柚子简笔画POLYVINYL ALCOHOL FIBER REINFORCED ENGINEERED CEMENTITIOUS COMPOSITES: MATERIAL DESIGN AND PERFORMANCES
Shuxin Wang and Victor C. Li方特生命之光
Department of Civil and Environmental Engineering, University of Michigan, USA
Abstract
Polyvinyl alcohol (PVA) fiber is considered as one of the most suitable polymeric fibers to be ud as the reinforcement of engineered cementitious composites (ECC), though the unique microstructure characteristics of PVA fiber add challenge to the material design. In this paper, the micromechanics bad design procedure for a PVA-ECC suitable for structural applications is described, and practical design considerations including requirements on interface bond, matrix toughness, and flaw system are outlined for achieving balanced composite performances. The properties of an exemplary PVA-ECC design are summarized, including tensile behavior and compressive strength development, bending respon, Young’s modulus, autogenous and drying shrinkage, and freeze-thaw durability.
1. INTRODUCTION
Engineered Cementitious Composites (ECC) is a unique reprentative of the new generation of high performance fiber reinforced cementitious composites, featuring high ductility and medium fiber content. Material engineering of ECC is constructed on the paradigm of the relationships between material microstructures, processing, material properties, and performance, where micromechanics is highlighted as the unifying link between composite mechanical performance and material microstructure properties [1]. The established micromechanics models guide the tailoring of composite constituents including fiber, matrix and interface for overall performance, and elevate the material design from trial-and-error empirical testing to systematic holistic "engineered" combination of individual constituents. The microstructure to composite performance linkage can be further extended to the structural performance level and integrate the material design into performance bad design concept for structures [2]. In that n, ECC embodies a material design approach in addition to being an advanced material and provides an additional degree of freedom in structural performance.
Polyvinyl alcohol (PVA) fiber emerged during a arch of low cost high performance fibers for ECC. The hydrophilic nature of PVA fiber impod great challenge in the composite design, as the fibers are apt to rupture instead of being pulled out becau of the tendency for the fiber to bond strongly t
o cementitious matrix. Careful engineering in fiber geometry, fiber/matrix interface and matrix properties is of vital importance to achieve high ductility in PVA-ECC. To guide the tailoring process, micromechanical models accounting for the uniqueness of PVA-fiber were developed.
The objective of this paper is to provide a performance summary of an exemplary PVA-ECC. As large scale applications of ECC are emerging, the data collected here may rve as reference for structural engineers. To limit the paper length, the composite modeling and design considerations will be only briefly described. 2. COMPOSITE DESIGN GUIDELINE
The primary performance target in ECC design is tensile ductility. In addition, high strength and Young’s modulus, tight crack width, and high durability are also preferred in general. In most applications, tensile strain capacity 2% is considered sufficient. In terms of strength, it is desirable if ECC compressive strength is comparable with regular high strength concrete. As to crack width, previous study [3] suggested that permeability would be in the same order of sound concrete when the crack width is below 80 -100 µm.
One dominant variable governing strength is water to binder ratio w/cm . Low w/cm is also beneficial to maintain mixture consistency and facilitate fiber distribution. As long as ductility is not compromid, low w/cm is desired in mix design. In this exemplary mix, w/cm =0.24 is ud.
Tensile ductility may be gauged by the strain-hardening index J b ’/J tip , where J b ’ is the complementary energy of the fiber bridging stress vs. opening σ(δ) relation (as defined by Eqn. 1, where σ0 is the peak bridging stress and δ0 is the corresponding crack opening), and J tip is the matrix toughness. J b ’/J tip >1 is esntial to achieve strain-hardening, and higher J b ’/J tip leads to more saturated multiple cracking.
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(1)
Modeling of σ(δ) relation is one esntial piece of ECC micromechanics, since it not only connects the fiber, matrix and interface properties to J b ’ but also provides the estimation of maximum crack width before failure. Due to significant slip-hardening respon during pullout, PVA-fiber may be pulled out from both sides across the crack, in contrast to one-way pullout typically obrved in steel
and other polymeric fibers. In addition, matrix spalling at the fiber exits has to be accounted for accurate estimation of crack opening. A complete model of σ(δ) relation for PVA-ECC can be found in [4]. In connection to J b ’, a very strong interface bond leads to fiber rupture at small crack opening, resulting in small J b ’. On the other hand, a very weak interface may cau low bridging strength and large crack opening. Parametric study indicates that to satisfy the crack width performance target (below 80 µm) the optimal interface properties should be in the range of 1.5 - 2.5 MPa for frictional stress and below 1.5 N/m for interface fracture energy, given adequate fiber diameter and length for easy fiber dispersion. Unfortunately, the bond properties of PVA fiber without any treatment are far above the optimal values.
Two approaches were adopted to reduce the excessive interface bond. On the fiber aspect, surface coating by oil was investigated. With increa of oiling content, both frictional stress and interface fracture energy decrea significantly. Conquently, J b ’ increas with oiling content for a given matrix, as shown in Figure 1, where fiber volume fraction 2.0% was ud. On the matrix aspect, fly ash was introduced. High volume fraction of fly ash tends to reduce both the interface bond and matrix toughness. Figure 2 shows effect of ASTM Type F fly ash content on J b ’/J tip , where oiling content 1.2% was ud for fiber and the fiber volume fraction was 2.0%. Besides, fly ash improves m
ixture workability and material sustainability. However, high content of fly ash leads to slower strength development at early age. It was found that fly ash to cement at 1.2 provided best overall performances, and this proportion is ud in the following exemplary mix.
Equally as important as the interface bond tailoring is the matrix properties control. Matrix toughness below 12 N/m is preferred. Meanwhile, the matrix cracking strength must be limited not to exceed σ0. Matrix flaw size distribution control may become necessary to ensure saturated multiple cracking. The determination of optimal flaw size can be found in [4]. However, if the matrix tensile strength without macro defects can be controlled to be below the variation range of σ0, saturated multiple cracking will be guaranteed. For the optimal interface properties propod above and PVA fiber volume fraction of 2.0%, the preferred matrix tensile strength without macro defects is 5.0 MPa.
3. PVA-ECC PERFORMANCES
The mix proportions of the exemplary PVA-ECC (referred as M45) is given in Table 1. The volume fraction of fiber is 2%. ASTM Type I portland cement and low calcium ASTM class F fly ash were ud. Large aggregates were excluded in ECC mix design, and only fine sand was incorporated. The silica sand ud here had a maximum grain size of 250 µm and an average size of 110 µm. The PVA fiber had a diameter of 39 µm, a length of 12 mm, and overall Young’s modulus of 25.8 MPa. The apparent fiber strength when embedded in cementitious matrix was 900 MPa. The fiber surface was treated with oil coating to reduce interface bond and the oiling content is 1.2%.
Fly ash / cement
J b '/J t i p
Figure 2: Effect of fly ash content on J
b ’/J tip Oiling Agent Content (%)
C o m p l e m e n t a r y E n e r g y J b ' (N /m )
Figure 1: Effect of fiber surface oiling
content on J b ’
Table 1: Mix proportions of PV A-ECC (kg/m 3)
3.1 Tensile behavior
Tensile behavior was measured by direct uniaxial tension test. The coupon specimen measured 304.8 mm by 76.2 mm by 12.7 mm. Deformation was recorded with a gage length of 180 mm. Figure 3 shows the development of tensile strain capacity over age. The strain capacity at 24 hours after casting is about 2.3%. At early age, the strain capacity increas with time and reaches above 4% after 7 days, and later it decreas and then plateaus after about 30 days. The change in strain capacity reflects the evolutions of matrix toughness and interfacial bond properties. Figure 4 prent
s the typical tensile stress-strain curves after 24 hours and 90 days, where the tensile strength increas from 3.0 to 5.4 MPa.
Crack width in ECC prior to failure is limited by the crack opening corresponding to maximum bridging stress. Figure 5 shows the crack width development of PVA-ECC M45 at age of 28 days. The crack width remains under 60 µm until strain capacity is exhausted at 3.5%.
The robustness of tensile ductility may be enhanced by controlling pre-existing flaw size distribution in matrix [4], e.g. introduction of sufficient number of macro artificial flaws as crack initiators to ensure saturated multiple cracking. Figure 6 shows
Cement Sand Class F Fly ash Water Superplasticizer PVA Fiber
583 467 700 298 19 26
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24 hours T e n s i l e S t r e s s (M P a )
Strain (%)
Figure 4: Typical tensile stress-strain curves at 24 hours and 90 days
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Age (day)
Figure 3: Age dependency of tensile strain
capacity
C r a c k W i d t h (µm )T e n s i l e S t r e s s (M P a )
Figure 5: Crack width development
热搜是什么在哪里看a t of tensile stress-strain curves of PVA-ECC M45 containing 7% by volume of polypropylene beads with 4 mm diameter. The age at test is 90 days. Consistent tensile performance with strain capacity exceeding 3% is demonstrated. Figure 7 shows the saturated multiple cracking pattern with average spacing less than 2 mm.
3.2 Flexural behavior
Flexural respon was measured by four point bending test. The beam specimen measures 304.8 mm (length) by 76.2 mm (width) by 25.4 mm (depth), and the bending test span configuration is 101.6 mm by 76.2 mm by 101.6 mm. Figure 8 shows the typical flexural respons at 24 hours and 90 days. Significant deflection-hardening can be en and the corresponding flexural strength is 11 and 16 MPa respectively. Figure 9 shows the cracking pattern of the tensile face at the constant moment ction (90 days), and the average crack spacing is below 1.5 mm.
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Figure 9: Multiple cracking pattern under
bending 0
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0510152024 hours
90 days
Displacement (mm)B e n d i n g
S t r e s s (M P a )
Figure 8: Flexural behavior of PVA-ECC
Figure 7: Multiple cracking pattern of PVA-ECC under uniaxial tension
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e n s i l e S t r e s s (M P a )
Strain (%)
Figure 6: Robust tensile behavior of PVA-ECC with flaw size distribution control
3.3 Compressive strength
Compressive strength was measured using 75 mm (diameter) by 150 mm (height) cylinders. Figure 10 shows the strength development of PVA-ECC M45 up to 8 months. Rapid strength gain was en in the first 14 days and the 14 days strength is about 65 MPa. After that, the strength gain was much slower and at 8 months the strength is 75 MPa.
Similar to high strength concrete, PVA-ECC M45 exhibits nearly linear behavior under compression prior to failure. The Young’s modulus is 20.4 MPa as measured at 28 days, and the peak stress reaches at 0.43% strain.
3.4 Freeze-thaw resistance
The test of freeze-thaw durability followed ASTM C666 Procedure A. The specimens were expod to freeze-thaw cycles 14 days after casting. Figure 11 shows the dynamic modulus measured during 300 freeze-thaw cycles, and no deterioration was obrved. After 300 cycles, the tensile strain capacity was recorded as 2.8 ± 0.6%, only slightly decread from 3.0 ± 0.5% measured from specimens of the same age (14 weeks) not subjected to freeze-thaw condition. The compressive strength after 300 cycles was 60.7 ± 2.1 MPa, which was 22% lower than control specimens cured in room temperature.
3.5 Shrinkage behavior
Drying shrinkage was measured according to ASTM C157/C157M-99 and C596-01. The specimens were demolded after one day and stored in water for 2 days before they were moved to different relative humidity environments and the measurement was started. Free drying shrinkage deformation was monitored until hygral equilibrium was reached. Figure 12 shows the drying shrinkage of PVA-ECC M45 under various relative humidity. Due to the high binder content in ECC, the drying shrinkage of PVA-ECC M45 is about 80% higher than normal structural concrete.
Autogenous deformation is the bulk deformation of a clod, isothermal, cementitious material system not subjected to external forces [5]. Depending on material composition and hardening stage, it can be either expansion or shrinkage. The measurement of autogenous deformation of PVA-ECC M45 ud dilatometer following Jenn and Hann [6]. The mixture was cast under vibration into corrugated polyethylene tube with 25 mm diameter. The
D y n a m i c M o d u
l u s (G P a )Freeze-Thaw Cycles
Figure 11: Freeze-thaw durability
C o m p r e s s i v e S t r e n g t h (M P a )
Age (day)
Figure 10: Compressive strength
development
