15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
IN-SITU TESTING OF BRICK MASONRY WALLS STRENGTHENED
WITH CFRP FABRIC
Gostič, Samo1; Bosiljkov, Vlatko2; Jarc Simonič, Mojca3
1
PhD, Building and Civil Engineering Institute ZRMK Ljubljana, Slovenia, [email protected]
2
PhD, Assistant Professor, University of Ljubljana, Faculty of Civ. and Geodetic Engineering, [email protected]
3
Building and Civil Engineering Institute ZRMK Ljubljana, Slovenia, [email protected]
New requirements for strengthening buildings of cultural heritage assets, apart from its
efficiency demands also reversibility of proposed methods. In this regard, one of the most
promising methods is application of carbon reinforced polymers (CFRP) fabric to the surface
of the wall. Within the framework of European FP7 research project PERPETUATE new
computation models for masonry and strengthening techniques will be developed. To support
validation of models various test on masonry specimens will be performed.
In this article experimental results of in-situ shear tests of strengthened clay brick masonry
walls with CFRP fabric will be presented. In load bearing walls with different thickness of 30
cm and 45 cm respectively, positioned within the building dated from around 1935 built with
solid bricks in low strength lime-cement mortar, cuts were made to isolate six 100 cm wide
and 200 cm high specimens. For the purpose of this study, two configurations of positions of
CFRP stripes were compared with unstrengthened specimens: walls with strips of fabric
placed on masonry surface in two diagonal directions and walls with strips placed in several
horizontal levels providing the confinement effect to masonry brick rows. Specimens were
tested by horizontal cyclic loading under the constant vertical load.
Following experimental tests, different failure mechanisms were observed and the
contribution of applied CFRP strips to load bearing capacity and ductility has been found to
be different depending from failure mechanisms. The advantage of the horizontally applied
confinement demonstrated significant increase of both ductility and energy dissipation. The
most important conclusion is that the new innovative strengthening approach favourably
influence the behaviour of slender wall and that in the design of retrofitted clay masonry
buildings the calculation models must check all possible failure mechanisms and not only the
shear mechanism.
Keywords: masonry, FRP, strengthening, in-situ test, shear strength, ductility
Theme: Research and testing
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
INTRODUCTION
Many of masonry buildings are classified as the most valuable architectural monuments of
cultural heritage as standalone buildings or as city aggregates. Unfortunately unreinforced
brick masonry (URM) showed low seismic resilience during past earthquakes. Main problems
are poor connection of load bearing walls and low shear strength of masonry walls. Several
conventional strengthening methods to overcome these problems were developed in the past.
The most common are: changing of weak mortar in joints, jacketing of URM walls with
reinforced concrete and binding the walls with steel ties. Each of these mentioned methods
has its own advantages and disadvantages. They are all disruptive to residents, realization
takes a lot of time, and some of the methods significantly change seismic characteristics of
building. In the case of cultural heritage buildings this methods are not suitable because of the
appearance alteration as well as the need to retain the original historic material. Strengthening
methods using new materials (Fibre Reinforced Polymers) promise to overcome those
problems. After application FRPs can be removed from the original walls if so later required
by the heritage conservation.
One of the first studies of effect of strengthening masonry wall by fibres was done by Croci
et.al. (1987). Triantafillou's researches include wide spectra of strengthening with FRPs. The
experimental work focused on masonry, led to proposal of equations for masonry
strengthened with FRP. Valluzzi (2002) performed series of diagonal tests on differently
reinforced walls. Double sided, diagonal strengthening with GFRP showed to be the most
efficient method.
In the present experimental work the in-plane shear tests were performed in-situ on masonry
walls. Investigation was focused on diagonal and horizontal strengthening with CFRP fabric
stripes.
EXPERIMENTAL CAMPAIGN
In-situ tests were performed on the (typical) old masonry building from 1930-ies. Load
bearing masonry walls were made with solid clay bricks (295 × 140 × 65 mm) and weak lime
mortar with coarse sand (Dmax= 8 mm). The same materials are common for cultural heritage
buildings from that period. Walls were of two thicknesses: 30 cm and 45 cm and on each
thickness three specimens were prepared: one unreinforced specimen, one strengthened with
diagonal stripes and one with combined horizontal and vertical stripes. Specimens were
prepared by cutting walls on 2.0 m high and 1.0 m wide pieces with wire saw to ensure low,
undamaging vibrations and smooth sides. As load set-up enabled 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 area designated for gluing was prepared by removing plaster and grinding
the loose parts. The edges were rounded on appropriate places to avoid CFRP fibbers bend
cracking. Unevenness of the surface was corrected with epoxy based mortar (BE-POX CL/21)
in thickness up to 5 mm. After that the wet lay-up technique was used to apply CFRP to the
wall. Sheet C-240 (from S&P) cut on 10 cm wide stripes with weight of 300 g/m2 were used.
They were bonded with S&P epoxy resin 55 on both sides of the masonry. The strengthening
was carried out by the company GRAS from Ljubljana.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
Figure 1: Three configurations (D, un-strengthened and H) were prepared for testing
Basic materials have been tested to determine compressive strength of brick, mortar, tensile
strength of FRP fabric as well as some other characteristics. Samples of bricks were randomly
taken from different locations and tested. Compression strength was 20,1 MPa with quite high
standard deviation (11,72 MPa). CFRP has E modulus 240 GPA with tensile strength
3800 MPa.
COMPRESSION TESTS
To determine the compressive strength and modulus of elasticity tests on two brick wall
samples (T30 and T45) were performed. Dimensions of the first were (width/height/thickness)
100 x 100 x 30 cm and the other 100 x 85 x 45 cm. The result of the investigations gave the
compressive strength of the wall which had to be determined also to set the vertical pre-stress
of shear tested walls. In-situ compression tests were carried out with a hydraulic jack of
1300 kN capacity in a range up to 335 kN (Figure 2).
Preparations of the samples were similar to those for shear tests - samples were cut -isolated
from existing walls with wire saw, cleared of a plaster and evened on the top surface with
cement mortar. With the system of steel ties and displacement controlled hydraulic jack the
samples were tested in-place. Vertical and horizontal deformations were measured with
LVDTs mounted on both sides of the wall (Figure 3).
Sample T30 was tested with monotonically increasing load up to failure. Sample T45 was
released at 45% of the maximum load and then loaded again up to failure. Mechanical
characteristics gained in the compression tests are presented in Table 1.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
Table 1 Results of compression tests
T30
T45
average
fmc
MPa
0,92
0,73
0,83
Ew
MPa
519,92
754,54
637,23
0,65
0,50
0,58
157,74
250,93
204,34
0,30
0,33
0,32
w
Gc
MPa
Gc/Ew
T 30
300
250
load [kN]
200
150
100
50
0
-4
-2
0
-50
2
4
6
8
10
12
def. [‰]
T 45
350
load [kN]
300
250
200
150
100
50
0
0
Figure 2: Compression test set up
1
def. [‰]
2
3
Figure 3: Diagrams of vertical
deformations for T30 and T45
SHEAR TESTS
Six 2 m high and 1 m wide specimens with thicknesses 30 cm and 45 cm were cut out from
load bearing walls for shear tests. Un-strengthened specimens for comparison were labelled
O30 and O45. Diagonally strengthened specimens were D30 and D45 while H30 and H45 had
vertical and horizontal reinforcement. At the middle of wall height (Figure 4) the horizontal
load of hydraulic jack was applied separating wall into upper and bottom ‘specimen’ of the
wall each with ratio h/l = 1.0. The specimens were thus tested as elements with symmetrically
fixed ends into the surrounding masonry. Walls were additionally loaded with vertical force
(with second hydraulic jack) to reach stress level at 30% of compressive masonry strength to
simulate load of two more stories above tested walls.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
Figure 4: Shear test set-up
Figure 5: Shear test
instrumentation set-up
displacement [mm]
Displacements and deformations were measured with linear variable differential transducers
(LVDTs, Figure 5). Vertical pre-stress load (V) and horizontal load (H) were measured with
load cells. Loading during tests was displacement controlled and it was progressing in steps 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 previously achieved.
40
30
20
10
0
-10
-20
-30
-40
0
10
20
step
30
40
Figure 6: Loading protocol
EXPERIMENTAL RESULTS
Un-reinforced specimens O30 and O45 started to show first cracks at 70..80% of max load (or
about 2..3 mm of horizontal displacement). They failed by propagation of diagonal cracks to
width of 12 mm (Figure 7) after reaching maximum load (34 kN for O30 and 61 kN for O45)
and continuing till ultimate displacement of 10 mm. On the Figure 8 the rotation versus
horizontal load with its envelope is shown together with the pattern of cracks at the end of
test.
Cracks propagation was efficiently obstructed by the CFRP strips, which resulted in
formation of many minor cracks for specimens D. First diagonal cracks occurred at 60% of
max load (or about 5..8 mm of horizontal displacement). FRP stripes in diagonal
configuration detached on uneven parts of surface where the weak part for detachment was in
the brick and not in the glue (Figure 9). Load reached during D30 test was even lower (30 kN)
than for un-reinforced while D45 reached 73 kN (Figure 10).
The system of vertical and horizontal stripes was the most effective. Failure mechanism
showed compressive failure within the FRP confinement and shear cracks after reaching load
59 kN and 84 kN (H30 and H45) with ultimate displacements of 34 mm and 45 mm
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
respectively. There were local detachments from the surface and even local rupture of stripes
but the specimens finally fail by masonry crushing in compression zones (Figure 11).
O30
40
35
horizontal load [kN]
30
25
20
15
10
5
0
0
5
10
15
rotation [mm/m]
Figure 7: Diagonal cracks on
unreinforced specimen
Figure 8: Envelope of rotation vs. load
and the pattern of the cracks
D 45
80
horizontal load [kN]
70
60
50
40
30
20
10
0
0
Figure 9: Failure of FRP
stripes (detachment)
5
10
rotation [mm/m]
15
Figure 10: Envelope of rotation vs. load
and the pattern of the cracks
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
H30
70
horizontal load [kN]
60
50
40
30
20
10
0
0
10
20
30
rotation [mm/ m]
Figure 11: Local detachment of FRP
stripes from the surface
Figure 12: Envelope of rotation vs. load
and the pattern of the cracks
To compare results obtained on walls of different dimensions we calculated stress as
horizontal load divided by horizontal cross section area (Figure 13, for upper-z and lower-s
part of wall separately). The best results both in terms of strength and ductility were gained
with H configuration of vertical and horizontal stripes (green lines).
0.2
o30_s
D30_s
H30_s
0.15
o45_s
stress [MPa]
D45_s
H45_s
0.1
o30_z
D30_z
H30_z
0.05
o45_z
D45_z
0
H45_z
0
10
20
30
rotation [mm/m]
40
50
Figure 13: Hysteresis envelopes for all shear tested walls
For study of effectiveness the average values of strengthening configuration was compared to
average values of URM walls. The biggest increase of shear strength and ultimate
displacement was achieved by configuration H (Figure 14).
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
Figure 14: Effectiveness of CFRP strengthening configurations D and H
CONCLUSION
The application of CFRP stripes for in-plane strengthening of masonry have been tested insitu. Tests were performed on walls of two thicknesses under constant vertical load and with
half-cyclic displacement controlled horizontal load. With the method we obtained good
results for strength and ductility and due to reversibility of application it is suitable for
strengthening the cultural heritage buildings. Two different configurations of CFRP stripes
have been tested and compared with un-reinforced specimen. Best results were attained for
walls strengthened with vertical and horizontal stripes (config.H). The average increase of
shear strength over un-reinforced specimen was 150% with 380% increase of ultimate
displacement. Stripes in diagonal configuration did not perform so well because the failure
mechanism was governed by detachment of FRP from the masonry surface. The un-reinforced
masonry failed in diagonal shear while the horizontally and vertically reinforced specimens
failed by masonry compressive failure within the FRP confinement combined with diagonal
shear cracks.
ACKNOWLEDGEMENTS
The results 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.
REFERENCES
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 epoxy-bonded
FRP laminates", Journal of Composites for Construction 2 (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