- Email: [email protected]
Water permeability of Eco-Friendly Ductile Cementitious Composites (EDCC) under an applied compressive stress
Cement and Concrete Composites 107 (2020) 103500
Contents lists available at ScienceDirect
Cement and Concrete Composites journal homepage: http://www.elsevier.com/locate/cemconcomp
Water permeability of Eco-Friendly Ductile Cementitious Composites (EDCC) under an applied compressive stress Qiannan Wang a, *, Nemkumar Banthia b, Wei Sun c, Chunping Gu d a
School of Civil Engineering and Architecture, Zhejiang University of Science and Technology, Hangzhou, Zhejiang, 310023, China Department of Civil Engineering, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada c School of Materials Science & Engineering, Southeast University, Nanjing, Jiangsu, 211189, China d College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou, Zhejiang, 310023, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Permeability Eco-Friendly Ductile Cementitious Composite (EDCC) Water flow Compressive stress Critical stress level
Permeability of concrete is one of its most important characteristics determining the durability. In this study, water permeability of a new class of fiber reinforced concrete materials called Eco-Friendly Ductile Cementitious Composites (EDCC) was studied, with and without an applied compressive stress. Hollow-core specimens were used and permeability tests were performed under full flow-equilibrium conditions. Four applied stress levels of 0.3fu, 0.4fu, 0.5fu and 0.6fu were investigated, where fu is the compressive strength of the EDCC in question. Permeability tests under identical conditions were carried out on plain control specimens without fiber rein forcement (termed Plain Cementitious Composites, PCC). The results indicated that the permeability of un stressed specimens declined over time due to the continuous hydration. A ‘critical’ compressive stress level (fcc) was identified and defined, which when exceeded, a dramatic increase in the coefficient of permeability occurred. The ‘critical’ compressive stress level (fcc) was noted to be between 0.5fu ~ 0.6fu for EDCC and 0.4fu ~ 0.5fu for PCC. In other words, EDCC was more damage tolerant than PCC, and even when fcc was exceeded, the impact of stress on the permeability of EDCC was far less pronounced compared to PCC.
1. Introduction Concrete carries flaws and micro-cracks both in the material and at the interfaces even before an external load is applied. Under an applied load, distributed micro-cracks propagate, coalesce and align themselves to produce macro-cracks, resulting in greatly compromised durability of concrete structures [1]. Fortunately, the micro and macro-fracturing processes can be favorably modified by adding short, randomly distributed fibers of various suitable materials, resulting in fiber rein forced concrete. During the last two decades, significant efforts have been made towards developing high performance fiber reinforced cementitious composites (HPFRCC) that demonstrate significant in creases in crack growth resistance as manifested by stress-strain curves that depict large ultimate strains. Engineered Cementitious Composites (ECC) is one promising type of HPFRCCs, depicting elasto-plastic and strain-hardening response with very tight crack widths [2]. These unique behaviors result from an elaborate design using a micro-mechanical model taking into account the interactions among fiber, matrix and the fiber-matrix interface [3].
The typical fiber used in ECC is polyvinyl alcohol (PVA) fiber with a diameter of 39 μm and a length of 6–12 mm [4]. The main problem of using PVA fiber in producing ECC is that PVA fiber tends to develop a very strong chemical bond with the matrix due to the presence of the hydroxyl group in its molecular chains. This high chemical bond leads to probable fiber ruptures which limits the tensile strain capacity [5]. In order to achieve strain-hardening behavior and high ductility, the strength of the chemical bond should be reduced. Li et al. [6] found that the fiber/matrix interfacial bond could be reduced by applying oil coating to the fiber surface. As a result, it is now generally accepted to prepare ECC with oil-coated PVA fibers. Moreover, matrix fracture toughness has to be limited to achieve strain-hardening. Therefore, the production of standard ECC mixtures has been restricted to the use of fine aggregate such as microsilica sand [7]. Plus, ECC materials contain considerably higher cement contents than conventional concrete. Such matrices and the use of surface-treated PVA fibers result in undesired high material costs and processing complications. They also increase the carbon footprint of ECC materials and make them unsustainable. Preliminary work [8,9] was reported on a new class of ECC-like
* Corresponding author. E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.cemconcomp.2019.103500 Received 30 September 2018; Received in revised form 10 September 2019; Accepted 26 December 2019 Available online 28 December 2019 0958-9465/© 2019 Elsevier Ltd. All rights reserved.
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
materials with ‘non-oiled’ PVA fibers and natural sand (1.19 mm maximum grain size) under the guidance of micromechanical principles. These materials also carried large amounts of supplementary cementi tious materials (SCMs). With their high strain tolerance and minimal amounts of cement, we are calling these materials Eco-Friendly Ductile Cementitious Composites (EDCC). EDCCs are arguably an evolved variation on ECC and depict similar constitutive characteristics. Due to its superior resistance to cracking, ECC is expected to demonstrate improved durability than normal concrete, especially under external loading. Permeability of concrete is regarded as a basic indicator of its durability [10]. Extensive work has been carried out to understand the permeability of normal concrete under an applied compressive stress [11,12]. It is shown that for normal concrete, the response is highly dependent on the level of applied compressive stress. At low levels of compressive stress, some researchers report a modest decrease in permeability [13,14], likely occurring due to pore-compression. This phenomenon however does not appear to be universal as some researchers observed no decrease in permeability under an applied compressive stress [15]. One universal agreement, however, is in the fact that when the compressive stress reaches a certain threshold value, often called the ‘critical’ stress level, permeability in creases rapidly [13,16]. This is often explained by the creation of in ternal cracks that abruptly coalesce when the ‘critical’ stress level is attained causing a rapid increase in the permeability. Lepech and Li [17] measured the permeability of a typical ECC mixture which was called ECC M45, and found its permeability coeffi cient at 28d was 8.18 � 10 12 m/s. This value is lower than the permeability coefficient of regular concrete, resulting from the relatively low water content of ECC. Several other studies have also studied the permeability of ECC under tension [17–20]. It has been generally demonstrated that due to the narrow cracks, the permeability of ECC remained low even after the formation of numerous microcracks and tensile strain of up to 3%. No data exists on performance of ECC under compressive loading. The present study aims to investigate the permeability of EDCC under an applied compressive stress. The specific objectives are: (1) to conduct permeability tests on EDCC specimens under full flowequilibrium conditions; (2) to monitor the evolution of EDCC’s perme ability at very early ages (2d to 5d); and (3) to investigate the effects of compressive stress and fiber on permeability of EDCC.
Table 2 Properties of PVA fibers.
Table 1 Mixture constituents and proportions (kg/m3) of EDCC and PCC. 385 770 77 462 333 1.7 0
Elastic modulus, GPa
PVA
13
8
40
1560
40
The permeability test setups were adapted from Banthia and Bhar gava [21]. A schematic representation of the apparatus designed for carrying out water permeability tests on concrete with the presence of an applied compressive stress is shown in Fig. 2. In a typical test, two permeability cells were assembled with iden tical specimens in them. One of the cells (Cell A) was placed in a uni versal testing machine (UTM) where a constant compressive stress could be applied. The other cell (Cell B) remained outside the UTM under the condition of no stress. Water was allowed to permeate through both the cells under identical flow conditions. Early studies with asphaltic and pervious concretes [22,23] indicated that permeability of concrete decreased slightly as the water pressure increased. However, the dif ference in permeability measured under different water pressures became insignificant when the water pressure exceeded 10 psi (0.07 MPa). One of the concerns with water pressure is that in young concrete with poorly developed strengths, internal damage may occur at higher water pressures. Equally relevant is the fact that at low water pressures, the accuracy of the test results may be compromised. Based on some trial tests, a constant inflow water pressure of 50 psi (0.35 MPa) was adopted for permeability tests in this study. This water pressure was 3% of the compressive strength (11.11 MPa) for the tested specimens at 48 h. Therefore, it was assumed to have no deleterious effect on the properties and deformations of the tested specimens. The setup uses cylindrical specimens that are 102 mm in diameter and 204 mm long with a 73 mm diameter hollow cylindrical core. Such specimens were cast with both EDCC and PCC, demolded 24 h after casting, and then assembled in the permeability cells [21]. Tests started 48 h after casting and were carried out until the age of 5d. No stress was applied on either specimen at first. At the age of 3d, a certain compressive load was applied on the specimen in Cell A to observe if there is any change in permeability caused by loading. The load was kept constant for one day and removed from the specimen at the age of 4d. The test was then carried on for another day, until the age of 5d. Four applied stress levels of 0.3fu, 0.4fu, 0.5fu and 0.6fu, where fu represents the compressive strength (determined with hollow cylindrical
Cylindrical specimens (75 mm in diameter and 150 mm long) were
PCC
Tensile strength, MPa
2.4. Permeability test
2.2. Compressive strength test
385 770 77 462 333 2 26
Diameter, μm
The uniaxial tensile test was performed on contoured specimens at the age of 28d. A closed-loop controlled Instron testing system was used in displacement-controlled mode to conduct the tensile test (Fig. 1). The specimens were gripped using mechanical wedge grips. The grip jaws must be carefully adjusted before test to make sure the specimen ends were aligned in vertical. When loading a specimen, the bottom grip jaw was locked open, the specimen was placed and clamped securely within the upper grip jaw first, and then the lower grip jaw was unlocked, providing clamping force on the specimen. The dog-bone shaped spec imens could avoid fractures at the gripping ends during test. Two LVDTs were connected to a data acquisition system to measure displacements over a gauge length of 60 mm. The displacement rate used was 0.002 mm/s.
Based on preliminary tests [8], the mixture constituents and pro portions of EDCC with “non-oiled” PVA fibers and natural sand were determined and are given in Table 1. The volume fraction of PVA fiber was 2% and its properties are shown in Table 2. Plain Cementitious Composite (PCC) without the fiber served as the control. Notice a slightly reduced amount of superplasticizer (1.7 l/m3) in PCC.
EDCC
Length, mm
2.3. Uniaxial tensile test
2.1. Material composition
Cement, ASTM Type I Fly ash, Class C Silica fume Natural sand, maximum size of 1.19 mm Water Superplasticizer (l/m3) PVA fiber
Density, kg/m3
cast to determine the compressive strength. The specimens were cured in water at 20 � C until tested. The ends of cylinders were ground before testing to ensure a flat and parallel surfaces and a better contact with the loading platen.
2. Test program
Ingredient
Type of fiber
2
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Fig. 1. Uniaxial tensile test set-up: (a) specimen configuration; (b) testing system.
Fig. 2. Setup for water permeability tests.
specimens) of EDCC/PCC at the age of 3d, were investigated. The mass of the water permeated through the two cells was collected in separate collection reservoirs and measured as a function of time. The water collected was related to the coefficient of water permeability (KW ) by applying Darcy’s law [24]: KW ¼
Q⋅lnðr2 =r1 Þ 2πh⋅△H
Eq. (1) clearly applies to flow conditions that are under equilibrium (input volume ¼ output volume) and hence it is critical that the mea surements be carried out only after the conditions of equilibrium are established. Typically, it took approximately 1 h after the start of the test to achieve condition of full flow-equilibrium. Data were recorded only after equilibrium was established.
(1)
3. Results and discussion
where, KW equals the coefficient of water permeability (m/s); Q equals the rate of water flow (m3/s); r1 and r2 are the inner radius and outer radius of the specimen (m); h equals the height of the specimen (m); and ΔH is the difference in hydraulic head between the inner and the outer sides of the specimen (m).
3.1. Compressive strength The compressive strengths of EDCC and PCC specimens as a function of age are shown in Fig. 3. The addition of PVA fibers did not alter the 3
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
compressive strength much. EDCC marginally exceeded the strength of PCC at 28d. Overall, the compressive strengths of EDCC and PCC were very close. 3.2. Uniaxial tensile performance The uniaxial tensile stress-strain curves of EDCC and PCC specimens at 28d are presented in Fig. 4. As expected, PCC specimens depicted a brittle response with a low tensile strength of about 1.83 MPa. On the other hand, EDCC specimens exhibited an elasto-plastic behavior with minor strain-hardening and a tensile strength of 4.43 MPa. After the first cracking in EDCC, the load continued to increase concurrently with multiple cracking, which contributed to the inelastic strain capacity as high as 3.89%. The crack opening measured after unloading was be tween 40–80 μm, and the crack spacing was about 2 mm as shown in Fig. 5. The tensile properties of EDCC were equivalent to typical ECC, which has a tensile strength of 4–6 MPa and a strain capacity of 3–5% [2]. Unlike regular ECC mixtures, the EDCC specimens in this study contained silica fume and high volume of fly ash. The addition of silica fume improved the fiber dispersion and led to a higher fiber-matrix interface frictional bond [25], both of which likely benefited the stain-hardening behavior. Increasing the fly ash content will reduce the fiber-matrix adhesive bond as well as the matrix toughness, while increasing the interfacial frictional bond, in favor of attaining high tensile strain capacity [26]. Therefore, the EDCC formulation allowed for both a reduced cement content and the use of non-oiled PVA fibers and natural sand without reducing the final strain capacity.
Fig. 4. Uniaxial tensile performance of EDCC and PCC specimens.
3.3. Permeability of unstressed specimens The permeability plots of the unstressed EDCC and PCC specimens, each accompanying a stressed specimen in each test, are presented in Fig. 6. Each point in the plots represents an average permeability coef ficient measured at 15 min interval. When equilibrium conditions are not established in a test, the permeability plots often tend to be jiggly. Very stable curves for unstressed specimens in Fig. 6 indicate that equilibrium flow conditions in these tests can be assumed. Notice also that the permeability plots for both EDCC and PCC in unstressed con ditions exhibited a steady decline. This decline in permeability can be assumed to be entirely due to continuous hydration in the specimens. Hydration processes are expected to be very active in these young specimens and significant truncation in the permeability paths and increased tortuosity in cracks are expected [27]. Likewise, the
Fig. 5. Cracking pattern of EDCC specimen after unloading.
microstructure of the paste is expected to get denser as the hydration progresses. Not surprisingly, therefore, the permeability of both EDCC and PCC decreased and the compressive strength increased (Fig. 3) over time as the hydration continued. Permeability of concrete is a highly variable property and is sensitive to factors such as material composition, casting and curing conditions and test details. One thing noticeable in the plots of Fig. 6 is that while various specimens started with widely different permeability co efficients at the age of 2d, plots essentially merged at 5d and the dif ferences diminished. Different specimens would carry different surface flaws at the start of the test induced by casting and demolding, which would tend to undergo healing and pore-blockage with time. This would, in time, reduce variability and bring specimen responses closer to each other. The averaged permeability plots of unstressed EDCC and PCC spec imens are shown in Fig. 7. The permeability coefficient of EDCC at 2d was 1.50 � 10 10 m/s, and it decreased gradually to 3.82 � 10 11 m/s by the age of 5d. The average permeability of PCC specimens was 2.59 � 10 10 m/s at the age of 2d, which was higher than the EDCC specimens. The difference vanished almost entirely by the age of 5d. The perme ability of PCC specimens at 5d was 4.26 � 10 11 m/s, which was very close to that of EDCC. The higher initial permeability of PCC may be due to the microcracks caused by casting and demolding, which were then healed to some extent during the permeability tests. The initiation and propagation of such cracks were likely inhibited in EDCC by the fibers, resulting in a lower permeability.
Fig. 3. Compressive strengths of EDCC and PCC. 4
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Fig. 6. Permeability plots of unstressed (a) EDCC specimens and (b) PCC specimens.
permeability of different specimens. When EDCC specimens were subjected to a compressive stress level of 0.6fu, there was an instant increase in permeability, as seen in Fig. 8 (d). The permeability coefficient rose from 8.58 � 10 11 m/s to 1.23 � 10 10 m/s. Interestingly, there was also an instant decrease in perme ability (from 8.30 � 10 11 m/s to 7.60 � 10 11 m/s) when the load was removed at the age of 4d. This meant that the cracks caused by loading closed to some extent when the stress was removed. Also, the hydration continued after the load was removed. 3.5. Permeability of PCC specimens under compression When a compressive stress level of 0.3fu or 0.4fu was applied to PCC specimens, no obvious change in the permeability was observed (Fig. 9). This is consistent with EDCC. However, when a stress of 0.5fu was applied, there was a sudden jump in the permeability. Also, as seen in the case of EDCC, the permeability decreased when the specimen was unloaded. When the compressive stress reached 0.6fu, the permeability of PCC increased by almost an order of magnitude. It is conceivable that as the load approached the peak value, microcracks propagated and interconnected with each other thus increasing the permeability [13]. As seen in Figs. 8(d) and 9(c), and especially Fig. 9(d), after a sudden jump in permeability due to loading at the age of 3d, permeability decreased rapidly in the first few hours of loading. The possible reasons for such instability including load relaxation, water hammer effect and self-healing, were discussed in Ref. [10].
Fig. 7. Averaged permeability plots of unstressed EDCC and PCC.
3.4. Permeability of EDCC specimens under compression The permeability plots of EDCC specimens under different levels of compressive stress are shown in Fig. 8 where the results from the com panion unstressed specimens are also shown for comparison. There was no obvious change in the permeability of EDCC caused by a compressive stress up to 0.5fu. Likewise, there seemed to be no difference in permeability when the specimens were unloaded after one day of loading with stress up to 0.5fu. This is consistent with the finding of Yang et al. [15], who studied the water permeability of concrete under deviatoric stress. On the other hand, a decrease in the permeability of concrete under low compressive stress could also be expected due to the consolidation or closing of pore and microcracks. The different re sponses of permeability at low levels of compressive stress, as discussed in Ref. [10], are caused by the composition, microstructure and porosity of concrete. It may be noticed that the permeability of specimen loaded under stress of 0.4fu (Fig. 8(b)) seemed to be higher than the one loaded under stress of 0.5fu (Fig. 8(c)). This was only related to the variable perme ability coefficients between different specimens. As discussed above, permeability of concrete is highly variable especially at early ages. Therefore, for early-aged concrete, it’s more reliable to study the effects of stress on permeability with the same specimen rather than comparing
3.6. Further discussion 3.6.1. Critical compressive stress level (fcc) As was done previously [10], here we define a ‘critical’ compressive stress level (fcc) at which a large increase in the permeability is expected. fcc is dependent not just on the composition of the material but also on its constitutive response. As the permeability plots in Figs. 8 and 9 show, the fcc was between 0.5fu ~0.6fu for EDCC specimens, and between 0.4fu ~0.5fu for the companion PCC specimens. This observation is in agreement with Banthia and Bhargava [16] who investigated the water permeability of fiber reinforced concrete and compared it to control without fibers. They also found a similar phenomenon to exist and concluded that the addition of fiber resulted in an increase in fcc. Hoseini et al. [11] reviewed the effect of fibers on the permeability of stressed concrete and concluded that permeability in concrete is affected more by cracks in the paste than by the paste/aggregate interface. The addition of fibers leads to changes in the crack profile whereby, instead of a few large cracks, a multitude of closely spaced microcracks form, especially 5
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Fig. 8. Permeability plots of EDCC specimens under compressive stress of (a) 0.3fu; (b) 0.4fu; (c) 0.5fu and (d) 0.6fu.
for EDCC with PVA fibers.
an enhanced damage tolerance in EDCC. Unlike normal concrete or fiber reinforced concrete, EDCC exhibits multiple cracking and tight crack widths under an applied stress. As shown in Fig. 5, the crack opening of EDCC after tensile test was between 40–80 μm. Even though the cracks opened up during loading, the crack widths were still much lower compared to normal concrete under similar stress conditions. Therefore, EDCC can offer much greater water tightness under stress compared to plain concrete. So far as ΔKU is concerned, ΔKU was much smaller compared with ΔKL for both EDCC and PCC specimens under stress levels beyond fcc.. This was due to two reasons: first, the cracks caused by loading couldn’t close completely when the specimen was unloaded, implying there was some permanent damage; and second, some self-healing occurred at the surfaces of the cracks during loading [29–31]. As a result, the crack opening was reduced and the permeability decreased accordingly before unloading, which also led to a smaller ΔKU compared with ΔKL. Moreover, Yang et al. [32] found that cracks with widths under 50 μm could be healed completely, and the extent of this self-healing di minishes with an increasing crack width. When the width exceeded 150 μm, the cracks could likely not heal. As discussed above, EDCC had much smaller crack widths under stress, therefore, the extent of self-healing in EDCC is expected to be higher than in normal concrete. The tight crack widths of EDCC under stress not only make it less permeable compared with PCC, they also promote self-healing, which are of great significance for improving structural durability.
3.6.2. Effects of PVA fibers on permeability of stressed specimens The effect of compressive stress on permeability can be further described by a change in permeability coefficient (ΔK). Let ΔKL represent an increase in permeability due to loading, and let ΔKU represent a decrease in permeability due to subsequent unloading. Acknowledging that there may exist an instability in the permeability plots due to loading as discussed above, the stressed permeability at the age of 3.125d (3 h after the stress was applied) was adopted as a representative permeability coefficient after loading [10]. When the specimens were loaded at stress levels which were lower than fcc, stress showed no in fluence on permeability. Therefore, ΔKL and ΔKU were both zero for these stress levels. The results of ΔKL and ΔKU for EDCC and PCC spec imens are given in Fig. 10. For PCC specimens, permeability increased by 2.57 � 10 11 m/s for a stress level of 0.5fu and 1.04 � 10 10 m/s for a stress level of 0.6fu, respectively. Clearly, the effect of stress on permeability got more remarkable as the stress level increased from 0.5fu to 0.6fu. It has been reported that the change of permeability caused by cracking is propor tional to the cube of the crack width [28]. In other words, permeability is highly sensitive to the crack width. A higher stress level leads to growing crack widths in the material, resulting in a rapidly increasing permeability. As shown in Fig. 10, the increase in permeability due to loading can be significantly reduced by adding fibers. The permeability of EDCC only increased by 1.54 � 10 11 m/s at a stress level of 0.6fu, which was almost a full order of magnitude lower than that of PCC. This indicated 6
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Fig. 9. Permeability plots of PCC specimens under compressive stress of (a) 0.3fu; (b) 0.4fu; (c) 0.5fu and (d) 0.6fu.
Cementitious Composites (EDCC). Permeability of EDCC was measured under a compressive stress and compared with plain (unreinforced) cementitious composites (termed PCC). The following conclusions were drawn: (1) With a high volume of fly ash and proper content of silica fume, EDCC mixture prepared with non-oiled PVA fibers and natural sand can attain strain-hardening behavior with an average strain capacity approaching 4%. (2) For both EDCC and PCC specimens under no stress, the perme ability coefficient exhibited a steady decline over time. This was contributed to ongoing hydration at early ages. The permeability of PCC was higher than EDCC at 2d, but the difference between the two diminished by the age of 5d. (3) A ‘critical’ compressive stress level (fcc) is defined in this study. This is the stress level at which there is a sudden increase in the permeability coefficient upon stress application. The critical compressive stress level (fcc) was noted to be between 0.5fu ~ 0.6fu for EDCC and 0.4fu ~ 0.5fu for PCC. Once the critical stress is removed, the permeability coefficient showed a sudden drop. (4) Compressive stress showed a much less pronounced effect on permeability of EDCC than PCC even under a critical compressive stress. The inherently tight crack widths of EDCC make it less permeable and promote the self-healing process in it. These fea tures of EDCC suggest that it is a much more durable construction material, especially under stress.
Fig. 10. Change in permeability coefficient caused by compressive stress.
4. Conclusions The study describes the development of low carbon footprint, elastoplastic cementitious composites called Eco-Friendly Ductile 7
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Declaration of competing interest
[14] I. Yurtdas, S.Y. Xie, N. Burlion, J.F. Shao, J. Saint-Marc, A. Garnier, Deformation and permeability evolution of petroleum cement paste subjected to chemical degradation under temperature, Transp. Porous Media 86 (3) (2011) 719–736. [15] H. Yang, Y. Jia, S.Y. Xie, J.F. Shao, An experimental and numerical investigation of the mechanical behaviour of a concrete and of its permeability under deviatoric loading, Transp. Porous Media 102 (3) (2014) 427–454. [16] N. Banthia, A. Bhargava, Permeability of stressed concrete and role of fiber reinforcement, ACI Mater. J. 104 (1) (2007) 70–76. [17] M.D. Lepech, V.C. Li, Water permeability of engineered cementitious composites, Cement Concr. Compos. 31 (10) (2009) 744–753. [18] H. Liu, Q. Zhang, C. Gu, H. Su, V. Li, Influence of microcrack self-healing behavior on the permeability of Engineered Cementitious Composites, Cement Concr. Compos. 82 (2017) 14–22. [19] H. Liu, Q. Zhang, C. Gu, H. Su, V.C. Li, Influence of micro-cracking on the permeability of engineered cementitious composites, Cement Concr. Compos. 72 (2016) 104–113. [20] C. Wagner, B. Villmann, V. Slowik, V. Mechtcherine, Water permeability of cracked strain-hardening cement-based composites, Cement Concr. Compos. 82 (2017) 234–241. [21] A. Bhargava, N. Banthia, Measurement of concrete permeability under stress, Exp. Tech. 30 (5) (2006) 28–31. [22] K. Hall, J. Cruz, H. Ng, Effects of testing time and confining pressure on fallinghead permeability tests of hot-mix asphalt concrete, Transp. Res. Record J. Transp. Res. Board 1723 (1) (2000) 92–96. [23] Y. Qin, H. Yang, Z. Deng, J. He, Water permeability of pervious concrete is dependent on the applied pressure and testing methods, Adv. Mater. Sci. Eng. 2015 (2015) 1–6. [24] Y. Fang, Z. Wang, Y. Zhou, Time-dependent water permeation behavior of concrete under constant hydraulic pressure, Water Sci. Eng. 1 (4) (2008) 61–66. [25] J. Zhou, S. Qian, G. Ye, O. Copuroglu, K.V. Breugel, V.C. Li, Improved fiber distribution and mechanical properties of engineered cementitious composites by adjusting the mixing sequence, Cement Concr. Compos. 34 (3) (2012) 342–348. [26] S. Wang, V.C. Li, Engineered cementitious composites with high-volume fly ash, ACI Mater. J. 104 (3) (2007) 233–241. [27] H.N. Atahan, O.N. Oktar, M.A. Tas¸demir, Effects of water–cement ratio and curing time on the critical pore width of hardened cement paste, Constr. Build. Mater. 23 (3) (2009) 1196–1200. [28] V. Picandet, A. Khelidj, H. Bellegou, Crack effects on gas and water permeability of concretes, Cement Concr. Res. 39 (6) (2009) 537–547. [29] D. Palin, H.M. Jonkers, V. Wiktor, Autogenous healing of sea-water exposed mortar: quantification through a simple and rapid permeability test, Cement Concr. Res. 84 (2016) 1–7. [30] G. Yildirim, M. Sahmaran, M. Balcikanli, E. Ozbay, M. Lachemi, Influence of cracking and healing on the gas permeability of cementitious composites, Constr. Build. Mater. 85 (2015) 217–226. [31] H.-W. Reinhardt, M. Jooss, Permeability and self-healing of cracked concrete as a function of temperature and crack width, Cement Concr. Res. 33 (7) (2003) 981–985. [32] Y. Yang, M.D. Lepech, E.H. Yang, V.C. Li, Autogenous healing of engineered cementitious composites under wet–dry cycles, Cement Concr. Res. 39 (5) (2009) 382–390.
The authors declare that there are no known conflicts of interest associated with the submitted work and there has been no financial support for this work that could have influenced its outcome. Acknowledgements This work was supported by the Chinese Scholarship Council (CSC) and the Fundamental Research Funds for the Provincial Universities of Zhejiang (Grant no. 2019QN16), the experimental work was carried out in the materials lab, Department of Civil Engineering, University of British Columbia, Canada. References [1] N. Banthia, Fiber Reinforced Concrete, ACI SP-142ACI, Detroit, MI, 1994, pp. 91–119. [2] V.C. Li, From micromechanics to structural engineering-the design of cementitious composites for Civil engineering applications, Proc. Jpn. Soc. Civ. Eng. 10 (471) (1993) 37–48. [3] V.C. Li, C.K.Y. Leung, Steady-state and multiple cracking of short random fiber composites, J. Eng. Mech. 118 (11) (1992) 2246–2264. [4] E.H. Yang, S. Wang, Y. Yang, V.C. Li, Fiber-bridging constitutive law of engineered cementitious composites, ACT 6 (1) (2008) 181–193. [5] T. Kanda, Interface property and apparent strength of high-strength hydrophilic fiber in cement matrix, J. Mater. Civ. Eng. 10 (1) (1998) 5–13. [6] V.C. Li, C. Wu, S. Wang, A. Ogawa, T. Saito, Interface tailoring for strain-hardening polyvinyl alcohol-engineered cementitious composite (PVA-ECC), ACI Mater. J. 99 (5) (2002) 463–472. [7] M. Sahmaran, M. Lachemi, K.M.A. Hossain, R. Ranade, V.C. Li, Influence of aggregate type and size on ductility and mechanical properties of engineered cementitious composites, ACI Mater. J. 106 (3) (2009) 308–316. [8] Q. Wang, N. Banthia, W. Sun, Development of Engineered Cementitious Composites with Non-oiled Polyvinyl Alcohol Fibers and Natural Sand, Fiber Concrete, Prague, Czech Republic, 2013. [9] S. Soleimani-Dashtaki, S. Soleimani, Q. Wang, N. Banthia, C.E. Ventura, Effect of high strain-rates on the tensile constitutive response of Ecofriendly Ductile Cementitious Composite (EDCC), Procedia Eng. 210 (2017) 93–104. [10] Q. Wang, N. Banthia, W. Sun, Water permeability of repair mortars under an applied compressive stress at early ages, Mater. Struct. 51 (1) (2018) 6. [11] M. Hoseini, V. Bindiganavile, N. Banthia, The effect of mechanical stress on permeability of concrete: a review, Cement Concr. Compos. 31 (4) (2009) 213–220. [12] Y. Zhang, M. Zhang, Transport properties in unsaturated cement-based materials—a review, Constr. Build. Mater. 72 (2014) 367–379. [13] M. Choinska, A. Khelidj, G. Chatzigeorgiou, G. Pijaudier-Cabot, Effects and interactions of temperature and stress-level related damage on permeability of concrete, Cement Concr. Res. 37 (1) (2007) 79–88.
8
Contents lists available at ScienceDirect
Cement and Concrete Composites journal homepage: http://www.elsevier.com/locate/cemconcomp
Water permeability of Eco-Friendly Ductile Cementitious Composites (EDCC) under an applied compressive stress Qiannan Wang a, *, Nemkumar Banthia b, Wei Sun c, Chunping Gu d a
School of Civil Engineering and Architecture, Zhejiang University of Science and Technology, Hangzhou, Zhejiang, 310023, China Department of Civil Engineering, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada c School of Materials Science & Engineering, Southeast University, Nanjing, Jiangsu, 211189, China d College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou, Zhejiang, 310023, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Permeability Eco-Friendly Ductile Cementitious Composite (EDCC) Water flow Compressive stress Critical stress level
Permeability of concrete is one of its most important characteristics determining the durability. In this study, water permeability of a new class of fiber reinforced concrete materials called Eco-Friendly Ductile Cementitious Composites (EDCC) was studied, with and without an applied compressive stress. Hollow-core specimens were used and permeability tests were performed under full flow-equilibrium conditions. Four applied stress levels of 0.3fu, 0.4fu, 0.5fu and 0.6fu were investigated, where fu is the compressive strength of the EDCC in question. Permeability tests under identical conditions were carried out on plain control specimens without fiber rein forcement (termed Plain Cementitious Composites, PCC). The results indicated that the permeability of un stressed specimens declined over time due to the continuous hydration. A ‘critical’ compressive stress level (fcc) was identified and defined, which when exceeded, a dramatic increase in the coefficient of permeability occurred. The ‘critical’ compressive stress level (fcc) was noted to be between 0.5fu ~ 0.6fu for EDCC and 0.4fu ~ 0.5fu for PCC. In other words, EDCC was more damage tolerant than PCC, and even when fcc was exceeded, the impact of stress on the permeability of EDCC was far less pronounced compared to PCC.
1. Introduction Concrete carries flaws and micro-cracks both in the material and at the interfaces even before an external load is applied. Under an applied load, distributed micro-cracks propagate, coalesce and align themselves to produce macro-cracks, resulting in greatly compromised durability of concrete structures [1]. Fortunately, the micro and macro-fracturing processes can be favorably modified by adding short, randomly distributed fibers of various suitable materials, resulting in fiber rein forced concrete. During the last two decades, significant efforts have been made towards developing high performance fiber reinforced cementitious composites (HPFRCC) that demonstrate significant in creases in crack growth resistance as manifested by stress-strain curves that depict large ultimate strains. Engineered Cementitious Composites (ECC) is one promising type of HPFRCCs, depicting elasto-plastic and strain-hardening response with very tight crack widths [2]. These unique behaviors result from an elaborate design using a micro-mechanical model taking into account the interactions among fiber, matrix and the fiber-matrix interface [3].
The typical fiber used in ECC is polyvinyl alcohol (PVA) fiber with a diameter of 39 μm and a length of 6–12 mm [4]. The main problem of using PVA fiber in producing ECC is that PVA fiber tends to develop a very strong chemical bond with the matrix due to the presence of the hydroxyl group in its molecular chains. This high chemical bond leads to probable fiber ruptures which limits the tensile strain capacity [5]. In order to achieve strain-hardening behavior and high ductility, the strength of the chemical bond should be reduced. Li et al. [6] found that the fiber/matrix interfacial bond could be reduced by applying oil coating to the fiber surface. As a result, it is now generally accepted to prepare ECC with oil-coated PVA fibers. Moreover, matrix fracture toughness has to be limited to achieve strain-hardening. Therefore, the production of standard ECC mixtures has been restricted to the use of fine aggregate such as microsilica sand [7]. Plus, ECC materials contain considerably higher cement contents than conventional concrete. Such matrices and the use of surface-treated PVA fibers result in undesired high material costs and processing complications. They also increase the carbon footprint of ECC materials and make them unsustainable. Preliminary work [8,9] was reported on a new class of ECC-like
* Corresponding author. E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.cemconcomp.2019.103500 Received 30 September 2018; Received in revised form 10 September 2019; Accepted 26 December 2019 Available online 28 December 2019 0958-9465/© 2019 Elsevier Ltd. All rights reserved.
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
materials with ‘non-oiled’ PVA fibers and natural sand (1.19 mm maximum grain size) under the guidance of micromechanical principles. These materials also carried large amounts of supplementary cementi tious materials (SCMs). With their high strain tolerance and minimal amounts of cement, we are calling these materials Eco-Friendly Ductile Cementitious Composites (EDCC). EDCCs are arguably an evolved variation on ECC and depict similar constitutive characteristics. Due to its superior resistance to cracking, ECC is expected to demonstrate improved durability than normal concrete, especially under external loading. Permeability of concrete is regarded as a basic indicator of its durability [10]. Extensive work has been carried out to understand the permeability of normal concrete under an applied compressive stress [11,12]. It is shown that for normal concrete, the response is highly dependent on the level of applied compressive stress. At low levels of compressive stress, some researchers report a modest decrease in permeability [13,14], likely occurring due to pore-compression. This phenomenon however does not appear to be universal as some researchers observed no decrease in permeability under an applied compressive stress [15]. One universal agreement, however, is in the fact that when the compressive stress reaches a certain threshold value, often called the ‘critical’ stress level, permeability in creases rapidly [13,16]. This is often explained by the creation of in ternal cracks that abruptly coalesce when the ‘critical’ stress level is attained causing a rapid increase in the permeability. Lepech and Li [17] measured the permeability of a typical ECC mixture which was called ECC M45, and found its permeability coeffi cient at 28d was 8.18 � 10 12 m/s. This value is lower than the permeability coefficient of regular concrete, resulting from the relatively low water content of ECC. Several other studies have also studied the permeability of ECC under tension [17–20]. It has been generally demonstrated that due to the narrow cracks, the permeability of ECC remained low even after the formation of numerous microcracks and tensile strain of up to 3%. No data exists on performance of ECC under compressive loading. The present study aims to investigate the permeability of EDCC under an applied compressive stress. The specific objectives are: (1) to conduct permeability tests on EDCC specimens under full flowequilibrium conditions; (2) to monitor the evolution of EDCC’s perme ability at very early ages (2d to 5d); and (3) to investigate the effects of compressive stress and fiber on permeability of EDCC.
Table 2 Properties of PVA fibers.
Table 1 Mixture constituents and proportions (kg/m3) of EDCC and PCC. 385 770 77 462 333 1.7 0
Elastic modulus, GPa
PVA
13
8
40
1560
40
The permeability test setups were adapted from Banthia and Bhar gava [21]. A schematic representation of the apparatus designed for carrying out water permeability tests on concrete with the presence of an applied compressive stress is shown in Fig. 2. In a typical test, two permeability cells were assembled with iden tical specimens in them. One of the cells (Cell A) was placed in a uni versal testing machine (UTM) where a constant compressive stress could be applied. The other cell (Cell B) remained outside the UTM under the condition of no stress. Water was allowed to permeate through both the cells under identical flow conditions. Early studies with asphaltic and pervious concretes [22,23] indicated that permeability of concrete decreased slightly as the water pressure increased. However, the dif ference in permeability measured under different water pressures became insignificant when the water pressure exceeded 10 psi (0.07 MPa). One of the concerns with water pressure is that in young concrete with poorly developed strengths, internal damage may occur at higher water pressures. Equally relevant is the fact that at low water pressures, the accuracy of the test results may be compromised. Based on some trial tests, a constant inflow water pressure of 50 psi (0.35 MPa) was adopted for permeability tests in this study. This water pressure was 3% of the compressive strength (11.11 MPa) for the tested specimens at 48 h. Therefore, it was assumed to have no deleterious effect on the properties and deformations of the tested specimens. The setup uses cylindrical specimens that are 102 mm in diameter and 204 mm long with a 73 mm diameter hollow cylindrical core. Such specimens were cast with both EDCC and PCC, demolded 24 h after casting, and then assembled in the permeability cells [21]. Tests started 48 h after casting and were carried out until the age of 5d. No stress was applied on either specimen at first. At the age of 3d, a certain compressive load was applied on the specimen in Cell A to observe if there is any change in permeability caused by loading. The load was kept constant for one day and removed from the specimen at the age of 4d. The test was then carried on for another day, until the age of 5d. Four applied stress levels of 0.3fu, 0.4fu, 0.5fu and 0.6fu, where fu represents the compressive strength (determined with hollow cylindrical
Cylindrical specimens (75 mm in diameter and 150 mm long) were
PCC
Tensile strength, MPa
2.4. Permeability test
2.2. Compressive strength test
385 770 77 462 333 2 26
Diameter, μm
The uniaxial tensile test was performed on contoured specimens at the age of 28d. A closed-loop controlled Instron testing system was used in displacement-controlled mode to conduct the tensile test (Fig. 1). The specimens were gripped using mechanical wedge grips. The grip jaws must be carefully adjusted before test to make sure the specimen ends were aligned in vertical. When loading a specimen, the bottom grip jaw was locked open, the specimen was placed and clamped securely within the upper grip jaw first, and then the lower grip jaw was unlocked, providing clamping force on the specimen. The dog-bone shaped spec imens could avoid fractures at the gripping ends during test. Two LVDTs were connected to a data acquisition system to measure displacements over a gauge length of 60 mm. The displacement rate used was 0.002 mm/s.
Based on preliminary tests [8], the mixture constituents and pro portions of EDCC with “non-oiled” PVA fibers and natural sand were determined and are given in Table 1. The volume fraction of PVA fiber was 2% and its properties are shown in Table 2. Plain Cementitious Composite (PCC) without the fiber served as the control. Notice a slightly reduced amount of superplasticizer (1.7 l/m3) in PCC.
EDCC
Length, mm
2.3. Uniaxial tensile test
2.1. Material composition
Cement, ASTM Type I Fly ash, Class C Silica fume Natural sand, maximum size of 1.19 mm Water Superplasticizer (l/m3) PVA fiber
Density, kg/m3
cast to determine the compressive strength. The specimens were cured in water at 20 � C until tested. The ends of cylinders were ground before testing to ensure a flat and parallel surfaces and a better contact with the loading platen.
2. Test program
Ingredient
Type of fiber
2
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Fig. 1. Uniaxial tensile test set-up: (a) specimen configuration; (b) testing system.
Fig. 2. Setup for water permeability tests.
specimens) of EDCC/PCC at the age of 3d, were investigated. The mass of the water permeated through the two cells was collected in separate collection reservoirs and measured as a function of time. The water collected was related to the coefficient of water permeability (KW ) by applying Darcy’s law [24]: KW ¼
Q⋅lnðr2 =r1 Þ 2πh⋅△H
Eq. (1) clearly applies to flow conditions that are under equilibrium (input volume ¼ output volume) and hence it is critical that the mea surements be carried out only after the conditions of equilibrium are established. Typically, it took approximately 1 h after the start of the test to achieve condition of full flow-equilibrium. Data were recorded only after equilibrium was established.
(1)
3. Results and discussion
where, KW equals the coefficient of water permeability (m/s); Q equals the rate of water flow (m3/s); r1 and r2 are the inner radius and outer radius of the specimen (m); h equals the height of the specimen (m); and ΔH is the difference in hydraulic head between the inner and the outer sides of the specimen (m).
3.1. Compressive strength The compressive strengths of EDCC and PCC specimens as a function of age are shown in Fig. 3. The addition of PVA fibers did not alter the 3
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
compressive strength much. EDCC marginally exceeded the strength of PCC at 28d. Overall, the compressive strengths of EDCC and PCC were very close. 3.2. Uniaxial tensile performance The uniaxial tensile stress-strain curves of EDCC and PCC specimens at 28d are presented in Fig. 4. As expected, PCC specimens depicted a brittle response with a low tensile strength of about 1.83 MPa. On the other hand, EDCC specimens exhibited an elasto-plastic behavior with minor strain-hardening and a tensile strength of 4.43 MPa. After the first cracking in EDCC, the load continued to increase concurrently with multiple cracking, which contributed to the inelastic strain capacity as high as 3.89%. The crack opening measured after unloading was be tween 40–80 μm, and the crack spacing was about 2 mm as shown in Fig. 5. The tensile properties of EDCC were equivalent to typical ECC, which has a tensile strength of 4–6 MPa and a strain capacity of 3–5% [2]. Unlike regular ECC mixtures, the EDCC specimens in this study contained silica fume and high volume of fly ash. The addition of silica fume improved the fiber dispersion and led to a higher fiber-matrix interface frictional bond [25], both of which likely benefited the stain-hardening behavior. Increasing the fly ash content will reduce the fiber-matrix adhesive bond as well as the matrix toughness, while increasing the interfacial frictional bond, in favor of attaining high tensile strain capacity [26]. Therefore, the EDCC formulation allowed for both a reduced cement content and the use of non-oiled PVA fibers and natural sand without reducing the final strain capacity.
Fig. 4. Uniaxial tensile performance of EDCC and PCC specimens.
3.3. Permeability of unstressed specimens The permeability plots of the unstressed EDCC and PCC specimens, each accompanying a stressed specimen in each test, are presented in Fig. 6. Each point in the plots represents an average permeability coef ficient measured at 15 min interval. When equilibrium conditions are not established in a test, the permeability plots often tend to be jiggly. Very stable curves for unstressed specimens in Fig. 6 indicate that equilibrium flow conditions in these tests can be assumed. Notice also that the permeability plots for both EDCC and PCC in unstressed con ditions exhibited a steady decline. This decline in permeability can be assumed to be entirely due to continuous hydration in the specimens. Hydration processes are expected to be very active in these young specimens and significant truncation in the permeability paths and increased tortuosity in cracks are expected [27]. Likewise, the
Fig. 5. Cracking pattern of EDCC specimen after unloading.
microstructure of the paste is expected to get denser as the hydration progresses. Not surprisingly, therefore, the permeability of both EDCC and PCC decreased and the compressive strength increased (Fig. 3) over time as the hydration continued. Permeability of concrete is a highly variable property and is sensitive to factors such as material composition, casting and curing conditions and test details. One thing noticeable in the plots of Fig. 6 is that while various specimens started with widely different permeability co efficients at the age of 2d, plots essentially merged at 5d and the dif ferences diminished. Different specimens would carry different surface flaws at the start of the test induced by casting and demolding, which would tend to undergo healing and pore-blockage with time. This would, in time, reduce variability and bring specimen responses closer to each other. The averaged permeability plots of unstressed EDCC and PCC spec imens are shown in Fig. 7. The permeability coefficient of EDCC at 2d was 1.50 � 10 10 m/s, and it decreased gradually to 3.82 � 10 11 m/s by the age of 5d. The average permeability of PCC specimens was 2.59 � 10 10 m/s at the age of 2d, which was higher than the EDCC specimens. The difference vanished almost entirely by the age of 5d. The perme ability of PCC specimens at 5d was 4.26 � 10 11 m/s, which was very close to that of EDCC. The higher initial permeability of PCC may be due to the microcracks caused by casting and demolding, which were then healed to some extent during the permeability tests. The initiation and propagation of such cracks were likely inhibited in EDCC by the fibers, resulting in a lower permeability.
Fig. 3. Compressive strengths of EDCC and PCC. 4
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Fig. 6. Permeability plots of unstressed (a) EDCC specimens and (b) PCC specimens.
permeability of different specimens. When EDCC specimens were subjected to a compressive stress level of 0.6fu, there was an instant increase in permeability, as seen in Fig. 8 (d). The permeability coefficient rose from 8.58 � 10 11 m/s to 1.23 � 10 10 m/s. Interestingly, there was also an instant decrease in perme ability (from 8.30 � 10 11 m/s to 7.60 � 10 11 m/s) when the load was removed at the age of 4d. This meant that the cracks caused by loading closed to some extent when the stress was removed. Also, the hydration continued after the load was removed. 3.5. Permeability of PCC specimens under compression When a compressive stress level of 0.3fu or 0.4fu was applied to PCC specimens, no obvious change in the permeability was observed (Fig. 9). This is consistent with EDCC. However, when a stress of 0.5fu was applied, there was a sudden jump in the permeability. Also, as seen in the case of EDCC, the permeability decreased when the specimen was unloaded. When the compressive stress reached 0.6fu, the permeability of PCC increased by almost an order of magnitude. It is conceivable that as the load approached the peak value, microcracks propagated and interconnected with each other thus increasing the permeability [13]. As seen in Figs. 8(d) and 9(c), and especially Fig. 9(d), after a sudden jump in permeability due to loading at the age of 3d, permeability decreased rapidly in the first few hours of loading. The possible reasons for such instability including load relaxation, water hammer effect and self-healing, were discussed in Ref. [10].
Fig. 7. Averaged permeability plots of unstressed EDCC and PCC.
3.4. Permeability of EDCC specimens under compression The permeability plots of EDCC specimens under different levels of compressive stress are shown in Fig. 8 where the results from the com panion unstressed specimens are also shown for comparison. There was no obvious change in the permeability of EDCC caused by a compressive stress up to 0.5fu. Likewise, there seemed to be no difference in permeability when the specimens were unloaded after one day of loading with stress up to 0.5fu. This is consistent with the finding of Yang et al. [15], who studied the water permeability of concrete under deviatoric stress. On the other hand, a decrease in the permeability of concrete under low compressive stress could also be expected due to the consolidation or closing of pore and microcracks. The different re sponses of permeability at low levels of compressive stress, as discussed in Ref. [10], are caused by the composition, microstructure and porosity of concrete. It may be noticed that the permeability of specimen loaded under stress of 0.4fu (Fig. 8(b)) seemed to be higher than the one loaded under stress of 0.5fu (Fig. 8(c)). This was only related to the variable perme ability coefficients between different specimens. As discussed above, permeability of concrete is highly variable especially at early ages. Therefore, for early-aged concrete, it’s more reliable to study the effects of stress on permeability with the same specimen rather than comparing
3.6. Further discussion 3.6.1. Critical compressive stress level (fcc) As was done previously [10], here we define a ‘critical’ compressive stress level (fcc) at which a large increase in the permeability is expected. fcc is dependent not just on the composition of the material but also on its constitutive response. As the permeability plots in Figs. 8 and 9 show, the fcc was between 0.5fu ~0.6fu for EDCC specimens, and between 0.4fu ~0.5fu for the companion PCC specimens. This observation is in agreement with Banthia and Bhargava [16] who investigated the water permeability of fiber reinforced concrete and compared it to control without fibers. They also found a similar phenomenon to exist and concluded that the addition of fiber resulted in an increase in fcc. Hoseini et al. [11] reviewed the effect of fibers on the permeability of stressed concrete and concluded that permeability in concrete is affected more by cracks in the paste than by the paste/aggregate interface. The addition of fibers leads to changes in the crack profile whereby, instead of a few large cracks, a multitude of closely spaced microcracks form, especially 5
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Fig. 8. Permeability plots of EDCC specimens under compressive stress of (a) 0.3fu; (b) 0.4fu; (c) 0.5fu and (d) 0.6fu.
for EDCC with PVA fibers.
an enhanced damage tolerance in EDCC. Unlike normal concrete or fiber reinforced concrete, EDCC exhibits multiple cracking and tight crack widths under an applied stress. As shown in Fig. 5, the crack opening of EDCC after tensile test was between 40–80 μm. Even though the cracks opened up during loading, the crack widths were still much lower compared to normal concrete under similar stress conditions. Therefore, EDCC can offer much greater water tightness under stress compared to plain concrete. So far as ΔKU is concerned, ΔKU was much smaller compared with ΔKL for both EDCC and PCC specimens under stress levels beyond fcc.. This was due to two reasons: first, the cracks caused by loading couldn’t close completely when the specimen was unloaded, implying there was some permanent damage; and second, some self-healing occurred at the surfaces of the cracks during loading [29–31]. As a result, the crack opening was reduced and the permeability decreased accordingly before unloading, which also led to a smaller ΔKU compared with ΔKL. Moreover, Yang et al. [32] found that cracks with widths under 50 μm could be healed completely, and the extent of this self-healing di minishes with an increasing crack width. When the width exceeded 150 μm, the cracks could likely not heal. As discussed above, EDCC had much smaller crack widths under stress, therefore, the extent of self-healing in EDCC is expected to be higher than in normal concrete. The tight crack widths of EDCC under stress not only make it less permeable compared with PCC, they also promote self-healing, which are of great significance for improving structural durability.
3.6.2. Effects of PVA fibers on permeability of stressed specimens The effect of compressive stress on permeability can be further described by a change in permeability coefficient (ΔK). Let ΔKL represent an increase in permeability due to loading, and let ΔKU represent a decrease in permeability due to subsequent unloading. Acknowledging that there may exist an instability in the permeability plots due to loading as discussed above, the stressed permeability at the age of 3.125d (3 h after the stress was applied) was adopted as a representative permeability coefficient after loading [10]. When the specimens were loaded at stress levels which were lower than fcc, stress showed no in fluence on permeability. Therefore, ΔKL and ΔKU were both zero for these stress levels. The results of ΔKL and ΔKU for EDCC and PCC spec imens are given in Fig. 10. For PCC specimens, permeability increased by 2.57 � 10 11 m/s for a stress level of 0.5fu and 1.04 � 10 10 m/s for a stress level of 0.6fu, respectively. Clearly, the effect of stress on permeability got more remarkable as the stress level increased from 0.5fu to 0.6fu. It has been reported that the change of permeability caused by cracking is propor tional to the cube of the crack width [28]. In other words, permeability is highly sensitive to the crack width. A higher stress level leads to growing crack widths in the material, resulting in a rapidly increasing permeability. As shown in Fig. 10, the increase in permeability due to loading can be significantly reduced by adding fibers. The permeability of EDCC only increased by 1.54 � 10 11 m/s at a stress level of 0.6fu, which was almost a full order of magnitude lower than that of PCC. This indicated 6
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Fig. 9. Permeability plots of PCC specimens under compressive stress of (a) 0.3fu; (b) 0.4fu; (c) 0.5fu and (d) 0.6fu.
Cementitious Composites (EDCC). Permeability of EDCC was measured under a compressive stress and compared with plain (unreinforced) cementitious composites (termed PCC). The following conclusions were drawn: (1) With a high volume of fly ash and proper content of silica fume, EDCC mixture prepared with non-oiled PVA fibers and natural sand can attain strain-hardening behavior with an average strain capacity approaching 4%. (2) For both EDCC and PCC specimens under no stress, the perme ability coefficient exhibited a steady decline over time. This was contributed to ongoing hydration at early ages. The permeability of PCC was higher than EDCC at 2d, but the difference between the two diminished by the age of 5d. (3) A ‘critical’ compressive stress level (fcc) is defined in this study. This is the stress level at which there is a sudden increase in the permeability coefficient upon stress application. The critical compressive stress level (fcc) was noted to be between 0.5fu ~ 0.6fu for EDCC and 0.4fu ~ 0.5fu for PCC. Once the critical stress is removed, the permeability coefficient showed a sudden drop. (4) Compressive stress showed a much less pronounced effect on permeability of EDCC than PCC even under a critical compressive stress. The inherently tight crack widths of EDCC make it less permeable and promote the self-healing process in it. These fea tures of EDCC suggest that it is a much more durable construction material, especially under stress.
Fig. 10. Change in permeability coefficient caused by compressive stress.
4. Conclusions The study describes the development of low carbon footprint, elastoplastic cementitious composites called Eco-Friendly Ductile 7
Q. Wang et al.
Cement and Concrete Composites 107 (2020) 103500
Declaration of competing interest
[14] I. Yurtdas, S.Y. Xie, N. Burlion, J.F. Shao, J. Saint-Marc, A. Garnier, Deformation and permeability evolution of petroleum cement paste subjected to chemical degradation under temperature, Transp. Porous Media 86 (3) (2011) 719–736. [15] H. Yang, Y. Jia, S.Y. Xie, J.F. Shao, An experimental and numerical investigation of the mechanical behaviour of a concrete and of its permeability under deviatoric loading, Transp. Porous Media 102 (3) (2014) 427–454. [16] N. Banthia, A. Bhargava, Permeability of stressed concrete and role of fiber reinforcement, ACI Mater. J. 104 (1) (2007) 70–76. [17] M.D. Lepech, V.C. Li, Water permeability of engineered cementitious composites, Cement Concr. Compos. 31 (10) (2009) 744–753. [18] H. Liu, Q. Zhang, C. Gu, H. Su, V. Li, Influence of microcrack self-healing behavior on the permeability of Engineered Cementitious Composites, Cement Concr. Compos. 82 (2017) 14–22. [19] H. Liu, Q. Zhang, C. Gu, H. Su, V.C. Li, Influence of micro-cracking on the permeability of engineered cementitious composites, Cement Concr. Compos. 72 (2016) 104–113. [20] C. Wagner, B. Villmann, V. Slowik, V. Mechtcherine, Water permeability of cracked strain-hardening cement-based composites, Cement Concr. Compos. 82 (2017) 234–241. [21] A. Bhargava, N. Banthia, Measurement of concrete permeability under stress, Exp. Tech. 30 (5) (2006) 28–31. [22] K. Hall, J. Cruz, H. Ng, Effects of testing time and confining pressure on fallinghead permeability tests of hot-mix asphalt concrete, Transp. Res. Record J. Transp. Res. Board 1723 (1) (2000) 92–96. [23] Y. Qin, H. Yang, Z. Deng, J. He, Water permeability of pervious concrete is dependent on the applied pressure and testing methods, Adv. Mater. Sci. Eng. 2015 (2015) 1–6. [24] Y. Fang, Z. Wang, Y. Zhou, Time-dependent water permeation behavior of concrete under constant hydraulic pressure, Water Sci. Eng. 1 (4) (2008) 61–66. [25] J. Zhou, S. Qian, G. Ye, O. Copuroglu, K.V. Breugel, V.C. Li, Improved fiber distribution and mechanical properties of engineered cementitious composites by adjusting the mixing sequence, Cement Concr. Compos. 34 (3) (2012) 342–348. [26] S. Wang, V.C. Li, Engineered cementitious composites with high-volume fly ash, ACI Mater. J. 104 (3) (2007) 233–241. [27] H.N. Atahan, O.N. Oktar, M.A. Tas¸demir, Effects of water–cement ratio and curing time on the critical pore width of hardened cement paste, Constr. Build. Mater. 23 (3) (2009) 1196–1200. [28] V. Picandet, A. Khelidj, H. Bellegou, Crack effects on gas and water permeability of concretes, Cement Concr. Res. 39 (6) (2009) 537–547. [29] D. Palin, H.M. Jonkers, V. Wiktor, Autogenous healing of sea-water exposed mortar: quantification through a simple and rapid permeability test, Cement Concr. Res. 84 (2016) 1–7. [30] G. Yildirim, M. Sahmaran, M. Balcikanli, E. Ozbay, M. Lachemi, Influence of cracking and healing on the gas permeability of cementitious composites, Constr. Build. Mater. 85 (2015) 217–226. [31] H.-W. Reinhardt, M. Jooss, Permeability and self-healing of cracked concrete as a function of temperature and crack width, Cement Concr. Res. 33 (7) (2003) 981–985. [32] Y. Yang, M.D. Lepech, E.H. Yang, V.C. Li, Autogenous healing of engineered cementitious composites under wet–dry cycles, Cement Concr. Res. 39 (5) (2009) 382–390.
The authors declare that there are no known conflicts of interest associated with the submitted work and there has been no financial support for this work that could have influenced its outcome. Acknowledgements This work was supported by the Chinese Scholarship Council (CSC) and the Fundamental Research Funds for the Provincial Universities of Zhejiang (Grant no. 2019QN16), the experimental work was carried out in the materials lab, Department of Civil Engineering, University of British Columbia, Canada. References [1] N. Banthia, Fiber Reinforced Concrete, ACI SP-142ACI, Detroit, MI, 1994, pp. 91–119. [2] V.C. Li, From micromechanics to structural engineering-the design of cementitious composites for Civil engineering applications, Proc. Jpn. Soc. Civ. Eng. 10 (471) (1993) 37–48. [3] V.C. Li, C.K.Y. Leung, Steady-state and multiple cracking of short random fiber composites, J. Eng. Mech. 118 (11) (1992) 2246–2264. [4] E.H. Yang, S. Wang, Y. Yang, V.C. Li, Fiber-bridging constitutive law of engineered cementitious composites, ACT 6 (1) (2008) 181–193. [5] T. Kanda, Interface property and apparent strength of high-strength hydrophilic fiber in cement matrix, J. Mater. Civ. Eng. 10 (1) (1998) 5–13. [6] V.C. Li, C. Wu, S. Wang, A. Ogawa, T. Saito, Interface tailoring for strain-hardening polyvinyl alcohol-engineered cementitious composite (PVA-ECC), ACI Mater. J. 99 (5) (2002) 463–472. [7] M. Sahmaran, M. Lachemi, K.M.A. Hossain, R. Ranade, V.C. Li, Influence of aggregate type and size on ductility and mechanical properties of engineered cementitious composites, ACI Mater. J. 106 (3) (2009) 308–316. [8] Q. Wang, N. Banthia, W. Sun, Development of Engineered Cementitious Composites with Non-oiled Polyvinyl Alcohol Fibers and Natural Sand, Fiber Concrete, Prague, Czech Republic, 2013. [9] S. Soleimani-Dashtaki, S. Soleimani, Q. Wang, N. Banthia, C.E. Ventura, Effect of high strain-rates on the tensile constitutive response of Ecofriendly Ductile Cementitious Composite (EDCC), Procedia Eng. 210 (2017) 93–104. [10] Q. Wang, N. Banthia, W. Sun, Water permeability of repair mortars under an applied compressive stress at early ages, Mater. Struct. 51 (1) (2018) 6. [11] M. Hoseini, V. Bindiganavile, N. Banthia, The effect of mechanical stress on permeability of concrete: a review, Cement Concr. Compos. 31 (4) (2009) 213–220. [12] Y. Zhang, M. Zhang, Transport properties in unsaturated cement-based materials—a review, Constr. Build. Mater. 72 (2014) 367–379. [13] M. Choinska, A. Khelidj, G. Chatzigeorgiou, G. Pijaudier-Cabot, Effects and interactions of temperature and stress-level related damage on permeability of concrete, Cement Concr. Res. 37 (1) (2007) 79–88.
8