COMPARISON OF IN-SITU AND LABORATORY TESTS OF BRICK
MASONRY WALLS STRENGTHENED WITH CARBON FIBRE
REINFORCED POLYMER FABRIC
Samo GOSTIČ
Title
University or Affiliation
Address
email address*
PhD
Building and Civil Engineering Institute ZRMK
Dimičeva 12, Ljubljana, Slovenia
[email protected]*
Mojca JARC SIMONIČ
Title
University or Affiliation
Address
email address
B.Sc.
Building and Civil Engineering Institute ZRMK
Dimičeva 12, Ljubljana, Slovenia
[email protected]
Vlatko BOSILJKOV
Title
University or Affiliation
Address
email address
Assist. Prof.
University of Ljubljana, Faculty of Civil and Geodetic Engineering
Jamova 2, Ljubljana, Slovenia
[email protected]
Roko ŽARNIĆ
Title
University or Affiliation
Address
email address
Prof.
University of Ljubljana, Faculty of Civil and Geodetic Engineering
Jamova 2, Ljubljana, Slovenia
[email protected]
Abstract
Shear failure of masonry walls is a critical failure mechanism that in many cases leads to the
sudden collapse of entire buildings during earthquakes. To improve seismic resistance of brick
masonry walls the reinforcement of masonry with carbon fibre reinforced polymer (CFRP)
fabric is presented as innovative and efficient strengthening technique. In order to develop the
most efficient way of applying CFRP fabric to brick masonry walls, in-situ and laboratory
tests were carried out and their results are compared. Laboratory tests were carried out on
sixteen walls with different reinforcement configuration (diagonally strengthened,
horizontally and vertically strengthened, only horizontally strengthened and un-strengthened)
and six walls of different configurations were tested in-situ. Specimens were tested under
constant vertical load and by displacement controlled horizontal cyclic loading. Laboratory
test specimens were tested as single fixed, while in-situ specimens were tested as doublefixed. CFRP strengthening favourably influenced the mechanism of wall behaviour in all
cases. It significantly increased ultimate displacement, ductility and dissipated energy. The
un-reinforced masonry typically failed in diagonal shear. Diagonally reinforced masonry in
laboratory failed in compression due to rocking, while horizontally reinforced specimens
resisted high shear and compressive stresses and failed due to masonry compressive failure
within the FRP confinement. Failure mechanism of in-situ tested specimens showed
compressive failure within the FRP confinement and shear cracks. In general the behaviour of
horizontally reinforced masonry exhibited much higher ductility and energy dissipation in
comparison to diagonally strengthened walls.
Keywords: ductility, FRP, masonry, shear test, stiffness, strength, strengthening
Page 1 of 8
1. Introduction
Major part of existing stocks of public and residential buildings that were built in the last
millennia represents brickwork masonry. Unfortunately unreinforced brick masonry (URM)
exhibited low seismic resilience during past earthquakes. Shear failure of masonry walls is a
common problem that in many cases leads to the sudden collapse of entire buildings during
earthquakes. Several conventional strengthening methods to overcome these problems were
developed in the past. One of them is also the new method with application of carbon fibre
reinforced polymer (CFRP) fabric reinforcement. By attaching FRP stripes of fabric to the
masonry the load bearing capacity and ductility of walls is greatly enhanced. At the same time
the wall stiffness which could lead to unfavourable redistribution of load is not increased. One
of the first studies of effect of strengthening masonry wall by fibres was done by Croci et.al.
[1]. Triantafillou's research [2] includes wide spectra of composite materials for
strengthening. The experimental work focused on masonry led to proposal of design
equations for masonry strengthened with FRP. Valluzzi [3] performed series of diagonal tests
on differently reinforced walls. Double sided, diagonal strengthening with GFRP showed to
be the most efficient method.
2. Experimental program
As it is not possible to reliably determine the wall characteristics needed for the efficient nonlinear seismic analysis (shear strength, displacement capacity...) only based on brick and
mortar characteristics it is necessary to perform the shear and compression test on the
masonry specimens. The critical elements of masonry to fail in shear during an earthquake are
parts of walls between openings, so for our testing campaign dimensions of specimens are
chosen to represent such elements. On the other hand the dimensions of the specimens have
also to satisfy the restraints of test equipment capacity. The most realistic parameters
regarding seismic resistance can be obtained by testing actual building walls in-situ with
original materials, geometry and also with usual geometry imperfections. Because such in-situ
tests are destructive by their nature and also complicated to perform, they are often replaced
with tests on specimens built and tested in laboratory conditions.
2.1
Specimens
2.1.1 In-situ tests
In-situ tests were performed on the typical old masonry building from early 30’s. Load
bearing masonry walls were made with solid clay bricks with dimensions 295× 140×65 mm
and weak lime mortar (L:S=1:5 and Dmax= 8 mm). For each thickness of walls we prepared
specimens by cutting them on 2.0 m high and 1.0 m wide pieces. As horizontal load was
applied in the middle of such wall we were actually testing two walls with symmetrically
fixed ends (ratio h/l=1.0).
Walls were of two thicknesses: 30 cm and 45 cm and for each thickness we prepared one
unreinforced specimen, one strengthened with diagonal stripes and one with horizontal and
vertical stripes. As load set-up allow only one direction of applying horizontal force the
diagonal stripes were glued only on the ‘tensile’ diagonal of the wall (Figure 1).
Surface of the wall in area designated for gluing was prepared with removing plaster and
grinding the loose parts. Unevenness of the surface was corrected with epoxy based mortar in
thickness up to 5 mm. After that the wet lay-up technique was used to apply CFRP to the wall.
Page 2 of 8
Figure 1. Configuration of CFRP reinforcement and test set-up for in-situ tests
(of unreinforced wall; from left: strengthened diagonally, horizontally and unreinforced)
2.1.2 Laboratory tests
Sixteen walls (height/width/thickness=126/106/12 cm) have been made on reinforced
concrete base footing. They were made of contemporary solid clay brick (250×120×65 mm).
Mortar used to build wall specimens was a mixture of cement, lime and sand (Dmax = 4 mm)
in a volume ratio of 1:2:6. All specimens were cured for at least 1 year before test.
(A) Diagonally
(B) Horizontally and vertically
(C) Horizontally
Figure 2. Configuration of CFRP reinforcement for laboratory tests
Four walls were left unreinforced. Others were strengthened with unidirectional carbon fibres
in different configurations; 6 diagonally, 3 horizontally and 3 horizontally with wide strips on
the sides. Strips were applied on front and back side. Wet lay-up technique was used to apply
CFRP to the wall. The panels have been cleaned with abrasion before epoxy primer was
applied. Epoxy adhesive, combined with filler was applied to bond the CFRP on the surface
of the wall. Finally the top coat of epoxy adhesive was applied to ensure saturation of the
fibres.
2.2
Test Setup
Due to the capacity of testing equipment there were some differences in test setup and
execution of the in-situ and laboratory tests. For in-situ tests the horizontal load of hydraulic
jack (1000 kN) was applied at the middle of wall height (Figure 1) separating wall into upper
and bottom ‘specimen’ of the wall. The specimens were thus tested as elements with
symmetrically fixed ends into the surrounding masonry (Figure 4). Walls were additionally
loaded with vertical force to reach stress level at 30% of compressive masonry strength.
In the laboratory the walls were tested as the shear cantilevers in the test frame as shown
below (Figure 3). Free end was at the bottom of the test frame, where vertical and also
Page 3 of 8
horizontal load is transmitted into the panel. A combination of vertical compression (400 kN
which was about 25 % of compressive strength) and in-plane shear load was applied to
specimen.
Figure 3. Masonry wall during testing
(wall is inserted up-side-down)
Figure 4. In-situ configuration
Principle of scales held vertical load constant during the experiment. The shear load was
applied to the wall by a horizontal hydraulic jack, which was displacement controlled by
computer software.
2.3
Loading sequence
40
30
20
10
0
-10
-20
-30
-40
displacement [mm]
displacement [mm]
The horizontal load during both types of experiments was displacement controlled with some
differences. Horizontal loading during laboratory testing increased in steps 0.5 mm, 1.0, 2.0,
4.0 mm, etc with each loading step repeated cyclic three times with the same amplitude and
velocity (Figure 5). Shear loading was provided by two way acting servo-hydraulic actuator
of 250 kN capacity. Actuator was fixed to the supporting frame, which transferred the shear
load to the RC plate of the laboratory.
0
10
20
step
30
40
Figure 5. Loading protocol in lab tests
40
30
20
10
0
-10
-20
-30
-40
0
10
20
step
30
40
Figure 6. Loading protocol for in-situ tests
Loading during in-situ tests was progressing with one repetition to the step (to 0.5 mm, 1.0,
1.5, 2.0, 3.0 mm etc) and release near zero (Figure 6). Loading was stopped when lateral force
in the current step could not reach 80% of maximum force achieved in the test.
2.4
Instrumentation
In both cases we measured horizontal displacements and deformations with linear variable
differential transducers (LVDTs). Masonry deformations were also measured using LVDTs
attached diagonally and vertically along the height of the wall. Load cells were used to
measure vertical (pre-stress) and horizontal load.
2.5
Materials
Basic materials have been tested to determine compressive strength of brick, mortar, tensile
strength of FRP fabric as well as some other characteristics listed in the table below. The
differences between characteristics of in-situ and laboratory masonry were dramatically apart.
Contemporary materials and careful mason work (especially use of better mortar) when
building laboratory specimens ended in 15-times higher compressive strength and 9-times
higher elastic modulus than in case of old masonry tested in-situ.
Page 4 of 8
Table 1. Mechanical characteristics of materials (laboratory tests).
MATERIAL PROPERTY
Brick
Compressive strength
Mortar
CFRP
IN-SITU
32 MPa
Tensile strength
5.34 MPa
Compressive strength
6.77 MPa
Flexural strength
2.08 MPa
Tensile strength
3400 MPa
Young's modulus
Masonry
LAB
Compressive strength
Young's modulus
ν
Gc
230 GPa
12.40 MPa
METHOD
20.1 MPa EN 772-1
n/a
~0.5 MPa EN 1015-11
n/a
3800 MPa ASTM D 3039/D 3039M
240 GPa
0.83 MPa EN 1052-1
5.74 GPa
0.64 GPa
0.12
0.49
2.29 GPa
0.21 GPa
For strengthening laboratory specimens the 10 mm and 50 mm stripes were used together
with appropriate epoxy resin. In case of the in-situ tests the stripes were 100 mm wide.
3. Results of masonry shear tests
3.1
Failure mechanism
Most of the tested masonry walls failed by propagation of diagonal cracks. In the laboratory
the failure mode started as a combination of shear and flexural mode. First cracks occurred in
corners of the panel due to the rocking of the wall. Behaviour of strengthened panels was
strongly related to configuration of CFRP reinforcement. The predominant mode of failure in
type (B) and (C) strengthening was flexural mode, which resulted in local failure of the wall
toe. Shear cracks started to develop at approx. 70 % of maximum displacement.
Crushing of brick inside FRP confinement
Detachment of diagonal or due to compression
Figure 7. Failure modes of FRP reinforced masonry (left two in lab, right two in-situ)
Cracks propagation was efficiently obstructed by the CFRP reinforcement, which resulted in
appearance of many new minor cracks. Diagonally strengthened walls showed flexural and
diagonal crack development. Toe crushing was the main cause of failure for this type of
strengthening.
For in-situ tests first diagonal cracks occurred at 70-80% of max load (or about 2-3 mm of
horizontal displacement). After that cracks propagated and became wider until the end of the
test. All walls failed in shear and the tests were stopped after compression failure of masonry
Page 5 of 8
within the FRP confinement. FRP stripes in diagonal configuration first detached on uneven
parts of the surface then progressed to the anchored end. The system of vertical and horizontal
stripes was effective until the end of loading thou there were local detachments from the
surface and even local rupture of stripes. The detachment of FRP stripes occurred mostly in
the brick and not in glue or their bond.
3.2
Shear strength of masonry
Typical response of combined compression-shear tests performed on section of wall is
presented in the form of rotation vs. horizontal load on the diagrams bellow (Figure 8 and
Figure 9). Results are shown for horizontally and vertically reinforced walls. Envelopes of
such results had been collected for ‘positive’ and ‘negative’ direction of forced displacement
in the case of laboratory tests or for lower and upper part of wall in the case of in-situ tests.
An example of such envelopes is represented below (Figure 10) where crack patterns at
different stages of loading are also shown.
150
Z03
70
60
horizontal load [kN]
100
50
horizontal load [kN]
H30
0
-50
50
40
30
20
10
-100
0
-150
-15
-10
-5
0
5
rotation [mm/m]
10
0
15
5
10
15
20
25
30
rotation [mm/m]
Figure 8. Results for lab tests
Figure 9. Results for in-situ tests
Figure 10. Envelope of rotation-force relationship for H system on 30 cm walls
with corresponding development of cracks pattern.
Page 6 of 8
35
40
Shape of diagrams from the laboratory testing (Figure 8) are mostly indicating ‘rocking’
behaviour at first half of test followed with hysteresis opening and failure on one side of
loading direction. The direction of first loading cycle was not affecting later damage
development or failure mode.
In-situ walls indicated constant development of cracks with highly non-elastic deformations
(Figure 9). The crack patterns and damage propagation is similar for in-situ and laboratory
tests. We compared cracks patterns with typical points on the envelope curve (sample is
shown for test of H30 on Figure 10). To compare results obtained on walls of different
dimensions (laboratory and in-situ) we calculated stress as horizontal load divided by
horizontal cross section area. Below are results shown for all laboratory specimens (Figure 11,
two envelopes: positive and negative for each specimen) and in-situ tests (Figure 12, two
envelopes for each specimen: upper and lower part of wall).
Figure 11. Results of laboratory shear tests
Figure 12. Results of in-situ shear tests on masonry
Specimens prepared and tested in the laboratory yielded much higher load bearing capacity
(from 0,7 to 1,1 MPa) compared to old masonry on the site (from 0,1 to 0,2 MPa). That was
due to better materials used and careful mason work. From results we can also conclude that
reinforcement with the CFRP glued in diagonal direction of walls is not effective solution.
The best results were gained with CFRP fabric glued in vertical and horizontal direction
(green curves: z08, z13, z15, H30 and H45 on diagrams above). For laboratory or in-situ tests
the effect of horizontal stripes confining the wall was decisive for higher load bearing
capacity and especially for higher ultimate rotation capacity.
For study of FRP effectiveness the average values of three walls for each strengthening
configuration (H lab, H+V lab, D lab) was compared to average values of 3 unreinforced
walls in case of laboratory tests. Average of two walls of each strengthening configuration (H
in-situ, D in-situ) was compared with average of 2 unreinforced walls for in-situ comparison.
The biggest increase of shear strength (approx. 150%) was attained by horizontal
strengthening of (weak) walls on site (Figure 13). The effect of horizontal strengthening on
laboratory walls was modest 120%. Diagonal configuration did contribute just a little to shear
strength (~5%) and about 110% to ultimate rotation. The highest increase (380%) of ultimate
rotation was attained with applying horizontal (and vertical) CFRP stripes of fabric on
masonry on site and it was also high (around 200%) for configurations H and H+V tested in
the laboratory.
4. Conclusions
To improve seismic resistance of brick masonry walls the method with application of
reinforcement with carbon fibre reinforced polymer (CFRP) fabric is presented. To develop
the most efficient way of applying CFRP fabric to brick masonry walls, in-situ and laboratory
Page 7 of 8
tests were carried out and their results are compared. Results show different effectiveness
depending on the configuration of reinforcement. CFRP strengthening favourably influenced
Figure 13. Effectiveness of CFRP strengthening variants
the mechanism of wall behaviour in all cases. The biggest gain was the increase of ultimate
displacement (or rotation) especially for system with vertical and horizontal stripes. Stripes in
diagonal configuration did not perform so well. The primary mode of failure of diagonally
strengthened wall tested in the lab was attributed to the exceedence of the compressive
strength at the wall toes (due to rocking), which resulted in a localized compression failure of
masonry. In-situ tests for diagonal configuration were governed by peeling of the stripes from
the masonry.
The difference in boundary conditions between in-situ and laboratory test set-ups proved not
to be so significant. The quality of basic masonry material had the significant impact on
behaviour of reinforced specimens. Better results of strengthening effectiveness were gained
on initially weaker (in-situ) masonry. Best results were attained for walls strengthened with
vertical and horizontal stripes. The increase of shear strength for lab specimens was 120% and
150% for in-situ compared to reference unreinforced specimens.
5. Acknowledgements
The results of in-situ tests have been achieved in the project PERPETUATE
(www.perpetuate.eu), funded by the European Commission in the Seventh Framework
Programme (FP7/2007-2013), under grant agreement n° 244229
The results of laboratory tests were funded by the Ministry of higher education, research and
technology of Republic of Slovenia under grant no. Z2-3411.
6. References
[1]
[2]
[3]
Croci, G., D'Ayala, D., D'Asdia, P., Palombini, F., 1987, "Analysis on shear walls
reinforced with Fibers.", IABSE Symp. On Safety and Quality Assurance of Civ. Engrg.
Struct., Int. Assoc. For Bridge and Struct., Lisbon, Portugal
Triantafillou, Thanasis C., 1998, "Strengthening of masonry structures using epoxybonded FRP laminates", Journal of Composites for Construction (2) May, 96-104,
ASCE
Valluzi M.R., Tinazzi D., Modena C., 2002, "Shear behavior of masonry panels
strengthened by FRP laminates", Construction and Building materials, 16, 409-416
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