Construction

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Construction and Building Materials 22 (2008) 504–512

and Building

MATERIALS

High temperature resistance of normal strength and autoclavedhigh strength mortars incorporated polypropylene and steel fibers

Serdar Aydın *, Halit Yazıcı, Bulent Baradan

Department of Civil Engineering, Engineering Faculty, Dokuz Eylul University, Buca 35160, _Izmir, Turkey

Received 21 February 2006; received in revised form 2 November 2006; accepted 20 November 2006Available online 11 January 2007

Abstract

The effects of high temperatures up to 900 �C on normal strength, water cured and autoclave cured high strength mortars were inves-tigated within the scope of this study. Mechanical properties such as, compressive strength, flexural strength, modulus of elasticity, andweight loss of the specimens were determined. The effect of polypropylene (PP) and steel fibers incorporation on high temperature behav-ior of high strength mortars were also investigated. Microstructure investigations were also carried out by SEM analyses. Test resultsindicated that compressive strength of all mixtures increased with increasing temperature up to 300 �C. However, above 300 �C all spec-imens showed explosive disintegration except normal strength mortars, and high strength mortars with PP fibers. It seems that, PP fiberinclusion is effective in preventing the explosive spalling behavior of high strength mortars. On the other hand, steel fibers did notimprove high temperature resistance. To prevent spalling, minimum PP dosage for autoclave and water cured high strength mortarshas been determined as 0.2% and 0.1%, respectively.� 2006 Elsevier Ltd. All rights reserved.

Keywords: High temperature; Autoclave curing; High strength mortar; Steel fiber; Polypropylene fiber; Mechanical properties; SEM

1. Introduction

High performance concrete and mortars with highstrength and durability properties have been graduallyreplacing normal strength concrete, especially in structuresexposed to severe loading and environmental conditions.As they become more commonly used, the chance of beingexposed to the high temperatures also increases. Theadvantages of high performance concrete result from theimprovement of internal structure of the material as com-pared with that of normal mortar and concrete. The densemicrostructure of high performance concrete and mortarsensures a high strength and a very low permeability.

Today, generally high pressure steam curing whichknown as ‘‘autoclave curing’’ has been employed in themanufacture of precast products when any of the followingcharacteristics are desired: high early strength, high dura-

0950-0618/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2006.11.003

* Corresponding author. Tel.: +90 232 412 7044; fax: +90 232 412 7253.E-mail address: serdar.aydin@deu.edu.tr (S. Aydın).

bility, and reduced drying shrinkage and moisture move-ment [1]. These concrete products are manufactured byadding silica in a reactive form in appropriate quantities.At an optimum silica content of 30%, the autoclavedcement paste is significantly stronger than the normallycured paste. In practice, silica can be added to cement pastein the form of slag, fly ash, silica fume or siliceous fineaggregate [2]. However, the dense microstructure of auto-clave cured or water cured high performance concreteseems to be a disadvantage in the situation where the fireendurance is a necessity. The absence of voids whichrelieves the internal stress creates a major problem. Thisproblem can be solved by fiber addition to the mixture.However, only a relatively small number of studies havebeen carried out on high performance concrete and mor-tars subjected to high temperature and they also revealedcontrary results. Chan et al. [3] studied the effects of hightemperature on normal and high strength concrete up to1200 �C. Test results indicated that high strength concretesuffered marginally smaller loss of compressive strength

Table 1Physical, chemical and mechanical properties of cement, slag and silicafume

Cement Slag (GGBFS) Silica fume (SF)

Chemical composition (%)

SiO2 18.69 35.71 92.26Al2O3 5.00 14.52 0.89Fe2O3 3.49 0.80 1.97CaO 63.12 32.13 0.49MgO 1.09 9.39 0.96Na2O 0.29 – 0.42K2O 0.76 – 1.31SO3 2.95 – 0.33Cl� 0.010 – 0.09Loss on ignition 3.56 – –Insoluble residue 0.38 – –Free CaO (%) 1.27 – –

Physical properties of cement

Specific gravity 3.13Initial setting time (min) 130Final setting time (min) 210Volume expansion (mm) 1.00Specific surface (m2/kg) 380

Compressive strength (MPa)

2 days 29.97 days 43.228 days 51.9

Pozzolanic activity index (%)

GGBFS (28 days) 108SF (28 days) 115

Specific surface

GGBFS (m2/kg)-Blaine 485SF (m2/kg)-Nitrogen Ab. 20,000

S. Aydın et al. / Construction and Building Materials 22 (2008) 504–512 505

but a greater worsening of the permeability-related durabil-ity than normal strength concrete. Chan et al. [4] preparedthree high strength concretes mixtures incorporating steelfiber and polypropylene fiber independently. Steel fibersreduced the deterioration of concrete to some extent. Poly-propylene fibers used to prevent spalling of high strengthconcrete did not lead to a marked degradation in residualstrength even if it evaporated at high temperatures. Lieand Kodur [5] studied thermal and mechanical propertiesof steel fibre-reinforced concrete at elevated temperatures.They concluded that concrete heat resistance could be per-formed by incorporating steel fibres. On the other hand,Hertz [6] stated that the presence of steel fibers did notreduce the risk of explosion. Moreover, specimens withthe highest steel fiber content were most likely to explode.And also, fire tests on reinforced column specimens real-ized by Kodur et al. [7] showed that incorporation of poly-propylene fibers extended fire resistance time of highstrength concrete. This result may be attributed to the for-mation of voids left after the evaporation of polypropylenefibers. Furthermore, the presence of steel or polypropylenefibers and the type of aggregate in concrete influence theextent of spalling. The extent of spalling was highest inthe column which was made with siliceous aggregate. Thepresence of carbonate aggregate helped in minimizingspalling. The minimum spalling occurred on the columnwhich was containing polypropylene fibers. Bilodeauet al. [8] investigated the required amount of polypropylenefibers for preventing the spalling of lightweight concretesubjected to hydrocarbon fire. Results from this studyshow that close to 3.5 kg of the 20 mm polypropylene fibersper cubic meter of concrete is required to prevent the spall-ing of a low w/c lightweight concrete made with a silicafume-blended cement when subjected to hydrocarbon fire.However, only 1.5 kg of finer 12.5 mm fibers per cubicmeter has been found sufficient. Diederichs et al. [9] andNishida et al. [10] showed that deleterious spalling of highstrength concrete can be greatly reduced by adding smallquantities (on the order of 0.1% by volume) of fibres madefrom a low melting-point polymer to the concrete.

It is well known that, concretes densified by means of sil-ica fume are more likely to explode. However, it was pos-sible to compose a concrete with a silica content of 10%by weight of cement that did not show a significantlyincreased risk of explosion compared to a similar concretewith no silica fume [6]. On the other hand, this limit is givenas 5% by Poon et al. [11].

However, the effects of high temperature on autoclave-cured mortars have not been investigated widely.

2. Experimental work

Ordinary Portland cement, ground granulated blast fur-nace slag (GGBFS), silica fume, crushed basalt with a max-imum size of 4 mm, a superplasticizer as well as steel andpolypropylene fibers were the main materials of thisresearch. The physical, chemical and strength characteris-

tics of Portland cement (CEM I 42,5N) are given in Table1. GGBFS has been procured from Iskenderun iron-steelplant, Turkey. The chemical composition and other prop-erties of GGBFS and silica fume are also presented inTable 1. Physical properties and grading of basalt used inthis study are given in Table 2. A sulfonated naphthaleneformaldehyde based superplasticizer on complying withASTM C 494-99 [12], type F and EN 934-2 [13] has beenused. The length and aspect ratio of steel fibers were70 mm and 50, respectively. Steel fiber was used 1% by vol-ume. The length of PP fibers was 12 mm and their aspectratio was 240. In the initial stage of experiments, optimumPP fiber ratios which prevent explosive spalling behaviorwere determined. According to these initial test results, inthe second stage 0.1% and 0.2% PP fiber contents were cho-sen for water cured and autoclave cured mortars,respectively.

Four types of mortar mixtures were prepared, normalstrength (NS), high strength (HS), high strength with 1%steel fiber (HSSF), and high strength with polypropylenefiber (HSPP) with various PP dosages. No silica fumeand superplasticizer were used in the NS mortar. Additionof steel and PP fibers to high strength mortars (HSPP andHSSF) has also increased the superplasticizer demand. Inorder to keep the workability constant, these mixtures have

Table 2Grading and physical properties of basalt

Sieve size (mm) % Passing

4 100.02 73.01 46.60.5 28.80.25 13.8

Physical properties

Bulk specific gravity 2.75Water absorption (%) 0.78

Unit weight (kg/m3)

Loose 1546Compacted 1819

506 S. Aydın et al. / Construction and Building Materials 22 (2008) 504–512

more superplasticizer than the others. The material compo-sitions of all mixtures are given in Table 3.

The mixtures were prepared by a Hobart mixer. Testspecimens were cast from the same batch into the prismatic(40 · 40 · 160 mm) and cylindrical (50/100 mm) steelmolds. The specimens were kept in the molds for 12 h atroom temperature of about 20 �C. After demolding, onegroup of specimens was kept in water 20 ± 2 �C until test-ing. The other group high strength mortar was autoclavedat 210 �C and 2.0 MPa for 6 h. Maximum temperature andpressure levels were reached after 1.5 h. After completionof their curing periods, the specimens which were subjectedto heat treatment, were cooled in laboratory atmosphere.Water cured and autoclave cured specimens were subjectedto the high temperatures at the age of 28 and 2 days,respectively.

After the curing period, three prismatic and three cylin-der specimens from each mixture were exposed to 300, 600and 900 �C for three hours in oven. All of the water curedspecimens exposed to temperature in saturated condition.The heating rate was set at 10 �C/min. Curing historyand high temperatures subjected to specimen are given inTable 4. The hot mortar specimens were slowly cooled inlaboratory conditions. At the end of the cooling period,the prismatic specimens were subjected to flexural strength

Table 3Mixture proportion

Component NS HS HSSFa

Cement (kg/m3) 375 335 335Blast furnace slag (kg/m3) 124 168 168Silica fume (kg/m3) – 168 168Water (kg/m3) 248 203 203Basalt (kg/m3) 1490 1345 1345Superplasticizer (L/m3) – 13.6 20.0Steel fiber (kg/m3) – – 78Polypropylene fiber (kg/m3) – – –Fiber volume (%) – – 1Water/cement 0.66 0.61 0.61Water/binder 0.50 0.30 0.30Aggregate/binder 3.0 2.0 2.0Flow (mm) 111 107 108

a Mass not adjusted to steel fibre addition.

test. The specimens were loaded from their mid span andthe clear distance between simple supports was 120 mm.The two broken pieces left from flexural test were subjectedto compressive strength test. The modulus of elasticity wasdetermined on cylinder specimens using with a 10 mm longstrain gage. The flexural strength, compressive strength andmodulus of elasticity test results were compared with thetest results of non-exposed control specimens. SEM analy-sis has been implemented on water cured and autoclavecured HSPP mortars exposed to temperature of 600 and900 �C after cooling period. Samples were taken from innerpart of the mortar specimens (�10 mm from the edge) andcoated with gold prior to examination.

3. Results and discussion

The effect of PP dosage on high temperature behavior ofHS mortars for both water and autoclave curing was inves-tigated on mortar specimens containing 0.4, 0.3, 0.2 and0.1% PP in the first stage of study. This experimental pro-gram was implemented at 600 �C and 900 �C which causeexplosive spalling in HS specimens. The mechanical prop-erties of mixtures before and after high temperature expo-sure are presented in Table 5. A 0.1% PP fiber volume(0.9 kg/m3) content seems to be a proper ratio to preventspalling for water cured mortars. This finding is in confor-mity with Diederichs et al. [9] and Nishida et al. [10] results.On the other hand, in autoclave curing case, to preventexplosive spalling, 0.2% PP fiber volume (1.8 kg/m3) isrequired. This difference possibly results from the denserstructure of the autoclaved mortars. It is well known thatif there is enough fine silica in the mixtures autoclave pro-vides perfect conditions for complete and acceleratedhydration media for hydration process especially with com-bined mineral admixtures [14]. Residual compressivestrength of water cured and autoclave cured specimensare given in Figs. 1 and 2, respectively. It is obvious thatincrease in PP dosage beyond 0.2% did not change themechanical performance considerably. Due to this finding,

HSPP1 HSPP2 HSPP3 HSPP4

335 335 335 335168 168 168 168168 168 168 168203 203 203 2031345 1345 1345 134515.3 17.0 18.3 20.0– – – –0.9 1.8 2.7 3.60.1 0.2 0.3 0.40.61 0.61 0.61 0.610.30 0.30 0.30 0.302.0 2.0 2.0 2.0108 108 109 108

Table 4Curing history and high temperatures subjected to specimens

Mixtures Curing High temperature

First stage

HSPP1, HSPP2, HSPP3, HSPP4 Standard (20 �C in water) 600 and 900 �C for 3 hAutoclaving (2 MPa, 210 �C)

Second stage

NS, HS, HSPP1, HSSF Standard (20 �C in water) 300, 600 and 900 �C for 3 hHS(ac), HSPP2(ac), HSSF(ac) Autoclaving (2 MPa, 210 �C)

Table 5Mechanical properties of mortars that contain different amounts (0.1–0.4%) of PP fiber

Temperature (�C) Compressive strength (MPa) Flexural strength (MPa)

0.1 0.2 0.3 0.4 0.1 0.2 0. 3 0. 4

Water cured mortars (28 days)

20 94.7 94.1 96.5 96.0 11.9 12.6 12.0 12.0600 127.7 121.4 119.9 123.9 7.7 7.4 8.5 8.4900 30.2 34.4 33.2 37.0 3.5 5.0 4.3 4.1

Autoclave cured mortars (2 days)

20 115.0 116.2 115.9 121.0 16.5 16.9 16.7 20.2600 Exp. 125.1 110.8 112.7 Exp. 10.3 10.8 12.7900 Exp. 33.5 36.5 45.9 Exp. 5.6 5.2 5.4

0

20

40

60

80

100

120

140

160

20 600 900

Temperature, ˚C

Res

idua

l com

pres

sive

str

engt

h, %

0.1% 0.2% 0.3% 0.4%

Fig. 1. The effect of PP fiber content on residual compressive strength ofwater cured mortars.

0

20

40

60

80

100

120

20 600 900Temperature, ˚C

Res

idua

l com

pres

sive

str

engt

h, %

0.1% 0.2% 0.3% 0.4%

Fig. 2. The effect of PP fiber content on residual compressive strength ofautoclave cured mortars.

S. Aydın et al. / Construction and Building Materials 22 (2008) 504–512 507

in the second stage of this research optimum PP contentswere chosen as 0.1% and 0.2% for water cured and auto-clave cured specimens, respectively.

The compressive strength, flexural strength, and modu-lus of elasticity of all mortar specimens before and afterhigh temperatures are presented in Table 6. It can be seenfrom Table 6, improvement on compressive strength byautoclave curing in 2 days is between 23% and 54% com-pared to the water cured samples of 28 days age. Similarly,the increase in flexural strength is about 39–44%. Steel andPP fiber addition to the HS mortar caused increases incompressive and flexural strength for water curing. Auto-clave curing also increased the flexural and compressivestrength of HS mortars containing steel fibers. On the otherhand, using PP fiber decreased the compressive strengthdue to melting of PP. From the point of modulus of elastic-ity, SF incorporation increased the modulus of elasticity,whereas PP fibers decreased for both autoclave and stan-dard water curing cases.

It must be mentioned here that, 50% of total binder con-sists of pozzolanic materials (25% GBFS + 25% silicafume) in HS mortars. In other words, although HS mortarsshowed extremely high compressive strengths, cement dos-age of these mixtures was merely 335 kg/m3. However inmost cases cement content of high strength concrete (over100 MPa compressive strength) is more than 500–600 kg/m3.

Test results showed that, above 300 �C, all mortar spec-imens except NS and both water cured and autoclave curedHSPP mixtures exploded, causing degradation of all spec-imens. Even though, the water cured HSPP1 specimenswere in saturated condition with 25% silica fume (by binder

Table 6Mechanical properties of mixtures before and after high temperature exposure

Maximum temperature (�C) Compressive strength (MPa) Flexural strength (MPa) Modulus of elasticity (GPa)

NS HS HSSF HSPPa NS HS HSSF HSPPa NS HS HSSF HSPPa

Water cured mortars (28 days)

20 49.0 90.7 111.4 94.7 9.6 11.8 13.0 11.9 34.0 46.0 54.2 44.7300 61.2 139.9 188.3 134.2 10.3 12.3 12.0 12.9 25.5 30.4 33.6 30.9600 55.9 Exp. Exp. 127.7 7.9 Exp. Exp. 7.7 13.0 Exp. Exp. 15.0900 10.4 Exp. Exp. 30.2 0.8 Exp. Exp. 3.5 7.0 Exp. Exp. 8.5

Autoclave cured mortars (2 days)

20 – 139.3 163.0 116.2 – 17.0 18.1 16.9 – 50.4 56.2 48.6300 – 155.5 209.7 148.4 – 9.3 13.9 13.2 – 32.2 38.4 35.7600 – Exp. Exp. 125.1 – Exp. Exp. 10.3 – Exp. Exp. 19.7900 – Exp. Exp. 33.5 – Exp. Exp. 5.6 – Exp. Exp. 10.4

a In water cured 0.1% PP fiber, in autoclave cured 0.2% PP fiber were used.

508 S. Aydın et al. / Construction and Building Materials 22 (2008) 504–512

weight) did not explode up to 900 �C. Spalling, especiallyexplosive type, when subject to fire or rapid temperaturerise is a major disadvantage of high performance concrete(HPC). However, incorporation of polypropylene fiberscreated voids that permit the release of water vapour pres-sure by burning off within the matrix of HPC. These resultshave been shared by other researchers [4,8–10,13,15].Although PP fibers had been melted during autoclave cur-ing at about 210 �C before high temperature exposure inoven, it seems that similar mechanism to prevent the spall-ing is also valid for autoclaved high strength mortar.

3.1. Residual compressive strength

Relative residual compressive strength of water curedand autoclave cured (ac) mortar specimens are presentedin Fig. 3. Relative residual compressive strength showsthe ratio between strength of exposed specimens and con-trol specimens. The residual compressive strength of allspecimens has been increased about 12–69% at 300 �C.The strength gain up to 300 �C may be due to the reliefof pressures by drying, which also creates greater van derwalls forces, resulting a closer configuration of capillarypores. The strength gain may also be explained with the

0

20

40

60

80

100

120

140

160

180

0 300 600 900Temperature (˚C)

Res

idua

l com

pres

sive

str

engt

h (%

)

NS HS HSSF HSPP1HS-ac HSSF-ac HSPP2-ac

Fig. 3. Residual compressive strength vs high temperatures.

formation of tobermorite which was formed by the reac-tion between the unhydrated blast furnace slag particlesand lime [11]. Furthermore, it can be observed fromFig. 3, strength gain at this temperature level is greaterfor water cured mortars than autoclave cured ones. Itmay be due to autoclave cured mortar have less lime andunhydrated blast furnace slag than water cured ones.Moreover, water cured specimens were in saturated condi-tion which possibly caused extra curing effect at elevatedtemperatures. The highest increment was observed forwater cured HSSF specimens. This may be attributed tothe incorporation of steel fibers to the high performancemortars, aimed to constrain the volume change of mortarsdue to the rapid temperature changes so as to reduce theinitiation and propagation of micro-defects in mortars.However, this finding was not supported by flexuralstrength and modulus of elasticity test results.

As seen in Fig. 3., NS and HSPP mortars both watercured and autoclave cured did not exhibit a strength lossup to 600 �C. This behavior may be attributed to the posi-tive effect of slag on high temperature resistance of cementbased binders due to the lesser content of calcium hydrox-ide [2,11,16–18]. Mixes containing larger amounts of cal-cium hydroxide, the compressive strength can suffer attemperatures above 300 �C, owing to increased micro-cracking around the calcium hydroxide crystals [2].Besides, the decomposition of calcium hydroxide into limeand water vapor above 350 �C may lead to serious damagedue to lime expansion during the cooling period [19,20].Furthermore, this positive finding which is in contrast withthe literature may also be related to the basalt aggregateused in this study. Since it is well known that, concretespecimens with aggregates such as, limestone, granite, bar-ite, dolomitic or siliceous aggregates may lose 40% or moreof its compressive strengths at 600 �C produced with nor-mal Portland cement and with or without pozzolanic mate-rials [3,11,21–23]. However, as an alternative usingfirebrick as an aggregate which is thermally stable and pro-vide good bonding with cement paste, and 65% slagreplaced cement paste as a binder, only 20% compressivestrength loss has been observed [16].

0

10

20

30

40

50

60

70

80

90

100

0 300 600 900

Temperature (˚C)

Res

idua

l mod

ulus

of

elas

tici

ty (

%)

NS HS HSSF HSPP1

HS-ac HSSF-ac HSPP2-ac

Fig. 5. Residual modulus of elasticity vs high temperatures.

S. Aydın et al. / Construction and Building Materials 22 (2008) 504–512 509

The residual compressive strength of polypropyleneincorporated high strength water cured mortar is higherthan the autoclave cured ones at all temperatures. Theresidual compressive strength of NS, HSPP1 andHSPP2(ac) were 114%, 135% and 108%, respectively at600 �C. When the temperature elevated to 900 �C, the resid-ual compressive strength of NS, HSPP1 and HSPP2(ac) hasdropped to 21%, 32% and 29%, respectively.

3.2. Residual flexural strength

The variations of residual flexural strength ratio of spec-imens after exposure to high temperature are shown inFig. 4. It can be seen from Fig. 4, in contrast to compres-sive strength, all autoclave cured mixtures and steel fiberincorporated water cured specimens exhibited flexuralstrength loss at 300 �C. When the temperature elevated to600 �C, flexural strength loss of HSPP mixture for bothwater and autoclave cured is much higher than NS mortar.On the other hand, at 900 �C residual strength ratio ofHSPP mixtures is greater than NS specimens. It is obviousthat, the deteriorating effect of elevated temperatures onflexural strength of mortar specimens was more severe thancompressive strength. This may be due to the fact that, thedestructive effect of micro-cracks that form at elevated tem-peratures was more apparent in the case of tensile stresscreated in flexural test. This result has also been observedby Culfik and Ozturan [24]. However, the gap betweenresidual flexural strength and compressive strength dimin-ishes at 900 �C. This may be also attributed to the increas-ing porosity and destruction of C–S–H matrix due to thehigh temperature which results dramatic reductions incompressive strength.

3.3. Residual modulus of elasticity

Relative residual modulus of elasticity of water curedand autoclave cured mortar specimens are presented inFig. 5. The deteriorating effect of elevated temperatureson modulus of elasticity was more severe than compressiveand flexural strength cases. All mixtures lost considerable

0

20

40

60

80

100

120

0 300 600 900

Temperature (˚C)

Res

idua

l fle

xura

l str

engt

h (%

)

NS HS HSSF HSPP1HS-ac HSSF-ac HSPP2-ac

Fig. 4. Residual flexural strength vs high temperatures.

amounts of modulus of elasticity even at 300 �C, in con-trast with the cases of compressive and flexural strength.At 600 �C, although there are almost no significantamounts of compressive strength loss, there is a greatreduction in modulus of elasticity. According to mixturetype residual modulus of elasticity is between 34% and40% at this temperature level.

3.4. Weight loss

Weight loss of specimens due to the high temperatureeffect is presented in Fig. 6. The maximum loss observedin water cured specimens possibly due to the higher mois-ture content and porosity of water cured specimens. Also,weight loss of NS mortar is greater than HS mortar. Thismay also be explained with the higher porosity of NSspecimens.

3.5. Microstructure investigation

SEM analyses were implemented in order to analyze themicrostructure of the mixtures. Fig. 7 shows SEM imagesof water cured HSPP1 and autoclave cured HSPP2 mortars

0

2

4

6

8

10

12

14

16

0 300 600 900

Temperature (˚C)

Wei

ght

loss

(%

)

NS HS HSSF HSPP1HS-ac HSSF-ac HSPP2-ac

Fig. 6. Weight loss of water cured and autoclave cured mortar at 300, 600and 900 �C.

Fig. 7. SEM analysis of unheated mortar: (a) HSPP1; (b) HSPP2(ac).

Fig. 8. SEM analysis of mortars at 600 �C: (a) HSPP1; (b) HSPP2(ac).

510 S. Aydın et al. / Construction and Building Materials 22 (2008) 504–512

at 20 �C. As shown in figure, matrix phase of HSPP1 mor-tar have PP fibers and small voids while the autoclavecured HSPP2 mortars consist of highly denser matrix phasewith melted PP fiber gaps. Polypropylene fibers had losttheir solid structure when autoclaving at 210 �C. SEManalyses of specimens that exposed to 600 �C are shownin Fig. 8. At this temperature, PP fibers in water curedHSPP1 mortar readily melt and volatilized, creating addi-tional pores and small channels in the mortar. Fig. 8 showsvoids of melted PP fibers for HSPP1 and HSPP2(ac). Com-

Fig. 9. SEM analysis of mortars at 90

pared with unheated specimen, no visible changes weredetermined in the structure at this temperature. This resultwas supported by the compressive strength test in whichalmost all mixtures showed no reduction in strength at600 �C. However, SEM images did not present any evi-dence about loss of flexural strength and modulus of elas-ticity. Fig. 9 shows SEM images of HSPP1 and HSPP2(ac)mortars after 900 �C. As shown in Fig. 9, the space ratio inmatrix increase significantly both water cured and auto-clave cured mortars. Besides, small, rounded formations

0 �C: (a) HSPP1; (b) HSPP2(ac).

S. Aydın et al. / Construction and Building Materials 22 (2008) 504–512 511

were observed in water cured HSPP1. Crystals withrounded shape may be b-C2S, which is one of decomposi-tion products of C–S–H at elevated temperatures, as con-firmed by the literature [25]. On the other hand,autoclave cured HSPP2 mortar exhibit glassy like structurewith pores. These findings explain the great reduction inmechanical properties.

4. Conclusions

Using high volume of pozzolanic materials (25%GBFS + 25% silica fume) in binder phase of HS mixtures,high mechanical properties could be achieved under watercuring at 28 days (compressive strength up to 111 MPa).And, mechanical properties could be improved further byautoclave curing in a much shorter curing period (2 days)(compressive strength up to 163 MPa). These mixtureshave merely 335 kg/m3 cement dosage.

Compressive and flexural strength of HS mortarincreased by using steel fibers and increased slightly byusing PP fibers at 20 �C. On the other hand, steel fibersalso increased the compressive and flexural strength whilePP fibers caused some reduction in compressive and flex-ural strength under autoclave curing before high temper-ature exposure. This behavior may be attributed to themelting of PP fibers which creates some pores in thematrix.

All mixtures showed compressive strength gain at 300 �Cexposure due to closer configuration of capillary pores inthe denser cement paste matrix. Moreover, further hydra-tion of unhydrated cement and pozzolanic reaction betweenGGBFS and lime may also increase the compressivestrength at 300 �C. This behavior was clearer in water curedspecimens than autoclaved mixtures possibly due to thehigher maturity of autoclaved specimens and higher mois-ture content of water cured specimens before exposure.

From the point of flexural strength, strength loss ofautoclaved HS mortars was higher than water cured spec-imens at 300 �C. However, this difference almost dimin-ished at 600 and 900 �C levels possibly due to the mostof the micro-cracking having take place at high tempera-tures of 600 �C and beyond.

When the temperature elevated to 600 �C, destruction ofexplosive nature has been occurred in HS and HSSF mix-tures. It seems that, PP fibers are effective in preventingexplosion of HS mortars both water and autoclave curingcases. Whereas, steel fibers did not change the behaviorof the HS mixtures. However, required PP dosage to pre-vent spalling increased from 0.1% to 0.2% in the case ofautoclave curing due to the denser microstructure of thespecimens.

Normal strength mortar exhibited a higher compressivestrength loss than high strength water cured and autoclavecured mortar at 900 �C. SEM images showed that thestrength loss at this temperature results of coarsening ofvoids in matrix phase and decomposition of C–S–H struc-ture of high strength mortar.

The destructive effect of elevated temperatures on flex-ural strength and modulus of elasticity of mortar specimenswas more distinctive than compressive strength case. Allmixtures lost considerable modulus of elasticity values evenat 300 �C, distinctly from the case of compressive strength.This may be due to the micro-crack formation which affectsdeformation and tensile strength characteristics adversely.

Test results indicate that, using PP fibers in highstrength and ultra-high strength autoclaved mortars is apromising development which conserves almost all com-pressive strength characteristics without spalling up to600 �C temperature even with a high silica fume contentin a saturated condition.

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