PULLOUT BEHAVIOUR OF POLYVINYL ALCOHOL FIBER FROM CEMENTITIOUS MATRIX DURING PLASTIC STATE

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1 BEFIB212 Fibre reinforced concrete Joaquim Barros et al. (Eds) UM, Guimarães, 212 PULLOUT BEHAVIOUR OF POLYVINYL ALCOHOL FIBER FROM CEMENTITIOUS MATRIX DURING PLASTIC STATE Jianzhong Liu *, Changfeng Li *, Jiaping Liu *, Zhiqian Yang * and Gong Cui * * State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Research Institute of Building Science, No.12, Beijing West Road, Nanjing, Jiangsu, China lichangfeng@cnjsjk.cn, web page: Keywords: PVA fiber, pullout behaviour, plastic state, matrix properties, embedded length. Summary: Toughening and reinforcing effect of polyvinyl alcohol (PVA) fibers to cementitious materials during plastic stage depends on the fiber-matrix interfacial bonding properties. Equipment for testing pull out behaviour of PVA fibers from cementitious matrix during plastic state was developed to understand the reinforcement of PVA fibers. Difference between polypropylene (PP) and PVA fibers, effect of fiber embedded length and matrix properties on pullout behaviour of PVA fibers was studied. Results showed that, PVA fiber showed better bond properties with the matrix, and the debonding displacement of PP and PVA fiber was similar, about at the.5mm. The higher the matrix strength was, the greater the bonding strength between PVA fiber and matrix would be, in other words, interfacial bonding strength and fiber energy consumption increased. With the increase of fiber embedded length, frictional force and slip distance between fiber and matrix increased, while the ratio of pullout displacement to embedded length decreased, which indicated the increase of embedded length did not cause the expected effect to bond properties. All studies in this paper provide experimental data to understand the high performance PVA fiber and to further make full use of it. 1 INTRODUCTION Plastic shrinkage cracking is a common problem in concrete construction, which can cause the performance of concrete reduced [1]. In particular, concrete with wide surfaces such as bridge slabs, pavements and parking lot floors are affected by restraint, high rates of evaporation, and high temperatures during the initial placing, so the plastic shrinkage cracking before the cementitious composite has hardened completely often happens. Randomly dispersed synthetic fibers, such as PVA fiber, can be effective in controlling plastic shrinkage cracking of cementitious composites. These fibers produce a bridging force across the width of a crack and prevent its propagation [2, 3]. In order to fully utilize the performance of synthetic fibers, careful consideration is necessary to study the effect and its bond properties with matrix. Cementitious composites exhibit the general characteristics of brittle matrix composites; that is, the failure of the matrix precedes the fiber failure, thus allowing the fibers to bridging the propagating crack. During the initial stage of loading, the PVA fiber and cementitious composites bear the stress together, and the matrix is the main bearer of stress; after the cracking of matrix, instead, the fibers become the main bearer of stress and bridge the cracks. The fiber/matrix bond strongly affects the ability of fibers to stabilize crack propagation in the matrix. The interfacial bonding properties, such as fiber debonding and sliding at the interface, have a significant influence on the carrying and deformation capacity of cementitious composites. Thus, the study of interface between fiber and matrix is an effective measure to obtain better mechanical properties of fiber reinforced cementitious composite. Many researchers have investigated and modeled the effect of the interfacial bond on composite properties such as crack resistance [4] and durability [5]. Among the existing methods to evaluate the bonding properties, single fiber pull-out test

2 is mostly used, due to its loading condition between fiber and matrix is similar to that during the cracking of cementitious composites [6-8]. However, because of the limitation of experimental methods, the micro pull-out curves of PVA fiber from cement matrix was hard to obtain. Little research has been conducted to quantitatively evaluate their interfacial properties, especially in the plastic state cementitious matrix. The main scope of this study is to assess the pull-out behavior of PVA fibers from plastic state cementitous matrix. A new apparatus (load range: -2N) has been designed and shown to be effective in obtaining the pull-out curves of PVA fibers from cement matrix. Difference between PP and PVA fibers was analyzed, and effect of matrix properties and embedded length on bond properties between PVA fiber and cement matrix was also studied. The study makes it possible to better understand of the role of PVA fibers in improving the properties of brittle cementitious composites in plastic state, and to develop rational analytical models to describe bond in PVA fiber reinforced cementitous composites. 2 Experimental procedures 2.1 Raw materials and mix proportion The cement used was P.Ⅱ52.5 ordinary Portland cement produced by Onada Corp. in Jiangsu Province, China. The fibers (PP and PVA fibers) and PCA Ⅳ-B (a polycarboxylate-based superplasticizer) were provided by Jiangsu Bote New Materials Co., Ltd in Jiangsu Province, China. The properties of fibers were tested by GB/T Synthetic Fiber Used in Cement Concrete and Mortar in China, and were shown in Table 1. The mix proportions were shown in Table 2. After the mixing of cementitous composites, the fibers was embedded into the specimens, then all of them were cured in the temperature of 2 and the humidity of 4%, until the tests performed. Table 1: Properties of fibers Code Diameter /μm Tensile strength /MPa Elasticity modulus /GPa Elongation at break /% Image PP PVA Table 2: Mix proportions No. W/C Cement / kg/m 3 Water / kg/m 3 PCAⅣ-B / % Test methods The pull-out tests were performed in a composite system with a capacity of 2N, as shown in Fig. 2. When the tests began, the pull-out specimens were gripped by outer frame and lifting device, fibers were gripped by pneumatic grip. Care was taken to ensure zero fiber free length to avoid elastic stretching of the free fiber, which would affect the load-displacement curves of PVA fiber from cement matrix. The displacement control was performed by the actuator, at a rate of.1 mm/s. The load was 2

3 sensed by a load cell and the pull-out displacement of the fiber was sensed by the movement of the actuator, then the pull-out load-displacement curves would be shown in the computer and could be output. The composite system was controlled by computer software; all the procedures performed can ensure the effectiveness and accuracy of test results. The bond strength τ can be calculated by the peak load of fiber pullout, as shown in Equation (1). P dl max ( 1) Where P max was the peak load of fiber pulling out from cement matrix, d was the diameter of fiber, l f was the length of fiber. f (a) Schematic diagram (b) Real photo Figure 1: Pull-out test apparatus 3 Results and discussion 3.1 Difference between PP and PVA fibers Differences between PP and PVA fibers in bond properties with cement matrix were studied, and the bond mechanism was also analyzed. Figure 2 showed the pullout load-displacement curves of PP, PVA fibers from cement matrix, Figure 3 showed the bond strength between PP, PVA fibers and cement matrix, Figure 4 present the SEM images of PP and PVA fiber after pulling out from cement matrix. 3

4 Bond strength / MPa W/C=.2 EL=6mm T=6h PP W/C=.2 EL=6mm T=6h PVA 4 3 W/C=.3 EL=6mm T=9h PVA W/C=.3 EL=6mm T=9h PP (a) W/C=.2 (b) W/C=.3 Figure 2: Pullout load-displacement curves of PP and PVA fibers from cement matrix From Figure 2, for the two water-cement ratio (.3 and.2), the same rules can be drawn to study the difference between PP and PVA fibers, not only in the shape of the complete pullout loaddisplacement curve, but also in the value of peak load. First, the interfacial debonding load of PVA fiber from cement matrix was slightly higher than that of PP fiber, and the debonding displacement of PP and PVA fiber was similar, about at the.5mm. Second, the frictional force after debonding between PVA fibers and cement matrix was obviously higher than that of PP fiber. With the increase of pullout displacement, the frictional force and the value of peak load increased, which was different from PP fiber W/C=.3 EL=6mm PP W/C=.3 EL=6mm PVA Time after mixing /h Figure 3: Bond strength between fibers and cement matrix From Figure 3, two conclusions can be obtained for studying the bond strength of PP, PVA fibers and cement matrix. First, before the 5h from the placing of cement matrix, the bond strength between PP, PVA fiber and cement matrix was similar, and the changes of bond strength with time was not obvious. Second, after the 5h from the placing of cement matrix, the bond strength between PVA fibers and cement matrix increased significantly, and was obviously higher than PP fiber. The different law of bond strength between PVA fibers and cement matrix before and after 5h from the placing of cement matrix may be due to the state and performance of cement matrix. The higher bond properties of PVA fibers with cement matrix to PP fibers may be due to the excellent properties of PVA fibers. 4

5 Bond strength / MPa (a) PP fiber (b) PVA fiber Figure 4 : SEM micrographs of PP and PVA fibers As shown in Figure 4, little cement hydration products can be seen on the surface of PP fiber, while there was a layer of hydrated cement products on the surface PVA fiber. The bond mechanism of PVA fiber to the cement matrix seemed to be different from that of PP fiber. Combined with the properties of fibers and the bond properties between fiber and matrix shown in Figure 1-2 and 1-3, it can be concluded that the chemical bond between PP fiber and matrix was weak. The main bond mechanism of PP fiber to cement matrix may be energy dissipation and pull-out stress damping by deformation of fiber [9]. Compared with PP fibers, due to the special surface and hydrophilic nature of PVA fibers, good adhesion of PVA fibers to cement matrix can be drawn from SEM image, which would effectively transfer the applied load at the interface. So, better bonding properties of PVA fibers to PP fibers can be obtained. 3.2 Effect of matrix properties Effect of matrix properties on pullout behavior of PVA fibers from cement matrix was studied. Figure 5(a) and (b) showed the pullout load-displacement curves of PVA fibers from matrix and the bond strength between PVA fibers and matrix, respectively, with the three water-cement ratios. 6 5 W/C=.2 EL=6mm T=6h W/C=.3 EL=6mm W/C=.25 EL=6mm W/C=.2 EL=6mm 4 W/C=.25 EL=6mm T=6h W/C=.3 EL=6mm T=6h Time after mixing / h (a) Load-displacement curve (b) Bond strength Figure 5: Effect of W/C ratios on bond properties between PVA fibers and cement matrix From Figure 5(a) and (b), it was found that with the decrease of water-cement ratio, the shape of pullout load-displacement curve of PVA fibers from cement matrix was different, the frictional force and 5

6 bond strength between PVA fibers and cement matrix increased. In general, the debonding of PVA fiber from matrix occurred at the small displacement, often lower than.5mm, then the frictional force would followed. For the water-cement ratio of.3, the change of load with the increase of pullout displacement of PVA fiber was not obvious, with constant level. For the water-cement ratio lower than.3, with the increase of pullout displacement, the load increased significantly, and the lower the water-cement ratio was, the higher increase would be obtained. However, with the decrease of watercement ratio, the pullout displacement decreased. The cementitious composites with lower watercement ratio were usually brittle, which would show brittle failure and cause the decrease of pullout displacement of PVA fiber. From the study on effect of water-cement ratios on bond properties between PVA fibers and cement matrix, a view can be given, with the decrease of water-cement ratio, or the increase of matrix strength, shorter PVA fiber should be applied to fully utilize the performance of PVA fiber. 3.3 Effect of fiber embedment length Pullout tests were carried out for three different embedded lengths (3, 6 and 9 mm) of PVA fibers in cement matrix. Figure 6 showed the pullout load-displacement curves of PVA fibers from cement matrix for the three different embedded lengths, Figure 7 present the bond strength for the three different embedded lengths W/C=.2 T=6h EL=9mm W/C=.2 T=6h EL=6mm W/C=.2 T=6h EL=3mm (a) W/C= W/C=.3 T=7h EL=9mm W/C=.3 T=7h EL=6mm W/C=.3 T=7h EL=3mm (b) W/C=.3 Figure 6: Pullout load-displacement curves for three different embedded lengths From Figure 6, it was found that with the increase of embedded length, some similarities and differences in the pullout load-displacement curves of PVA fibers from cement matrix can be drawn. The first similarity was the fiber debonding displacement was about.5mm, the debonding load was between 5-9 cn; the second similarity was the shape of three pullout curves for each water-cement ratio was similar. For the water-cement ratio of.3, the pullout load corresponding to frictional sliding increased significantly, while the increase for the water-cement ratio of.2 was not obvious. The differences were with the increase of fiber embedded length, the pullout load and displacement increased, while the ratio of pullout displacement to embedded length decreased. 6

7 Bond strength / MPa W/C=.2 T=5h W/C=.2 T=6h W/C=.2 T=7h Embedded length / mm Figure 7: Bond strength for three different embedded lengths From Figure 7, it was found that with the increase of embedded length, the bond strength between PVA fibers and cement matrix decreased slightly, although the peak load increased. In general, the increase in friction force between the fiber and matrix due to the abrasion of PVA fiber while sliding out of the cement matrix, and with the increase in embedded fiber length and the decrease of watercement ratio, the abrasion effect tended to increase. But the phenomenon of decrease of bond strength indicated that the increase of embedded length did not cause the expected effect to bond properties. 4 Conclusions From the experimental results obtained in this study, the following conclusions can be drawn: 1) PVA fibers showed better bond properties with matrix to PP fibers, and the bond mechanism of PVA fiber to the cement matrix seemed to be different from that of PP fiber. The main bond mechanism of PP fiber to cement matrix may be energy dissipation and pull-out stress damping by deformation of fiber. Compared with PP fibers, due to the special surface and hydrophilic nature of PVA fibers, good adhesion and bond properties of PVA fibers to cement matrix can be drawn. 2) With the decrease of water-cement ratio, the shape of pullout load-displacement curve of PVA fibers from cement matrix was different, the frictional force and bond strength between PVA fibers and cement matrix increased. However, with the decrease of water-cement ratio, the pullout displacement decreased. Thus with the decrease of water-cement ratio, or the increase of matrix strength, shorter PVA fiber should be applied to fully utilize the performance of PVA fiber. 3) With the change of embedded length, the fiber debonding displacement was about.5mm, the debonding load was between 5-9 cn; the shape of three pullout curves for each watercement ratio was similar. The pullout load and displacement increased, while the ratio of pullout displacement to embedded length decreased, which indicated the increase of embedded length did not cause the expected effect to bond properties. ACKNOWLEGEMENT The study of this paper is financially supported by National Basic Research Program of China (973 Program) (Grant No. 2CB73581), National Natural Science Foundation of China (Grant No. 5984) and Natural Science Foundation of Jiangsu Province (Grant No. BK211835). The 7

8 research work was performed at Jiangsu Bote New Materials Co, Ltd. in China. The authors would like to express their gratitude for the financial and technical support that made this project possible. REFERENCES [1] J.H.J. Kim, C.Gi. Park, S.W. Lee, et al., Effects of the geometry of recycled PET fiber reinforcement on shrinkage cracking of cement-based composites, Compos., 39, (28). [2] T. Kanda and V.C. Li, Interface property and apparent strength of high-strength hydrophilic fiber in cement matrix, J. Mater. Civ. Eng., (1), 5-13(1998). [3] S. Singh, A. Shukla, R. Brown, Pullout behavior of polypropylene fibers from cementitious matrix, Cem. Concr. Res., 34(), (24). [4] B. Mobasher, C.Y. Li, Effect of interfacial properties on the crack propagation in cementitious composites, Adv. Cem. Based Mater., 4, 93 5(1996). [5] A. Bentur, Role of interfaces in controlling the durability of fiber-reinforced cements, J. Mater. Civ. Eng., 1 (1), 2-7(2). [6] V.C. Li, Y. Wang and S. Backer, Effect of inclining angle, bundling and surface treatment on synthetic fibre pull-out from a cement matrix, Compos., 21(2), (199). [7] Y.W. Chan, V.C. Li, Age effect on the characteristics of fibre/cement interfacial properties, J. Mater. Sci., 32, (1997). [8] T. Abu-Lebdeh, S. Hamoush, W. Heard, et al., Effect of matrix strength on pullout behavior of steel fiber reinforced very-high strength concrete composites, Constr. Build. Mater., 25(1), 39-46(211). [9] H.R. Pakravan, M. Jamshidi, M. Latifi, Performance of fibers embedded in a cementitious matrix, J. Appl. Polym. Sci., 116, (2). 8