The aim of this work is to describe bonding properties between surface treated polymer fibers and a cement matrix. In order to increase an interaction between the matrix and fiber surfaces, two fiber types having approx. 0.5 mm in diameter were modified by mean of oxygen plasma treatment. Surface physical changes of treated fibers were examined using SEM morphology observation and interfacial adhesion mechanical tests. The principle of mechanical tests rested on a single fiber pulling out from the matrix (cement paste, CEM I 42.5 R, w/c 0.4). The embedded length was equal to 50 % of original fiber length (50 mm), where the fiber free-end displacement and force resisting to the displacement were monitored. It was pointed out that interfacial shear stress needed to break the bond between the modified fibers and the matrix increased almost by 15–65 % if compared to reference fibers. When the fiber free-end displacement reached to 3.5 mm, the shear strength increased almost twice

Horová, Tereza

Prošek, Zdeněk

Trejbal, Jan

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/APP.2017.13.0130Acta Polytechnica CTU Proceedings 13:130–134, 2017 © Czech Technical University in Prague, 2017available online at http://ojs.cvut.cz/ojs/index.php/appPULLOUT BEHAVIOR OF OXYGEN PLASMA TREATEDPOLYMER FIBERS FROM CEMENT MATRIXJan Trejbala,∗, Tereza Horováa, Zdeněk Prošeka,ba Czech Technical University in Prague, Faculty of Civil Engineering, Thákurova 7, 166 29 Prague, CzechRepublicb University Centre for Energy Efficient Buildings of Technical University in Prague, Třinecká 1024,273 43 Buštěhrad, Czech Republic∗ corresponding author: jan.trejbal@fsv.cvut.czAbstract. The aim of this work is to describe bonding properties between surface treated polymerfibers and a cement matrix. In order to increase an interaction between the matrix and fiber surfaces,two fiber types having approx. 0.5 mm in diameter were modified by mean of oxygen plasma treatment.Surface physical changes of treated fibers were examined using SEM morphology observation andinterfacial adhesion mechanical tests. The principle of mechanical tests rested on a single fiber pullingout from the matrix (cement paste, CEM I 42.5 R, w/c 0.4). The embedded length was equal to50 % of original fiber length (50 mm), where the fiber free-end displacement and force resisting to thedisplacement were monitored.It was pointed out that interfacial shear stress needed to break the bond between the modifiedfibers and the matrix increased almost by 15–65 % if compared to reference fibers. When the fiberfree-end displacement reached to 3.5 mm, the shear strength increased almost twice.Keywords: Cement composites, fiber reinforcement, plasma treatment, morphology, pullout.1. IntroductionA fiber-reinforced concrete (FRC) is a composite mate-rial containing fibers in an amount of 1 % of concretevolume standardly. The role of the fibers is to promotea crack distribution and to reduce crack widths. FRCmechanical properties are dependent on (i) propertiesof each components (by which we mean mainly cementmatrix as a continual phase and fibers as reinforce-ment) and (ii) a mutual interaction between the twobasic phases. The strong interaction characterized bythe bond and the adhesion between the two phasesplays an important role in the mechanical performanceof the FRC [1–3].The FRC may be reinforced with polymer fibers.The use of polymer instead of more widespread steelfibers brings a number of benefits: a corrosion resis-tance, a lower propensity to a balling fibers creationduring a mixing process, a frugality to hosses andnozzles of shotcrete devices, etc [4].Despite the above mentioned benefits, polymerfibers (including polypropylene, polyethylene, Nylon,PVA, etc.) have their significant limitations. This isprimarily about poor adhesion and wettability to thecement matrix due to their chemical inertness, lowsurface energy and smooth surfaces [5, 6].The smooth fiber surfaces can not guarantee re-quired strong adhesion with the matrix. After thefull debonding, when fibers are pulled out from thematrix, a weak abrasion effect results to FRC integrityinfraction. To ensure reinforcing effect, high amountof fibers can be applied. Unfortunately, this accessis inappropriate due to mixture workability reducingeffect [3]. On the other hand, an interfacial bondstrength enhancement seems to be the most effectiveway to improve FRC mechanical performance with-out undesirable side effects, especially in the field ofpolymer fiber reinforcement [7–9].In order to modify polymer fiber surfaces, severaltypes of treatment may be applied, e.g. chemical (e.g.use of high alkali solutions) and physical (e.g. mechan-ical roughening). Unfortunately, the first mentionedmethod we can not considered to be effective and en-vironmentally friendly due to high time consumption(often crossing tens of hours), requirement of hightemperature (almost 100 ◦C) and need to use aggres-sive chemical substances [9–11]. Next, it is difficultto control the mechanical methods and moreover, itseffect has a very strong impact on fiber mechanicalproperties [12, 13].To modify polymer fiber surfaces effectively,a plasma treatment seems to be good alternative toconventional methods mentioned earlier. An effect ofthe plasma treatment is twofold: chemical as well asphysical. The chemical treatment rests in a polymersurface activation, when active polar groups are im-plemented onto fiber surfaces. As a consequence, thesurface is modified from hydrophobic to hydrophilicand thus its reactiveness with polar liquid (water con-tained in cement paste) is increased [8, 14]. Nextto that, the polymer surface is roughened via ionbombardment. In the case of the FRC containingpolymer fibers, the second mentioned phenomenon isvery important to achieve strong adhesion between130vol. 13/2017 Polymer fiber pullout behaviorFigure 1. Principle of physical surfaces treatment (from own resources).Name Material Diameter Young modulus Length Mass Density[-] [-] [µm] [GPa] [mm] [kg/m3]Concrix polyolefin 500 ≥ 10 50 910BeneSteel PP, PE 480 5.17 ± 0.50 55 ∼913Table 1. Summary of basic fiber parameters.the two materials when the fiber is pulled out from thematrix (called as bridging effect). In this study, wefocus primarily just to the physical modifications andtheir effect. The theoretical principle of the plasmamodification is imaged in the Figure 1.As a proper indicator of the interaction rate be-tween the modified fibers and the matrix, mechanicaltests should be done. Indirect mechanical methodshave been applied – for example bending tests of thecomposite material [8]. Nevertheless, pullout tests ofthe single fiber from the matrix provide clear infor-mation about the interaction between the two materi-als. Therefore, we focused to the second mentionedmethod. Moreover, the effect of ion bombardmentmay be examined using SEM morphology investiga-tion and AFM surface scanning [8, 15].2. Materials2.1. Polymer fibersTwo types of polymer macro-fibers were used. Bothof them are standardly used as randomly distributedand oriented reinforcement in the field of the FRCs toreduce creation and development of shrinkage cracksand to preserve construction integrity. Concrix fiberscomposing from two layers (shell and high-moduluscore) were made from polyolefin (a specific type wasnot provided by the manufacturer) in Switzerland.BeneSteel fibers composing from PE (polyethylene)and PP (polypropylene) mixture were made in theCzech Republic. The basic fiber parameters providedby their manufacturers are summarized in Table 1.2.2. Cement samplesCement prismatic samples composed of Portland ce-ment CEM I 42.5 R (water to cement ratio w/c 0.4)having dimensions equal to 25×20×25mm were madeand stored for 15 days (unmolded after 1 day of hard-ening) in standard laboratory conditions. A fiberembedded length was equal to 25 mm. The sample iscaptured in Figure 2.Figure 2. Cement specimen with single fiber forpullout tests.3. Methods3.1. Plasma modificationIn order to modify fiber surfaces physically, the oxygencold plasma treatment was done using Tesla VT 214device. The duration of plasma treatment was 30 and480 seconds. The first mentioned duration (30 seconds)should guarantee required surface changes, but onlyto a minimal extend. On the other hand, the longermentioned duration was established as the longestpossible time (when the fiber were treated even longer,their melting temperature was reached). The powerof RF source was 100 W, gas pressure 20 Pa.3.2. SEM morphology analysisTo asses physical changes onto fiber surfaces properly,the scanning electron microscope observation was doneby Zeiss Merlin (Carl Zeiss Microscopy GmbH). Usingthe plasma sputtering system (BOC Edward Scan-coats Six), the fibers were coated by thin gold layerto eliminate surface charging during SEM analysis.3.3. Pullout testsAss a proper indicator of the interaction of both refer-ence and plasma treated fibers and the cement matrix,pullout tests of the single fiber were realized. The dis-placement controlled testing took place on the loadingframe Web Tiv Ravestein FP100 at the constant rateof 0.8 mm/min until reaching 60 N and 0.6 mm/minfurther. As a result of the experiment, a dependencebetween the fiber free-end displacement and the force131J. Trejbal, T. Horová, Z. Prošek Acta Polytechnica CTU ProceedingsFigure 3. SEM images of reference (left), 30 s (middle) and 480 s (right) plasma treated Concrix fibers.Figure 4. SEM images of reference (left), 30 s (middle) and 480 s (right) plasma treated BeneSteel fibers.resisting to the displacement was recorded. To pro-vide values normalized by the fiber surface area, theinterfacial shear stress was calculated. We focusedto two stages: (i) when the pullout force reached itsmaximum and (ii) when the fiber free-end was dis-placed by 3.5 mm. The mentioned shear stress wascalculated asτmax =FmaxCf le(1)andτ3.5 =F3.5Cf le(2)where Fmax and F3.5 represent the maximal forceand the force needed to reach the fiber free-end dis-placement of 3.5 mm, Cf is a fiber circumference andle is a fiber embedded length.4. Results4.1. SEM morphology analysisThe morphology changes onto Concrix and BeneSteelfiber surfaces are shown in the Figure 3 and Figure 4,respectively. The SEM images with magnificence of5k showed that both types of untreated fibers hadsmooth surfaces having tiny grooves that originatedprobably from a production technology. The samecan be said of fibers modified 30 seconds by plasmatreatment. This treatment time turned out to be tooshort to achieve fiber surfaces physical changes viathe ion bombardment but despite of this, tiny mor-phology changes were revealed. On the other side,the surfaces were significantly changed after 480 sec-onds of modifications. In the case of Concrix fibers,their surfaces were slightly damaged. A violation ofthe surfaces by scoops was observed. In the case ofBeneSteel fibers, their surfaces after 480 seconds ofplasma treatment can be described as significantlyroughened by longitudinal grooves, if compared toreference fibers.The roughening of fiber surfaces has twofold impor-tance, as mentioned below.• Increase of the surface area. Before full debonding,when is the FRC exposed to tensile load, chemicalbonds between the two materials are realized on alarger area and thus a stronger mutual interactionis ensured.• Increase of the shear stress. After full debonding,when are fibers pulled out from the matrix, theshear strength is increased due to the fiber surfaceroughness.4.2. Pullout testsPullout tests of single fibers from cement matrix re-vealed the significant cohesion increase in the bothstages – before and after the full debonding.Before the full debonding, the maximal shear stress(τmax) was increased by ca. 10 and 60 % regardlessto the duration of the plasma modification in the caseof Concrix and BeneSteel fibers, respectively. Thisphenomenon can be attributed to the increase of thecontact area and to the activation of the surfaces by132vol. 13/2017 Polymer fiber pullout behaviorConcrix ESBeneSteel2.00.41.20.81.6Duration of plasma treatment [s]0 30 480 0 30 480τ      [MPa]maxFigure 5. Maximal interfacial shear stresses for various treatment duration.Concrix ESBeneSteel2.00.41.20.81.6uration of plas a treat ent [s]Duration of plasma treatment [s]0 30 480 0 30 480τ     [MPa]3.5Figure 6. Interfacial shear stresses at 3.5 mm pullout for various treatment duration.functional polar groups. The results are graphicallyshown in the Figure 5.In this study, we focused especially on physicalfiber surface changes, so we put more emphasis to theshear stress (τ3.5) monitored when fibers pulled outfrom the matrix by 3.5 mm (after the full debonding).The results revealed that the shear stress increasedafter 30 seconds of the plasma treatment by ca. 70and 80 % in the case of Concrix and BeneSteel fibers,respectively. After 480 seconds of the treatment, another shear stress increase was recorded. Concretelyby 15 % identical for both fiber types as is shown inFigure 6. These phenomenons were caused by theincrease of fiber surface roughness.5. ConclusionsTwo types of polyolefin macro-fibers having ca.0.5 mm in diameter were surface treated by meanof the cold low-pressure oxygen plasma treatment inorder to achieve their surface physical changes viaion bombardment and thus to enhance an interactionbetween their surfaces and the cement matrix.The fibers were exposed to plasma by 30 and480 seconds. To asses the plasma modification im-pact onto fiber surfaces, the scanning electron mi-croscopy enabling an observation of morphology andsingle fiber pullout tests from the cement matrix(prismatic cement specimens composed of Portlandcement CEM I 42.5 R, w/c ratio 0.4, dimensions25×20×25mm, embedded length 50 % of the fiberoriginal length) pointing out on an interaction betweenthe two materials directly were done. Interfacial shearstresses were calculated to mutual compare and nor-malize provided values. The stresses were assessedinto two stages – when the value reached to its maxi-mum (before the full debonding) and when the fiberfree-end was pulled out from the matrix by 3.5 mm.Because we concerned primarily to the physical fiber133J. Trejbal, T. Horová, Z. Prošek Acta Polytechnica CTU Proceedingssurface modification, we focused on the shear stresseswhen was the fiber completely debonded from thematrix in detail. Based on the two experiments, wefound out that:• Fiber surfaces were roughened via ion bombard-ment especially in the case of modification lasting480 seconds. Compared to reference fibers charac-terized by the smooth surface, modified sampleswere etched. Grooves and other surface damageswere revealed (scales, depressions) using SEM.• The cohesion between the two materials was sig-nificantly increased. The maximal interfacial shearstress increased by ca. 10 and 60 % for both treat-ment times (30 and 60 seconds) of plasma modifi-cation in the case of Concrix and BeneSteel fibers,respectively. After the full debonding, the cohesionbetween both fiber types was increased by 70–80 %,if compared to reference fibers.All in all, it was demonstrated that plasma treat-ment is an effective method to modify polymer fiberssurfaces in order to enhance their interaction with thecement matrix, as it was found also in [5, 8, 14]. Thismodifications may bring many benefits in the fieldof FRC production. To extend the plasma modifica-tion into the industrial use, it will be necessary toaccelerate the treatment process.AcknowledgementsThis work was financially supported by CzechTechnical University in Prague – SGS projectSGS16/201/OHK1/3T/11, by the Czech Science Founda-tion research project 15-12420S and by the Ministry ofEducation, Youth and Sports within National Sustainabil-ity Programme I, project No. LO1605.References[1] V. Li. Engineered cementitious composites - tailoredcomposites through micromechanical modeling. Journalof Advanced Concrete Technology 1(3):64–91, 2003.doi:10.3151/jact.1.215.[2] V. Li. Engineered cementitious composites (ecc) –material, structural, and durability performance. InConcrete Construction Engineering Handbook, chap. 24.CRC Press, 2008.[3] P. Tatnall. Fiber-reinforced concrete. In Significanceof Tests and Properties of Concrete and Concrete-Making Materials, chap. 49. ASTM International, 2006.[4] M. Luňáček, P. Suchánek, R. Bader. Application ofbicomponent synthetic macro fibres concrix intunneling. Tunnel 8:54–56, 2012.[5] V. Li, H. Stang. Interface property characterizationand strengthening mechanisms in fiber reinforced cementbased composites. Advanced Cement Based Materials6(1):1–20, 1997. doi:10.1016/S1065-7355(97)90001-8.[6] A. Naaman, H. Reinhardt. High performance fiberreinforced cement composites: Classification andapplications. Materials and Structures 36(10):389–401,2003. doi:10.1007/BF02479507.[7] J. Trejbal, L. Kopecký, S. Potocký, J. Fládr. Theeffect of glass fiber reinforcement on flexural strength oflime-based mortars. In 53rd conference on experimentalstress analysis, pp. 450–453. Czech Technical Universityin Prague, 2015.[8] J. Trejbal, L. Kopecký, P. Tesárek, et al. Impact ofsurface plasma treatment on the performance of petfiber reinforcement in cementitious composites. Cementand Concrete Research 89:276–287, 2016.doi:10.1016/j.cemconres.2016.08.018.[9] V. Machovič, V. Lapčák, L. Borecká, et al.Microstructure of interfacial transition zone between petfibres and cement paste. Acta Geodyn Geomater10(1):121–127, 2013. doi:10.13168/AGG.2013.0012.[10] B. Wei, H. Cao, S. Song. Tensile behavior contrast ofbasalt and glass fibers after chemical treatment.Materials and Design 31:4244–4250, 2010.doi:10.1016/j.matdes.2010.04.009.[11] M. Troëdec, A. Rachini, C. Peyratout, et al.Influence of chemical treatments on adhesion propertiesof hemp fibres. Journal of Colloid and Interface Science356:303–310, 2011. doi:10.1016/j.jcis.2010.12.066.[12] A. Tagnit-Hamou, Y. Vanhove, N. Petrov.Microstructural analysis of the bond mechanismbetween polyolefin fibers and cement pastes. Cementand Concrete Research 35(2):364–370, 2005.doi:10.1016/j.cemconres.2004.05.046.[13] L. Yan, R. Pendleton, C. Jenkins. Interfacemorphologies in polyolefin fiber reinforced concretecomposites. Composites Part A: Applied Science andManufacturing 29(5–6):643–650, 1998.doi:10.1016/S1359-835X(97)00114-0.[14] J. Trejbal, V. Šmilauer, A. Kromka, et al.Wettability enhancement of polymeric and glass microfiber reinforcement by plasma treatment. In Nanocon2015 7th international conference on nanomaterials –research and aplication – conference proceedings, pp.315–320. Tanger, 2015.[15] M. Sharma, S. Gao, E. Mäder, et al. Carbon fibersurfaces and composite interphases. Composites Scienceand Technology 102(6):35–50, 2014.doi:10.1016/j.compscitech.2014.07.005.134

PULLOUT BEHAVIOR OF OXYGEN PLASMA TREATED POLYMER FIBERS FROM CEMENT MATRIX

https://ojs.cvut.cz/ojs/index.php/APP/article/download/4649/4468

PULLOUT BEHAVIOR OF OXYGEN PLASMA TREATED POLYMER FIBERS FROM CEMENT MATRIX

Abstract

Similar works

Full text

Available Versions

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)