innovative technique of textile reinforced mortar (trm ...eprints.brighton.ac.uk/18018/1/final paper...
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1
Innovative technique of textile reinforced mortar (TRM) for flexural strengthening of
reinforced concrete (RC) beams
A. Baiee, I. Rafiq and A. Lampropoulos, University of Brighton, UK
ABSTRACT
Performance degradation of existing RC structures due to aging and environmental effects made
strengthening of RC structures a global issue. Bond between the substrate member and new
strengthening layer is considered a threshold for any successful strengthening technique. This study
explores the efficiency of using cementitious high strength connectors in preventing the debonding of
TRM strengthening layer from strengthened RC beams. The experimental program includes two parts. In
first part, the effect of strength, ratio, diameter and distribution of connectors, for smooth and rough
surfaces, on the tensile bond strength are examined. The applicability of these connectors is investigated
in second part by means of RC beams strengthened with TRM comprising four and eight textile basalt
fibre layers. The results demonstrate that the inclusion of cementitious connectors changed the failure
mode from debonding failure to desired flexural failure. The proposed improvement exhibited increasing
to cracking and ultimate load up to about 140% and 93%, respectively. However, a significant reduction
in ductility of all strengthened beams was observed in comparing with the control specimen.
NOMENCLATURE
C cement
G coarse gravel
S sharp sand
S.S silica sand
S.P superplasticizer
W water
fc cubic compressive strength
ft splitting tensile strength
P applied compressive load
fb splitting bond strength
ρc ratio of cementitious connectors
Aco area of cementitious connectors
Ac area of interface
n number of connectors.
d diameter of connector (mm).
D and L the short and long dimensions of half
cylinder (mm), respectively
μδ ductility index
δu deflection at ultimate load
δy deflection at steel yielding
1. INTRODUCTION
Many of the existing RC structures were
constructed based on old design codes that do
not supply the safety requirements
recommended by present design codes
especially for seismic and fire aspects.
Fibre Reinforced Polymer (FRP) composites
has been widely used for strengthening RC
structures. This technique has favourable
properties such as high strength to weight ratio
and resistivity to corrosion but also has
drawbacks mainly attributed to the organic
epoxy resins used to bind the fibres [1]. In
addition, epoxy has many weak points such as
poor fire resistance; high cost; and
inapplicability on wet surfaces or at low
temperatures; hazards for the manual worker;
poor thermal compatibility with the base
concrete [2,3,4,5].
Therefore, inorganic binder (cementitious
matrix) has been investigated to be used instead
of epoxy. Recently, textile fibres cementitious
technique has been investigated for
strengthening masonry and RC structures. This
technique is based on using textile polymer
fibres bonded with the substrate concrete by
using cementitious matrix [3]. There are
different forms of textile cementitious
composite; Textile Reinforced Mortar (TRM),
Textile Reinforced Concrete (TRC), Fibre
Reinforced Cementitious Mortar (FRCM) and
Mineral Based Composites (MBC). All these
forms have similar components with a slight
difference in property of the cementitious
2
matrix. This study adopted TRM form as a
textile strengthening technique.
Most of studies that investigated using TRM
in strengthening masonry structure explained a
debonding failure mode of the strengthening
layer [6,7,8]. Therefore, anchors to improve the
bond strength have been investigated.
Garmendia et al. [8] and Ismail and Ingham [9]
found using anchors can changed the failure
mode from de-bond to basalt breakage of
strengthening layer. However, Bernat et al. [10]
observed a reduction in the enhancement of the
capacity of masonry brick walls strengthened
with carbon textile reinforced mortar in
presence of carbon spike anchors. That
reduction may be attributed to the insufficient
penetration of anchors within the mortars which
in turns affect the adhesive bond between the
textile fibres and the wall.
In field of RC structures, similar debonding
observation of the TRM strengthening layer
was reported by many researchers
[3,11,12,13,14]. Some researchers studied
improve the bond by improving the adhesive
properties of the matrix [4,14,15]. The
enhancement of the strengthened beams was
attributed to the increasing the adhesive bond of
the matrix because of increasing the calcium
silicate hydrate (C–S–H) with reducing the
amounts of ettringite in form of Al2O3-Fe2O3-tri
(AFt) or calcium hydroxide (Ca(OH)2).
However, the improvement in bond strength
was insufficient to achieve the desired
enhancement of the strengthened members.
Therefore, some researchers investigated using
anchors to improve the bond strength [11, 16,
17]. Inspite of the achived enhancement but its
still unsufficient to provide the required bond
between the substrate concrete and
strengthening layer. That may be due to lack of
interlocking between these anchors and the
mortar.
In general, bond between old concrete and
new cementitious layer depends mainly on the
adhesive bond (chemical bond) and physical
bond of the interfacial zone (interface) between
the two layers. Chemical bond is resulted from
a chemical forces during the hydration of
cement and forming calcium silicate hydrate
(C–S–H) [18]. As the (C-S-H) increases at the
interface, the chemical bond strength increases.
Many researchers found that using cementitious
matrix with high compressive strength can
exhibit high bond strength with old concrete
[19,20,21,22].
The physical bond depends mainly on the
mechanical interlocking between the old and
new concrete. The interlocking is highly
effected by the surface preparation of the
substrate concrete. Many researchers found
that higher roughness can lead to high bond
strength [23,24,25,26,27,28,29,30].
It can be stated that bond between the
substrate member and new strengthening layer
is considered a threshold for any successful
strengthening technique. Therfore, the present
work investigate improving the bond strength
of strengthened RC beams with TRM by
enhancing the chemical and physical bond of
the interface. The chemical enhancement
includes using high strength mortar as a matrix
for TRM. While, high strength cementitious
connectors is investigated to enhance the
physical bond.
2. EXPERIMENTAL PROGRAME
2.1 METHODOLOGY
The experimental work consisted two parts;
the first part deals with investigation the effect
of cementitious connectors on the splitting
bond strength of hybrid cylinders. The
objective of this part was to determine the
connector ratio which can explain bond
strength approximately equals to the splitting
tensile strength of the substrate concrete.
Hybrid cylinders are made up of half cylinder
concrete which casted and cured for 28 days
and half mortar casted after preparation the
surface of the cured half concrete cylinder.
The second part included casting five RC
beams and after 28 days water curing, the TRM
strengthening layer was casted and cured in
water for 28 days. All RC beams were tested
3
under three points flexural loading. The
investigated parameters included; strength of
the mortar of TRM (normal and high strength
mortar) and amount of textile basalt fibre (four
and eight layer). In addition, the mechanical
properties of substrate concrete and TRM
strengthening layer were determined.
2.2 MATERIALS PROPERTIES
Ordinary Portland cement type CEMI 52.5N
that satisfies BS EN 197-1:2011 standards for
Portland cement were used for casting concrete
of beams, half cylinders and mortars of TRM.
Crushed gravel with10mm maximum aggregate
size analogous to BS EN 12620:2002+A: 2008
was used. Sharp sand with maximum size
aggregate 4mm satisfying BS EN
12620:2002+A:2008 was used for casting
concrete. For mortar, fine silica sand that
satisfies the requirements of BS EN
12620:2002+A:2008 was used. Fosroc
Auracast200 high performance concrete
superplasticizer based on polycarboxylate
polymers was used to improve the workability
of mortar. Textile basalt fibres with opening
meshes size 5mm were used for composing the
TRM strengthening layer, table 1 presents the
main properties of these fibres.
Table 1: Properties of textile basalt fibres Thickness (mm) 0.6 – 0.7
Melting point (C˚) 1350
Type of coating Styrene-acrylic latex
Maximum load (N/5cm) 1873.86 (warp)
4416.29 (weft)
Elongation at beak (%) 4.26 (warp)
10.26 (weft)
2.3 MIX PROPORTIONS
Mix proportions of cylinders samples and RC
beams were designed to provide concrete grade
C40/50. Cubic specimens (100×100×100) mm
that satisfies BS EN 12390-3:2009 were used to
assess the compressive strength. Splitting
tensile test of cylinders (100 mm diameter and
200mm length) was carried out to determine the
tensile strength of the concrete at 28 days
according to BS EN 12390-6:2009. Standard
slump test was carried out according to BS EN
12350-2:2009 to assess the workability of
concrete. The obtained slump values of the
mixes ranged between 75 to 100 mm.
Two different mortar grades were used in this
study; M50 with compressive strength about 50
MPa for NSM and M100 for HSM. To provide
sufficient penetration of mortar inside the holes
of the substrate concrete and textile basalt fibre,
high workability should be provided. Flow
table test that satisfy the BS-EN 1015-3:1999
was conducted to assess the workability. The
results of the mortar flow ranged between 225
and 250 mm. The mix proportions and
mechanical properties of concrete and mortars
are listed in table 2.
2.4. BOND BETWEEN CONCRETE
AND MORTAR
Hybrid cylinders were tested under splitting
compressive load to assess the bond strength
between old concrete and new mortar. Hybrid
cylinders are made up of half cylinder normal
strength concrete (NSC) which casted and
curing for 28 days and half mortar casted after
preparation the surface of the cured half
cylinder. The hybrid specimens cured again for
28 days before testing. The process of casting
and testing is shown in Fig. 1. In this work, the
ratio of connectors is the total holes area
divided by the contact area (interface area)
between the old concrete and new mortar.
Different ratios; 0.85, 1.7, 2.5, 5 and 10 of the
connectors were investigated.
…………. (1)
………….. (2)
Splitting bond strength is evaluated according
to the following equation:
………………. (3)
4
Fig. 1. Process of casting and testing of hybrid cylinder
Two surface preparations were adopted; as
casted (AC) and rough (R). The diameter of the
cementitious connector ranged between 6mm
and 12mm, with an average depth equal to
10mm. These dimensions were selected after
many trials to avoid destroying the substrate
half cylinder concrete, see Fig.2. Rough surface
was achieved by roughening the half concrete
cylinders using needle scaler hammer for
removing 1-2 mm from the surface. Also,
normal strength mortar investigated to assess
the effect of mortar properties on the bond
strength. Control specimen represents the
results of splitting tensile strength of normal
strength concrete. The configurations of the
tested hybrid cylinders are reported in table 3.
As casted Rough
As casted with connectors Rough with connectors
Fig. 2. Prepared surface texture of the substrate concrete
Table 2: Mix proportions and mechanical properties of the adopted mixes
Mix C
(kg/m3)
G
(kg/m3)
S
(kg/m3)
S.S
(kg/m3)
S.P.
(kg/m3)
W
(kg/m3)
Fc -
7days
(MPa)
Fc -
28days
(MPa)
Ft
(MPa)
NSC 450 800 785 -- -- 225 44.1 54.4 3.78
NSM 540 -- -- 1350 15 270 40.3 51.3 3.1
HSM 1050 -- -- 1050 45 210 85.3 104.8 5.7
Table 3: Configuration of hybrid cylinders
Specimen Surface Connectors’ ratio
(%)
CONT -- --
R0 R --
AC0 AC --
RONSM R --
ACM0.85-6C AC 0.85
ACM0.85-6E AC 0.85
ACM1.75 AC 1.75
ACM2.5 AC 2.5
ACM5.0 AC 5
ACM10 AC 10
RM1.75 R 1.75
RM0.85-6C R 0.85
RM0.85-6E R 0.85
NSC
HSM
5
3ɸ 8mm 2ɸ 6mm
P
345mm 345mm
750mm 150mm
150
mm
2.5 RC BEAMS STRENGTHENED WITH
BASALT TRM
Normal strength concrete (NSC) beams with
dimensions (150x150x750mm) reinforced with
three bars of diameter 8mm in longitudinal
direction and 6mm diameter for shear
reinforcement were adopted. The yield stress of
the bars was 380 MPa. The reinforcement
details are illustrated in Fig.3
After preparing the required reinforcement of
the RC beams, concrete was casted in steel
moulds.
Fig. 3. steel reinforcement details of RC beams
After water curing period (28 days), needle
scaler hammer was used to removing 1-2 mm
from the surface of RC beams before casting
the TRM layer. After that, hammer drill
machine was used to make holes inside
substrate concrete beams with diameter equal to
7mm. The applied ratio of connectors was 8.75,
see Fig.4.
Roughnening of surface
Drilling the holes
Fig. 4. Process of preparing RC beams
Five RC beams were tested under three-point
flexural loading. One RC beams without
strengthening (CON) to serve as a control
beam. One beam strengthening with normal
strength TRM and without connectors (B1).
One beam was strengthened with four layers of
high strength mortar TRM and with connectors
(B2). Like this beam, two beams (B3) and (B4)
with eight strengthening layers was tested to
investigate the effect of number of layers on the
performance of the strengthened beams. For all
strengthened beams, prefixed of basalt fibre and
then casting mortar was used. Only B3 a hand-
lay-up method was applied for casting the TRM
strengthening layer, see Fig. 5. Table 4 presents
the details of the investigated beams. Zwick
device with ultimate load 200kN was used for
testing with rate of loading 0.25 mm/min, see
Fig. 6.
Table 4: Detailed of the tested beams Beam Type of
mortar
Ratio
connectors
No. of
layers
CON --- 0 0 B1 NSM 0 4
B2 HSM 0.0875 4
B3 HSM 0.0875 8
B4 HSM 0.0875 8
a. pre-fixed of fibres b. hand-lay-up
Fig. 5. Details of casting TRM on RC beams
6
Fig. 6. Test set up of RC specimens
3. TEST RESULTS AND DISCUSSION
3.1 BOND TEST
Three specimens for each case were adopted.
HSM specimens exhibited 20% increasing in
bond strength in comparison with NSM
counterparts. This attributed to the presence of
high cement content in the HSM mixture,
which leads to increase the C-S-H components.
HSM explained bond with the particles of
substrate concrete higher than the bond
between components of substrate concrete as
shown in Fig. 8. The obtained results are listed
in table 5.
The results showed that the lowest bond
strength was observed for AC0 specimens
because of untreated surface (as cast) is usually
weak. However, the inclusion of connectors
improves the bond strength because of
increasing the contact area at the interface and
reaching stronger layer than the surface.
Adhesive bond of smooth surface without
connectors depends only on the chemical
interaction between old concrete and new
mortar. While in presence of connectors, the
adhesion bond will depend on the tensile
strength of connectors in addition to the
chemical interaction. That means as the tensile
strength of the connectors increased, the
adhesive bond increased.
In addition, it was found a negligible effect of
the distribution of the connectors on the bond
strength. All connectors exhibited perfect bond
inside the substrate concrete, see Fig.8. Despite
the achieved enhancement of bond because of
application the cementitious connectors, but
they still provide bond strength lower that
rough surface without connectors. Specimen
ACM1.75 explained about 58% of the rough
surface counterpart (RM1.75). That explains
the importance of removing the outer thin layer
of the substrate member.
Table 5: Summary of bond test results
Specimen fb (MPa) C.O.V (%)
CONT 3.78 4
R0 2.85 2
AC0 1.0 18
RONSM 2.37 4
ACM0.85-6C 1.57 4
ACM0.85-6E 1.6 13
ACM1.75 1.71 5
ACM2.5 1.74 12
ACM5 2.03 9
ACM10 2.29 2
RM1.75 2.91 13
RM0.85-6C 3.04 6
RM0.85-6E 3.51 6
However, for rough surface, the results
showed that the distribution of connectors can
affect the bond strength. All specimens with
connectors explained higher bond strength than
counterparts without connectors.
The highest achieved bond strength was with
using rough surface and the connectors
distributed at the edges of the interface. Despite
the high coefficient of variation (c.o.v.) of most
results, it can be stated rough surface with
inclusion cementitious connectors can provide
bond strength equal to the bond strength
between the components of substrate concrete.
3.2 RC STRENGTHENED BEAMS
Flexural failure was observed for all tested
beams except B1, where it exhibited de-
bonding of the strengthening layer at the mid
span before reaching ultimate load. The de-
bonding of strengthening layer started at load
equal to 50.2kN. While, B2 explained perfect
bond between the substrate RC and TRM
strengthening layer until reaching the ultimate
tensile capacity of the basalt fibre. Fig. 9 shows
the crack pattern at failure load of the tested
beams.
LVDT-1
LVDT-2
Actuator
Support
Support
345mm 345mm
7
ROHSM AC0 RONSM ACM0.0085-6C ACM0.0085-6E ACM0.0175
ACM0.025 ACM0.05 ACM0.1 RM0.0175 RM0.0085-6C RM0.0085-6E
Fig. 8: Failure mode of bond specimens
The strengthening layer (TRM) of beam B2
explained a flexural failure and the failure was
due to fail of basalt fibre mesh. While a de-
bonding failure was observed at the mid span of
beam B1, see Fig9. This de-bonding reduced
the enhancement of the strengthening layer to
the ultimate strength from 30% to 21%, see
Fig.10. In addition, the debonding limited the
enhancement up to 21%. Where, there is no
expected enhancement if the amount of basalt
fibres increased.
The highest increasing in ultimate capacity
about 93% was observed for beam B3.
However, beam B4 exhibited lesser increasing
in ultimate capacity about 79% than beam B3.
Both beams were strengthened with the same
properties of connectors and textile fibres. That
was due to the difference in method of
application the textile layers, which affect the
location of fibres within mortar. The textile
fibres in beam B3 were closer each to other,
which lead to loading them at the same time
and presenting higher tensile strength than
beam B4. This explains the effect of fibre
position within the matrix on the tensile
strength of the TRM layer. All results are
reported in table 6.
Moreover, beams strengthening with eight
layers of textile basalt fibres exhibited cracking
load (load at first crack) higher than the
ultimate load of the control beam. That means it
can be convert a RC beams from cracked
members under ultimate loading to un-cracked
member under the same amount of loading
which enhanced the durability of the
strengthened members.
All strengthened beams explained higher initial
stiffness than control beam, see Fig. 11. The
ductility of the tested beams is calculated based
on deflection ductility index based on the
following equations:
μδ = δu / δy ……….. (4)
Table 6: Results of tested beams
Specimen Pc
a
(kN)
∆cb
(mm)
Puc
(kN)
∆ud
(mm) Pc/ Pco
e ∆c /∆co
f Pu/ Puo
g ∆u /∆uo
h Failure mode
CON 22.1 1.2 45.5 12.1 -- -- -- -- Flexural
B1 29.6 1.6 54.9 4.7 1.34 1.33 1.21 0.39 Debond-flexural
B2 31 1.5 59 4.1 1.4 1.25 1.3 0.34 Flexural
B3 53.2 2.5 87.6 8.3 2.41 2.08 1.93 0.69 Flexural
B4 51.7 2.3 81.6 5 2.34 1.92 1.79 0.41 Flexural a. Load at first crack (kN). b. Deflection at first crack (mm). c. Ultimate load (kN). d. Deflection at ultimate load
(kN). e. Load at first crack of control beam (kN). f. Deflection at first crack of control beam (mm). g. Ultimate
load of control beam (kN). h. Deflection at ultimate load of control beam (mm).
8
Fig. 9. Failure mode of strengthening RC beams
Fig. 10. Enhancement of strengthening
All strengthened beams explained lower
ductility than control beam due to the increase
the cross section of the strengthened beams as
shown in table 7. Beam B1 explained higher
ductility than beam B2 because of the
debonding which leads to reduce the cross
section of the strengthened beams. In addition,
the stiffness of TRM with HSM is higher than
NSM counterpart. Beam B3 and B4 exhibited
approximately same ductility because both
presented high bond strength until ultimate
load.
Table 7: Ductility of the tested beams
Specimen δu (mm) δy (mm) μδ
CON 11.56 1.2 9.6
B1 2.8 0.93 3
B2 2.2 1.1 2
B3 5.6 1.85 3
B4 3.1 1.15 2.75
Fig. 11. Load deflection curves of the tested
0
50
100
150
CON B1 B2 B3 B4
Incre
asin
g in
lo
ad
(%
)
Beams
ultimate load
cracking load
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15
Load
[kN
]
Deflection [mm]
CON B1
B2 B3
B4
CON
B1 B2
B3 B4
9
4. CONCLUSIONS
The main conclusions that can be drawn are
as following:
- The application of cementitious
connectors improves the bond strength
of smooth surfaces. However, this
improvement is less effective than
rough surface. It is effective with rough
surface to achieve bond strength higher
than substrate concrete.
- High strength mortar exhibits higher
bond strength up to about 20% than
normal strength mortar counterparts.
- Application of high strength
cementitious connectors improves the
bond strength between the substrate
concrete and TRM strengthening layer.
This improvement depends on the
properties and amount of textile fibres.
- Strengthening RC beams with eight
layers of textile basalt fibre increase the
ultimate load up to 93% and the
cracking load up to 140%.
- All strengthened beams explained lower
ductility than control beams because of
the increasing in cross section of the
strengthened beam. The reduction in
ductility affected by properties of the
bond and TRM strengthening layer.
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