A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

THE PULTRUSION TECHNOLOGY FOR THE PRODUCTION OF FABRIC-CEMENT COMPOSITES (a)

Alva PELED and (b)Barzin MOBASHER (a) Structural Engineering Department, Ben Gurion University, Beer Sheva, Israel, e-mail:[email protected] (b) Department of Civil and Environmental Engineering, Arizona State University, Tempe, AZ, USA, e-mail:[email protected] ABSTRACT Use of reinforcement in thin cement based elements is essential in order to improve the tensile and flexural performance. The reinforcements can be either short fibers or continuous reinforcement, in a fabric form. Practical use of fabric-cement composites requires an industrial cost-effective production process. The objective of this study was to develop the pultrusion technique as a novel industrial cost-effective method for the production of prefabricated high performance thin-sheet fabric-reinforced cement composites. Woven fabrics made from low modulus polyethylene and glass meshes were used to produce the pultruded cement composites. The influence of fabric cell opening, application of pressure during the process, and cement-based matrix modification were examined. The mechanical behavior of the pultruded fabric-cement components was found to be relatively high obtaining strain hardening behavior even for fabrics with low modulus of elasticity. The best performance was achieved for glass fabric composites when high content of fly ash replacing the cement. The intensity of the applying pressure significantly affects the mechanical behavior of the pultruded composite. The promising combination of fabric reinforcement in cement composite products and the pultrusion process is expected to lead to an effective novel technique to produce a new class of high performance fabric-cement composite materials. Keywords Fabric, cement, composite, glass fiber, polyethylene fiber, pultrusion, processing INTRODUCTION The past decade has seen an increased use of prefabricated cement-bonded fiberboard around the world. Such elements are used for wall panels, exterior siding, pressure pipes, and roofing and flooring tiles. Use of reinforcement in these elements is essential in order to improve the tensile and flexural performance. The reinforcements can be either as short fibers or as continuous reinforcements, in a fabric form. Recently, several researchers began to examine the potential of thin sheet cement-based products reinforced with hand lay-up of fabrics, showing very promising results [1-6]. In addition to ease of manufacturing, fabrics provide benefits such as excellent anchorage and

chopped strand mat

A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

bond development. Peled et al [3] found that the flexural strength of cement-based composite products with low modulus polyethylene fabrics is almost two times higher than the strength of composites reinforced with straight continuous polyethylene yarns. The main explanation for the improved performance of fabric reinforced cement composites is the enhanced matrix-reinforcement bond due to mechanical anchoring. The mechanical anchoring is provided by the non-linear geometry of the individual yarns within the fabric, induced by the fabric structure [4,7]. These results for cement-based composites differ from the behavior of polymer composites, where straight yarns lead to optimal results while non-linear yarns reduce the reinforcing effectiveness [8]. Practical use of fabric-cement composites requires an industrial cost-effective production process, which has not been developed yet. The pultrusion process is a natural candidate for this purpose, as it is based on a relatively simple set up using low cost equipment while assuring uniform production. Pultrusion has been examined to produce cement composites with continuous filaments (filament winding technique) by several researchers, exhibiting significantly improved performance. Cement composites containing 5% (AR) unidirectional glass fibers produced by pultrusion achieved tensile strength of 50 MPa [9], compared to an average tensile strength of about 6-10 MPa of conventional GFRC (Glass Fiber Reinforced Cement) composites. Pultrus ion products reinforced with PAN-based carbon continuous filaments achieve superior flexural strength of about 600 MPa with 16% content by volume [10] and 800 MPa with 23% content by volume [11]. The challenge in developing an industrial production method to produce thin sheet cement elements with fabrics is not only technological, but also scientific, as it has been demonstrated in numerous circumstances that the production method can have substantial impact on the properties of the final product. It was reported that even when keeping the same matrix and fibers, changing the process could significantly affect the properties of the composite [12-14]. The objective of this study was to develop a novel industrial cost-effective method, the pultrusion technique, for the production of prefabricated high performance thin-sheet fabricreinforced cement composites. Woven fabrics made from low modulus polyethylene as well as glass meshes were used to produce the pultruded cement composites in this study. The influence of the opening of the fabric as well as the applying pressure during the pultrusion process was studied. Also, various mixtures containing superplasticizer and fly ash as replacement for cement were examined in order to find the appropriate rheology for the pultrusion process. A microstructural analysis was conducted and correlated with the mechanical performance of the composite. EXPERIMENTAL Two types of fabrics were used for this study: bound fabric and woven fabric (plain weave). In bounded fabrics a perpendicular set of yarns (warp and weft) are glued together at the junction points. In woven fabrics the warp and the fill (weft) yarns pass over and under each other. The bounded fabric was made from multifilament AR with 2 yarns per cm, in both directions of the fabric. The woven fabric was made from monofilament polyethylene (PE) with 22 yarns per cm in the reinforcimg direction (warp yarns) and 5 yarns per cm in the perpendicuar direction (fill yarns). The AR glass fibers were with tensile strength of 1270-2450 MPa, elasticity modulus of 78,000 MPa and filament diameter of 13.5 microns. The PE fibers were with tensile strength of 260 MPa, 1760 MPa modulus of elasticity and 0.25 mm diameter. The pultrusion process was used to produce the fabric -cement specimens in this work. In this process the fabrics were passed through a slurry infiltration chamber, and then pulled through a set of rollers to squeeze the paste in the openings of the fabric, removed excessive paste, and formed

biaxial fabric

A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

composite laminates. The pultrusion set up is presented in Figure 1. By using this method fabriccement sheets with width of 20 cm, length of 33 cm and thickness of about 1 cm were produced. Each cement board was made with 7 layers of fabrics, giving reinforcement content of about 4.5% and 9.5% by volume of AR glass fibers and PE fibers, respectively. After forming the sample through the rollers, a pressure was applied on top of the fabric cement laminates to improve penetration of the matrix in between the opening of the fabrics. A constant pressure of 15 KPa due to a 900 N load applied on the surface of the fabric-cement sheet was applied for both fabric types. Most of this pressure was removed 1 hour after the pultrusion process, with a 100 N load up to 24 hours (1.7 KPa) from the pultrusion process. In the case of the AR glass fabric a load of 100 N was studied to examine the effect of the applying pressure on the mechanical performance of the composite. Note that the intensity of this applied pressure is limited and cannot reach high levels, as the matrix is still fresh at this point and elevated pressures can remove most of the matrix in between the fabric laminates. This results in an insufficient matrix content to help bind the laminates. In this study, 900 N was the highest pressure examined which still kept the cement matrix in between the laminates of the fabric.

Impregnated fabric Impregnated cement bath Composite laminates Pressure Cylinders Figure 1: Set-up of the pultrusion process Controlling the rheological properties of the cement mixture is an important factor during the pultrusion process. The mixture should be sufficiently fluid to enable the fabric to transfer through the cement slurry but dense enough so that it will remain on the fabric when it leaves the cement bath. In order to develop a mixture with optimal rheology for the pultrusion process addition of fly ash as well as superplasticizer were used to produce the fabric cement laminates. The mix designs used in this study are presented in Table 1. In all cases the water/binder ratio by weight was 0.4. The rheology properties of the fresh mixtures were measured by shear rheometery 10 minutes after starting mixing. This was the average time required to produce the pultruded sheet. Table 2 presents the experimental program for all tested systems. Table 1: Ingredients of the pultruded mixtures Volume Fraction % Batch #

#1

#2

#3

Cement Silica Fume Fly Ash Superplasticizer

42 5 --0.1

42 5 --0.2

19 5 24 0.05

biaxial fabric

A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

After 24 hours from the pultrusion process the remaining 100 N loads were removed and then the fabric -cement laminates were cut to specimens having width of 250 mm and length of 180 mm. There after all specimens were cured for 3 days in 80ºC steam and then stored in room environment until testing in tension, 7 days after the pultrusion process. For studying the mechanical performance of the pultruded laminates closed loop control direct tensile tests were performed on a MTS testing machine with a rate of 0.5 mm/min. The composite laminates were examined along the pultrusion process. The tensile stress versus strain was recorded. Typical stress-strain curves representing the tensile behavior of individual composites were chosen for comparison. Attention was given to formation of distributed cracks throughout the tensile testing. The microstructure of the pultruded specimens was studied by Scanning Electron Microscopy (SEM) and correlated with the mechanical performance. Fragments of specimens obtained after tensile tests were dried at 60°C and gold-coated for observation under the SEM. Table 2: Experimental program of the different fabric systems Fabric type AR-Glass AR-Glass AR-Glass AR-Glass Polyethylene

Batch # 1 1 2 3 2

Pressure N 100 900 900 100 900

RESULTS AND DISCUSSION Influences of processing technology The pultrusion process was successfully used in this study and several fabric-cement sheets were produced with relative ease. Figure 2 shows an example of such pultruded specimen reinforced with AR glass fabric. The smooth surface and aesthetically appealing of this pultruded specimen is clearly seen in this figure. This smooth surface would only require painting when these elements are assembled in practice.

Figure 2: Specimen of cement composite with glass fabrics produced by the pultrusion process. Comparison of the tensile behavior of AR glass fabric produced with the pultrusion method compares with the conventional GFRC produced by the spray process containing short glass fibers is presented in Figure 3. The benefit of using the pultruded fabric laminates is obvious in this figure. A

biaxial fabric

A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

significant superior tensile behavior in both, strength and toughness is seen for the pultruded specimen compares with much poorer response of the conventional GFRC. This improvement in tensile behavior of the pultruded composite is greater than four folds, indicating the advantages of using fabrics with the pultrusion process. Effects of fabric types Two fabrics were used in this study with low and high modulus of elasticity, PE and glass, respectively. The spacing between the yarns in the fabric was much smaller for the polyethylene fabrics as compared to the glass fabrics, i.e., the polyethylene fabrics were relatively dense having less then 1 mm opening between the yarns in the fabric. A good penetration of the cement matrix was obtained even for the PE fabric with the finest opening (less than 1 mm), as presented in Figure 4. This figure presents SEM micrographs of the fabrics embedded in the cement matrix, indicating that the pultrusion process can successfully be used for relatively dense fabric s, which can open the use of fabric for wide range of fabric structures and products. The tensile behavior of both fabric systems is shown in Figure 5. A strain hardening behavior is clearly seen for both fabrics although the polyethylene fabric is made from low modulus fibers. Greatest tensile strength is observed for the AR glass fabric, a tensile strength of above 20 MPa is seen compared to only 12 MPa for the PE fabric. The increased tensile performance of the AR glass fabric composites can be attributed to the high modulus of elasticity of this glass fabric. The ductile behavior of the PE fabrics is clear in this figure.

Tensile stress, MPa

20

Fabrics Pultrusion process Vf=4.5%

16 12 8

Short fibers Spray process Vf=5%

4 0 0

0.01

0.02

0.03

Strain mm/mm

Figure 3: Tensile behavior of AR glass fabric produced with the pultrusion method compares with GFRC produced by the spray process containing short glass fibers

A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

AR-glass

PE

Figure 4: SEM micrograph of PE and AR glass fabrics embedded in the cement matrix

Tensile Stress, MPa

24 AR glass

20

AR glass

16 12 PE

8

PE 4 0 0

0.03

0.06

0.09

0.12

Strain, mm/mm

Figure 5: Tensile responses of cement composites with AR glass fabric and PE fabric Effect of pressure After forming the fabric-cement composite with the pultrusion process an additional pressure was applied on top of the laminates to improve penetration of the matrix in between the opening of the fabrics. In order to understand the effects induced by applying pressure on the mechanical behavior of the pultruded fabric-cement laminates, two different pressures were examined for AR glass fabric composite at 100 N and 900 N. Figure 6 shows the effect of pressure on the cement penetration, and the difference in the penetration of the cement matrix is clearly observed. In Figure 6a no pressure was applied, whereas in Figure 6b the applied pressure was 15 KPa. It is observed that the matrix did not penetrate fully to the opening of the fabric when no pressure was applied. However, a good penetration is seen in between the opening of the fabric when 15 KPa pressures (900 N) were applied. This indicates that nominal pressure is needed after the pultrusion process in order to help with the penetration of the cement matrix in between the gaps of the fabric and fill up the mesh opening.

A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

(a)

(b) Figure 6: Effect of pressure on the cement penetration: (a) no pressure was applied, (b) 900 N (15 KPa) pressure was applied Figure 7 clearly shows that the intensity of the pressure significantly influences the tensile response of the composite. Increasing the pressure from 100 N to 900 N improves the tensile strength of the element by about 80%. However, the ductility of the low pressure composite is much greater than that of the high pressure composite. 20 Tensile Stress, MPa

900 N 16 100 N 12 8 4 0 0

0.01

0.02

0.03

0.04

0.05

0.06

Strain, mm/mm

Figure 7: The effect of the intensity of the pressure on the tensile behavior of AR glass fabric composites Such differences in the mechanical behavior of the two composites can occur due to poorer penetration of the matrix in between the opening of the fabric of the low pressed composite, leading to reduction in the bond strength between the fabric and the cement matrix. This difference in bonding was observed by SEM, as shown in Figure 8, which presents the AR glass fabric embedded in the cement matrix for specimens loaded with 100 N (Figure 8a) and 900 N (Figure 8b) pressures. The observations focused on the corner of the mesh, where the perpendicular yarns are connecting together to form the fabric. A larger gap between the fabric and matrix is observed for the composite with the lower pressure, indicating poorer bonding of this laminate composite

A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

(Figure 8a). Much smaller gap between the matrix and the fabric is seen for the high intensity pressure (Figure 8b), indic ating improved bonding with this system.

Matrix

fabric

(b) 900 N

(a) 100 N

fabric

Matrix

Figure 8: SEM micrographs at the fabric-matrix interface near the corner of the fabric opening for the different pressures: (a) 100 N and (b) 900 N (X500). During the tensile tests successive cracks were developed along the width of the specimen, as expected. The spacing between the cracks can give indication on the bonding between the fabric and the cement, the finer the spacing the greater the bonding. Such differences in crack spacing were observed for the different pultruded specimens with the high and the low pressure, applied after the pultrusion process (Figure 9). The crack spacing of the high pressure specimen 100isNmuch smaller, only 10.2 mm, than that of the low pressure specimen, which is 16.8 mm, suggesting better bond between the fabric and the cement matrix when the pressure is increased. This correlates with the SEM observations (Figure 8) and can explain the improved mechanical behavior of the composite with the higher pressure (Figure 7).

900 N

Figure 9: Fabric -cement composites produced with different pressures after tensile test Matrix modification Controlling the rheological properties of the cement mixture is an important factor during the pultrusion process. In order to develop a mixture with optimal rheology, addition of superplasticizer (SP) and fly ash were evaluated (Table 1). AR glass fabric composites with 0.1% and 0.2% SP were produced by the pultrusion process. The tensile results indicated that the composite with the increased content of SP performed better than that with the lower content of SP, the tensile strength of the specimen with the high content SP was about 21 MPa compared with only 16 MPa of the low content SP specimen. This improvement is in about 25% and can be due to improved rheological properties with the increased SP content. Measurement of the viscosity of the fresh cement mixtures with various SP contents indicated a reduction in shear stress from 3672 N/m2 to 856 N/m2 when

A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

the SP content decreased from 0.2% to 0.1% by volume. Better fluidity of the fresh matrix can lead to better penetration of the cement in between the fabric opening. Cement laminate composites with AR glass fabrics and high content of fly ash were also produced by the pultrusion process and examined in tension. In this case, 60% by volume of cement was replaced by fly ash. The results clearly indicate an improvement in the mechanical behavior of the cement composite compared to similar composites without fly ash, as clearly presented in Figure 10. This suggests that the use of fly ash as replacement for cement can be uniquely beneficial for the pultrusion process, when glass fabrics are used. A tensile strength of about 25 MPa at a strain capacity of about 5% is observed with these fly ash materials. This represents a tensile strength of more than eight times, and a strain capacity more than 400 times the plain cement based materials. Compared to GFRC, these materials are as much as five times stronger and have about six times more strain capacity (Figure 3). The ductility is therefore more than an order of magnitude increased as compared to GFRC containing 5% AR glass fibers. Such improvement by the fly ash can be attributed to the improved rheology properties of the fresh mixture and improvement in the durability of the glass fabric due to the presence of fly ash. The rheology of the fresh mixture indicated a shear stresses of 586 N/m2 with the fly ash in comparison with shear stresses of 3672 N/m2 of the control mixture without fly ash.

Tensile Stress, MPa

25 With FA

20 15 10

No FA

5 0 0

0.01

0.02

0.03

0.04

0.05

0.06

Strain, mm /mm Figure 10: Effect of fly ash addition on the tensile behavior of the fabric-cement composites

CONCLUSIONS The pultrusion technique for the production of fabric-cement products requires relatively simple set up using low cost equipment while allowing good control of laminates alignment and giving relatively smooth surface. The tensile behavior of the pultruded composite with glass fabrics was significantly improved compares with conventional GFRC made with the spray process. An improvement greater than 4 folds in tensile strength of the pultruded composite was observed.

A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

The mechanical behavior of the pultruded fabric-cement components was impressive with a relatively high strength and exhibiting strain hardening behavior even for fabrics with low modulus of elasticity. The best performance was achieved for glass fabric composites when high content of fly ash was replaced with the cement at the rate of 60% by volume. This observation suggests that the use of fly ash as replacement for cement can be uniquely beneficial for the pultrusion process, when glass fabrics are used. A pressure is required on top of the fabric-cement laminates after the pultrusion process in order to enable penetration of the cement in between the opening of the fabric. The intensity of this pressure was found to significantly affect the mechanical behavior of the pultruded composites. Increasing the pressure significantly improves the tensile strength due to improvement in bond strength between the fabric and the cement matrix. The promising combination of fabric reinforcement in cement composite products and the pultrusion process is expected to lead to an effective novel technique to produce a new class of high performance fabric-cement composite materials. Such breakthrough may open the way for a multitude of new products and applications. ACKNOWLEDGEMENTS The authors would like to thank Nippon Electric Glass Co., Ltd. for their cooperation and for the effort they made to provide the glass fabrics used in this study. REFERENCES 1.

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A. Peled, and B. Mobasher, “Cement Based Pultruded Composites with Fabrics,” Proceedings , 7th International Symposium on Brittle Matrix Composites (BMC7), Warsaw, Poland, pp. 505-514, 2003.

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