EXTRUSION OF ENGINEERED CEMENT-BASED COMPOSITE MATERIAL

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1 EXTRUSION OF ENGINEERED CEMENT-BASED COMPOSITE MATERIAL Don de Koker and GPAG van Zijl Department of Civil Engineering, University of Stellenbosch, Republic of South Africa Abstract Discontinuous fiber reinforced Engineered Cement-based Composites prepared by extrusion molding introduce advantages in cement product processing. Extrusion is a plastic-forming method whereby several structural shapes can be manufactured. Extrusion molding of cement products entails their formation under high shear and compressive forces. This leads to performance-enhancing densification of the material, while it also has the potential of producing products of superior geometrical tolerance and has a beneficial influence on the fiber orientation. There is evidence that short fibers are aligned by extrusion, leading to significantly improved mechanical properties of the material. Fiber alignment manifests in the main direction of the extrusion mechanism. Consequently, the mechanical properties of the fiber composite are improved in that direction. The aim of this paper is to substantiate the effectiveness of extrusion with respect to fiber orientation and interfacial bonding, hence forth leading to enhanced mechanical properties. 1. Introduction The primary function of fibers in Engineered Cement-based Composites (ECC) is to bridge, arrest and divert micro-cracks developing in concrete. It is this mechanism through which fibers increase the tensile strength of concrete [1]. In the post-peak region of the tensile stress-strain behavior, the number of fibers per unit area of the cracked section plays a governing role. This number of fibers bridging a crack and the primary failure mode of either these fibers, or the matrix-fiber interfacial, bond governs the ductile performance of the ECC. Given a sufficient number of crack-bridging fibers and favorable fiber pull-out at the 1301

cracked section, multiple cracking is cultivated, characterized by pseudo strainhardening, i.e. a sustained increase in load capacity beyond the first matrix cracking resistance.

Through micro-mechanics based modeling, it has been shown that material and geometrical properties of the fibers, as well as stiffness and strength properties of the matrix and the fiber-matrix

2 cracked section, multiple cracking is cultivated, characterized by pseudo strainhardening, i.e. a sustained increase in load capacity beyond the first matrix cracking resistance. Thus an increase in peak tensile strength and higher post-peak ductility are affected, which distinguish ECC from normal Fiber Reinforced Concrete (FRC) [2]. Through micro-mechanics based modeling, it has been shown that material and geometrical properties of the fibers, as well as stiffness and strength properties of the matrix and the fiber-matrix interaction determine the multiple cracking, pseudohardening tensile response of ECC. In this paper the role played in determining the superior mechanical behavior by the manner in which ECC products are manufactured, is highlighted. In cast specimens the fiber parameters which determine ECC toughness and strength include fiber volume fraction, the fiber aspect ratio (length/diameter), the fiber tensile strength and Young s modulus. With regard to the cement-based matrix the major role players are the matrix tensile strength, Young s modulus, density and, importantly, the aggregate grading. In the matrix-fiber interfacial zone the potential enhanced composite properties depend on the matrix density, interfacial bond and the orientation of the fibers. In extruded ECC products these same parameters remain operational, but the altered realizations of parameters like fiber orientation, as well as the matrix-fiber interfacial zone density, improve the tensile mechanical behavior of such products. However, to realize these improvements, successful extrusion of ECC is required, for which the rheology of the batch and the manufacturing parameters must be well controlled to prevent defects such as edge tearing, voids, fiber clogging and other discontinuities. These defects result in reduced performance of the cured composite. (a) (b) Fig. 1: Extrusion manufacturing process for ECC plate- and pipe members. In this paper, the main influences on (as well as consequences of) the mentioned fiber orientation and interfacial bond properties are reported at the hand of a study of the fast growing pool of reported research activities on ECC internationally, and also from 1302

3 experience gained in preliminary experimentation with crude, small-scale extrusion apparatus of both piston type and auger type. 2. Research Significance Fiber breakage is known to produce more brittle tensile behavior than fiber pull-out from ECC matrices. This is illustrated in Figure 2 by two specimens tested in another project of the same research programme reported here [3]. The Figure shows direct tensile test results of similar specimens, containing PVA fibers of length 13 mm and diameter mm. However, for one specimen, 70% of the cement was replaced by fly-ash, in effect lowering the tensile strength. Fiber pull-out accompanied this weaker, but tough response, while fiber breakage was audible in the case of the stronger matrix. It is clear that the latter, stronger matrix has lower deformability. Although both specimens were produced by standard casting and vibrating, this example illustrates that brittle failure is associated with stronger matrices. In the case of extruded ECC, products of lower porosity are produced, associated with stronger matrices. Thereby, the same effect as shown in Figure 2 may arise, by comparing the tensile mechanical responses of identical matrices, but from different fabrication processes of casting (weak, tough) and extrusion (strong, brittle) Stress [MPa] With Fly-Ash Strain [%] Fig. 2: Illustration of effect of Fiber failure mode for a strong matrix compared to a weak mix (with fly ash). A simplified analysis of the performance of the specimens reflected in Figure 1, but under more general loading producing stress gradients, reveals that the tough uniaxial tensile response may cause a modulus of rupture (MOR) of several times the composite tensile strength. However, this cannot be achieved by the less ductile matrix, as tensile softening degradation upon higher strain of fibers far from the neutral axis, limits the 1303

4 ultimate resistance in flexure. This is illustrated in Figure 3, where stress distributions in the failing cross-sections of a ductile, as well as a brittle beam under 3 point bending are schematized. Whereas a MOR of several times the tensile strength may be realized in the ductile material, the magnification in a brittle material is less. For the two matrices of Figure 2 the following relations are approximately true: cu / tu = 6.5, E=350 cu, where cu is the composite compressive strength, tu the composite ultimate tensile strength and E, the composite Young s modulus. Sectional equilibrium and ultimate moment calculation with the simplified stress distribution for tough tensile response in Figure 3 reveal that an ultimate tensile strain F = 2.9% is required to reach the full potential in bending, i.e. to realize stress transfer in all fibers in the cross-section at ultimate resistance. This is achieved by the fly ash (FA) specimen in Figure 2, but not by the stronger matrix without FA. The calculation predicts a ratio of MOR / tu =4.5. Experimental results reported by Peled and Shah (2003) [4] show this ratio to be 3.5 for extrudates containing FA, but only 2.0 for stronger extrudates, with no FA. As their results were obtained by the comparison of bending results only, i.e. by comparing fiberreinforced composite flexural response to that of matrices without fiber reinforcement, the uniaxial tensile response is not known, which prevents complete validation of the above analytical prediction. Nevertheless, the reduction in the bending:axial resistance ratio shown by their results clearly to proves that this trend accompanies increased brittleness. The rest of this paper studies the mechanisms which influence, or even govern, the tensile response of ECC, with particular reference to the influence of extrusion on fiber orientation and fiber-matrix interfacial zone densification. Stress-strain response in failing cross-section MOR Tough tensile response MOR Brittle tensile response Fig. 3: Illustration of tensile strain-limited flexural resistance of brittle material. 3. Fiber Orientation The advantage of using extrusion in cement product processing is that material is formed under high shear and high compressive forces. A further advantage specific to fiber reinforced cement products is that, with properly designed dies and a properly controlled mix, fibers can be aligned in the load-bearing direction. 1304

5 In standard cast and vibration technique, fibers orientate randomly, unless influenced by the geometrical boundaries of the specimen [1]. Individual fibers can also interact to determine the final orientation, but aggregate particle size grading and distribution probably has the largest influence on the orientation. Fiber dispersion and orientation could be determined respectively by means of counting the number of fibers per unit area with the aid of an optical microscope, or from the fiber cross-sectional shape [5], as schematized in Figure Fiber Distribution Whereas influences on the fiber orientation in ECC are studied in this section, fiber distribution is actually a consequence of fiber orientation, although phenomena like fiber clumping, an extreme case of distribution, do influence the orientation thereof. Fiber distribution runs parallel with fiber orientation in the sense that both distribution and orientation of fibers are affected by the boundaries, aggregate grading and manufacturing process. The number of fibers per unit cross-sectional area is an important factor in the peak tensile resistance and post-peak ductility of the ECC. In order to predict the number of fibers per unit cross-sectional area of ECC, the commonly used equation is of the form: Vf N 1 = A f (1) Matrix x y Fig. 4: Illustration of Fiber Orientation where N 1 = number of fibers per unit area V f = volume fraction of fibers in concrete A f = cross-sectional area of fibers = orientation factor 1305

6 3.2 Manufacturing Process ECC may be manufactured by different manufacturing processes: cast molding, piston extrusion and auger extrusion. Specimens or products of the different processes exhibit different fiber orientations. Cast specimens exhibit random distribution of fibers, except when boundaries come into play [1]. In extruded products the fibers are directed in the load-bearing direction of the mechanism driving it. This has not been quantified in the current research program, but was observed in preliminary studies with crude, simple piston and auger extrusion facilities. A marked tendency was observed of fibers to orientate in the axial direction in piston extrusion and in the helix pattern of the auger driving mechanism, as opposed to random orientation in cast specimens. Cracks develop perpendicular to the direction of the tensile stress. Like steel reinforcement, fibers are optimal if they are imbedded in the matrix in the direction of the tensile stress and bridge the crack at a perpendicular angle. The mechanical properties of the fiber composites are therefore enhanced in the direction of the fiber alignment. The ductility should therefore be optimized by means of favorable fiber orientation in order to enhance the pseudo strain-hardening phenomenon. An anomaly exists here in terms of the so-called snubbing factor, which describes the increased fiber pull-out resistance when a fiber bridges a crack at an angle. However, it is postulated that by directing fibers to bridge cracks perpendicularly, the lower resistance to fiber pull-out avoids fiber breakage, enabling tough post initial cracking response accompanied by fiber pull-out. 3.3 Influence of Aggregate on Fiber Orientation and Distribution The size of the aggregate particles has a significant influence on the distribution of the fibers and the fiber orientation, Fig. 5. The fibers in ECC mortar mixes are only separated by fine aggregate particles which are allowed to move freely between the fibers. In conventional FRC, all aggregate particles which are bigger than the average distance between fibers will cause the fibers to become concentrated in balls (fiber balls) and give rise to irregular distribution of the fibers. This effect of fiber clumping will increase in proportion to particle size and has a negative influence on the properties of the concrete. The Figure cultivates an insight into the micro-mechanical phenomenon of improved ductile behavior for increased amounts of fly-ash and fine graded aggregate in ECC. This phenomenon is illustrated in the paper by Song and Van Zijl (2004) [3], where large proportions of fly ash was intended and treated as well rounded fine aggregate. Coarse aggregates are not used, as they tend to adversely affect the unique ductile behavior of the composite. 1306

7 (a) (b) (c) Fig. 5: The influence of particle size on fiber distribution, fiber orientation and workability. 4. Interfacial Bond Through extrusion the fiber packing and the matrix is densified whereby the interfacial bond between the fibers and the matrix is strengthened. It has been argued in the previous section that the performance and properties of fiber cement composites depend on the fiber orientation, which in turn is governed by the method of processing. In this section, the governing role of the fabrication process on the characteristics of the fibermatrix interface is discussed. 4.1 Review of earlier studies The extent to which the fibers may contribute to the strength of concrete depends on the fiber aspect ratio (L/d), the volume fraction of fibers (V f ) and the fiber bond factor or frictional shear resistance, enhanced by the snubbing factor g as follows [6]: u = ½ g L/d V f (2) Analogous to eq. (2) the fiber factor F has been formulated by Narayanan and Darwish (1990) [7] as F = L/d V f (3) with a bond factor accounting for fiber sectional shape, geometrical deformations such as crimped or hooked fibers, or indentations. With the fiber factor (F) it is easy to access the fiber properties contribution to the ECC strength, and predict what influence any adjustments to fiber properties might have on the extrudate. 1307

8 4.2 Fiber Bond Factor ( ) Many of the earlier methods of measuring the bond developed between fiber and cement-based matrices were generally based on simple pull-out tests in which one or both sides of the wire were embedded in the matrix and the wire was subjected to direct tension by restraining matrix blocks in compression. Mechanical anchorage of fibers by indentation, crimping or hooding would obviously enhance the bond significantly. Regression analysis was derived from these anchorage tests and conservative bond factor values assigned to different shaped fibers, as: =0.5 for round fibers, =0.75 for hooked fibers and =1.0 for indented fibers. 4.3 Aspect Ratio (L/d) Contradictory behavior in different fiber geometries for various manufacturing techniques could be explained by the differences in bond strength and matrix properties in different systems, and altering the mode of failure of the fibers until the mechanical performance of the extruded composite is optimized. Extruded composites show more benefit from a lower fiber aspect ratio than what cast composites does. This is due to matrix densification and accompanied increased fiber factor F, causing the tendency of fiber breakage for relatively longer fibers, where longer shear transfer is possible. 4.4 Tensile Strength of Fibers A requirement to exploit the full interfacial bond potential is that the fiber tensile strength does not limit this resistance. Therefore, it should correspond to whatever ultimate capacity is required from the manufactured composite. This required proportionality between fiber and matrix strength stems from the optimization of the primary mode of failure. Ideally, the probability of fibers pulling out and that of fiber fracture should be equal. Hence, strong matrix properties for high-strength applications require high-strength fibers to withstand the load applied. Hence, higher matrix strength through lower porosity (in extruded ECC), requires a higher strength fiber. 4.5 Effect of Fiber Length Whereas increasing fiber length improves the mechanical behavior of cast composites, it reduces the behavior of extruded composites. Optimization of interfacial bond strength could only be achieved when the fiber length is such that it provides enough resistance to fiber pull-out, yet have a high enough fiber modulus of rupture to avoid fiber fracture. = fiber tension f = fiber tensile strength L = fiber length (a) max < f (b) max = f (c) max > f L < L crit L = L crit L > L crit Fig. 6: Definition of critical fiber length (L crit ) - Maidl (1996) [8]. 1308

9 Failure mode is analogues to the fiber length. If the fiber length used in the mix design is less than the critical fiber length, fiber pull-out would occur. When the fiber length is longer than the critical fiber length, then fiber fracture is the primary mode of failure. 5. Conclusions At the onset of a comprehensive research program to characterize the influence on ECC mechanical behavior when subjected to extrusion, this paper has reported the potential of beneficial increased alignment of fibers, accompanied by matrix fiber interface strengthening through densification by the high pressure formation process. Thereby, fiber mode of failure may be altered to cause rupture, in turn causing strong, but brittle composite tensile response. The inability of such brittle tensile response to exploit the post first cracking plateau, or even strain hardening for stress redistribution to its full potential in loading cases causing stress gradients, was illustrated by simplified analysis. Subsequently, a mixture of experience gained through preliminary experimentation on crude extrusion facilities of both piston and auger action types, as well as research results from the literature was presented towards identifying the major mechanisms at play in extrusion processing of ECC. 1. Fiber breakage generates higher composite peak strength, whereas fiber pull-out leads to increased ductility. 2. The type of manufacturing process of ECC has a significant influence on the fiber orientation. Orientation of fibers is mainly in the load bearing direction of the process driving the mechanism. While piston extrusion aligns the fibers in the direction of extrusion, auger extrusion aligns the fibers diagonally. This leads to enhanced mechanical properties in the direction of main fiber orientation. 3. Interfacial bonding between fibers and the matrix is coupled with the primary mode of failure. Depending on the interfacial bond, an optimal fiber length must be chosen. Since extrusion produces high compaction of ECC, reducing the porosity, the optimal fiber length for extruded composites is shorter than that for cast composites, in terms of improved mechanical performance. 4. Extrusion is sensitive to different fiber lengths for the same fiber volume ratio. The optimal fiber length not only relates to the die opening geometry, but is also bound by the preferred mode of failure. 5. Extrusion provides an increase in shear resistance between the fibers and the matrix. If no fly ash is used in the mix, the shearing resistance of the fiber-matrix interface is increased to such an extent that fiber breakage may become the dominating failure mechanism. If fly ash is introduced in the extrusion mix, the lower matrix strength counteracts this mechanism, hence allowing fiber pull-out as the primary mode of failure, enabling favorable composite strain-hardening behavior. 1309

10 Acknowledgements The authors acknowledge the support of the South African Cement and Concrete Institute, Infraset Infrastructure Products, as well as the Technology and Human Resources in Industry Programme of the South African Ministry of Trade and Industry. We also express our gratitude for fiber material sponsored by Kuraray Co. Ltd. References 1. Lee, C.D. and Soroushian, P., Distribution and Orientation of Fibers in Steel Fiber Reinforced Concrete, ACI Materials Journal, 87(5), pp Li, V.C., Wang, S. and Wu, C., Tensile Strain-Hardening Behavior of Polyvinyl Alcohol Engineered Cement-based Composite (PVA-ECC), ACI Materials Journal, 98(6), pp Gao Song and Van Zijl, Tailoring ECC for Commercial Application, Submitted for publication in 6 th Rilem Simposium on Fiber Reinforced Concrete BEFIB (2004). 4. Peled, A. and Shah, S.P., Processing Effects in Cementitious Composites: Extrusion and Casting, ASCE Journal of Materials in Civil Engineering, 15(2), pp Li, V.C., Takashima, H., Miyagai, K. and Hashida, T., A design approach for the mechanical properties of polypropylene discontinuous fiber reinforced cement-based composites by extrusion molding, Engineering Fracture Mechanics 70, pp Bentur, A. and Mindess, S., Fibre Reinforced Cementitious Composites, Elsevier Applied Science. 7. Narayanan, R. and Darwish, I.Y., Design chart for reinforced and prestressed fiber concrete elements, The Structural Engineer, 68(2), pp Maidl, B.R., Steel Fibre Reinforced Concrete, Ernst & Sohn, VCH publishing. 1310

DUCTILE ENGINEERED CEMENTITIOUS COMPOSITE ELEMENTS FOR SEISMIC STRUCTURAL APPLICATION

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