Adhesion problems between FRP reinforcement and masonry support in
existing building strengthening
S. Briccoli Bati, L. Rovero and U. Tonietti
Dipartimento di Costruzioni
Florence University
Italy
ABSTRACT
The key factor for evaluating mechanical behaviour in masonry reinforced with composite materials is the bond strength
between composite material and masonry. In this paper, the said strength was tested using shear test on brick reinforced
with CFRP (Carbon Fiber Reinforced Polymer) of varying dimensions.
The results made it possible to point out the influence played by the dimensions of the reinforcement sheet on the collapse
load, ultimate slip, quality of the load-displacement diagram, and collapse modes. Lastly, identification of the local strains
along the reinforcement sheet enabled us to formulate hypotheses regarding the local bond-slip relationship and then on
the transfer mode of the stresses from the reinforcement to the brick
Introduction
The technique of bonding with composite materials having a polymeric matrix reinforced with long carbon fibers (CFRP)
has become very widespread in the consolidation of masonry structural elements. The correct use of composite materials
as reinforcement for masonry structures is subordinated to a knowledge of the mechanical behaviour of the reinforced
masonry system, with particular reference to the collapse modes that emerge due to a detachment of the reinforcement
from the masonry support. The key factor characterizing the mechanical behaviour of the CFRP masonry reinforcement is,
therefore, the bond strength, i.e. the maximum load that can be transferred from the masonry to the composite material in
order to avoid brittle collapse due to a loss of bonding.
In the literature, many research projects of an experimental and theoretical nature exist that study the bond between
CFRP reinforcement and structural elements made of concrete [1] [2] [3]. These studies are aimed at defining the
experimental testing typology appropriate for describing the phenomenon, identifying the influential parameters, clarifying
empirical models and numerical analyses, and defining theoretical models based on fracture mechanics.
As far as the literature on masonry is concerned, there are many fewer experimental results available, and the theoretical
models proposed are derived from ones determined from experimentation on concrete [4]. The existing studies on
masonry structural elements reinforced with CFRP [5] [6] [7] have pointed out that the collapse mode is almost always
determined by de-bonding of a thin layer of brick adjacent to the masonry-to-adhesive interface, while a loss of adhesion
of the bonding almost never occurs. This observation emphasizes the fact that the bonding capacity must thus depend
significantly also on the resistance of the support. Such a circumstance is the base of several theoretical formulations,
born for the concrete, that connect the resistance to the detachment with the fracture energy released during the removing
process [3]. Besides the support tensile resistance, other factors affecting the bond strength are, obviously, the
dimensions of the reinforcement sheet, the ratio between sheet width and support width, the reinforcement stiffness, since
everyone able to guide the stress distribution at the interface. Moreover another important aspect concerning the bond
strength, revealed by all the experiments carried out, is that an effective bond length exists beyond which the collapse load
does not increase.
In this paper we report the results of an extensive experimental investigation aimed at studying the strength of the brickCFRP bonding, using adhesion test on bricks reinforced with CFRP of varying dimensions.
Test apparatus
The test setup selected to determine the adhesion between brick and CFRP is a double-shear push test, as defined in Yao
et al. 2004. The test specimen consisted of a single brick block on the larger sides of which were glued two reinforcement
sheets made up of a single sheet of fiber soaked in an epoxy matrix only in the parts in contact with the brick. While the
brick block was immobilized on the higher small side, the reinforcement sheet rotated around a steel cylinder connected to
the load cell and then to the hydraulic jack by means of a spherical hinge.
Four displacement transducers measured the vertical displacements of the steel cylinder and then of the end of the sheet,
and two transducers controlled that there were no displacements at the bonded base. The action exerted by the hydraulic
jack subjected the reinforcement sheets to traction, and triggered a state of compression on the block (Fig.1)
A
B
Figure 1.: Specimen scheme; test apparatus drawing; a specimen during the test
Mechanical characteristics of the materials
The mechanical characteristics of the brick employed in the specimens were determined by uniaxial compression, direct
tensile and bending tests.
The compression tests were performed on prismatic brick specimens of dimensions 56x15x30 mm3
(length×height×width), made from brick blocks provided by the “Laterizi S. Marco” firm (Venice, Italy). The direct tensile
tests were conducted on 40x10x80 mm3 prismatic brick specimens with symmetrically-arranged notches measuring
approximately 1 cm in length. The bending tests were carried out on 120x250x55 mm3 prismatic brick specimens. The
mechanical parameters determined by means of these tests are reported in Table 1. The mechanical characteristics of the
MBrace FRP system (CFRP), supplied by the MAC S.p.A. firm (Treviso, Italy) which also provided the technical
specifications, are reported in Table 2. This system consisted of "MBrace Fibers C-130" high-strength unidirectional
carbon fibre sheets (0,16 mm thick), an "MBrace" primer, and a two-component, epoxy-base adhesive.
Bulk density
Young's modulus
3
[kg/m ]
1800
[MPa]
1785
Compressive
strength
[MPa]
17,39
Direct tensile
strength
[MPa]
1,7
Bending tensile
strength
[MPa]
3,53
Table 1. The mechanical characteristics of the bricks
Bulk
density
Fiber
Primer
Adhesive
[kg/l]
1820
1067
1020
Tension
Young's
modulus
[MPa]
230000
>700
>3000
Bending
Young's
modulus
[MPa]
>580
>3500
Direct tensile
strength
[MPa]
>3430
>12
>50
Bending
tensile
strength
[MPa]
Ultimate strain
>24
>24
[%]
1,5
3
2,5
Table 2. Technical specifications of the Mbrace system components (MAC S.p.A)
Test Specimens
All specimens were obtained by utilizing brick with the standard dimensions of 55x120x250 mm3 and CFRP sheets with
constant width and different lengths: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160 and 200mm. In
order to give statistical validity to the experimental data, three specimens of each type were carried out. Totally 51
specimens were tested.
Failure modes
In all tests, the collapse of the specimens happened unexpectedly (brittle), and was characterized by the following fracture
modes which occurred in combined manner:
1.
removal of a considerable portion of brick, of a semi-truncated conical shape, located on constrained edge of the brick
(end A of the reinforcement in Figure 1)
2.
removal of a considerable portion of brick, of a semi-truncated conical shape, located on the end of the reinforcement
bonded on the brick (end B of the reinforcement in Figure 1)
3.
de-bonding in the brick of a thin layer of brick below the reinforcement between the ends A and B of the reinforcement
The fracture mode (3) consisted of the de-cohesion of a thin layer of brick below the one impregnated with primer is the
principal fracture mode that characterizes all specimens and describes the mechanical phenomenon. The fracture mode
(3) could be attributed to an excess of shear stress in the brick (Figure 2 C).
The fracture mode (1) consisted of the detaching of a prism of brick, of a semi-truncated conical shape, in correspondence
with the constrained edge of the brick could be attributed to the sliding action of the reinforcement spread within the brick,
generating tri-axial stress that were responsible for the fractures on the inclined surfaces. The shape, geometry and
dimensions of the wedge varied slightly with the dimensions of the reinforcement, whereas the angle that the two
generating lines formed on the glued surface was approximately constant (Figure 2 A).
It is reasonable to believe that the fracture mode (2), consisted of the detaching of a brick prism, of a semi-truncated
conical shape, in correspondence with end of the reinforcement bonded on the brick, was a secondary phenomenon that
occurs instantly after the other previously-described collapse modes (Figure 2 B).
Figure 2.: Failure modes
Collapse loads
Figure 3 provides a histogram that compares the average collapse loads the values of all the types of specimens and in
Table 3 these values are summarized. The histogram pointed out that the effective length, beyond which the load remains
constant, is about 110mm.
Strain distributions in the reinforcement
In all specimens, electrical resistance strain gauges were bonded to the sheet in a longitudinal direction every 15 mm in
order to evaluate the strain distribution of the reinforcement.
Figure 4 shows the position of the strain gauges for the specimens with 40x160 mm reinforcement sheet and the strain
path in the longitudinal direction of the sheet with respect to different load level. It could be observed that strains up to
70% of the load involved the sheet only up to 50 mm from the loaded end the values of the micro-strain are lower than
3000; the micro-strains are 8000 and 10000 for 85% and 100% of the load. For loads with increases greater than 85%, the
strains, with lower values, involved further parts of the sheet. For all load levels the micro-strain are recorded only in
100mm length of the sheet, that is, within the effective bond length.
.
load (daN)
1200
1000
800
600
400
200
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 200
length of reinforcement (mm)
Figure 3. Histogram of the average collapse loads
width x length of
reinforcement (mm)
40x 10
40x 20
40x 30
40x 40
40x 50
40x 60
40x 70
40x 80
40x 90
40x 100
40x 110
40x 120
40x 130
40x 140
40x 150
40x 160
40x 200
Collapse load
(N)
2390
4490
5390
5910
6680
6465
7900
7760
8140
9065
9775
9340
10130
10135
10960
10030
10460
Table 3. Average collapse loads
The mean shear stress distribution between the brick and the reinforcement was evaluated by imposing the equilibrium
condition of the sheet with reference to the distance between two contiguous strain gauges:
τ frp ( x) = t frp E frp
ε k − ε k −1
xk − xk −1
(1)
where ε k and ε k−1 are the strain values corresponding to positions i and I-1, and xk and xk−1 are the corresponding
distance from the specimen loaded end; t frp is the sheet thickness, and Efrp is the Young modulus of the sheet. The slip
between the brick and the reinforcement was evaluated by integral of the strains:
s frp ( x) =
k
(ε i +1 − ε i )( xi +1 − xi )
(2)
i =1
x = ( xk + xk −1 )
(3)
Figure 5 shows the mean shear stress versus load as a percentage of the ultimate load plotted at different strain-gauge
positions.
The diagrams show that, at load levels of less than 70% of the ultimate load, the shear stress between the reinforcement
and the brick occupied a 50-mm length of the sheet near the loaded end. When the load exceeded 85% of the ultimate
load, the shear stress reached the peak in correspondence with the strain-gage position near the loaded end of the sheet.
Figure 6 shows the shear stress-slip diagram determined by (2) for specimens with 40x160 mm sheet.
The influence of reinforcement width
micro-strain
In support of the assertions concerning the 40 mm width reinforcement sheet it is interesting to compare the results
obtained for reinforcement sheet having different width [7]. By the results reported in figure 6 it is evident that the collapse
load and the effective length depend on the width of the sheet and on the ratio between the sheet’s and the support’s
widths.
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
10% load
30% load
50% load
70% load
85% load
100% load
0
50
100
150
200
strain gages position (mm)
Figure 4. Position of the strain gauges for the specimens with 40x160 mm sheet and the strain path with respect to
different load level.
10
9
sg1-2
sg3-4
sg5-6
sg7-8
sg9-10
shear stress (MPa)
8
7
sg2-3
sg4-5
sg6-7
sg8-9
sg10-11
6
5
4
3
2
1
0
0
0,2
0,4
0,6
0,8
1
load level (P/Pu)
1,2
Figure 5. Mean shear stress versus load as a percentage of the ultimate load plotted at different strain-gauge positions
(specimens with 40x160 mm sheet)
shear stress (MPa)
10
9
8
7
6
5
4
3
2
1
0
0
0,02
0,04
0,06
0,08
slip (mm)
0,1
Figure 6. Shear stress-slip diagram (specimens with 40x160 mm sheet)
collapse load (daN)
1200
1000
800
width 10mm
width 20mm
width 40mm
600
400
200
0
10
30
50
70
100
140
lenght of sheet (mm)
Figure 7: Histogram of collapse load for three different width of the sheets.
Acknowledgments
The authors gratefully acknowledge the financial support provided from RELUIS
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