Tensile Behavior of Fabric Cement-Based Composites ... - CiteSeerX

Tensile Behavior of Fabric Cement-Based Composites: Pultruded and Cast A. Peled1 and B. Mobasher2 Abstract: There is a growing interest in the use of fabrics as reinforcements for cement composites due to their superior performance in comparison to other cementitious composites. This paper compares the effects of two processing methods, casting and pultrusion, on the tensile properties of fabric-cement composites. Four fabric types were used, including bonded glass mesh, woven polyvinylalcohol, woven polyethylene, and warp knitted weft insertion polypropylene. The evolution of crack spacing and crack width as a function of applied strain as well as stiffness degradation were correlated with tensile responses of various composites. Pullout tests and microstructural analysis were conducted to better understand the tensile behavior. The advantages of using pultrusion are clear. Pultruded fabric-cement composites exhibited improved mechanical performance, especially those that incorporated knitted fabrics made from multifilament yarns with an open junction point. This improved performance is due to the improved bonding by the impregnation of cement paste during pultrusion, which helped fill the spaces between the filaments of the bundled yarns. DOI: 10.1061/共ASCE兲0899-1561共2007兲19:4共340兲 CE Database subject headings: Cements; Composite materials; Fibers; Tensile loads; Fabrics; Bonding; Pultrusion.

Introduction New cement-based composite systems with enhanced performance can be obtained by impregnating cement paste or mortars into a fabric. New types of reinforcements, potential production technologies, and the mechanical properties of the composites have been studied recently 共Peled et al. 1999, 1998; Peled and Bentur 2000; Kruger et al. 2003; Meyer and Vinkler 2003; Häußler-Combe et al. 2004; Mobasher and Pivacek 1998; Mobasher et al. 1997兲. In addition to the ease of manufacturing, fabrics provide benefits such as excellent anchorage and bond development 共Peled et al. 1999兲, and improve composite behavior 共Peled et al. 1998; Peled and Bentur 2000兲. The mechanical properties of composites incorporating continuous yarns produced by filament winding processes have been reported by Mobasher et al. and others 共Mobasher and Pivacek 1998; Mobasher et al. 1997; Kazuhisa et al. 1998; Nishigaki et al. 1991兲. Results indicate that cement composites containing 5% 关alkali resistance AR兲兴 unidirectional glass fibers achieved a tensile strength of 50 MPa 共Mobasher and Pivacek 1998; Mobasher et al. 1997兲; conventional glass fiber-reinforced cement 共GFRC兲 composites have an average tensile strength of only 6 – 10 MPa. Similar products reinforced with polyacrylnitril 共PAN兲-based carbon continuous filaments achieve superior flexural strength of about 600 MPa with 1

Senior Lecturer, Structural Engineering Dept., Ben Gurion Univ., Beer Sheva 84105, Israel. E-mail: [email protected] 2 Professor, Dept. of Civil and Environmental Engineering, Arizona State Univ., Tempe, AZ 85287-5306. E-mail: [email protected] Note. Associate Editor: Houssam A. Toutanji. Discussion open until September 1, 2007. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on June 28, 2005; approved on April 24, 2006. This paper is part of the Journal of Materials in Civil Engineering, Vol. 19, No. 4, April 1, 2007. ©ASCE, ISSN 0899-1561/2007/4-340–348/$25.00.

16% content by volume 共Kazuhisa et al. 1998兲 and 800 MPa with 23% content by volume 共Nishigaki et al. 1991兲. The concepts of filament winding were recently extended to a pultrusion process that could be used for mechanical impregnation of fabrics in an industrial setting 共Mobasher et al. 2005; Peled and Mobasher 2005; Peled et al. 2004兲. Results of various studies suggest that the manufacturing process significantly affects the properties of the composite 共Igarashi et al. 1996; Delvasto et al. 1986; Peled and Shah 2003兲. Using the same processing method, materials, and fiber, Igarashi et al. 共1996兲 found that increasing the processing time of the fresh mixture influences the fiber-matrix bond strength due to changes in the interfacial microstructure. Delvasto et al. 共1986兲 found that the flexural response performance of the composite depended on the applied pressure after casting. Pressed composites showed an increase in flexural strength but a reduction in the postcracking response. Peled and Shah 共2003兲 found significant differences in the properties of cast and extruded composites with similar matrix and fiber. A more fundamental understanding of the effects of pultrusion on fabric cement-based composites will aid both design and fabrication. This paper compares the tensile properties of cement composites reinforced with various fabric types and made by two different processing methods, casting and pultrusion. The study correlates composite behavior with bond and microstructural characteristics. Multiple cracking characterizes the tensile behavior of fabriccement composites. The nature of multiple cracking and the resulting stress-strain curve, toughness, and strength are dependent on the properties of the fabric and cement matrix, the interface bond, and the fabric anchorage. Therefore, microstructural features such as crack spacing, width, and density as well as the composite stiffness in the postcrack range are chosen to correlate the effect of materials and processing methods to changes in tensile strength, strain capacity, and ductility. Factors including fabric type, structure, and processing methods are addressed in the context of tensile stress, crack spacing,

340 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / APRIL 2007

Table 1. Properties and Structure of Yarns Made up the Fabrics Yarn type PE PP PVA Glass

Yarn nature

Strength 共MPa兲

Modulus of elasticity 共MPa兲

Filament size 共mm兲

Bundle diameter 共mm兲

Number of filaments

Monofilament Bundle Bundle Bundle 共coated兲

260 500 920 1,360

1,760 6,900 36,000 78,000

0.250 0.040 0.025 0.014

0.25 0.40 0.80 0.80

1 100 200 —

crack width, and stiffness degradation as a function of applied strain. The pultruded and cast composites incorporated bonded glass mesh fabric, woven polyvinylalcohol 共PVA兲 and polyethylene 共PE兲 fabric, and warp knitted weft insertion polypropylene 共PP兲 fabric. Pullout tests were conducted to examine the bond between the fabric and the cement matrix. Finally, a qualitative examination of the microstructure of the composites was conducted using optical and scanning electron microscopy and correlated with the mechanical properties of the composite and interface bond.

Experimental Program Fabric Types Three types of fabrics were used in this study: Bonded, warp knitted weft insertion, and woven. In bonded fabrics, a perpendicular set of yarns 共warp and weft兲 are glued together at junction points. In weft insertion warp knitted fabric, yarns in the warp direction are knitted into stitches to assemble straight yarns in the weft direction; these are the reinforcing yarns in the composite. In woven fabrics, the warp and the fill 共weft兲 yarns pass over and under each other. The bonded fabric was made from AR glass fibers with four yarns per cm in both directions; it was entirely coated with sizing. The weft insertion knitted fabric was composed of multifilament polypropylene 共PP兲 with eight yarns per cm in the reinforcing direction and 0.8 stitches per cm in the perpendicular direction. Two woven fabrics were studied: One made from multifilament PVA with 5.5 yarns per cm in both directions, and one made from monofilament PE with 22 yarns per cm in the reinforcing direction and six yarns per cm in the perpendicular direction. Table 1 presents the properties and geometry of the various yarns. The variety of fabrics selected allows the comparison of different structures 共woven, knitted, and bonded兲 and yarn arrangements 共mono- versus multifilament, twisted, and coated兲.

Pultrusion Process As shown in Fig. 1, during pultrusion fabric passes through a slurry infiltration chamber, and then is pulled through a set of rollers to squeeze the paste between the fabric openings while simultaneously removing excess paste. The fabric-cement composite laminate sheets were then formed on a plate shaped mandrel, resulting in samples with width 25 mm, length 250 mm, and thickness 7 – 10 mm 共depending on fabric type兲. Each cement board was made with six layers of fabric, resulting in reinforcement content by volume of about 6% for the AR glass, 9% for the PP and PE, and 11% for the PVA fabric. After forming the samples, a constant pressure of 15 kPa due to a 900 N load was applied on top of the fabric-cement laminates to improve matrix penetration between the yarn and fabric openings. This pressure was reduced within 1 h of pultrusion to a 100 N load 共1.7 kPa兲 and maintained up to 24 h. Note that the volume fractions were calculated based on the bundle diameter and assume no penetration of the cement matrix between the filaments of the bundle. Casting Process Casting specimens were prepared by hand lay-up of fabric layers in the cement matrix. Similar to the pultruded specimens, a six layer fabric laminate of dimensions 250⫻ 25⫻ 10– 13 mm was prepared that provided reinforcing yarns by volume in the range of 3–6%, depending on yarn and specimen type. Thicker specimens were prepared due to increased matrix content, resulting in a lower volume content of reinforcing yarns as compared to pultruded specimens.

Composite Preparation Composite specimens were prepared by pultrusion or by lay-up of fabrics 共casting兲 in a cement matrix. The effect of the processing methods on composite properties was determined by studying the composite under tension as well as by pullout. The matrix used throughout the study was made from 42% cement, 5% silica fume, 0.1% superplasticizer, and 50% water by volume; the water/cement ratio by weight was 0.37. Specimens for Tension Tests Tension specimens were prepared for all fabric types by both pultrusion and casting.

Fig. 1. The pultrusion process

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Specimens for Pullout Tests Specimens for pullout tests were only prepared from AR glass and PP fabrics. For one set of specimens, the fabrics were impregnated in a cement bath using pultrusion and then embedded by hand lay-up in the cement matrix. For the other set of specimens, the pultrusion process was skipped and the fabrics were directly embedded in the matrix. A single layer of fabric was placed in the center of each specimen, which was 25 mm wide by 8 mm thick, with a fabric width of 10 mm. Two embedded lengths of PP fabric were studied, 7.6 and 12.7 mm. The embedded length of the glass fabric was 12.7 mm. A tensile load of approximately 1.7 N per cm width of fabric was applied during processing to keep the yarn and fabric parallel to the longitudinal axis. The tension was released after 24 h. Curing All specimens, cast and pultruded, for tensile and pullout tests, were demolded 24 h after casting. Five replicate tensile fabriccement laminates were prepared from each system. All specimens were cured for three days at 80° C, 100% RH, and then stored in room environment until pullout or tension testing, seven days after processing.

Testing Tensile Tests The mechanical performance of the pultruded and cast laminates was studied using closed loop control direct tensile tests performed on a MTS testing machine with a capacity of 89 kN. The rate of cross head displacement was set at 0.008 mm/ s. Metal plates of dimensions of 25⫻ 30⫻ 1 mm were glued onto the gripping edges of the specimen to minimize localized damage and provide better load transfer from the hydraulic grips. At least five replicate samples were tested in each fabric and processing category; the reported results reflect the average and standard deviation values. Typical stress-strain curves representing the tensile behavior of individual composites were compared. Note that no failure occurred with the PP, PVA, and PE composites, and the tests ended after reaching a strain level of 0.06. Several parameters were calculated: 共1兲 The initial modulus of the composite 共at the elastic region兲; 共2兲 the BOP 共stress at the end of elastic region兲; 共3兲 the postcracking tensile strength; 共4兲 the toughness 共area under the load-displacement curve, up to 0.07 strain when applicable兲; and 共5兲 the processing efficiency factor 共the ratio between the postcracking tensile strengths of the pultruded and cast composites兲. The efficiency factor was calculated to compare the processing systems with different fabrics. Crack Spacing Measurements Throughout the tensile testing, parallel distributed cracks formed along the length of the specimen. The continuous formation of these cracks was recorded at 15 s intervals using a high resolution camera and a digital frame grabber. The images were processed with the digital processing toolbox of MATLAB using a two-step approach to quantitatively measure the crack spacing and density as a function of the applied strain. During the first step, newly formed cracks on each image were traced and added to data from the previous loading increment. The second step measured the

spacing between the traced cracks. The crack spacing was measured in pixels, and the image was calibrated using conventional techniques to convert pixel size to length measures 共Stang et al. 1990; Mobasher et al. 1990兲. The average crack spacing at each strain level was correlated with the stress-strain response. The stress-strain graph was used to calculate the tangent stiffness at several designated strain levels. Results were used to correlate the stiffness degradation with the crack spacing data. For more details, see Mobasher et al. 共2005兲. Pullout Tests Pullout tests formed the basis of the evaluation of the interaction between the fabrics and the cement matrix. The tests were conducted in an Instron testing machine at a crosshead rate of 0.25 mm/ s. Fabrics of width 10 mm were embedded in the cement matrix. To compare the pullout behavior of different density fabrics, only a part of the fabric equivalent to eight pulling yarns was subjected to load for each embedded fabric. In most cases the yarns perpendicular to the pullout direction remained in the matrix after the pullout test. Therefore, in addition to the shear stresses developed at the matrix-yarn interface, the pullout resistance includes the forces required to break the junctions between the yarns. The test continued until complete pull out of the yarns occurred. Load-slip curves were recorded. In all cases, a 25.4 mm free length of fabric separated the pulled yarns and the grips. This was necessary for handling purposes. The effect of the free length compliance on the pullout curves was measured using specimens of several lengths, and the compliance was calibrated accordingly 共Mobasher et al. 1990兲. The reported results are the average values of at least six specimens tested under the same conditions. Microstructure Characteristics The microstructures of the different composites were characterized and correlated with their mechanical properties. For these observations, specimen fragments obtained after tensile and pullout tests were dried at 60° C and gold-coated. Microstructural features such as matrix penetration between the opening of the fabric, between the filaments of the bundle, and between the stitches of the knitted fabric were evaluated using a scanning electron microscope 共SEM兲. Attention was given to the processing effects, focusing on impregnated and unimpregnated fabrics.

Results Tensile Behavior Fabric Type Table 2 presents a summary of the average tensile results of the different composites. Fig. 2共a兲 provides the tensile behavior of the different fabrics for the cast system. The glass composites show improved strength 共11.5 MPa兲 with relatively stiff behavior, as compared to the more ductile behavior and relatively lower strength of the PP, PVA, and PE composites 共9.5, 7.9, and 5.0 MPa, respectively兲共Table 2兲. The trend is quite different for the pultruded specimens 关Fig. 2共b兲 and Table 2兴; the PP composite outperformed the other fabrics in both strength and toughness. This is despite having a lower relative modulus of elasticity than the AR glass fabric 共Table 1兲. The improved performance of PP is


Table 2. Tensile Properties of the Composites Modulus of elasticity 共MPa兲

First crack stress 共MPa兲

Tensile strength 共MPa兲

Processing efficiency factor 共pultrusion/cast兲

Fabric type

Pultruded

Cast

Pultruded

Cast

Pultruded

Cast

First crack

strength

PP PE PVA Glass

7,316 4,040 5,557 6,121

2,371 2,236 6,513 1,995

4.6 3.5 2.7 3.9

2.7 3.2 2.9 3.3

23.8 7.0 12.2 15.4

9.5 5.0 7.9 11.5

1.70 1.09 0.93 1.18

2.51 1.40 1.54 1.34

mainly at large strains above a value of 0.03. All the ductile composites, PVA, PP, and PE, were loaded to strain levels as high as 0.06 with no apparent failure. These trends imply the advantages of pultrusion for production of PP fabric composites, which will be further discussed in subsequent sections. The AR glass, PP, and PE cast specimens yielded similar values for the initial modulus 共Table 2兲 that are also representative of the matrix modulus, suggesting that the fabric has little effect. In pultruded samples, however, fabric type significantly influences the initial modulus. The PP and PE fabric yield the highest and lowest moduli, respectively. A low modulus for PE composite systems is expected due to the low modulus 共⬃2 GPa兲 of the fabric 共Table 1兲. The improved modulus of the PP composite is even greater than that of the AR glass composite even though the AR glass fiber, at 70 GPa, is much stiffer than the PP fiber at 6 GPa 共Table 1兲. An improved bond is expected for PVA fabric in both pultruded and cast composites because of the enhanced bonding of this fabric 共Kanda and Li 1998兲, which can lead to an enhanced modulus even for the cast system, as it does here. The cast fabric specimens show only a small difference in their BOP values. The pultruded PP fabric specimens, however, have much higher BOP values as compared to both cast PP fabric and other pultruded fabric specimens. This supports the beneficial aspects of pultrusion for PP fabric composites. Processing Fig. 3 compares casting and pultrusion for all fabrics. The pultruded composite always performed better than the cast one, although the level of improvement varies. PP fabric pultruded specimens exhibit superior tensile behavior in both strength and toughness as compared to the much poorer response of cast specimens 关Fig. 3共a兲兴. For example, the tensile strength of the PP pultruded composite is more than twice that of the cast composite. For AR glass fabric systems, an improvement in strength is observed for the pultruded composite but with more brittle behavior

Fig. 2. Stress-strain response of the composites with the different fabrics produced with the: 共a兲 cast process; 共b兲 pultrusion process

关Fig. 3共b兲兴. Pultrusion improves PVA composites mainly at high strains 关Fig. 3共c兲兴, and a relatively small influence is observed with PE composites 关Fig. 3共d兲兴. The efficiency factors calculated for these specimens emphasize the improved behavior of all pultruded composites as compared to the cast composites 共Table 2兲. The efficiency factor was calculated as the ratio between the pultruded and cast tensile strength values for each fabric system. All efficiency values are greater than 1.0, with a value of 2.5 for the PP composite and values of 1.5 and lower for the others. In addition, the pultrusion process yields a 50% improvement for the PVA fabric, while a smaller change in efficiency is observed for the PE and glass systems 共Table 2兲. The BOP efficiency values, calculated as the ratio between the pultruded and cast BOP values for each fabric, show better initial behavior of the pultruded PP fabric composite, suggesting better first crack behavior in this case. Since the pultruded and cast composites have the same number of fabric layers but different volume fractions of yarns, and assuming that the fabrics carry most of the loads at the ultimate point, one can use tensile load to compare the two processing systems. Fig. 4 compares tensile load versus strain for all composites. Pultrusion of PP composites results in 30% improved tensile loads at large strains 关Fig. 4共a兲兴. Similar behavior with a marginal influence at low strain is observed with PVA composites 关Fig. 4共c兲兴. The PE composite exhibits no difference between the two processing systems 关Fig. 4共d兲兴, however, and the cast glass fabric specimens performed better than the pultruded specimens, since the latter are more brittle 关Fig. 4共b兲兴. Crack Formation Throughout the tensile loading cycle, successive cracks were developed and photographically recorded by a digital frame grabber at regular time intervals. The calculated crack spacing based on these images was correlated with the applied strains and plotted together with the stress-strain responses in Fig. 3. In general, lower crack spacing at the end of the test suggests a stronger bond between the fabric and the cement matrix. Such lower crack spacing is clearly observed in all pultruded systems. The crack spacing of pultruded specimens throughout all strain ranges is significantly lower than that of cast composites, suggesting better bonding and improved mechanical behavior for the pultruded composites. Fabric type also plays a role. The PP fabric provides the highest change in crack spacing behavior when the cast and pultruded specimens are compared 关Fig. 3共a兲兴. At a strain value of 0.03, the crack spacing of the pultruded composite is less than 10 mm, whereas for the cast composite it is more than 20 mm, suggesting that pultrusion leads to a significant improvement in bonding. Much smaller differences in crack spacing are observed


Fig. 3. Comparison of stress-strain response and crack spacing behavior of the pultruded and cast composites for the various fabrics: 共a兲 PP; 共b兲 glass; 共c兲 PVA; and 共d兲 PE

when comparing pultruded and cast specimens for AR glass, PE, and PVA fabric specimens implying lower contribution to the bond by the pultrusion process. Crack width as a function of applied strain was tested on several PP specimens 共Fig. 5兲. The width of the crack through the entire loading is significantly larger for the cast composite than the pultruded one. At a strain value of about 1.5%, the cast crack width is 0.4 mm, but only 0.1 mm in the pultruded specimen. At a higher strain, 6%, this difference is even more evident, showing with crack width values of 1 and 0.2 mm for cast and pultruded specimens, respectively. These observations confirm the improved bonding of the PP fabric with the pultrusion process.

Stiffness One can also compare the composites’ ability to allow a fine distribution of cracking while maintaining a desired stiffness by plotting stiffness degradation as a function of crack spacing. Fig. 6 shows a comparative evaluation of the correlation of crack spacing with the tangent stiffness for PE, AR glass, PVA, and PP composites. In general, tangent stiffness and crack spacing decrease as a function of applied strain. For both processing methods, the AR glass composite exhibits the stiffest values due to its high modulus of elasticity. The stiffness magnitudes and degradation trends of the PP and PE composites produced by casting are

Fig. 4. Comparison of tensile load-strain response of the pultruded and cast composites for the various fabrics: 共a兲 PP; 共b兲 glass; 共c兲 PVA; and 共d兲 PE 344 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / APRIL 2007

Fig. 7. Pullout behavior of pultruded and cast specimens: 共a兲 glass; 共b兲 PP fabrics Fig. 5. Effect of crack width during loading of the PP fabric composite, for cast and pultruded specimens

similar, as expected due to their similar range of elasticity modulus 共Table 1兲. In the pultruded composites, however, the PP composite exhibits higher stiffness values than the PE system during the entire loading. The stiffness values and rate of degradation as a function of crack spacing are greater with the pultruded composites for the PP and AR glass systems. The reduction in stiffness values at low strains 共high crack spacing兲 are much more significant the pultruded than the cast composites regardless of fabric. At high strains, pultruded PP composites exhibit the greatest reduction. These results suggest that pultrusion does positively impact stiffness and crack spacing, particularly for PP composites. Pullout Behavior To better understand the effects of the processing parameters on mechanical properties, the pullout resistance of PP and AR glass fabrics was studied. Fig. 7 compares the pullout behavior of cast and pultruded 共impregnated and nonimpregnated兲 fabrics, and Table 3 presents the average maximum pullout loads. The production process exerts no significant influence on glass fabric pullout resistance 关Fig. 7共a兲 and Table 3兴. The fabric pullout resistance of the PP pultruded composites, however, is much greater than that of the cast ones 关Fig. 7共b兲兴. The results presented in Table 3 indicate that maximum pullout loads for both embedded lengths of the impregnated PP fabric are as much as twice

that of the nonimpregnated fabric. The average pullout loads of the AR glass composites are relatively low compared to PP 共Table 3兲. The improved pullout resistance of pultruded PP composites as compared with the cast may result in their increased tensile performance 关Figs. 3共a兲兴. The above differences between fabric types and processing methods may be related to differences at the interface developed with the different systems, as discussed in the next section. Microstructure Characteristics Fig. 8 presents SEM micrographs of all fabric types embedded in the cement matrix that show the geometry of their yarns, and the penetration of cement in them. The AR glass is a bonded fabric made from multifilament fiber bundles. Since the bundle is completely covered with sizing, however, matrix penetration occurs only between the interstitial yarn spaces 关Fig. 8共a兲兴. The PE is a woven fabric made of monofilament yarns; hence cement penetration only occurs between fabric spaces 关Fig. 8共b兲兴. The dense weave of the PVA fabric with its large bundles limits cement paste penetration between fabric openings as well as between the filaments of the bundles 关Fig. 8共c兲兴. Moreover, the woven structure allows the matrix to fill the spaces between the filaments only on the side of the bundle adjacent to the matrix; the other side is in contact with the transverse bundle. The bundles in the PP knit fabric are smaller and connected by loops, resulting in a relatively open fabric structure 关Fig. 8共d兲兴. This allows good penetrability of

Fig. 6. Effect of stiffness degradation versus crack spacing of the different fabrics produced by: 共a兲 cast; 共b兲 pultrusion JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / APRIL 2007 / 345

Table 3. Pullout Results of PP and Glass Fabrics

PP fabric PP fabric Glass fabric

Maximum pullout resistance 共N兲

Embedded length 共mm兲

Pultruded

Cast

7.6 12.7 12.7

239 271 146

118 174 184

cement paste between the filaments of the bundle and in their openings. These observations support the profound effect of processing on PP fabric. Fig. 9 provides a close up view of PP fabrics embedded in the cement matrix for both pultrusion and casting. In the pultruded specimen, good penetration of the cement matrix has occurred between the reinforcing filaments of the bundle 关Figs. 9共b兲 and 3共a兲兴 and between the loops and stitches 关Fig. 9共d兲兴. Much poorer penetration is observed with the cast composite; there are empty spaces between the filaments of the bundle 关Fig. 9共a兲兴, and the loops and the gaps between the filaments of the stitches are relatively empty 关Fig. 9共c兲兴. Note that since these stitches are located perpendicular to the loading direction during tension and pullout, their proper filling with paste improves anchorage to the cement matrix. These observations correlate well with the improved pullout behavior for the PP pultruded system 共Table 3兲 and the high tensile behavior of this composite 关Fig. 3共a兲兴.

Discussion The results obtained in this study indicate that the PP fabric was most affected by the different processing methods. Greater overall tensile strength and stiffness response, lower crack spacing, smaller crack width, and higher pullout resistance were found for the pultruded PP fabric as compared to cast. Significant stiffness degradation during tensile tests was observed for the cast PP composite as compared with pultruded, indicating higher damage during loading. These findings indicate that better interaction between fabric and matrix leads to the improved performance of the pultruded PP composite. The PVA specimens were also affected by pultrusion, but not as much as PP. The pultruded PVA composites exhibited enhanced mechanical performance as illustrated by higher tensile response, lower crack spacing, and greater stiffness 关Fig. 3共c兲 and Table 2兴. For the glass fabric, pultrusion was so detrimental that a higher load-strain response was measured for the cast specimen 关Fig. 4共b兲兴. The PE specimens did not exhibit a significant difference between the two processing meth-

ods as regards overall mechanical behavior 关Figs. 3共d兲 and 4共d兲兴. These differences in the behavior of the PE, glass, PVA, and PP composites are associated with the differences in the structure of the fabrics, the yarns that make up the fabrics, and their inherent bonding characteristics. The relatively open knit type PP fabric is made of bundles and connected at the junction points by stitches; the PVA is also made of bundles, but it is a dense, woven fabric. The PE fabric is woven but made from monofilament yarns, and the bonded type AR glass fabric contains bundles that were coated with sizing prior to fabric production. Note that knitting more easily produces open fabric structures than does weaving. When the bundled PP knit fabric is used to produce a composite by casting, the penetrability of the matrix into the spaces between the filaments of the bundles is relatively low. This is due to the presence and tightening effect of the bulky stitches, which strongly hold the filaments in the bundle and prevent them from being opened. The inferior penetration reduces filament-matrix interaction and the reinforcement potential of individual filaments. When this bundled knit fabric is pultruded, however, the high shear stresses help fill the spaces between the filaments as well as the loops of the stitches 关Figs. 8共d兲 and 9兴, leading to improved mechanical anchoring of the fabric to the cement matrix 关Fig. 7共b兲兴 and enhanced mechanical performance 关Fig. 3共a兲兴. The yarns of the PVA fabric are also multifilament and thus expected to be filled by pultrusion. But since the fabric is densely woven 关Fig. 8共c兲兴 and the bundles contact the paste only on one side while the other side is in full contact with the transverse bundle, complete penetration of the matrix between the filaments of the reinforcing yarns does not occur. The excessive weave density also leads to poor penetration between fabric openings and the filaments of the transverse bundles, reducing mechanical anchoring. The lack of stitches 共as opposed to knit fabric兲 also limits the mechanical anchoring. Even the intensive process of pultrusion cannot help much with filling the fabric spaces. Note that the number of filaments in one PVA bundle 共200兲 is greater than that of a PP bundle 共100兲共Table 1兲, which also causes less efficient matrix penetration between the inner filaments of the PVA bundle. But, due to the chemical bond of PVA with cement, the mechanical bond is relied upon to a lesser degree. All these observations may explain why the PVA fabric does not exhibit a significant difference in strength dependent on the processing method. The situation is quite different with the PE woven fabric, which is made from monofilament yarn. As such, the cement can only penetrate between the openings of the fabric, and no improved penetration of the matrix is required 关Fig. 8共b兲兴. Therefore, pultrusion does not significantly influence the mechanical performance of the PE composite 关Fig. 3共d兲兴. Similarly, the bundled

Fig. 8. SEM micrographs of the different fabric in the cement matrix: 共a兲 glass; 共b兲 PE; 共c兲 PVA; and 共d兲 PP 346 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING © ASCE / APRIL 2007

Fig. 9. Close look at PP fabric in the cement matrix made by the cast and pultruded methods: 共a兲 reinforcing yarns cast; 共b兲 reinforcing yarns pultruded; 共c兲 loops cast; and 共d兲 loops pultruded

yarns of AR glass fabrics were coated with sizing prior to fabric production, bonding the filaments together and filling the spaces between them. Thus, one cannot force the paste between the filaments, and pultrusion does not offer any advantage 关Fig. 8共a兲兴. In view of the above discussion, it can be concluded that pultrusion is highly valuable in the production of fabric cement composites only when the fabric has a multifilament structure such that the intensity of the pultrusion process is required to impregnate the spaces between the filaments with the matrix. However, the bundle and fabric structure must also be open enough to allow penetration of the cement between the filaments. In these cases the composite action between the matrix and the individual fine filaments is activated, leading to a higher efficiency of the fibers

as compared to other systems as well as inducing a strain hardening composite even in relatively low modulus yarns, which results in a composite with a strength that outperforms those obtained by much higher modulus yarns.

Conclusions 1.

2.

The results of the present study indicate that the production process of a fabric-cement composite should be adapted to the structure of the fabric to optimize its reinforcing efficiency. When reinforcements consist of multifilament bundles


3.

4.

5.

stitched together in a weft insertion warp knitted fabric, intensive shear forces are needed during processing to open up the spaces between the filaments and fill them with the matrix. The pultrusion process is effective in doing so, resulting in a much better pullout resistance, better utilization of the filaments to maximize their efficiency, and improved tensile performance. Mobilization of filaments in the pultrusion process results in a strain hardening composite even when the modulus of the yarn is relatively low, as the PP fabric demonstrates. This can be used for mobilizing low modulus yarn fabrics to obtain high performance cementitious composites. When dense woven fabrics with a large number of filaments are involved, the matrix cannot penetrate between the filaments even when intensive methods such as pultrusion are used, as shown by the PVA fabric in this study. In fabrics composed of monofilament yarns or of bundles of filaments coated with sizing, pultrusion offers no advantage from a mechanical point of view, as there are no fine spaces into which the matrix can be forced to penetrate. Thus, casting and pultrusion gave similar results.

Acknowledgments The writers would like to thank Nippon Electric Glass Co., Ltd., Kuraray America, Inc., and Karl Mayer, Ltd., for providing the fabrics used in this study. The National Science Foundation, Program No. 0324669-03, and the U.S. Israel Binational Science Foundation 共BSF兲 No. 2002232 are acknowledged for their financial support in this research.

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