Study on mechanical properties of cost-effective polyvinyl alcohol engineered cementitious composites (PVA-ECC)

Construction and Building Materials 78 (2015) 397–404

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Study on mechanical properties of cost-effective polyvinyl alcohol engineered cementitious composites (PVA-ECC) Zuanfeng Pan a,⇑, Chang Wu b, Jianzhong Liu c, Wei Wang b, Jiwei Liu b a

College of Civil Engineering at Tongji University, Shanghai 200092, China School of Civil Engineering at Southeast University, Nanjing 210096, China c State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Research Institute of Building Science Co., Ltd., Nanjing 210008, China b

g r a p h i c a l a b s t r a c t

 Feasibility of use of unoiled PVA fibers

6

5

5

4

σ (MPa)

and hybrid PVA fibers in ECC were studied.  Four-point bending test, uniaxial tensile and compressive tests were carried out.  Three typical mixes of PVA-ECC with different cost and performance were proposed.

Oile d P VA fibe r

3 2 1

Article history: Received 12 May 2014 Received in revised form 26 October 2014 Accepted 27 December 2014 Available online 17 January 2015 Keywords: Engineered cementitious composites (ECC) Polyvinyl alcohol (PVA) fiber Mix proportion Pseudo strain hardening Tensile strain Compressive strength

0 .02

0.04

0.0 6 0.08 δ (m m)

0.10

0 .1 2

0.14

0 -0.5 0

M21-1 M21-2 M21-3

M17-1 M17-2 M17-3 1

2

3

Tensile strain (%)

4

5

6

Uniaxial tensile stress-strain response of PVA-ECC

Engineered cementitious composites (ECC) which shows prominent tensile ductility and toughness, and fine multiple cracking, meets the high requirements of safety and durability in developing sustainable infrastructures. Currently, the cost of oiled polyvinyl alcohol (PVA) fiber widely used in ECC is very high. The price of regular unoiled PVA fiber is relatively lower, however, the tensile ductility of unoiled PVAECC may be limited. Based upon the micromechanics model, the feasibility of use of unoiled PVA fibers and hybrid PVA fibers in ECC were studied, and the mix proportion was redesigned through parametric analysis. The four-point bending test, uniaxial tensile test and uniaxial compressive test were carried out to characterize the mechanical behavior of ECC with 21 mix proportions. According to the cost and performance of PVA-ECC, three typical mixes were proposed: M7 with low cost, relatively low tensile ductility and reinforced by unoiled PVA fibers, M17 with moderate cost, relatively high tensile ductility and reinforced by hybrid PVA fibers and M21 with high cost, high tensile ductility and reinforced by oiled PVA fibers. In practical applications, the determination of mix depends on the structural performance requirements. Ó 2015 Elsevier Ltd. All rights reserved.

As one of a family of high performance fiber reinforced cement composite (HPFRCC) [1], engineered cementitious composites (ECC) developed by Li et al. [2–4] based on the basic principle of micromechanics and fracture mechanics is a high toughness com-

http://dx.doi.org/10.1016/j.conbuildmat.2014.12.071 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

2

a b s t r a c t

1. Introduction

⇑ Corresponding author.

3

1

σ(δ ) curves for different type of PVA-ECC

i n f o

4

Unoiled PV A f iber

0 0.00

a r t i c l e

Stress (MPa)

h i g h l i g h t s

posite material with pseudo strain hardening and multiple cracking properties. The ECC consists of cement, mineral admixture, fine aggregates (maximum grain size is usually 0.15 mm), water, admixtures which are used to enhance the strength and workability, less than 2% volume of short fibers. ECC exhibits multiple cracks formed uniformly over the length of the specimen, and the opening of each crack is usually controlled to be less than 100 lm, subsequently, the ultimate tensile strain can reach over 2.0%. In 2006, the RILEM TC HFC technical committee decided to

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Z. Pan et al. / Construction and Building Materials 78 (2015) 397–404

emphasize the unique tensile strain-hardening response of this material, and gave the more descriptive name strain hardening cementitious composites (SHCC) to this class of materials [5,6]. Ultra high performance fiber reinforced cementitious composites (UHPFRCC) [7], whose name comes from the excellent performance under tensile loading conditions, was developed in Japan. This class of materials has been made in China in recent years, named as ultra high toughness cementitious composites (UHTCC) [8,9] respect to the excellent toughness of the material. Li et al. [4] initially made up ECC with high-modulus polyethylene (PE) fiber (PE-ECC), then, Kanda and Li [10] used the environmentally and friendly polyvinyl alcohol (PVA) fibers with excellent alkali resistance to produce ECC (PVA-ECC). The cost of PVA fiber is approximately 1/8 that of high-modulus PE fiber, and its tensile strength and elastic modulus are higher than that of polypropylene (PP) fiber [2]. Unoiled PVA fiber may be ruptured in a cementitious matrix due to the strong chemical bonding to cement hydrates; therefore, the interface is engineered by applying oil coating to the surface of fiber to decrease the bond. The oiling content is 1.2% by fiber weight. Currently, oiled PVA fiber is widely used in ECC [11–13]. At present, the oiled PVA fiber is mainly produced by Kuraray Co. Ltd., Japan, and its cost is very high. As a result, it is very difficult to put ECC into large-scale practical engineering. It is worth studying on the cost-effective PVA-ECC. In China, the tensile strength and elastic modulus of regular unoiled PVA fiber are close to the oiled PVA fiber, and its cost is relatively lower, about 1/8 that of oiled PVA fiber. The reason for unoiled PVA-ECC not exhibiting good pseudo strain-hardening performance is the strong chemical bonding to cement hydrates. Due to the difference of physical properties and interface bonding properties between regular unoiled PVA fiber and the oiled PVA fiber produced by Kuraray, the proportion of mixture of unoiled PVA-ECC needs to be redesigned. According to the design theory of ECC material, the feasibility of unoiled PVA fibers and hybrid PVA fibers were discussed at first, and the optimization scheme of the mix proportion of cost-effective PVA-ECC was presented through parametric analysis. Totally 21 groups of specimens were designed to investigate the influences of fly ash replacement, water–binder ratio, sand–binder ratio and fiber volume fraction on the tensile strain capacity and compressive strength, through four-point bending test, uniaxial tensile test and uniaxial compressive test. According to the performance and cost of ECC material, three typical mixes were proposed. The development of cost-effective PVA-ECC helps cut the cost of ECC, and the widespread use of ECC becomes possible.

tensile capacity due to the fiber being ruptured or pulled out. This criterion expression is as follows:

rfc 6 r0

ð1Þ

(2) Steady-state cracking criterion Based on a J-integral analysis of a steady state crack, Marshall and Cox [17] concluded that the presence of steady-state cracking was reflected by the energy balance provided by the following expression.

J tip ¼ rss dss 

Z

dss

rðdÞdd

ð2Þ

0

and

rss 6 r0

ð3Þ

where Jtip is the crack tip toughness, which can be approximated as the cementitious matrix toughness if fiber volume fraction is less than 5%; rss is the steady state cracking stress; dss is the crack opening corresponding to rss. Eqs. (2) and (3) implies the steady-state cracking criterion, which is the crack tip toughness Jtip should be less than the complementary energy J0b calculated from the r–d curve, as schematically illustrated in Fig. 1. The requirement is the area of the shaded part Jtip is less than that of hatched part J0b .

J tip 6 r0 d0 

Z

d0

0

rðdÞdd ¼ J0b

ð4Þ

If the absorbed energy in the rising phase of r–d curve is insufficient or the crack tip toughness is too high, the steady-state cracking criterion is hard to be satisfied. Lin et al. [16] modified the previous fiber-bridging model, and the modified model could reflect the effects of the chemical bonding of the fiber/matrix interface and fiber breakage on the relationship between fiber bridging stress and crack opening width. So this model can predict the bridging behavior of strong chemical bonding fiber such as PVA fiber. According to the conditions for pseudo strain hardening, both the values of r0/rfc and J0b /Jtip must be greater than 1.0. Considering the discreteness of properties of fiber and matrix, Kanda and Li [18] proposed the performance indicators of pseudo strain hardening, to assure that ECC can steadily exhibit strain hardening and multiple cracking. For PVA-ECC, the requirement of J0b /Jtip > 3 and r0/rfc > 1.45 should be meet. Actually, the energy criterion is difficult to be satisfied if unoiled PVA fiber is used, however, the value of J0b /Jtip will increase with the increment of water cement ratio or fly ash replacement which reduces Jtip. 2.2. Feasibility of unoiled PVA-ECC

2. Feasibility analysis of unoiled PVA-ECC

One of the main keys to produce ECC is the physical and chemical properties of reinforcing fiber. The comparison of mechanical

2.1. Design guidelines of ECC composition The design theory of ECC composition is constructed on the basis of micromechanics and fracture mechanics. Li et al. [2,14] proposed the fiber-bridging model for short random fiber reinforced cementitious composites, and accordingly, Li [15] put forward pseudo strain-hardening model of this kind of material under uniaxial tension which can exhibit strain hardening and multiple cracking. This micromechanical model was updated by Lin et al. [16] recently. The conditions for pseudo strain-hardening model used as design guidelines for ECC material tailoring are categorized in first cracking stress and steady state cracking criterions as following, (1) First cracking stress criterion The condition requires that the first cracking strength rfc must be less than maximum fiber-bridging capacity on each potential crack plane r0 otherwise; the initially cracking section will lose

σ

σ0 σ ss

J b' J tip

δ ss

δ0

δ

Fig. 1. Typical relationship between fiber bridging stress and crack opening width.

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Z. Pan et al. / Construction and Building Materials 78 (2015) 397–404 Table 1 Comparison of parameters of mechanical properties of PVA fiber. Type

Diameter (lm)

Length (mm)

Elongation (%)

Density (g/cm3)

Elastic modulus (MPa)

Nominal strength (MPa)

Unoiled REC-15

26 39

12 12

7 7

1.3 1.3

36.3 42.8

1560 1620

Table 2 Parameters of fiber bridging model. Type

Vf (%)

Frictional bond (MPa)

Chemical bond (J/m2)

Apparent strength (MPa)

Unoiled REC-15

2.0 2.0

2.0 1.0

4.5 1.5

1052 1092

properties of typical unoiled PVA fiber [19] and REC-15 oiled PVA fiber [5] is shown in Table 1. The unoiled PVA fiber is produced by the Jiangsu Subote New Materials Co., Ltd., while the REC-15 fiber produced by Kuraray Co., Ltd., is surface-coated by oil (1.2% by fiber weight). The hydrophilic characteristic of PVA fiber may lead to great chemical and frictional bonding effect, thereby fiber being easily broken when ECC member is subjected to tension. However, the water–cement ratio and cement/fly ash ratio can be adjusted to improve the bonding effect. The performance indicators of pseudo strain hardening of two types of PVA-ECC need to be quantitatively evaluated. The parameters used in the model is shown in Table 2, and the bonding strength of fiber/matrix is obtained from current typical composition of ECC [20]. Based on the fiber-bridging model developed by Lin et al. [16], the relationship between fiber-bridging stress and crack opening displacement of the two types of PVA-ECC is shown in Fig. 2. Fig. 2 indicates that the peak stress of unoiled PVA fiber is more than that of oiled fiber, due to the great frictional and chemical bond, however, the corresponding crack opening is much smaller, and therefore the complementary energy is smaller. By the integral operation, the complementary energy J0b of unoiled PVA fiber and oiled PVA fiber are 3.3 and 17.6 J/m2, respectively. Jtip of matrix generally varies from 2 to 6 J/m2. Here, it is assumed that Jtip equals to 3 J/m2 due to the high water–cement ratio [20,21]. Then the performance indicators of pseudo strain hardening J0b /Jtip for the two types of fibers are 1.1 and 5.9, respectively. That is why oiled PVA-ECC can usually exhibit the phenomenon of steady-state cracking propagation and pseudo strain hardening. While unoiled PVA-ECC may behave with unsteady-state multiple cracking, even

without multiple cracking, due to the discreteness of properties of fiber and matrix. To meet the requirement of performance indicator of pseudo strain hardening, the proportion of mixture of ECC needs to be optimized. Jtip can be reduced by increasing water–binder ratio or replacing a larger portion of cement by fly ash. However, it should be noted that low Jtip will lead to a low first crack strength, which is undesirable for the serviceability of structure. In addition, the frictional and chemical bond can be reduced as the water–binder ratio increases, and thereby J0b increases. Increase of fly ash replacement reduces the chemical bond of fiber/matrix, and J0b can increase to some extent. Increase of fiber volume fraction raises the peak bridging stress and J0b , thereby, it can promote ECC to behave with pseudo strain hardening. However, due to the smaller diameter of unoiled PVA fiber, if Vf P 1.5%, it is hard to mix during the experimental observation. Vf is the fiber volume fraction. Therefore, if only unoiled PVA fiber is used, Vf should be less than 1.5%. While unoiled and oiled PVA fibers are both used to produce hybrid PVA-ECC, Vf can be appropriately increased. 2.3. Feasibility of hybrid PVA-ECC According to the findings by Li et al. [20,21], Gd = 1.5 J/m2 and

s0 = 1.0 MPa for oiled PVA fiber in ECC with high volumes of fly ash, while Gd = 2.5 J/m2 and s0 = 1.5 MPa for unoiled PVA fiber. Gd and s0 are the interface chemical bond and frictional bond, respectively. It is assumed that the total fiber volume fraction equals to 1.6%, and the relationship between the fiber-bridging stress and crack opening displacement of the hybrid PVA-ECC which is calculated by the modified pseudo strain-hardening model by Lin et al. [16] is shown in Fig. 3. The effects of different fiber volume fraction (Vfu + Vfo = 1.6%) on fiber bridging peak stress and complementary energy are shown in Figs. 4 and 5, respectively. Vfu and Vfo are the volume fractions of unoiled and oiled PVA fibers, respectively. Fig. 3 indicates that, with the increase of unoiled PVA fiber, the r(d) curve becomes steeper, the peak bridging stress increases gradually, while the corresponding opening width became smaller.

4 5

3 Vfu, Vfo Oile d P VA fibe r

3

σb (MPa)

σ (MPa)

4

2

0 0.00

1

Unoiled PV A f iber

1

0 .02

0.04

0.0 6 0.08 δ (m m)

0.10

0, 1.6% 0.2%, 1.4% 0.4%, 1.2% 0.6%, 1.0% 0.8%, 0.8% 1.0%, 0.6% 1.2%, 0.4% 1.4%, 0.2% 1.6%, 0

2

0 .1 2

Fig. 2. r(d) curves for different type of PVA-ECC.

0.14

0 0.00

0.03

0.06 0.09 δ (mm)

Fig. 3. Effect of Vfu on r(d) curve.

0.12

0.15

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Z. Pan et al. / Construction and Building Materials 78 (2015) 397–404

4.0

3. Experimental program 3.1. Mix proportion of cost-effective PVA-ECC

σ0 (MPa)

3.6 3.2 2.8 2.4 2.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Totally 21 groups of PVA-ECC with different mix proportions were designed to investigate the effects of water–cement ratio, fly ash replacement, sand–binder ratio, fiber volume fraction etc. on the mechanical properties, as shown in Table 4. The mortar matrix used in this study consisted of Portland cement (P.II.42.5), fine sand (silica sand, average size of 110 lm, and maximum size of 300 lm), and where indicated, fly ash (first-grade, supplied by Nanjing Thermal Power Plant). The admixture is a polycarboxylic-type, high-performance water reducer, produced by Jiangsu Subote New Materials Co., Ltd. The dimensional information and mechanical properties of unoiled and oiled PVA fibers are listed in Table 1. Cement, silica sand and fly ash, were first mixed at low speed for approximately one minute and a half. Water and water-reducing admixture were then added into the dry mixture and mixed for another 3 min. Once a consistent mixture was reached, PVA fibers were slowly added into the mortar mixture until all fibers were uniformly distributed in the cement paste. The whole mixing procedure typically took about 10 min.

Vfu (%) 3.2. Four-point bending test Fig. 4. Effect of Vfu on fiber-bridging peak stress.

3.2.1. General information of test All beam specimens were cast with the same size of 350  50  15 mm (length  width  depth). The specimens were demoulded for 1 day to secure a hardened state, and then stored in a moist room where the ambient temperature was 25 °C and the relative humidity was approximately 95% for 28 days. There were

16

Table 4 Matrix mix proportion of PVA-ECC (weight ratios).

2

J b' (J/m )

12

8

4 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Vfu (%) Fig. 5. Effect of Vfu on complementary energy.

Table 3 Performance indicators of pseudo strain hardening of unoiled and oiled PVA fibers. Case

1 2 3 4 5

Vf (%) Unoiled

Oiled

– 2.0 – 0.6% –

2.0 – 1.6 1.0 1.0

r0 (MPa)

J0b (m2)

J0b /Jtip

r0/rfc

3.87 4.74 3.11 3.20 1.94

17.60 3.30 14.22 8.48 8.88

5.90 1.10 4.74 2.83 2.96

1.29 1.58 1.04 1.07 0.65

This is mainly due to the strong bond between unoiled PVA fibers and the matrix. If Vfu 6 0.8%, J0b of composites increases as Vfu decreases, and the shape of r(d) curve is mainly controlled by the content of oiled PVA fiber. However, if Vfu > 0.8%, J0b stays relatively low, and the shape of r(d) curve are mainly controlled by the content of unoiled PVA fiber. The performance indicators of pseudo strain hardening for different types and volume fractions of PVA fibers are shown in Table 3. If only 1.0% oiled PVA fibers are mixed, although the requirement of the energy criterion can be met, but the requirement of the strength criterion cannot be met. The requirements of pseudo strain hardening can be basically met if 1.0% oiled PVA fibers and 0.6% unoiled PVA fibers are used.

Composition Cement Fly Sand– ash binder ratio

Water– binder ratio

Waterreducing admixture

Vf (%)

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20 M21

0.30 0.30 0.30 0.30 0.30 0.28 0.32 0.28 0.32 0.28 0.32 0.30 0.30 0.30 0.30 0.30 0.28 0.26 0.28 0.28 0.28

0.0054 0.0052 0.0050 0.0048 0.0047 0.0083 0.0045 0.0080 0.0042 0.0078 0.0040 0.0050 0.0050 0.0020 0.0050 0.0054 0.0029 0.0032 0.0020 0.0025 0.0028

1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 0.0 1.2 1.4 0.6 0.6 – – –

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.2 1.8 2.4 3.0 3.6 1.8 1.8 2.4 2.4 3.0 3.0 2.4 2.4 1.8 1.8 1.8 2.4 2.4 2.4 2.4 2.4

0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.30 0.42 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36

Unoiled Oiled

Note: Binder comprises cement and fly ash; Vf is fiber volume fraction.

100

100

100

Fig. 6. Test setup of four-point bending.

– – – – – – – – – – – – – – – – 1.0 1.0 1.0 1.6 2.0

Z. Pan et al. / Construction and Building Materials 78 (2015) 397–404 Table 5 Comparison of flexural behavior of specimens. Composition

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20 M21

Vf (%) Unoiled

Oiled

1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 – 1.2 1.4 0.6 0.6 – – –

– – – – – – – – – – – – – – – – 1.0 1.0 1.0 1.6 2.0

Ultimate deflection (mm)

Flexural strength (MPa)

Tensile strain capacity (%)

3.11 4.51 4.77 6.62 5.65 1.62 4.71 2.66 4.85 2.97 3.97 6.06 1.54 0.30 2.71 4.16 22.22 18.81 14.68 25.92 40.03

5.52 5.18 5.02 4.74 4.71 5.77 5.06 5.64 4.73 5.24 4.69 5.41 6.30 6.03 5.18 6.02 8.56 9.26 6.92 10.24 11.96

0.33 0.48 0.51 0.71 0.60 0.17 0.50 0.28 0.52 0.32 0.42 0.65 0.16 0.02 0.29 0.44 2.37 2.01 1.57 2.76 4.25

three specimens for each group. The experimental parameters included water– cement ratio, the amount of fly ash, sand–binder ratio, type of fiber and fiber volume fraction. The mix proportion of each specimen is shown in Table 4. Each specimen was loaded with four-point bending in a hydraulic servo loading system, and the test setup is shown in Fig. 6. The specimens were loaded by a displacement control with loading rate of 0.5 mm/min in order to obtain the softening behavior of the member. A linear variable differential transformer (LVDT) was placed at the center of the beam to measure the displacement at the midspan. 3.2.2. Test results 3.2.2.1. Failure characteristics and load–displacement curves. At the beginning of loading, all specimens were in the elastic stage. With increasing external loading, a single small crack generally tended to open at the location with flaw in the

(a)

(c)

401

constant moment region. After that, the flexural behavior of beams can be divided into four types: brittle fracture, quasi-brittle fracture, unsteady-state multiple cracking crack and steady-state multiple cracking. The measured flexural strength and ultimate deflections of all specimens are shown in Table 5. The phenomenon of typical multiple cracking of specimens under bending, and typical load–displacement curves for specimens are shown in Figs. 7 and 8, respectively. For the composition without PVA fibers, the first crack occurred at the midspan and propagated quickly to the top, and then the specimens fractured abruptly. For specimens with high sand–binder ratio and low water–binder ratio, take Specimen M13 for example, the multiple cracking was not observed, however, the bearing capacity was not lost after cracking. After arriving at the peak load, the load decreased gradually with the displacement increased due to the bridging action of PVA fibers. The overall flexural behavior showed quasi-brittle fracture failure mode. For specimens with high fiber volume fraction, high water–binder ratio or large fly ash replacement, such as specimens M4 and M7, after initial cracking, the load decreased a little with the increment of displacement. Due to the bridging action of fibers at the interfaces of cracks, the slip hardening between the fibers and matrix occurred, and the external load increased with the deflection with a relatively low slope due to flexural stiffness degradation resulting from multiple cracking along the beam. When the ultimate bridging stress at the crack surface was reached, and the width of the major crack in the midspan increased considerably, indicating that tension softening occurred and then the load decreased with the increment of deflection. Although there were multiple cracks in such specimens, but the fibers ruptured prematurely due to the strong bond effect between the fibers and matrix, the multiple cracks were specious and unsaturated. For specimens with hybrid PVA fibers or oiled fibers, such as specimens M17 and M21, after cracking, the initial cracks continued to propagate towards the top surface of the beam without significant increase of crack width, accompanied by multiple tiny cracks parallel to the dominant crack occurring throughout the constant moment region. Meanwhile, the distribution of multiple tiny cracks also extended from the midspan to both supports of the beams during the loading process, showing distinct difference for specimens with unoiled PVA fibers. Slight strain hardening behavior and multiple cracking along the specimens after initial cracking can be observed. After that, the bridging action between the fibers and cementitious matrix cannot sustain the tensile stresses caused by the external moment, and the load was decreased gradually after the maximum load. In the descent branch of load–displacement curve, unstable opening of the dominant crack was observed. Finally, the ECC beam specimens failed by pulling out and fracture of fibers, and significant opening of the major crack in the midspan. The maximum crack width, xm of specimens M17, M19 and M21 were observed during the loading process. xm of specimen M17 corresponding to the peak flexural strength is about 140 lm, while xm of specimen M21 is 110 lm. xm of specimen M19 is approximately the same as that of specimen M17.

(b)

(d)

Fig. 7. Phenomenon of multiple cracking of specimens under bending.

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Z. Pan et al. / Construction and Building Materials 78 (2015) 397–404

(a)

(b) Fig. 8. Typical load–displacement curves of specimens.

(a)

(b)

Fig. 10. Phenomenon of multiple cracking of specimens under uniaxial tension.

Fig. 9. Loading device of uniaxial tensile test.

3.2.2.2. Analysis of influence factors. The test results of specimens M1 to M5 indicate that the flexural strength decreases by 14.7% as the fly ash replacement ratio is increased from 1.2 to 3.6, while the deflection at the midspan is increased, as shown in Table 5. The reason is that both the matrix toughness and interface bonding may be weakened as the fly ash replacement increases, leading to reduction of flexural strength. Specimens with more fly ash replacement tend to be multiply cracking; therefore their ultimate deflections can reach a higher level. From the test results of specimens M6, M2 and M7, and M8, M3 and M9 in Table 5, both matrix toughness and interface bonding reduce as the water–binder ratio increases. Except for the difference that both the fly ash replacement and water–binder ratio are relatively large, the ultimate deflection at the midspan may decrease from the comparison of specimens M4 and M11. This is because the fiber/matrix interface bonding strength is too low due to the high fly ash replacement and water–binder ratio, which leads to lowering the maximum bridging stress, subsequently the specimen gets into the softening stage. The test results of M12, M3 and M13 demonstrate that the ultimate deflection at the midspan decreases by 74.6% as the sand–binder ratio increases from 0.30 to 0.42. The increment of silica sand will increase the matrix toughness, which prevents the steady-state multiple cracking. The flexural strength of specimens are enhanced as the fiber volume fraction increases, resulting from increasing of bridging stress of the cross section. However, if there is no PVA fiber, the flexural strength is greater, which is contrary to the above analysis. The reason lies in the cementitious composites without fibers has good fluidity, low gas content, and high density, therefore the flexural strength is relatively high. Table 5 indicates that as the fiber volume fraction increases, the ultimate deflection at the midspan increases. Cai and Xu [22] and Qian and Li [23] proposed effective methods for predicing the ultimate tensile strain using the four-point bending test results. It is worth noting that the inverse method [23] is not recommended for use for tensile strain

capacity less than 1.0%. The calculated tensile strain capacity of each specimen by the inverse method [22] is shown in Table 5. For specimens M17 to M21, the predicted tensile strain capacity by the method [23] are 2.45%, 2.06%, 1.66%, 3.01% and 4.70%, respectively, which are close to the values by the method [22]. 3.3. Uniaxial tensile test The uniaxial tensile test serves two objectives: to search for an optimal and practical composition of PVA-ECC and to verify the derived tensile strain capacity through four-point bending test. Only compositions M17 and M21 were selected to be tested. The cross section in the middle part of each dumbbell-shaped specimen was 30  30 mm. These specimens were cured for 28 days in the same environment as the flexural specimens were in. There were six specimens for each group. The specimens were tested by a displacement control in a 30 kN capacity hydraulic servo loading system, and the loading rate was 0.2 mm/min. The 80 mm length in the middle part of each specimen was selected to be measured, and two LVDTs were placed at the front and back of each specimen, as shown in Fig. 9. The phenomenon of multiple cracking of specimens and the recorded stress– strain curves of compositions M17 and M21 are shown in Figs. 10 and 11, respectively. Both compositions M17 and M21 exhibited multiple cracking with many sub-parallel cracks cross the specimens during strain hardening. For composition M17, the strain hardening effect is more obvious at the beginning of cracking. Later, as the unoiled PVA fibers started to be fractured, the trend of strain hardening gradually disappeared. For the oiled PVA-ECC, the strain hardening effect is not so distinct at the beginning of cracking, and the stress–strain curve was approximately horizontal. As the displacement was increased, the slip hardening between the oiled PVA fibers and matrix worked, therefore, the characteristics of strain hardening became more apparent. xm of specimens M17 and M21 were observed. xm of specimen M17 corresponding to the peak tensile strength is about 110 lm, while xm of specimen M21 is approximately 90 lm.

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3.4. Uniaxial compressive test 3.4.1. Compressive strength Totally 13 groups of cubic specimens (70.7  70.7  70.7 mm) were made for studying the influences of fly ash replacement, water–binder ratio, sand–binder ratio, and fiber volume fraction on the compressive strength of the cementitious composites. These specimens were cured for 28 days in the same environment as the flexural specimens were in. There were three specimens for each group. All specimens were tested by a displacement control in a 3000 kN capacity hydraulic servo loading system, and the loading rate was 0.3 mm/min. The effects of fly ash replacement, water–binder ratio, sand–binder ratio, and fiber volume fraction on compressive strength is shown in Fig. 12. The influence of fiber volume fraction on compressive strength originates in two opposite aspects. The positive effect is that the compressive strength can be improved by the constraint of lateral expansion under loading which is contributed to the increased resistance of fiber bridging to microcrack sliding and extending. The negative effect is that the number of pores will increase and the density will get worse as the fiber volume fraction increases, which leads to strength degradation. Unoiled fibers may tend to ‘‘ball’’ in the mix if the fiber content increases. From Fig. 12(d), the combined effect of fibers results in decreasing compressive strength with increasing fiber content.

Fig. 11. Uniaxial tensile stress–strain response of PVA-ECC.

Table 6 Results of uniaxial tensile test. Composition

efc (%)

rfc (MPa)

etu (%)

rcu (MPa)

e0 tu1 (%)

e0 tu2 (%)

M17 M21

0.022 0.023

3.02 3.44

2.61 4.46

3.51 4.39

2.37 4.25

2.45 4.70

Note: e0 tu1 and e0 tu2 are the derived ultimate tensile strains by the inverse methods proposed by Cai and Xu [22] and Qian and Li [23], respectively.

3.4.2. Elastic modulus Two groups of prism specimens (100  100  300 mm) with the compositions M17 and M18 were made for obtaining the elastic modulus of hybrid PVA-ECC. There were three specimens for each group. All specimens were tested by a force control in a 3000 kN capacity hydraulic servo loading system, and the loading rate was 0.3 MPa/s. Two gauges were placed at both sides of each specimen, and the average of the two strain readings was regarded as the strain of each specimen. The elastic modulus of ECC, as in concrete, depends on the amount of aggregates. However, presence of more aggregates will considerably reduce the tensile strain capacity, as seen from Table 5. In addition, there is no coarse aggregate in ECC; therefore, the elastic modulus of ECC is usually lower than that of concrete. The mean values of measured elastic modulus for M17 and M18 are 17.6 and 18.8 GPa, respectively.

4. Cost effectiveness analysis of PVA-ECC The average value of measured first crack strength rfc, strain efc, peak stress rcu and ultimate tensile strain etu are tabulated in Table 6. From the comparison of etu by the tensile test and four-point bending test, the difference is very small, which proves that four-point bending test method is a simple and practical method to estimate the tensile properties of ECC.

(a)

(c)

The cost and tensile properties of different type of PVA-ECC are shown in Table 7. Composition M0 represents the plain concrete, and its cost per cubic meter is assumed to be 1.0 for easily

(b)

(d) Fig. 12. Effect of each parameter on compressive strength.

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Table 7 Costs and tensile properties of different type of PVA-ECC. Composition

M0 M7 M17 M19 M20 M21

Vf (%) Unoiled (%)

Oiled (%)

– 1.3 0.6 – – –

– – 1.0 1.0 1.6 2.0

Cost

Tensile strain capacity (%)

1.0 2.0 6.5 6.0 9.5 11.5

0.01 0.50 2.37 1.57 2.76 4.25

comparison. It’s worth noting that the cost of each composition is calculated from the prices of materials in the Chinese market. According to the costs and tensile and compressive properties of different type of PVA-ECC, three typical mixes were proposed: M7 with low cost, relatively low tensile ductility and reinforced by unoiled PVA fiber, M17 with moderate cost, relatively high tensile ductility and reinforced by hybrid PVA fiber and M21 with high cost, high tensile ductility and reinforced by oiled PVA fiber. The determination of mix depends on the structural performance requirements in practical applications. 5. Conclusions It is believed that ECC has a bright application prospect in civil engineering for its high ductility and high toughness in tension. At present, the use of ECC in China is limited by the extremely high cost of oiled PVA fibers. To reduce the production cost of ECC, the feasibility of unoiled PVA-ECC and hybrid PVA-ECC were studied, and the mix proportion was redesigned through parametric analysis based upon the micromechanics model. According to the performance indicators of strain-hardening model and test results of mechanical properties of cementitious composites, the following conclusions can be drawn: (1) The tensile strain capacity of cementitious composites with unoiled PVA fibers is significantly larger than that of plain concrete. However, this type of cementitious composites is difficult to exhibit steady-state multiple cracking. Four-point bending test results and uniaxial compressive test results indicate that the increase of water–cement ratio and fly ash replacement is in favor of strain-hardening and multiple cracking, but it also reduces the flexural strength and compressive strength of the cementitious composites. (2) To further increase the tensile ductility of ECC, unoiled PVA fibers were mixed with oiled ones at a proper proportion to develop hybrid PVA-ECC. Hybrid PVA-ECC may reduce the complementary energy of ECC as well as maintain the maximum bridging stress of cross-section, and can achieve the strain hardening and steady-state multiple cracking. (3) Reduction of sand–cement ratio can enhance the flexural ductility; however, it may decrease the compressive strength. In addition, there is no coarse aggregate in ECC, therefore the elastic modulus of ECC is usually lower than that of conventional concrete. (4) According to the performance and cost of ECC with different compositions, three typical mixes were proposed: M7 with low cost, relatively low tensile ductility and reinforced by unoiled PVA fibers, M17 with moderate cost, relatively high tensile ductility and reinforced by hybrid PVA fibers and M21 with high cost, high tensile ductility and reinforced by oiled PVA fibers. In practical applications, the determination of mix depends on the structural performance requirements. The development of hybrid ECC helps cut costs of ECC, and the widespread use of ECC becomes possible.

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