP = Ground pumice,
MK = Metakaolin.
Table 9 The mineralogical structures of concretes by semi-quantitative analysis.
Mineralogy C 750 °C
C 5GP 750 °C 5GP
5MK + 5GP
750 °C 5MK + 5GP
Amorphous Phase
C = Control concrete, GP = Ground pumice, MK = Metakaolin.
C
Aggregate
Aggregate
Aggregate
Aggregate
Aggregate
Aggregate
Aggregate
Interface
Interface
Interface
Inter face
Interface
Inter face
Inter face
Interface
Inter
face
Matrix
Matrix
Matrix
Matrix Matrix
Matrix
Matrix Matrix
Matrix Cracks
C-S-H
Space
Space Space
Crack
s
Crack
s
C-S-H
C-S-H C-S-H
C-S-H
GP
C-S-H
750 oC
Fig 10 SEM micrographs of HSCs exposed to 25 °C, 500 °C and 750 °C temperatures.