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Three dimensional (3D) fabrics as reinforcements for cement-based composites
Accepted Manuscript Three dimensional (3D) fabrics as reinforcements for cement-based composites Eliyahu Adiel Sasi, Alva Peled PII: DOI: Reference:
S1359-835X(15)00132-3 http://dx.doi.org/10.1016/j.compositesa.2015.04.008 JCOMA 3907
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
3 April 2014 11 December 2014 12 April 2015
Please cite this article as: Sasi, E.A., Peled, A., Three dimensional (3D) fabrics as reinforcements for cement-based composites, Composites: Part A (2015), doi: http://dx.doi.org/10.1016/j.compositesa.2015.04.008
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Three dimensional (3D) fabrics as reinforcements for cement-based composites Eliyahu Adiel Sasi1, Alva Peled2
1
Material Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel
e-mail: [email protected], 2
Structural Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel e-mail: [email protected],
Abstract This research studied the flexural behavior of cement-based elements reinforced with 3D fabrics. The effects of the through-thickness (Z direction) yarns were examined in terms of four parameters: (i) yarn properties, (ii) varying the composite content of (i.e., coverage by) high-performance aramid yarn, (iii) treatment of the fabric with epoxy, and (iv) 2D and 3D fabric composites were compared. Overall, the 3D fabric composites performed better than the 2D fabric composites, which tended to delaminate. Our results indicate that even though the Z yarns are not oriented in the direction of the applied loads, 3D fabrics still have potential applications as reinforcements for cement-based composites. Indeed, the Z yarns hold the entire fabric together, which leads to improved mechanical anchoring and mechanical properties particularly when the fabric has been treated with epoxy, i.e., to create a stiff reinforcing unit. Key words: A. Fabrics/textiles; A. 3-Dimensional reinforcement; B. Mechanical properties; Cement composites
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Introduction
Recent years have seen a considerable increase in interest in TRC (textile reinforced cement/concrete) composites and in methods for improving their mechanical performance [1-2]. Research has shown that TRCs possess superior tensile strength, toughness, ductility and energy absorption properties [3-6]. The composite in two dimensional (2D) fabrics is reinforced in directions parallel, but not orthogonal, to the fabric plane. If using 2D fabrics to reinforce a composite, laminated composites should be prepared using several fabric layers. However, laminated composites are sensitive to failure by delamination and characterized by poor shear and split resilience properties under static, dynamic or impact loads. The wide variety of fabric structures enabled by advances in modern textile technology has conferred great flexibility on fabric design. Thus it is now also possible to produce three-dimensional (3D) fabrics with reinforcement in the plane normal to the fabric. Reinforced in three orthogonal directions, 3D fabrics can both 1
reduce the chances of composite failure by delamination and also enhance composite shear strength, and therefore, they are expected to markedly improve the mechanical properties of cement-based composites. To produce 3D fabrics, several methods, such as knitting, weaving, and braiding, among others, can be used. An attractive option for cement-based composites is double needle bar warp knitting, because it allows for an open structure that, in turn, facilitates better penetration by the cement. Warp knitting technology can be used to create 3D fabric structures by connecting two separate, independent 2D knitted fabrics together with a third set of yarns along the thickness of the fabric. The connecting yarns, referred to as spacer yarns, provide both stabilization and reinforcement. Recently, 3D spacer fabrics were developed for use in cement-based products [7]. Several studies of the behavior of cement-based composites with 3D fabric reinforcement demonstrated the potential applicability of these types of reinforcement in the cement field [8-10]. These studies focused mainly on 3D fabrics in which the through-thickness (Z direction) yarns were of low moduli, and as such, they were used for stabilization purposes only. The objective of this research was to study the flexural behavior of cement-based composites reinforced with 3D fabrics, the latter of which were produced by stitching 2D fabrics together in the Z direction using different yarns. Specifically, this study examined: (i) Z-direction yarn properties, e.g., high performance of aramid vs. low performance of polyester, (ii) Z-direction yarn content in terms of its highperformance aramid, 50% and 100%, (iii) treatment of the yarn with epoxy to increase fabric stiffness and reinforcement efficiency, and (iv) 2D fabric vs. the 3D fabric composite made from the same AR glass X, Y yarns.
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Experimental program
2.1
Preparation of specimens
2.1.1 Fabrics For this work, we used 3D warp knitted fabric structures in which two sets of independent 2D knitted fabrics were stitched together in the Z direction of the fabric using a third set of yarns (Fig. 1). The warp and weft yarns (the yarns along the X and Y directions) were stitched together with loops of fine multifilament polyester (PES), leaving square openings of 0.8 × 0.8 cm in the sections of 2D knitted fabric. The 3D fabrics used in this research were all made with multifilament alkali resistant (AR) glass yarns (Cem-FIL® grade) along the X (weft) and Y (warp) directions. The spacer yarns in the Z direction were made from two different yarn types (Table 1). The first, used mainly for stabilization, was monofilament polyester (PES), and the second, used to provide reinforcement, was a high-performance multifilament yarn of aramid. Three different types of fabric were prepared (Fig. 2): i. 2D fabric (2D) – made with AR glass along the weft and warp directions exactly as the 3D fabric but without the Z direction yarns; ii. 3D Reference (3D REF) – in which the spacer yarns comprised 2
PES only, i.e., without high-performance reinforcing spacer yarns; iii. 3D Aramid (3D Ar 100 or 3D Ar 50) – in which, in addition to PES, high-performance reinforcing aramid yarns were located along the Z direction. For 3D aramid, two fabrics with different aramid yarn contents were made. The first (3D Ar 100) contained the maximum possible content of spacer aramid yarns such that no gaps were evident between them to provide 100% reinforcing aramid yarns along the Z direction. In the second 3D aramid fabric (3D Ar 50), the aramid yarns were arranged in a "one in, one out" formation, i.e., the fabric knit alternated between an aramid yarn and then a gap with the width of one yarn to provide 50% reinforcing aramid yarns along the Z direction. Two sets of the fabrics described above were tested, one as it was received while the second was first treated with epoxy. Treatment with epoxy was difficult due to the complexity of fabric structure. Treatment with epoxy comprised coating all the yarns in all directions with the epoxy using a brush, applying enough epoxy such that it penetrated (i.e., saturated) the yarns but without filling in the spaces between the X and Y direction yarns (in 2D and 3D fabrics) or the space between two 2D fabric sections in a 3D composite. Ensuring these spaces remained free of epoxy enabled maximum cement matrix penetration of the fabrics. Sikadur® 52, a low-viscosity high-strength epoxy, was selected among other examined coating materials as it provided the best filling of the yarns while leaving the 3D fabric spaces open. The epoxy treatment conferred on the fabric greater stiffness and improved its reinforcement capacity, as the epoxy treated bundle behaved as a single unit in which loads were carried by all the filaments together. All fabrics were produced specifically for this research at the Institut für Textiltechnik der RWTH Aachen University (ITA). 2.1.2 Composites Eight composite systems, four with and four without epoxy, were prepared from the three different fabric types discussed above. In all composites, the matrix comprised a cement paste (water and cement only) with a 0.4 water/cement ratio using CEM II 42.5 N/B-V to keep the system as simple as possible. The 3D fabric specimens were prepared by filling the bottom portion of the mold with a thin layer of cement paste on which we placed the 3D fabric; the rest of the mold was then filled until the fabric was completely covered (Fig. 3). For the 2D fabric specimens, we prepared sandwich-type composites, in which the two layers of fabric were located at the top and bottom of the composite and separated by a layer of cement paste. To situate the 2D fabric layers within the matrix, a thin layer of cement paste was first cast in the bottom of the mold, the first 2D layer was laid down on that, more cement was added on top of the first fabric layer before the second 2D layer was positioned and then covered in cement to fill up the mold to the top. This method provided a two-ply fabric composite similar to the 3D fabric composite but without the through-thickness connecting yarns of the composite. During the cement paste casting, a vibration procedure was applied using a 3,600 VPM vibration table to ensure good penetration of the matrix between the openings of the fabric and bundle filaments. After casting, the composites were left to harden for 3
24 h, demolded, and then cut into specimens measuring 330 × 40 × 26 mm (length × width × thickness). The specimens were cured at 100% relative humidity for 13 days and then another 2 days at room environment until flexure testing 16 days after casting.
2.2
Test procedure
2.2.1 Tensile test The tensile properties of the 2D fabrics (without cement) were examined in both the weft and warp directions. A sample of 2D fabric was cut into 185 mm long slices with four yarns along the loading direction. Aluminum foil was attached with epoxy to both ends of the specimen. The test was executed on an Instron tensile machine with closed loop operation and load cell capacity of 100 kN. The crosshead velocity was fixed to 0.5 mm/min. Four specimens were tested in both the warp and weft directions. For each system a typical curve (i.e., one whose values were close to average) was chosen for comparison. 2.2.2 Four-point bending All the composite systems were tested by four point bending (Instron tensile machine as in 2.2.1), the setup of which entailed a support span of 300 mm and loading span of 100 mm. The crosshead velocity was fixed to 0.5 mm/min. A linear variable differential transformer (LVDT) with a range of ±15 mm was connected to the bottom surface of the specimen to measure deflection. A camera was placed in front of the specimen to photograph its side view during the bending tests to record crack pattern and mode of failure. For specimens without epoxy, the test was stopped at 12 mm deflection and while for those with epoxy it was terminated at 16 mm. In all composites, the AR glass yarns were located along the specimen length relative to the load direction of the crosshead, where the Z yarns were stitched into the composite from top to bottom (i.e., they spanned the composite thickness). The warp direction was the reinforcing direction in all composites. For each system four specimens were tested and load vs. deflection curves were recorded. The stresses and toughness (calculated as the areas under the stress–deflection curves) were calculated along with their standard deviations. For each system, a typical representative curve (i.e., one whose values were close to average) was chosen for comparison. In addition, to better understand the cracking behaviour and evolution of the different composites during flexural loading a photoelastic sheet was attached to the side of each specimen prior to loading. Images of the specimens that recorded their photoelastic responses to loading were captured every 30 s by a digital camera. For this measurement composites of 3D REF and Ar 100 with and without epoxy were examined up to the loading limit of the photoelastic sheet, or ~6 mm deflection. The stress concentrations that developed on the specimen during loading were monitored by capturing the corresponding behaviour of the photoelastic sheet affixed to the composite. When a crack was initiated and opened in the composite, the stress
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concentration was depicted on the photoelastic sheet, i.e., in this way it was possible to detect the location and development of cracks during stress testing. 2.2.3 Microstructure characteristics (SEM) A FEI Quanta 200 scanning electron microscope (SEM) was used to observe the microstructures of the different 3D fabric composites to evaluate cement matrix penetration between the loops, the openings in the fabric, the filaments of the bundle, and the fabric junctions. The observations were of composite cross sections, i.e., bundle cross sections, and along the bundle lengths embedded in the cement matrices. A precision diamond-tipped water cooled saw was used to slice the specimens and expose the bundle cross sections. The cross sectioned samples were initially grinded in three steps using grinding paper of 320, 500 and 1200 grit sizes, after which they were further polished with 10-, 2.5- and 0.5-m diamond paste. The samples were then cleaned with alcohol in a supersonic cleaner for 30 sec. For observations along the embedded bundle, the composite was split to minimize damage to yarns, fabrics and interface, the sample was neither grinded nor polished. Six composite systems were observed by SEM: 2D, 3D REF and 3D Ar 100 with and without epoxy.
3. Results and Discussion 3.1 Fabrics Detailed geometries of the three fabrics, showing the top view of a junction between warp and weft yarns and the loops connecting them, are presented in Fig. 4. The weft and warp yarns are connected together by the loops to form the fabric, but the warpknitted production method of the fabric entails wrapping the loops around the warp yarn along its length, in the end tightening the filaments of the warp bundle together. The weft yarn, on the other hand, is relatively free of loops along most of its length, i.e., between the fabric junction points, to provide a more open bundle structure. When such fabrics are used as reinforcements in a cement-based matrix, differences in bundle opening are expected to affect cement penetration and thus composite performance. Therefore, the three fabrics differed by the material makeup and number of the weft loops wrapped around the warp yarn. For example, the weft yarn of the 2D fabric contained only one loop type, fine multifilament PES (16.7 tex, Table 1, Fig. 4a). The 3D REF fabric incorporated two loops around the warp yarn (Fig. 4b), one the fine multifilament PES for connection purposes (similar to the 2D fabric) and the second a monofilament PES (Table 1) used as Z direction stabilization and spacer yarns. Finally, the 3D Ar fabric had three loop types (Fig. 4c), two of which were PES as in the 3D REF composite and the third of which was high performance aramid oriented in the Z direction. These loops, on the one hand, influence yarn connection strength and thus fabric stability, stiffness, and its overall properties and performance. On the other hand, their tightening effect at the junctions and around the warp yarn can affect the penetration of cement particles between the bundle filaments. Increasing the number of loops around the warp yarn is expected both to limit cement matrix penetration and also to lead to stiffer and stronger fabric. These two effects can influence the cement-based composite structural behavior. Note that not only are the 5
type and number of loops of interest, but also the properties of the yarns oriented in the Z direction. We therefore compared three conditions based on the Z direction yarns: no yarns (2D fabric), low modulus yarn (PES in 3D REF) or high modulus yarn (aramid of 3D Ar). These conditions are also expected to affect composite reinforcement ability. The tensile properties of the plain (i.e., not in cement) 2D AR glass fabric were tested along the warp and weft directions. For the tensile stress calculation of the 2D fabric the cross section of the glass yarn was calculated based on its specific density, 2.86 gr/cm3 and tex, 2400 gr/Km, giving a yarn cross section area of 8.9610-3 cm2. Note that each tested fabric contained 4 yarns along the loading direction. (The complicated geometries of the 3D fabrics precluded applying these tests to them). Typical tensile stress curves for each fabric direction are shown in Fig. 5. In general, the fabrics performed significantly better in the warp direction than in the weft direction. Average and standard deviations of ultimate tensile strength (UTS) and strain for the warp and weft directions were 746±32 and 490±52 MPa for UTS and 1.8±0.1 and 1.4±0.1 for strain, respectively, showing that in the warp direction, UTS was 52% higher and strain was 28% higher on average than in the weft direction. This finding can be explained based on the geometry of the warp knitted fabric, which, in turn, influences the bundle geometry. As mentioned, multifilament polyester loops in the warp direction wrapped around and tightened the warp bundle filaments together (Fig. 4). In contrast, the weft yarns contained no loops and therefore, the bundle was free, which left more gaps between the filaments. The tightening effect of the loops effectively protected the bundle filaments in the warp direction, and therefore, they were less exposed to external damage. In addition, there was also greater friction between the warp-direction filaments, which thus functioned more efficiently as one collective unit. On the other hand, due to their looser array yarns in the weft direction exhibited lower friction between filaments and correspondingly less efficient behavior, and they were more exposed to external damages. Although these results are based directly on our findings regarding 2D fabrics, it seems safe to assume that 3D fabrics would exhibit similar trends toward improved properties in the warp direction. In this work the warp direction was the reinforcing direction of the composite.
3.2 Composite flexural behaviour The average flexural properties of all tested composites with 2D and 3D fabrics with and without epoxy are presented in Table 2 with their standard deviations. The table presents the average values of modulus of rupture (MOR), deflection at MOR and toughness calculated as the area under the stress-deflection curve up to deflection at 15% of maximum stress (MOR). The flexural stress vs. the deflection curves of composites reinforced with 2D fabric or with 3D fabric with or without aramid Z yarns (3D Ar or 3D REF, respectively) are compared in Fig. 6, with and without epoxy treatment. All 3D fabric composites, namely, 3D Ar 100, 3D Ar 50 and 3D REF, exhibited greater flexural responses than the composite reinforced with the 2D fabric, regardless of whether they were treated 6
with epoxy. For the composites without epoxy treatment (Fig. 6a), the 3D composite performed better than the 2D composite, but no significant difference was observed between the different 3D fabric composites (3D Ar 50, 3D Ar 100 and 3D REF), all three of which had similar flexural properties, mainly in terms of MOR and deflections at MOR but also, to some extent, in toughness (Table 2). This suggests that compared to 2D fabrics, the 3D fabrics are better reinforcements for cementbased elements. Specifically, composite flexural performance was improved by the presence of the Z direction yarns, but their properties (low or high tensile strength and modulus) and makeup made only negligible contributions to composite flexural strength. The observed differences between the 2D and 3D fabric composites, therefore, may relate to the complex geometries of the 3D fabrics (Fig. 1 and Fig. 4), which result in a more efficient reinforcing unit with better mechanical anchoring within the cement matrix compared with the 2D fabric system. To facilitate stronger and stiffer connections between the yarns in the Z direction with those in the Y and X directions, i.e., along the composite plane, the 2D and 3D fabrics were treated with epoxy. The epoxy treatment reinforces the fabric composites and stiffens their 3D structures such that during loading, all the yarns in all directions work together in unison as a single, rigid unit. The flexural stress vs. deflection curves of all epoxy treated composites reinforced with 2D and 3D fabrics are compared in Fig. 6b. As for the non-epoxy treated composites, here, too, the superior performances of the 3D compared to the 2D fabric composites were obvious. Moreover, the magnitude of the improvement in performance (e.g., in terms of strength, deflection at peak and toughness) of the 3D fabric composites over the 2D fabric composite was much greater after epoxy treatment. In contrast to the non-epoxy systems, however, the type and content of yarns in the Z direction in the epoxy treated systems significantly influenced composite behavior. Specifically, the greater the number of high-performance Z yarns in the composite, the higher its flexural strength. Therefore, the flexural strength of the 3D REF fabric, which lacked aramid yarns, was the lowest while for the 3D Ar 100 fabric it was the highest. Moreover, the observed improvements in the flexural strengths of the 3D fabric composites was more than 70% higher than that found for the 2D fabric composite. Fig. 7 summarizes and compares the average flexural strength and toughness values of all tested composites reinforced with 2D or 3D fabrics with and without epoxy and the properties of the plain cement paste without reinforcement. Note that for the epoxy fabric composites, the test was terminated at 16 mm deflection. Significant improvements of up to one order of magnitude in MOR and toughness were achieved for the composites relative to the plain cement paste. Fig. 7 clearly shows that the properties of the 2D fabric composites with and without epoxy were similar, with the epoxy treated system exhibiting only a slightly higher MOR than the untreated system. In contrast, significant differences were observed in the flexural properties of the 3D fabric composites between systems with and without epoxy, such that the former exhibited much better performance than the latter. For the 3D fabric composites, the epoxy treatment conferred strength increases of about 40%, 60% and 7
100% for the 3D REF, 3D Ar 50 and 3D Ar 100 composites, respectively, compared with the composites without epoxy. More pronounced is the approximately three-fold improvement in toughness in the composites with epoxy relative to those without epoxy (Fig. 7, Table 2). Furthermore, the deflection at peak, 6 mm for the non-epoxy system, reached 13 mm for the 3D Ar 100 epoxy treated composite. These results indicate that under epoxy treatment conditions, when the 3D fabric is rigid and behaves as a single unit, the Z yarns, although they are not oriented in the reinforcing direction, contribute to overall composite behavior, and as such, their properties and contents are significant. In 3D fabrics that have not been treated with epoxy, however, the contributions by the Z yarns – whose main role is to hold the two fabrics together at the faces – to composite behavior are much less significant, and therefore, they contribute less to composite reinforcement. Due to the difficulties associated with precisely cutting equal-sized pieces of the composites for testing, the number of AR glass yarns in the applied load direction of individual samples was not always the same for all specimens. To overcome this drawback and to emphasize the significant structural role fulfilled by the aramid yarns, the flexural strengths and toughness values are given per single reinforcing yarn (Fig. 8), i.e., the initially obtained MOR and toughness values were divided by the number of actual reinforcing yarns. The average MOR and strength values per single reinforcing yarn are presented vs. the aramid yarn content for the 3D fabric composites for the epoxy and non-epoxy treated systems. Zero Z yarn content refers to the 3D REF composite that lacked aramid yarns. Composite flexural strength and toughness increased linearly with the content of high modulus aramid yarns, a trend that was evident for both the epoxy treated and non-treated systems, but the advantage of 3D fabric treatment with epoxy was again obvious. These trends clearly show the reinforcing benefit conferred by the Z direction yarns of the fabric, especially when they are high modulus yarns. In addition, also noticeable was the lower consistency of the results for the epoxy treated composites, which were highly scattered compared with those for the non-epoxy system.
3.3 Microstructure characteristics 3.3.1 Fabric geometry SEM images of non-epoxy treated 2D, 3D REF, and 3D Ar 100 fabrics embedded in the cement matrix (Fig. 9) are of fabric junctions (similar to Fig. 4). The connecting loops of the multifilament PES are clearly visible in the 2D fabric composite (Fig. 9a). In the 3D fabrics, in contrast, these loops are not as clear, but the spacer yarn loops, i.e., the monofilament PES yarn (Fig. 9b-c) and the aramid yarn (Fig. 9c), are visible. The weft yarns of the fabrics in all three composites are clearly observed whereas the warp yarn can only be seen for the 2D fabric composite. For both 3D fabrics, the warp yarn is not visible as it is hidden behind the loops and the weft yarns. The filaments of the weft bundle in the 2D fabric are spread out and open (Fig. 9a) relative to the more compact and denser nature of the 3D Ar 100 fabric (Fig. 9c), which also contains the PES and aramid stitches. The density and openness of the weft bundle in the 3D REF sample is somewhere between these two. The observed differences in yarn openness 8
are expected to influence cement penetration and composite performance. In all composites, cement matrix penetration into the open spaces in the fabric, which can promote mechanical anchoring of the fabric within the cement matrix, was evident. Cement matrix penetration between the weft yarn filaments, however, was negligible in all composites. Nevertheless, SEM images of the impressions made by the non-epoxy treated fabrics in the cement matrix (Fig. 10a-c) clearly show imprints that correspond with the loops, indicating that the cement matrix penetrated the open spaces formed by the loops of the knitted fabric. Moreover, the impressions of weft yarn filaments are evident in the SEM images of all the matrices, a finding that is the most pronounced for the 2D fabric composite, in which the use of a single set of stitches (multifilament PES) to connect the weft and warp yarns (Figs. 4a and 9a) left a significant portion of the weft yarn free of connector yarn (Fig. 10a). The imprints made by these filaments in the matrix suggest that the matrix penetrated at least the perimeter of the bundle filaments. The higher the level of matrix penetration, the better the bonding and mechanical anchoring of the bundle filaments and loops within the matrix, depending the quantity of matrix that penetrates the fabric. Note that the wavy shape of the stitches in the warp direction may further increase the anchoring of the fabric in the cement matrix. Cement matrix penetration of weft yarn filaments (without epoxy) was examined at a fabric junction and far from the junction (examined locations are labelled with rectangles in Fig. 11a). The penetration of the cement between the filaments was higher at the location far from the fabric junction (Fig. 11b) compared to at the junction (Fig. 11c). The presence at the junction of warp and weft yarns, one on top of the other, and the loops prevents the spaces between the filaments from being filled by the cement paste, which reduces bonding. A similar trend was also observed for the warp yarn. These results indicate the importance of the fabric junctions in terms of fabric penetrability by the cement matrix – for cases in which the yarns have not been treated with epoxy – and how that influences bonding and overall composite mechanical performance. Indeed, the influence of the junctions is even more pronounced for the 3D fabrics, whose increased numbers of loops and yarns at junctions (PES and aramid, Fig. 4c) can severely reduce cement penetration. This critical influence of fabric structure must be considered when dealing with the reinforcing efficiency and bonding of 3D fabrics. Examination of the non-epoxy treated aramid yarns (Z direction yarns) in the composite shows good penetration of the cement paste between the filaments of the aramid bundle both along the length of the filaments (Fig. 12a) and in a cross-section of the bundle (Fig. 12b). Thus, the use of non-epoxy treated high-performance aramid yarns may further improve the mechanical performances of composites with 3D fabric compared to similar 3D composites that use low-performance non-aramid yarns (Table 2).
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3.3.2 Effects of epoxy treatment The above observations clearly show that for 3D fabric, penetration of the cement matrix between bundle filaments is relatively low due to the fabric’s complexity and the tightening function played by the loops, which together reduce the reinforcement efficiency of the fabric. One way to improve fabric reinforcing efficiency is to treat it (i.e., saturate it; section 2.1.1) with epoxy, which by connecting all bundles and filaments as a single unit causes them to carry the applied loads together. The bundle filaments of fabrics that have been treated with epoxy, however, are impenetrable by the cement matrix because the epoxy has filled in the spaces normally filled by cement as clearly observed in Fig. 13. Thus, the epoxy treatment effectively nullifies concerns about cement penetration. Figure 13 shows epoxy treated 3D Ar 100 fabric embedded in a cement matrix (Fig. 13a) and the imprint of this fabric in the cement matrix (Fig. 13b). Both images clearly show how the bundle yarns and loops have been covered, and any spaced filled in, by the epoxy. This strong epoxy-based interconnection of warp, weft, Z yarns and loops provides stiff and strong junctions. In addition, the penetration of openings in the fabric by the cement matrix is also very clear (Fig. 13a), indicating strong mechanical anchoring of the entire fabric unit in the matrix. Furthermore, the loops around the warp yarns confer on the reinforcing unit in the warp direction a complex shape, i.e., with alternating narrow and wide areas along its length (Fig. 13b), which, compared to a smooth and straight shape, can promote much stronger anchoring of the warp reinforcing unit in the matrix and therefore improved mechanical performance. However, due to the stiff epoxy-based connections of the warp, weft and Z yarns at the junction and the related anchoring mechanism, these regions of the composite are expected to be characterized by high stress, increasing their sensitivity to cracking. This is clearly visible in Fig. 14, which shows a junction of 3D Ar 100 epoxy treated fabric embedded in cement matrix. In both plates of Fig. 14, cracks are visible at the fabric-cement intersection. The aramid loop warping around the glass filaments is clearly seen in Fig. 14b, which also shows the good epoxy penetration between the filaments of the aramid loop and glass warp yarn, again indicative of the strong connection between the yarns constituting the 3D fabric. Both plates of Fig. 14 also show gaps between epoxy coated yarn and matrix at the junctions that may also affect composite mechanical properties. In summary, for composites reinforced with 3D fabric treated in epoxy, the fabric, together with the Zdirection aramid yarns anchored in the cement matrix, can be considered as a single 3D unit, leading to the improved mechanical performance of the epoxy-treated 3D fabric composites (Fig. 6b, 6 and Table 2). However, the epoxy-matrix bond is not perfect, and as a result, cracking is more likely to happen in epoxy treated fabric and cement composites, a potential outcome that could limit the reinforcing efficiency of such composites.
3.4 Cracking 3.4.1 Crack patterns and failure behavior The crack patterns of the 2D, 3D REF and 3D Ar 100 composites without and with epoxy treatment are presented in Figs. 15 and 16. Multiple cracking is clearly visible 10
for the 3D fabric composites in both the epoxy and non-epoxy treated cases. Such multiple cracking, however, is not as clearly evident in the 2D fabric composite. Composite failure behavior was further investigated by comparing the failure mechanisms at the end of the testing periods of composites with the 2D fabric and 3D Ar 100 fabric treated in epoxy (Fig. 17). The 2D fabric composite exhibited complete failure with a clearly visible, single wide crack and separation of the fabric from the matrix, i.e., delamination. The delamination is indicative of the fabric’s low reinforcing efficiency and of poor fabric-matrix bonding. In contrast, the 3D Ar 100 composite with through-thickness aramid yarns underwent multiple cracking accompanied by damage to the bottom (tensile) zone of the composite. Some delamination between fabric and matrix can be seen at the top (compression) zone of the specimen, but this was not as severe compared with the cracking behaviour of the 2D fabric composite. Note that for the 3D fabric a deflection of about 16 mm was reached at the end of testing. Such failure behaviour indicates that the 3D fabric has higher reinforcing efficiency, i.e., better bonding and mechanical anchoring of the 3D fabric within the cement matrix compared with the 2D fabric. The observed failure mechanisms correlate well with the overall flexural behaviours of the composites (Fig. 6) and the SEM observations, which together indicate that the 3D fabric composite has a strong anchoring mechanism. Crack development and subsequent crack patterns differed somewhat between the systems with and without epoxy treatment. For the non-epoxy treated composites (Fig. 15), both the 3D REF and 3D Ar 100 composites developed cracks from bottom to top along relatively straight paths. This suggests that a bending type of failure was probably the reason for cracking in the 3D fabric composites without epoxy. For the epoxy treated composites (Fig. 16), however, although bending cracks can be observed, other cracks also developed diagonally from the bottom to the top of the composite to the points where the loads were applied, suggesting in this case that a shear failure mechanism was more likely. For the non-treated epoxy composites the developed loads are relatively low and thus the reinforcement by the 2D as well as the 3D fabrics was sufficient to sustain the shear stresses and the failure is a bending type. In the epoxy treated composites the loads are much greater and the shear reinforcement in both 2D and 3D fabrics was not enough and a shear type of failure was occurred. Thus, these observations suggest that different failure mechanisms – depending on the stiffness and reinforcing efficiency of the 3D fabric – were responsible for the observed cracking behaviors. Specifically, fabric that was pre-treated with epoxy behaved as a whole, single unit within the composite. In contrast, yarn bundles and filaments in the non-epoxy treated 3D fabrics exhibited at least partially separate behavior that resulted in lower reinforcing efficiency and relatively low load bearing capacity. Further research is required in order to better understand the mechanisms that influence failure mechanisms of 3D fabric cement-based composites.
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3.4.2 Crack monitoring – photoelastic Fig. 18 shows the photoelastic responses of 3D REF (without aramid yarns) during the different stages of loading (Fig. 18a-b, 10 sequential images each). The response stages are numbered consecutively on the stress-deflection curve (Fig. 18c-d). For the composite without the epoxy, the single small peak in the first image (Fig. 18a) indicates the initiation of the first crack in the composite at a very early loading stage (#1 in Fig. 18c). Likewise, the two peaks evident in the second image (Fig. 18a) show that two cracks had developed by this stage of loading (#2 in Fig. 18c). In each subsequent image (3, 4 and 5, Fig. 18a), the number of peaks increases by one, i.e., more cracks appeared with the increase in load indicating multiple cracking behaviour. Beyond this stage of loading, although the peaks become wider making it increasingly difficult to distinguish between the developing cracks, the photoelastic images still clearly show that more cracks developed. Indeed, the abrupt, small drops observed in the stress-strain curve (Fig. 18c) also support multiple cracking. Thus in the 3D REF composite without epoxy, the cracks built up gradually and moderately (fringes formed slowly), such that they started from the composite’s weak point and then developed gradually along its length. In contrast, the photoelastic responses of the 3D REF composite with epoxy treated fabric indicate that the cracking mechanisms and stress distributions of the two composites differed. In the early loading stages (#1-2 in Fig. 18b and 18d), a multitude of small peaks are evident along the entire length of the specimen, implying that soon after loading began, many tiny cracks were initiated together. In the third stage, only the crack at the centre of the specimen grows (image 3 in Fig. 18b), after which a second crack grows left of centre followed by the growth of a third crack to the right of the centre crack (#4-5 in Fig. 18b). Image 6 shows that at a deflection of 1.7 mm, a fourth peak grew (Fig. 18d), but beyond this loading stage the lines of the concentrated stresses merge together. As with the non-epoxy treated composite, here it is also difficult to distinguish between the different stress regions. But the entire region of the concentrated stresses (fringes) widened as loading continued, suggesting that although it is difficult to distinguish between the different cracks on the photoelastic sheet, the number and size of the cracks both grew. In addition, crack development is also noticeable on the stress-deflection curve (Fig. 18d), where it appears as sharp drops. When comparing the stage 10 photoelastic sheet responses of the two systems with and without epoxy, newly developed cracks are not apparent. At this stage, deflection of the non-epoxy system is ~1.8 mm whereas for the epoxy system it is about 4.5 mm. Beyond this stage, the dominant mechanism may be crack widening. Similar behaviour can be observed in Fig. 19 showing the photoelastic responses of the Ar 100 composites with and without epoxy, i.e., the composites that contain aramid yarns in the through-thickness, or Z, direction. Numerous tiny, visible cracks were immediately initiated in the epoxy composite systems during the initial loading stages, but the non-epoxy composite system exhibited an individual and gradual cracking pattern. In both cases, however, crack saturation occurred at deflections of 12
about 2 mm (Fig. 19c-d). Note that here the epoxy treated composite system exhibited shear failure that, although not recorded by the photoelastic sheet measurements, was believed to influence overall composite cracking behaviour (Fig. 16c). To summarise, based on the photoelastic measurements, several differences in crack development were observed between the epoxy and non-epoxy composites: (i) crack initiation – in the epoxy system, cracks were initiated together along the entire length of the specimen at a very early stage of loading; in contrast, cracks in the non-epoxy system were created individually and separately during loading; (ii) crack development – cracks initiated in the epoxy system grew during continuous loading, starting from the centre of the specimen to both sides gradually; in the non-epoxy system, cracks began at the weak point of the specimen and then developed along its length gradually; (iii) crack saturation – for the 3D REF composite only, the crack opening and widening mechanism began at a much greater deflection for the epoxy system compared to the non-epoxy system. These observations may indicate that the anchoring mechanism between fabric and matrix in the epoxy treated system – in which the fabric behaves as a whole unit, and therefore, matrix and fabric work together – results in a much stronger and tougher composite compared to the nonepoxy composite (Figs. 6, 7 and Table 2).
4
Conclusions
This study examined the flexural behavior of cement-based composites reinforced with 3D fabric, focusing on yarns situated in the through-thickness (Z direction) of the fabric influenced the composite. Based on the results obtained in this work, it can be concluded that compared with 2D fabrics, 3D fabrics are highly beneficial reinforcements for cement-based composites. Composites constructed with the 2D fabrics were typically low performance and often underwent delamination. In contrast, the complex geometries of the 3D fabrics facilitated stronger anchoring mechanisms with the cement matrix and good connections between the yarns along the plane surfaces of the fabric, thereby leading ultimately to a more efficient reinforcing unit. Treatment of the 3D fabric with epoxy significantly improved composite performance. Indeed, the epoxy treatment is critical for 3D fabrics due to their very complex fabric structure. The epoxy treatment binds the different elements of the fabric together such that they behave as a single, whole unit in all three directions, carrying the applied loads together. The differences in crack initiation and development revealed by the photoelastic measurements demonstrated this idea. For the epoxy systems, cracks were initiated together immediately when loading began along the entire length of the specimen, i.e., reflecting behaviour of one whole fabric unit. These initial, tiny cracks then grew during continuous loading, starting from the centre of the specimen and progressing in both directions gradually. In the non-epoxy system, the cracks were created individually and separately during loading starting from the weak point of the specimen, i.e., different behaviours of individual yarns. 13
The improved flexural performance of the composite reinforced with the epoxy treated fabrics can be related to two main mechanisms: (i) the greater reinforcing efficiency of the yarns as they work together during loading when connected by the epoxy vs. the lower reinforcing efficiency of the non-treated epoxy fabrics, in which the bundles and the filaments act separately, resulting in a lower load bearing capacity; (ii) stronger mechanical anchoring between epoxy treated fabric and cement matrix that was clearly observed by SEM. However, the bond between epoxy and matrix was not perfect, such that cracking at the fabric junction was more likely, a finding that may limit the reinforcing efficiency of the epoxy treated composites and therefore, that must be further investigated. The yarns in the Z direction of the fabric, i.e., composite thickness, were found to strongly influence composite performance: (i) high performance Z yarns such as aramid greatly improved the strength and toughness of the cement-based composite compared to low performance yarns such as PES; (ii) the greater the relative content of high-performance yarns in the Z direction, the better composite performance. The influence of Z direction yarns was more pronounced when the fabric was treated in epoxy before its inclusion in the cement composite. In conclusion, 3D fabrics have immense potential in the reinforcement of cementbased composites even though the Z yarns are not in the direction of the applied load. The Z yarns hold the whole fabric together to confer on the fabric good mechanical anchoring properties. That anchoring can be easily improved when the bundles within the fabric are treated with epoxy (i.e., coated, saturated and glued with epoxy), resulting in a reinforcing component that behaves as a single unit to confer improved mechanical performance on the composite.
5
Acknowledgments
The authors would like to acknowledge Israel Science Foundation GRANT NO. 147/11 for the financial support of this research and the Institut für Textiltechnik der RWTH Aachen University (ITA), Germany, for fabric production.
6
References
[1] Brameshuber, W., Textile reinforced Concrete: State of the Art Report of RILEM Committee 201-TRC, RILEM Publications, Bagneux, France, (2006). [2] Bentur A. and Mindess, S., 2002. Fibre reinforced cementitious composites. Spoon Press, London, New York. [3] Peled A. and Bentur A., 2003. Fabric structure and its reinforcing efficiency in textile reinforced cement composites. Composites Part A. 34:07-118. [4] Peled A. and Mobasher B., 2005. Pultruded fabric-cement composites. ACI Mater J. 102:15-23. [5] Brameshuber W. Brockmann T., and Banholzer B., 2006. Material and bonding 14
characteristics for dimensioning and modeling of textile reinforced concrete (TRC) elements. Mater Struct. 39:749-763. [6] Naaman, A.E., 2003. Strain hardening and deflection hardening fiber reinforced cement composites. In A.E. Naaman and H.W. Reinhardt (eds.), Fourth International Workshop on High Performance Fiber Reinforced Cement Composites (HPFRCC 4), RILEM Publications, 2003, Bagneux, France; pp. 95-113. [7] Hanisch V., Kolkmann A., Roye A., and Gries T., 2006. Influence of machine settings on mechanical performance of yarns and textile structures. In Hegger et al (eds), Proceedings of the 1st International RILEM Symposium (Textile Reinforced Concrete ICTRC) RILEM TC201-TRC; pp. 13-22. [8] Roye A., Gries T., and Peled A., 2004. Spacer fabric for thin walled concrete elements. In: de Prisco et al (eds), Fiber Reinforced Concrete – BEFIB, PRO 39, RILEM; pp. 1505-1514. [9] Naaman N., 2010. Textile reinforced cement composites: competitive status and research directions. In: Brameshuber (ed) International RILEM Conference on Materials Science (MatSci) I; pp. 3-22. [10] Peled A., Zhu D., and Mobasher B., 2011. Impact Behavior of 3D Fabric Reinforced Cementitious Composites. HPFRCC6, Ann Arbor, June 2011. [11] Zhu D., Peled A., and Mobasher B., 2011. Dynamic tensile testing of fabriccement composites. Construction and Building Materials J. 25(1):385-395.
15
List of Figures Fig. 1: 3D warp knitted fabric: a) schematic description of the production method, (b) 3D fabric Fig. 2: Images of the different fabrics, top and side views, including schematic description of the 3D fabrics with the two aramid yarn contents. Fig. 3: (a) Casting of the 3D fabric specimen, (b) scheme of the 3D composite along its length at the end of casting. Fig. 4: Stereoscope images of plain fabric junction: (a) 2D fabric, (b) 3D REF fabric, (c) 3D Ar (aramid) fabric, not in cement. Fig. 5: Tensile stress vs. strain along the weft and warp directions of plain (without cement) 2D fabrics (AR glass). Fig.6: Flexural behavior of the composites with 2D fabric and with 3D fabrics with (Ar 50 and Ar 100) or without (REF) aramid yarns: (a) without epoxy; (b) with epoxy. Fig. 7: Comparison of the different fabric-cement composites with and without epoxy treatment: (a) MOR; (b) toughness, related to the cement paste matrix. Fig. 8: Tensile properties per single reinforcing yarn vs. aramid Z yarn content (a) flexural strength, (b) toughness. Fig.9: SEM images of the three fabric types (without epoxy) embedded in the cement matrix, top view: (a) 2D; (b) 3D REF; (c) 3D Ar 100. Fig. 10: SEM images of fabric imprints in the cement matrix: (a) 2D, (b) 3D REF, (c) 3D Ar 100 fabric composites. Fig.11: Cement penetration between filaments of weft yarn in a 3D REF composite at different locations: (a) general view showing the two regions observed in b and c (red rectangles); (b) far from a junction; (c) at the junction. Fig.12: Aramid yarn penetration by cement paste: (a) top view; (b) bundle cross section. Fig.13: 3D Ar 100 fabric with epoxy embedded in cement matrix (a) top view of fabric junction, (b) loop print in the matrix Fig. 14: Fabric junction of 3D Ar fabric with epoxy in cement matrix: (a) top view; (b) view of fabric cross section exposing the warp glass yarns with the aramid loop around it, Back Scattering mode. Fig.15: Crack pattern of the different composites without epoxy: (a) 2D, (b) 3D REF and (c) 3D Ar 100 Fig. 316: Crack patterns of the different composites with epoxy: (a) 2D, (b) 3D REF and (c) 3D Ar 100 Fig.17: Failure mechanisms of composites at the end of testing: (a) 2D fabric composite, and (b) 3D Ar 100 composite, both with epoxy. 16
Fig.18: 3D REF composites with and without epoxy: (a and b) photoelastic responses, (c and d) flexural vs. deflection curves annotated with numbers corresponding to stages of photoelastic response monitoring. Fig. 19: 3D Ar100 composites without and with epoxy: (a and b) photoelastic responses, (c and d) flexural vs. deflection curves annotated with numbers corresponding to stages of photoelastic response monitoring.
17
Table 1: Properties of the yarns used to manufacture the reinforced fabrics. Tensile strength
Tex
Elastic Modulus [GPa]
[gr/Km]
Function within the fabric
0222
20
0022
Weft and Warp
8553
20
062
Reinforced yarn along Z axis
PES monofilament
-
-
66.4
Spacers
PES multifilament
-
-
16.7
Loops
Fiber type
[MPa] AR glass Cem-FIL© grade Aramid Technora T-240
Table 2: Flexural properties of the different composites: average MOR, deflection at MOR and toughness values, along with their standard deviations.
Sample type
MOR [MPa]
Deflection at MOR [mm]
Toughness [MPamm] (at 15% of MOR)
Paste
1.56±0.3
0.057±0.004
1.5±0.44
2D
7.23±0.42
2.99±0.58
26.49±4.05
12.11±0.67
6.36±0.64
57.37±6.23
Ar 50
13.4±1.17
6.26±0.17
65.87±6.35
Ar 100
13.28±0.33
6.09±0.38
77.88±14.23
9.7±1.31
3.05±0.94
24.18±14.45
3D Ref
16.95±0.96
11.3±2.38
169.28±59.94
Ar 50
20.96±1.3
10.95±2.11
193.98±43.02
Ar 100
25.72±2.65
13.37±1.19
239.5±34.1
Without 3D Ref Epoxy
2D With Epoxy
18
(a) Spacer, Z yarn 2D fabrics
3D fabric (b)
Fig. 4: 3D warp knitted fabric: a) schematic description of the production method, (b) 3D fabric
19
Fabric Name
Top view
Side view
2D
3D REF
3D Ar 100 3D Ar Top View
Ar 100
Ar 50
Fig. 5: Images of the different fabrics, top and side views, including schematic description of the 3D fabrics with the two aramid yarn contents.
20
330 mm 3D fabric
2 mm 22 mm 2 mm
Cement paste
(a)
(b)
Fig. 3: (a) Casting of the 3D fabric specimen, (b) scheme of the 3D composite along its length at the end of casting.
Multi Filament PES
Monofilament PES
AR glass AR glass
Multifilament PES Monofilament PES
(a)
(b)
Aramid
(c)
Fig. 4: Stereoscope images of plain fabric junction: (a) 2D fabric, (b) 3D REF fabric, (c) 3D Ar (aramid) fabric, not in cement.
21
Fig.5: Tensile stress vs. strain along the weft and warp directions of plain (without cement) 2D fabrics (AR glass).
22
(a)
(b)
Fig.6: Flexural behavior of the composites with 2D fabric and with 3D fabrics with (Ar 50 and Ar 100) or without (REF) aramid yarns: (a) without epoxy; (b) with epoxy.
23
(a)
(b) Fig. 7: Comparison of the different fabric-cement composites with and without epoxy treatment: (a) MOR; (b) toughness, related to the cement paste matrix.
24
(b)
(a)
Fig. 8: Tensile properties per single reinforcing yarn vs. aramid Z yarn content (a) flexural strength, (b) toughness.
(a)
(b)
(c)
Fig.9: SEM images of the three fabric types (without epoxy) embedded in the cement matrix, top view: (a) 2D; (b) 3D REF; (c) 3D Ar 100.
25
(a)
(b)
(c) Fig. 10: SEM images of fabric imprints in the cement matrix: (a) 2D, (b) 3D REF, (c) 3D Ar 100 fabric composites.
26
(a)
(b)
(c)
Fig.11: Cement penetration between filaments of weft yarn in a 3D REF composite at different locations: (a) general view showing the two regions observed in b and c (red rectangles); (b) far from a junction; (c) at the junction.
(a)
(b)
Fig.12: Aramid yarn penetration by cement paste: (a) top view; (b) bundle cross section. 27
(a)
(b)
Fig.13: 3D Ar 100 fabric with epoxy embedded in cement matrix (a) top view of fabric junction, (b) loop print in the matrix
Glass filaments
Void
Gap Aramid
Monofilament
PES
Aramid (a)
(b)
Fig. 14: Fabric junction of 3D Ar fabric with epoxy in cement matrix: (a) top view; (b) view of fabric cross section exposing the warp glass yarns with the aramid loop around it, Back Scattering mode.
28
(a)
(b)
(c)
Fig.15: Crack pattern of the different composites without epoxy: (a) 2D, (b) 3D REF and (c) 3D Ar 100
(a)
(b)
(c)
Fig. 616: Crack patterns of the different composites with epoxy: (a) 2D, (b) 3D REF and (c) 3D Ar 100
29
(a)
(b)
Fig.17: Failure mechanisms of composites at the end of testing: (a) 2D fabric composite, and (b) 3D Ar 100 composite, both with epoxy.
30
Without epoxy
(a)
(c)
With epoxy
(b)
(d)
Fig.18: 3D REF composites with and without epoxy: (a and b) photoelastic responses, (c and d) flexural vs. deflection curves annotated with numbers corresponding to stages of photoelastic response monitoring.
31
Without epoxy
With epoxy
(a)
(b)
(c)
(d)
Fig.19: 3D Ar100 composites without and with epoxy: (a and b) photoelastic responses, (c and d) flexural vs. deflection curves annotated with numbers corresponding to stages of photoelastic response monitoring.
32
S1359-835X(15)00132-3 http://dx.doi.org/10.1016/j.compositesa.2015.04.008 JCOMA 3907
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
3 April 2014 11 December 2014 12 April 2015
Please cite this article as: Sasi, E.A., Peled, A., Three dimensional (3D) fabrics as reinforcements for cement-based composites, Composites: Part A (2015), doi: http://dx.doi.org/10.1016/j.compositesa.2015.04.008
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Three dimensional (3D) fabrics as reinforcements for cement-based composites Eliyahu Adiel Sasi1, Alva Peled2
1
Material Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel
e-mail: [email protected], 2
Structural Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel e-mail: [email protected],
Abstract This research studied the flexural behavior of cement-based elements reinforced with 3D fabrics. The effects of the through-thickness (Z direction) yarns were examined in terms of four parameters: (i) yarn properties, (ii) varying the composite content of (i.e., coverage by) high-performance aramid yarn, (iii) treatment of the fabric with epoxy, and (iv) 2D and 3D fabric composites were compared. Overall, the 3D fabric composites performed better than the 2D fabric composites, which tended to delaminate. Our results indicate that even though the Z yarns are not oriented in the direction of the applied loads, 3D fabrics still have potential applications as reinforcements for cement-based composites. Indeed, the Z yarns hold the entire fabric together, which leads to improved mechanical anchoring and mechanical properties particularly when the fabric has been treated with epoxy, i.e., to create a stiff reinforcing unit. Key words: A. Fabrics/textiles; A. 3-Dimensional reinforcement; B. Mechanical properties; Cement composites
1
Introduction
Recent years have seen a considerable increase in interest in TRC (textile reinforced cement/concrete) composites and in methods for improving their mechanical performance [1-2]. Research has shown that TRCs possess superior tensile strength, toughness, ductility and energy absorption properties [3-6]. The composite in two dimensional (2D) fabrics is reinforced in directions parallel, but not orthogonal, to the fabric plane. If using 2D fabrics to reinforce a composite, laminated composites should be prepared using several fabric layers. However, laminated composites are sensitive to failure by delamination and characterized by poor shear and split resilience properties under static, dynamic or impact loads. The wide variety of fabric structures enabled by advances in modern textile technology has conferred great flexibility on fabric design. Thus it is now also possible to produce three-dimensional (3D) fabrics with reinforcement in the plane normal to the fabric. Reinforced in three orthogonal directions, 3D fabrics can both 1
reduce the chances of composite failure by delamination and also enhance composite shear strength, and therefore, they are expected to markedly improve the mechanical properties of cement-based composites. To produce 3D fabrics, several methods, such as knitting, weaving, and braiding, among others, can be used. An attractive option for cement-based composites is double needle bar warp knitting, because it allows for an open structure that, in turn, facilitates better penetration by the cement. Warp knitting technology can be used to create 3D fabric structures by connecting two separate, independent 2D knitted fabrics together with a third set of yarns along the thickness of the fabric. The connecting yarns, referred to as spacer yarns, provide both stabilization and reinforcement. Recently, 3D spacer fabrics were developed for use in cement-based products [7]. Several studies of the behavior of cement-based composites with 3D fabric reinforcement demonstrated the potential applicability of these types of reinforcement in the cement field [8-10]. These studies focused mainly on 3D fabrics in which the through-thickness (Z direction) yarns were of low moduli, and as such, they were used for stabilization purposes only. The objective of this research was to study the flexural behavior of cement-based composites reinforced with 3D fabrics, the latter of which were produced by stitching 2D fabrics together in the Z direction using different yarns. Specifically, this study examined: (i) Z-direction yarn properties, e.g., high performance of aramid vs. low performance of polyester, (ii) Z-direction yarn content in terms of its highperformance aramid, 50% and 100%, (iii) treatment of the yarn with epoxy to increase fabric stiffness and reinforcement efficiency, and (iv) 2D fabric vs. the 3D fabric composite made from the same AR glass X, Y yarns.
2
Experimental program
2.1
Preparation of specimens
2.1.1 Fabrics For this work, we used 3D warp knitted fabric structures in which two sets of independent 2D knitted fabrics were stitched together in the Z direction of the fabric using a third set of yarns (Fig. 1). The warp and weft yarns (the yarns along the X and Y directions) were stitched together with loops of fine multifilament polyester (PES), leaving square openings of 0.8 × 0.8 cm in the sections of 2D knitted fabric. The 3D fabrics used in this research were all made with multifilament alkali resistant (AR) glass yarns (Cem-FIL® grade) along the X (weft) and Y (warp) directions. The spacer yarns in the Z direction were made from two different yarn types (Table 1). The first, used mainly for stabilization, was monofilament polyester (PES), and the second, used to provide reinforcement, was a high-performance multifilament yarn of aramid. Three different types of fabric were prepared (Fig. 2): i. 2D fabric (2D) – made with AR glass along the weft and warp directions exactly as the 3D fabric but without the Z direction yarns; ii. 3D Reference (3D REF) – in which the spacer yarns comprised 2
PES only, i.e., without high-performance reinforcing spacer yarns; iii. 3D Aramid (3D Ar 100 or 3D Ar 50) – in which, in addition to PES, high-performance reinforcing aramid yarns were located along the Z direction. For 3D aramid, two fabrics with different aramid yarn contents were made. The first (3D Ar 100) contained the maximum possible content of spacer aramid yarns such that no gaps were evident between them to provide 100% reinforcing aramid yarns along the Z direction. In the second 3D aramid fabric (3D Ar 50), the aramid yarns were arranged in a "one in, one out" formation, i.e., the fabric knit alternated between an aramid yarn and then a gap with the width of one yarn to provide 50% reinforcing aramid yarns along the Z direction. Two sets of the fabrics described above were tested, one as it was received while the second was first treated with epoxy. Treatment with epoxy was difficult due to the complexity of fabric structure. Treatment with epoxy comprised coating all the yarns in all directions with the epoxy using a brush, applying enough epoxy such that it penetrated (i.e., saturated) the yarns but without filling in the spaces between the X and Y direction yarns (in 2D and 3D fabrics) or the space between two 2D fabric sections in a 3D composite. Ensuring these spaces remained free of epoxy enabled maximum cement matrix penetration of the fabrics. Sikadur® 52, a low-viscosity high-strength epoxy, was selected among other examined coating materials as it provided the best filling of the yarns while leaving the 3D fabric spaces open. The epoxy treatment conferred on the fabric greater stiffness and improved its reinforcement capacity, as the epoxy treated bundle behaved as a single unit in which loads were carried by all the filaments together. All fabrics were produced specifically for this research at the Institut für Textiltechnik der RWTH Aachen University (ITA). 2.1.2 Composites Eight composite systems, four with and four without epoxy, were prepared from the three different fabric types discussed above. In all composites, the matrix comprised a cement paste (water and cement only) with a 0.4 water/cement ratio using CEM II 42.5 N/B-V to keep the system as simple as possible. The 3D fabric specimens were prepared by filling the bottom portion of the mold with a thin layer of cement paste on which we placed the 3D fabric; the rest of the mold was then filled until the fabric was completely covered (Fig. 3). For the 2D fabric specimens, we prepared sandwich-type composites, in which the two layers of fabric were located at the top and bottom of the composite and separated by a layer of cement paste. To situate the 2D fabric layers within the matrix, a thin layer of cement paste was first cast in the bottom of the mold, the first 2D layer was laid down on that, more cement was added on top of the first fabric layer before the second 2D layer was positioned and then covered in cement to fill up the mold to the top. This method provided a two-ply fabric composite similar to the 3D fabric composite but without the through-thickness connecting yarns of the composite. During the cement paste casting, a vibration procedure was applied using a 3,600 VPM vibration table to ensure good penetration of the matrix between the openings of the fabric and bundle filaments. After casting, the composites were left to harden for 3
24 h, demolded, and then cut into specimens measuring 330 × 40 × 26 mm (length × width × thickness). The specimens were cured at 100% relative humidity for 13 days and then another 2 days at room environment until flexure testing 16 days after casting.
2.2
Test procedure
2.2.1 Tensile test The tensile properties of the 2D fabrics (without cement) were examined in both the weft and warp directions. A sample of 2D fabric was cut into 185 mm long slices with four yarns along the loading direction. Aluminum foil was attached with epoxy to both ends of the specimen. The test was executed on an Instron tensile machine with closed loop operation and load cell capacity of 100 kN. The crosshead velocity was fixed to 0.5 mm/min. Four specimens were tested in both the warp and weft directions. For each system a typical curve (i.e., one whose values were close to average) was chosen for comparison. 2.2.2 Four-point bending All the composite systems were tested by four point bending (Instron tensile machine as in 2.2.1), the setup of which entailed a support span of 300 mm and loading span of 100 mm. The crosshead velocity was fixed to 0.5 mm/min. A linear variable differential transformer (LVDT) with a range of ±15 mm was connected to the bottom surface of the specimen to measure deflection. A camera was placed in front of the specimen to photograph its side view during the bending tests to record crack pattern and mode of failure. For specimens without epoxy, the test was stopped at 12 mm deflection and while for those with epoxy it was terminated at 16 mm. In all composites, the AR glass yarns were located along the specimen length relative to the load direction of the crosshead, where the Z yarns were stitched into the composite from top to bottom (i.e., they spanned the composite thickness). The warp direction was the reinforcing direction in all composites. For each system four specimens were tested and load vs. deflection curves were recorded. The stresses and toughness (calculated as the areas under the stress–deflection curves) were calculated along with their standard deviations. For each system, a typical representative curve (i.e., one whose values were close to average) was chosen for comparison. In addition, to better understand the cracking behaviour and evolution of the different composites during flexural loading a photoelastic sheet was attached to the side of each specimen prior to loading. Images of the specimens that recorded their photoelastic responses to loading were captured every 30 s by a digital camera. For this measurement composites of 3D REF and Ar 100 with and without epoxy were examined up to the loading limit of the photoelastic sheet, or ~6 mm deflection. The stress concentrations that developed on the specimen during loading were monitored by capturing the corresponding behaviour of the photoelastic sheet affixed to the composite. When a crack was initiated and opened in the composite, the stress
4
concentration was depicted on the photoelastic sheet, i.e., in this way it was possible to detect the location and development of cracks during stress testing. 2.2.3 Microstructure characteristics (SEM) A FEI Quanta 200 scanning electron microscope (SEM) was used to observe the microstructures of the different 3D fabric composites to evaluate cement matrix penetration between the loops, the openings in the fabric, the filaments of the bundle, and the fabric junctions. The observations were of composite cross sections, i.e., bundle cross sections, and along the bundle lengths embedded in the cement matrices. A precision diamond-tipped water cooled saw was used to slice the specimens and expose the bundle cross sections. The cross sectioned samples were initially grinded in three steps using grinding paper of 320, 500 and 1200 grit sizes, after which they were further polished with 10-, 2.5- and 0.5-m diamond paste. The samples were then cleaned with alcohol in a supersonic cleaner for 30 sec. For observations along the embedded bundle, the composite was split to minimize damage to yarns, fabrics and interface, the sample was neither grinded nor polished. Six composite systems were observed by SEM: 2D, 3D REF and 3D Ar 100 with and without epoxy.
3. Results and Discussion 3.1 Fabrics Detailed geometries of the three fabrics, showing the top view of a junction between warp and weft yarns and the loops connecting them, are presented in Fig. 4. The weft and warp yarns are connected together by the loops to form the fabric, but the warpknitted production method of the fabric entails wrapping the loops around the warp yarn along its length, in the end tightening the filaments of the warp bundle together. The weft yarn, on the other hand, is relatively free of loops along most of its length, i.e., between the fabric junction points, to provide a more open bundle structure. When such fabrics are used as reinforcements in a cement-based matrix, differences in bundle opening are expected to affect cement penetration and thus composite performance. Therefore, the three fabrics differed by the material makeup and number of the weft loops wrapped around the warp yarn. For example, the weft yarn of the 2D fabric contained only one loop type, fine multifilament PES (16.7 tex, Table 1, Fig. 4a). The 3D REF fabric incorporated two loops around the warp yarn (Fig. 4b), one the fine multifilament PES for connection purposes (similar to the 2D fabric) and the second a monofilament PES (Table 1) used as Z direction stabilization and spacer yarns. Finally, the 3D Ar fabric had three loop types (Fig. 4c), two of which were PES as in the 3D REF composite and the third of which was high performance aramid oriented in the Z direction. These loops, on the one hand, influence yarn connection strength and thus fabric stability, stiffness, and its overall properties and performance. On the other hand, their tightening effect at the junctions and around the warp yarn can affect the penetration of cement particles between the bundle filaments. Increasing the number of loops around the warp yarn is expected both to limit cement matrix penetration and also to lead to stiffer and stronger fabric. These two effects can influence the cement-based composite structural behavior. Note that not only are the 5
type and number of loops of interest, but also the properties of the yarns oriented in the Z direction. We therefore compared three conditions based on the Z direction yarns: no yarns (2D fabric), low modulus yarn (PES in 3D REF) or high modulus yarn (aramid of 3D Ar). These conditions are also expected to affect composite reinforcement ability. The tensile properties of the plain (i.e., not in cement) 2D AR glass fabric were tested along the warp and weft directions. For the tensile stress calculation of the 2D fabric the cross section of the glass yarn was calculated based on its specific density, 2.86 gr/cm3 and tex, 2400 gr/Km, giving a yarn cross section area of 8.9610-3 cm2. Note that each tested fabric contained 4 yarns along the loading direction. (The complicated geometries of the 3D fabrics precluded applying these tests to them). Typical tensile stress curves for each fabric direction are shown in Fig. 5. In general, the fabrics performed significantly better in the warp direction than in the weft direction. Average and standard deviations of ultimate tensile strength (UTS) and strain for the warp and weft directions were 746±32 and 490±52 MPa for UTS and 1.8±0.1 and 1.4±0.1 for strain, respectively, showing that in the warp direction, UTS was 52% higher and strain was 28% higher on average than in the weft direction. This finding can be explained based on the geometry of the warp knitted fabric, which, in turn, influences the bundle geometry. As mentioned, multifilament polyester loops in the warp direction wrapped around and tightened the warp bundle filaments together (Fig. 4). In contrast, the weft yarns contained no loops and therefore, the bundle was free, which left more gaps between the filaments. The tightening effect of the loops effectively protected the bundle filaments in the warp direction, and therefore, they were less exposed to external damage. In addition, there was also greater friction between the warp-direction filaments, which thus functioned more efficiently as one collective unit. On the other hand, due to their looser array yarns in the weft direction exhibited lower friction between filaments and correspondingly less efficient behavior, and they were more exposed to external damages. Although these results are based directly on our findings regarding 2D fabrics, it seems safe to assume that 3D fabrics would exhibit similar trends toward improved properties in the warp direction. In this work the warp direction was the reinforcing direction of the composite.
3.2 Composite flexural behaviour The average flexural properties of all tested composites with 2D and 3D fabrics with and without epoxy are presented in Table 2 with their standard deviations. The table presents the average values of modulus of rupture (MOR), deflection at MOR and toughness calculated as the area under the stress-deflection curve up to deflection at 15% of maximum stress (MOR). The flexural stress vs. the deflection curves of composites reinforced with 2D fabric or with 3D fabric with or without aramid Z yarns (3D Ar or 3D REF, respectively) are compared in Fig. 6, with and without epoxy treatment. All 3D fabric composites, namely, 3D Ar 100, 3D Ar 50 and 3D REF, exhibited greater flexural responses than the composite reinforced with the 2D fabric, regardless of whether they were treated 6
with epoxy. For the composites without epoxy treatment (Fig. 6a), the 3D composite performed better than the 2D composite, but no significant difference was observed between the different 3D fabric composites (3D Ar 50, 3D Ar 100 and 3D REF), all three of which had similar flexural properties, mainly in terms of MOR and deflections at MOR but also, to some extent, in toughness (Table 2). This suggests that compared to 2D fabrics, the 3D fabrics are better reinforcements for cementbased elements. Specifically, composite flexural performance was improved by the presence of the Z direction yarns, but their properties (low or high tensile strength and modulus) and makeup made only negligible contributions to composite flexural strength. The observed differences between the 2D and 3D fabric composites, therefore, may relate to the complex geometries of the 3D fabrics (Fig. 1 and Fig. 4), which result in a more efficient reinforcing unit with better mechanical anchoring within the cement matrix compared with the 2D fabric system. To facilitate stronger and stiffer connections between the yarns in the Z direction with those in the Y and X directions, i.e., along the composite plane, the 2D and 3D fabrics were treated with epoxy. The epoxy treatment reinforces the fabric composites and stiffens their 3D structures such that during loading, all the yarns in all directions work together in unison as a single, rigid unit. The flexural stress vs. deflection curves of all epoxy treated composites reinforced with 2D and 3D fabrics are compared in Fig. 6b. As for the non-epoxy treated composites, here, too, the superior performances of the 3D compared to the 2D fabric composites were obvious. Moreover, the magnitude of the improvement in performance (e.g., in terms of strength, deflection at peak and toughness) of the 3D fabric composites over the 2D fabric composite was much greater after epoxy treatment. In contrast to the non-epoxy systems, however, the type and content of yarns in the Z direction in the epoxy treated systems significantly influenced composite behavior. Specifically, the greater the number of high-performance Z yarns in the composite, the higher its flexural strength. Therefore, the flexural strength of the 3D REF fabric, which lacked aramid yarns, was the lowest while for the 3D Ar 100 fabric it was the highest. Moreover, the observed improvements in the flexural strengths of the 3D fabric composites was more than 70% higher than that found for the 2D fabric composite. Fig. 7 summarizes and compares the average flexural strength and toughness values of all tested composites reinforced with 2D or 3D fabrics with and without epoxy and the properties of the plain cement paste without reinforcement. Note that for the epoxy fabric composites, the test was terminated at 16 mm deflection. Significant improvements of up to one order of magnitude in MOR and toughness were achieved for the composites relative to the plain cement paste. Fig. 7 clearly shows that the properties of the 2D fabric composites with and without epoxy were similar, with the epoxy treated system exhibiting only a slightly higher MOR than the untreated system. In contrast, significant differences were observed in the flexural properties of the 3D fabric composites between systems with and without epoxy, such that the former exhibited much better performance than the latter. For the 3D fabric composites, the epoxy treatment conferred strength increases of about 40%, 60% and 7
100% for the 3D REF, 3D Ar 50 and 3D Ar 100 composites, respectively, compared with the composites without epoxy. More pronounced is the approximately three-fold improvement in toughness in the composites with epoxy relative to those without epoxy (Fig. 7, Table 2). Furthermore, the deflection at peak, 6 mm for the non-epoxy system, reached 13 mm for the 3D Ar 100 epoxy treated composite. These results indicate that under epoxy treatment conditions, when the 3D fabric is rigid and behaves as a single unit, the Z yarns, although they are not oriented in the reinforcing direction, contribute to overall composite behavior, and as such, their properties and contents are significant. In 3D fabrics that have not been treated with epoxy, however, the contributions by the Z yarns – whose main role is to hold the two fabrics together at the faces – to composite behavior are much less significant, and therefore, they contribute less to composite reinforcement. Due to the difficulties associated with precisely cutting equal-sized pieces of the composites for testing, the number of AR glass yarns in the applied load direction of individual samples was not always the same for all specimens. To overcome this drawback and to emphasize the significant structural role fulfilled by the aramid yarns, the flexural strengths and toughness values are given per single reinforcing yarn (Fig. 8), i.e., the initially obtained MOR and toughness values were divided by the number of actual reinforcing yarns. The average MOR and strength values per single reinforcing yarn are presented vs. the aramid yarn content for the 3D fabric composites for the epoxy and non-epoxy treated systems. Zero Z yarn content refers to the 3D REF composite that lacked aramid yarns. Composite flexural strength and toughness increased linearly with the content of high modulus aramid yarns, a trend that was evident for both the epoxy treated and non-treated systems, but the advantage of 3D fabric treatment with epoxy was again obvious. These trends clearly show the reinforcing benefit conferred by the Z direction yarns of the fabric, especially when they are high modulus yarns. In addition, also noticeable was the lower consistency of the results for the epoxy treated composites, which were highly scattered compared with those for the non-epoxy system.
3.3 Microstructure characteristics 3.3.1 Fabric geometry SEM images of non-epoxy treated 2D, 3D REF, and 3D Ar 100 fabrics embedded in the cement matrix (Fig. 9) are of fabric junctions (similar to Fig. 4). The connecting loops of the multifilament PES are clearly visible in the 2D fabric composite (Fig. 9a). In the 3D fabrics, in contrast, these loops are not as clear, but the spacer yarn loops, i.e., the monofilament PES yarn (Fig. 9b-c) and the aramid yarn (Fig. 9c), are visible. The weft yarns of the fabrics in all three composites are clearly observed whereas the warp yarn can only be seen for the 2D fabric composite. For both 3D fabrics, the warp yarn is not visible as it is hidden behind the loops and the weft yarns. The filaments of the weft bundle in the 2D fabric are spread out and open (Fig. 9a) relative to the more compact and denser nature of the 3D Ar 100 fabric (Fig. 9c), which also contains the PES and aramid stitches. The density and openness of the weft bundle in the 3D REF sample is somewhere between these two. The observed differences in yarn openness 8
are expected to influence cement penetration and composite performance. In all composites, cement matrix penetration into the open spaces in the fabric, which can promote mechanical anchoring of the fabric within the cement matrix, was evident. Cement matrix penetration between the weft yarn filaments, however, was negligible in all composites. Nevertheless, SEM images of the impressions made by the non-epoxy treated fabrics in the cement matrix (Fig. 10a-c) clearly show imprints that correspond with the loops, indicating that the cement matrix penetrated the open spaces formed by the loops of the knitted fabric. Moreover, the impressions of weft yarn filaments are evident in the SEM images of all the matrices, a finding that is the most pronounced for the 2D fabric composite, in which the use of a single set of stitches (multifilament PES) to connect the weft and warp yarns (Figs. 4a and 9a) left a significant portion of the weft yarn free of connector yarn (Fig. 10a). The imprints made by these filaments in the matrix suggest that the matrix penetrated at least the perimeter of the bundle filaments. The higher the level of matrix penetration, the better the bonding and mechanical anchoring of the bundle filaments and loops within the matrix, depending the quantity of matrix that penetrates the fabric. Note that the wavy shape of the stitches in the warp direction may further increase the anchoring of the fabric in the cement matrix. Cement matrix penetration of weft yarn filaments (without epoxy) was examined at a fabric junction and far from the junction (examined locations are labelled with rectangles in Fig. 11a). The penetration of the cement between the filaments was higher at the location far from the fabric junction (Fig. 11b) compared to at the junction (Fig. 11c). The presence at the junction of warp and weft yarns, one on top of the other, and the loops prevents the spaces between the filaments from being filled by the cement paste, which reduces bonding. A similar trend was also observed for the warp yarn. These results indicate the importance of the fabric junctions in terms of fabric penetrability by the cement matrix – for cases in which the yarns have not been treated with epoxy – and how that influences bonding and overall composite mechanical performance. Indeed, the influence of the junctions is even more pronounced for the 3D fabrics, whose increased numbers of loops and yarns at junctions (PES and aramid, Fig. 4c) can severely reduce cement penetration. This critical influence of fabric structure must be considered when dealing with the reinforcing efficiency and bonding of 3D fabrics. Examination of the non-epoxy treated aramid yarns (Z direction yarns) in the composite shows good penetration of the cement paste between the filaments of the aramid bundle both along the length of the filaments (Fig. 12a) and in a cross-section of the bundle (Fig. 12b). Thus, the use of non-epoxy treated high-performance aramid yarns may further improve the mechanical performances of composites with 3D fabric compared to similar 3D composites that use low-performance non-aramid yarns (Table 2).
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3.3.2 Effects of epoxy treatment The above observations clearly show that for 3D fabric, penetration of the cement matrix between bundle filaments is relatively low due to the fabric’s complexity and the tightening function played by the loops, which together reduce the reinforcement efficiency of the fabric. One way to improve fabric reinforcing efficiency is to treat it (i.e., saturate it; section 2.1.1) with epoxy, which by connecting all bundles and filaments as a single unit causes them to carry the applied loads together. The bundle filaments of fabrics that have been treated with epoxy, however, are impenetrable by the cement matrix because the epoxy has filled in the spaces normally filled by cement as clearly observed in Fig. 13. Thus, the epoxy treatment effectively nullifies concerns about cement penetration. Figure 13 shows epoxy treated 3D Ar 100 fabric embedded in a cement matrix (Fig. 13a) and the imprint of this fabric in the cement matrix (Fig. 13b). Both images clearly show how the bundle yarns and loops have been covered, and any spaced filled in, by the epoxy. This strong epoxy-based interconnection of warp, weft, Z yarns and loops provides stiff and strong junctions. In addition, the penetration of openings in the fabric by the cement matrix is also very clear (Fig. 13a), indicating strong mechanical anchoring of the entire fabric unit in the matrix. Furthermore, the loops around the warp yarns confer on the reinforcing unit in the warp direction a complex shape, i.e., with alternating narrow and wide areas along its length (Fig. 13b), which, compared to a smooth and straight shape, can promote much stronger anchoring of the warp reinforcing unit in the matrix and therefore improved mechanical performance. However, due to the stiff epoxy-based connections of the warp, weft and Z yarns at the junction and the related anchoring mechanism, these regions of the composite are expected to be characterized by high stress, increasing their sensitivity to cracking. This is clearly visible in Fig. 14, which shows a junction of 3D Ar 100 epoxy treated fabric embedded in cement matrix. In both plates of Fig. 14, cracks are visible at the fabric-cement intersection. The aramid loop warping around the glass filaments is clearly seen in Fig. 14b, which also shows the good epoxy penetration between the filaments of the aramid loop and glass warp yarn, again indicative of the strong connection between the yarns constituting the 3D fabric. Both plates of Fig. 14 also show gaps between epoxy coated yarn and matrix at the junctions that may also affect composite mechanical properties. In summary, for composites reinforced with 3D fabric treated in epoxy, the fabric, together with the Zdirection aramid yarns anchored in the cement matrix, can be considered as a single 3D unit, leading to the improved mechanical performance of the epoxy-treated 3D fabric composites (Fig. 6b, 6 and Table 2). However, the epoxy-matrix bond is not perfect, and as a result, cracking is more likely to happen in epoxy treated fabric and cement composites, a potential outcome that could limit the reinforcing efficiency of such composites.
3.4 Cracking 3.4.1 Crack patterns and failure behavior The crack patterns of the 2D, 3D REF and 3D Ar 100 composites without and with epoxy treatment are presented in Figs. 15 and 16. Multiple cracking is clearly visible 10
for the 3D fabric composites in both the epoxy and non-epoxy treated cases. Such multiple cracking, however, is not as clearly evident in the 2D fabric composite. Composite failure behavior was further investigated by comparing the failure mechanisms at the end of the testing periods of composites with the 2D fabric and 3D Ar 100 fabric treated in epoxy (Fig. 17). The 2D fabric composite exhibited complete failure with a clearly visible, single wide crack and separation of the fabric from the matrix, i.e., delamination. The delamination is indicative of the fabric’s low reinforcing efficiency and of poor fabric-matrix bonding. In contrast, the 3D Ar 100 composite with through-thickness aramid yarns underwent multiple cracking accompanied by damage to the bottom (tensile) zone of the composite. Some delamination between fabric and matrix can be seen at the top (compression) zone of the specimen, but this was not as severe compared with the cracking behaviour of the 2D fabric composite. Note that for the 3D fabric a deflection of about 16 mm was reached at the end of testing. Such failure behaviour indicates that the 3D fabric has higher reinforcing efficiency, i.e., better bonding and mechanical anchoring of the 3D fabric within the cement matrix compared with the 2D fabric. The observed failure mechanisms correlate well with the overall flexural behaviours of the composites (Fig. 6) and the SEM observations, which together indicate that the 3D fabric composite has a strong anchoring mechanism. Crack development and subsequent crack patterns differed somewhat between the systems with and without epoxy treatment. For the non-epoxy treated composites (Fig. 15), both the 3D REF and 3D Ar 100 composites developed cracks from bottom to top along relatively straight paths. This suggests that a bending type of failure was probably the reason for cracking in the 3D fabric composites without epoxy. For the epoxy treated composites (Fig. 16), however, although bending cracks can be observed, other cracks also developed diagonally from the bottom to the top of the composite to the points where the loads were applied, suggesting in this case that a shear failure mechanism was more likely. For the non-treated epoxy composites the developed loads are relatively low and thus the reinforcement by the 2D as well as the 3D fabrics was sufficient to sustain the shear stresses and the failure is a bending type. In the epoxy treated composites the loads are much greater and the shear reinforcement in both 2D and 3D fabrics was not enough and a shear type of failure was occurred. Thus, these observations suggest that different failure mechanisms – depending on the stiffness and reinforcing efficiency of the 3D fabric – were responsible for the observed cracking behaviors. Specifically, fabric that was pre-treated with epoxy behaved as a whole, single unit within the composite. In contrast, yarn bundles and filaments in the non-epoxy treated 3D fabrics exhibited at least partially separate behavior that resulted in lower reinforcing efficiency and relatively low load bearing capacity. Further research is required in order to better understand the mechanisms that influence failure mechanisms of 3D fabric cement-based composites.
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3.4.2 Crack monitoring – photoelastic Fig. 18 shows the photoelastic responses of 3D REF (without aramid yarns) during the different stages of loading (Fig. 18a-b, 10 sequential images each). The response stages are numbered consecutively on the stress-deflection curve (Fig. 18c-d). For the composite without the epoxy, the single small peak in the first image (Fig. 18a) indicates the initiation of the first crack in the composite at a very early loading stage (#1 in Fig. 18c). Likewise, the two peaks evident in the second image (Fig. 18a) show that two cracks had developed by this stage of loading (#2 in Fig. 18c). In each subsequent image (3, 4 and 5, Fig. 18a), the number of peaks increases by one, i.e., more cracks appeared with the increase in load indicating multiple cracking behaviour. Beyond this stage of loading, although the peaks become wider making it increasingly difficult to distinguish between the developing cracks, the photoelastic images still clearly show that more cracks developed. Indeed, the abrupt, small drops observed in the stress-strain curve (Fig. 18c) also support multiple cracking. Thus in the 3D REF composite without epoxy, the cracks built up gradually and moderately (fringes formed slowly), such that they started from the composite’s weak point and then developed gradually along its length. In contrast, the photoelastic responses of the 3D REF composite with epoxy treated fabric indicate that the cracking mechanisms and stress distributions of the two composites differed. In the early loading stages (#1-2 in Fig. 18b and 18d), a multitude of small peaks are evident along the entire length of the specimen, implying that soon after loading began, many tiny cracks were initiated together. In the third stage, only the crack at the centre of the specimen grows (image 3 in Fig. 18b), after which a second crack grows left of centre followed by the growth of a third crack to the right of the centre crack (#4-5 in Fig. 18b). Image 6 shows that at a deflection of 1.7 mm, a fourth peak grew (Fig. 18d), but beyond this loading stage the lines of the concentrated stresses merge together. As with the non-epoxy treated composite, here it is also difficult to distinguish between the different stress regions. But the entire region of the concentrated stresses (fringes) widened as loading continued, suggesting that although it is difficult to distinguish between the different cracks on the photoelastic sheet, the number and size of the cracks both grew. In addition, crack development is also noticeable on the stress-deflection curve (Fig. 18d), where it appears as sharp drops. When comparing the stage 10 photoelastic sheet responses of the two systems with and without epoxy, newly developed cracks are not apparent. At this stage, deflection of the non-epoxy system is ~1.8 mm whereas for the epoxy system it is about 4.5 mm. Beyond this stage, the dominant mechanism may be crack widening. Similar behaviour can be observed in Fig. 19 showing the photoelastic responses of the Ar 100 composites with and without epoxy, i.e., the composites that contain aramid yarns in the through-thickness, or Z, direction. Numerous tiny, visible cracks were immediately initiated in the epoxy composite systems during the initial loading stages, but the non-epoxy composite system exhibited an individual and gradual cracking pattern. In both cases, however, crack saturation occurred at deflections of 12
about 2 mm (Fig. 19c-d). Note that here the epoxy treated composite system exhibited shear failure that, although not recorded by the photoelastic sheet measurements, was believed to influence overall composite cracking behaviour (Fig. 16c). To summarise, based on the photoelastic measurements, several differences in crack development were observed between the epoxy and non-epoxy composites: (i) crack initiation – in the epoxy system, cracks were initiated together along the entire length of the specimen at a very early stage of loading; in contrast, cracks in the non-epoxy system were created individually and separately during loading; (ii) crack development – cracks initiated in the epoxy system grew during continuous loading, starting from the centre of the specimen to both sides gradually; in the non-epoxy system, cracks began at the weak point of the specimen and then developed along its length gradually; (iii) crack saturation – for the 3D REF composite only, the crack opening and widening mechanism began at a much greater deflection for the epoxy system compared to the non-epoxy system. These observations may indicate that the anchoring mechanism between fabric and matrix in the epoxy treated system – in which the fabric behaves as a whole unit, and therefore, matrix and fabric work together – results in a much stronger and tougher composite compared to the nonepoxy composite (Figs. 6, 7 and Table 2).
4
Conclusions
This study examined the flexural behavior of cement-based composites reinforced with 3D fabric, focusing on yarns situated in the through-thickness (Z direction) of the fabric influenced the composite. Based on the results obtained in this work, it can be concluded that compared with 2D fabrics, 3D fabrics are highly beneficial reinforcements for cement-based composites. Composites constructed with the 2D fabrics were typically low performance and often underwent delamination. In contrast, the complex geometries of the 3D fabrics facilitated stronger anchoring mechanisms with the cement matrix and good connections between the yarns along the plane surfaces of the fabric, thereby leading ultimately to a more efficient reinforcing unit. Treatment of the 3D fabric with epoxy significantly improved composite performance. Indeed, the epoxy treatment is critical for 3D fabrics due to their very complex fabric structure. The epoxy treatment binds the different elements of the fabric together such that they behave as a single, whole unit in all three directions, carrying the applied loads together. The differences in crack initiation and development revealed by the photoelastic measurements demonstrated this idea. For the epoxy systems, cracks were initiated together immediately when loading began along the entire length of the specimen, i.e., reflecting behaviour of one whole fabric unit. These initial, tiny cracks then grew during continuous loading, starting from the centre of the specimen and progressing in both directions gradually. In the non-epoxy system, the cracks were created individually and separately during loading starting from the weak point of the specimen, i.e., different behaviours of individual yarns. 13
The improved flexural performance of the composite reinforced with the epoxy treated fabrics can be related to two main mechanisms: (i) the greater reinforcing efficiency of the yarns as they work together during loading when connected by the epoxy vs. the lower reinforcing efficiency of the non-treated epoxy fabrics, in which the bundles and the filaments act separately, resulting in a lower load bearing capacity; (ii) stronger mechanical anchoring between epoxy treated fabric and cement matrix that was clearly observed by SEM. However, the bond between epoxy and matrix was not perfect, such that cracking at the fabric junction was more likely, a finding that may limit the reinforcing efficiency of the epoxy treated composites and therefore, that must be further investigated. The yarns in the Z direction of the fabric, i.e., composite thickness, were found to strongly influence composite performance: (i) high performance Z yarns such as aramid greatly improved the strength and toughness of the cement-based composite compared to low performance yarns such as PES; (ii) the greater the relative content of high-performance yarns in the Z direction, the better composite performance. The influence of Z direction yarns was more pronounced when the fabric was treated in epoxy before its inclusion in the cement composite. In conclusion, 3D fabrics have immense potential in the reinforcement of cementbased composites even though the Z yarns are not in the direction of the applied load. The Z yarns hold the whole fabric together to confer on the fabric good mechanical anchoring properties. That anchoring can be easily improved when the bundles within the fabric are treated with epoxy (i.e., coated, saturated and glued with epoxy), resulting in a reinforcing component that behaves as a single unit to confer improved mechanical performance on the composite.
5
Acknowledgments
The authors would like to acknowledge Israel Science Foundation GRANT NO. 147/11 for the financial support of this research and the Institut für Textiltechnik der RWTH Aachen University (ITA), Germany, for fabric production.
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References
[1] Brameshuber, W., Textile reinforced Concrete: State of the Art Report of RILEM Committee 201-TRC, RILEM Publications, Bagneux, France, (2006). [2] Bentur A. and Mindess, S., 2002. Fibre reinforced cementitious composites. Spoon Press, London, New York. [3] Peled A. and Bentur A., 2003. Fabric structure and its reinforcing efficiency in textile reinforced cement composites. Composites Part A. 34:07-118. [4] Peled A. and Mobasher B., 2005. Pultruded fabric-cement composites. ACI Mater J. 102:15-23. [5] Brameshuber W. Brockmann T., and Banholzer B., 2006. Material and bonding 14
characteristics for dimensioning and modeling of textile reinforced concrete (TRC) elements. Mater Struct. 39:749-763. [6] Naaman, A.E., 2003. Strain hardening and deflection hardening fiber reinforced cement composites. In A.E. Naaman and H.W. Reinhardt (eds.), Fourth International Workshop on High Performance Fiber Reinforced Cement Composites (HPFRCC 4), RILEM Publications, 2003, Bagneux, France; pp. 95-113. [7] Hanisch V., Kolkmann A., Roye A., and Gries T., 2006. Influence of machine settings on mechanical performance of yarns and textile structures. In Hegger et al (eds), Proceedings of the 1st International RILEM Symposium (Textile Reinforced Concrete ICTRC) RILEM TC201-TRC; pp. 13-22. [8] Roye A., Gries T., and Peled A., 2004. Spacer fabric for thin walled concrete elements. In: de Prisco et al (eds), Fiber Reinforced Concrete – BEFIB, PRO 39, RILEM; pp. 1505-1514. [9] Naaman N., 2010. Textile reinforced cement composites: competitive status and research directions. In: Brameshuber (ed) International RILEM Conference on Materials Science (MatSci) I; pp. 3-22. [10] Peled A., Zhu D., and Mobasher B., 2011. Impact Behavior of 3D Fabric Reinforced Cementitious Composites. HPFRCC6, Ann Arbor, June 2011. [11] Zhu D., Peled A., and Mobasher B., 2011. Dynamic tensile testing of fabriccement composites. Construction and Building Materials J. 25(1):385-395.
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List of Figures Fig. 1: 3D warp knitted fabric: a) schematic description of the production method, (b) 3D fabric Fig. 2: Images of the different fabrics, top and side views, including schematic description of the 3D fabrics with the two aramid yarn contents. Fig. 3: (a) Casting of the 3D fabric specimen, (b) scheme of the 3D composite along its length at the end of casting. Fig. 4: Stereoscope images of plain fabric junction: (a) 2D fabric, (b) 3D REF fabric, (c) 3D Ar (aramid) fabric, not in cement. Fig. 5: Tensile stress vs. strain along the weft and warp directions of plain (without cement) 2D fabrics (AR glass). Fig.6: Flexural behavior of the composites with 2D fabric and with 3D fabrics with (Ar 50 and Ar 100) or without (REF) aramid yarns: (a) without epoxy; (b) with epoxy. Fig. 7: Comparison of the different fabric-cement composites with and without epoxy treatment: (a) MOR; (b) toughness, related to the cement paste matrix. Fig. 8: Tensile properties per single reinforcing yarn vs. aramid Z yarn content (a) flexural strength, (b) toughness. Fig.9: SEM images of the three fabric types (without epoxy) embedded in the cement matrix, top view: (a) 2D; (b) 3D REF; (c) 3D Ar 100. Fig. 10: SEM images of fabric imprints in the cement matrix: (a) 2D, (b) 3D REF, (c) 3D Ar 100 fabric composites. Fig.11: Cement penetration between filaments of weft yarn in a 3D REF composite at different locations: (a) general view showing the two regions observed in b and c (red rectangles); (b) far from a junction; (c) at the junction. Fig.12: Aramid yarn penetration by cement paste: (a) top view; (b) bundle cross section. Fig.13: 3D Ar 100 fabric with epoxy embedded in cement matrix (a) top view of fabric junction, (b) loop print in the matrix Fig. 14: Fabric junction of 3D Ar fabric with epoxy in cement matrix: (a) top view; (b) view of fabric cross section exposing the warp glass yarns with the aramid loop around it, Back Scattering mode. Fig.15: Crack pattern of the different composites without epoxy: (a) 2D, (b) 3D REF and (c) 3D Ar 100 Fig. 316: Crack patterns of the different composites with epoxy: (a) 2D, (b) 3D REF and (c) 3D Ar 100 Fig.17: Failure mechanisms of composites at the end of testing: (a) 2D fabric composite, and (b) 3D Ar 100 composite, both with epoxy. 16
Fig.18: 3D REF composites with and without epoxy: (a and b) photoelastic responses, (c and d) flexural vs. deflection curves annotated with numbers corresponding to stages of photoelastic response monitoring. Fig. 19: 3D Ar100 composites without and with epoxy: (a and b) photoelastic responses, (c and d) flexural vs. deflection curves annotated with numbers corresponding to stages of photoelastic response monitoring.
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Table 1: Properties of the yarns used to manufacture the reinforced fabrics. Tensile strength
Tex
Elastic Modulus [GPa]
[gr/Km]
Function within the fabric
0222
20
0022
Weft and Warp
8553
20
062
Reinforced yarn along Z axis
PES monofilament
-
-
66.4
Spacers
PES multifilament
-
-
16.7
Loops
Fiber type
[MPa] AR glass Cem-FIL© grade Aramid Technora T-240
Table 2: Flexural properties of the different composites: average MOR, deflection at MOR and toughness values, along with their standard deviations.
Sample type
MOR [MPa]
Deflection at MOR [mm]
Toughness [MPamm] (at 15% of MOR)
Paste
1.56±0.3
0.057±0.004
1.5±0.44
2D
7.23±0.42
2.99±0.58
26.49±4.05
12.11±0.67
6.36±0.64
57.37±6.23
Ar 50
13.4±1.17
6.26±0.17
65.87±6.35
Ar 100
13.28±0.33
6.09±0.38
77.88±14.23
9.7±1.31
3.05±0.94
24.18±14.45
3D Ref
16.95±0.96
11.3±2.38
169.28±59.94
Ar 50
20.96±1.3
10.95±2.11
193.98±43.02
Ar 100
25.72±2.65
13.37±1.19
239.5±34.1
Without 3D Ref Epoxy
2D With Epoxy
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(a) Spacer, Z yarn 2D fabrics
3D fabric (b)
Fig. 4: 3D warp knitted fabric: a) schematic description of the production method, (b) 3D fabric
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Fabric Name
Top view
Side view
2D
3D REF
3D Ar 100 3D Ar Top View
Ar 100
Ar 50
Fig. 5: Images of the different fabrics, top and side views, including schematic description of the 3D fabrics with the two aramid yarn contents.
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330 mm 3D fabric
2 mm 22 mm 2 mm
Cement paste
(a)
(b)
Fig. 3: (a) Casting of the 3D fabric specimen, (b) scheme of the 3D composite along its length at the end of casting.
Multi Filament PES
Monofilament PES
AR glass AR glass
Multifilament PES Monofilament PES
(a)
(b)
Aramid
(c)
Fig. 4: Stereoscope images of plain fabric junction: (a) 2D fabric, (b) 3D REF fabric, (c) 3D Ar (aramid) fabric, not in cement.
21
Fig.5: Tensile stress vs. strain along the weft and warp directions of plain (without cement) 2D fabrics (AR glass).
22
(a)
(b)
Fig.6: Flexural behavior of the composites with 2D fabric and with 3D fabrics with (Ar 50 and Ar 100) or without (REF) aramid yarns: (a) without epoxy; (b) with epoxy.
23
(a)
(b) Fig. 7: Comparison of the different fabric-cement composites with and without epoxy treatment: (a) MOR; (b) toughness, related to the cement paste matrix.
24
(b)
(a)
Fig. 8: Tensile properties per single reinforcing yarn vs. aramid Z yarn content (a) flexural strength, (b) toughness.
(a)
(b)
(c)
Fig.9: SEM images of the three fabric types (without epoxy) embedded in the cement matrix, top view: (a) 2D; (b) 3D REF; (c) 3D Ar 100.
25
(a)
(b)
(c) Fig. 10: SEM images of fabric imprints in the cement matrix: (a) 2D, (b) 3D REF, (c) 3D Ar 100 fabric composites.
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(a)
(b)
(c)
Fig.11: Cement penetration between filaments of weft yarn in a 3D REF composite at different locations: (a) general view showing the two regions observed in b and c (red rectangles); (b) far from a junction; (c) at the junction.
(a)
(b)
Fig.12: Aramid yarn penetration by cement paste: (a) top view; (b) bundle cross section. 27
(a)
(b)
Fig.13: 3D Ar 100 fabric with epoxy embedded in cement matrix (a) top view of fabric junction, (b) loop print in the matrix
Glass filaments
Void
Gap Aramid
Monofilament
PES
Aramid (a)
(b)
Fig. 14: Fabric junction of 3D Ar fabric with epoxy in cement matrix: (a) top view; (b) view of fabric cross section exposing the warp glass yarns with the aramid loop around it, Back Scattering mode.
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(a)
(b)
(c)
Fig.15: Crack pattern of the different composites without epoxy: (a) 2D, (b) 3D REF and (c) 3D Ar 100
(a)
(b)
(c)
Fig. 616: Crack patterns of the different composites with epoxy: (a) 2D, (b) 3D REF and (c) 3D Ar 100
29
(a)
(b)
Fig.17: Failure mechanisms of composites at the end of testing: (a) 2D fabric composite, and (b) 3D Ar 100 composite, both with epoxy.
30
Without epoxy
(a)
(c)
With epoxy
(b)
(d)
Fig.18: 3D REF composites with and without epoxy: (a and b) photoelastic responses, (c and d) flexural vs. deflection curves annotated with numbers corresponding to stages of photoelastic response monitoring.
31
Without epoxy
With epoxy
(a)
(b)
(c)
(d)
Fig.19: 3D Ar100 composites without and with epoxy: (a and b) photoelastic responses, (c and d) flexural vs. deflection curves annotated with numbers corresponding to stages of photoelastic response monitoring.
32