Construction and Building Materials 249 (2020) 118760

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Construction and Building Materials

journal homepage: www.elsevier.com/locate/conbuildmat

Direct shear behavior of cementitious mortar reinforced by carbon fiber

textile
Ngoc Hieu Dinh, Hai Van Tran, Kyoung-Kyu Choi ⇑
School of Architecture, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, South Korea

h i g h l i g h t s

Direct shear behavior of Textile-reinforced cementitious mortar was investigated.

The TRM material showed ductile behavior when subjected to direct shear.

A crack-shear slip model for TRM was developed and verified with the test results.

a r t i c l e i n f o a b s t r a c t

Article history: This study experimentally investigated the characteristics of carbon fiber textile-reinforced mortar (TRM)
Received 18 October 2019 under direct shear. The main test parameters of this study include the fiber reinforcement ratio of carbon
Received in revised form 19 December 2019 fiber textile, the surface treatment used between mortar matrix and the fiber textile, and the inclination
Accepted 14 March 2020
angle of the fiber filaments. Three fiber reinforcement ratio of carbon fiber textile of (0.17, 0.35, and 0.53)
Available online 23 March 2020
% in the mortar matrix were investigated. Two different surface treatment methods were employed to
improve the bond performance of carbon fiber textiles in the mortar matrix: epoxy impregnation only,
Keywords:
and sand coating after epoxy impregnation. Two inclination angles of fiber filaments of 0°/90° and –
Shear behavior
Textile-reinforced mortar
45°/45° were investigated. The test results showed that the textile reinforcement within cementitious
Epoxy impregnation mortar matrix affected the failure modes and the shear performance of test specimens in terms of crack-
Carbon fiber textiles ing and ultimate shear stress, crack deformation, and residual shear stress in the pre- and post-peak
Residual shear stress stages. Moreover, a crack-shear slip model for cementitious mortar reinforced by carbon fiber textile
was developed considering the material characteristics of the mortar and the fiber textile, and its predic-
tion showed good agreement with the experimental results.
Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction their low resistance capacity to high temperature [3], incompati-

bility with concrete surfaces, etc.
Over the past decades, there has been continuously rising In the early 2000 s, for structural strengthening and seismic ret-
demand for the retrofitting of existing reinforced concrete (RC) rofitting of concrete and masonry structures, the innovative
and masonry structures, not only in seismic regions, but also in textile-based composite was introduced as textile-reinforced mor-
non-seismic regions, due to aging deterioration, environment- tar (TRM) consisting of textile fiber reinforcement embedded in
induced degradation, lack of maintenance, or the demand to meet cementitious mortar matrix. The acronym fabric-reinforced
current design building codes. Externally bonded Fiber-reinforced cementitious matrix (FRCM) or textile reinforced concrete (TRC)
polymers (FRPs) have been widely used as composite material to was also utilized as similar material in several works of literature
provide additional flexural, shear, or torsional strength and con- and research reports [4,5]. Due to their excellent advantages,
finement applications for concrete structures due to their benefits including for example low-cost, high-temperature resistant expo-
that include low invasiveness, good corrosion resistance, high sure, ability to be applied in moist or low-temperature environ-
strength-to-weight ratio, and ease of application [1,2]. Despite ment, and compatibility with concrete substrates [6], TRM and
many advantages, the application of FRPs is also limited, due to TRC technology have emerged as potential solutions in the precast
industry, and the construction of load-bearing structures. TRM-
⇑ Corresponding author. based prefabricated components and light-weight components
E-mail addresses: dnhieu@dut.udn.vn (N.H. Dinh), haivan993@gmail.com could be used for exterior cladding system and façade panels,
(H.V. Tran), kkchoi@ssu.ac.kr (K.-K. Choi).

https://doi.org/10.1016/j.conbuildmat.2020.118760
0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
2 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760

thin-walled shell structures, sandwich elements, small and Fig. 1 depicts the setups of flow tests of the fresh mortar mix
medium-size panels, fatigue and impact-resistant infrastructure, and compressive tests of mortar specimens, respectively. The
and transportation structures and support systems. workability and air content of the mortar mix proportion were
Numerous studies have been performed for the purpose of determined by standard tests using flow table in accordance with
application of the TRM system to flexural strengthening [7,8], ASTM C230 [28] and ASTM C185 [29], respectively. Table 1 sum-
shear strengthening [9–12], confinement of axially loaded concrete marizes the results of the flow and air content of fresh mortar
[13,14], seismic retrofitting of RC members [15], and masonry mix, which were 220 mm and 5.65%, respectively. Moreover, after
infilled RC frames [16]. In addition, various existing studies have casting and curing at room temperature (RT; 25 °C), uniaxial com-
been performed to investigate the mechanical behavior of textile pressive tests were performed on five cube mortar specimens of
fiber composite materials, focusing on the tensile strength of size 50 mm  50 mm  50 mm in accordance with the Korean
TRM, and bonding behavior of textiles to the cementitious matrix, standard test method KS L 5105 Standard [30], and the average
or the cementitious matrix to structural substrates [4; 17–19]. compressive strength of mortar was 41.53 MPa at the 28th day.
Recently, TRM-based products have been manufactured using cast- For textile reinforcement, commercial carbon fiber textiles
ing, laminating, or spraying technique [20,21], and have been made of T700S carbon fiber yarns in two orthogonal directions
newly used to reduce transportation and installation costs [22]. were employed. Fig. 2 depicts the geometrical and mechanical
Nonetheless, so far, the current research [7–19] have not suffi- characteristics of T700S yarns. The carbon fiber textiles are made
ciently investigated the effects of the surface treatment methods of T700S carbon fiber yarns with a spacing of 10 mm between
and fiber orientation on the pure shear behavior [23–26] of the yarns. Each T700S fiber yarn consists of 24,000 filaments and
cementitious matrix strengthened by fiber textile, which plays a 12,000 filaments in the warp and weft directions, respectively,
vital role in structural shear strengthened purposes or the struc- with a filament diameter of 7 mm. The density of yarn is 1.80 g/
tural behavior of components fabricated from textile-reinforced m3, and the approximate thickness of each layer of carbon fiber
cementitious composite. textile is 0.21 mm. In addition, the values of the mean tensile
The primary objective of this study is to experimentally investi- strength and elastic modulus of the T700S fiber yarn provided by
gate the behavior of cementitious mortar reinforced with carbon the manufacturer were (4,900 and 230,000) MPa, respectively.
fiber textiles under direct shear. The shear tests were carried out
based on the Fédération Internationale de la Précontrainte (FIP)
standard [27]. The main test parameters include the fiber rein-
forcement ratio of carbon fiber textile, surface treatment method, 2.2. Specimen details and test parameters
and inclination angle of fiber filaments. Three different fiber rein-
forcement ratios of (0.17, 0.35, and 0.53) % in the mortar matrix Fig. 3 and Table 2 depict the geometrical details of the test spec-
were investigated; two different surface treatment methods imens and the test program, respectively. All test specimens were
(epoxy impregnation only, and sand coating after epoxy impregna- fabricated in the form of a rectangular prismatic shape of height
tion) were adopted to improve the bond performance of carbon 140 mm, length 300 mm, and thickness 60 mm. The carbon fiber
fiber textiles in the mortar matrix; and two inclination angles of textiles were located at the central plane of the test specimens
fiber filaments of 0°/90° and –45°/45° were investigated. Based so that the warp yarns perpendicular to the shear plane. Prior to
on the test results, the shear characteristics of test specimens were casting specimens, the fiber layers were linked together by using
analyzed and investigated in terms of various parameters, namely cable ties, and the surface treatment techniques were performed.
the failure modes, shear crack propagation, cracking shear stress, The paste was poured into the mold in two layers. After the first
ultimate shear stress, crack deformation, and residual shear stress. layer of mortar of 30 mm thickness was poured, the carbon fiber
In addition, a crack-shear slip model was proposed based on the textiles were slightly pressed into the first mortar layer so that
test results, considering the material characteristics of the mortar the mortar protruded through all the perforations between fiber
and the fiber textile. yarns, followed by the application of the final mortar layer. The
specimens were demoulded after 24 h, and cured for 28 days at
laboratory temperature of 25 °C and 75% relative humidity, before
2. Experimental program conducting experiments. In addition, 5 mm deep notches were
sawed around the vertical middle plane of the test specimens to
2.1. Materials properties induce cracking along the intended shear failure plane, as illus-
trated in Fig. 3.
In this study, a standard mortar mix proportion was employed Three test parameters were investigated in this study: (a) the
with a design compressive strength of 40 MPa at the 28th day. fiber reinforcement ratio of fiber textiles; (b) the surface treatment
Table 1 details the mix proportion. The mortar mix proportion con- methods of fiber textiles; and (c) the inclination angle of fiber fila-
sisted of Type I Portland cement, and granular sand with a maxi- ments. In total, seven series of test specimens were investigated.
mum particle size of 1.63 mm. The water-to-cement ratio was Table 2 provides the details of the test series according to the main
0.5, while that of cement-to-sand was 0.4. The mix work lasted test parameters. Except for the SL-02 series, four replicas of test
for (3–5) min. A super-plasticizer was used as an admixture with specimens were fabricated and tested for each series. Based on
a dosage of cement content of (0.3–0.5) % during mix work, to the combination of the test parameters, a total of twenty-seven
achieve adequate workability. shear test specimens were fabricated and tested.

Table 1
Mortar mix proportion.

Water Cement Sand W/C(a) C/S(b) Superplasticizer Air content Actual compressive strength
3 3 3
(kg/m ) (kg/m ) (kg/m ) (kg/m3) (%) (MPa)
270 540 1,350 0.5 0.4 1.62 – 2.70 5.65 41.53
(a)
water-to-cement ratio.
(b)
cement-to-sand ratio.
 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760 3

and SL-06 had the number of (2, 4, and 6) layers of carbon fiber tex-
tile in the mortar matrix, respectively. The corresponding fiber
reinforcement ratios of fiber reinforcement in the mortar matrix
for SL-02, SL-04, and SL-06 were (0.17, 0.35, and 0.53) %, respec-
tively. Note that the fiber reinforcement ratio was calculated as
the ratio of the total area of warp yarns to the area of the shear
plane (refer to Fig. 3). Moreover, in these series, the amount of
epoxy resin used for epoxy impregnation was 148 g/m2, which
was used in common with previous research by Donnini et al.
[4]. In addition, the angle of inclination of the carbon fiber fila-
ments was 0°/90° (Fig. 4 (a)).
The second series, labeled S-NT, S-EP, and S-ES, was tested to
investigate the effect of surface treatment used for textile rein-
forcement on the shear performance of the mortar matrix. The
aim of surface treatment is to improve the bond performance
and stress transfer action of the carbon fiber textile by filling the
(a) Flow tests (b) Compressive strength tests
spaces between the fiber filaments within the mortar matrix, in
Fig. 1. Test setups of flow tests and compressive strength tests. order to sustain the high level of loading [31–33]. In this study, a
Bisphenol-A type epoxy resin with a low viscosity of (700–1,100)
cps at 25 °C was used as a common polymer filler in composite
T700S Carbon materials [4,6,7,9]. The amidoamine hardener was adopted as cur-
Warp yarns
10 mm fiber yarn Warp yarn Weft yarn ing agent. In detail, no surface treatment was employed in the S-NT
Properties series (control series); meanwhile, the medium level of epoxy resin
No. of filament 24k 12k impregnation with a weight of 148 g/m2 was employed for surface
Filament treatment in the S-EP series according to Donnini et al. [4] (Fig. 4
7 (a)); and a medium level of epoxy resin impregnation with a
diameter (µm)
Mesh size weight of 148 g/m2 incorporating sand coating with a weight of
10 mm

10 672 g/m2 was employed for surface treatment in the S-ES series
(mm)
Thickness according to Donnini et al. [4] (Fig. 4 (b)). Note that the epoxy
0.21 impregnation was conducted by using a plastic spatula, and the
(mm)
epoxy resin and sand were weighed before application [4]. For
Density (g/m3) 1.80
the S-ES series, after curing of the epoxy resin, the surface of the
Modulus of carbon fiber textiles was coated with sand having a diameter of
elasticity 230,000 250 lm, by using a high-speed spray gun (sandblasting) [34].
Weft yarns
(MPa) Moreover, in the second series, four layers of fiber textiles were
Tensile strength employed for each test specimen.
4,900
(MPa) Finally, in the third series, labeled SA-90 and SA-45, the effect of
the angle of inclination of the fiber filaments on the shear perfor-
Fig. 2. Details of the carbon yarns used in the present study. mance of mortar matrix was investigated. SA-90 and SA-45 series
correspond to inclination angles of 0°/90° and –45°/45°, respec-
tively. Moreover, in these series, the medium level of epoxy
Weft yarns
impregnation with a weight of 148 g/m2 was used for surface treat-
Notch
ment [4], and four layers of fiber textiles were employed (Fig. 4
Test (c)).
Warp yarns
specimen
2.3. Test setup and measuring instruments

In order to investigate the shear behavior of cementitious com-

posite materials, previous researchers have proposed various test
methods: the Z-type push-off tests [35,36], the Japanese Society
140

of Civil Engineering (JSCE) standard tests [37–38], and the Fédéra-

.5
147 tion Internationale de la Précontrainte (FIP) standard tests [27]. In
5
the present study, the shear tests were carried out based on the FIP
standard test setup with one shear failure plane, due to several
.5 300
147 Unit: mm advantages: the shear plane is theoretically subjected to pure shear
60 force, the occurrence of bending moment is minimized [39] and
specimens are easy to construct. Fig. 5 (a) depicts a schematic of
the experimental setup and the details of measuring instruments.
Fig. 3. Geometrical details of test specimens. The shear tests were carried out by using a universal testing
machine (UTM) with a loading capacity of 1,000 kN. Fig. 5 shows
the system of steel supports and steel plates by which the shear
The first series, labeled SL-00, SL-02, SL-04, and SL-06, was load was produced. The vertical loads P were applied relatively
tested to investigate the effect of fiber reinforcement ratio of car- close to the notches of the specimens, to create a narrow region
bon fiber textiles on the shear performance of the mortar matrix. of shear force. The distance between a pair of vertical loads P
The SL-00 series was control specimens without carbon fiber tex- was maintained as [d / 6 = 23 mm], where d is the height of spec-
tile embedded in the mortar matrix; meanwhile, SL-02, SL-04, imens, according to the recommendation of Bažant and Pfeiffer
4 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760

Table 2
Details of test series.

Test series Number of carbon Surface treatment method Angle of inclination Number oftest Test parameter
fiber textile layers of the textile specimens
SL-00 0 – 0°/90° 4 Fiber reinforcement ratio
SL-02 2 Epoxy impregnation 0°/90° 3
SL-04(a) 4 Epoxy impregnation 0°/90° 4
SL-06 6 Epoxy impregnation 0°/90° 4
SS-NT 4 – 0°/90° 4 Surface treatment method
SS-EP(a) 4 Epoxy impregnation 0°/90° 4 of fiber textile
SS-ES 4 Epoxy impregnation+ sand blasting 0°/90° 4
SA-90(a) 4 Epoxy impregnation 0°/90° 4 Angle of inclination of the textiles
SA-45 4 Epoxy impregnation –45°/45° 4
(a)
the same series.

Epoxy resin: 148 g/m2

Epoxy resin: 148 g/m2 + Sand: 672 g/m2 Epoxy resin: 148 g/m2

(a) Epoxy resin impregnation (b) Epoxy resin impregnation (c) Epoxy resin impregnation
(series SL-02, SL-04, and SL- + Sand coating (series SS-ES) (series SA-45)
06)
Fig. 4. Surface treatment used for carbon fiber textile.

Applied load
Load cell
Steel plate

Rollers
Test specimen
P Steel supports

LVDT 1
Shear
failure plane LVDT 3
140

LVDT 2
Steel supports P

Rollers

Steel plate
23
12 150 Unit: mm

(a) Schematic of test setup (b) Photograph of test setup

Fig. 5. Shear test setup and measuring instruments.

[40], in order to prevent local shear of the mortar under the steel performed using the displacement control method with a constant
supports prior to producing shear fracture. The shear tests were velocity rate, so that peak loads were achieved in about 5 min
 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760 5

(±30 s), according to the recommendation of Bažant and Pfeiffer cracks in the vicinity of the notches (shear plane). The shear frac-
[40]. ture zones featured numerous initial tensile microcracks, which
In regard to measurement instrument, horizontal deformation later interconnected together by shearing.
of the shear plane (crack width) was measured as the average In the case of SL-00 (Fig. 6 (a)), the initial cracks formed basi-
value obtained by two highly sensitive linear variable displace- cally from the notch in the vertical direction normal to the maxi-
ment transducers (LVDTs) installed in the horizontal direction mum principal stress. With increasing slip across the shear plane,
(LVDT 1 and LVDT 2); and vertical deformation of the shear plane specimen SL-00 exhibited brittle failure with the diagonal crack
(crack slip) was measured by using LVDT 3 installed in the vertical lastly connected to the loading point. Moreover, at the final failure,
direction. According to the specifications from the manufacturer, SL-00 showed a wide shear fracture zone surrounding the shear
the LVDTs used in this test have a maximum capacity of 5 mm, plane; and no cracks were observed out of the loading plane
and a sensitivity of 2,000  10-6 mm. In addition, Fig. 5 (b) shows (Fig. 6 (a)). The presently observed fracture characteristics were
the system of light aluminum frames that was mounted directly on analogous to those observed by Bažant and Pfeiffer [40].
the specimens to keep the LVDTs at fixed positions. The loading Nonetheless, in the case of specimens reinforced by carbon fiber
was recorded during experiments by a load cell with a capacity textiles (Fig. 6 (b)–(d)), the discrepancy of failure modes was
of 100 kN. observed. The area of shear fracture zones was narrower, and shear
crack propagation was mainly concentrated in the shear plane
coinciding with the preformed notches. In the cases of textile rein-
3. Crack patterns and failure modes forced specimens with surface treatment (series SL-02, SL-04, SL-
06), splitting cracks occurred out of the loading plane, as shown
Fig. 6 depicts the typical crack patterns and failure modes in Fig. 6 (b) and (c), which were mainly attributed to the separation
observed for each series in this study. Overall, under shear loads, between the carbon fiber textiles and the mortar matrix. In partic-
the failure modes of all test specimens were governed by the shear ular, in SL-06, the vertical shear crack width is narrow, and the final

Loading Front view

point Diagonal
crack

Initial vertical Initial

crack vertical crack
Inclined crack
Loading
point
Splitting crack
Diagonal
crack Top view
Initial vertical (c) SL-06
Front view crack
Front view

Top view

(a) SL-00

Front view
No splitting crack
Top view

(d) SS-NT
Initial
vertical crack
Fiber rupture
and slippage
Splitting crack
Top view

(b) SL-02, SL-04 (e) Fracture of carbon fiber meshes

Fig. 6. Observed crack patterns of TRM test specimens.
6 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760

failure was induced by inclined cracks that propagated across the cially altered the failure modes of the test specimens subjected
weak plane originating from the steel support at the level of crack to shear in comparison to plain mortar specimens. Also, the adop-
slip of about 3.5 mm (Fig. 6 (c)). These phenomena were observed tion of a high fiber reinforcement ratio up to 0.53% within the mor-
in three out of the four specimens in the SL-06 series. The alter- tar matrix and surface treatment techniques of textile could
ation of failure mode observed in the SL-06 series could be attrib- mitigated the crack propagation horizontally.
uted to the very high shear stress carried by the fibers across the
shear plane leading to the severe splitting of the outer mortar lay-
4. Shear behavior obtained from test results
ers in the vicinity of the shear plane (refer to Fig. 6 (c)), which
results in the formation of critical diagonal crack debonding. In
In this study, the shear stress–crack slip and shear stress–crack
the cases of textile reinforced specimens without surface treat-
width of the test specimens were addressed (refer to Figs. 8 and 9).
ment (series SS-NT), splitting cracking did not occur out of the
The shear stress was defined as the ratio of the applied force and
loading plane, as shown in Fig. 6 (d). It is noteworthy that despite
the cross-sectional area of the shear plane. In the ascending branch
the difference of fiber orientation within the mortar matrix, the
of loading, the shear test parameters of each specimen were
crack patterns and failure modes observed in the SA-45 series were
obtained from the test results in terms of the peak load, cracking
analogous to those observed in the SL-04 series. This could be
(scr) and ultimate shear stress (su), crack slip, and crack width at
attributed to the fact that the same amount of fiber reinforcement
ultimate shear stress. In the post-peak stage, the shear test param-
was adopted within the mortar matrix.
eters of each specimen were obtained from the test results in terms
Moreover, Fig. 6 (e) exhibits the fracture of carbon fiber textiles
of residual shear stress (sre). It should be noted that the cracking
inside the mortar matrix at failure. Overall, the failure of carbon
shear stress of the specimen was determined at the first cracking
fiber textiles was mainly concentrated at the location of the shear
of the mortar matrix, and the ultimate shear stress was determined
plane, and manifested by fiber rupture and slippage induced by
at the peak load.
principal shear stress.
Tables 3 and 4 summarize the shear parameters obtained from
From the direct shear tests, the crack propagation at the shear
test results and the corresponding coefficient of variation (COV) in
plane of all mortar test specimens reinforced by carbon fiber textile
the pre- and post-peak stages, respectively. Overall, the addition of
could be investigated by using the relations between crack opening
carbon fiber textiles embedded in mortar matrix significantly
displacement (crack width) and crack vertical displacement (crack
enhanced both the strength and deformation capacity in shear
slip), as depicted in Fig. 7. Overall, the curves in Fig. 7 consist of two
compared to non-fiber-reinforced specimens, as shown in Table 3.
branches corresponding to the pre- and post- peak stages. In all
Hereafter, the effects of various test parameters on the shear
TRM specimens, in the pre-peak stage prior to reaching ultimate
behavior of test specimens are analyzed and discussed in detail.
shear stress, the test specimens showed steeper slope in the crack
propagation curves, in comparison to those in the post-peak stage.
In particular, in the post-peak stage, the SL-06 series (Fig. 7 (c)) and 4.1. Effect of the fiber reinforcement ratio
SS-ES (Fig. 7 (e)) series showed flatter slopes of crack propagation
than the rest of the series, which represented the reduction in Figs. 8 and 9 depict the shear stress–crack slip and shear stress–
crack width. crack width relations of the SL-00, SL-02, SL-04, and SL-06 series,
Thus, from the obtained test results, it could be found that the respectively. From these figures, as expected, plain mortar speci-
addition of carbon fiber textiles within the mortar matrix benefi- mens (SL-00 series) exhibited brittle behavior and sudden failure

Fig. 7. Crack propagation curves of textile reinforced cementitious mortar specimens.

 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760 7

Fig. 8. Shear stress–crack slip relationships of the SL-00, SL-02, SL-04, and SL-06 series.

Fig. 9. Shear stress–crack width relationships of the SL-00, SL-02, SL-04, and SL-06 series.
8 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760

Table 3
Test results of shear parameters in the pre-peak stage.

Test specimen Peak load Cracking shear stress, Ultimate shear stress, Crack slip at ultimate Crack width at
scr su shear stress ultimate shear stress
Mean (kN) COV Mean (MPa) COV Mean (MPa) COV Mean (mm) COV Mean (mm) COV
SL-00 35.61 0.05 5.48 0.05 5.48 0.05 0.03 0.17 0.01 0.15
SL-02 45.91 0.02 4.94 0.24 7.06 0.02 0.27 0.22 0.29 0.41
SL-04SS-EPSA-90 55.63 0.05 7.17 0.21 8.19 0.07 0.26 0.31 0.23 0.29
SL-06 76.37 0.06 10.48 0.12 11.75 0.06 0.45 0.25 0.25 0.26
SS-NT 53.44 0.03 5.81 0.27 8.22 0.03 0.21 0.37 0.19 0.14
SS-ES 61.63 0.02 8.75 0.15 9.48 0.02 0.27 0.28 0.19 0.27
SA-45 67.54 0.07 4.17 0.19 10.39 0.07 0.43 0.13 0.42 0.06

Table 4
Test results of residual shear stress in the post-peak stage.

Test specimen At crack slip of 1 mm At crack slip of 3 mm

Mean (MPa) COV sre/su Mean (MPa) COV sre/su
SL-00 – – – – –
SL-02 3.58 0.11 0.51 4.13 0.19 0.58
SL-04SS-EPSA-90 5.49 0.08 0.67 5.99 0.13 0.73
SL-06 8.92 0.09 0.76 9.60 0.13 0.82
SS-NT 3.73 0.33 0.45 3.48 0.14 0.42
SS-ES 5.95 0.06 0.63 7.10 0.02 0.75
SA-45 5.05 0.07 0.49 3.89 0.06 0.37

at a very low level of crack slip and crack width (Fig. 8 (a) and 9 stress of (7.06, 8.56, and 11.75) MPa, respectively (see Table 3), cor-
(a)). The shear stress–displacement response showed almost linear responding to roughly (28.8, 56.2, and 114.4) % higher than that of
trend up to the ultimate shear stress, and then the load-carrying SL-00. Generally, the textile reinforcement was effective even after
capacity was instantaneously lost beyond the ultimate shear stress. the mortar matrix had cracked to carry the additional imposed
In contrast, SL-02, SL-04, and SL-06 showed ductile behavior with shear stress; and the cracking shear stress as well as ultimate shear
increased deformation capacity in the post-peak stage. After reach- stress could be increased according to the fiber reinforcement ratio
ing the cracking stress corresponding to the first cracking of the in the mortar matrix.
mortar matrix, the stress increased more and finally reached the Fig. 11 compares the mean values of cracking slip and crack
ultimate shear stress. Moreover, Figs. 8 and 9 show that after ulti- width corresponding to the ultimate shear stress of the SL-00,
mate shear stress, due to the fiber rupture and the slippage SL-02, SL-04, and SL-06 series. Overall, due to the enhancement
between the mortar matrix and the fiber textiles, the shear in strength and deformation capacity, mortar specimens reinforced
stress–crack slip and stress–crack width curves reveal irregular with fiber textiles show a significant difference in crack slip and
shapes in the post-peak stage. crack width at ultimate shear stress, in comparison to those of
In detail, Fig. 10 presents the mean values of cracking shear plain mortar specimens. With high fiber content, the SL-06 series
stress and ultimate shear stress for the SL-00, SL-02, SL-04, and show greater crack slip at ultimate shear stress than do SL-02
SL-06 series. Fig. 10 (a) shows that the SL-00 and SL-02 series and SL-04 (Fig. 11 (a)). Meanwhile, crack width at the ultimate
reveal almost the same average cracking shear stress of (5.48 and shear stress of SL-02, SL-04, and SL-06 shows almost the same
4.94) MPa, respectively; meanwhile, SL-04 and SL-06 reveal appre- value (Fig. 11 (b)).
ciably higher average cracking stress of (7.17 and 10.48) MPa, Fig. 12 depicts the effect of fiber reinforcement ratio on the
respectively (see Table 3), corresponding to roughly (30.8 and residual shear stress of TRM specimens in the post-peak stage at
91.2) % higher than that of SL-00. Fig. 10 (b) shows that SL-02, the crack slip of (1 and 3) mm, respectively. Moreover, Table 4
SL-04, and SL-06 reveal appreciably higher average ultimate shear provides the ratio of residual shear stress to ultimate shear stress

Fig. 10. Effect of the fiber reinforcement ratio on the cracking and ultimate shear stress of TRM specimens.
 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760 9

Fig. 11. Effect of the fiber reinforcement ratio on the crack slip and crack width corresponding to the ultimate shear stress of TRM specimens.

surface treatment (SS-EP and SS-ES series) for textile reinforce-

ment could sustain more residual shear stress in comparison to
those without surface treatment (SS-NT series); and in the case
of SS-NT, severe fiber rupture at failure was observed, resulting
in irregular shapes of shear stress–crack slip and shear stress–crack
width curves (Figs. 13 and 14) . In particular, the specimens with
textiles coated with sand after epoxy impregnation (SS-ES series)
show slightly higher ultimate shear stress than do those obtained
from the SS-NT and SS-EP series.
In detail, Fig. 15 compares the mean values of cracking shear
stress and ultimate shear stress between the SS-NT, SS-EP, and
SS-ES series. Fig. 15 (a) shows that the SS-EP and SS-ES series
Fig. 12. Effect of the fiber reinforcement ratio on the residual shear stress of TRM
specimens. reveal higher average cracking stress than that of SS-NT. Mean-
while, Fig. 15 (b) shows that the SS-NT and SS-EP series reveal
almost similar average ultimate shear stress of (8.22 and 8.19)
(sre / su). Fig. 12 and Table 4 show that the increase of fiber rein- MPa, respectively, which values are slightly lower by 15% than that
forcement ratio could, it appears, increase the residual shear stress of the SS-ES series. These observations are attributed to the greater
by increasing the transfer of shear forces from the matrix to the bond improvements in carbon fibers and mortar matrix attained by
entire carbon filaments. For example, at 3 mm level of crack slip, spreading sand on the epoxy coating to provide stronger mechan-
SL-04 and SL-06 could sustain the mean residual shear stress of ical anchoring [33]. In addition, Table 3 indicates that the mean
(6.50 and 9.59) MPa, respectively, corresponding to roughly (57.4 values of crack slip and crack width corresponding to the ultimate
and 132.2) % higher than that of SL-02. This can be explained by shear stress of the SS-NT, SS-EP, and SS-ES series show almost the
the bridging effects of the fiber filaments over the shear cracks pre- same.
venting crack widening across the shear plane. Fig. 16 depicts the effect of surface treatment on the residual
shear stress. Moreover, Table 4 also provides the ratio of residual
4.2. Effect of the surface treatment methods shear stress to ultimate shear stress (sre / su). Fig. 16 and Table 4
show that the surface treatment methods appear to improve the
Figs. 13 and 14 show the effects of the surface treatment of tex- TRM shear performance by filling the spaces between the filaments
tile reinforcement on the shear stress–crack slip and shear stress– with epoxy resin coating [41–43], which results in the increase of
crack width relations, respectively. Generally, the specimens with residual shear stress. For example, at 1 mm level of crack slip,

Fig. 13. Effect of the surface treatment method on shear stress–crack slip relationships.
10 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760

Fig. 14. Effect of the surface treatment method on shear stress–crack width relationships.

Fig. 15. Effect of the surface treatment method on the cracking and ultimate shear stress of TRM specimens.

crack width relations, respectively. Overall, the change of inclina-

tion angle of the fiber filament within the mortar matrix affected
the shear behavior of TRM specimens. The specimens of SA-45 ser-
ies having the inclination angle of fiber filament of –45°/45° could
sustain higher ultimate shear stress than that of SA-90 having the
inclination angle of 0°/90°. Conversely, in the post-peak stage, the
residual shear stress carried by the SA-45 series was slightly less
than that of the SA-90 series.
In detail, Fig. 18 compares the mean values of cracking shear
stress and ultimate shear stress between the SA-45 and SA-90 ser-
ies. Fig. 18 (a) shows that the SA-45 series reveal lower average
cracking shear stress than that of SA-90; however, Fig. 18 (b)
Fig. 16. Effect of the surface treatment method on the residual shear stress of TRM
specimens. shows that SA-45 reveals average ultimate shear stress of
10.39 MPa, which is 27% higher than that of the SA-90 series. This
is because, in the case of the fiber inclination angle of ± 45°, the
SS-EP and SS-ES could sustain the mean residual shear stress of carbon fiber yarns in two perpendicular axes work together to
(5.49 and 5.95) MPa, respectively, corresponding to roughly resist the applied load [44]. In addition, Fig. 19 compares the mean
(47.18 and 59.52) % higher than that of SS-NT. Even at 3 mm level values of crack slip and crack width corresponding to the ultimate
crack slip, the SS-ES series could sustain the mean residual shear shear stress of the SA-45 and SA-90 series. Overall, the use of car-
stress of 7.10 MPa with a COV of 0.02, which was 18.53% higher bon fiber textile having inclination angles of ± 45° increases the
than that recorded in the case of the SS-ES series. This phe- cracking slip and crack width corresponding to ultimate shear
nomenon is mainly attributed to the improved bonding by impreg- stress. Table 3 indicates that the mean values of crack slip at ulti-
nating the carbon fiber filaments with epoxy resin (SS-EP series) mate shear stress in the cases of SA-45 and SA-90 series are (0.43
and coating with sand (SS-ES series). and 0.26) mm, respectively; the mean values of crack width at ulti-
mate shear stress in the cases of SA-45 and SA-90 series are (0.42
4.3. Effect of the inclination angle of fiber filaments and 0.23) mm, respectively.
Moreover, Table 4 also provides the ratio of residual shear stress
Fig. 17 (a) and (b) depict the effects of the inclination angle of to ultimate shear stress (sre / su). It was found that the residual
fiber filaments on the shear stress–crack slip and shear stress– shear stress recorded in the case of the SA-45 series is slightly less
 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760 11

Fig. 17. Shear stress–crack slip and shear stress–crack width relationships of the SA-45 and SA-90 series.

Fig. 18. Effect of the incline angle of fiber filaments on the cracking and ultimate shear stress of TRM specimens.

Fig. 19. Effect of the incline angle of fiber filaments on the crack slip and crack width corresponding to the ultimate shear stress of TRM specimens.

than that of SA-90. Further investigations are needed to clarify this The shear strength contributed by cementitious mortar can be
mechanism. evaluated as:

V M ¼ sM  A ð2Þ
5. Prediction of the shear strength of cementitious mortar
reinforced by carbon fiber textile where, A is the area of the shear plane, and sM is the effective shear
stress of cracked cementitious material, which can be evaluated
The shear strength, corresponding to the ultimate shear stress, based on Vecchio and Collins [45]:
of cementitious mortar reinforced by carbon fiber textile could
qffiffiffiffi
be predicted as the contribution of the shear strength provided 0
0:18 f c
by cementitious mortar ðV M Þ and the shear strength provided by sM ¼ ð3Þ


the textile V f : 0:31 þ a24w
g þ16

0
where, f c is the compressive strength of mortar, w [=0.01 mm]
V ¼ VM þ Vf ð1Þ is the crack width corresponding to cracking shear stress of the
12 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760

SL-00 series (refer to Table 3), and ag is the maximum aggregate

size.
The shear strength contributed by fiber textiles can be evalu-
ated considering ACI 549.4R-13 [46] and ACI 440.2R [47], as:

V f ¼ nAf v f f v ðsina þ cosaÞ ð4Þ

f f v ¼ Ef efe ð5Þ

efe ¼ 0:004 6 0:75 efu ð6Þ

where, n is the number of fiber textile layers; Af v is the total
cross-sectional area of fiber yarns in the warp direction, which is
perpendicular to the shear plane; a is the inclination angle of fiber
Fig. 20. Idealized trilinear relation after cracking of TRM specimens subjected to
filaments to the horizontal axis; f f v ; Ef ; efe and efu are the effective direct shear.
tensile stress at ultimate state, elastic modulus, effective strain
level and ultimate strain of the fiber yarn, respectively. In the pre-
sent study, Ef ð¼ 230; 000 MPaÞ and efu ð¼ 0:021Þ as material prop- From the experimental results, the fiber reinforcement ratios of
erties were directly obtained from the manufacturer (refer to textile reinforcement contribute to the improvement of cracking
Fig. 2). and ultimate shear stress. Thus, from the experimental results,
Table 5 shows the comparison between the experimental and Fig. 8 shows that the cracking shear stress (scr) of TRM specimens,
the predicted shear strength of test specimens. Overall, the pre- corresponding to the initial cracks at the shear plane, is determined
dicted results based on Eqs. (1)–(6) agree well with the shear to be 80% of the ultimate shear stress (su). The ultimate shear stress
strength obtained from the test results. The mean ratio of experi- (su) can be evaluated as ½su ¼ V=A, where V is the total shear
mental to predicted shear strength is 1.09 with a coefficient of vari- strength carried by cementitious mortar and the textiles calculated
ation of 0.06. Thus, for up to six layers of carbon fiber textile used from Eq. (1), and A is the area of the shear plane. Moreover, based
as reinforcement within the mortar matrix in the present study, on the experimental results, the crack slip (du) corresponding to the
the shear strength of test specimens could be anticipated consider- ultimate shear stress is determined to be 0.3 mm.
ing the material characteristics of the mortar, as well as the fiber In the post-peak stage, based on the experimental results, the
textile. For the higher fiber reinforcement ratio, further investiga- sharp drop from the ultimate shear stress to residual shear stress
tions need to be performed to understand the strengthening effect (sre) stops when the crack slip reaches approximately 0.5 mm.
of the fiber textile material. Fig. 8 shows that after that, sre is stably maintained in all of the
TRM specimens, until the crack slip reaches approximately 4 mm
before the initial fiber rupture and slippage. Thus, the values of
6. Idealized crack-shear slip model for textile reinforced mortar deq and do could be determined to be (0.5 and 4) mm, respectively.
specimens In addition, the residual shear stress (sre) at 0.5 mm crack slip (deq)
is evaluated so that the fracture energy from the model is equal to
In this study, the crack–shear slip model for textile reinforced the fracture energy obtained from the test results. In the proposed
mortar specimens is proposed based on the test results. The case model, the fracture energy is derived directly as a function of fiber
study is the TRM specimens with different fiber reinforcement reinforcement ratio based on the test results. Fig. 21 illustrates the
ratio using epoxy impregnation as surface treatment for textile relationship between fiber reinforcement ratio and fracture energy
reinforcement (SL-02, SL-04, and SL-06 series). The proposed corresponding to 0.5 mm crack slip (Gf 0.5). The fracture energy of
model was developed based on the study of Jongvivatsakul et al. TRM specimens (Gf 0.5, N/mm) can be calculated from Eq. (7) based
[48] for steel fiber-reinforced concrete. on regression analysis:
Fig. 20 depicts the idealized trilinear relation representing the
Gf 0:5 ¼ 9:373q þ 2:646 ð7Þ
crack-shear slip model. Before the cracking shear stress (scr), crack
slip is assumed to be zero [48]. After cracking, the shear stress
increases to the ultimate shear stress (su), corresponding to crack
slip of du. After ultimate shear stress, the shear stress suddenly
decreases to residual shear stress (sre), which is maintained from
the crack slip of deq, to the crack slip at failure (do).

Table 5
Comparison between the experimental and the predicted shear strength of test
specimens.

Test specimen Vtest Vpredict Vtest/Vpredict

(kN) (kN)
SL-00 35.61 30.11 1.18
SL-02 45.91 42.01 1.09
SL-04SS-EPSA-90 55.63 53.90 1.03
SL-06 76.37 65.80 1.16
SS-NT 53.44 53.90 0.99
SS-ES 61.63 53.90 1.14
SA-45 67.54 63.76 1.06
Mean value 1.09
Fig. 21. Relationship between fiber reinforcement ratio and fracture energy
COV 0.06
corresponding to 0.5 mm crack slip.
 N.H. Dinh et al. / Construction and Building Materials 249 (2020) 118760 13

Fig. 22. Comparison between the proposed model and the experimental data.

where, q (%) is the fiber reinforcement ratio. Note that the frac- ing shear stress and an average 27% higher ultimate shear stress
ture energy is calculated from the area under the shear stress– than that of the SA-90 series; however, in the post-peak stage,
crack slip curves, as shown in Fig. 8. the residual shear stress carrying capacity of the SA-45 series
Fig. 22 compares the experimental data and proposed crack- is slightly less than that of the SA-90 series. In addition, the
shear slip model. Overall, the results from the model show good crack displacement including crack width and crack slip of the
agreement with the test results for the cases of the SL-02, SL-04, SA-45 series also show an increasing trend, in comparison to
and SL-06 series. Thus, for the tested ranges of parameters and the SA-90 series.
the specific type of mortar, the proposed model can be used to 5. The ultimate shear stress of cementitious mortar reinforced by
explain the behavior of cementitious mortar reinforced by carbon carbon fiber textile is predicted considering the material char-
fiber textile under direct shear. However, further investigation acteristics of the mortar and the fiber textile, and shows good
and calibration should be carried out to use the proposed model agreement with the experiment results.
for different types of mortar. 6. The idealized trilinear model is proposed based on the test
results to simulate the crack–shear slip of cementitious mortar
7. Conclusions reinforced by carbon fiber textile considering the fiber rein-
forcement ratios, and overall its prediction shows good agree-
The present study investigates the direct shear characteristics of ment with the experiment results.
textile reinforced mortar specimens. The main test parameters
include the fiber reinforcement ratio, the surface treatment meth- CRediT authorship contribution statement
ods used for fiber textile, and the inclination angle of the fiber fil-
aments. Based on the test results, the primary conclusions that can Ngoc Hieu Dinh: Conceptualization, Methodology, Writing -
be drawn are as follows: original draft. Hai Van Tran: Investigation, Data curation.
Kyoung-Kyu Choi: Methodology, Supervision, Writing - review &
1. Overall, the failure modes of all test specimens are mostly gov- editing, Funding acquisition.
erned by the shear cracks in the vicinity of the notches. Com-
pared to plain mortar specimens, specimens reinforced by Declaration of Competing Interest
carbon fiber textiles show a narrower area of shear fracture
zones, which are mainly concentrated at the preformed The authors declare that they have no known competing finan-
notches. In the cases of specimens reinforced by carbon fiber cial interests or personal relationships that could have appeared
textile with surface treatment, splitting cracks occur out of to influence the work reported in this paper.
the loading plane, while in the cases of specimens reinforced
by carbon fiber textile without surface treatment, this phe- Acknowledgment
nomenon does not occur.
2. Compared to plain mortar specimens, all cementitious mortar This research was supported by a grant (19CTAP-C152105-01)
specimens reinforced by carbon fiber textile show ductile from the Infrastructure and Transportation Technology Promotion
behavior with increased deformation capacity and additional Research Program, funded by the Ministry of Land, Infrastructure
residual shear stress carrying capacity in the post-peak stage. and Transport of the Korean Government.
The use of higher fiber reinforcement ratios of textile reinforce-
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