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Construction and Building Materials 22 (2008) 2269–2275

and Building

MATERIALS

The effect of high temperature on compressive strengthand splitting tensile strength of structural lightweight concrete

containing fly ash

Harun Tanyildizi *, Ahmet Coskun

Department of construction education, Firat University Elazig, Turkey

Received 23 June 2006; accepted 12 July 2007Available online 5 November 2007

Abstract

In this study, the effect of high temperature on compressive and splitting tensile strength of lightweight concrete containing fly ash wasinvestigated experimentally and statistically. The mixes incorporating 0%, 10%, 20% and 30% fly ash were prepared. After being heatedto temperatures of 200, 400 and 800 �C, respectively, the compressive and splitting tensile strength of lightweight concrete was tested.This article adopts Taguchi approach with an L16 (45) to reduce the numbers of experiment. Two control factors (percentage of flyash and heating degree) for this study were used. The level of importance of these parameters on compressive and splitting tensilestrength was determined by using analysis of variance (ANOVA) method.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Lightweight concrete; Mechanical properties; High temperature; Fly ash; Analysis of variance (ANOVA)

1. Introduction

The demand for structural lightweight concrete in manyapplications of modern construction is increasing, owing tothe advantage that lower density results in a significantbenefit in terms of load-bearing elements of smaller crosssections and a corresponding reduction in the size of thefoundation [1]. Lightweight aggregates are broadly classi-fied in to two types-natural (pumice, diatomite, volcaniccinders, etc.) and artificial (perlite, expanded shale, clay,slate, sintered PFA, etc.). Lightweight concrete can easilybe produced by utilizing natural lightweight aggregatei.e., pumice or perlite aggregate [2]. Structural lightweightconcrete has its obvious advantages of higher strength/weight ratio, better tensile strain capacity, lower coefficientof thermal expansion due to air voids in the lightweightaggregate [3,4].

0950-0618/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2007.07.033

* Corresponding author. Fax: +90 424 2367064.E-mail address: htanyildizi@firat.edu.tr (H. Tanyildizi).

In view of the global sustainable development, it isimperative that supplementary cementing materials be usedin replace of cement in the concrete industry. The mostworldwide available supplementary cementing materialsare silica fume (SF), a by-product of silicon metal, andfly ash (FA), a by-product of thermal power stations [5].It is estimated that approximately 600 million tons of FAare available worldwide now, but at present, the currentworldwide utilization rate of FA in concrete is about 10%[6]. Due to the rapid economic development and thegrowth in the world population consumption of energyover the world, the FA has significantly increased. Airand environment pollution has became a problem, thus,the idea of using waste material has gained popularity.FA and SF are two of the most common concrete ingredi-ents due to their pozzolanic properties [6,7].

The effects of high temperature on the mechanical prop-erties of concrete have been investigated since the middle ofthe 20 century [7–14]. The fire resistance capacity of con-crete is complicated because not only is concrete a compos-ite material with components having different thermal

Table 2Levels of the variables used in the experiments

Variable Level 1 Level 2 Level 3 Level 4

Heating degree, H (�C) 20 200 400 800Fly ash, A (%) 0 10 20 30

2270 H. Tanyildizi, A. Coskun / Construction and Building Materials 22 (2008) 2269–2275

characteristics, it also has properties that depend on mois-ture and porosity [12]. A number of research studies indi-cated that the addition of silica fume highly densities thepore structure of concrete, which can result in explosivespelling due to the build-up of pore pressure by steam[13]. Since the evaporation of physically absorbed waterstarts at 80 �C which induces thermal cracks, such con-cretes may show inferior performance as compared to pureconcretes at elevated temperatures [14]. With the growth ofengineering structures, applications of structural light-weight concrete are increased day by day. Therefore, ques-tions about the performance of structural lightweightconcrete at high temperature need to examined.

One of the main purposes of this study is to investigatethe effect of experimental parameters on compressive andsplitting tensile strength of lightweight concrete exposedto high temperature. The effect of the experimental param-eters on compressive and splitting tensile strength was eval-uated statistically and the level of significance of theparameters affecting compressive and splitting tensilestrength was determined by using analysis of variance(ANOVA) method.

2. Experimental study

2.1. Materials

ASTM Type I Portland cement (PC) which produced asTS EN 197-1-CEM 42.5 in Turkey was used in this study.Fly ash (F class according to ASTM C618) from Orhaneli(Bursa, Turkey) power plant was selected for this work.The chemical analysis properties of the cement and flyash are presented in Table 1. Scoria aggregate was obtainedfrom natural deposits in Elazig city (Turkey). Lightweightaggregate was scoria aggregate with a specific gravity of 2,a maximum grain size 16 mm and water absorption bymass of 23%.

2.2. Design of experiments

Design of experiments is a power analysis tool for mod-eling and analyzing the influence of process variables oversome specific variable [15]. The most important stage in thedesign of experiment lies in the selection of the control fac-

Table 1The chemical properties of cement and fly ash

Bulk oxide % by mass

Portland cement Fly ash

SiO2 21.12 48.53Al2O3 5.62 24.61Fe2O3 3.24 7.59CaO 62.94 9.48MgO 2.73 2.28LOI 1.42 1.69Specific surface area (cm2/g) 3430 2836Specific gravity (g/cm3) 3.10 2.27

tors. As many as possible should be included, so that itwould be possible to identify non-significant variables atthe earliest opportunity [16]. In general, the compressiveand splitting tensile strength experimental parameters ismainly depend on the manufacturing conditions employed.Table 2 shows the detail of the variables used in the exper-iment. It is noted that there is parameters at 4 levels. Only16 experiments are needed to study the entire experimentalparameters using the L16 (45) orthogonal array.

2.3. Mix proportions

The weight of used materials in the mix design is given inTable 3. In mixtures containing fly ash, 0%, 10%, 20% and30% of Portland cement by weight was replaced with flyash. A superplasticizer was used to improve the workabil-ity. The cubic specimens (100 · 100 · 100 mm) wereprepared to determine the effect of different temperatureson compressive strength. The cylinder specimens(100 · 200 mm) were prepared to determine the effect ofdifferent temperatures on splitting tensile strength. Foreach temperature, three specimens were prepared.

2.4. Curing and heating regimes

The specimens were demolded 24 h after the casting andplaced in a water tank at 20 �C. After 28 days water curing,they were heated in an electric furnace up to 200, 400 and800 �C. Each temperature was maintained for 1 h toachieve the thermal steady state [17]. The specimens wereallowed to cool naturally to room temperature.

3. Results

3.1. Experimental finding

The changes of mechanical properties of concrete sub-ject to high temperature are dependent on materials as wellas environmental factors (such as initial strength beforeexposure to high temperature, moisture content, and so

Table 3Mixture proportion of concretes

Designationof mixture

Cement(kg/m3)

Fly Ash(kg/m3)

W/C PumiceAggregates(kg/m3)

Superplasticizer(kg/m3)

M 500 – 0.77 820 6H 450 50 0.77 805 6A 400 100 0.77 795 6E 350 150 0.77 780 6

Table 4Retained compressive strength after exposed to high temperatures

Retained compressive strength (%) Temperature (�C)

20 200 400 800

M 100 97.95 92.6 36.13H 100 98.34 86.99 39.44A 100 93.77 84.76 43.64E 100 91.09 80.23 42.15

Table 5Retained splitting tensile strength after exposed to high temperatures

Retained splitting tensile strength (%) Temperature (�C)

20 200 400 800

M 100 87.84 81.94 23.55H 100 88.81 83.72 26.03A 100 90.27 85.55 42.04E 100 91.85 84.63 43.15

H. Tanyildizi, A. Coskun / Construction and Building Materials 22 (2008) 2269–2275 2271

on) [18]. In this study, the specimens were used to investi-gate the most unfavorable situation (high temperatures orfire) for structural lightweight concrete. The cubic speci-mens (100 · 100 · 100 mm) for compressive strength andthe cylinder specimens (100 · 200 mm) for splitting tensilestrength were prepared. After they were heated in an elec-tric furnace up to 200, 400 and 800 �C, the specimens weretests. The compressive strength and splitting tensilestrength results after exposed to high temperature are givenin Figs. 1 and 2. The test results indicated that each temper-ature range had a distinct pattern of strength loss. It can beseen form Figs. 1 and 2 that the compressive strength andsplitting tensile strength of structural lightweight concretedrops with temperature starting from 200 �C in this study.

It can be seen from Figs. 1 and 2 that the compressivestrength decreased with the increase of temperature. Thereduction in compressive strength can be attributed to thedriving out of free water and fraction water of hydrationof concrete due to high temperatures or fire. Dehydrationof concrete causes a decrease in its strength, elasticmodulus, coefficient of thermal expansion and thermalconductivity [19]. An increase in strength was observed inFA concrete due to the formation of tobermorite [20].

0

10

20

30

40

50

60

0 20 200 400 800 1000Temperature (ºC)

Com

pres

sive

str

engt

h (M

Pa) M

HAE

Fig. 1. Compressive strength results after exposure to high temperature.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 20 200 400 800 1000Temperature (ºC)

Split

ting

tens

ile s

tren

gth

(MPa

) MHAE

Fig. 2. Splitting tensile strength results after exposure to hightemperature.

Tables 4 and 5 show the retained compressive strengthand splitting tensile strength after exposed to high temper-atures. It can be seen from Table 4 that the compressivestrength value drops sharply to 63.87% compared to thatof specimens unfired after 800 �C. It can be seen fromTable 5 that the splitting tensile strength value drops shar-ply to 76.45% compared to that of specimens unfired after800 �C. These losses in compressive strength and splittingtensile strength show the occurrence of micro and macrocracks in structural lightweight concrete because of hightemperature. The decrease in the compressive strengthand splitting tensile strength decreased with increased offly ash content; in the other words, the fly ash increasedthe resistance of concrete against high temperature.

3.2. The surface characteristics of samples

For concrete subject to 200 �C, color does not change,while straw yellow, off white when the concrete are exposedto temperature of 400 and 800 �C, respectively. Therefore,by combining changes in rules of strength, color, and tem-perature during fire, the retained compressive strength canbe inferred primarily. This will provide some reference forstructure in practice.The reduction in compressive strengthand the change in color of concrete result from the changein structure and composition of concrete during firing.Some complicated processes of shrinkage, decomposition,expansion, and crystal destruction occur during fire. Crys-tal shape transformation of SiO2 results in the increase ofvolume up to 0.85%. Dehydration, followed by the hydro-scopic process of Ca(OH)2, also makes volume expansive.Moreover, expansion caused by temperature rise andshrinkage caused by dehydration of cement paste ulti-mately results in a volume change of 0.5% [21].

Fig. 3 shows the surface character of samples afterexposed to high temperature. The surface cracks startedto appear at round 400 �C and continued to grow till thefinal rise in temperature up to 800 �C. Many surface cracks

Fig. 3. Surface character of samples after high temperature.

Temperature (ºC)

Split

ting

tens

ile s

tren

gth

(MPa

)

9008007006005004003002001000

3.5

3.0

2.5

2.0

1.5

1.0

Fig. 5. Plot of splitting tensile strength values in relation to thetemperature.

2272 H. Tanyildizi, A. Coskun / Construction and Building Materials 22 (2008) 2269–2275

occur on sample B after 800 �C. However, few cracks hap-pen on sample A after 400 �C. The crack decreased with theincrease of fly ash content and the same time increased withthe increase of temperature. It can be said that in the build-ings damaged by fire, by examining the color of concretesurface, we can have some ideas about the change in theconcrete strength [22].

3.3. Statistical modeling

In this part of study, the statistical relation (model)between compressive strength, splitting tensile strengthand temperature for predicting compressive strength, split-ting tensile strength values is introduced. Regression anal-ysis method is usually used to obtain this type of relation.In this study, non-linear regression analysis was used toestablish a model between the experimentally obtained val-ues and temperature (see Figs. 4 and 5).

The statistical model of the compressive strength wasexpressed as

Compressive strength

y1 ¼ 43:48þ 0:00033x� 0:000041x2 ð1ÞThe statistical model of the compressive strength was

expressed as

Temperature (ºC)

Com

pres

sive

str

engt

h (M

Pa)

9008007006005004003002001000

50

40

30

20

10

Fig. 4. Plot of compressive strength values in relation to the temperature.

Splitting tensile strength

y2 ¼ 3:288� 0:000218x� 0:000003x2 ð2Þwhere, y1 is compressive strength, y2 is splitting tensilestrength, x is temperature. The high correlation coefficient(0.93 and 0.94) obtained form regression analysis indicatedthe suitability of the used relation (power function) and thecorrectness of the calculated constants Eq. (1) was used toestimate the compressive strength values without experi-mentation with 7% deviation from the experimental results.Eq. (2) was used to estimate the splitting tensile strengthvalues without experimentation with 6% deviation fromthe experimental results.

3.4. Statistical analysis of the results

In this study, ANOVA and F-test were performed to seestatistically significant process parameters and percent con-tribution of the parameters on the compressive and split-ting tensile strength of structural lightweight concrete.Larger F value indicates that the variation of the processparameter makes a big change on the performance charac-teristics [22–24].

In the ANOVA, a loss function is used to calculate thedeviation between the experimental value and the desiredvalue. Some characteristics, such as bond strength, com-pressive strength and splitting tensile strength, also donot negative values. Such characteristics are called largerthe better type quality characteristics. Larger the character-istics gives the better performance (percentage of fly ashand heating degree) in compressive and splitting tensilestrength of structural lightweight concrete. Therefore,the ‘‘larger is better (LB)’’ loss function for this studywas selected to obtain the optimal conditions. The lossfunction Lij of LB performance characteristic can beexpressed as

Lij ¼1

n

Xn

k¼1

1

y2ijk

ð3Þ

where Lij is the loss function of the ith performance charac-teristic in the jth experiment, n the number of tests, and yijk

22

24

26

28

30

32

34

Mea

n S/

N ra

tio (d

B)

H. Tanyildizi, A. Coskun / Construction and Building Materials 22 (2008) 2269–2275 2273

the experimental value of the ith performance characteris-tics in the jth experiment at the kth test.

The loss function is further transformed into a signal-to-noise (S/N) ratio for determining the performance charac-teristic deviating from the desired value. The S/N ratio nij

for the ith performance characteristic in the jth experimentcan be expressed as

nij ¼ �10 logðLijÞ ð4ÞThe control factors and the levels given in Table 2 were

used for calculating the S/N ratio. The S/N ratios of thecompressive and splitting tensile strength were calculatedfor each level of the experimental parameters given inTable 2. The S/N ratio results are shown in Tables 6 and7. Figs. 6 and 7 show the S/N response graph of each levelof the experimental parameters for the compressive andsplitting tensile strength.

Based on the S/N and ANOVA method, the maximumthe compressive and splitting tensile strength was obtained

Table 6S/N ratio of compressive strengtha

Control factor Mean S/N ratios (dB)

Level 1 Level 2 Level 3 Level 4

Heating degree,H (�C)

32.765003b 32.341159 31.457683 24.856977

Fly ash, A (%) 29.293325 30.013837 30.902372 31.211288b

a Overall mean S/N ratio = 30.355205 dB.b Optimum level.

Table 7S/N ratio of splitting tensile strengtha

Control factor Mean S/N ratios (dB)

Level 1 Level 2 Level 3 Level 4

Heating degree, H (�C) 10.457483b 9.511354 8.937922 0.687851Fly ash, A (%) 6.265016 6.754062 8.160670 8.414862b

a Overall mean S/N ratio = 7.398652 dB.b Optimum level.

Table 8Results of ANOVA for compressive strength

Control factor Degrees of freedom (f) Sum of square

Heating Degree, H (�C) 3 164.788437Fly Ash, A (%) 3 9.105564Error 9 2.527251Total 15 176.421252

Table 9Results of ANOVA for splitting tensile strength

Control factor Degrees of freedom (f) Sum of square

Heating Degree, H (�C) 3 244.896595Fly Ash, A (%) 3 13.255927Error 9 15.805915Total 15 273.958436

at 20 �C and 30% fly ash admixture (Tables 6 and 7 andFigs. 6 and 7). These findings were consistent with theexperimental results. The results of ANOVA for the com-pressive and splitting tensile strength are presented inTables 8 and 9.

The strongly effective parameters on compressive andsplitting tensile strength are obtained at 800 �C and 30%fly ash admixture. The analysis indicated that the experi-mental error was very low level (1.43% and 5.77%). Figs.8 and 9 show the relative importance of the experimental

20

H1 H2 H3 H4 A1 A2 A3 A4

Compressive strength parameters level

Fig. 6. S/N response graph of experimental parameters for compressivestrength.

H1 H2 H3 H4 A1 A2 A3 A4

0

2

4

6

8

10

12

14

Splitting tensile strength parameters level

Mea

n S/

N ra

tio (d

B)

Fig. 7. S/N response graph of experimental parameters for splitting tensilestrength.

(SSA) Variance (VA) FA0 Contribution (%)

54.929479 195.613882 93.4062283.035188 10.808858 5.1612630.280806 – 1.432509

– – 100

(SSA) Variance (VA) FA0 Contribution (%)

81.632198 46.481953 89.3918794.418642 2.516006 4.8386631.756213 – 5.769457

– – 100

Fly Ash. A (%)

5.16%Error

1.43%

Heating Degree. H

(ºC)93.41%

Fig. 8. The effect of experimental parameters on compressive strength.

Fly Ash. A(%)

4.84%Error

5.77%

Heating Degree. H

(°C)89.39%

Fig. 9. The effect of experimental parameters on splitting tensile strength.

2274 H. Tanyildizi, A. Coskun / Construction and Building Materials 22 (2008) 2269–2275

parameters used in this study on the compressive and split-ting tensile strength. As shown clearly from the figure, theheating degree has an utmost importance on compressiveand splitting tensile strength.

4. Conclusion

In this study, the effect of experimental parameters,namely heating degree and fly ash, on the compressiveand splitting tensile strength is investigated experimentallyand statistically. Specific findings of this research includethe following:

� It is found experimentally that the highest the compres-sive and splitting tensile strength results were obtainedfrom the specimens with 30% fly ash.

� The compressive strength value drops sharply to 63.87%compared to that of specimens unfired after 800 �C. Thesplitting tensile strength value drops sharply to 76.45%compared to that of specimens unfired after 800 �C.� The fly ash is prevented the decrease of concrete strength

against high temperature. It shows that fly ash contrib-utes to the interfacial properties mainly by the pozzola-nic effect.� Color changes were observed on concrete surface

because of the effect of high temperature. Therefore,we can have some ideas about the change in the concretestrength.� Based on the S/N, the optimum parameters for the com-

pressive and splitting tensile strength were obtainedfrom specimens containing 30% fly ash and 20 �C. Basedon ANOVA and F-test, the most effective parameters onthe compressive and splitting tensile strength were foundas heating degree and fly ash, respectively.

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