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Review of potential structural applications of hybrid fiber Engineered Cementitious Composites
Construction and Building Materials 36 (2012) 216–227
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Review
Review of potential structural applications of hybrid fiber Engineered Cementitious Composites M. Maalej a, S.T. Quek b, S.F.U. Ahmed c,⇑, J. Zhang b, V.W.J. Lin b, K.S. Leong b a
Department of Civil & Environmental Engineering, College of Engineering, University of Sharjah, Sharjah, United Arab Emirates Department of Civil Engineering, National University of Singapore, Singapore c Department of Civil Engineering, Curtin University, Perth, Western Australia, Australia b
h i g h l i g h t s " Hybrid fiber ECC exhibited significant enhancement to the structural response. " Hybrid fiber ECC exhibited significant improvement in impact energy absorption. " Multiple cracking with fine crack widths led to reduced corrosion activities. " Multiple cracking delayed the debonding of the FRP reinforcing sheet. " Hybrid fiber ECC eliminated spalling of concrete cover due to high fracture energy.
a r t i c l e
i n f o
Article history: Received 20 February 2012 Received in revised form 17 April 2012 Accepted 24 April 2012 Available online 23 June 2012 Keywords: Engineered Cementitious Composites Hybrid fiber Corrosion Strengthening Masonry Impact
a b s t r a c t This paper reviews some of the recent research work focusing on assessing the performance of hybrid fiber Engineered Cementitious Composite (ECC) materials in a number of potential structural applications. A summary of the design and characteristics of such materials is presented followed by a review of recent potential applications of hybrid fiber ECC. The reviewed applications include the use of hybrid fiber ECC for designing impact and blast resistant protective panels, strengthening of unreinforced masonry (URM) walls, strengthening of RC beams, and enhancing corrosion durability of RC beams. The review demonstrates that hybrid fiber ECC can significantly enhance the performance of structures incorporating these materials. Ó 2012 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . The critical fiber volume fraction concept . . . . Blast/impact resistant panels. . . . . . . . . . . . . . . Strengthening of unreinforced masonry walls . Effective FRP-strengthening of RC beams . . . . . Corrosion-resistant RC beams . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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⇑ Corresponding author. E-mail address: [email protected] (S.F.U. Ahmed). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.04.010
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1. Introduction In recent years, it has been demonstrated that Engineered Cementitious Composites (ECCs) can be designed to exhibit pronounced tensile strain-hardening by adding to the cement-based matrix a relatively low volume (typically 62%) of short randomly-distributed fibers of specific type and property. The resulting ECC composites are characterized by their high tensile strain capacity, fracture energy and notch insensitivity, making them ideal materials for various structural applications. Under uniaxial tension, sequentially developed parallel cracks contribute to the inelastic strain at increasing stress level. The ultimate tensile strength and strain capacity can be as high as 5 MPa and 4%, respectively (see Fig. 1). The latter is two orders of magnitude higher than that of normal or ordinary fiber reinforced concrete. It has been suggested that, for a number of structural application, the use of more than one type of fiber as reinforcement results in a hybrid fiber composite that is better able to meet the material performance requirements then monofiber composite for the given application [1–5]. For instance, to use ECC in protective structures, the material is required to possess sufficient strength to resist penetration, while at the same time it is required to absorb a large amount of energy, thereby minimizing fragmentation and reducing the velocities of the fragments (i.e. need material of sufficiently high strain capacity). For using ECC in corrosion-resistant structures, a low crack width is required to reduce the ingress of aggressive substances reaching the steel reinforcement, while at the same time a high strain capacity is also required to prevent delamination and concrete cover spalling [6]. Further, given that monofiber ECCs containing high modulus fibers (e.g. steel or carbon fibers) normally exhibit high ultimate strength, low crack width and low
strain capacity [7], while those containing low modulus fibers (e.g. polyvinyl alcohol and polyethylene fibers) exhibit opposite behaviors [6,8], it becomes clear that a hybrid-fiber ECC with proper volume ratio of high and low modulus fibers can be designed to achieve an optimal balance between ultimate strength, crack width and strain capacity [9] (see Fig. 2), and is therefore better able to meet the functional requirement for these applications. In the sections to follow, the performance of hybrid fiber ECC in a number of structural applications is reviewed with the objective of assessing the potential of these materials in providing better functionality, in particular, in applications involving enhancing the impact- and blast-resistance of structures, improving the strength and ductility of FRP-strengthened beams and enhancing the corrosion durability of RC members. 2. The critical fiber volume fraction concept A micromechanical model for the design of hybrid fiber ECC was develop by Ahmed et al. [2] on the basis of fracture mechanics and deformation mechanism taking into account the effects of hybrid fibers. This micromechanical model for hybrid-fiber ECC is an extension of an earlier model proposed by Li and Leung [10] for monofiber composites. An important component of the micromechanical model is the composite bridging law (rcd) schematically depicted in Fig. 3. The rc–d law describes the constitutive relationship between the traction rc acting across a matrix crack plane and the separation distance d of the crack faces (i.e. COD: crack opening displacement) in a singly pre-cracked uniaxial tensile specimen loaded quasi-statically to complete failure. Micromechanical modelling of the bridging law in discontinuous randomly-distributed fiber reinforced cementitious composites has been carried out by Li [11] and Maalej et al.
5
σc
Tensile Stress (MPa)
σc 4
σcu
3
δ
2
σc
1 0 0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
δ
Strain
δ
L f /2
*
Fig. 3. Composite bridging law.
Fig. 1. Uniaxial tensile stress–strain curves of ECC from coupon specimen tests [1].
σ
For a given V f1 crit
min
Vf = Vf1+V f2
σcu σfc
min
Vf2
Fig. 2. Effect of hybrid reinforcement in the strain hardening behavior of ECC [9].
Multiple-cracking Vf2
Fig. 4. First-crack strength (rfc) and ultimate bridging strength (rcu) for different volume fractions of fiber 2; intersection of rfc and rcu yields the minimum volume fraction of fiber 2 required to achieve strain-hardening behavior for a given volume fraction of fiber 1.
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[12]. When no fiber rupture occurs in the composite, the rc-d relationship is given by [11]:
Vf1 =0.5% (ST fibre)
8 h qffiffiffiffi i > Pre-peak rc d curve for d 6 d < g ro 2 dd dd ¼ h i2 > : g ro 1 2d for d 6 d 6 Lf =2 Post-peak rc d curve L
Vf2 (%)
rc
3
f
2 PE1.5% (Vf > Vfcrit) PE0.8% (Vf ≈ Vfcrit )
1
PE0.2% (Vf < Vfcrit )
ð1a; bÞ where g is a factor which accounts for a fiber/matrix local frictional effect (called snubbing) due to the misalignment between the fiber axis and the normal crack plane, d⁄ = Lf(s/Ef)(Lf/df)/(1 + g) is the COD corresponding to the stage when debonding is completed for all fibers, and at which all fibers are pulling-out of the matrix and ro = Vfs(Lf/df)/2, in which s, df, Lf, Ef and Vf are the fiber/matrix interfacial bond strength, and the fiber diameter, length, modulus and volume fraction, respectively. No chemical bond between fibers and matrix is assumed in this model. For a hybrid fiber ECC system, the principle of superposition has been adopted to develop the composite bridging law (rc–d) and predict the first crack strength (rfc) and the ultimate bridging strength (rcu) of the hybrid fiber composite. The first crack strength (rfc) may be obtained from a consideration of the balance of stress intensity factors associated with the crack tip fracture toughness (Ktip), the applied tensile loading (KL) and the fiber bridging stress (KB) [10]:
K L þ K B ¼ K tip
0 0.01
0
0.03
0.02
Crack Size, C (m) Fig. 5. Design chart for hybrid fiber ECC. For a given crack size and volume fraction of fiber 1, the minimum volume fraction of fiber 2 can be obtained (note: C indicates the crack radius).
Table 2 Example mix proportions of hybrid fiber ECC [2]. Series
Vf < Vf Vf >
Volume fraction of fiber, Vf (%)
V crit f V crit f V crit f
Mix proportions (by weight)
Steel
PE
Cement
Silica fume
Water
Superplastizer
0.5
0.2
1
0.1
0.28
0.02
0.5
0.8
0.5
1.5
ð2Þ
The stress intensity factor due to fiber bridging (KB) in hybrid fiber ECC is calculated from the pre-peak (K 1B1 ) and the post-peak (K 2B1 ) contribution of fiber 1 and the pre-peak (KB2) contribution of fiber 2 as follows:
ð3Þ
where the subscripts 1 and 2 refer to fiber 1 and 2, respectively. The ultimate bridging strength (rcu) of the hybrid fiber ECC is given by the sum of the peak bridging stress of fiber 2 (g2ro2) and the post-peak bridging stress of fiber 1 at a COD equal to d2 :
rcu ¼ g 2 ro2 þ g 1 ro1 1
2d2 Lf 1
ST0.5+PE1.5 (Vf > Vf crit)
Vf1=0.5% (ST fibre)
5
Stress (MPa)
K B ¼ ðK 1B1 þ K 2B1 Þ þ K B2
6
4 ST0.5+PE0.8 (Vf ≈ Vfcrit )
3 ST0.5+PE0.2 (Vf < Vfcrit )
2 1
2
0
ð4Þ
0
1
2
3
4
5
6
Strain (%)
The model assumed that strain-hardening behavior could be achieved in a hybrid fiber composite when the maximum bridging strength imposed by the fiber/matrix interaction exceeds the first crack strength at which new matrix cracks can propagate (multiple cracking). For a given volume fraction of one type of fiber (Vf1), the above condition (rcu = rfc) illustrated in Fig. 4 led to identify the existence of a minimum volume fraction of the other type of fiber (V min f 2 ), and a minimum size of initial matrix crack (C) necessary for the composite to exhibit strain-hardening behavior. The parametric values for the fibers, matrix and fiber/matrix interface used in the model are given in Table 1. The fracture toughness, Km, of the cement paste matrix was determined according to the two-parameter fracture model [13]. The average value was measured to be about 0.37 MPa m1/2 and the matrix elastic modulus was 18.0 GPa [2]. It has been estimated from the model that the critical fiber volume fraction, V crit f , for strain-hardening behavior in a hybrid fiber composite containing steel (ST) and
Fig. 6. Tensile stress–strain response and multiple cracking of hybrid fiber ECCs for various fiber volume fractions.
polyethylene (PE) fibers is equal to 0.6% PE for 0.5% ST. For different crack sizes, C, different volume fractions of fiber 2 can be obtained for a given volume fraction of fiber 1. Fig. 5 illustrates a sample design chart for Vf1 equal to 0.5% ST where the crack size dependency was included. This type of design chart can be used to design hybrid fiber composites for strain-hardening and multiple-cracking behaviors. When using this kind of chart, however, one should be aware that excessive increase of fiber volume fraction may lead to interface bond deterioration and matrix property degradation [7]. To test the validity of the proposed critical fiber volume fraction concept, a series of specimens with three different volume combinations of hybrid fiber reinforcement (see Table 2) were tested
Table 1 Properties of fiber, matrix and fiber/matrix interface [2]. Fiber type
Length (mm)
Diameter (lm)
Young’s modulus (GPa)
Tensile strength (MPa)
Bond strength (MPa)
Snubbing coefficient
ST (fiber-1) PE (fiber-2)
13 12
160 39
200 66
2500 2610
4.20 1.02
0.75 0.8
Note: Poisson’s ratio (v) = 0.2; crack size (c) = 10 mm; assumed values.
M. Maalej et al. / Construction and Building Materials 36 (2012) 216–227
Indent Depth (mm)
ECC50 30
219
ECC75
FRC100 RC100*
20
10 ECC100 0 0
4
8
12
16
Impact Number Fig. 7. Indent depth against number of impacts for specimens of different materials and thicknesses (RC100 refers to an RC specimen of 100 mm thickness) [4].
under uniaxial tension. The results from these tests (shown in Fig. 6) indicate that the hybrid fiber composite with V f < V crit failed f by the development of a large single crack in the middle of the specimen. The observed strain capacity was significantly lower than those with V f P V crit f . For the hybrid fiber composite with Vf close but greater than V crit f , a strain-hardening response has been observed, confirmed by the appearance of multiple cracks. This composite exhibited a tensile strain capacity close to 2%. In contrast, the hybrid fiber composite with V f > V crit exhibited prof nounced tensile strain-hardening and multiple-cracking behaviors with a strain capacity of about 4%. The material design guidelines from the above model had successfully led to ultra ductile hybrid fiber composites. After first crack, the load continued to rise without fracture localization. Sequentially developed parallel cracks contributed to the inelastic strain at increasing stress level. The ultimate tensile strength and strain capacity were up to 5 MPa and 4%, respectively.
Fig. 10. Load–deflection responses of wall components in Series I tests [16].
3. Blast/impact resistant panels In view of the high ductility and fracture energy of the hybrid fiber ECC as discussed above, Zhang et al. [4] proposed the use of this material for the construction of blast-resistant protective panels. The performance of the hybrid fiber ECC in these applications can be evaluated experimentally using drop weight impact tests [14] to simulate the impact by large mass at relatively low strain rate and low impact velocity. When a structure is impacted by a blast wave or a large projectile at low impact velocity, the global response of the target is likely to dominate where tensile and flexural stresses must be sustained at large deformation without crack localization (unlike the case of impact by a small projectile at high
Fig. 8. Damage development of RC100 panel on impact face: (a) 1st impact, (b) 2nd impact, and (c) 3rd impact [4].
Fig. 9. Damage development of ECC 100 panel on impact face: (a) 1st impact, and (b) 10th impact [4].
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Fig. 11. Load–deflection responses of wall components in Series II tests [16]. Fig. 13. Failure of unreinforced masonry wall under impact loading [16].
velocity where the load application is spatially concentrated and the local response effects dominate). An experiment similar to that performed by Banthia et al. [14] had been conducted by Zhang et al. [4] to evaluate the damage and failure mode of hybrid-fiber ECC panels caused by large projectiles or fragments. One of the specific aims of the experiment was to quantify the extent to which hybrid-fiber ECC improves the resistance of blast panels against impact loading. For this purpose, drop weight tests were conducted on full-scale hybrid-fiber ECC blast/shelter panels (2 m 1 m 0.05–0.1 m) to study their response and performance under impact loading. Conventional steel reinforced concrete (RC) and steel fiber reinforced concrete (FRC) blast panels were also tested to identify the advantages of using hybrid fiber ECC in this application. The impact resistance of blast panels of different materials was evaluated in terms of the extent of damage, energy absorption capacity and residual resistance
against multiple impacts. The drop weight impact test results showed that the hybrid-fiber ECC panels exhibit lesser damage, significantly-improved impact resistance against multiple impacts and improved ductility and energy absorption capacity compared to both RC and FRC counterparts. The response of the hybrid fiber ECC panels to drop weight impact was characterized as follows: Much smaller indent depth and crater size on the impact face (by as much as 80% and 50%, respectively). The indent depths of the crater for all the panels under multiple impacts are summarized in Fig. 7. Fig. 8 shows the damage on the impact face of the RC panel after the first three impacts. Similar indent depth as the 1st impact on RC panel was induced on the hybrid fiber ECC panels after ten impacts, as shown in Fig. 9.
Table 3 Summary of Series I tests results [16]. Specimen
Observed failure mode
Ultimate load-carrying capacity (kN)
Deflection (mm)
Energy absorption capacity (J)
REF PSE20 PSE34 PDE34 PSD8 PDD8
Tensile flexure Compression flexure Compression flexure Buckling-bonding Punching shear Shear de-bonding
11.94 78.26 105.5 122.7 146.9 206.6
0.74 9.95 9.60 5.54 4.35 4.53
5.28 666.8 836.9 522.0 398.7 621.9
Fig. 12. Typical cracking pattern of the ECC-strengthening layers (a) without steel mesh (b) with steel mesh [16].
M. Maalej et al. / Construction and Building Materials 36 (2012) 216–227
Fig. 14. Typical load versus time graph for the first three impacts from Series III tests [16].
Much smaller exit crater on the distal face after perforation with the structure remaining largely intact. Ductile failure process characterized by large deformation limit prior to serious scabbing and total failure. Large improvement in the cracking behavior with a larger volume of material involved in energy absorption for better resistance. Zhang et al. [4] had also adopted a Single Degree of Freedom (SDOF) concept to analyze the global behavior, especially the energy absorption capacity of RC and hybrid fiber ECC panels. From the analysis, the higher ultimate resistance and maximum allowable deflection of the hybrid fiber ECC panels appeared to be the main parameters contributing to the higher energy absorption capacity and hence the enhanced global resistance of the hybrid fiber ECC panels over their RC counterparts. The results of the above study and those of a previous study [3] on the strain rate effect of hybrid fiber ECC and its impact resistance against high velocity impact from small-mass projectile provided reliable evidence for the advantages of using hybrid fiber ECC materials in protective structures. 4. Strengthening of unreinforced masonry walls Apart from the mechanical properties of hybrid fiber ECC mentioned thus-far, recent investigations conducted by Li and Li [15] have also verified that the strain hardening and multiple cracking characteristics of Engineered Cementitious Composites make them outstanding repair materials. This motivated the application of hybrid fiber ECC in the strengthening of unreinforced masonry walls against out-of-plane loadings [16]. In their study, Maalej et al. [16] carried out three series of laboratory tests to assess the extent to which hybrid fiber ECC can enhance the out-of-plane resistance
221
of the strengthened masonry walls. Three series of wall panels were subjected to three types of load patterns namely patch load, uniformly-distributed load and low-velocity projectile impact load. The experimental program included a total of 18 masonry wall panels each measuring 1000 1000 mm in plan and 100 mm in thickness excluding the hybrid fiber ECC layer. All walls were fabricated using solid clay bricks each having dimensions of 215 100 70 mm. The test specimens were grouped into three series, with Series I and II tests focusing on quasi-static loading, and Series III tests focusing on impact loading. Each series of test consisted of two unreinforced masonry walls to serve as control specimen and four strengthened masonry walls. Four strengthening configurations were studied, namely, (a) single-face of 34 mm-thick hybrid fiber ECC-strengthening layer (SE34), (b) double-face of 34 mm-thick hybrid fiber ECC-strengthening layer (DE34) each, (c) single-face of 34 mm-thick hybrid fiber ECCstrengthening layer with 8 mm-diameter steel mesh (SD8), and (d) double-face of 34 mm-thick hybrid fiber ECC-strengthening layer with 8 mm-diameter steel mesh (DD8) each. In Series I test, an additional reinforcing configuration, single-face of 20 mm-thick hybrid fiber ECC-strengthening layer (SE20) was included to investigate the effects of varying the thickness of the strengthening layer. Each strengthened wall panel was identified using a combination of four to five characters. The first character, P (Patch), U (Uniformly distributed) or I (Impact) referred to the type of loading and the three to four characters that followed referred to the reinforcement configurations as described above. The results from the quasi-static tests (Series I and II) had shown significant improvements in the ultimate load-carrying capacity and the ductility of the unreinforced masonry walls with the hybrid fiber ECC-strengthening systems as shown in Figs. 10 and 11, respectively. The results summarized in Table 3 for Series I indicated that the hybrid fiber ECC-strengthening system had increased the failure loads and deflection capacities of the walls by 6.5–17.3 times and 5.8–13.4 times, respectively, relative to those of the unreinforced masonry walls. Likewise, from Series II test results, the failure loads and deflection capacities were increased by 10.7–22 times and 4.2–15.9 times, respectively, relative to those of the unreinforced masonry walls. The energy absorption for Series I and II masonry walls were significantly increased as well from 75 to 158 times and from 53 to 279 times, respectively, relative to those of the unreinforced masonry walls. In general, the hybrid fiber ECC-strengthening layer with or without steel mesh displayed very good stress distribution with well-distributed cracks beneath the loading area stretching out towards the support in a radial pattern as shown in Fig. 12. The importance of retrofitting an unreinforced masonry walls was demonstrated in Series III tests where an unreinforced masonry wall exhibited sudden and therefore catastrophic failure upon first impact loading. The wall was perforated by the projectile
Fig. 15. Impact and distal face of ISE34 after perforation [16].
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M. Maalej et al. / Construction and Building Materials 36 (2012) 216–227
Fig. 16. Impact and distal face of IDE34 after perforation [16].
Section A-A Fig. 17. Hybrid fiber ECC (shaded area) surrounding main reinforcement bars in RC beam [1].
well-distributed radial pattern. It was also observed that the hybrid fiber ECC-strengthening layer without steel mesh had very densely distributed cracks, while the hybrid fiber ECC-strengthening layer with steel mesh had cracks that were sparsely distributed, indicating that the stresses were largely distributed by the steel mesh rather than the hybrid fiber ECC-strengthening layer.
100
ECC-2
80
A2
Load (kN)
upon impact and shattered into a few pieces as shown in Fig. 13. On the other hand, all hybrid fiber ECC-strengthened masonry walls were able to withstand multiple impacts before perforation—five for ISE34, nine for ISD8, nine for IDE34 and 18 for IDD8. Furthermore, during the impact, no fragments ejection was observed at the surface with the hybrid fiber ECC-strengthening layer. This observation demonstrated the hybrid fiber ECC’s ability to prevent fragmentations due to the impact, which may help reduce human injuries in the event of blast/explosion. A typical impact load versus time graphs obtained from the first three impact loadings applied to the strengthened wall from Series III is shown in Fig. 14. In general, two peak loads could be identified from each of the impacts. The primary peak load occurred within 2 ms upon impact, corresponding to the instant when the projectile struck the specimen accelerating it downwards, and the secondary peak load took place when the accelerating specimen lost its kinetic energy and rebounded upward, increasing its contact pressure with the on-coming projectile. Figs. 15 and 16 show typical damage on the impact and distal faces for two reinforced wall panels, one with single face and another with double face reinforcement. All the hybrid fiber ECCstrengthened masonry wall panels remained structurally intact and showed only localized damage after being perforated. In general, all reinforced walls had cracks on the distal side that propagated from the impact point towards the support edges. Under point-load impact, the panel tried to bend close to a conical shape with high hoop stress. The cementitious matrix of the hybrid fiber ECC material being brittle then fractured but the cracks were arrested by the bridging action of the fibers, resulting in a
ECC-1 A1
60
40
20
0 0
10
20
30
40
Midspan deflection (mm) Fig. 18. Load–deflection curves of beams A1, A2, ECC-1, and ECC-2 [1].
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M. Maalej et al. / Construction and Building Materials 36 (2012) 216–227
(a) Cracking patterns of beam A2
(b) Cracking patterns of beam ECC-2
Fig. 19. Cracking patterns in beams A2 and ECC-2 around the loading point [1].
P/2 ∅6
P/2
mm @ 80mm c/c FO
250
720
45mm clear cover
560
720
250
2500 Fig. 20. Details of medium-scale beam specimens (all dimensions in mm) [5].
Fig. 21. Corrosion status of steel in (a) OPCC-2 beam and (b) FGC-3 beam [5].
5. Effective FRP-strengthening of RC beams The applications of Fiber Reinforced Polymer (FRP) composites to concrete structures had been studied intensively over the past few years in view of the many advantages that FRPs possess. While FRPs had been shown to be effective in strengthening RC beams, strength increases had generally been associated with reductions in the beams’ deflection capacities due to premature debonding. Debonding failure modes occurred mainly due to interfacial shear and normal stress concentrations at FRP-cut off points and at flexural cracks along the RC beam. Maalej and Leong [1] suggested that if the quasi-brittle concrete material which surrounds the main flexural reinforcement was replaced with a ductile layer of hybrid fiber ECC as shown in Fig. 17, then it would be possible to delay the debonding failure mode and hence increase the deflection capacity of the strengthened beam. If hybrid fiber ECC was introduced in a RC member, increased number of cracks with smaller crack widths would be expected to form on the beam tensile face rather than fewer but wider
cracks in the case of an ordinary concrete beam. Such distributed finer cracks would be expected to reduce crack-induced stress concentration and result in a more efficient stress distribution in the FRP layer. The objectives of the above-study were to investigate both experimentally and numerically the structural performance of FRP-strengthened RC beams incorporating a ductile hybrid fiber ECC layer around the main flexural reinforcement. The load-carrying and deflection capacities as well as the maximum FRP strain at failure were used as criteria to evaluate the performance. The experimental program included two series of RC beams where one series consisted of two ordinary RC beams (beams A1 and A2) and another series consisted of two hybrid fiber ECC layered beams (ECC-1 and ECC-2). In each series, one specimen was strengthened using externally-bonded carbon fiber reinforced polymer (CFRP) sheets while the second was kept as a control1 in
1
Specimens A1 and ECC1 are control specimens without any CFRP reinforcement.
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M. Maalej et al. / Construction and Building Materials 36 (2012) 216–227
FOSS Reading (με)
2000 OPCC-2 (1686 με)
1600 1200
240
800 400
210
FGC-2 (277 με )
0
25
50
75
100
125
∅ 6 mm @ 120mm
Fiber opc cable 3 -16mm
300
FGC-3 (442 με )
0
2 -13mm FOSS
150
Time (days) Fig. 22. Corrosion-induced tensile strain in concrete as measured by FOSS gauge [5].
between the CFRP and the concrete beam. This delayed intermediate crack-induced interfacial debonding and resulted in higher strengthening ratio and higher deflection capacity and, therefore, a more effective use of the CFRP material. Further, despite the large distance between the support and the CFRP cut-off point in beam ECC-2, plate-end debonding or concrete cover peeling were not observed. This is expected due to the high fracture energy of the hybrid fiber ECC material. The above experimental results showed that the hybrid fiber ECC had indeed delayed debonding of the CFRP and resulted in effective use of the CFRP material. With the use of hybrid fiber ECC as a ductile layer, RC beams can effectively be strengthened while minimizing loss in deflection capacity.
6. Corrosion-resistant RC beams
Fig. 23. Load–deflection response of un-corroded and corroded beams (a) OPCC beams (b) FGC beams [5].
order to compare its load–deflection behavior under third-point loading with the strengthened specimen. It could be seen from Fig. 18 that the CFRP strengthened beams with a hybrid fiber ECC layer (e.g. beam ECC-2) depicted higher load-carrying capacity compared to their ordinary reinforced concrete counterparts (e.g. beam A2). If expressed in term of strengthening ratio (SR, defined as the strength of beam with CFRP reinforcement divided by the strength of control beam), beam ECC-2 had a strengthening ratio of about 1.43, compared to 1.28 for beam A2. Also, beam ECC-2 was noted to have a significantly higher deflection capacity (29.6 mm) at peak load compared to beam A2 (21.9 mm). On the cracking behavior, both ECC-1 and ECC-2 showed a considerable number of fine cracks compared to the ordinary RC beams (beam A1 and A2) as revealed in Fig. 19. As a result, the crack spacings were consequently much smaller in the hybrid fiber ECC beams (particularly in ECC-2) than in the ordinary RC beam. These multiple but fine cracks played a major role in reducing crack-induced stress concentration resulting in more efficient stress distribution in the CFRP sheet and a better stress transfer
As an alternate method of improving the corrosion resistance of RC beams exposed to aggressive substances, Maalej and Li [6] proposed a new design for RC flexural members where part of the concrete which surrounds the main flexural reinforcement is replaced with a strain hardening ECC material as shown in Fig. 17. This alternate design with layered ECC had been referred to as Functionally-Grade Concrete (FGC). It was suggested that the ECC material in FGC beams could provide two levels of protection. First, it could prevent the migration of aggressive substances into the concrete, therefore, preventing reinforcement corrosion. Second, in the extreme case when corrosion initiates, accelerated corrosion due to longitudinal cracks would be reduced (if not eliminated), and spalling and delamination problems common to many of today’s RC structures would be prevented. This is expected due to the high strain capacity and fracture resistance of the ECC material. Maalej et al. [5] adopted the above concept to prepare a series of FGC beams where the main longitudinal reinforcements were surrounded by a hybrid fiber reinforced mortar material exhibiting strain hardening and multiple cracking under third-point flexural loading. The hybrid fiber reinforcement consisted of high modulus (steel) and low modulus (PVA) fibers used with respective volume fractions of 1% and 1.5%. The objective of the study was to evaluate the effectiveness of the FGC concept in retarding the corrosion of steel reinforcement in RC beams and reducing the tendency of the concrete cover to delaminate as measured by a concrete embeddable fiber optic strain sensor (FOSS). The effects of steel loss and corrosion damage on the flexural response of RC beams were also evaluated. The experimental program included 2 control RC beams (OPCC beams) made of ordinary Portland cement concrete and 3 FGC beams with geometrical detail as shown in Fig. 20. At any given time, the FGC beams were found to exhibit lower level of steel loss than the OPCC beam (see Fig. 21). It was also observed that an FGC beam took about 70% longer time to achieve the same level of induced steel loss compared to an OPCC beam. The
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Fig. 24. Typical cracking patterns of corroded beams (a) OPCC-2 at about peak load; (b) close-up of region highlighted in (a) after failure; (c) FGC-3 at about peak load; (d) close-up of region highlighted in (c) after failure [5].
1000
Estimated amount of corrosion [g]
Plain mortar SHFRCC (PE fiber)
100 2
Icorr (μA/cm )
SHFRCC (hybrid steel-Pe fiber)
10 1 0
10
20
30
40
50
60
70
80
90
100
0.1 0.01 0.001
Time (days)
Fig. 25. Progress of corrosion current (Icorr values) of the beams made with plain mortar, ECC containing polyethylene (PE) fiber and ECC containing hybrid steel–PE fiber [19].
better performance of the FGC beams over the OPCC beam was also evident from the absence of any corrosion-induced cracks or damage and the lowest tendency for the concrete cover to delaminate as measured by a concrete embeddable FOSS (see Fig. 22). The
(a) Plain mortar
(b) ECC containing polyethylene (PE)
(c) ECC containing hybrid steel-PE fiber Fig. 26. Corrosion induced damage of beams after 14 weeks of accelerated corrosion test [19].
Time [weeks] Mortar
FRCC
HFRCC
Fig. 27. Amount of corrosion (estimated by Faraday’s law) with progress of corrosion in a long term accelerated corrosion test for 1 year (note: FRCC denotes ECC containing polyethylene (PE) fiber and HFRCC denotes ECC containing hybrid steel–PE fibers) [20].
load–midspan displacement response of the test specimens indicated that for the same level of steel loss, the FGC specimens exhibited higher residual load and deflection capacities compared to their OPCC counterpart (see Fig. 23). While a corroded OPCC beam was found to experience widening of corrosion-induced cracks, delamination and spalling during loading, no such behavior was observed in the FGC beams (see Fig. 24). The FGC concept was found in the above study to be very effective in preventing corrosion-induced damage in RC beams and minimizing the loss in the beam’s load and deflection capacities. Similar results had also been reported in another study [17] where FGC beams were subjected to an accelerated corrosion regime following initial pre-cracking under service load, and where some of the specimens were subjected to corrosion under sustained loading. The latter study indicated that the development of multiple fine flexural cracks in the FGC beams (as opposed to few large cracks in an ordinary RC beam) did not accelerate the corrosion process, but rather it had slowed it down resulting in reduced corrosion damage. This outcome was consistent with results published by Tsukamoto [18] who studied the tightness of plain and fiber reinforced concrete and found that the flow rate (of water) scales with the third power of crack width (d3) and under a certain crack width (critical crack width2) no further flow occurred.
2
Which is in the range of 0.10–0.15 mm for the material studied.
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M. Maalej et al. / Construction and Building Materials 36 (2012) 216–227
Week
Mortar
Hybrid fiber ECC
Mono fiber ECC
th
30
th
40
nd
52
2.0
(a) MT0 MT1 MT2 MT3 MT4
1.5 1.0 0.5 0.0 0
20
40
Expansion of crack width (mm)
Expansion of crack width (mm)
Fig. 28. Time-dependent change of corrosion induced damage in the specimens made with plain mortar, ECC containing PE fiber and hybrid steel–PE fiber ECC [20].
2.0
(b) FR0 FR1 FR3 FR4 FR5
1.5 1.0 0.5 0.0 0
60
20
Time (week)
40
60
Expansion of crack width (mm)
Time (week) 2.0
(c)
1.5 HF0 HF1 HF3 HF5
1.0 0.5 0.0 0
20
40
60
Time (week) Fig. 29. Change of the width of corrosion induced longitudinal crack in the beams having different initially given crack widths in (a) mortar, (b) ECC containing PE fiber (represented as FR in the figure) and (c) ECC containing steel–PE hybrid fiber (represented as HR in the figure) [21].
In more recent studies [19–21], the corrosion durability and the resistance to corrosion-induced damage of steel reinforced beams incorporating hybrid steel–polyethylene fiber ECC and polyethylene monofiber ECC were studied. Ahmed and Mihashi [19] evaluated the corrosion resistance of cracked steel reinforced beams made with mono and hybrid fiber ECC materials as well as plain mortar. The monofiber ECC contained polyethylene (PE) fiber of 1.5% by volume, while the hybrid fiber ECC contained both steel and PE fibers of 0.75% each with a total fiber volume fraction of 1.5%. The beam specimens were 100 100 400 mm in dimension and were pre-cracked under three-point bending with a maximum load of about 80% of their corresponding calculated ultimate load. The mono and hybrid fiber ECC beams exhibited numerous multiple cracks with an average crack width of 0.05 mm and 0.026 mm, respectively, while the mortar beam exhibited a single crack of 0.2 mm width. The corrosion resistance of the beam specimens was measured electrochemically using mini sensors placed on reinforcing bar in each beam. The test results indicated improved corrosion resistance of the (steel reinforced) hybrid fiber ECC beams in terms of lowest measured corrosion current (Icorr values) in comparison to the ordinary mortar and monofiber ECC (Fig. 25) counterparts. Better resistance to corrosion-induced damage of the hybrid fiber ECC specimens was also observed in this study as shown in Fig. 26. The improved performance of the hybrid fiber ECC specimens could be attributed to the better tensile strain
Fig. 30. Typical corrosion induced cracking in (a) mortar, (b) ECC containing PE fiber and (c) ECC containing steel–PE hybrid fiber beams [21].
M. Maalej et al. / Construction and Building Materials 36 (2012) 216–227
hardening characteristics of the hybrid fiber ECC material in comparison to the monofiber ECC and mortar. In a follow-up investigation, Mihashi et al. [20] subjected (steel reinforced) hybrid fiber ECC, monofiber ECC and plain mortar beams to accelerated corrosion for 1 year. The corrosion current was measured and used to calculate the steel loss according to Faraday’s second law. The authors reported superior corrosion resistance of the hybrid fiber ECC beams in terms of lowest steel loss (Fig. 27) and least corrosion induced damage (Fig. 28), in comparison to the monofiber ECC and plain mortar counterparts. Mihashi et al. [21] also monitored the corrosion-induced cracking of cracked beams containing mortar, monofiber ECC and hybrid fiber ECC. The width of longitudinal cracks, which formed in the beams due to steel corrosion, was measured during the accelerated corrosion test. The results indicated increased crack width in the mortar specimens with increased corrosion exposure time. No such increase was observed in the hybrid fiber ECC specimen (Fig. 29). This was also evident by comparing the corrosion induced damage of the hybrid fiber ECC beams and mortar beams at any given time (see Fig. 30). 7. Concluding remarks In reviewing the potential structural applications of hybrid fiber Engineered Cementitious Composites (ECCs), it becomes clear that the ability of the material to strain harden through the development of multiple fine cracks had imparted significant enhancement to the structural response of the element. For elements that were subjected to impact loading, the multiple cracking and the large strain capacity had resulted in a significant improvement in the element capacity to absorb impact energy, and therefore minimize structural damage. For corrosion protection and FRP strengthening of RC members, the fine cracking had led to reduced corrosion activities in the member and delayed debonding of the FRP reinforcing sheet indicative of reduced stress concentration. Further, the high fracture energy of the hybrid fiber ECC material had practically eliminated spalling, scabbing, and fragment ejection in the hybrid fiber ECC panels that were subjected to drop weight impact, and had also prevented corrosion-induced concrete cover spalling in the RC beams that were subjected to accelerated corrosion, and peeling of the concrete cover in the FRP strengthened beams. The high strain capacity, fracture energy and damage tolerance of the hybrid fiber ECC as well as the characteristics of its microcraking response make the material highly desirable in numerous other potential applications. For instance, more recent test results [22] that have not been discussed in the present paper, have shown that the use of hybrid fiber ECC in the plastic zone of type-2 beam column connections as a replacement of concrete and partial replacement of transverse (confinement) reinforcement can significantly enhance the joint shear resistance, energy absorption capacity, and cracking response, thereby, enhancing the joint seismic resistance, and reducing reinforcement congestion and construction complexity. While hybrid fiber ECC has such significant potential for use in numerous applications in the construction industry, its actual universal exploitation will depend, first, on the ability to prepare
227
and place hybrid fiber ECC mixes that are of consistent fresh and hardened properties in ways that do not significantly deviate from common techniques. Second, as fibers used in hybrid fiber ECC mixes are of such high performance (and cost impact), fiber materials meeting specific design requirement as per available micromechanical models shall become readily available at costs that can be justified based on life-cycle cost analysis. In this regard, the use of more than one type of fiber (leading to a hybrid fiber composite) will offer additional flexibility in the design of the composite that meet the specific requirement for a given application while at the same time reducing the associated life cycle cost. References [1] Maalej M, Leong KS. Engineered cementitious composites (ECCs) for effective FRP-strengthening of RC beams. Compos Sci Technol 2005;65(7–8):1120–8. [2] Ahmed SFU, Maalej M, Paramasivam P. Analytical model for tensile strainhardening and multiple-cracking behavior of hybrid fiber engineered cementitious composites. ASCE J Mater Civ Eng 2007;19(7):527–39. [3] Maalej M, Quek ST, Zhang J. Behavior of hybrid-fiber engineered cementitious composites subjected to dynamic tensile loading and projectile impact. ASCE J Mater Civ Eng 2005;17(2):143–52. [4] Zhang J, Maalej M, Quek ST. Performance of hybrid-fiber ECC blast/shelter panels subjected to drop weight impact. ASCE J Mater Civ Eng 2007;19(10):855–63. [5] Maalej M, Ahmed SFU, Paramasivam P. Corrosion durability and structural response of functionally-graded concrete beams. J Adv Concr Technol 2003;1(3):307–16. [6] Maalej M, Li VC. Introduction of strain hardening engineered cementitious composites in the design of reinforced concrete flexural members for improved durability. ACI Struct J 1995;92(2):167–76. [7] Li VC, Wu HC, Maalej M, Mishra DK, Hashida T. Tensile behavior of engineered cementitious composites with discontinuous random steel fibers. J Am Ceram Soc 1996;79(1):74–8. [8] Maalej M, Hashida T, Li VC. Effect of fiber volume fraction on the off-crackplane fracture energy in strain hardening engineered cementitious composites. J Am Ceram Soc 1995;78(12):3369–75. [9] Ahmed SFU, Maalej M. Tensile strain hardening behaviour of hybrid steel– polyethylene fibre reinforced cementitious composites. J Constr Build Mater 2009;23(1):96–106. [10] Li VC, Leung CKY. Steady state and multiple cracking of short random fiber composites. ASCE J Eng Mech 1992;118(11):2246–64. [11] Li VC. Post-crack scaling relations for fiber reinforced cementitious composites. ASCE J Mater Civ Eng 1992;4(1):41–57. [12] Maalej M, Li VC, Hashida T. Effect of fiber rupture on tensile properties of short fiber composites. ASCE J Eng Mech 1995;121(8):903–13. [13] Jenq YS, Shah SP. Two parameter fracture model for concrete. ASCE J Eng Mech 1985;111(10):1227–41. [14] Banthia N, Mindess S, Bentur A, Pigeon M. Impact testing of concrete using a drop weight impact machine. Exp Mech 1989;29(2):63–9. [15] Li M, Li VC. Behaviour of ECC/concrete layered repair system under drying shrinkage conditions. J Restor Build Monument 2006;2:143–60. [16] Maalej M, Lin VWJ, Nguyen MP, Quek ST. Engineered cementitious composites for effective strengthening of unreinforced masonry walls. Eng Struct (Elsevier, UK) 2010;32(8):2432–9. [17] Maalej M, Chhoa CY, Quek ST. Effect of cracking, corrosion and repair on the frequency response of RC beams. Constr Build Mater (Elsevier) 2010;24(5):719–31. [18] Tsukamoto M. Tightness of fiber concrete. Darmstadt concrete. Ann J Concr Concr Struct 1990;5:215–25. [19] Ahmed SFU, Mihashi H. Corrosion durability of strain hardening fiber reinforced cementitious composites. Aust J Civ Eng 2010;8(1):27–39. [20] Mihashi H, Ahmed SFU, Kobayakawa A. Corrosion of reinforcing steel in fiber reinforced cementitious composites. J Adv Concr Technol 2011;9(2):159–67. [21] Ahmed SFU, Mihashi H, Kobayakawa A. Influence of crack widths on corrosion of reinforcing steel bar in fiber reinforced cementitious composites. In: The proceedings of RILEM conference on Advances in construction materials through science and engineering, Hong Kong; 2011. [22] Al-Qudah SM. Enhancing the seismic behavior of beam-column joints using engineered cementitious composites (ECCs). Master thesis, Department of Civil and Environmental Engineering, University of Sharjah; 2011.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Review
Review of potential structural applications of hybrid fiber Engineered Cementitious Composites M. Maalej a, S.T. Quek b, S.F.U. Ahmed c,⇑, J. Zhang b, V.W.J. Lin b, K.S. Leong b a
Department of Civil & Environmental Engineering, College of Engineering, University of Sharjah, Sharjah, United Arab Emirates Department of Civil Engineering, National University of Singapore, Singapore c Department of Civil Engineering, Curtin University, Perth, Western Australia, Australia b
h i g h l i g h t s " Hybrid fiber ECC exhibited significant enhancement to the structural response. " Hybrid fiber ECC exhibited significant improvement in impact energy absorption. " Multiple cracking with fine crack widths led to reduced corrosion activities. " Multiple cracking delayed the debonding of the FRP reinforcing sheet. " Hybrid fiber ECC eliminated spalling of concrete cover due to high fracture energy.
a r t i c l e
i n f o
Article history: Received 20 February 2012 Received in revised form 17 April 2012 Accepted 24 April 2012 Available online 23 June 2012 Keywords: Engineered Cementitious Composites Hybrid fiber Corrosion Strengthening Masonry Impact
a b s t r a c t This paper reviews some of the recent research work focusing on assessing the performance of hybrid fiber Engineered Cementitious Composite (ECC) materials in a number of potential structural applications. A summary of the design and characteristics of such materials is presented followed by a review of recent potential applications of hybrid fiber ECC. The reviewed applications include the use of hybrid fiber ECC for designing impact and blast resistant protective panels, strengthening of unreinforced masonry (URM) walls, strengthening of RC beams, and enhancing corrosion durability of RC beams. The review demonstrates that hybrid fiber ECC can significantly enhance the performance of structures incorporating these materials. Ó 2012 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5. 6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . The critical fiber volume fraction concept . . . . Blast/impact resistant panels. . . . . . . . . . . . . . . Strengthening of unreinforced masonry walls . Effective FRP-strengthening of RC beams . . . . . Corrosion-resistant RC beams . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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⇑ Corresponding author. E-mail address: [email protected] (S.F.U. Ahmed). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.04.010
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1. Introduction In recent years, it has been demonstrated that Engineered Cementitious Composites (ECCs) can be designed to exhibit pronounced tensile strain-hardening by adding to the cement-based matrix a relatively low volume (typically 62%) of short randomly-distributed fibers of specific type and property. The resulting ECC composites are characterized by their high tensile strain capacity, fracture energy and notch insensitivity, making them ideal materials for various structural applications. Under uniaxial tension, sequentially developed parallel cracks contribute to the inelastic strain at increasing stress level. The ultimate tensile strength and strain capacity can be as high as 5 MPa and 4%, respectively (see Fig. 1). The latter is two orders of magnitude higher than that of normal or ordinary fiber reinforced concrete. It has been suggested that, for a number of structural application, the use of more than one type of fiber as reinforcement results in a hybrid fiber composite that is better able to meet the material performance requirements then monofiber composite for the given application [1–5]. For instance, to use ECC in protective structures, the material is required to possess sufficient strength to resist penetration, while at the same time it is required to absorb a large amount of energy, thereby minimizing fragmentation and reducing the velocities of the fragments (i.e. need material of sufficiently high strain capacity). For using ECC in corrosion-resistant structures, a low crack width is required to reduce the ingress of aggressive substances reaching the steel reinforcement, while at the same time a high strain capacity is also required to prevent delamination and concrete cover spalling [6]. Further, given that monofiber ECCs containing high modulus fibers (e.g. steel or carbon fibers) normally exhibit high ultimate strength, low crack width and low
strain capacity [7], while those containing low modulus fibers (e.g. polyvinyl alcohol and polyethylene fibers) exhibit opposite behaviors [6,8], it becomes clear that a hybrid-fiber ECC with proper volume ratio of high and low modulus fibers can be designed to achieve an optimal balance between ultimate strength, crack width and strain capacity [9] (see Fig. 2), and is therefore better able to meet the functional requirement for these applications. In the sections to follow, the performance of hybrid fiber ECC in a number of structural applications is reviewed with the objective of assessing the potential of these materials in providing better functionality, in particular, in applications involving enhancing the impact- and blast-resistance of structures, improving the strength and ductility of FRP-strengthened beams and enhancing the corrosion durability of RC members. 2. The critical fiber volume fraction concept A micromechanical model for the design of hybrid fiber ECC was develop by Ahmed et al. [2] on the basis of fracture mechanics and deformation mechanism taking into account the effects of hybrid fibers. This micromechanical model for hybrid-fiber ECC is an extension of an earlier model proposed by Li and Leung [10] for monofiber composites. An important component of the micromechanical model is the composite bridging law (rcd) schematically depicted in Fig. 3. The rc–d law describes the constitutive relationship between the traction rc acting across a matrix crack plane and the separation distance d of the crack faces (i.e. COD: crack opening displacement) in a singly pre-cracked uniaxial tensile specimen loaded quasi-statically to complete failure. Micromechanical modelling of the bridging law in discontinuous randomly-distributed fiber reinforced cementitious composites has been carried out by Li [11] and Maalej et al.
5
σc
Tensile Stress (MPa)
σc 4
σcu
3
δ
2
σc
1 0 0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
δ
Strain
δ
L f /2
*
Fig. 3. Composite bridging law.
Fig. 1. Uniaxial tensile stress–strain curves of ECC from coupon specimen tests [1].
σ
For a given V f1 crit
min
Vf = Vf1+V f2
σcu σfc
min
Vf2
Fig. 2. Effect of hybrid reinforcement in the strain hardening behavior of ECC [9].
Multiple-cracking Vf2
Fig. 4. First-crack strength (rfc) and ultimate bridging strength (rcu) for different volume fractions of fiber 2; intersection of rfc and rcu yields the minimum volume fraction of fiber 2 required to achieve strain-hardening behavior for a given volume fraction of fiber 1.
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M. Maalej et al. / Construction and Building Materials 36 (2012) 216–227
[12]. When no fiber rupture occurs in the composite, the rc-d relationship is given by [11]:
Vf1 =0.5% (ST fibre)
8 h qffiffiffiffi i > Pre-peak rc d curve for d 6 d < g ro 2 dd dd ¼ h i2 > : g ro 1 2d for d 6 d 6 Lf =2 Post-peak rc d curve L
Vf2 (%)
rc
3
f
2 PE1.5% (Vf > Vfcrit) PE0.8% (Vf ≈ Vfcrit )
1
PE0.2% (Vf < Vfcrit )
ð1a; bÞ where g is a factor which accounts for a fiber/matrix local frictional effect (called snubbing) due to the misalignment between the fiber axis and the normal crack plane, d⁄ = Lf(s/Ef)(Lf/df)/(1 + g) is the COD corresponding to the stage when debonding is completed for all fibers, and at which all fibers are pulling-out of the matrix and ro = Vfs(Lf/df)/2, in which s, df, Lf, Ef and Vf are the fiber/matrix interfacial bond strength, and the fiber diameter, length, modulus and volume fraction, respectively. No chemical bond between fibers and matrix is assumed in this model. For a hybrid fiber ECC system, the principle of superposition has been adopted to develop the composite bridging law (rc–d) and predict the first crack strength (rfc) and the ultimate bridging strength (rcu) of the hybrid fiber composite. The first crack strength (rfc) may be obtained from a consideration of the balance of stress intensity factors associated with the crack tip fracture toughness (Ktip), the applied tensile loading (KL) and the fiber bridging stress (KB) [10]:
K L þ K B ¼ K tip
0 0.01
0
0.03
0.02
Crack Size, C (m) Fig. 5. Design chart for hybrid fiber ECC. For a given crack size and volume fraction of fiber 1, the minimum volume fraction of fiber 2 can be obtained (note: C indicates the crack radius).
Table 2 Example mix proportions of hybrid fiber ECC [2]. Series
Vf < Vf Vf >
Volume fraction of fiber, Vf (%)
V crit f V crit f V crit f
Mix proportions (by weight)
Steel
PE
Cement
Silica fume
Water
Superplastizer
0.5
0.2
1
0.1
0.28
0.02
0.5
0.8
0.5
1.5
ð2Þ
The stress intensity factor due to fiber bridging (KB) in hybrid fiber ECC is calculated from the pre-peak (K 1B1 ) and the post-peak (K 2B1 ) contribution of fiber 1 and the pre-peak (KB2) contribution of fiber 2 as follows:
ð3Þ
where the subscripts 1 and 2 refer to fiber 1 and 2, respectively. The ultimate bridging strength (rcu) of the hybrid fiber ECC is given by the sum of the peak bridging stress of fiber 2 (g2ro2) and the post-peak bridging stress of fiber 1 at a COD equal to d2 :
rcu ¼ g 2 ro2 þ g 1 ro1 1
2d2 Lf 1
ST0.5+PE1.5 (Vf > Vf crit)
Vf1=0.5% (ST fibre)
5
Stress (MPa)
K B ¼ ðK 1B1 þ K 2B1 Þ þ K B2
6
4 ST0.5+PE0.8 (Vf ≈ Vfcrit )
3 ST0.5+PE0.2 (Vf < Vfcrit )
2 1
2
0
ð4Þ
0
1
2
3
4
5
6
Strain (%)
The model assumed that strain-hardening behavior could be achieved in a hybrid fiber composite when the maximum bridging strength imposed by the fiber/matrix interaction exceeds the first crack strength at which new matrix cracks can propagate (multiple cracking). For a given volume fraction of one type of fiber (Vf1), the above condition (rcu = rfc) illustrated in Fig. 4 led to identify the existence of a minimum volume fraction of the other type of fiber (V min f 2 ), and a minimum size of initial matrix crack (C) necessary for the composite to exhibit strain-hardening behavior. The parametric values for the fibers, matrix and fiber/matrix interface used in the model are given in Table 1. The fracture toughness, Km, of the cement paste matrix was determined according to the two-parameter fracture model [13]. The average value was measured to be about 0.37 MPa m1/2 and the matrix elastic modulus was 18.0 GPa [2]. It has been estimated from the model that the critical fiber volume fraction, V crit f , for strain-hardening behavior in a hybrid fiber composite containing steel (ST) and
Fig. 6. Tensile stress–strain response and multiple cracking of hybrid fiber ECCs for various fiber volume fractions.
polyethylene (PE) fibers is equal to 0.6% PE for 0.5% ST. For different crack sizes, C, different volume fractions of fiber 2 can be obtained for a given volume fraction of fiber 1. Fig. 5 illustrates a sample design chart for Vf1 equal to 0.5% ST where the crack size dependency was included. This type of design chart can be used to design hybrid fiber composites for strain-hardening and multiple-cracking behaviors. When using this kind of chart, however, one should be aware that excessive increase of fiber volume fraction may lead to interface bond deterioration and matrix property degradation [7]. To test the validity of the proposed critical fiber volume fraction concept, a series of specimens with three different volume combinations of hybrid fiber reinforcement (see Table 2) were tested
Table 1 Properties of fiber, matrix and fiber/matrix interface [2]. Fiber type
Length (mm)
Diameter (lm)
Young’s modulus (GPa)
Tensile strength (MPa)
Bond strength (MPa)
Snubbing coefficient
ST (fiber-1) PE (fiber-2)
13 12
160 39
200 66
2500 2610
4.20 1.02
0.75 0.8
Note: Poisson’s ratio (v) = 0.2; crack size (c) = 10 mm; assumed values.
M. Maalej et al. / Construction and Building Materials 36 (2012) 216–227
Indent Depth (mm)
ECC50 30
219
ECC75
FRC100 RC100*
20
10 ECC100 0 0
4
8
12
16
Impact Number Fig. 7. Indent depth against number of impacts for specimens of different materials and thicknesses (RC100 refers to an RC specimen of 100 mm thickness) [4].
under uniaxial tension. The results from these tests (shown in Fig. 6) indicate that the hybrid fiber composite with V f < V crit failed f by the development of a large single crack in the middle of the specimen. The observed strain capacity was significantly lower than those with V f P V crit f . For the hybrid fiber composite with Vf close but greater than V crit f , a strain-hardening response has been observed, confirmed by the appearance of multiple cracks. This composite exhibited a tensile strain capacity close to 2%. In contrast, the hybrid fiber composite with V f > V crit exhibited prof nounced tensile strain-hardening and multiple-cracking behaviors with a strain capacity of about 4%. The material design guidelines from the above model had successfully led to ultra ductile hybrid fiber composites. After first crack, the load continued to rise without fracture localization. Sequentially developed parallel cracks contributed to the inelastic strain at increasing stress level. The ultimate tensile strength and strain capacity were up to 5 MPa and 4%, respectively.
Fig. 10. Load–deflection responses of wall components in Series I tests [16].
3. Blast/impact resistant panels In view of the high ductility and fracture energy of the hybrid fiber ECC as discussed above, Zhang et al. [4] proposed the use of this material for the construction of blast-resistant protective panels. The performance of the hybrid fiber ECC in these applications can be evaluated experimentally using drop weight impact tests [14] to simulate the impact by large mass at relatively low strain rate and low impact velocity. When a structure is impacted by a blast wave or a large projectile at low impact velocity, the global response of the target is likely to dominate where tensile and flexural stresses must be sustained at large deformation without crack localization (unlike the case of impact by a small projectile at high
Fig. 8. Damage development of RC100 panel on impact face: (a) 1st impact, (b) 2nd impact, and (c) 3rd impact [4].
Fig. 9. Damage development of ECC 100 panel on impact face: (a) 1st impact, and (b) 10th impact [4].
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Fig. 11. Load–deflection responses of wall components in Series II tests [16]. Fig. 13. Failure of unreinforced masonry wall under impact loading [16].
velocity where the load application is spatially concentrated and the local response effects dominate). An experiment similar to that performed by Banthia et al. [14] had been conducted by Zhang et al. [4] to evaluate the damage and failure mode of hybrid-fiber ECC panels caused by large projectiles or fragments. One of the specific aims of the experiment was to quantify the extent to which hybrid-fiber ECC improves the resistance of blast panels against impact loading. For this purpose, drop weight tests were conducted on full-scale hybrid-fiber ECC blast/shelter panels (2 m 1 m 0.05–0.1 m) to study their response and performance under impact loading. Conventional steel reinforced concrete (RC) and steel fiber reinforced concrete (FRC) blast panels were also tested to identify the advantages of using hybrid fiber ECC in this application. The impact resistance of blast panels of different materials was evaluated in terms of the extent of damage, energy absorption capacity and residual resistance
against multiple impacts. The drop weight impact test results showed that the hybrid-fiber ECC panels exhibit lesser damage, significantly-improved impact resistance against multiple impacts and improved ductility and energy absorption capacity compared to both RC and FRC counterparts. The response of the hybrid fiber ECC panels to drop weight impact was characterized as follows: Much smaller indent depth and crater size on the impact face (by as much as 80% and 50%, respectively). The indent depths of the crater for all the panels under multiple impacts are summarized in Fig. 7. Fig. 8 shows the damage on the impact face of the RC panel after the first three impacts. Similar indent depth as the 1st impact on RC panel was induced on the hybrid fiber ECC panels after ten impacts, as shown in Fig. 9.
Table 3 Summary of Series I tests results [16]. Specimen
Observed failure mode
Ultimate load-carrying capacity (kN)
Deflection (mm)
Energy absorption capacity (J)
REF PSE20 PSE34 PDE34 PSD8 PDD8
Tensile flexure Compression flexure Compression flexure Buckling-bonding Punching shear Shear de-bonding
11.94 78.26 105.5 122.7 146.9 206.6
0.74 9.95 9.60 5.54 4.35 4.53
5.28 666.8 836.9 522.0 398.7 621.9
Fig. 12. Typical cracking pattern of the ECC-strengthening layers (a) without steel mesh (b) with steel mesh [16].
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Fig. 14. Typical load versus time graph for the first three impacts from Series III tests [16].
Much smaller exit crater on the distal face after perforation with the structure remaining largely intact. Ductile failure process characterized by large deformation limit prior to serious scabbing and total failure. Large improvement in the cracking behavior with a larger volume of material involved in energy absorption for better resistance. Zhang et al. [4] had also adopted a Single Degree of Freedom (SDOF) concept to analyze the global behavior, especially the energy absorption capacity of RC and hybrid fiber ECC panels. From the analysis, the higher ultimate resistance and maximum allowable deflection of the hybrid fiber ECC panels appeared to be the main parameters contributing to the higher energy absorption capacity and hence the enhanced global resistance of the hybrid fiber ECC panels over their RC counterparts. The results of the above study and those of a previous study [3] on the strain rate effect of hybrid fiber ECC and its impact resistance against high velocity impact from small-mass projectile provided reliable evidence for the advantages of using hybrid fiber ECC materials in protective structures. 4. Strengthening of unreinforced masonry walls Apart from the mechanical properties of hybrid fiber ECC mentioned thus-far, recent investigations conducted by Li and Li [15] have also verified that the strain hardening and multiple cracking characteristics of Engineered Cementitious Composites make them outstanding repair materials. This motivated the application of hybrid fiber ECC in the strengthening of unreinforced masonry walls against out-of-plane loadings [16]. In their study, Maalej et al. [16] carried out three series of laboratory tests to assess the extent to which hybrid fiber ECC can enhance the out-of-plane resistance
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of the strengthened masonry walls. Three series of wall panels were subjected to three types of load patterns namely patch load, uniformly-distributed load and low-velocity projectile impact load. The experimental program included a total of 18 masonry wall panels each measuring 1000 1000 mm in plan and 100 mm in thickness excluding the hybrid fiber ECC layer. All walls were fabricated using solid clay bricks each having dimensions of 215 100 70 mm. The test specimens were grouped into three series, with Series I and II tests focusing on quasi-static loading, and Series III tests focusing on impact loading. Each series of test consisted of two unreinforced masonry walls to serve as control specimen and four strengthened masonry walls. Four strengthening configurations were studied, namely, (a) single-face of 34 mm-thick hybrid fiber ECC-strengthening layer (SE34), (b) double-face of 34 mm-thick hybrid fiber ECC-strengthening layer (DE34) each, (c) single-face of 34 mm-thick hybrid fiber ECCstrengthening layer with 8 mm-diameter steel mesh (SD8), and (d) double-face of 34 mm-thick hybrid fiber ECC-strengthening layer with 8 mm-diameter steel mesh (DD8) each. In Series I test, an additional reinforcing configuration, single-face of 20 mm-thick hybrid fiber ECC-strengthening layer (SE20) was included to investigate the effects of varying the thickness of the strengthening layer. Each strengthened wall panel was identified using a combination of four to five characters. The first character, P (Patch), U (Uniformly distributed) or I (Impact) referred to the type of loading and the three to four characters that followed referred to the reinforcement configurations as described above. The results from the quasi-static tests (Series I and II) had shown significant improvements in the ultimate load-carrying capacity and the ductility of the unreinforced masonry walls with the hybrid fiber ECC-strengthening systems as shown in Figs. 10 and 11, respectively. The results summarized in Table 3 for Series I indicated that the hybrid fiber ECC-strengthening system had increased the failure loads and deflection capacities of the walls by 6.5–17.3 times and 5.8–13.4 times, respectively, relative to those of the unreinforced masonry walls. Likewise, from Series II test results, the failure loads and deflection capacities were increased by 10.7–22 times and 4.2–15.9 times, respectively, relative to those of the unreinforced masonry walls. The energy absorption for Series I and II masonry walls were significantly increased as well from 75 to 158 times and from 53 to 279 times, respectively, relative to those of the unreinforced masonry walls. In general, the hybrid fiber ECC-strengthening layer with or without steel mesh displayed very good stress distribution with well-distributed cracks beneath the loading area stretching out towards the support in a radial pattern as shown in Fig. 12. The importance of retrofitting an unreinforced masonry walls was demonstrated in Series III tests where an unreinforced masonry wall exhibited sudden and therefore catastrophic failure upon first impact loading. The wall was perforated by the projectile
Fig. 15. Impact and distal face of ISE34 after perforation [16].
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Fig. 16. Impact and distal face of IDE34 after perforation [16].
Section A-A Fig. 17. Hybrid fiber ECC (shaded area) surrounding main reinforcement bars in RC beam [1].
well-distributed radial pattern. It was also observed that the hybrid fiber ECC-strengthening layer without steel mesh had very densely distributed cracks, while the hybrid fiber ECC-strengthening layer with steel mesh had cracks that were sparsely distributed, indicating that the stresses were largely distributed by the steel mesh rather than the hybrid fiber ECC-strengthening layer.
100
ECC-2
80
A2
Load (kN)
upon impact and shattered into a few pieces as shown in Fig. 13. On the other hand, all hybrid fiber ECC-strengthened masonry walls were able to withstand multiple impacts before perforation—five for ISE34, nine for ISD8, nine for IDE34 and 18 for IDD8. Furthermore, during the impact, no fragments ejection was observed at the surface with the hybrid fiber ECC-strengthening layer. This observation demonstrated the hybrid fiber ECC’s ability to prevent fragmentations due to the impact, which may help reduce human injuries in the event of blast/explosion. A typical impact load versus time graphs obtained from the first three impact loadings applied to the strengthened wall from Series III is shown in Fig. 14. In general, two peak loads could be identified from each of the impacts. The primary peak load occurred within 2 ms upon impact, corresponding to the instant when the projectile struck the specimen accelerating it downwards, and the secondary peak load took place when the accelerating specimen lost its kinetic energy and rebounded upward, increasing its contact pressure with the on-coming projectile. Figs. 15 and 16 show typical damage on the impact and distal faces for two reinforced wall panels, one with single face and another with double face reinforcement. All the hybrid fiber ECCstrengthened masonry wall panels remained structurally intact and showed only localized damage after being perforated. In general, all reinforced walls had cracks on the distal side that propagated from the impact point towards the support edges. Under point-load impact, the panel tried to bend close to a conical shape with high hoop stress. The cementitious matrix of the hybrid fiber ECC material being brittle then fractured but the cracks were arrested by the bridging action of the fibers, resulting in a
ECC-1 A1
60
40
20
0 0
10
20
30
40
Midspan deflection (mm) Fig. 18. Load–deflection curves of beams A1, A2, ECC-1, and ECC-2 [1].
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(a) Cracking patterns of beam A2
(b) Cracking patterns of beam ECC-2
Fig. 19. Cracking patterns in beams A2 and ECC-2 around the loading point [1].
P/2 ∅6
P/2
mm @ 80mm c/c FO
250
720
45mm clear cover
560
720
250
2500 Fig. 20. Details of medium-scale beam specimens (all dimensions in mm) [5].
Fig. 21. Corrosion status of steel in (a) OPCC-2 beam and (b) FGC-3 beam [5].
5. Effective FRP-strengthening of RC beams The applications of Fiber Reinforced Polymer (FRP) composites to concrete structures had been studied intensively over the past few years in view of the many advantages that FRPs possess. While FRPs had been shown to be effective in strengthening RC beams, strength increases had generally been associated with reductions in the beams’ deflection capacities due to premature debonding. Debonding failure modes occurred mainly due to interfacial shear and normal stress concentrations at FRP-cut off points and at flexural cracks along the RC beam. Maalej and Leong [1] suggested that if the quasi-brittle concrete material which surrounds the main flexural reinforcement was replaced with a ductile layer of hybrid fiber ECC as shown in Fig. 17, then it would be possible to delay the debonding failure mode and hence increase the deflection capacity of the strengthened beam. If hybrid fiber ECC was introduced in a RC member, increased number of cracks with smaller crack widths would be expected to form on the beam tensile face rather than fewer but wider
cracks in the case of an ordinary concrete beam. Such distributed finer cracks would be expected to reduce crack-induced stress concentration and result in a more efficient stress distribution in the FRP layer. The objectives of the above-study were to investigate both experimentally and numerically the structural performance of FRP-strengthened RC beams incorporating a ductile hybrid fiber ECC layer around the main flexural reinforcement. The load-carrying and deflection capacities as well as the maximum FRP strain at failure were used as criteria to evaluate the performance. The experimental program included two series of RC beams where one series consisted of two ordinary RC beams (beams A1 and A2) and another series consisted of two hybrid fiber ECC layered beams (ECC-1 and ECC-2). In each series, one specimen was strengthened using externally-bonded carbon fiber reinforced polymer (CFRP) sheets while the second was kept as a control1 in
1
Specimens A1 and ECC1 are control specimens without any CFRP reinforcement.
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FOSS Reading (με)
2000 OPCC-2 (1686 με)
1600 1200
240
800 400
210
FGC-2 (277 με )
0
25
50
75
100
125
∅ 6 mm @ 120mm
Fiber opc cable 3 -16mm
300
FGC-3 (442 με )
0
2 -13mm FOSS
150
Time (days) Fig. 22. Corrosion-induced tensile strain in concrete as measured by FOSS gauge [5].
between the CFRP and the concrete beam. This delayed intermediate crack-induced interfacial debonding and resulted in higher strengthening ratio and higher deflection capacity and, therefore, a more effective use of the CFRP material. Further, despite the large distance between the support and the CFRP cut-off point in beam ECC-2, plate-end debonding or concrete cover peeling were not observed. This is expected due to the high fracture energy of the hybrid fiber ECC material. The above experimental results showed that the hybrid fiber ECC had indeed delayed debonding of the CFRP and resulted in effective use of the CFRP material. With the use of hybrid fiber ECC as a ductile layer, RC beams can effectively be strengthened while minimizing loss in deflection capacity.
6. Corrosion-resistant RC beams
Fig. 23. Load–deflection response of un-corroded and corroded beams (a) OPCC beams (b) FGC beams [5].
order to compare its load–deflection behavior under third-point loading with the strengthened specimen. It could be seen from Fig. 18 that the CFRP strengthened beams with a hybrid fiber ECC layer (e.g. beam ECC-2) depicted higher load-carrying capacity compared to their ordinary reinforced concrete counterparts (e.g. beam A2). If expressed in term of strengthening ratio (SR, defined as the strength of beam with CFRP reinforcement divided by the strength of control beam), beam ECC-2 had a strengthening ratio of about 1.43, compared to 1.28 for beam A2. Also, beam ECC-2 was noted to have a significantly higher deflection capacity (29.6 mm) at peak load compared to beam A2 (21.9 mm). On the cracking behavior, both ECC-1 and ECC-2 showed a considerable number of fine cracks compared to the ordinary RC beams (beam A1 and A2) as revealed in Fig. 19. As a result, the crack spacings were consequently much smaller in the hybrid fiber ECC beams (particularly in ECC-2) than in the ordinary RC beam. These multiple but fine cracks played a major role in reducing crack-induced stress concentration resulting in more efficient stress distribution in the CFRP sheet and a better stress transfer
As an alternate method of improving the corrosion resistance of RC beams exposed to aggressive substances, Maalej and Li [6] proposed a new design for RC flexural members where part of the concrete which surrounds the main flexural reinforcement is replaced with a strain hardening ECC material as shown in Fig. 17. This alternate design with layered ECC had been referred to as Functionally-Grade Concrete (FGC). It was suggested that the ECC material in FGC beams could provide two levels of protection. First, it could prevent the migration of aggressive substances into the concrete, therefore, preventing reinforcement corrosion. Second, in the extreme case when corrosion initiates, accelerated corrosion due to longitudinal cracks would be reduced (if not eliminated), and spalling and delamination problems common to many of today’s RC structures would be prevented. This is expected due to the high strain capacity and fracture resistance of the ECC material. Maalej et al. [5] adopted the above concept to prepare a series of FGC beams where the main longitudinal reinforcements were surrounded by a hybrid fiber reinforced mortar material exhibiting strain hardening and multiple cracking under third-point flexural loading. The hybrid fiber reinforcement consisted of high modulus (steel) and low modulus (PVA) fibers used with respective volume fractions of 1% and 1.5%. The objective of the study was to evaluate the effectiveness of the FGC concept in retarding the corrosion of steel reinforcement in RC beams and reducing the tendency of the concrete cover to delaminate as measured by a concrete embeddable fiber optic strain sensor (FOSS). The effects of steel loss and corrosion damage on the flexural response of RC beams were also evaluated. The experimental program included 2 control RC beams (OPCC beams) made of ordinary Portland cement concrete and 3 FGC beams with geometrical detail as shown in Fig. 20. At any given time, the FGC beams were found to exhibit lower level of steel loss than the OPCC beam (see Fig. 21). It was also observed that an FGC beam took about 70% longer time to achieve the same level of induced steel loss compared to an OPCC beam. The
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Fig. 24. Typical cracking patterns of corroded beams (a) OPCC-2 at about peak load; (b) close-up of region highlighted in (a) after failure; (c) FGC-3 at about peak load; (d) close-up of region highlighted in (c) after failure [5].
1000
Estimated amount of corrosion [g]
Plain mortar SHFRCC (PE fiber)
100 2
Icorr (μA/cm )
SHFRCC (hybrid steel-Pe fiber)
10 1 0
10
20
30
40
50
60
70
80
90
100
0.1 0.01 0.001
Time (days)
Fig. 25. Progress of corrosion current (Icorr values) of the beams made with plain mortar, ECC containing polyethylene (PE) fiber and ECC containing hybrid steel–PE fiber [19].
better performance of the FGC beams over the OPCC beam was also evident from the absence of any corrosion-induced cracks or damage and the lowest tendency for the concrete cover to delaminate as measured by a concrete embeddable FOSS (see Fig. 22). The
(a) Plain mortar
(b) ECC containing polyethylene (PE)
(c) ECC containing hybrid steel-PE fiber Fig. 26. Corrosion induced damage of beams after 14 weeks of accelerated corrosion test [19].
Time [weeks] Mortar
FRCC
HFRCC
Fig. 27. Amount of corrosion (estimated by Faraday’s law) with progress of corrosion in a long term accelerated corrosion test for 1 year (note: FRCC denotes ECC containing polyethylene (PE) fiber and HFRCC denotes ECC containing hybrid steel–PE fibers) [20].
load–midspan displacement response of the test specimens indicated that for the same level of steel loss, the FGC specimens exhibited higher residual load and deflection capacities compared to their OPCC counterpart (see Fig. 23). While a corroded OPCC beam was found to experience widening of corrosion-induced cracks, delamination and spalling during loading, no such behavior was observed in the FGC beams (see Fig. 24). The FGC concept was found in the above study to be very effective in preventing corrosion-induced damage in RC beams and minimizing the loss in the beam’s load and deflection capacities. Similar results had also been reported in another study [17] where FGC beams were subjected to an accelerated corrosion regime following initial pre-cracking under service load, and where some of the specimens were subjected to corrosion under sustained loading. The latter study indicated that the development of multiple fine flexural cracks in the FGC beams (as opposed to few large cracks in an ordinary RC beam) did not accelerate the corrosion process, but rather it had slowed it down resulting in reduced corrosion damage. This outcome was consistent with results published by Tsukamoto [18] who studied the tightness of plain and fiber reinforced concrete and found that the flow rate (of water) scales with the third power of crack width (d3) and under a certain crack width (critical crack width2) no further flow occurred.
2
Which is in the range of 0.10–0.15 mm for the material studied.
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Week
Mortar
Hybrid fiber ECC
Mono fiber ECC
th
30
th
40
nd
52
2.0
(a) MT0 MT1 MT2 MT3 MT4
1.5 1.0 0.5 0.0 0
20
40
Expansion of crack width (mm)
Expansion of crack width (mm)
Fig. 28. Time-dependent change of corrosion induced damage in the specimens made with plain mortar, ECC containing PE fiber and hybrid steel–PE fiber ECC [20].
2.0
(b) FR0 FR1 FR3 FR4 FR5
1.5 1.0 0.5 0.0 0
60
20
Time (week)
40
60
Expansion of crack width (mm)
Time (week) 2.0
(c)
1.5 HF0 HF1 HF3 HF5
1.0 0.5 0.0 0
20
40
60
Time (week) Fig. 29. Change of the width of corrosion induced longitudinal crack in the beams having different initially given crack widths in (a) mortar, (b) ECC containing PE fiber (represented as FR in the figure) and (c) ECC containing steel–PE hybrid fiber (represented as HR in the figure) [21].
In more recent studies [19–21], the corrosion durability and the resistance to corrosion-induced damage of steel reinforced beams incorporating hybrid steel–polyethylene fiber ECC and polyethylene monofiber ECC were studied. Ahmed and Mihashi [19] evaluated the corrosion resistance of cracked steel reinforced beams made with mono and hybrid fiber ECC materials as well as plain mortar. The monofiber ECC contained polyethylene (PE) fiber of 1.5% by volume, while the hybrid fiber ECC contained both steel and PE fibers of 0.75% each with a total fiber volume fraction of 1.5%. The beam specimens were 100 100 400 mm in dimension and were pre-cracked under three-point bending with a maximum load of about 80% of their corresponding calculated ultimate load. The mono and hybrid fiber ECC beams exhibited numerous multiple cracks with an average crack width of 0.05 mm and 0.026 mm, respectively, while the mortar beam exhibited a single crack of 0.2 mm width. The corrosion resistance of the beam specimens was measured electrochemically using mini sensors placed on reinforcing bar in each beam. The test results indicated improved corrosion resistance of the (steel reinforced) hybrid fiber ECC beams in terms of lowest measured corrosion current (Icorr values) in comparison to the ordinary mortar and monofiber ECC (Fig. 25) counterparts. Better resistance to corrosion-induced damage of the hybrid fiber ECC specimens was also observed in this study as shown in Fig. 26. The improved performance of the hybrid fiber ECC specimens could be attributed to the better tensile strain
Fig. 30. Typical corrosion induced cracking in (a) mortar, (b) ECC containing PE fiber and (c) ECC containing steel–PE hybrid fiber beams [21].
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hardening characteristics of the hybrid fiber ECC material in comparison to the monofiber ECC and mortar. In a follow-up investigation, Mihashi et al. [20] subjected (steel reinforced) hybrid fiber ECC, monofiber ECC and plain mortar beams to accelerated corrosion for 1 year. The corrosion current was measured and used to calculate the steel loss according to Faraday’s second law. The authors reported superior corrosion resistance of the hybrid fiber ECC beams in terms of lowest steel loss (Fig. 27) and least corrosion induced damage (Fig. 28), in comparison to the monofiber ECC and plain mortar counterparts. Mihashi et al. [21] also monitored the corrosion-induced cracking of cracked beams containing mortar, monofiber ECC and hybrid fiber ECC. The width of longitudinal cracks, which formed in the beams due to steel corrosion, was measured during the accelerated corrosion test. The results indicated increased crack width in the mortar specimens with increased corrosion exposure time. No such increase was observed in the hybrid fiber ECC specimen (Fig. 29). This was also evident by comparing the corrosion induced damage of the hybrid fiber ECC beams and mortar beams at any given time (see Fig. 30). 7. Concluding remarks In reviewing the potential structural applications of hybrid fiber Engineered Cementitious Composites (ECCs), it becomes clear that the ability of the material to strain harden through the development of multiple fine cracks had imparted significant enhancement to the structural response of the element. For elements that were subjected to impact loading, the multiple cracking and the large strain capacity had resulted in a significant improvement in the element capacity to absorb impact energy, and therefore minimize structural damage. For corrosion protection and FRP strengthening of RC members, the fine cracking had led to reduced corrosion activities in the member and delayed debonding of the FRP reinforcing sheet indicative of reduced stress concentration. Further, the high fracture energy of the hybrid fiber ECC material had practically eliminated spalling, scabbing, and fragment ejection in the hybrid fiber ECC panels that were subjected to drop weight impact, and had also prevented corrosion-induced concrete cover spalling in the RC beams that were subjected to accelerated corrosion, and peeling of the concrete cover in the FRP strengthened beams. The high strain capacity, fracture energy and damage tolerance of the hybrid fiber ECC as well as the characteristics of its microcraking response make the material highly desirable in numerous other potential applications. For instance, more recent test results [22] that have not been discussed in the present paper, have shown that the use of hybrid fiber ECC in the plastic zone of type-2 beam column connections as a replacement of concrete and partial replacement of transverse (confinement) reinforcement can significantly enhance the joint shear resistance, energy absorption capacity, and cracking response, thereby, enhancing the joint seismic resistance, and reducing reinforcement congestion and construction complexity. While hybrid fiber ECC has such significant potential for use in numerous applications in the construction industry, its actual universal exploitation will depend, first, on the ability to prepare
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and place hybrid fiber ECC mixes that are of consistent fresh and hardened properties in ways that do not significantly deviate from common techniques. Second, as fibers used in hybrid fiber ECC mixes are of such high performance (and cost impact), fiber materials meeting specific design requirement as per available micromechanical models shall become readily available at costs that can be justified based on life-cycle cost analysis. In this regard, the use of more than one type of fiber (leading to a hybrid fiber composite) will offer additional flexibility in the design of the composite that meet the specific requirement for a given application while at the same time reducing the associated life cycle cost. References [1] Maalej M, Leong KS. Engineered cementitious composites (ECCs) for effective FRP-strengthening of RC beams. Compos Sci Technol 2005;65(7–8):1120–8. [2] Ahmed SFU, Maalej M, Paramasivam P. Analytical model for tensile strainhardening and multiple-cracking behavior of hybrid fiber engineered cementitious composites. ASCE J Mater Civ Eng 2007;19(7):527–39. [3] Maalej M, Quek ST, Zhang J. Behavior of hybrid-fiber engineered cementitious composites subjected to dynamic tensile loading and projectile impact. ASCE J Mater Civ Eng 2005;17(2):143–52. [4] Zhang J, Maalej M, Quek ST. Performance of hybrid-fiber ECC blast/shelter panels subjected to drop weight impact. ASCE J Mater Civ Eng 2007;19(10):855–63. [5] Maalej M, Ahmed SFU, Paramasivam P. Corrosion durability and structural response of functionally-graded concrete beams. J Adv Concr Technol 2003;1(3):307–16. [6] Maalej M, Li VC. Introduction of strain hardening engineered cementitious composites in the design of reinforced concrete flexural members for improved durability. ACI Struct J 1995;92(2):167–76. [7] Li VC, Wu HC, Maalej M, Mishra DK, Hashida T. Tensile behavior of engineered cementitious composites with discontinuous random steel fibers. J Am Ceram Soc 1996;79(1):74–8. [8] Maalej M, Hashida T, Li VC. Effect of fiber volume fraction on the off-crackplane fracture energy in strain hardening engineered cementitious composites. J Am Ceram Soc 1995;78(12):3369–75. [9] Ahmed SFU, Maalej M. Tensile strain hardening behaviour of hybrid steel– polyethylene fibre reinforced cementitious composites. J Constr Build Mater 2009;23(1):96–106. [10] Li VC, Leung CKY. Steady state and multiple cracking of short random fiber composites. ASCE J Eng Mech 1992;118(11):2246–64. [11] Li VC. Post-crack scaling relations for fiber reinforced cementitious composites. ASCE J Mater Civ Eng 1992;4(1):41–57. [12] Maalej M, Li VC, Hashida T. Effect of fiber rupture on tensile properties of short fiber composites. ASCE J Eng Mech 1995;121(8):903–13. [13] Jenq YS, Shah SP. Two parameter fracture model for concrete. ASCE J Eng Mech 1985;111(10):1227–41. [14] Banthia N, Mindess S, Bentur A, Pigeon M. Impact testing of concrete using a drop weight impact machine. Exp Mech 1989;29(2):63–9. [15] Li M, Li VC. Behaviour of ECC/concrete layered repair system under drying shrinkage conditions. J Restor Build Monument 2006;2:143–60. [16] Maalej M, Lin VWJ, Nguyen MP, Quek ST. Engineered cementitious composites for effective strengthening of unreinforced masonry walls. Eng Struct (Elsevier, UK) 2010;32(8):2432–9. [17] Maalej M, Chhoa CY, Quek ST. Effect of cracking, corrosion and repair on the frequency response of RC beams. Constr Build Mater (Elsevier) 2010;24(5):719–31. [18] Tsukamoto M. Tightness of fiber concrete. Darmstadt concrete. Ann J Concr Concr Struct 1990;5:215–25. [19] Ahmed SFU, Mihashi H. Corrosion durability of strain hardening fiber reinforced cementitious composites. Aust J Civ Eng 2010;8(1):27–39. [20] Mihashi H, Ahmed SFU, Kobayakawa A. Corrosion of reinforcing steel in fiber reinforced cementitious composites. J Adv Concr Technol 2011;9(2):159–67. [21] Ahmed SFU, Mihashi H, Kobayakawa A. Influence of crack widths on corrosion of reinforcing steel bar in fiber reinforced cementitious composites. In: The proceedings of RILEM conference on Advances in construction materials through science and engineering, Hong Kong; 2011. [22] Al-Qudah SM. Enhancing the seismic behavior of beam-column joints using engineered cementitious composites (ECCs). Master thesis, Department of Civil and Environmental Engineering, University of Sharjah; 2011.