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Surface modification of polyethylene fiber by ozonation and its influence on the mechanical properties of Strain-Hardening Cementitious Composites
Composites Part B 177 (2019) 107446
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Surface modification of polyethylene fiber by ozonation and its influence on the mechanical properties of Strain-Hardening Cementitious Composites Zeyu Lu, Ran Yin, Jie Yao *, Christopher K.Y. Leung Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyethylene (PE) fiber Strain-hardening cementitious composites (SHCC) Fiber surface treatment Ozonation Interfacial properties
Polyethylene (PE) fibers are widely used to develop high strength Strain-Hardening Cementitious Composites (SHCC). Unlike Polyvinyl Alcohol (PVA) fiber, which has relatively low tensile strength and strong bond with matrix, the high tensile strength of the PE fiber is not fully utilized in the system, due to the hydrophobic nature of the fibers. In this study, a promising fiber treatment method by using ozone and ozone-derived hydroxyl radicals is developed to modify the surface properties of PE fibers, aiming to improve the fiber/matrix in teractions and then the tensile performance of resulting SHCC. Firstly, feasibility and mechanism of ozonation are revealed by competition kinetic technology, and the XPS results indicate that only hydroxyl groups can be introduced to the PE fiber by optimizing the ozone (O3) concentration and treating time, and surface roughness of the PE fiber is also increased due to the etch effect from ozonation. For the tensile performance of SHCC, compared to the composites with pristine fibers, PE fibers treated with O3 for 30 and 60 min can improve the ultimate tensile strain of SHCC by 1.3 and 2.5 times, respectively. This significant enhancement in deformation capacity of SHCC is attributed to the increased chemical bond and frictional bond after fiber treatment, as the results from single fiber pullout tests show that the fiber/matrix frictional bond is increased from 2.35 MPa (pristine PE) to 3.13 MPa (O3 (30 min)/PE) and 3.38 MPa (O3 (60 min)/PE). The research outcomes provide a novel way on surface treatment for PE fibers to improve the tensile performance of SHCC.
1. Introduction Generally speaking, quasi-brittle cement-based materials have two intrinsic drawbacks: brittleness and low flexural/tensile strength and strain. The addition of steel rebars, polymer or fibers has been the most common way to improve the ductility and tensile performance of cement-based materials [1–5]. For instance, macro-defect free (MDF) cement is a high-strength cement-polymer composites produced by mixing cement and polymer (commonly polyvinyl alcohol, PVA), under high shear force, modest pressure and temperature [6,7]. The flexural strength of MDF can easily reach up to 150–300 MPa, which is around 30 times higher than the normal concrete. The mechanism behind is that hydroxyl groups (-OH) of PVA can chemically interact with the calcium and aluminate phases released during the hydration reaction of cement, to form a strong cross-linked polymer-cement matrix [8–10]. Moreover, through the molecular dynamic simulation, the interfacial properties between the PVA and cement are found to be strengthened due to the H-bonds interaction between the –OH in PVA and silicate oxygen atoms in the C–S–H [11]. Besides MDF, Fiber Reinforced Cementitious
Composites (FRCC) have been rapidly developed in the past few de cades. Through incorporating fibers into cementitious matrix, such as polypropylene (PP), polyethylene (PE), polyvinyl alcohol (PVA) or carbon fiber, the flexural/tensile strength, ductility and toughness of FRCC can be improved by preventing and/or controlling the initiation, propagation and coalescence of cracks. Specifically, a type of high per formance FRCC named Strain Hardening Cementitious Composites (SHCC) have been developed since 1990’s by Li and co-workers under the guidance of micromechanics, have been developed by Li et al. [12]. This novel material is distinguished by the strain-hardening behavior up to several percent strain as well as outstanding crack control (maximum crack width below 60 μm). With the addition of synthetic fibers (PVA, PE, ETC.) is distinguished by the strain hardening behavior up to several percent [13–15]. However, PVA fiber often ruptures during the pullout process from cementitious matrix, because the tensile strength of PVA fiber (1 GPa, KURALON K-II REC 15 [16]) is not sufficient to tolerate the high interfacial bond coming from the chemical interaction between the –OH in PVA fiber and cement hydrates. To this end, oil coating is usually applied to reduce the bond between the PVA fiber and cementitious
* Corresponding author. E-mail addresses: [email protected] (Z. Lu), [email protected] (R. Yin), [email protected] (J. Yao), [email protected] (C.K.Y. Leung). https://doi.org/10.1016/j.compositesb.2019.107446 Received 15 May 2019; Received in revised form 4 September 2019; Accepted 12 September 2019 Available online 13 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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matrix. However, for those fibers with high strength but inert surface properties (e.g., PE fibers), it is of importance to improve the fiber/ matrix interfacial bond to fully utilize the high strength of PE fibers, and finally enhance the mechanical behavior of SHCC. Inspired from the chemical composition of PVA fiber, if –OH can be introduced on the surface of PE fibers after treatment, a stronger bond between the PE fibers and cementitious matrix is expected to be achieved [17]. In fact, various kinds of surface treatments, such as chemical or physical surface coating, ion/plasma beams and surface oxidization, have been used to modify the properties of fibers with inert surface to make them rougher or capable of higher chemical reactivity [16,18–20]. Among these treatments, surface coating is the most popular method due to the easy operation process, which only needs to immerse the fiber in a solution with coating materials under mechanically mixing and specific conditions (e.g., certain temperature, ultrasonication, etc.), following by lifting and drying. For example, Lu et al. [21] coated a thin SiO2 layer onto carbon fibers through the condensation and polymerization of the tetraethyl orthosilicate under alkaline conditions to enhance the inter facial bond between the carbon fiber and cementitious matrix. Relying on the reaction between the surficial-coated SiO2 on carbon fiber and Ca (OH)2 in cement hydrates, the chemical debonding energy and frictional bond were 10 and 2 times of the pristine carbon fiber, respectively. Yang et al. [22] coated carbon nano-fibers (CNFs) on the surface of cluster-like PE fibers through hydrophobic interactions, and indicated that the interface transition zone between the PE fibers and matrix was strengthened by filling nano-pores and cross-linking nano-cracks, lead ing to a 15.0% and 20.0% enhancement in tensile strength and tensile strain capacity of PE-SHCC, respectively. However, based on the au thors’ experience, the surface coating process is not effective to treat cluster-like fibers, especially for those located in the inside of the cluster [17]. Without extra pressure to push coating materials into the inside of fiber cluster, only the mechanical mixing is not enough to make all cluster-like fibers fully coated. More importantly, the dispersion of nanomaterials themselves is a key problem, as discussed in Ref. [22], and it might become even worse if mixed with fibers. The non-uniform coating of cluster-like PE fibers can lead to unstable and volatile rein forcing efficiency in SHCC. The cluster-like PE fibers are anticipated to be oxidized more uniformly and effectively. Inspired from the strong interfacial bond between the PVA polymer/ fiber and cementitious matrix, it is believed that the introduction of hydroxyl groups into PE fiber is an efficient way to improve the inter action between the PE fibers and SHCC matrix. Ozone (O3) is a powerful oxidant with a redox potential of 2.07 V, which is usually generated on-site through electrolysis of oxygen in labscale and industrial applications [23,24]. Due to the higher oxidation power, many inorganic (e.g. metals, sulfide, bromide etc.) and organic substances can be oxidized by ozone, through direct ozonation or the oxidation by the secondary reactive species (e.g., hydroxyl radicals) derived from ozone. However, the ozonation for PE fibers has not yet been reported and whether the ozonation can introduce oxygen groups, especially for hydroxyl groups, to PE fibers remains unknown. The generated ozone can be directly supplied in the gas form and it can also be easily dissolved in water. To the authors’ best knowledge, the gas form of ozone released in water will generate many air bubbles with pressure to push ozone into the inside of the cluster of the target (e.g., PE fiber). By optimizing the ozone concentration and treating time, it is believed that the cluster-like PE fibers have a great potential to be oxidized more uniformly and effectively. The aim of this study is to activate the surface properties of PE fibers by ozonation to improve the chemical and physical bond with cemen titious matrix, and finally to improve the reinforcing efficiency of PE fibers in SHCC. Firstly, PE fibers were oxidized by controlling the ozonation time under an optimized ozone concentration. The mecha nism and feasibility of ozonation to PE fibers for introducing hydroxyl groups were analyzed through competition kinetic technology, and the surface properties of treated PE (O3/PE) fibers were characterized by
SEM/EDX, XPS and surface roughness test. Secondly, the O3/PE fibers were used to fabricate SHCC to investigate the influence of fiber ozon ation on the tensile performance of PE-SHCC, in terms of tensile strength, ultimate tensile strain and crack width control. A digital image processing method was adopted to investigate the crack pattern of SHCC under tension. The interfacial microstructure between the O3/PE fibers and cementitious matrix was then analyzed by SEM. In addition, the single fiber pullout tests were conducted to validate the bond between the fiber and matrix, and gave a deep insight on how efficient of O3 treatment can strengthen the interaction between the fibers and matrix. 2. Experimental preparation 2.1. Raw materials Portland cement subjecting to the requirement of type 52.5 N (Green Island@, Hong Kong) and silica fume (SF, Elkem® Microsilica 920U) were used as the binder to fabricate the SHCC. Silica sand with a mean diameter of 150 μm was used as the fine aggregate. A polycarboxylatebased agent with 28.0% solid ratio was used as the superplasticizer for the SHCC. In addition, 2.0 vol % of PE fiber was added for all mixes to produce the SHCC, and the physical properties of the PE fiber (Quan tumeta®, China) are shown in Table 1, while Table 2 shows the mix proportions of all mixes. 2.2. Samples preparation 2.2.1. Fabrication and characterization of O3/PE fiber Fig. 1 shows the schematic diagram of the ozonation set-up for PE fibers. The ozonation treatment for PE fibers (18.3 g, 2 vol% for one batch of sample) was conducted in a 2L glass reactor containing ultra pure water, and the ozone was generated using an ozone generator (10K–2U, Enaly, China) at a constant flow rate for 30 min and 60 min, respectively, named as O3(30 min)/PE fiber and O3(60 min)/PE fiber. It should be pointed out that the ozone bubbles are beneficial in making PE fiber clusters and ozone fully contacted so that even the PE fibers inside the cluster can also be fully oxidized. In addition, competition kinetic technology was applied to determine the steady-state concentrations of dissolved ozone and hydroxyl radicals generated during the ozonation process [26]. Specifically, nitrobenzene (NB) and benzoic acid (BA) were used as two probe compounds to determine the steady-state con centrations of ozone and hydroxyl radicals. 1-μM NB and BA were spiked to the glass reactor containing PE fiber and ultrapure water before ozonation starts. Samples from two probes were collected at different time intervals, quenched with sodium sulfite at a sulfite-to-ozone molar ratio around 1.2:1, and analyzed for the remaining probe compound concentrations using ultra-performance liquid chromatography (UPLC) (VP series, Shimadzu) equipped with a Waters symmetry C18 column and a UV–Vis detector. Eluents of water (pH 3, adjusted using phos phoric acid) and methanol (55:45, v/v %) were used to separate the NB and BA at a flow rate of 1.0 mL/min. In addition, X-ray photoelectron spectroscopy (XPS, Physical Electronics 5600 multi-technique system) was employed to investigate the carbon atomic composition of the PE fiber and O3/PE fiber. Furthermore, surface morphologies and element analysis of the testing fibers were characterized by Scanning Electronic Microscopy (SEM, JEOL6700F) equipped with EDX.
Table 1 Physical properties of the PE fiber [25].
2
Length (mm)
Diameter (μm)
Tensile strength (MPa)
Elastic Modulus (GPa)
Density (g/ cm3)
12
24
3000
120
0.97
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2.3. Fabrication of PE and O3/PE fibers reinforced SHCC
Table 2 Mix proportions. Specimen PE-SHCC O3(30 min)/PEFRCC O3(60 min)/PEFRCC
Binder
Sand
w/c
PCE
PE or O3/PE fibers (vol%)
Cement
SF
1 1
0.25 0.25
0.375 0.375
0.20 0.20
0.03 0.03
2.0 2.0
1
0.25
0.375
0.20
0.03
2.0
To achieve a better dispersion of fibers in the SHCC matrix, solid materials, including cement particles, silica fume and sand were firstly mixed at a low speed for 5 min. After that, water containing predissolved PCE superplasticizer was added into the mixture at once and then mixed in a high speed for the other 5 min. After that, fibers were added and mixed at a low speed for 3 min and then at a high speed for the other 3 min. During this process, the fresh mixture on the wall of the mixing container was scraped down to ensure the fiber volume fraction is 2%. Finally, the specimens were cast into pre-oiled dog-bone molds [27] and covered with a thin plastic sheet to prevent the water evapo ration. After 24 h curing in an ambient environment, the specimens were demolded and placed into a curing room (25 � C/RH 95%) for 13 days before the test. In addition, without special instruction, three samples were conducted to get the average result for each test.
Note: SF, Sand, w/c and PCE are by weight of the cement.
2.3.1. Samples for single fiber pullout test The mold used for single fiber pullout test is shown in Fig. 2. The first step is to use 2 horizontal screws to secure two ‘bottom parts’ and one ‘middle part’ together. The ‘middle part’ is in U shape, so after being sandwiched by two bottom parts, there is a gap between two ‘bottom parts’ which can be cast into matrix. The second step is to place PE fibers on the pre-cut notches on the top surface of two ‘bottom parts’, and then stick a very thin layer of tape to prevent fiber sliding and moving. The third step is to use 6 vertical screws to secure two ‘upper parts’ on the ‘bottom parts’, so the fiber will be clamped. The last step is to cast the matrix into the gap, and then vibrate the mold until all the visible air bubbles trapped in the mold are expelled out (the mold is made of polymethyl methacrylate which is transparent, so it can be observed from side view that whether there is air bubble trapped in the matrix). The sample will be demolded after 48 h and then cured in curing room (25 � C/RH 95%) for another 12 days. Since the sample prepared by this
Fig. 1. Ozonation set-up for PE fibers.
Fig. 2. The mold and sample size for single fiber pullout test. 3
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mold is a long continuous rectangle with many fibers through it, it will be cut into many small pieces with only one fiber through each piece at least 24 h before the test. The protruded fiber on one side was cut off with a sharp blade. Since thickness of the middle part is 3 mm, the fiber embedded length is also 3 mm.
increases to 30 min and 60 min, the relative content of C–OH functional groups is continuously increased to 15.9% and 38.0%, respectively. The XPS results not only confirm the transformation of C–C bond to C–OH bond after O3 treatment, but also indicate that only –OH groups can be generated when the ozonation time is less than 60 min. Therefore, the chemical activated and physical roughened surface of PE fiber with O3 treatment has a great potential to strengthen the interfacial bond be tween the fiber and SHCC matrix, which will be deeply discussed in the section 3.2 and 3.3. However, it should be pointed out that further in crease in the ozonation time will lead to a stronger oxidation of PE fiber, which cannot ensure only hydroxyl groups being introduced to the O3/PE fiber. In order to quantify how is the surface roughness condition of PE fiber before and after O3 treatment, surface roughness parameters of PE and O3/PE fiber were characterized and imaged by a 3D Surface Metrology (Bruker NPFLEX) with the mode of Vertical Scanning Inter ferometry (VSI). Three roughness measurements were conducted on eight testing areas of each sample and the average value was calculated in terms of Ra (average roughness calculated over the entire measured array) and Rq (root mean square roughness). As displayed in Fig. 5, the altitude difference in vertical direction of PE fiber is only one third of O3(30 min)/PE fiber, and more color patterns can be seen in Fig. 5b, indicating that O3 has a strong etch effect on the surface of PE fiber. To be more specific, Table 3 lists the variation of the two important roughness parameters before and after ozonation treatment, and it in dicates that both parameters of O3/PE fibers are more than three times higher than that of PE fiber, revealing that the surface roughness of PE fibers can be greatly improved by O3 treatment, which lays a foundation for a stronger physical friction between treated PE fiber and cementi tious matrix during fiber pullout process. The mechanism behind the ozonation treatment for PE fibers, to the authors’ best knowledge, is believed to be contributed by both ozone itself and the reaction species (e.g. hydroxyl radicals) derived from ozone. Under our test conditions, the concentrations of ozone and ozonederived hydroxyl radicals were quantified using competition kinetic
3. Results and discussion 3.1. Feasibility and mechanism of O3 treatment for PE fibers In this study, the oxidation of PE fiber is controlled by different ozonation time under a constant ozone flow, and the surface properties of PE fiber after O3 treatment, including chemical composition, surface morphology and roughness, are characterized by XPS, SEM and rough ness test, respectively. Fig. 3 shows the morphology and elementary composition of PE and O3/PE fiber. It is to see that the O3 treatment makes the surface of PE fiber rougher due to the etch effect of oxidation process [28], which is beneficial to improving the friction bond between the fiber and matrix during the fiber pullout process. From Fig. 3, the oxygen wt. % of the O3/PE fiber increases with the increasing ozonation time, which can reach up to 6.92 wt % and 11.10 wt % after 30 min and 60 min of ozonation treatment, respectively, indicating that the PE fiber can be variously oxidized by the O3 treatment. In order to quantitatively determine the content of carbon-oxygen species of PE fiber with different ozonation time, XPS was conducted to investigate the chemical state of the carbon atoms in PE and O3/PE fiber. Fig. 4 shows the XPS C1s spectra of PE fiber and O3/PE fiber after 30 min and 60 min of ozonation. PE fiber only shows a single peak at 284.8 eV, corresponding to the C–C vibration, as shown in Fig. 4a. However, the asymmetric shape of the C1s peaks of the O3/PE fiber reveals the existence of bonds other than those of pristine PE fiber. By deconvoluting the C1s spectrum, hydrox ylation is the major reason leading to the asymmetry of the C1s peak. Specifically, with the increase of ozonation time, the level of oxidation of PE fiber is also increased, which can be reflected by the higher in tensity of C–OH appeared at 286.3 eV. When the ozonation time
Fig. 3. SEM/EDX result of (a) PE fiber (b) O3(30 min)/PE fiber (c) O3(60 min)/PE fiber. 4
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Fig. 4. XPS C1s spectra of (a) PE fiber, (b) O3(30 min)/PE fiber and (c) O3 (60 min)/PE fiber.
and 4.63 � 10 5 M respectively. While in the presence of PE fiber, the pseudo first order rate constants of NB and BA degradation decreased to 1.112 and 1.683, respectively. Accordingly, as shown in Table 4, the calculated steady-state concentrations of hydroxyl radicals and ozone decreased to 4.752 � 10 12 M and 1.033 � 10 5 M respectively. The differences on the ½HO�ss and ½O3 �ss between the two cases (i.e., with or without PE fiber) were thus calculated to be 1.303 � 10 12 M and 3.597 � 10 5 M respectively, which were the quantities of hydroxyl radicals and ozone that were consumed by PE fiber.
technology [29]. Nitrobenzene (NB) and benzoic acid (BA) were used as probe compounds, and their degradation kinetics were used to quantify the ozone and hydroxyl radicals in the absence and presence of PE fiber. NB exhibits high reactivity towards hydroxyl radicals (k ¼ 3.9 � 109 M 1s 1) but hardly reacts with ozone [30]. BA reacts rapidly with hydroxyl radicals (k ¼ 5.9 � 109 M 1s 1) and also reactive towards ozone (k ¼ 1.2 M 1s 1). Therefore, the degradation of NB and BA during ozonation process can be expressed by Eqs. (1) and (2): rNB ¼ ko;NB ½NB� ¼ kHO;NB ½HO�ss ½NB�
Eq. 1
rBA ¼ ko;BA ½BA� ¼ kHO;BA ½HO�ss ½BA� þ kO3;BA ½O3 �ss ½BA�
Eq. 2
3.2. Tensile properties of SHCC with treated PE fibers Fig. 7 shows the direct tension sample configuration and strain-stress curves of PE-SHCC and O3/PE-SHCC under direct tension. There were three samples tested for each mix, but some results were not represen tative, therefore, only two representative curves for each mix are pre sented. Two important parameters, i.e. ultimate tensile strength and tensile strain capacity, are summarized in Table 5. It should be pointed out that the ultimate tensile strength and tensile strain capacity may not be at the same point in Fig. 7, because some curves (e.g. PE–SHCC–2) show a very high tensile load at the early stage of the curve, after that point, the ductility of the sample seems still increasing although the tensile load will not exceed that point again. In this case, the ultimate tensile strength is chosen to be the maximum tensile load, while the tensile strain capacity is chosen from the later part of curves (e.g. for
where rNB and rBA are the degradation rate of NB and BA respectively. ko;NB and ko;BA are the observed rate constants of NB and BA degradation, which can be obtained by plotting the concentration of residual NB and BA versus reaction time. kHO;NB , kHO;BA , and kO3;BA are the second order rate constants of NB and BA toward hydroxyl radicals and ozone, which can be obtained from literature and have been listed above. ½HO�ss and ½O3 �ss are the steady-state concentrations of hydroxyl radicals and ozone respectively. Fig. 6 shows the time-dependent degradation kinetics of NB and BA during ozonation in the presence and absence of PE fiber. In the absence of PE fiber, the pseudo first-order rate constants of NB and BA degradation were 1.417 and 2.147 min 1 respectively. Based on Eqs. (1) and (2), the ½HO�ss and ½O3 �ss were calculated to be 6.056 � 10 12 M 5
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Fig. 5. Surface roughness image of (a) PE fiber (b) O3(30 min)/PE fiber (c) O3(60 min)/PE fiber. Table 3 Surface roughness parameters of the PE,O3(30 min)/PE and O3 (60 min)/PE fiber. Roughness parameters
PE fiber (nm)
O3/PE fiber (30 min) (μm)
O3/PE fiber (60 min) (μm)
Ra Rq
300.7 368.5
1.24 1.39
2.57 3.00
PE–SHCC–2, the tensile strain capacity is 2.1% instead of 1.1%). From Table 5, it reveals that after the ozonation treatment for PE fiber, ulti mate tensile strength decreases, while the tensile strain capacity in creases. For the decrease in the ultimate tensile strength, some reasons from the published paper [12] may due to the stochastic intrinsic from sample to sample, for example, the ultimate tensile strength will significantly decrease if there is a large size flaw in the sample (so the fiber volume fraction will decrease in that section). And also, since ozonation treatment improves the interfacial bond between the PE fiber and SHCC matrix, the other plausible explanation for the decrease of the ultimate tensile strength may be because of the fiber rupture, but further study is needed to prove this conjecture. For the tensile strain capacity, from Table 5, it can be found that the tensile strain capacity of O3(30 min)/PE-SHCC and O3(60 min)/PE-SHCC is 3.2% and 4.9%, respectively, which is 1.5 and 2.3 times higher than that of pristine PE-SHCC (2.1%). The tensile strain capacity is the most important parameter in practical design. SHCC with larger tensile strain capacity
Fig. 6. The time-dependent degradation kinetics of NB and BA during ozona tion in the presence and absence of PE fiber.
stands for the better energy absorption, crack width control, durability, etc. Here, the reason for the improvement of tensile strain capacity is the densified microstructure of fiber/matrix interface. Fig. 8 shows the interfacial microstructure between the PE fiber and SHCC matrix with 6
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However, after 30 min of ozonation treatment, the surface property of PE fiber is modified by introducing –OH group and fiber becomes hy drophilic, therefore, a more densified interface between O3(30 min)/PE fiber and SHCC matrix can be observed in Fig. 8d, e and f. The interfacial microstructures of O3(60 min)/PE-SHCC are displayed separately in Fig. 9 as they are quite different from that of PE-SHCC and O3(30 min)/PE-SHCC. It can be seen that the interface of O3(60 min)/ PE-SHCC is denser than O3(30 min)/PE-SHCC, no obvious gap can be observed and the fiber seems to be fully covered by cement hydrates. More cement hydration products can be found on the surface of O3(60 min)/PE fiber, as shown in Fig. 9d. This results from the hydroxyl groups introduced by the ozonation treatment, which can chemically
Table 4 The calculated steady-state concentrations of hydroxyl radicals and ozone dur ing ozonation in the presence and absence of PE fiber. [HO]ss w.o. PE fiber w. PE fiber Difference (G1-G2)
6.056 � 10 4.752 � 10 1.303 � 10
[O3]ss 12
M 12 M 12 M
4.63 � 10 5 M 1.033 � 10 5 M 3.597 � 10 5 M
and without ozonation (30 min) treatment. Since the PE fiber has inert surface and is hydrophobic, incompact interface (Fig. 8a and b) and big gaps (Fig. 8c) between the fiber and matrix can be clearly observed.
Fig. 7. The direct tension test sample and tensile performance of (a) PE-SHCC, (b) O3(30 min)/PE-SHCC and (c) O3(60 min)/PE-SHCC. 7
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Table 5 Tensile properties of PE and O3/PE fiber reinforced FRCC. Sample
Ultimate tensile strength (MPa)
Tensile strain capacity (%)
PE–SHCC–1 PE–SHCC–2 Average O3(30min)/ PE–SHCC–1 O3(30min)/ PE–SHCC–2 Average O3(60 min/PE)– SHCC–1 O3(60min)/ PE–SHCC–2 Average
7.2 7.8 7.5 6.0
2.0 2.1 2.1 3.6
5.5
2.8
5.8 6.3
3.2 4.6
6.0
4.9
6.2
4.8
where Pmax is the maximum fiber pullout force obtained from the test, df is the fiber diameter (24 μm), Lf is the fiber embedded length (3 mm) and η is the effective fiber volume friction which can be considered as 0 here. The effective interfacial stress reveals the bond quantitively, including both chemical and frictional bond. The average τeff for pristine PE, O3 (30 min)/PE and O3 (60 min)/PE fibers are 2.35 MPa, 3.13 MPa and 3.38 MPa, respectively, therefore, it validates the idea that the ozonation strengthen the bond between the PE fibers and cementitious matrix. From Fig. 10, it can be seen that the variation of the testing data is quite large, two reasons are given in the following: (1) the sample size is too small. The diameter of the fiber is only 24 μm and the thickness of the matrix is only 3 mm which makes the sample very difficult to cast and test. Effects such as misalignment of the fiber to the loading direc tion, bad compacting of the matrix which introduces voids near the fiber/matrix interface, intrinsic error of the testing setup, etc. are all very likely to occur and thus influence the results; (2) since the scale of the sample is very small, the variation of the material properties also influences the results. For example, the average size of the quartz sand used in the matrix is 150 μm, much larger than the diameter of the fiber, therefore, the interface along the longitudinal direction of the fiber may be inhomogeneous and thus influences the testing results. From litera tures, the results of single fiber pullout tests also show very large vari ation [23–25]. Therefore, further study, such as improving the test setup or increasing the sample number, is needed to make the results more convincing. Fig. 11 shows the SEM photos of pristine PE fiber and O3(30 min)/PE fiber after pullout test. It can be seen that the pristine PE fiber has a very smooth surface, while there are many small particles adhering on the surface of O3(30 min)/PE fiber. This phenomenon also provides strong evidence that the ozonation is very effective to strengthen the fiber/ matrix bond.
interact with the calcium phase in C–S–H through ionic bond, as dis cussed in the section 3.1, and finally strengthen the bond between the fiber and matrix. Moreover, through the comparison among Fig. 8c, f and Fig. 9c, the surface of fiber becomes rougher with the increase of ozonation treatment time, so it double confirmed that the frictional bond between the fiber and matrix can be improved with the increasing time of ozonation treatment. In summary, the ozonation treatment is an effective method to chemically and physically activate the surface properties of PE fiber by introducing hydroxyl groups to PE fibers. After the fiber treatment, the ultimate tensile strength varies within the error range, but a significant improvement on the tensile strain capacity can be achieved. 3.3. Single fiber pull-out test results Single fiber pullout test was conducted in order to measure the fiber/ matrix bond directly. Five samples for PE and O3(30min)/PE and ten samples for O3(60min)/PE have been tested. Some samples failed during the installment to the test machine, so only a part of sample results are presented in the Fig. 10. The sample test setup and maximum single fiber pullout forces are summarized in Fig. 10, which can be used to calculate the effective interfacial stress by Eq. (3) [10,22]:
τeff ¼
Pmax
πdf Lf ð1 þ ηÞ
�
Pmax
πdf Lf
3.4. Crack width control Another advantage of improving the fiber/matrix bond is that the crack width can be smaller. In the section 3.2, during the direct tension test, photos were taken every 0.5% increment of strain, and then a digital image processing method was used to obtain the average crack width. Fig. 12 shows the grayscale images of PE-SHCC and O3(30 min)/ PE-SHCC under different tensile strains. The average crack width can be calculated through the following equation:
Eq. 3
Fig. 8. Interfacial microstructures between the SHCC matrix and (a, b, c) pristine PE fiber (d, e, f) O3 (30 min)/PE fiber. 8
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Fig. 9. Interfacial microstructures between the FRCC matrix and (a, b, c) O3 (60 min)/PE fiber and (d) fracture surface morphology of O3(60 min)/PE fiber.
Fig. 10. Test setup and maximum single fiber pullout force.
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Fig. 11. Morphology of (a) PE fiber (b, c) O3(30 min)/PE fiber after single fiber pull out test.
adding 1% elongation so the total length is 80 � ð1 þ 1%Þ ¼ 80:8mm. Terefore, the size of every pixels is 80:8 =m mm which will be denoted as Cof. in Eq. (4). By adopting this approach, the average crack width are summerized in Fig. 13. From Fig. 13, it can be observed that the average crack width of O3(30 min)/PE-SHCC is generally smaller than PE-SHCC, which indicates that strengthening the bond between the PE fiber and matrix is also beneficial for crack width control. 4. Conclusions In this study, a novel fiber treatment method by using ozone was developed to modify the surface properties of PE fibers. Hydroxyl groups (-OH) were introduced to PE fiber and made the hydrophobic PE fiber become hydrophilic, so more cement hydrates could be precipitated on the surface of fiber to form a densified interfacial microstructure. The
Fig. 12. Grayscale image of (a) PE-SHCC (b) O3(30 min)/PE-SHCC under different tensile strains.
Average crack opening width ¼
Total area of black pixals *Cof Crack number*sample width
Eq. 4
The Cof. in Eq. (4) means the correponding size of the pixels. For example, the grayscale image of PE-SHCC under 1% tensile strain is consist of ½m �n� pixelss. The original length of the sample is 80 mm,
Fig. 13. The development of average crack opening width and average crack spacing for PE-SHCC and O3(30 min)/PE-SHCC. 10
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newly introduced –OH functional groups can provide extra ionic and H bond between the PE fiber and cementitious matrix, which was bene ficial for strengthening the fiber/matrix interface. Meanwhile, the etching effect of ozonation made the surface of fiber rougher, which also improved the frictional bond between the fiber and matrix during fiber pullout process. In addition, direct tension behavior of SHCC with pristine PE fiber and O3/PE fiber are investigated, and the results show that the tensile strain capacity of O3/PE-SHCC is much higher than that of PE-SHCC. To be more specific, the strain capacity for PE-SHCC is 2.1%, while for 30 min and 60 min O3/PE-SHCC was 3.2% and 4.9%, respectively. On the other hand, the results of single fiber pullout tests indicated that the effective interfacial stress between the fiber and ma trix was increased from 2.35 MPa (PE), 3.13 MPa (O3(30 min)/PE) and 3.38 MPa (O3(60 min)/PE), respectively, which validated that the ozonation treatment of fiber can strengthen the fiber/matrix bond, leading to a better tensile performance and strain-hardening behavior of SHCC.
[7] Hou D, Yu J, Wang P. Molecular dynamics modeling of the structure, dynamics, energetics and mechanical properties of cement-polymer nanocomposite. Compos B Eng 2019;162:433–44. [8] Ekincioglu O, Ozkul MH, Struble LJ, Patachia S. Optimization of material characteristics of macro-defect free cement. Cement Concr Compos 2012;34(4): 556–65. [9] Zhang W. Synthesis and fracture toughness of macro-defect-free (MDF) cement. 2014. [10] Yu J, Yao J, Lin X, Li H, Lam JY, Leung CK, et al. Tensile performance of sustainable strain-hardening cementitious composites with hybrid PVA and recycled PET fibers. Cement Concr Res 2018;107:110–23. [11] Zhou Y, Hou D, Manzano H, Orozco CA, Geng G, Monteiro PJ, et al. Interfacial connection mechanisms in calcium–silicate–hydrates/polymer nanocomposites: a molecular dynamics study. ACS Appl Mater Interfaces 2017;9(46):41014–25. [12] Li VC, Wang S, Wu C. Tensile strain-hardening behavior of polyvinyl alcohol engineered cementitious composite (PVA-ECC). ACI Mater J Am Concr Inst 2001; 98(6):483–92. [13] Li VC, Leung CK. Steady-state and multiple cracking of short random fiber composites. J Eng Mech 1992;118(11):2246–64. [14] Yao J, Leung CK. A new physical model for empirical fiber snubbing effect in cementitious composites based on large deflection beam theory. Cement Concr Compos 2018;96:238–51. [15] Lu C, Li VC, Leung CK. Flaw characterization and correlation with cracking strength in Engineered Cementitious Composites (ECC). Cement Concr Res 2018; 107:64–74. [16] Lu Z, Lu C, Leung CK, Li Z. Graphene oxide modified Strain Hardening Cementitious Composites with enhanced mechanical and thermal properties by incorporating ultra-fine phase change materials. Cement Concr Compos 2019;98: 83–94. [17] Yu K-Q, Yu J-T, Dai J-G, Lu Z-D, Shah SP. Development of ultra-high performance engineered cementitious composites using polyethylene (PE) fibers. Constr Build Mater 2018;158:217–27. [18] Pakravan HR, Memariyan F. Modification of low-surface energy fibers used as reinforcement in cementitious composites: a review. Polym Plast Technol Eng 2017;56(3):227–39. [19] Zhang W, Zou X, Wei F, Wang H, Zhang G, Huang Y, et al. Grafting SiO2 nanoparticles on polyvinyl alcohol fibers to enhance the interfacial bonding strength with cement. Compos B Eng 2019;162:500–7. [20] Farina I, Fabbrocino F, Colangelo F, Feo L, Fraternali F. Surface roughness effects on the reinforcement of cement mortars through 3D printed metallic fibers. Compos B Eng 2016;99:305–11. [21] Lu M, Xiao H, Liu M, Li X, Li H, Sun L. Improved interfacial strength of SiO2 coated carbon fiber in cement matrix. Cement Concr Compos 2018;91:21–8. [22] He S, Qiu J, Li J, Yang E-H. Strain hardening ultra-high performance concrete (SHUHPC) incorporating CNF-coated polyethylene fibers. Cement Concr Res 2017; 98:50–60. [23] Yin R, Fan C, Sun J, Shang C. Oxidation of iron sulfide and surface-bound iron to regenerate granular ferric hydroxide for in-situ hydrogen sulfide control by persulfate, chlorine and peroxide. Chem Eng J 2018;336:587–94. [24] Audran G, Marque SR, Santelli M. Ozone, chemical reactivity and biological functions. Tetrahedron 2018;74:6221–61. [25] Chen Y, Yu J, Leung CK. Use of high strength strain-hardening cementitious composites for flexural repair of concrete structures with significant steel corrosion. Constr Build Mater 2018;167:325–37. [26] Yin R, Ling L, Shang C. Wavelength-dependent chlorine photolysis and subsequent radical production using UV-LEDs as light sources. Water research; 2018. [27] Lu C, Lu Z, Li Z, Leung CK. Effect of graphene oxide on the mechanical behavior of strain hardening cementitious composites. Constr Build Mater 2016;120:457–64. [28] Meng T, Guo Q, Xi W, Ding W, Liu X, Lin N, et al. Effect of surface etching on the oxidation behavior of plasma chromizing-treated AISI440B stainless steel. Appl Surf Sci 2018;433:855–61. [29] Yin R, Sun J, Xiang Y, Shang C. Recycling and reuse of rusted iron particles containing core-shell Fe-FeOOH for ibuprofen removal: adsorption and persulfatebased advanced oxidation. J Clean Prod 2018;178:441–8. [30] Hoign�e J, Bader H. Rate constants of reactions of ozone with organic and inorganic compounds in water—II: dissociating organic compounds. Water Res 1983;17(2): 185–94.
Acknowledgement The authors would like to acknowledge the financial support from the China Ministry of Science and Technology under the Grant 2015CB655100, and the Natural Science Foundation of China under the Grant 51302104, and the Shanghai Tongji Gao Tingyao Environmental Science and Technology Development Foundation (STGEF). Support of the corresponding author by the Hong Kong PhD Fellowship is also gratefully acknowledged. The authors would like to express their appreciation to Dr. Jing YU at HKUST for useful scientific discussion, to Dr. Xinkun LU at Nano and Advanced Materials Institute Limited (Hong Kong SAR, China) for his help on MTS operation. The authors also thanks to the great helps from technicians from the Materials Charac terization and Preparation Facility at the HKUST, Dr. Borong SHI, Dr. Fanny L.Y. SHEK, Ms. Carrie W.Y. LAW, Mr. Nick K.C. HO, Ms. Yan ZHANG, Mr. Alex H.K. WONG and Ms. Christine P⋅Y. CHEUNG. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107446. References [1] Li Y, Li W, Deng D, Wang K, Duan WH. Reinforcement effects of polyvinyl alcohol and polypropylene fibers on flexural behaviors of sulfoaluminate cement matrices. Cement Concr Compos 2018;88:139–49. [2] Lu Z, Hanif A, Lu C, Sun G, Cheng Y, Li Z. Thermal, mechanical, and surface properties of poly (vinyl alcohol)(PVA) polymer modified cementitious composites for sustainable development. J Appl Polym Sci 2018;135(15):46177. [3] Yoo D-Y, Shin H-O. Bond performance of steel rebar embedded in 80–180 MPa ultra-high-strength concrete. Cement Concr Compos 2018;93:206–17. [4] Ferreira SR, Pepe M, Martinelli E, de Andrade Silva F, Toledo Filho RD. Influence of natural fibers characteristics on the interface mechanics with cement based matrices. Compos B Eng 2018;140:183–96. [5] Fu C, Ye H, Wang K, Zhu K, He C. Evolution of mechanical properties of steel fiberreinforced rubberized concrete (FR-RC). Compos B Eng 2019;160:158–66. [6] Birchall J, Howard A, Kendall K. Flexural strength and porosity of cements. Nature 1981;289(5796):388.
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Surface modification of polyethylene fiber by ozonation and its influence on the mechanical properties of Strain-Hardening Cementitious Composites Zeyu Lu, Ran Yin, Jie Yao *, Christopher K.Y. Leung Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyethylene (PE) fiber Strain-hardening cementitious composites (SHCC) Fiber surface treatment Ozonation Interfacial properties
Polyethylene (PE) fibers are widely used to develop high strength Strain-Hardening Cementitious Composites (SHCC). Unlike Polyvinyl Alcohol (PVA) fiber, which has relatively low tensile strength and strong bond with matrix, the high tensile strength of the PE fiber is not fully utilized in the system, due to the hydrophobic nature of the fibers. In this study, a promising fiber treatment method by using ozone and ozone-derived hydroxyl radicals is developed to modify the surface properties of PE fibers, aiming to improve the fiber/matrix in teractions and then the tensile performance of resulting SHCC. Firstly, feasibility and mechanism of ozonation are revealed by competition kinetic technology, and the XPS results indicate that only hydroxyl groups can be introduced to the PE fiber by optimizing the ozone (O3) concentration and treating time, and surface roughness of the PE fiber is also increased due to the etch effect from ozonation. For the tensile performance of SHCC, compared to the composites with pristine fibers, PE fibers treated with O3 for 30 and 60 min can improve the ultimate tensile strain of SHCC by 1.3 and 2.5 times, respectively. This significant enhancement in deformation capacity of SHCC is attributed to the increased chemical bond and frictional bond after fiber treatment, as the results from single fiber pullout tests show that the fiber/matrix frictional bond is increased from 2.35 MPa (pristine PE) to 3.13 MPa (O3 (30 min)/PE) and 3.38 MPa (O3 (60 min)/PE). The research outcomes provide a novel way on surface treatment for PE fibers to improve the tensile performance of SHCC.
1. Introduction Generally speaking, quasi-brittle cement-based materials have two intrinsic drawbacks: brittleness and low flexural/tensile strength and strain. The addition of steel rebars, polymer or fibers has been the most common way to improve the ductility and tensile performance of cement-based materials [1–5]. For instance, macro-defect free (MDF) cement is a high-strength cement-polymer composites produced by mixing cement and polymer (commonly polyvinyl alcohol, PVA), under high shear force, modest pressure and temperature [6,7]. The flexural strength of MDF can easily reach up to 150–300 MPa, which is around 30 times higher than the normal concrete. The mechanism behind is that hydroxyl groups (-OH) of PVA can chemically interact with the calcium and aluminate phases released during the hydration reaction of cement, to form a strong cross-linked polymer-cement matrix [8–10]. Moreover, through the molecular dynamic simulation, the interfacial properties between the PVA and cement are found to be strengthened due to the H-bonds interaction between the –OH in PVA and silicate oxygen atoms in the C–S–H [11]. Besides MDF, Fiber Reinforced Cementitious
Composites (FRCC) have been rapidly developed in the past few de cades. Through incorporating fibers into cementitious matrix, such as polypropylene (PP), polyethylene (PE), polyvinyl alcohol (PVA) or carbon fiber, the flexural/tensile strength, ductility and toughness of FRCC can be improved by preventing and/or controlling the initiation, propagation and coalescence of cracks. Specifically, a type of high per formance FRCC named Strain Hardening Cementitious Composites (SHCC) have been developed since 1990’s by Li and co-workers under the guidance of micromechanics, have been developed by Li et al. [12]. This novel material is distinguished by the strain-hardening behavior up to several percent strain as well as outstanding crack control (maximum crack width below 60 μm). With the addition of synthetic fibers (PVA, PE, ETC.) is distinguished by the strain hardening behavior up to several percent [13–15]. However, PVA fiber often ruptures during the pullout process from cementitious matrix, because the tensile strength of PVA fiber (1 GPa, KURALON K-II REC 15 [16]) is not sufficient to tolerate the high interfacial bond coming from the chemical interaction between the –OH in PVA fiber and cement hydrates. To this end, oil coating is usually applied to reduce the bond between the PVA fiber and cementitious
* Corresponding author. E-mail addresses: [email protected] (Z. Lu), [email protected] (R. Yin), [email protected] (J. Yao), [email protected] (C.K.Y. Leung). https://doi.org/10.1016/j.compositesb.2019.107446 Received 15 May 2019; Received in revised form 4 September 2019; Accepted 12 September 2019 Available online 13 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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matrix. However, for those fibers with high strength but inert surface properties (e.g., PE fibers), it is of importance to improve the fiber/ matrix interfacial bond to fully utilize the high strength of PE fibers, and finally enhance the mechanical behavior of SHCC. Inspired from the chemical composition of PVA fiber, if –OH can be introduced on the surface of PE fibers after treatment, a stronger bond between the PE fibers and cementitious matrix is expected to be achieved [17]. In fact, various kinds of surface treatments, such as chemical or physical surface coating, ion/plasma beams and surface oxidization, have been used to modify the properties of fibers with inert surface to make them rougher or capable of higher chemical reactivity [16,18–20]. Among these treatments, surface coating is the most popular method due to the easy operation process, which only needs to immerse the fiber in a solution with coating materials under mechanically mixing and specific conditions (e.g., certain temperature, ultrasonication, etc.), following by lifting and drying. For example, Lu et al. [21] coated a thin SiO2 layer onto carbon fibers through the condensation and polymerization of the tetraethyl orthosilicate under alkaline conditions to enhance the inter facial bond between the carbon fiber and cementitious matrix. Relying on the reaction between the surficial-coated SiO2 on carbon fiber and Ca (OH)2 in cement hydrates, the chemical debonding energy and frictional bond were 10 and 2 times of the pristine carbon fiber, respectively. Yang et al. [22] coated carbon nano-fibers (CNFs) on the surface of cluster-like PE fibers through hydrophobic interactions, and indicated that the interface transition zone between the PE fibers and matrix was strengthened by filling nano-pores and cross-linking nano-cracks, lead ing to a 15.0% and 20.0% enhancement in tensile strength and tensile strain capacity of PE-SHCC, respectively. However, based on the au thors’ experience, the surface coating process is not effective to treat cluster-like fibers, especially for those located in the inside of the cluster [17]. Without extra pressure to push coating materials into the inside of fiber cluster, only the mechanical mixing is not enough to make all cluster-like fibers fully coated. More importantly, the dispersion of nanomaterials themselves is a key problem, as discussed in Ref. [22], and it might become even worse if mixed with fibers. The non-uniform coating of cluster-like PE fibers can lead to unstable and volatile rein forcing efficiency in SHCC. The cluster-like PE fibers are anticipated to be oxidized more uniformly and effectively. Inspired from the strong interfacial bond between the PVA polymer/ fiber and cementitious matrix, it is believed that the introduction of hydroxyl groups into PE fiber is an efficient way to improve the inter action between the PE fibers and SHCC matrix. Ozone (O3) is a powerful oxidant with a redox potential of 2.07 V, which is usually generated on-site through electrolysis of oxygen in labscale and industrial applications [23,24]. Due to the higher oxidation power, many inorganic (e.g. metals, sulfide, bromide etc.) and organic substances can be oxidized by ozone, through direct ozonation or the oxidation by the secondary reactive species (e.g., hydroxyl radicals) derived from ozone. However, the ozonation for PE fibers has not yet been reported and whether the ozonation can introduce oxygen groups, especially for hydroxyl groups, to PE fibers remains unknown. The generated ozone can be directly supplied in the gas form and it can also be easily dissolved in water. To the authors’ best knowledge, the gas form of ozone released in water will generate many air bubbles with pressure to push ozone into the inside of the cluster of the target (e.g., PE fiber). By optimizing the ozone concentration and treating time, it is believed that the cluster-like PE fibers have a great potential to be oxidized more uniformly and effectively. The aim of this study is to activate the surface properties of PE fibers by ozonation to improve the chemical and physical bond with cemen titious matrix, and finally to improve the reinforcing efficiency of PE fibers in SHCC. Firstly, PE fibers were oxidized by controlling the ozonation time under an optimized ozone concentration. The mecha nism and feasibility of ozonation to PE fibers for introducing hydroxyl groups were analyzed through competition kinetic technology, and the surface properties of treated PE (O3/PE) fibers were characterized by
SEM/EDX, XPS and surface roughness test. Secondly, the O3/PE fibers were used to fabricate SHCC to investigate the influence of fiber ozon ation on the tensile performance of PE-SHCC, in terms of tensile strength, ultimate tensile strain and crack width control. A digital image processing method was adopted to investigate the crack pattern of SHCC under tension. The interfacial microstructure between the O3/PE fibers and cementitious matrix was then analyzed by SEM. In addition, the single fiber pullout tests were conducted to validate the bond between the fiber and matrix, and gave a deep insight on how efficient of O3 treatment can strengthen the interaction between the fibers and matrix. 2. Experimental preparation 2.1. Raw materials Portland cement subjecting to the requirement of type 52.5 N (Green Island@, Hong Kong) and silica fume (SF, Elkem® Microsilica 920U) were used as the binder to fabricate the SHCC. Silica sand with a mean diameter of 150 μm was used as the fine aggregate. A polycarboxylatebased agent with 28.0% solid ratio was used as the superplasticizer for the SHCC. In addition, 2.0 vol % of PE fiber was added for all mixes to produce the SHCC, and the physical properties of the PE fiber (Quan tumeta®, China) are shown in Table 1, while Table 2 shows the mix proportions of all mixes. 2.2. Samples preparation 2.2.1. Fabrication and characterization of O3/PE fiber Fig. 1 shows the schematic diagram of the ozonation set-up for PE fibers. The ozonation treatment for PE fibers (18.3 g, 2 vol% for one batch of sample) was conducted in a 2L glass reactor containing ultra pure water, and the ozone was generated using an ozone generator (10K–2U, Enaly, China) at a constant flow rate for 30 min and 60 min, respectively, named as O3(30 min)/PE fiber and O3(60 min)/PE fiber. It should be pointed out that the ozone bubbles are beneficial in making PE fiber clusters and ozone fully contacted so that even the PE fibers inside the cluster can also be fully oxidized. In addition, competition kinetic technology was applied to determine the steady-state concentrations of dissolved ozone and hydroxyl radicals generated during the ozonation process [26]. Specifically, nitrobenzene (NB) and benzoic acid (BA) were used as two probe compounds to determine the steady-state con centrations of ozone and hydroxyl radicals. 1-μM NB and BA were spiked to the glass reactor containing PE fiber and ultrapure water before ozonation starts. Samples from two probes were collected at different time intervals, quenched with sodium sulfite at a sulfite-to-ozone molar ratio around 1.2:1, and analyzed for the remaining probe compound concentrations using ultra-performance liquid chromatography (UPLC) (VP series, Shimadzu) equipped with a Waters symmetry C18 column and a UV–Vis detector. Eluents of water (pH 3, adjusted using phos phoric acid) and methanol (55:45, v/v %) were used to separate the NB and BA at a flow rate of 1.0 mL/min. In addition, X-ray photoelectron spectroscopy (XPS, Physical Electronics 5600 multi-technique system) was employed to investigate the carbon atomic composition of the PE fiber and O3/PE fiber. Furthermore, surface morphologies and element analysis of the testing fibers were characterized by Scanning Electronic Microscopy (SEM, JEOL6700F) equipped with EDX.
Table 1 Physical properties of the PE fiber [25].
2
Length (mm)
Diameter (μm)
Tensile strength (MPa)
Elastic Modulus (GPa)
Density (g/ cm3)
12
24
3000
120
0.97
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2.3. Fabrication of PE and O3/PE fibers reinforced SHCC
Table 2 Mix proportions. Specimen PE-SHCC O3(30 min)/PEFRCC O3(60 min)/PEFRCC
Binder
Sand
w/c
PCE
PE or O3/PE fibers (vol%)
Cement
SF
1 1
0.25 0.25
0.375 0.375
0.20 0.20
0.03 0.03
2.0 2.0
1
0.25
0.375
0.20
0.03
2.0
To achieve a better dispersion of fibers in the SHCC matrix, solid materials, including cement particles, silica fume and sand were firstly mixed at a low speed for 5 min. After that, water containing predissolved PCE superplasticizer was added into the mixture at once and then mixed in a high speed for the other 5 min. After that, fibers were added and mixed at a low speed for 3 min and then at a high speed for the other 3 min. During this process, the fresh mixture on the wall of the mixing container was scraped down to ensure the fiber volume fraction is 2%. Finally, the specimens were cast into pre-oiled dog-bone molds [27] and covered with a thin plastic sheet to prevent the water evapo ration. After 24 h curing in an ambient environment, the specimens were demolded and placed into a curing room (25 � C/RH 95%) for 13 days before the test. In addition, without special instruction, three samples were conducted to get the average result for each test.
Note: SF, Sand, w/c and PCE are by weight of the cement.
2.3.1. Samples for single fiber pullout test The mold used for single fiber pullout test is shown in Fig. 2. The first step is to use 2 horizontal screws to secure two ‘bottom parts’ and one ‘middle part’ together. The ‘middle part’ is in U shape, so after being sandwiched by two bottom parts, there is a gap between two ‘bottom parts’ which can be cast into matrix. The second step is to place PE fibers on the pre-cut notches on the top surface of two ‘bottom parts’, and then stick a very thin layer of tape to prevent fiber sliding and moving. The third step is to use 6 vertical screws to secure two ‘upper parts’ on the ‘bottom parts’, so the fiber will be clamped. The last step is to cast the matrix into the gap, and then vibrate the mold until all the visible air bubbles trapped in the mold are expelled out (the mold is made of polymethyl methacrylate which is transparent, so it can be observed from side view that whether there is air bubble trapped in the matrix). The sample will be demolded after 48 h and then cured in curing room (25 � C/RH 95%) for another 12 days. Since the sample prepared by this
Fig. 1. Ozonation set-up for PE fibers.
Fig. 2. The mold and sample size for single fiber pullout test. 3
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mold is a long continuous rectangle with many fibers through it, it will be cut into many small pieces with only one fiber through each piece at least 24 h before the test. The protruded fiber on one side was cut off with a sharp blade. Since thickness of the middle part is 3 mm, the fiber embedded length is also 3 mm.
increases to 30 min and 60 min, the relative content of C–OH functional groups is continuously increased to 15.9% and 38.0%, respectively. The XPS results not only confirm the transformation of C–C bond to C–OH bond after O3 treatment, but also indicate that only –OH groups can be generated when the ozonation time is less than 60 min. Therefore, the chemical activated and physical roughened surface of PE fiber with O3 treatment has a great potential to strengthen the interfacial bond be tween the fiber and SHCC matrix, which will be deeply discussed in the section 3.2 and 3.3. However, it should be pointed out that further in crease in the ozonation time will lead to a stronger oxidation of PE fiber, which cannot ensure only hydroxyl groups being introduced to the O3/PE fiber. In order to quantify how is the surface roughness condition of PE fiber before and after O3 treatment, surface roughness parameters of PE and O3/PE fiber were characterized and imaged by a 3D Surface Metrology (Bruker NPFLEX) with the mode of Vertical Scanning Inter ferometry (VSI). Three roughness measurements were conducted on eight testing areas of each sample and the average value was calculated in terms of Ra (average roughness calculated over the entire measured array) and Rq (root mean square roughness). As displayed in Fig. 5, the altitude difference in vertical direction of PE fiber is only one third of O3(30 min)/PE fiber, and more color patterns can be seen in Fig. 5b, indicating that O3 has a strong etch effect on the surface of PE fiber. To be more specific, Table 3 lists the variation of the two important roughness parameters before and after ozonation treatment, and it in dicates that both parameters of O3/PE fibers are more than three times higher than that of PE fiber, revealing that the surface roughness of PE fibers can be greatly improved by O3 treatment, which lays a foundation for a stronger physical friction between treated PE fiber and cementi tious matrix during fiber pullout process. The mechanism behind the ozonation treatment for PE fibers, to the authors’ best knowledge, is believed to be contributed by both ozone itself and the reaction species (e.g. hydroxyl radicals) derived from ozone. Under our test conditions, the concentrations of ozone and ozonederived hydroxyl radicals were quantified using competition kinetic
3. Results and discussion 3.1. Feasibility and mechanism of O3 treatment for PE fibers In this study, the oxidation of PE fiber is controlled by different ozonation time under a constant ozone flow, and the surface properties of PE fiber after O3 treatment, including chemical composition, surface morphology and roughness, are characterized by XPS, SEM and rough ness test, respectively. Fig. 3 shows the morphology and elementary composition of PE and O3/PE fiber. It is to see that the O3 treatment makes the surface of PE fiber rougher due to the etch effect of oxidation process [28], which is beneficial to improving the friction bond between the fiber and matrix during the fiber pullout process. From Fig. 3, the oxygen wt. % of the O3/PE fiber increases with the increasing ozonation time, which can reach up to 6.92 wt % and 11.10 wt % after 30 min and 60 min of ozonation treatment, respectively, indicating that the PE fiber can be variously oxidized by the O3 treatment. In order to quantitatively determine the content of carbon-oxygen species of PE fiber with different ozonation time, XPS was conducted to investigate the chemical state of the carbon atoms in PE and O3/PE fiber. Fig. 4 shows the XPS C1s spectra of PE fiber and O3/PE fiber after 30 min and 60 min of ozonation. PE fiber only shows a single peak at 284.8 eV, corresponding to the C–C vibration, as shown in Fig. 4a. However, the asymmetric shape of the C1s peaks of the O3/PE fiber reveals the existence of bonds other than those of pristine PE fiber. By deconvoluting the C1s spectrum, hydrox ylation is the major reason leading to the asymmetry of the C1s peak. Specifically, with the increase of ozonation time, the level of oxidation of PE fiber is also increased, which can be reflected by the higher in tensity of C–OH appeared at 286.3 eV. When the ozonation time
Fig. 3. SEM/EDX result of (a) PE fiber (b) O3(30 min)/PE fiber (c) O3(60 min)/PE fiber. 4
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Fig. 4. XPS C1s spectra of (a) PE fiber, (b) O3(30 min)/PE fiber and (c) O3 (60 min)/PE fiber.
and 4.63 � 10 5 M respectively. While in the presence of PE fiber, the pseudo first order rate constants of NB and BA degradation decreased to 1.112 and 1.683, respectively. Accordingly, as shown in Table 4, the calculated steady-state concentrations of hydroxyl radicals and ozone decreased to 4.752 � 10 12 M and 1.033 � 10 5 M respectively. The differences on the ½HO�ss and ½O3 �ss between the two cases (i.e., with or without PE fiber) were thus calculated to be 1.303 � 10 12 M and 3.597 � 10 5 M respectively, which were the quantities of hydroxyl radicals and ozone that were consumed by PE fiber.
technology [29]. Nitrobenzene (NB) and benzoic acid (BA) were used as probe compounds, and their degradation kinetics were used to quantify the ozone and hydroxyl radicals in the absence and presence of PE fiber. NB exhibits high reactivity towards hydroxyl radicals (k ¼ 3.9 � 109 M 1s 1) but hardly reacts with ozone [30]. BA reacts rapidly with hydroxyl radicals (k ¼ 5.9 � 109 M 1s 1) and also reactive towards ozone (k ¼ 1.2 M 1s 1). Therefore, the degradation of NB and BA during ozonation process can be expressed by Eqs. (1) and (2): rNB ¼ ko;NB ½NB� ¼ kHO;NB ½HO�ss ½NB�
Eq. 1
rBA ¼ ko;BA ½BA� ¼ kHO;BA ½HO�ss ½BA� þ kO3;BA ½O3 �ss ½BA�
Eq. 2
3.2. Tensile properties of SHCC with treated PE fibers Fig. 7 shows the direct tension sample configuration and strain-stress curves of PE-SHCC and O3/PE-SHCC under direct tension. There were three samples tested for each mix, but some results were not represen tative, therefore, only two representative curves for each mix are pre sented. Two important parameters, i.e. ultimate tensile strength and tensile strain capacity, are summarized in Table 5. It should be pointed out that the ultimate tensile strength and tensile strain capacity may not be at the same point in Fig. 7, because some curves (e.g. PE–SHCC–2) show a very high tensile load at the early stage of the curve, after that point, the ductility of the sample seems still increasing although the tensile load will not exceed that point again. In this case, the ultimate tensile strength is chosen to be the maximum tensile load, while the tensile strain capacity is chosen from the later part of curves (e.g. for
where rNB and rBA are the degradation rate of NB and BA respectively. ko;NB and ko;BA are the observed rate constants of NB and BA degradation, which can be obtained by plotting the concentration of residual NB and BA versus reaction time. kHO;NB , kHO;BA , and kO3;BA are the second order rate constants of NB and BA toward hydroxyl radicals and ozone, which can be obtained from literature and have been listed above. ½HO�ss and ½O3 �ss are the steady-state concentrations of hydroxyl radicals and ozone respectively. Fig. 6 shows the time-dependent degradation kinetics of NB and BA during ozonation in the presence and absence of PE fiber. In the absence of PE fiber, the pseudo first-order rate constants of NB and BA degradation were 1.417 and 2.147 min 1 respectively. Based on Eqs. (1) and (2), the ½HO�ss and ½O3 �ss were calculated to be 6.056 � 10 12 M 5
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Fig. 5. Surface roughness image of (a) PE fiber (b) O3(30 min)/PE fiber (c) O3(60 min)/PE fiber. Table 3 Surface roughness parameters of the PE,O3(30 min)/PE and O3 (60 min)/PE fiber. Roughness parameters
PE fiber (nm)
O3/PE fiber (30 min) (μm)
O3/PE fiber (60 min) (μm)
Ra Rq
300.7 368.5
1.24 1.39
2.57 3.00
PE–SHCC–2, the tensile strain capacity is 2.1% instead of 1.1%). From Table 5, it reveals that after the ozonation treatment for PE fiber, ulti mate tensile strength decreases, while the tensile strain capacity in creases. For the decrease in the ultimate tensile strength, some reasons from the published paper [12] may due to the stochastic intrinsic from sample to sample, for example, the ultimate tensile strength will significantly decrease if there is a large size flaw in the sample (so the fiber volume fraction will decrease in that section). And also, since ozonation treatment improves the interfacial bond between the PE fiber and SHCC matrix, the other plausible explanation for the decrease of the ultimate tensile strength may be because of the fiber rupture, but further study is needed to prove this conjecture. For the tensile strain capacity, from Table 5, it can be found that the tensile strain capacity of O3(30 min)/PE-SHCC and O3(60 min)/PE-SHCC is 3.2% and 4.9%, respectively, which is 1.5 and 2.3 times higher than that of pristine PE-SHCC (2.1%). The tensile strain capacity is the most important parameter in practical design. SHCC with larger tensile strain capacity
Fig. 6. The time-dependent degradation kinetics of NB and BA during ozona tion in the presence and absence of PE fiber.
stands for the better energy absorption, crack width control, durability, etc. Here, the reason for the improvement of tensile strain capacity is the densified microstructure of fiber/matrix interface. Fig. 8 shows the interfacial microstructure between the PE fiber and SHCC matrix with 6
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However, after 30 min of ozonation treatment, the surface property of PE fiber is modified by introducing –OH group and fiber becomes hy drophilic, therefore, a more densified interface between O3(30 min)/PE fiber and SHCC matrix can be observed in Fig. 8d, e and f. The interfacial microstructures of O3(60 min)/PE-SHCC are displayed separately in Fig. 9 as they are quite different from that of PE-SHCC and O3(30 min)/PE-SHCC. It can be seen that the interface of O3(60 min)/ PE-SHCC is denser than O3(30 min)/PE-SHCC, no obvious gap can be observed and the fiber seems to be fully covered by cement hydrates. More cement hydration products can be found on the surface of O3(60 min)/PE fiber, as shown in Fig. 9d. This results from the hydroxyl groups introduced by the ozonation treatment, which can chemically
Table 4 The calculated steady-state concentrations of hydroxyl radicals and ozone dur ing ozonation in the presence and absence of PE fiber. [HO]ss w.o. PE fiber w. PE fiber Difference (G1-G2)
6.056 � 10 4.752 � 10 1.303 � 10
[O3]ss 12
M 12 M 12 M
4.63 � 10 5 M 1.033 � 10 5 M 3.597 � 10 5 M
and without ozonation (30 min) treatment. Since the PE fiber has inert surface and is hydrophobic, incompact interface (Fig. 8a and b) and big gaps (Fig. 8c) between the fiber and matrix can be clearly observed.
Fig. 7. The direct tension test sample and tensile performance of (a) PE-SHCC, (b) O3(30 min)/PE-SHCC and (c) O3(60 min)/PE-SHCC. 7
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Table 5 Tensile properties of PE and O3/PE fiber reinforced FRCC. Sample
Ultimate tensile strength (MPa)
Tensile strain capacity (%)
PE–SHCC–1 PE–SHCC–2 Average O3(30min)/ PE–SHCC–1 O3(30min)/ PE–SHCC–2 Average O3(60 min/PE)– SHCC–1 O3(60min)/ PE–SHCC–2 Average
7.2 7.8 7.5 6.0
2.0 2.1 2.1 3.6
5.5
2.8
5.8 6.3
3.2 4.6
6.0
4.9
6.2
4.8
where Pmax is the maximum fiber pullout force obtained from the test, df is the fiber diameter (24 μm), Lf is the fiber embedded length (3 mm) and η is the effective fiber volume friction which can be considered as 0 here. The effective interfacial stress reveals the bond quantitively, including both chemical and frictional bond. The average τeff for pristine PE, O3 (30 min)/PE and O3 (60 min)/PE fibers are 2.35 MPa, 3.13 MPa and 3.38 MPa, respectively, therefore, it validates the idea that the ozonation strengthen the bond between the PE fibers and cementitious matrix. From Fig. 10, it can be seen that the variation of the testing data is quite large, two reasons are given in the following: (1) the sample size is too small. The diameter of the fiber is only 24 μm and the thickness of the matrix is only 3 mm which makes the sample very difficult to cast and test. Effects such as misalignment of the fiber to the loading direc tion, bad compacting of the matrix which introduces voids near the fiber/matrix interface, intrinsic error of the testing setup, etc. are all very likely to occur and thus influence the results; (2) since the scale of the sample is very small, the variation of the material properties also influences the results. For example, the average size of the quartz sand used in the matrix is 150 μm, much larger than the diameter of the fiber, therefore, the interface along the longitudinal direction of the fiber may be inhomogeneous and thus influences the testing results. From litera tures, the results of single fiber pullout tests also show very large vari ation [23–25]. Therefore, further study, such as improving the test setup or increasing the sample number, is needed to make the results more convincing. Fig. 11 shows the SEM photos of pristine PE fiber and O3(30 min)/PE fiber after pullout test. It can be seen that the pristine PE fiber has a very smooth surface, while there are many small particles adhering on the surface of O3(30 min)/PE fiber. This phenomenon also provides strong evidence that the ozonation is very effective to strengthen the fiber/ matrix bond.
interact with the calcium phase in C–S–H through ionic bond, as dis cussed in the section 3.1, and finally strengthen the bond between the fiber and matrix. Moreover, through the comparison among Fig. 8c, f and Fig. 9c, the surface of fiber becomes rougher with the increase of ozonation treatment time, so it double confirmed that the frictional bond between the fiber and matrix can be improved with the increasing time of ozonation treatment. In summary, the ozonation treatment is an effective method to chemically and physically activate the surface properties of PE fiber by introducing hydroxyl groups to PE fibers. After the fiber treatment, the ultimate tensile strength varies within the error range, but a significant improvement on the tensile strain capacity can be achieved. 3.3. Single fiber pull-out test results Single fiber pullout test was conducted in order to measure the fiber/ matrix bond directly. Five samples for PE and O3(30min)/PE and ten samples for O3(60min)/PE have been tested. Some samples failed during the installment to the test machine, so only a part of sample results are presented in the Fig. 10. The sample test setup and maximum single fiber pullout forces are summarized in Fig. 10, which can be used to calculate the effective interfacial stress by Eq. (3) [10,22]:
τeff ¼
Pmax
πdf Lf ð1 þ ηÞ
�
Pmax
πdf Lf
3.4. Crack width control Another advantage of improving the fiber/matrix bond is that the crack width can be smaller. In the section 3.2, during the direct tension test, photos were taken every 0.5% increment of strain, and then a digital image processing method was used to obtain the average crack width. Fig. 12 shows the grayscale images of PE-SHCC and O3(30 min)/ PE-SHCC under different tensile strains. The average crack width can be calculated through the following equation:
Eq. 3
Fig. 8. Interfacial microstructures between the SHCC matrix and (a, b, c) pristine PE fiber (d, e, f) O3 (30 min)/PE fiber. 8
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Fig. 9. Interfacial microstructures between the FRCC matrix and (a, b, c) O3 (60 min)/PE fiber and (d) fracture surface morphology of O3(60 min)/PE fiber.
Fig. 10. Test setup and maximum single fiber pullout force.
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Fig. 11. Morphology of (a) PE fiber (b, c) O3(30 min)/PE fiber after single fiber pull out test.
adding 1% elongation so the total length is 80 � ð1 þ 1%Þ ¼ 80:8mm. Terefore, the size of every pixels is 80:8 =m mm which will be denoted as Cof. in Eq. (4). By adopting this approach, the average crack width are summerized in Fig. 13. From Fig. 13, it can be observed that the average crack width of O3(30 min)/PE-SHCC is generally smaller than PE-SHCC, which indicates that strengthening the bond between the PE fiber and matrix is also beneficial for crack width control. 4. Conclusions In this study, a novel fiber treatment method by using ozone was developed to modify the surface properties of PE fibers. Hydroxyl groups (-OH) were introduced to PE fiber and made the hydrophobic PE fiber become hydrophilic, so more cement hydrates could be precipitated on the surface of fiber to form a densified interfacial microstructure. The
Fig. 12. Grayscale image of (a) PE-SHCC (b) O3(30 min)/PE-SHCC under different tensile strains.
Average crack opening width ¼
Total area of black pixals *Cof Crack number*sample width
Eq. 4
The Cof. in Eq. (4) means the correponding size of the pixels. For example, the grayscale image of PE-SHCC under 1% tensile strain is consist of ½m �n� pixelss. The original length of the sample is 80 mm,
Fig. 13. The development of average crack opening width and average crack spacing for PE-SHCC and O3(30 min)/PE-SHCC. 10
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newly introduced –OH functional groups can provide extra ionic and H bond between the PE fiber and cementitious matrix, which was bene ficial for strengthening the fiber/matrix interface. Meanwhile, the etching effect of ozonation made the surface of fiber rougher, which also improved the frictional bond between the fiber and matrix during fiber pullout process. In addition, direct tension behavior of SHCC with pristine PE fiber and O3/PE fiber are investigated, and the results show that the tensile strain capacity of O3/PE-SHCC is much higher than that of PE-SHCC. To be more specific, the strain capacity for PE-SHCC is 2.1%, while for 30 min and 60 min O3/PE-SHCC was 3.2% and 4.9%, respectively. On the other hand, the results of single fiber pullout tests indicated that the effective interfacial stress between the fiber and ma trix was increased from 2.35 MPa (PE), 3.13 MPa (O3(30 min)/PE) and 3.38 MPa (O3(60 min)/PE), respectively, which validated that the ozonation treatment of fiber can strengthen the fiber/matrix bond, leading to a better tensile performance and strain-hardening behavior of SHCC.
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Acknowledgement The authors would like to acknowledge the financial support from the China Ministry of Science and Technology under the Grant 2015CB655100, and the Natural Science Foundation of China under the Grant 51302104, and the Shanghai Tongji Gao Tingyao Environmental Science and Technology Development Foundation (STGEF). Support of the corresponding author by the Hong Kong PhD Fellowship is also gratefully acknowledged. The authors would like to express their appreciation to Dr. Jing YU at HKUST for useful scientific discussion, to Dr. Xinkun LU at Nano and Advanced Materials Institute Limited (Hong Kong SAR, China) for his help on MTS operation. The authors also thanks to the great helps from technicians from the Materials Charac terization and Preparation Facility at the HKUST, Dr. Borong SHI, Dr. Fanny L.Y. SHEK, Ms. Carrie W.Y. LAW, Mr. Nick K.C. HO, Ms. Yan ZHANG, Mr. Alex H.K. WONG and Ms. Christine P⋅Y. CHEUNG. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107446. References [1] Li Y, Li W, Deng D, Wang K, Duan WH. Reinforcement effects of polyvinyl alcohol and polypropylene fibers on flexural behaviors of sulfoaluminate cement matrices. Cement Concr Compos 2018;88:139–49. [2] Lu Z, Hanif A, Lu C, Sun G, Cheng Y, Li Z. Thermal, mechanical, and surface properties of poly (vinyl alcohol)(PVA) polymer modified cementitious composites for sustainable development. J Appl Polym Sci 2018;135(15):46177. [3] Yoo D-Y, Shin H-O. Bond performance of steel rebar embedded in 80–180 MPa ultra-high-strength concrete. Cement Concr Compos 2018;93:206–17. [4] Ferreira SR, Pepe M, Martinelli E, de Andrade Silva F, Toledo Filho RD. Influence of natural fibers characteristics on the interface mechanics with cement based matrices. Compos B Eng 2018;140:183–96. [5] Fu C, Ye H, Wang K, Zhu K, He C. Evolution of mechanical properties of steel fiberreinforced rubberized concrete (FR-RC). Compos B Eng 2019;160:158–66. [6] Birchall J, Howard A, Kendall K. Flexural strength and porosity of cements. Nature 1981;289(5796):388.
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