Impact of surface plasma treatment on the performance of PET fiber reinforcement in cementitious composites J. Trejbala , L. Kopeck´ya , P. Tes´areka , J. Fl´adra , J. Antoˇsa , M. Somra , V. Neˇzerkaa,∗ a
Faculty of Civil Engineering, Czech Technical University in Prague, Th´akurova 7, 166 29 Praha 6, Czech Republic
Abstract The purpose of this study is to demonstrate the influence of plasma treatment on the surface properties of PET fibers used as micro reinforcement in cementitious composites. The stress transfer across cracks that untreated fibers are able to accomplish can be enhanced via plasma treatment. The increase in surface roughness and the activation of polar groups result in the reduction of surface energy as observed during wetting angle measurements. The improvement in adhesion between primary (non-recycled) fibers and the surrounding matrix brought about a more pronounced strain hardening of the specimens tested in four-point bending. Plasma treatment of thinner fibers from recycled PET led to a lower capacity for transferring tensile stresses due to the reduced crosssectional area of the fibers. The considerable bridging force provided by the plasma treated primary PET fibers resulted in the prevention of excessive and abrupt cracking, and limited crack openings. Keywords: Plasma, Fiber Reinforcement (E), Microstructure (B), Bond Strength (C), Portland Cement (D)
1
1. Introduction
2
Fiber-reinforced concretes with discrete, short, and randomly distributed fibers have become
3
popular in the construction industry over the past few decades. In some cases, fibers are intended
4
to be the primary or even only reinforcement, while in other cases they only replace a portion
5
of conventional reinforcement provided by steel rebars [1, 2, 3]. In order to reinforce a cementi-
6
tious matrix, fibers must be easy to disperse in order to ensure uniform distribution, have suitable ∗
Corresponding author Email address:
[email protected] (V. Neˇzerka)
Preprint submitted to Cement and Concrete Research
August 18, 2016
7
mechanical properties, and must be durable even with a long-term exposure to an alkaline cemen-
8
titious matrix [4, 5]. All of the above mentioned properties have been reported for poly(ethylene
9
terephthalate) (PET) fibers [6, 7].
10
Machoviˇc et al. [8] have revealed that newly-formed multi-molecular layers enable chemi-
11
cal bonding between a PET surface and C-S-H gels in hydrated cements. Moreover, it has been
12
established that PET fibers do not have any influence on the hydration of Portland cement [6].
13
The use of PET fibers enhances concrete properties and utilizes waste produced by the disposal
14
of PET beverage containers effectively. Current worldwide production of PET products exceeds
15
6,700 million kg/year and a dramatic increase of production has been reported in China and In-
16
dia [5]. Recently, recycled PET has been utilized as replacement for conventional steel bars or
17
carbon fiber reinforced polymer (CFRP) strips [9, 10, 11], because of its corrosion-resistant nature
18
and low cost.
19
PET fibers as dispersed reinforcement in engineered cementitious composites (ECCs) can pro-
20
duce enormous deformations when compared to ordinary Portland cement concrete [12, 13, 14].
21
The key to ensuring the ductility in ECCs is fiber-bridging of cracks. To achieve this strain-
22
hardening behavior, matrix tensile strength must be lower than the maximum bridging stress that
23
can be transferred by fibers [15]. The hydrophobic surface of PET fibers results in poor adhesion
24
to any cementitious matrix and can therefore be very limiting [16, 17]. To overcome this, various
25
strategies have been employed to modify the surface structure of the fibers in order to make them
26
more hydrophilic. Strategies include wet chemical treatment, flame treatment, mechanical micro-
27
indentation, and various plasma treatments. Wet chemical treatment such as etching in an alkaline
28
environment may cause a significant loss of mechanical fiber strength and the etching intensity is
29
difficult to control. The disposal of waste products from such chemical modifications can intro-
30
duce environmental burdens [18]. Flame or heat treatment causes fibers to become brittle under
31
tensile stresses and can even trigger fatal polymer degradation. Mechanical micro-indentation is
32
usually too rough and can significantly reduce the fiber cross-sections and is extremely difficult to
33
implement on short fibers with small diameters [17]. Cold plasma treatment of fibers appears to
34
be the most suitable non-damaging and energy-efficient alternative, with easily controllable out-
2
35
comes [19].
36
Plasma is a medium consisting of electrons and positively charged ions and it exhibits quite
37
different properties than common substances so is therefore referred to as the fourth state of matter.
38
Plasma is quite rare on earth due to its high energy level, but 99% of the universe is assumed
39
to consist of plasma [20]. Plasma (ion beam) treatments have been employed since the 1980s
40
to modify the surface properties of polymers, primarily in the textile industry. By selecting an
41
appropriate feedstock gas and plasma exposure duration, rapid improvement in polymer surface
42
adhesion, wettability, or dyeability can be achieved without altering original bulk properties [21,
43
22]. The effect of PET fiber plasma treatment is twofold: ion bombardment makes a surface
44
rougher and the activation of polar groups on the fiber surface results in a reduction of surface
45
energy which promotes chemical bonding with surrounding matrix [23].
46
Oxygen plasma treatment can be also considered for improving PET fibers produced directly
47
by cutting beverage bottles. Such technology was proposed and tested by Fotti [24, 25] and the
48
outcomes of our study could contribute to cheap and environmentally friendly production [26]
49
of hydrophilic fibers with necessary properties. Such reinforcement could be used in concrete to
50
increase ductility in cover layers which protect primary structural rebar reinforcements and thus
51
enhance the durability of reinforced concrete structures.
52
The materials tested in our study are presented in Section 2. Our approach was a compre-
53
hensive experimental study focused on microscopy observations of a plasma treated fiber surface
54
(Section 3.1), an investigation of treatment impact on the wetting angle (Section 3.2), and the influ-
55
ence of plasma on the response of reinforced cementitious composites to mechanical loading (Sec-
56
tion 4). Particular attention was paid to the effect of fiber surface modification on crack-bridging
57
capacity with specimens subjected to four-point bending (Section 5).
58
2. Materials
59
Portland cement CEM I 42.5R produced at the Mokr´a Plant (Czech Republic) was used to
60
produce a cementitious matrix (cement paste) lacking any aggregates. The same water to cement
61
mass ratio, 0.4, was used for preparing all mixes.
3
62
20 mm long PET fibers were used as the dispersed micro reinforcement. This study focused
63
on the performance of two different sets of fibers from primary and recycled PET (Figure 1), with
64
diameters of 400 µm and 260 µm, respectively. Both kinds of the fibers, originally intended for
65
the production of brooms and brushes, were produced by Spokar Company (Czech Republic). The
66
longitudinal grooves along the fibers (Figure 2) resulted from the production process.
(a) primary PET
(b) recycled PET
Figure 1: PET fibers used as cementitious matrix reinforcement.
(a) primary PET
(b) recycled PET
Figure 2: Optical microscopy images of untreated PET fibers.
4
67
2.1. Prepared and tested specimens
68
Altogether, cement pastes with four types of reinforcing fibers and one lacking any reinforce-
69
ment were prepared simultaneously in order to obtain results independent of the composition of
70
the cementitious matrix. Each set was represented by 6 specimens. The first, reference mix (R)
71
consisted only of Portland cement and water. Pastes reinforced by primary untreated fibers (PU),
72
primary plasma treated fibers (PT), recycled untreated fibers (RU), and recycled plasma treated
73
fibers (RT) were prepared next. The volumetric fraction of reinforcing fibers was equal to 2% of
74
cement paste weight as suggested by Machoviˇc et al. [8] and Ochi et al. [7], who found that content
75
of PET fibers below 1.5% does not provide strain hardening. On the other hand, Khaloo et al. [27]
76
found that fiber-reinforced concrete strength might be reduced after exceeding a reasonable vol-
77
ume fraction of fibers in the mix, resulting in poor workability and compaction of fresh concrete.
78
Composition, water-cement ratio, and total porosities for all mixtures are summarized in Table 1.
79
There were no aggregates or additives in the mixtures, which enabled better identification of the
80
influence of each fiber type influence on effective properties. Table 1: Summary of prepared mixes. Mix
Cement
PET Fibers
Water / Cement
Total Porosity [%]
R
CEM I 42.5R
–
0.4
25.1 ± 0.7
PU
CEM I 42.5R
primary, untreated
0.4
23.5 ± 1.3
PT
CEM I 42.5R
primary, plasma treated
0.4
26.1 ± 0.2
RU
CEM I 42.5R
recycled, untreated
0.4
26.2 ± 0.4
RT
CEM I 42.5R
recycled, plasma treated
0.4
27.0 ± 0.3
81
Pastes were cast into prismatic molds (40 × 40 × 160 mm), which were compacted and un-
82
molded after 1 day of hardening. Curing took place in a controlled laboratory environment at
83
20±1 ◦ C and the specimens were submerged for 28 days in tap water in order to support a hy-
84
draulic reaction. Prior to testing, four specimens of each mixture were cut in half to be tested in
85
compression. The dynamic Young’s modulus was continuously measured using resonance method
86
on the uncut specimens. 5
87
Total porosity of the broken pieces was measured after a four-point bending test using the
88
pycnometric method. Each mix was represented with four specimens which had been crushed
89
and ground so that all pores were opened. It is clear from Table 1 that specimens reinforced with
90
plasma-treated fibers contained more pores. This can be attributed to the greater surface area of
91
the fibers and hence their increased ability to retain water. However, this effect is nearly negligible
92
with respect to the scatter of the measured data.
93
3. Plasma treatment
94
PET chains with covalent characteristics are chemically inert and hydrophobic. In order to
95
make PET fibers hydrophilic and increase their adhesion to the cementitious matrix, they were
96
subjected to oxygen cold plasma in a vacuum chamber using a Diener Femto PCCE device. Oxy-
97
gen plasma turned out to be very efficient in reducing the surface energy of polymers, as has been
98
¨ reported by Oktem et al. [28] and Mittal [29]. Oxygen or noble gases must be used during plasma
99
treatment in order to stabilize free radicals formed on the surface being treated [30].
100
During plasma treatment, the fibers were enclosed in a glass vessel covered by a Petri dish.
101
The dish has no influence on treatment results as demonstrated by Trejbal et al. [31] and pre-
102
vents blocking of plasma chamber vacuum pumps by stirred fibers. The most suitable treatment
103
duration established was 8 minutes and this exposure ensured a high level of surface roughening
104
with no melting, scorching, and warping of fibers. The process took place at a low gas pressure
105
equal 110 Pa, with a power supply of 100 W, at frequency of 40 Hz and with gas supply equal to
106
16.9 sccm.
107
3.1. Microscopy investigation
108
A scanning electron microscope (SEM) was used to investigate the surface of the PET fibers
109
before and after they were subjected to plasma treatment. Gold dust dispersed in the thickness of
110
10 nm over the surface of the investigated fibers provided the proper electric conductivity required
111
to produce high-quality SEM images.
112
The surface images with magnifications of 250× and 5 k× were taken. Small droplets and
113
longitudinal grooves, result of the fiber production process, are clearly visible in Figure 3. Plasma 6
114
treatment of 8 minutes led to the formation of tiny fissures and holes on the fiber surface (Figure 4).
115
An increase of fiber roughness is even more pronounced for fibers made of recycled PET. Expo-
116
sure to plasma resulted in the formation of a hair-like surface morphology and intense cracking
117
(Figure 5).
(a) magnification 250 ×
(b) magnification 5 k×
Figure 3: SEM images of untreated fibers from primary PET.
(a) magnification 250 ×
(b) magnification 5 k×
Figure 4: SEM images of fibers from primary PET after 8 minutes of plasma treatment.
7
(a) magnification 250 ×
(b) magnification 5 k×
Figure 5: SEM images of fibers from recycled PET after 8 minutes of plasma treatment.
118
3.2. Wetting angle measurement
119
In order to quantify surface properties changes due to plasma treatment, static wetting an-
120
gle measurements were conducted on the PET fiber specimens. As the hydrophobic character of
121
surfaces changes to hydrophilic, wetting angle decreases. A digital single-lens reflex camera was
122
used to optically measure contact angles on the surfaces of fibers half-submerged in distilled water.
123
Measurement equipment included a Canon EOS 600D DSLR camera with a Tamron 70–300 mm
124
objective lens, a grey neutral-density filter, a 50 mm focal length plano convex lens, and a Petri
125
dish filled with distilled water. Backlight illumination was provided by a LED lamp together with
126
a dispersing screen.
127
Wetting angle was evaluated for each fiber type on six fiber specimens separately using Nemetschek
128
Allplan CAD software. Tangents were placed into the images of the outlines of the fibers and ad-
129
hering water (Figure 6). Values obtained for each fiber are an average of two measurements on
130
mutually opposite sides of the fibers.
131
Results from wetting angle measurement demonstrate a positive impact of plasma treatment
132
on the surface of the fibers with respect to their hydrophilic behavior, regardless of PET fiber type
133
(primary or recycled). A summary of results is provided in Table 2.
8
Figure 6: Illustration of wetting angle measurement on primary fibers; from left to right: after 0, 30, 60, and 90 seconds of plasma treatment. Table 2: Influence of plasma treatment on the wetting angles of the PET fiber surfaces. Wetting Angle [◦ ]
Treatment Duration [min]
134
Primary PET
Recycled PET
0
65 ± 10
64 ± 8
4
27 ± 8
29 ± 10
8
24 ± 7
22 ± 6
16
26 ± 8
23 ± 7
4. Mechanical properties
135
Mechanical properties were investigated for the specimens described in Section 2.1. Using the
136
non-destructive resonance method, it was possible to track the evolution of the dynamic Young’s
137
modulus, Edyn , and investigate the influence of fibers on cement paste hardening. Such an ap-
138
proach is more reliable than evaluating elastic stiffness from force-displacement diagrams ob-
139
tained from quasi-static compression tests [32]. If required, a static modulus of elasticity, Estat ,
140
can be estimated according to the relationships determined by Rosell and Cantalapiedra [33] or
141
Malaikah et al. [34], who claim that the Edyn /Estat ratio ranges from 0.9 to 1.1.
142
Compressive and four-point bending tests were carried out using a VEB Rauenstein loading
143
frame with a 300 kN capacity load-cell. Loading force and displacement records provided by 9
144
LVDT sensors were gathered with a sampling frequency of 20 Hz. Both tests were displacement
145
controlled in order to capture the descending part of the force-displacement diagram after reaching
146
peak load. Special attention was paid to ductility enhancement and crack-bridging mechanisms,
147
since the main purpose of fiber reinforcement is to facilitate stress transfer across cracks that form
148
within very brittle cementitious matrix. This phenomenon is reflected mainly in post-cracking
149
hardening observed during four-point bending tests.
150
4.1. Dynamic Young’s modulus
151
The measurement of the dynamic Young’s modulus on prismatic specimens using the resonance
152
method is based on the longitudinal vibration equation for beams with a continuously distributed
153
mass and a free-free boundary condition. For details regarding acquisition and processing, the
154
reader is referred to [35].
155
Silva et al. [4] found that the addition of PET fibers in the amount of 0.8% by volume has no
156
impact on the modulus of elasticity. We reached the same conclusions with our measurements of
157
the specimens containing primary PET fibers. However, incorporating recycled fibers into cement
158
paste led to a reduction in elastic stiffness (Table 3). The same phenomenon was generally observed
159
in the case of pastes reinforced by plasma treated fibers. These retained more water on their
160
surfaces and their addition resulted in increased porosity (recall Table 1). It can be conjectured
161
that the increase in porosity is responsible for lower stiffness of specimens reinforced by plasma
162
treated fibers (Figure 7).
163
4.2. Uniaxial compression
164
A uniaxial compression test was conducted on prismatic 40 × 40 × 80 mm specimens (Fig-
165
ure 8) at a loading rate of 0.3 mm/min. Force-displacement diagrams are presented in Figures 9, 10,
166
and 11. Silva et al. [4] demonstrated that the addition of PET fibers into cementitious mortars
167
at volumes of 0.4 and 0.8% has no effect on compressive or flexural strength. However, these
168
conclusions contradict our findings, which are similar to results reported by Ochi et al. [7]: that
169
reinforcement of cement paste with PET fibers significantly increases compressive strength of the
170
material. The fibers enhance resistance of the reinforced composite to the lateral tension which
171
arises from the expansion of unconstrained specimens, thus suppressing transversal splitting. 10
Table 3: Development of the dynamic Young’s modulus the cement pastes reinforced by PET fibers. Mix
Average Dynamic Young’s Modulus [GPa] 7 days
14 days
21 days
28 days
R
21.1 ± 1.1
21.9 ± 0.7
22.3 ± 0.9
22.5 ± 1.1
PU
20.9 ± 0.4
22.0 ± 1.3
22.2 ± 1.6
22.6 ± 1.2
PT
21.4 ± 0.6
21.7 ± 1.4
22.0 ± 1.4
22.5 ± 1.9
RU
19.4 ± 1.3
21.0 ± 0.2
21.5 ± 0.6
21.6 ± 1.0
RT
19.9 ± 2.1
20.7 ± 2.0
21.0 ± 2.4
21.1 ± 2.5
23
dynamic Young’s modulus [GPa]
22.5 22 21.5 21 20.5 20
R PU PT RU RT
19.5 19 5
10
15
20
25
30
days of curing
Figure 7: Stiffness evolution between 7th and 28th day of curing.
11
172
However, the positive impact of PET inclusions on the resistance of a material subjected to
173
compression should be primarily attributed to their shapes. Choi et al. [25] demonstrated that addi-
174
tion of aggregates made of recycled PET resulted in deterioration of concrete compressive strength.
175
Kou et al. [36] conducted a similar study on concrete with partial replacement of conventional stone
176
aggregates with recycled PET granulate. They found that such concrete exhibited enhanced duc-
177
tility and resistance to chloride ion penetration, but its compressive strength and tensile splitting
178
strength were reduced.
179
Recycled plasma treated fibers, having smaller cross-sectional areas which were further re-
180
duced by treatment, failed as reinforcements because the weakened fibers were not capable of
181
efficient stress transfer across the cracks. The incorporation of weak recycled plasma treated PET
182
fibers resulted in a negligible increase in resistance to compressive loading when compared to the
183
reference mix (Table 4). On the other hand, the improvement of the interfacial bond between the
184
primary plasma treated fibers and the surrounding matrix resulted in an enhancement in material
185
resistance to uniaxial compression by 36% when compared with the reference mix. In order to
186
increase the compressive strength of cementitious composites, it appears that the incorporation of
187
plasma treated PET fibers with larger diameters into the mix is advantageous. Table 4: Results of uniaxial compression tests; Fmax and d0 denote the maximum force reached during a test and the corresponding displacement, respectively. Fmax
d0
[kN]
[mm]
R
59.5 ± 16.7
0.92 ± 0.13
PU
77.4 ± 7.7
1.04 ± 0.10
PT
76.7 ± 7.2
0.91 ± 0.08
RU
81.1 ± 10.8
0.93 ± 0.07
RT
61.9 ± 6.9
0.90 ± 0.07
mix
12
80 70 60
force [kN]
50 40 30 20
R-1 R-2 R-3 R-4 mean
10 0 0
0.5
1
1.5
displacement [mm]
Figure 8: Loading the reference specimen dur- Figure 9: Load-displacement diagrams from ing the uniaxial compression test.
compression tests on reference specimens.
90
90
PU-1 PU-2 PU-3 PU-4 mean
80 70
80 70 60
force [kN]
60
force [kN]
PT-1 PT-2 PT-3 PT-4 mean
50 40
50 40
30
30
20
20
10
10
0
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
0
displacement [mm]
0.2
0.4
0.6
0.8
1
1.2
1.4
displacement [mm]
(a) untreated fibers
(b) plasma treated fibers
Figure 10: Load-displacement diagrams from compression tests on specimens reinforced with primary fibers. 13
90
90
RU-1 RU-2 RU-3 RU-4 mean
80 70
70 60
force [kN]
60
force [kN]
RT-1 RT-2 RT-3 RT-4 mean
80
50 40
50 40
30
30
20
20
10
10
0
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
0
displacement [mm]
0.2
0.4
0.6
0.8
1
1.2
1.4
displacement [mm]
(a) untreated fibers
(b) plasma treated fibers
Figure 11: Load-displacement diagrams from compression tests on specimens reinforced with recycled fibers.
14
188
4.3. Four-point bending
189
Testing in four-point bending on 40 × 40 × 160 mm specimens was addressed to reveal the ef-
190
fect of fiber reinforcement on the response of cementitious materials to tensile stresses. LVDT sen-
191
sors, independent of loading frame deformation, were used to provide an accurate deflection mea-
192
surement. Specimens were symmetrically loaded and the span between cylindrical supports of
193
11 mm in diameter was equal to 120 mm and 60 mm for the fixed bottom and movable top sup-
194
ports, respectively. Even a slow loading rate of 0.05 mm/min was not sufficient to capture material
195
softening during bending of reference specimens (Figure 13). Their extremely brittle behavior was
196
a consequence of the lack of any reinforcement or aggregate that could interlock and provide the
197
opening crack with cohesion. On the other hand, the constant loading rate of 0.15 mm/min was
198
sufficient for tracking the loading-path beyond the elastic limit in all fiber-reinforced specimens.
199
The crack bridging was, in almost all cases, too strong to allow the specimen separate into two
200
parts and loading was stopped after reaching deflection of approximately 2 mm.
201
Figure 12 demonstrates pull-out failure for non-treated fibers caused by poor cohesion between
202
the smooth hydrophobic fiber surface and the surrounding matrix. Therefore, the capacity of the
203
fibers is not as fully utilized as in the case of treated fibers, which filed more frequently in tension.
204
This is reflected in the significant enhancement of post-peak resistance of specimens reinforced by
205
primary plasma-treated PET fibers. On the other hand, the elastic limit of such reinforced speci-
206
mens was lower than in the case of specimens containing untreated fibers. This can be attributed
207
to higher porosity of the material (recall Table 1).
208
The performance of both untreated fiber types was almost the same, not only in the terms of
209
maximum load but also regarding the hardening observed during the crack opening after reaching
210
maximum load, cf. Figures 14(a) and 15(a). When discussing treated fiber types, the primary ones
211
are apparently more efficiently used, and this is demonstrated by quite significant hardening (Fig-
212
ure 14(b)). The poor performance of recycled plasma treated fibers in comparison with other fiber
213
types is even more pronounced than it was during the uniaxial compression test. The specimens
214
reinforced by recycled plasma treated fibers exhibited a softening response and their ductility was
215
limited (Figure 15(b)).
15
3.5
3
force [kN]
2.5
2
1.5
1
R-1 R-2 R-3 R-4 mean
0.5
0 0
0.05
0.1
0.15
0.2
displacement [mm]
Figure 12: Four-point bending test and pull- Figure 13: Load-displacement diagrams from out failure of recycled untreated fibers during four-point bending tests on reference specithe crack opening.
216
mens.
5. Crack bridging
217
The activation and gradual rupture or pull-out failure of individual fibers can absorb and dis-
218
sipate external energy and stabilize crack propagation. Reduction of the crack opening prevents
219
water and harmful chemicals from entering the matrix, which in turn increases its durability. When
220
designed properly, fibers can efficiently control and arrest cracks promoted by mechanical load-
221
ing or shrinkage of the matrix [37, 38]. The addition of suitable fibers can significantly alter the
222
behavior of an intrinsically brittle cementitious matrix, creating a tough and ductile material [39].
223
In order to visualize strain concentrations and the behavior of cracks, digital image correlation
224
(DIC) [40, 41] was employed. This technique overcomes the limitations of conventional contact
225
measurement methods and allowed us to accomplish a full-field monitoring of displacement and
226
strain fields during the four-point bending test. To evaluate displacements and deformations of
227
DIC subsets within individual images, an artificial stochastic pattern had to be applied onto the
228
surface of specimens being tested (Figure 12). A non-commercial software, Ncorr [42], together 16
4.5
4
4
3.5
3.5
3
3
force [mm]
force [kN]
4.5
2.5 2 1.5
2.5 2 1.5
1
1
PU-1 PU-2 PU-3 PU-4 mean
0.5
PT-1 PT-2 PT-3 PT-4 mean
0.5
0
0 0
0.5
1
1.5
2
0
displacement [mm]
0.5
1
1.5
2
displacement [mm]
(a) untreated fibers
(b) plasma treated fibers
Figure 14: Load-displacement diagrams from four-point bending tests on specimens reinforced with primary fibers.
17
4.5
4
4
3.5
3.5
3
3
force [kN]
force [kN]
4.5
2.5 2 1.5
RT-1 RT-2 RT-3 RT-4 mean
2.5 2 1.5
1
1
RU-1 RU-2 RU-3 RU-4 mean
0.5
0.5
0
0 0
0.5
1
1.5
2
0
displacement [mm]
0.5
1
1.5
2
displacement [mm]
(a) untreated fibers
(b) plasma treated fibers
Figure 15: Load-displacement diagrams from four-point bending tests on specimens reinforced with recycled fibers.
18
229
with the in-house postprocessing tool, Ncorr post [43], was used for DIC calculation and analysis.
230
Cracking patterns were visualized with a map of maximum principal strain in which concentrations
231
can be attributed to the formation of cracks. Virtual extensometers that gather the magnitude of
232
displacement in two arbitrary points were employed to monitor the crack opening and deflection
233
of the tested specimens.
234
Images used for DIC analysis were taken with a DSLR camera Canon 70D at 10-second inter-
235
vals in uncompressed format (.raw), yielding approximately 35 px/mm definition. Perfect illumi-
236
nation provided by artificial lighting allowed for a short exposure time (1/125 sec), with the light
237
sensitivity (ISO) parameter set to 100. A focal length (zoom) of 55 mm with a distance between
238
the camera and the observed surface of 80 cm ensured a minimal lens distortion effect. Setting the
239
subset spacing and radius to 8 px and 32 px, respectively, yielded displacement and strain fields
240
consisting of 840 by 210 discrete values per specimen.
241
Despite the strain hardening present during bending of the majority of PU, PT and RU speci-
242
mens, no multiple cracking or branching of cracks was observed and tensile strain localized into a
243
single cohesive crack. The cracking patterns captured with DIC on selected chosen representative
244
specimens are presented in Figure 16. Cracks are visualized with a map of maximum principal
245
tensile strain and the numbered marks indicate the positions of virtual extensometers. The first
246
virtual extensometer (1) was located 3 mm from the bottom of the specimens, and the relative ex-
247
tensometer distance was 6 mm. Such a position and spacing allowed documentation of the crack
248
opening without bias from elastic deformations or boundary effects. A second extensometer (2)
249
was located in the vicinity of the supports in order to monitor the true deflection of the specimen.
250
The relationship between loading force action on the specimens during four-point bending and
251
crack opening is presented in Figure 17. The larger force needed for opening of the crack in the case
252
of the primary plasma treated fibers provides clear evidence of more efficient activation of the fibers
253
when compared to primary untreated fibers. The poor cohesion of the crack bridged by recycled
254
plasma treated fibers proves that these are not sufficiently strong enough to reinforce a cementitious
255
matrix. The same conclusions hold when interpreting the DIC measurement results based on the
256
dependence of the crack opening on specimen deflection (Figure 18). The enhanced capacity
19
257
of stress transfer across the cracks bridged by primary plasma-treated fibers ensured increased
258
bending stiffness. As a consequence, specimen deflection was two times lower than the deflection
259
of specimens reinforced by primary untreated or recycled plasma treated fibers.
260
6. Conclusion
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The investigation of the morphology and performance of primary and recycled PET fibers
262
when untreated and after the oxygen plasma treatment provided valuable findings to be considered
263
in the design of fiber-reinforced cementitious composites. Oxygen plasma treatment proved to be
264
very efficient in altering the surface of PET fibers as demonstrated by microscopy observations
265
and with wetting angle measurements. Surface roughening with ion bombardment together with
266
the activation of polar groups on fiber surfaces resulted in better adhesion of the fibers to the
267
cementitious matrix and stronger interfacial bonding.
268
269
270
271
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Based on the results presented here, it can be concluded that: 1. Plasma surface treatment of both fiber types, primary and recycled, significantly increased their roughness, as demonstrated by microscopy images, 2. The change of surface morphology made the fibers hydrophilic and led to a reduction of the wetting angle,
273
3. The elastic stiffness of cementitious pastes reinforced by plasma-treated fibers was lower
274
than those reinforced by untreated fibers due to higher water retention for such fibers and a
275
consequently higher porosity of the composite,
276
277
4. Reinforcement with plasma-treated fibers contributed to elevated compressive and flexural strength for primary fibers,
278
5. Compressive strength was not enhanced by the addition of recycled plasma treated fibers to
279
the reference paste and flexural strength was, if fact, reduced — these phenomena can be
280
attributed to an excessive reduction in the cross-sectional areas of the recycled fibers, which
281
were thinner than the primary ones,
282
283
6. Plasma treatment of primary fibers ensured better activation and increased hardening after reaching peak load during the four-point bending tests, and 20
(a) primary untreated fibers
(b) primary plasma treated fibers
(c) recycled untreated fibers
(d) recycled plasma treated fibers
21 bending of selected representative specimens. Figure 16: Crack development during four-point
4 3.5
3.5
3
vertical deflection [mm]
4
loading force [kN]
3 2.5 2 1.5 1
PU-1 PT-3 RU-3 RT-2
2.5 2 1.5 1 0.5
PU-1 PT-3 RU-3 RT-2
0.5
0 0
0
1
2
3
4
1
2
3
4
5
crack opening [mm]
0 5
crack opening [mm]
Figure 18: Relationship between the deflecFigure 17: Relationship between loading force tion of the specimen measured with virtual exand the crack opening measured with virtual tensometer 2 and the crack opening measured extensometer 1.
with virtual extensometer 1.
22
284
7. Performance of recycled fibers when subjected to tensile stresses was negatively affected by
285
plasma treatment, resulting in gradual softening of the specimens when subjected to four-
286
point bending.
287
The considerable bridging force provided by PET fibers, especially the plasma-treated primary
288
ones, resulted in the formation of cohesive cracks and thus prevented excessive and abrupt crack-
289
ing, thereby limiting the crack opening. A reduced crack opening promotes durability of fiber-
290
reinforced cementitious composites by preventing water and harmful chemicals from entering a
291
matrix. Better activation of fibers and gradual rupture combined with pull-out failure of individual
292
fibers provides absorption and dissipation of external energy and stabilizes crack propagation.
293
Our future work will be focused on providing more detail about the impact of plasma treatment
294
on various fibers and their behaviors when subjected to tensile stresses. We invite those interested
295
to read our follow-up paper, currently in preparation, which deals with the tensile strengths of
296
individual fibers before and after plasma surface modification as well as their pull-out strength
297
when used as reinforcement in a cementitious matrix.
298
Acknowledgment
299
This work was supported by the Czech Science Foundation [grant number 15-12420]. The
300
authors also thank the Center for Nanotechnology in Civil Engineering at the Faculty of Civil En-
301
gineering, CTU in Prague, the Joint Laboratory of Polymer Nanofiber Technologies of the Institute
302
of Physics Academy of Science of the Czech Republic, and the Faculty of Civil Engineering CTU
303
ˇ an Potock´y and Zdeˇnek Remeˇs (Institute of Physics of the Czech in Prague. Special thanks to Step´
304
Academy of Sciences) for their fruitful collaboration and to Stephanie Krueger, who provided
305
editing assistance.
306
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