High performance concrete under elevated temperatures

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

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Department of Civil Engineering, Bog˘aziçi University, _Istanbul, Turkey

h i g h l i g h t s

Performance of HPCs under elevated temperatures

Use of PP ﬁbers with air entraining admixture to decrease damage

Decreased spalling and increased residual strength

Complete disintegration of dense matrix under very high temperatures

Microstructural examination of cement paste–aggregate interface

a r t i c l e i n f o

Article history:

Received 4 February 2013

Received in revised form 28 February 2013

Accepted 2 March 2013

Available online 10 April 2013

Keywords:

High performance concrete

Elevated temperatures

Polypropylene ﬁbers

Air entraining admixture

ESEM

a b s t r a c t

In this study, PP ﬁbers and air entraining admixture (AEA) were used together in an high performance concrete (HPC) mix so as to create interconnected reservoirs in concrete and to improve ﬁre performance

of HPC For this reason, nine mixes of HPC incorporating blast furnace slag with 0.24 water-to-binder ratio and various PP and AEA contents were produced Specimens were cast in two different sizes in order

to see the effect of size and 18 series of specimens were obtained These series subjected to elevated tem-peratures (300 °C, 600 °C and 900 °C) with a heating rate of 10 °C/min and after air cooling, residual mass and compressive strength of specimens were determined The heated specimens were observed both at macro and micro scales to investigate the color changes, cracking and spalling of HPC at various temper-atures Also, thermogravimetric analyses were performed on powder samples from each nine mixes Results showed that addition of AEA diminished the decrease in residual strength but this result was found to be irregular after 300 °C for thick specimens The collaboration of AEA and PP ﬁbers decreased the risk of spalling of HPC Also, size of specimen was found to be important in deterioration of HPC

1 Introduction

Concrete with increased strength and durability has been

pri-marily used in special constructions such as high rise buildings,

infrastructures and nuclear power plants since it became

commer-cially available[1] Thenceforth, some of these HPC structures

ex-posed to severe ﬁre conditions have exhibited poor performance

The main reason of this insufﬁciency of HPC at high temperatures

is a result of the changes made in the composition of concrete

mixes Decrease in water to cementitious ratio, use of

supplemen-tary cementitious materials and plasticizers lead to impressive

improvements such as strength, rheology of fresh concrete,

imper-meability and compactness On the other hand, in most cases these

changes may lead to a decrease in ﬁre performance of HPC[2]

Lower water to cementitious materials ratio leads to lower porosity and this decreases permeability of concrete With the in-crease in temperature, water in the pores of concrete evaporates and consequently pressure within the cement paste increases Re-duced permeability of HPC limits the diffusion of water vapor from the concrete pores and therefore pore pressure continues to in-crease until the internal stresses reach the tensile strength of con-crete and eventually causes spalling[3]

Free water and moisture gradients inﬂuence the behavior of concrete at elevated temperatures and according to Hertz, they must be regarded as main reasons of spalling[4] Meyer-Ottens treats that tensile stresses caused by steam in the closed pores of normal concrete can reach the tensile strength of concrete with more than 3% moisture by weight[5] Hertz concluded traditional concrete with less than 3% moisture by weight will not spall and in the range of 3–4% moisture by weight has a risk of spalling, on the other hand, concrete with a dense microstructure (most of the HPC) may spall even when the moisture content is zero[4] Due

to the increased impermeability, only the crystal water arisen from

⇑ Corresponding author Address: Bog˘aziçi Üniversitesi, _Insßaat Mühendislig˘i

Bölümü, 34342 Bebek, _Istanbul, Turkey Tel.: +90 212 359 70 39, Mobile: +90 533

690 22 44; fax: +90 212 287 24 57.

E-mail addresses: abdullah.akca@boun.edu.tr (A.H Akca), Nilufer.ozyurt@boun.

edu.tr (N Özyurt Zihniog˘lu).

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dehydration of hydrates at high temperatures may cause the

spall-ing of concrete

Recently, many studies have focused on the contribution of dif-ferent materials such as fibers and mineral admixtures to improve fire resistance of HPC[2,6,7] Addition of PP fibers into HPC was found as an efficient way to avoid spalling of concrete Because,

PP ﬁbers melt in concrete above 170 °C and leave micro channels

in concrete and these channels form a network more permeable than cement matrix which contributes to outward migration of gases and water vapor and result in the reduction of pore pressure

[7–10] As a mineral admixture, inclusion of silica fume caused reduction in residual strength and spalling of concrete by densiﬁy-ing microstructure[11,12] On the contrary, addition of ﬂy ash or slag showed better performance and also in some studies, strength gain observed at temperatures ranged from 200 °C to 300 °C be-cause of tobermorite formation[13]

Furthermore, rapid heating of concrete is another factor which causes a high temperature difference between the deeper zone and the surface of a specimen and therefore explosive spalling may occur during heating[14] Anderberg stated that during his

Table 1

Properties of cement, slag and polypropylene ﬁbers.

Speciﬁc surface (cm 2

Table 2

Mix proportions.

Series W/B Cement (kg/m 3 ) Slag (kg/m 3 ) Water (kg/m 3 ) Sand (kg/m 3 ) SP a (kg/m 3 ) PP (dm 3 /m 3 ) AEA b (g/BA) c

a

SP stands for superplasticizer.

b

AEA stands for air entraining admixture.

c

BA stands for binder amount of 1 m 3

concrete mixture in kilogram.

0

100

200

300

400

500

600

700

800

900

1000

Time (Min)

Fig 1 Heating cycles.

Table 3

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

a

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experiments only rapid ﬁres have given rise to spalling[15] Re-lated to this, size and shape of concrete can be considered as fac-tors which directly affect the heating rate of concrete When a structural member subjected to high temperatures, the heat level

of thin parts increases rapidly and this may cause spalling due to rapid heating[4]

On the other hand, although they are widely used especially in HPC mixes, there are a limited number of studies on the effect of chemical admixtures on concrete under ﬁre conditions For example, as a chemical admixture, air entraining admixtures (AEAs) are used for producing air bubbles in concrete to improve resistance of concrete to damage caused by freezing and thawing situations Moreover, the entrained air enhances the workability and may reduce bleeding and segregation of concrete mixtures at fresh state and with the increase in air voids in the concrete ther-mal conductivity of the concrete decreases at the hardened state

[16] As Riley stated the surface of concrete with low thermal con-ductivity which acted as a refractory material would effectively produce an insulation layer to inner parts of concrete[17] In a study conducted by Seçer, AEA was used to entrain air into con-crete in the ratios of 4%, 6% and 8% in volume and these concon-cretes subjected to temperatures of 300 °C, 500 °C and 700 °C Results of the study showed that as the air content of concrete increased, reduction in the strengths of concretes subjected to high tempera-tures diminished

In this study, PP ﬁbers and AEA were used together in HPC so as

to form a more permeable network consisted of micro channels of melted PP fibers and entrained air voids in concrete at elevated temperatures Thus, it was aimed to permit the evacuation of gases and water vapor appeared due to heating To the authors’ knowl-edge this is the first study to discuss the combined effects of poly-propylene fibers and air entraining admixture on the properties of high performance concrete under elevated temperatures Consid-ering the moisture and size factors which influence the effect of temperature, moisture contents of HPC specimens were adjusted

Fig 2 (a) A PP ﬁber passing through an air void, (b) a micro-channel formed after

heating (a part of a melted PP ﬁber reaches to an entrained air void creating a

micro-channel).

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

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approximately to 3% by weight to simulate the most negative

con-dition before heating and concrete specimens with two different

sizes were prepared to see the effect of size at high temperatures

2 Experimental study

2.1 Materials and mix design

CEM I Type 42.5R Portland cement, ground granulated blast furnace slag (GGBS)

from Karabük, river sand, multiﬁlament polypropylene (PP) ﬁber, a high-range

water reducing admixture based on chains of modiﬁed polycarboxylate ether and

an air entraining admixture based on oil alcohol and ammonium salt were used

in the production of concrete It should be emphasized that no coarse aggregate

was used in the mixture The maximum size of the sand used was 1 mm The

prop-erties of cement, slag and PP ﬁber are presented in Table 1

Nine mixes of HPC with 0.24 water-to-binder ratio and various PP and AEA

con-tents were produced Mix proportions are shown in Table 2 F0A0 is control group and

represents concrete with no PP ﬁber and AEA F8, F16 specimens contain PP ﬁber at

8‰ and 16‰ of the volume of the concrete amount, respectively A0.5 and A1

speci-mens have AEA to binder ratio of 0.5‰ and 1‰ respectively Mixes were cast into

10 10 50 cm prisms and the specimens were kept in laboratory environment

for 24 h After demoulding, the specimens were labeled and then they were cured

in a water tank at 20 °C for 10 days After 10 days of curing period 10 10 50 cm

specimens were cut with a diamond blade in order to obtain 10 10 10 cm cubes

(represented by T10) and 10 10 5 cm prisms (represented by T5) and then these

specimens were kept in laboratory environment for 3 months.

At elevated temperatures, it is known that the extent of damage increases with

an increase in the moisture content and 3–4% moisture by weight was found to be

content becomes more signiﬁcant since HPC is much denser than normal concrete.

In this study, moisture contents of HPC specimens were aimed to adjust approxi-mately to 3% by weight to simulate the most negative condition Therefore, mois-ture contents of one specimen from each series were determined in accordance with BS1353 before exposure to elevated temperatures [20] All the specimens were found to have moisture content in the range of 2.7–4.4% which can be considered as

in the critical region.

2.2 Heating procedure

An electrical furnace that was capable to operate up to 1250 °C was used After the curing period, moisture content of specimens reached the desired value and specimens of each series were exposed to 300 °C, 600 °C and 900 °C temperatures for an hour in the furnace The heating rate was set to 10 °C/min which can be con-sidered the same as the average heating rate of standard ﬁre curve (ISO-834) for the ﬁrst 90 min It should be emphasized that this heating rate is detrimental because

of the thermal gradients between the outer part and the inner core of the specimens which cause to additional internal stresses in concrete and initiates spalling [4] At the end of the set exposure time, the hot concrete specimens were not taken out until the furnace cooled down to 100 °C with a cooling rate of 3 °C/min Fig 1 rep-resents the heating cycles.

2.3 Test procedures Mass measurements and compressive strength tests were performed on both unheated and heated concrete specimens Also, specimens were observed at both macro and micro scales.

2.3.1 Macroscopic and microscopic observation Assessment of ﬁre-damaged concrete begins with visual observation of color change, cracking and spalling of concrete Changes on visual appearance of concrete give information about the temperature which concrete has been exposed In the scope of this study, occurrence of spalling, crack patterns and color changes were examined at macro scale Additionally, changes on surface and interior part of the heated specimens were observed microscopically.

2.3.2 Measuring mass loss Mass of each specimen was measured before heating test and measured again after the heated specimens cooled down to room temperature Moreover, thermog-ravitmetric analyses were performed on powder samples obtained from each con-crete type by using a TA Instruments Q50 thermal analyzer The thermal analyzer heated the sample to 863 °C (which is the maximum heating capacity of the used testing machine) with a constant rate of 10 °C/min and simultaneously measured the mass of the sample Finally, thermogravimetric (TG) and differential thermo-gravimetric (DTG) curves of samples were drawn.

2.3.3 Residual strength measurement After exposure to high temperatures, three specimens of each series were sub-jected to compression test in accordance with BS 12390 to measure the residual compressive strength [21] The specimens with dimensions of 10 10 10 cm were tested as they were On the other hand, all the 10 10 5 cm specimens were cut using a diamond blade in order to obtain 5 5 5 cm cube specimens for com-pression test These dimensions were considered representative of the material since concrete produced in this study contained only ﬁne aggregates with a maxi-mum size of 1 mm and entrained air voids whose maximaxi-mum and average diameter were 500lm and 100lm, respectively The ﬁber length (12 mm) was also smaller than 1/3 of the smallest dimension of the 5 5 5 cm specimen This test method consisted of applying a compressive axial load to cube specimens at a constant rate

of 0.2 MPa/s until failure occurred.

3 Results and discussion The experimental test results obtained from compression tests and mass loss measurements and visual observations are discussed

in this section Summary of the results are given in the form of ta-bles and ﬁgures

3.1 Spalling Explosive spalling of some specimens were observed when the specimens exposed to 600 °C and spalling began at approximately

500 °C (This statement is done based on the sound of explosive spalling)[22] Partial spalling such as corner spalling and surface layer delamination was not observed in this study.Table 3shows IDs and number of specimens destroyed due to explosive spalling Fig 4 (a) Popouts (8X magniﬁcation) on sand particles heated to 900 °C (b)

aggregate cracking (6X magniﬁcation) on the specimen (T5F16A1) heated to 900 °C.

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As is seen inTable 3none of the specimens spalled at 300 °C, all the

non-ﬁbrous specimens were exploded at 600 °C and therefore

specimens from these series were not exposed to 900 °C

As a similar result to the ﬁndings of Han, no explosive spalling

was observed when PP ﬁbers were used except some specimens

with IDs T5F8A0 and T10F8A0 [23] Explosive spalling was

ob-served in these ﬁbrous specimens and this phenomenon can be

ex-plained by dense microstructure of HPC According to Peng (based

on experiments done using HPC with a compressive strength of

80 MPa), regardless of the interconnected channel system formed

by PP ﬁber melted above 170 °C, concrete at 0.24 water-to-binder

ratio was so dense that it could still keep the water pressure high

enough to result in explosive spalling[14] It should be noted that

while the specimens with 8‰ ﬁbers and no air entrainment

(T5F8A0 and T10F8A0) exploded, the air entrained specimens of

the same series (T5F8A0.5, T10F8A0.5, T5F8A1 and T10F8A1) were

not exploded as can be seen inTable 3 This result could be

attrib-uted to the effect of air entrainment

It is hypothesized that, the contribution of air entrainment to

resist against explosive spalling began with PP ﬁber addition

En-trapped and/or entrained air voids in concrete are almost closed

and as Hertz stated; if water vapor cannot escape from these closed

pores it causes increase in pore pressure and increases the risk of

spalling[4] In the air entrained PP ﬁber reinforced concrete, most

probably micro-channels were formed due to melting of PP ﬁbers

at above 170 °C and some of the closed pores connected to each

other by these micro-channels On the other hand, in non-ﬁbrous

air entrained concrete, absence of ﬁbers limited the ability of water

vapor to escape from the entrained air voids in HPC and thus

spall-ing occurred

To examine this effect, microstructures of concrete specimens

were examined by using an environmental scanning

elec-tron microscope (Philips XL30 ESEM-FEG/EDAX) and formed

micro-channels were observed InFig 2a, PP ﬁber passes through

an air void and inFig 2b, a melted PP ﬁber creates a micro-channel

by reaching an entrained air void Consequently, this result shows that the existence of both PP ﬁbers and entrained air voids in HPC may reduce the risk of explosive spalling

3.2 Color and cracking observation on the outer surfaces of the specimens

In all cases red discoloration was observed at 300 °C, gray dis-coloration was observed at 600 °C on the outer surface of the spec-imens and at 900 °C the surface colors of the specspec-imens were changed to whitish gray Ingham stated that red color change is a result of hydrated iron oxides present mostly in siliceous aggre-gates, pink to red discoloration is very important and has a struc-tural signiﬁcance because it means that temperature around

300 °C where the reduction in concrete strength mostly began was attained[24]

The deterioration of a structural member exposed to 300 °C can

be repairable On the other hand, whitening of a structural member indicates that temperature has exceeded 600 °C and it corresponds

to a serious loss in compressive strength After this amount of strength loss, concrete cannot be repairable anymore and it cannot withstand service loads

Outer surfaces of the specimens were examined visually and by using a stereomicroscope (Nikon SMZ1500) (Fig 3) Almost no cracks were observed on the outer surfaces of the specimens that were heated until 300 °C On the other hand, cracks with an open-ing goopen-ing up to 0.2 mm were observed on the surfaces of the spec-imens heated to 600 °C Surface cracks formed on the specspec-imens exposed to 900 °C were larger and the crack widths were in the range of 0.3–0.4 mm Moreover, siliceous sand particles were sep-arately heated to 900 °C and cracks and popouts on the surfaces Fig 5 Inner surfaces of specimens of T5F16A1 series (a) the specimen kept at room temperature (20 °C), specimens heated to (b) 300 °C, (c) 600 °C, (d) 900 °C

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were observed due to the volume change of sand particles (Fig 4a).

The effect of volume change of sand particles can also be seen in

Fig 4b

3.3 Color and cracking observation on the inner surfaces of the

specimens

On the other hand, color of the inner surface of specimens is less

inﬂuenced than that of outer surface at the same temperature

le-vel Color of cement paste in inner surfaces is prominently darker

than in outer surfaces at 300 °C and 600 °C,Fig 5b and c Moreover,

reddened ﬁne aggregates in the inner surface are lighter than outer

surface at these temperatures This could be due to the fact that the

exact same temperature may not be reached in the inner sections

of concrete

Furthermore, no cracking was observed on the inner surfaces of

the specimens which were heated up to 600 °C Inner cracks both

in aggregates and cement paste were only visible in the specimens

which were heated to 900 °C as seen in the Fig 5d Moreover,

widths of the cracks in the inner zone of the concrete were smaller

than that of the outer zone and were around 0.04 mm

The specimens heated to 900 °C decomposed and

decomposi-tion did not take place immediately One or two large cracks

(0.3–0.4 mm) were observed in the ﬁrst day (following removal

of the specimens from the furnace, Fig 6a), then, these cracks

turned into spider web-like cracks in the 2nd day (Fig 6b) and

ﬁ-nally complete disintegration of specimens were observed on the

3rd day (Fig 6c) In literature this phenomenon is explained by the decomposition of hydrates which starts at 400 °C and almost ends at 900 °C Therefore, most of the dehydration takes place and concrete loses almost all of its initial strength and stability

at 900 °C due to loss of all chemical water[25] Culﬁk and Ozturan Fig 6 Decomposition of a specimen heated to 900 °C.

Fig 7 Average mass losses of specimens.

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exposed their specimens to 900 °C They also reported hair-like

cracks in ﬁrst day and then complete disintegration of concrete

specimens 1 day after cooling period[26] Only six specimens of

two series (T5F8A1 and T5F16A0.5) did not decompose These

specimens were exposed to water during the cutting process and

most probably were healed when came contact with water

[27–29] This recovery can be attributed to regeneration of

some of C–S–H bonds on rehydration[28,29] Detailed information

about disintegration of these specimens will be given in

Section3.5

3.4 Mass losses After exposure to high temperatures mass losses of the speci-mens, resulting mainly from water and carbon dioxide transport and loss, were recorded Residual masses of each series can be seen

inFig 7except the spalled series Average mass losses of speci-mens exposed to 300 °C, 600 °C and 900 °C were 5.2%, 9.8% and 12.9%, respectively

Results of thermogravimetric analyses were similar Average mass losses of powder samples at 300 °C, 600 °C and 863 °C were

0.00 0.01 0.02 0.03 0.04 0.05

Deriv Weight (%/°C)

88 90 92 94 96 98 100

Weight (%)

Temperature (°C)

TGA Instrument: TGA Q50 V6.7 Build 203

Universal V4.7A TA Instruments

Fig 8 TGA and DTG curves (F0A0).

Table 4

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

Mixes Control strength Residual strength

–

a

ES stands for explosive spalling.

b

CD stands for complete disintegration.

Table 5

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

Mixes Control strength Residual strength

a

ES stands for explosive spalling.

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4.6%, 7.9% and 11.6%, respectively These results were smaller than

the residual mass values found by weighing the full-size

speci-mens This is expected since the full-size specimens included

closed pores which may entrap extra moisture when compared

to powder samples Moreover, the specimens heated in the furnace

were held at their maximum temperatures for an hour before they

left for cooling, while the powder samples exposed to TGA test

were cooled down immediately

Fig 8represents the TGA and Differential Thermo Gravimetry

(DTG) curves As shown in DTG curve, there are three peaks on the

graph and these peaks represent instantaneous mass losses with

temperature The ﬁrst peak is wider than others and starts with

the beginning of the heating (approximately 20 °C) and continues

until the complete evaporation of moisture of powder samples

(approximately 150 °C) Average mass loss of the samples is 3.6%

around this region The second peak appears between

approxi-mately 400 °C and 450 °C and corresponds to the loss of water from

portlandite[24,25,30–33] The instantaneous mass loss of the

sam-ples during second peak was 0.7% Finally, the third peak is the

high-est and appears between approximately 550 °C and 700 °C and corresponds to the loss of carbon dioxide ensuing from the carbon-ation products and the loss of water ensuing from the decomposition

of calcium silicate hydrates (C–S–H)[24,25,30–32] During the last peak mass of the samples reduced approximately 4% This explains the serious loss of strength after 600 °C TGA and DTG curves ob-tained for the other samples were not published here since they were very similar to the one given below for F0A0 sample

3.5 Residual strength Compressive strength tests were conducted on three specimens

of the control series before beginning heating cycles Compressive strength values ranged from 80 MPa to 130 MPa at ambient tem-perature for different mixes

After exposure to high temperatures, the residual compressive strengths of specimens were measured Residual strength mea-surements could not be conducted for non-ﬁbrous specimens after exposure to 600 °C, since all the specimens were ruined due to explosive spalling The overall results of the residual compression test show that the concrete specimens exposed to lower tures are stronger than the specimens exposed to higher tempera-tures as expected.Tables 4 and 5show the original and residual strength of all series

An important note should be given here.Tables 4 and 5show some residual strength values for the specimens heated up to

900 °C However, these values may be misleading since all of these specimens were found to disintegrate couple of days after testing The testing program was prepared such that specimens of T10 ser-ies were tested before T5 serser-ies All the specimens of T10 serser-ies were tested 1 day after heating The specimens showed some residual strength as is seen inTable 5 Unfortunately, after couple

of days the tested specimens were found completely disintegrated Having that in mind, it was decided to test 2 series of the 5 cm thick specimens immediately, while keeping the rest of them in the laboratory environment for 3 days to check if the same phe-nomenon will occur The specimens that were spared to be tested immediately were ﬁrst cut to obtain 5 5 5 cm cubes and then tested The specimens of these 2 series (T5F8A1 and T5F16A0.5) showed some residual strength as can be seen inTable 4, while the specimens kept in the laboratory environment were completely disintegrated in 3 days The specimens that were not

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

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

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

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disintegrated during and after the test were exposed to water

dur-ing the cuttdur-ing process and most probably were healed when came

contact with water[27–29]as explained before in the end of

Sec-tion3.3 This recovery can be attributed to regeneration of some of

C–S–H bonds on rehydration[28,29] The parts of these specimens

(T5F8A1 and T5F16A0.5) further observed for couple of months

after the test and no disintegration was observed This is an

impor-tant information and may be the subject of another project

Table 4shows that residual strength increases when air

entrain-ing admixture was used for all the specimens with a thickness of

5 cm when the specimens were heated to 300 °C and 600 °C,

respectively A similar comment cannot be made for the specimens

heated to 900 °C since most of them were completely

disinte-grated This result is also valid for some of the 10 cm thick

speci-mens Residual compressive strengths (%) of specimens at various

temperatures with different air entrainment are shown inFig 9

According to the results, air entrainment affects the residual

com-pressive strength of concrete However, air entrained non-ﬁbrous

specimens exploded above 600 °C With the absence of

micro-channels formed by melted PP ﬁbers, this situation can be

ex-plained by the increased pore pressure in the closed air voids of

concrete Air entrained specimens lost strength less than others

(except T10F16 series) at 300 °C for all specimens and residual

strengths percentages of T5 series increased with air entrainment

at 600 °C However, the positive effect of air entrainment was not

clear on the specimens of T10 series after 300 °C Also, there is

not a prominent difference between the residual strength

percent-ages of A0.5 and A1 series

All the ﬁbrous specimens withstood heating cycles and they

were subjected to compressive strength test Residual compressive

strengths (%) of specimens at various temperatures with different

PP ﬁber ratios are shown inFig 10 Although PP ﬁbers prevented

spalling of specimens, their existence in material adversely

af-fected the residual compressive strength of HPC Adding more PP

ﬁber into HPC mixes had negative effect on the residual strength

of the specimens and these results conﬁrm the ﬁndings given in lit-erature[10,34]

Residual compressive strengths (%) of specimens at various tem-peratures with different specimen sizes are shown inFig 11 Size of the specimens affected the compressive strength of HPC The spec-imens with 10 cm height showed better performance at elevated temperatures and this effect is clearer when temperature was in-creased up to 600 °C (The specimens heated up to 900 °C were as-sumed to represent no residual strength since they were disintegrated 3 days after the test) Specimens with 5 cm height re-tained 77% and 54% and 20% of their ambient strength at 300 °C,

600 °C and 900 °C, respectively On the other hand, specimens with

10 cm height retained 89% and 60% of their original strength for

300 °C, 600 °C The residual strength results measured are in agree-ment with the values given in Eurocode 2 for concrete with sili-ceous aggregates which retains 85%, 45% and 8% of its initial strength at 300 °C, 600 °C and 900 °C, respectively[35] The differ-ence in residual strengths between the specimens with 5 cm height and the specimens with 10 cm heights can be a result of rapid heat-ing of specimens with smaller size as mentioned by Hertz[4] 3.6 ESEM observations

Selected specimens were examined by using an environmental scanning electron microscope for better evaluating the effects of high temperatures on the microstructures of the specimens The regions which are considered as matrix–aggregate interface were especially chosen to be examined since hydration products are supposed to develop in these regions [36] Energy dispersive X-ray spectroscopy (EDX) analyses were carried out to detect CSH phases Phase regions with a Ca/Si ratio between 0.8 and 2.1 are considered to be CSH regions [36,37] The regions examined in the scope of this study had a Ca/Si ratio around 1, 7 This result

(c)

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

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is similar to the average Ca/Si atomic ratio reported by Djaknoun

[36].Fig 12represents ESEM pictures taken on a specimen kept

at room temperature (20 °C).Fig 12a and b shows

aggregate–ma-trix interface Aggregate–maaggregate–ma-trix interface of the specimens kept at

the room temperature represents a continuous structure with no

pores and cracks Cement hydration products have well deﬁned

crystal structure, with portlandite and CSH (Fig 12c)

Fig 13a and b represent ESEM pictures of a specimen heated to

300 °C.Fig 13a shows matrix aggregate interface.Fig 13b shows a

magniﬁed view of the cement matrix given in Fig 13a Cement

paste still has crystal structure characteristics, however is not as

distinctive as it was for unheated specimen

Fig 14 shows pictures taken on the specimens heated up to

600 °C.Fig 14a again shows aggregate–matrix interface Structure

of the hydration products is amorphous.Fig 14b shows the cracks

on the surface As can be seen on the ﬁgure, cracks are spread all

over the surface of the specimen due to the important amount of

water loss from the structure

Fig 15shows pictures of a specimen heated up until 900 °C As

mentioned before, almost all of these specimens disintegrated

cou-ple of days after being removed from the electrical furnace (the

specimen shown below is one of the specimens which was not

dis-integrated) Fig 15a shows porous cement–aggregate interface

Fig 15b represent, the cement structure which has amorphous

structure, Fig 15c shows a cracked sand particle and ﬁnally

Fig 15d shows the cracks spread all over the specimen

ESEM pictures show the effect of high temperatures on the microstructure of HPC Concrete microstructure is highly damaged with an increased temperature leading to failure of the specimens

4 Conclusions Behavior of HPC under high temperatures is different than nor-mal concrete due to very dense microstructure Precautions should

be taken to decrease the damage occur when HPC exposed to high temperature In this study air entraining admixture was used to-gether with polypropylene ﬁbers to create channels for evacuating water vapor Following conclusions were drawn as the result of this study

Spalling of HPC seems to be dependent on presence of PP ﬁber

in concrete Explosive spalling was observed especially in non-ﬁbrous specimens and began after 500 °C For other HPC

(a)

(b)

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

(a)

(b)

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

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