EFFECT OF AGING OF PET FIBRE ON THE MECHANICAL … · Keywords: Cement composite, PET fibre, Aging, Mechanical testing Poly(ethylene terephthalate) (PET) fibres have been used as

INTRODUCTION

The increasing amount of poly(ethylene terephtha-late) (PET) waste is a worldwide problem which hasinduced requirements for its recycling and reusing. Thatis why papers focused on utilization of PET fibres asmicro-reinforcement for fibre reinforced concretes havebeen published recently [1,2]. Quality of the micro-rein-forcement depends on many factors such as propertiesof cementitious matrix, fibre geometry, its surface, size,quantity, and rheological parameters.

Concrete represents a three-phase composite con-sisting of the cementitious matrix, fibres (and otheraggregates), and the interfacial transition zone (ITZ).Microstructure of the ITZ between a fibre and thematrix differs from the bulk cement paste and is a criti-cal point determining the concrete strength.

Addition of natural [3-6], steel [7], glass [8], poly-meric [9-12], and carbonaceous fibres [13] and car-bonaceous nanotubes [14] to mortars and concretes canmodify their parameters such as tensile strength, elasticmodulus and strength. For improvement of the cementcomposite properties by fibre addition, the fibres mustbe easily dispersed within the mixture, have suitablemechanical properties, and finally have persistent prop-erties in the highly alkaline matrix. Silva et al. [1] stud-ied using polyester fibres in the Portland cement matrixand have found that these fibres were loosing their

strength. Aging of PET fibres in the alkaline environ-ment of a cementitious matrix, where calcium hydrox-ide concentrates in the ITZ and pH reaches a value ofabout 12, was simulated by modifying the fibres in sat-urated calcium hydroxide solution at an increased tem-perature. Recycled PET fibres reacted with calciumhydroxide forming terephthalates on the fibre surface.After degradation, the tested PET fibres in the amountof 0.4 - 0.8 vol% had no significant effect on the com-pressive and flexural strength of mortars. Elastic modu-lus of the mortars was not affected by the PET fibresquantity. Firmness of the mortars expressed by the flex-ural strength was increased by the PET fibres addition.

Japanese authors [2] used the recycled PET fibresfrom waste PET bottles as a micro-reinforcement inconcrete. Good mixability of the fibres and concrete hasbeen found at the PET fibre concentration of 3 wt.%. Ithas been shown that PET fibres are alkali resistant andtheir price is comparable to steel fibres. At present, thefibres are used in Japan in tunnel construction.

The present work has studied chemical and thermaldegradation of PET fibres within a cementitious matrix.The chemical degradation has been simulated by alkalihydrolysis of the PET fibres in NaOH solution or asaturated Ca(OH)2 solution, which as a product ofcement hydration concentrates in the transition zonesurrounding the micro-reinforcement. Thermal aging ofthe PET fibres was realized by temperature cycles in

Original papers

172 Ceramics − Silikáty 52 (3) 172-182 (2008)

EFFECT OF AGING OF PET FIBRE ON THE MECHANICALPROPERTIES OF PET FIBRE REINFORCED CEMENT COMPOSITE

VLADIMÍR MACHOVIÈ*,***, JANA ANDERTOVÁ*, LUBOMÍR KOPECKÝ**, MARTIN ÈERNÝ***,LENKA BORECKÁ***, OLDØICH PØIBYL***, FRANTIŠEK KOLÁØ***, JAROSLAVA SVÍTILOVÁ***

*Institute of Chemical Technology Prague, Technická 5, 166 28, Prague, Czech Republic **Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, 166 29 Prague, Czech Republic

***Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic,V Holešovièkách 41, 182 09 Prague, Czech Republic

E-mail: [email protected]

Submitted January 9, 2008, accepted April 14, 2008

Keywords: Cement composite, PET fibre, Aging, Mechanical testing

Poly(ethylene terephthalate) (PET) fibres have been used as a dispersed micro-reinforcement in a cementitious matrix. Themicro-reinforcement in the fibre reinforced concrete absorbs tensile strain and prevents formation of microcracks originatingfrom concrete shrinkage. The present work has been aimed at studying the effects of aging PET fibres by alkali hydrolysis andtemperature cycles on the compressive and flexural strength of the PET-cement composite. Using differential scanningcalorimetry, infrared spectroscopy and water vapour sorption, chemical changes of PET fibres after their degradation havebeen characterized together with their effect on mechanical properties of the fibres and the resulting PET-cement composite.It has been found that the flexural strength of the composite has been increased by alkali hydrolysis of PET fibres in sodiumhydroxide.

a thermostated chamber under constant humidity. Thisway, the chemical degradation of the fibres duringhydration of the cementitious matrix under varyingweathering temperatures has been simulated, and simul-taneously the effect of changes in the surface qualityand adhesion of the fibres to concrete mixture in depen-dence on mechanical properties of the PET-cementcomposite.

EXPERIMENTAL

Samples were prepared only from a cementitiousmatrix without sand or gravel aggregate to enable detec-tion of minimum effect of the modified PET fibres onthe composite mechanical properties. The samples wereprepared by mixing an ordinary Portland cement withwater (w:c = 0.4) and the PET fibres (2 wt.%) with alength of 10 mm. The fibre is a discontinual technicalpolyester fibre produced by SLOVKORD, Inc., Senica,Slovakia. It has a circular section (see Figure 1) with adiameter 26.7 µm, and was provided as a strand con-taining 195 monofilaments.

Alkali hydrolysis of the PET fibres was achievedby 1 M NaOH solution or saturated Ca(OH)2 solution.The fibres were soaked in the solution, initially heatedto 90°C at a heating rate of 10°C per minute, and thenleft at an ambient temperature for three days. The fibreswere washed in distilled water after hydrolysis. ThePET fibre samples treated by alkali hydrolysis withCa(OH)2 and NaOH have been denoted as PET AHCaand PET AHNa, respectively.

Laboratory degradation of PET fibres simulatingaging by weathering were carried out in a thermostatedchamber Klimaprüfkammer 3533/15, Feutron Klima-simulation GmbH, Germany, at a constant relativehumidity of 70% in ten cycles under following agingconditions: sample PET A1 (25°C) - (50°C), samplePET A2: (-40°C) - (25°C), sample PET A3: (-40°C) -(50°C), sample PET A4: (-40°C) - (180°C). All samplesgained yellowish colour after degradation.

Infrared spectra were acquired using a NicoletNexus FTIR spectrometer equipped with a SpecacGolden Gate ATR accessory. The latter was fitted with adiamond crystal, and operated with single reflectionoptics at an interaction angle of 45° and a probe area of0.6 mm diameter; the sampling depth (at 1000 cm-1) wascalculated to be approximately 3 µm. The spectra wererecorded over the range 4000-650 cm-1, with a resolu-tion of 4 cm-1, and averaged over 32 scans. Subsequentmanipulation was carried out with Omnic 7.3 software.

The differential scanning calorimetry (DSC) mea-surements were performed on a Perkin Elmer DSC 7calorimeter at a heating rate of 10°C/min. Each thermo-gram was recorded from 25 to 350°C. From the meas-

ured heat of fusion (ΔHexp), an apparent crystallinitydegree, %CDSC, was determined according to the equa-tion %CDSC = 100 ΔHexp/ΔHo, where ΔHo is the heat offusion of an ideal PET crystal, for which a value of117.7 J/g was considered.

To study water vapour sorption onto PET fibres, agravimetric analyzer IGA-02 by Hiden Analytical Ltd.,Warrington, UK was used. Ultrahigh vacuum system ofthe instrument enables to measure adsorption anddesorption with a high precision. The instrument is fullythermostabilized with a deviation of 0.2 K. Weightmicrobalance has a long-term stability 1 µg with a de-viation of 0.2 µg. Prior to measuring, the fibre sampleswere degassed at a pressure < 10-5 Pa and temperature353 K. Adsorption was measured at a constant tempera-ture of 298 K. With respect to the microporous textureof the PET fibres studied, measurements of the CO2

sorption isotherm were done for determination of themicroporous surface and micropore distribution. TheIGA-02 gravimetric analyzer was used as well for thesorption measurements. Instrument setting and meas-urement conditions were same as at water vapour sorp-tion except for the sorption pressure. The CO2 sorptionwas measured within a pressure range of 0-0.1 MPa.

Microstructure and microtexture of the PET fibresurface were studied by a scanning electron microscopeXL30 ESEM-TMP (FEI Philips) with an EDAX micro-analyzer, providing high quality pictures in both highand low modes regimes as well as in the environmental(so called ESEM) mode.

Mechanical properties were measured using anInspekt 100 testing machine. Determination of the ten-sile strength of a bundle was done with a bundle fixedat both ends by belting a cam at a length of 500 mm ata feed speed of 9 mm/min.

The three-point bending test was done with a supportdistance of 80 mm (sample size 20×20×100 mm3) and ata feed speed of 0.2 mm/min. The experiments were car-

Effect of aging of PET fibre on the mechanical properties of PET fibre reinforced cement composite

Ceramics − Silikáty 52 (3) 172-182 (2008) 173

Figure 1. Micrograph of a PET fibre polished section.

27.28 µm

29.36 µm

29.10 µm

27.02 µm

23.90 µm

27.54 µm

28.32 µm

ried out at an ambient temperature after 40 days of thecomposite hydration. For the pressure test, feed speedof 0.05 mm/min was used (sample size 20×20×20 mm).The mechanical experiments were carried out with mul-tiple samples.

Porosimetric determinations were carried out usinga high-pressure mercury porosimeter PASCAL 240 byThermo Electron. The measurements were done withina pressure range of 0.1-200 MPa with a precision betterthan 0.2 %. According to Washburn's equation, the pres-sure range corresponds to a pore size ranging from 3.7to 7500 nm.

RESULTS AND DISCUSSION

Characterization of PET fibres

Poly(ethylene terephthalate) (PET) is one of themost versatile polymers used in films and fibres thanksto its high glass transition temperature (Tg) and slowcrystallization rate, which enable controlling its finalmorphology by various methods. PET belongs to orien-ted semicrystalline polymers containing both crystallineand amorphous fractions. These polymers can be wellcharacterized by Tg, melting temperature (Tm), anddegree of crystallization (WK). Based on these parame-ters, deformation behaviour of PET can be predicted.

The differential scanning calorimetry

The structural part of the chain –O–CH2–CH2–O–in a PET molecule can be arranged in the form of eithergauche or trans conformation isomers formed by innerrotation around a C–C bond [18]. The trans conforma-tion isomers appear mainly in the crystalline part of thepolymer. They have the lowest internal energy andclosest arrangement, whereas the gauche isomers formmainly amorphous zones of the polymeric chains.

The content of the trans conformation isomers inthe crystalline fraction, so called degree of crystalliza-tion, has been determined by DSC. The values deter-

mined are presented in Table 1 together with the mini-mum and maximum temperature of degradation of theoriginal PET fibre samples. All samples, except for afibre sample alkali hydrolyzed by sodium hydroxide,show a higher degree of crystallization compared to theoriginal fibres. It is evident from the table that the fibressubject to the minimum temperature of -40°C have thehighest degree of crystallization.

In the DSC thermograms, several endothermicpeaks appear within a region near the melting point (seeFigure 2 and Table 1). In the case of PET fibres, a lowertemperature peak is usually assigned to crystallites of asmaller size, characterized by a lower degree of perfec-tion [16]. For the original PET fibre sample, two peaksappear at the temperatures of 251 and 259°C. For thesamples aged by temperature cycles, except for the A3sample, only one peak is observed. The samplehydrolyzed by sodium hydroxide is characterized by acomplex thermogram, in which five peaks within themelting region between 252 and 266°C can be found.

Machoviè V., Andertová J., Kopecký L., Èerný M., Borecká L., Pøibyl O., Koláø F., Svítilová J.


Table 1. Content of the crystalline trans conformation isomers, minimum and maximum temperatures used for sample degradation,and melting temperatures observed in DSC thermograms of the original and modified PET fibres.

crystalline trans Tmin Tmax T1 T2 T3 T4 T5Sample (%) (°C) (°C) (°C) (°C) (°C) (°C) (°C)

PET 43.54 25 25 251 – 259 – –PET AHCa 46.32 25 90 250 – 259 –PET AHNa 41.97 25 90 252 256 258 261 266PET A1 48.02 25 50 – – – 261 –PET A2 53.18 -40 25 – 259 – –PET A3 49.01 -40 50 254 259 – –PET A4 49.82 -40 180 – 257 – – –

Figure 2. DSC thermograms of the original and modified PETfibres.

Temperature (°C)

End

o

50 250150 350100 300200

PET A4

400

PET A3

PET A2

PET A1PET AHNa

PET AHCa

PET

Infrared spectroscopy of PET fibres

Infrared spectroscopy is a valuable tool for analysisof oxygen functional groups in polymers. Using infra-red spectroscopy, Samson et al. [17] have found that thealkali hydrolysis causes scission of PET chains resultingin smaller fragments of ethyleneglycol and terephthalicacid.

Hydrolysis of PET fibres is enhanced at increasedhumidity above the glass transition temperature Tg. Itresults in an increased content of carboxylic and alcoholhydroxyl groups and shorter chains due to the reverseesterification [17]. Water diffuses into the amorphousregions of PET, where hydrolysis takes place. Thehydrolysis rate depends on the polymer morphology,crystallinity values, relative humidity, and temperature.The process of hydrolysis degradation, often denoted aschemicrystallization, involves changes in density due tothe scission of chains in the amorphous regions resul-ting in short chains with a sufficient mobility, which cancrystallize. The effect of chemicrystallization has been

detected also upon oxidative degradation. The increasein crystallinity upon degradation significantly affectsthe tensile strength. It has also been found that thedegree of crystallization affects the hydrolysis rate.

ATR (attenuated total reflectance) spectra of thealkali hydrolyzed and laboratory-aged PET fibre sam-ples within the spectral ranges of 3600-2750 cm-1 and1800-650 cm-1 are shown in Figures 3 and 4. The bandsat 3545 and 3430 cm-1, and the region 3380-3210 cm-1

have been assigned to hydroxyl groups of absorbedwater, OH stretching of diethyleneglycol end group, andAr–OH, respectively. The changes in content of hydro-xyl groups have been expressed by the ratio of bandareas A3545/(A3545 + A3430). The alkali hydrolyzed samplesand the laboratory-aged samples A3 and A4 showed anincrease in hydroxyl groups of absorbed water (seeFigure 5), which confirms an increased hydrophility ofthese fibre samples. The changes in content of carboxylgroups have been expressed by the ratio of band areasA1728/(A1728 + A1713). It is obvious from the results pre-sented in Figure 6 that the content of carboxyl groupshas been increased except for the alkali hydrolyzedsamples, where the carboxyl content has slightlydeclined.



Figure 3. ATR spectra of the original and modified PET fibreswithin the spectral range 3600-2750 cm-1; A: PET, B: PETAHCa, C: PET AHNa, D: PET A1, E: PET A2, F: PET A3,G: PET A4.

Wavenumbers (cm)

Abs

orba

nce

0.04

03600

0.06

0.02

3400 28003200

0.05

0.03

0.01

3000

3545 3430 3333 31033074

3057 2969 2914

2854

ABCDEFG

Figure 4. ATR spectra of the original and modified PET fibreswithin the spectral range 1800-650 cm-1; A: PET, B: PET AHCa,C: PET AHNa, D: PET A1, E: PET A2, F: PET A3. G: PET A4.

Wavenumbers (cm)

Abs

orba

nce

0.8

1800

1.2

0.4

1600 8001400

1.0

0.6

0.2

1200

1713

15781504

12601244

11211097

1018971

AB

CDEFG

1000

14081338

871849

791721

Figure 5. Relative abundance of hydroxyl groups in the origi-nal and modified PET fibres.

Hyd

roxy

l gro

ups

0.2

0.0PETA1

PETAHCa

PETA3

0.4

PETAHNa

PET PETA2

0.3

0.1

0.5

PETA4

Figure 6. Relative abundance of carboxyl groups in the origi-nal and modified PET fibres.

Car

boxy

l gro

ups

0.2

0.0PETA1

PETAHCa

PETA3

0.4

PETAHNa

PET PETA2

0.3

0.1

0.5

PETA4

Vibrational spectroscopy is a valuable tool forstudying polymer morphology, i.e., determination of thecrystalline and amorphous region in PET. Molecularconformational changes play an important role in deter-mination of PET structure. Rodriguez-Cabello et al. [16],e.g., determined the distribution of the trans/gaucheconformers by infrared spectroscopy using the 968 cm-1

band intensity of the trans isomer (O–CH2 stretchingvibration) and the 898 cm-1 band intensity of the gaucheisomer (glycol segment), relative to the 793 cm-1 bandintensity (CH aromatic deformation mode). Regardingthat the bands employed were not well resolved, bandseparation has been applied. For determination of thecontent of the trans and gauche conformation isomers,and thus the crystallinity changes of the PET fibres ana-lyzed in the present work, a two-phase conformationmodel has been used:

p1 I968/I793 + p2 I898/I793 = 1 (1)

where p1 and p2 are constants [16]. Figure 7 showsthe part of an ATR spectrum of the original PET fibresused for separation of the bands assigned to the transand gauche conformation isomers. The bands have been

separated using the Voigt function, and the p1 and p2

constants in Equation (1) found to be 0.4847 and0.7484, respectively.

Besides the content of the trans and gauche iso-mers in the amorphous region, infrared spectroscopyprovides information on the total content of the transconformation isomers (total trans) in both crystallineand amorphous regions of the polymer. The DSC resultsprovide the trans isomer content in the crystalline frac-tion only (crystalline trans). Subtracting the crystallinetrans (DSC) from the total trans (ATR) will provide thecontent of the trans isomers in the amorphous fraction(amorphous trans). Percentages of the individual iso-mers are given Table 2.

Mechanical properties of PET fibres

Rodrigues-Cabello et al. [16] have found that thecontent of the trans conformation isomers in the crys-talline region does not change significantly with in-creasing temperature, whereas the content of trans iso-mers in the amorphous zone decreases with increasingtemperature after reaching the glass transition tempera-ture (Tg = 75-80°C). This conformation transition con-nected with a loss of orientation of polymeric chains ofthe amorphous phase is reflected in a macroscopiceffect - Young's modulus, which is virtually constantbelow Tg and then decreases rapidly. Although mechani-cal behaviour can be controlled by the crystalline zonestructure of a polymer, it has been shown that in the caseof PET fibres it is influenced by the amorphous phasestructure. Similar finding has been done for mechanicalbehaviour of the degraded PET fibres in the presentwork. An example of deformation curves of the original(PET), alkali hydrolyzed with Ca(OH)2 (PET AHCa),and laboratory-aged (PET A1) fibres is presented in Fi-gure 8. From shapes of the deformation curves it is evi-dent that for the modified fibres, Young's modulus andmaximum stress decrease compared to the original sam-ple, whereas strain increases. Table 2 lists the mechani-cal parameters of the original and modified PET fibres



Table 2. Contents of the trans and gauche conformation isomers and mechanical parameters of the original and modified PETfibres.

crystalline total gauchec amorphous Young's maximum first failure last failuretransa transb (%) transd modulus EF stress σmax strain strain

Sample (%) (%) (%) (GPa) (MPa) (%) (%)

PET 43.54 85.96 23.52 42.42 10.6 ± 3.1 666.8 ± 30.1 10.79 11.50PET AHCa 46.32 66.38 22.83 20.06 9.1 ± 0.55 590.2 ± 38.6 11.79 13.90PET AHNa 41.97 77.32 21.66 35.35 8.8 ± 0.59 586.8 ± 32.8 12.21 14.31PET A1 48.02 69.46 39.20 21.44 5.1 ± 0.46 426.1 ± 37.8 18.30 18.61PET A2 53.18 67.01 37.72 13.83 7.1 ± 0.47 459.4 ± 50.0 11.52 13.90PET A3 49.01 70.01 25.89 21.00 9.3 ± 0.42 620.8 ± 42.1 12.20 12.82PET A4 49.82 60.23 37.14 10.41 5.0 ± 0.16 510.7 ± 36.3 17.40 19.88

a - DSC, b - FTIR, c - FTIR, d - FTIR total trans/DSC trans

Figure 7. Separation of the bands of the trans and gauche con-formation isomers in an ATR spectrum of the original PETfibres.

Wavenumbers (cm)

Abs

orba

nce

0.04

0800

0.06

0.02

850 950900

0.05

0.03

0.01

793 standard

968

0.08

0.07

together with the values of the strain at the first and lastfailure before fibre breakage. Figure 9 shows the rela-tions between Young's modulus and the content of thetrans and gauche conformation isomers. It is indicatedby (Table 2) and Figure 9 that Young's modulus and thestress before fibre failure increase with increasing con-tent of the trans isomers in the amorphous fraction ofthe PET fibres studied, whereas the gauche and crys-talline trans isomers show a reverse trend, in agreementwith literature [16]. It follows also from the results thatthe alkali hydrolyzed fibres and the thermally aged fibrePET A3 have about 90% strength of the original unde-graded PET fibre, whereas the thermally aged fibresPET A1, A2, A4 have 65-77 % strength of the originalundegraded PET fibre. The degraded PET fibres with ahigher strength have higher content of the amorphoustrans isomers and lower content of the gauche isomers.A simplified diagram of the structural changes in thecrystalline and amorphous fraction in PET fibres uponaging is depicted in Figure 10.



Figure 8. Strain-stress deformation curves of the original andselected modified PET fibres.

Strain (%)

Stre

ss (M

Pa)

200

0

600

400

50 10

300

100

500

15 20

700 PETPET AHCaPET A1

Figure 9. Relations between the Young's modulus and the con-tent of the trans and gauche conformation isomers.

Tran

s/G

auch

e co

nten

t (%

)

40

0

80

54 6

60

20

100

7 118 9 10

crystalline transtotal transgaucheamorphous trans

a)

b)

Figure 10. Diagram of the structural changes in the crystallineand amorphous fraction in original a) and modified PET fibreb), 1 - crystalline trans zone, 2 - amorphous trans zone.

Water vapour sorption onto PET fibres

Hydrophobic polymers absorb usually less than1 wt.% of water. For hydrophobic polymers such asPET, the sorbed water changes their physical propertiessuch as the glass transition temperature, dielectric con-stant, and Young's modulus [19]. Application of fibresin a composite matrix requires strong adhesion betweenthe fibre and matrix. Changes in the PET fibre surfaceproperties after their alkali hydrolysis and laboratory-aging can be studied by water vapour sorption. Figure11 shows that the degraded fibres are better water sor-bents than the original fibres. This can be explained byan increase in the specific surface of the fibre, and par-tially by formation of new organic groups (COOH andOH) with a higher affinity to water sorption. Significantcorrelation has not been found between the structural ormechanical parameters of the PET fibres and the con-tent of water adsorbed on the fibre surface.

Micropore distribution on the surface of the originaland alkali hydrolyzed PET fibres was studied by CO2

sorption (see Figure 12). Micropores with a diameter of7 nm prevail in the PET fibres. The surface occupied bymicropores is 36.56 m2/g in the original PET fibre, afteralkali hydrolysis with NaOH and Ca(OH)2 it has beenincreased to 37.82 and 45.34 m2/g, respectively.

Environmental scanning electron microscopy(ESEM) enables investigation of the surface structuremodifications. An increased appearance of defects onthe surface of the alkali hydrolyzed and laboratory-agedPET fibres is documented by the ESEM micrographs inFigure 13. The surface defects are not visible on theoriginal PET fibres, whereas small regular cracks arepresent on the surface of the alkali hydrolyzed PETfibres and irregular defects of a larger size can be foundon the aged PET fibres.

Porosity of cement composite

Porosity of the cement composites reinforced withthe original and alkali hydrolyzed PET fibres was stud-ied by mercury porosimetry. Pore distribution has beendetermined for the composites hydrated for 40 days.From Figure 14 it follows that the addition of the alkalihydrolyzed PET fibres decreases porosity compared tothe pure cement without micro reinforcement, and thatthe lowest porosity can be observed for the cement com-posite reinforced with the PET fibres alkali hydrolyzedwith NaOH. It can be assumed that these fibres have thehighest adhesion to the cementitious matrix and the ITZbetween the fibre and matrix the lowest content ofpores. Thus, this composite was expected to show thehighest firmness.

Compressive strength of the PET fibrereinforced cement composite

Mechanical properties of the cement compositedepend mainly on the content of fibres, their orienta-tion, and also on the quality of load transfer between thereinforcement and matrix. In the case of the compres-sive experiment, the presence of fibres in this materialdoes not improve significantly its compressive strength.A positive effect of the fibres was observed mainlyvisually; the samples with fibers disintegrate and stayedrelatively compact until high strains (up to strain ofabout 10 %), only containing number of smaller cracks.Compression stress-strain curves are depicted in Figu-re 15. Table 3 lists results of the compressive strengthcalculated from the maximum load before the firstmarked failure.

Bending test of the PET fibrereinforced cement composite

From the compression diagrams it has followedthat the presence of fibres in the cement material doesnot cause significant changes in the compressivestrength. The presence of PET fibres, however, marked-ly affects flexural strength. Figure 16 presents flexuralload-deflection curves for pure cement and the cement



Figure 11. Water vapour sorption isotherms for the original andmodified PET fibres.

P/P0

Mas

s ch

ange

(g/g

0)

0.02

0

0.04

0.20 0.4

0.03

0.01

0.8 10.6

A1A2A3PETAHNaAHCa

Figure 12. Sorption of CO2 on the original and alkali hydro-lyzed PET fibres, and determination of surface and microporedistribution.

Mas

s ch

ange

(g/g

0)

0.002

0

0.004

2000 400

0.003

0.001

800 1000600

AHCa, S micro = 45.34 m2/gAHNa, S micro = 37.82 m2/gPET, S micro = 36.56 m2/g

composite with the original PET fibres. The two curvesare quite different. The curve characterizing the purecement paste predicates of linear brittle behaviour. Theother curve is typical for cement composites. It can bedivided into three regions characterized by two loadvalues. The critical load F* is a load at the stage whenfirst cracks appear. The region I is characterized by rel-atively elastic behaviour before the appearance of firstcracks, where mainly the cementitious matrix con-tributes to the load capacity of the composite. Othermaxima within the region II between F* and Fmax indi-cate secondary crack occurrence. The peak load Fmax can



Figure 13. ESEM micrographs of the surface of the original and modified PET fibres; a) PET, b) PET AHCa, c) PET AHNa,d) PET A1, e) PET A2, f) PET A3. g) PET A4.

a)

b)

c)

d)

e)

f)

g)

be interpreted as the highest failure force and theregion II is the inelastic range until fracture, where onlyfibres carry tensile stresses. After the peak load Fmax

(region III), the mechanical behaviour is mostly dueto the presence of fibres in the composite. No suddenfailure but a continuous fall of the load has beenobserved. This behaviour underlines the major role ofthe debonding process in the ruin of the material [6].Figure 17 illustrates the final stage of the three-pointbending test, where all fibres break.

The results of the bending test expressed by theflexural modulus EC and flexural strength σb,max are pre-sented in Table 3. It is evident that the PET fibre addi-tion to cement has increased its flexural strength andmodulus. As obvious from Figure 18, the flexural mo-dulus of a PET-cement composite is linearly proportion-al to Young's modulus of a respective PET fibre withvarious aging degree obtained by alkali hydrolysis ortemperature cycles, with the highest modulus observedfor the composite reinforced with the original PETfibres. On the other hand, the dependence of the flexu-



Table 3. Mechanical properties of various PET-cement composites after 40 days of curing.

Compressive strength σc,max Flexural strength σb,max Flexural modulus EC

Sample (MPa) (MPa) (GPa)

cement 16.0 ± 4.8 1.0 ± 0.4 1.01cement-PET 19.6 ± 0.6 9.1 ± 0.3 3.43cement-PET AHCa 22.3 ± 4.3 7.3 ± 0.5 2.90cement-PET AHNa 24.8 ± 5.6 11.6 ± 0.7 2.81cement-PET A1 n.d. 6.6 ± 0.5 1.32cement-PET A2 n.d. 4.1 ± 0.4 2.27cement-PET A3 n.d. 10.0 ± 0.6 2.81cement-PET A4 n.d. 5.9 ± 0.4 1.62n.d. - not determined

Figure 14. Porosity of pure cement and PET-cement compositesreinforced with the original and alkali hydrolyzed PET fibres.

Tota

l por

osity

(%)

38

30C PET

AHCa96C PET96

46

C96

42

34

C PETAHNa96

40

32

48

44

36

Figure 15. Compression stress-strain curves for pure cementand PET-cement composites with the original and alkalihydrolyzed PET fibres.

Displacement (mm)

Load

(N)

8000

0

20000

16000

0.50.0 1.0

12000

4000

18000

1.5 2.5

22000

cement-PET AHCacement-PET AHNacement-PETcement

2.0

10000

14000

6000

2000

Figure 16. Flexural load-deflection curves for pure cement andPET-cement composite with the original PET fibres.

Deflection (mm)

Load

(N)

400

0

800

0.20.0 0.4

600

200

0.6 1.40.8

500

700

300

100

1.21.0

cement

cement/PET

F*

Fmax

I II III

Figure 17. Three-point bending test.

ral strength of various PET-cement composites on ten-sile strength of respective PET fibres see Figure 19 isambiguous. The composite reinforced with the fibreshydrolyzed with NaOH has the highest flexural strength(127 % of the strength of the composite with the origi-nal PET fibres). The composite reinforced with thesefibres is characterized with a very low porosity, and thusvery good adhesion to the cementitious matrix in theITZ region between the fibre and matrix. Also the com-posite with the thermally aged fibres A3 has higherstrength compared to the composite with the originalPET fibres (110 %). The composite with the PET fibreshydrolyzed with Ca(OH)2 has an 80% strength com-pared to the composite with the original PET fibres.Other composites with the thermally aged fibres PETA1, PET A2, and PET A4 show 73 %, 45 % and 65 %strengths. It follows from the results that the compositestrength depends mainly on the strength of the PETfibres.

CONCLUSION

The results can be summarised as follows:

Chemical and thermal degradation of PET fibres usedas micro-reinforcement in a cementitious matrix havebeen studied. Before addition to the cementitiousmatrix, the fibres were either alkali hydrolyzed withNaOH or Ca(OH)2 under elevated temperature, orlaboratory-aged using various temperature cycles.This treatment should have simulated aging of PETfibres in concrete in the course of its hydration aweathering.Infrared spectroscopy has revealed that the alkalihydrolysis induces scission of polymeric chains intosmaller fragments of ethyleneglycol and terephthalicacid, and increases the content of hydroxyl groups onthe PET fibre surface. The thermal aging of PETfibres has increased the content of carboxyl groupsand in some cases also hydroxyl groups.

DSC in combination with infrared spectroscopy wasused for determination of the degree of crystallizationand content of the trans and gauche conformation iso-mers in the original and modified PET fibres. ThePET fibre crystallinity determined using DSC is lowerin all aged samples except for the sample alkalihydrolyzed with NaOH. For all modified samples, thetotal content of the total and amorphous trans isomersafter the degradation has decreased compared to theoriginal PET fibres. The content of gauche isomershas increased in the thermally aged samples, whereasa moderate decrease has been observed for the alkalihydrolyzed samples.Young's modulus of the degraded PET fibres and thestress before breakage increases with increasing con-tent of the trans isomers in the amorphous fraction ofPET fibres, whereas the gauche and crystalline transcontents follow a reverse trend.The degraded PET fibres with a higher content of theamorphous trans isomers and lower content of thegauche isomers, namely the alkali hydrolyzed fibres,have about 90% strength of the original fibre. Thefibres aged by the temperature cycles with about 65-77 % strength of the original PET fibre contain ahigher amorphous fraction.The fibres attacked by Ca(OH)2 and NaOH show ahigher ability for H2O sorption than the original fibrethanks to an increased fibre surface and formation ofhydroxyl and carboxyl groups on the fibre surface.Similar trend has been observed for the content ofmicropores determined by carbon dioxide sorption.Porosity of the PET-cement composite decreases withthe PET fibre addition and with modification of theirsurface by alkali hydrolysis.The three-point bending has shown that a 2 wt.% ad-mixture of PET fibres in cement markedly increasesits maximum flexural strength and modulus. Thehighest strength has been obtained for the cementcomposite reinforced with the fibres alkali hydro-lyzed with NaOH. It can be assumed that the chemi-



Figure 18. Dependence of flexural modulus of various PET-cement composites on Young's modulus of respective PETfibres.

3.0

1.04

4.0

2.0

6 128

3.5

2.5

1.5

10

Figure 19. Dependence of the flexural strength of various PET-cement composites on the tensile strength of respective PETfibres.

σmax PET (MPa)

σ max

com

posi

te (M

Pa)

10

2400

12

6

450 700500

8

4

550 600 650

cal degradation increases adhesion of the fibre to thecementitious matrix in the interfacial transition zone,and thus improves mechanical properties of the PETfibre reinforced cement composite. It is expected thatthis easy chemical modification of the PET fibre sur-face will provide very sturdy while cheap concretesreinforced with recycled polymeric fibres.

Acknowledgement

The study has been supported by the grant No.106/05/2618 CEZ MSM 684077003 of the CzechScience Foundation. The authors are grateful to theemployees of the SLOVKORD, Inc., Senica, Slovakia forproviding the PET fibre samples. The assistance of thefollowing co-workers is gratefully acknowledged: IvanaŠedìnková (FTIR) and Pavel Straka (DSC).

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VLIV STÁRNUTÍ PET VLÁKEN NA MECHANICKÉVLASTNOSTI CEMENTOVÉHO KOMPOZITU

VYZTUŽENÉHO PET VLÁKNY

VLADIMÍR MACHOVIÈ*,***, JANA ANDERTOVÁ*,LUBOMÍR KOPECKÝ**, MARTIN ÈERNÝ***,LENKA BORECKÁ***, OLDØIC PØIBYL***,

FRANTIŠEK KOLÁØ***, JAROSLAVA SVÍTILOVÁ***

*Vysoká škola chemicko-technologická v Praze,Technická 5, 166 28, Praha 6

**Fakulta stavební ÈVUT,Thákurova 7, 166 29 Praha 6

***Ústav struktury a mechaniky hornin AV ÈR, v.v.i.,V Holešovièkách 41, 182 09 Praha 8

Polyetylenteraftalátová (PET) vlákna byla použita jakorozptýlená mikrovýztuž v cementové matrici. Mikrovýztuž vevláknobetonech absorbuje tahová namáhání a brání vznikumikrotrhlin vznikajících pøi smršt’ování betonu. Tato práce jezamìøena na studium vlivu stárnutí PET vláken alkalickouhydrolýzou a pomocí teplotních cyklù na pevnost cementovéhokompozitu v tlaku a ohybu. Pomocí DSC, infraèervené spektro-skopie a sorpce vodní parou a byly charakterizovány chemickézmìny PET vláken po jejich degradaci a jejich vliv na mecha-nické vlastnosti PET vláken a výsledného kompozitu PET-cement. Bylo nalezeno, že pevnost kompozitu v ohybu se zvýši-la po alkalické hydrolýze PET vláken hydroxidem sodným.

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