Proceedings of the 1 st Iberic Conference on Theoretical and Experimental Mechanics and Materials / 11 th National Congress on Experimental Mechanics. Porto/Portugal 4-7 November 2018. Ed. J.F. Silva Gomes. INEGI/FEUP (2018); ISBN: 978-989-20-8771-9; pp. 781-796. -781- PAPER REF: 7395 CHARACTERIZATION OF MATERIALS AND MASONRY ASSEMBLAGES FOR SEISMIC RESISTANT MASONRY INFILLS Luís M. Silva (*) , Graça Vasconcelos, Paulo B. Lourenço ISISE, Departamento de Engenharia Civil, Universidade do Minho, 4800-058 Azurém, Guimarães, Portugal (*) Email: [email protected]ABSTRACT This paper, presents the experimental work carried out at University of Minho to characterize the mechanical properties of the materials and masonry assemblages used in two new seismic resistant masonry infill systems. This information about mechanical properties of materials and masonry assemblages is important to understand the experimental results and for further numerical simulations and analytical analysis of masonry infill systems. Besides that, this characterization is important due to the novelty of these two systems, because there is no literature on the mechanical properties of same components used in these new solutions. These new systems are to be used in new construction in Portugal. On first part of paper are presented the solutions and it is presented and discussed the mechanical properties of masonry materials, masonry units and mortar. The mechanical properties include the compressive and flexural strength of mortar and compressive strength of units to different directions of loading. After that starts the presentation and discussion of mechanical properties in masonry assemblages, including the compressive strength in the direction normal to the bed joints based on uniaxial compression tests; tensile and shear strength based on diagonal compression test; flexural strength in the direction parallel and perpendicular to the bed joints based on flexural tests; and shear properties of interfaces through initial shear tests. All mechanical properties are obtained based on standard procedures following European standards. Keywords: Masonry infill, experimental characterization, compression, diagonal shear, flexural tests, initial shear. INTRODUCTION Masonry infill walls are a common solution for enclosures walls in many countries in southern Europe, mainly in Reinforced Concrete (RC) buildings, leading to the need of production of thousands of masonry units annually [1]. In the last 60 years, several studies [2], [3] have been conducted to understand the influence of the masonry infill walls in RC structure, and the seismic behaviour of these infill walls. According to several authors, the masonry infill walls contribute significantly to the performance of buildings in terms of interior quality, and have a positive influence in the lateral resistance, in the stiffness and in the energy dissipation in RC frames [4]. However, there are still problems in the behaviour of these walls especially when they are subjected to seismic action [5], because although they are considered non-structural, they contribute to withstand the seismic action. The recent earthquakes in Lefkada in 2003 [6], L’Aquila in 2009 [7], Van in 2011 and Emilia Romagna in 2012 [8], among others, showed that masonry infills walls can affect the global
Transcript
Proceedings of the 1st Iberic Conference on Theoretical and Experimental Mechanics and Materials /
11th National Congress on Experimental Mechanics. Porto/Portugal 4-7 November 2018.
Ed. J.F. Silva Gomes. INEGI/FEUP (2018); ISBN: 978-989-20-8771-9; pp. 781-796.
-781-
PAPER REF: 7395
CHARACTERIZATION OF MATERIALS AND MASONRY
ASSEMBLAGES FOR SEISMIC RESISTANT MASONRY INFILLS
Luís M. Silva(*)
, Graça Vasconcelos, Paulo B. Lourenço
ISISE, Departamento de Engenharia Civil, Universidade do Minho, 4800-058 Azurém, Guimarães, Portugal (*)
Masonry infill walls are a common solution for enclosures walls in many countries in
southern Europe, mainly in Reinforced Concrete (RC) buildings, leading to the need of
production of thousands of masonry units annually [1]. In the last 60 years, several studies
[2], [3] have been conducted to understand the influence of the masonry infill walls in RC
structure, and the seismic behaviour of these infill walls. According to several authors, the
masonry infill walls contribute significantly to the performance of buildings in terms of
interior quality, and have a positive influence in the lateral resistance, in the stiffness and in
the energy dissipation in RC frames [4]. However, there are still problems in the behaviour of
these walls especially when they are subjected to seismic action [5], because although they are
considered non-structural, they contribute to withstand the seismic action.
The recent earthquakes in Lefkada in 2003 [6], L’Aquila in 2009 [7], Van in 2011 and Emilia Romagna in 2012 [8], among others, showed that masonry infills walls can affect the global
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and local behaviour of the RC structures. These earthquakes allowed to observe that contemporary structures in reinforced concrete have a reasonable ability to withstand seismic loads, given that were designed for this purpose according to the current design codes [9]. Sometimes it is possible to observe some kind of damage like soft-story or short column associated with the improper use of masonry infill walls. In case of masonry infill walls, since they are considered non-structural walls, usually it is not made any safety check to the seismic actions. Eurocode 8 [9], is silent in this case and only present a simplified procedure for the calculation of the out-of-plane action, but does not provide design recommendations. On the other hand, this code considers that verification of the safety of non-structural elements is guaranteed if the relative displacements between floors are limited. However, states that appropriate measures should be taken to avoid brittle failure and premature disintegration of infill walls. Seismic events cited above allowed to observe significant damage occurring in this type of walls. The most common pathologies are the separation between masonry panels and structural elements, diagonal cracking, and out-of-plane partial or total collapse. This type of damage can put in danger human life and is also associated with considerable economic losses [10], [11], as happened in the Loma Prieta earthquake in 1989 and Northridge in 1994, where the costs associated with non-structural damage amounted to 30 million US dollars [12].
Despite masonry infills walls being a widely used construction solution in Portugal, the construction systems for this type of walls remains the same for many years, apart from same minor changes. In Portugal the masonry infill walls, had its great advent in the 60’s, with the massification of reinforced concrete structures. Since then until our days the masonry infills walls do not change, apart from same changes in thickness and introduction of thermal isolation between the leafs. The construction system remains based in cavity walls without any connection between leafs, constructed with horizontal perforated bricks and poor mortars. Masonry units have always been ceramic clay bricks, with horizontal perforation and high percent of voids with weak mechanical properties. In Figure 1, it is possible to see the standardized masonry units used in Portugal. However, their vulnerability under seismic actions is recognized by scientific community and proven whenever an earthquake occurs. Nevertheless, they continue to be used, because until now does not exist any seismic resistant system developed to be used in new construction, with clear design procedures and construction guidelines that contribute to reduction of seismic vulnerability.
Fig. 1 - Horizontal perforated masonry units used in Portugal.
To address the existing problems with masonry infill walls, it is necessary propose and study new systems to use in new construction that can withstand seismic action and propose design guidelines that can be used by structural designers.
To contribute to a change in this situation, two new seismic resistant masonry infill walls systems are present in this paper. These systems are to be used in new construction in Portugal. Besides that, an experimental characterization of materials and masonry assemblages is presented, and results are discussed in detail, since there is no literature on the mechanical behaviour of some of the materials that are used in these new systems.
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BRIEF DESCRIPTION OF PROPOSED SOLUTIONS
During the development of the new systems, three main types of solutions have been
identified for the development of innovative enclosure masonry systems. The main goal is to
solve the above-mentioned problems arising under the point of view of seismic behaviour.
Notwithstanding, problems and aspects also related to the service behaviour and to non-
mechanical behaviour of the infill walls, are tackled.
The main concepts on which the three systems rely are: (1) keeping the enclosure wall rigidly
attached (adherent) to the frame, but using either or both robust units and internal (mainly
steel rebars, as in reinforced load bearing masonry) or external (mainly reinforced plasters)
reinforcements; (2) keeping the enclosure wall rigidly attached (adherent) to the frame, or
slightly disconnected, but allowing the internal deformation of the wall to occur, by means of
special devices, special units, or special sliding or deformable vertical or horizontal joints; or
(3) disconnecting the enclosure system from the top beam and/or from the columns, in order
to allow relative displacements between the wall and the frame to occur without interactions.
It is also possible create hybrid systems using more than one of the solution described above.
At University of Minho were created two systems called UMSystem1, and UMSystem2. The
first system was the UMSystem1 also called Uniko System. This system is composed of a
single-leaf masonry wall, with 100mm thickness, made with a vertical perforated clay
masonry unit. This unit use interlocking along the head joint. It was decided to place the
masonry units aligned in the vertical direction creating a continuous vertical interlocking joint
(see Figure 2(a)), to take advantage of sliding between masonry units, improving the energy
dissipation. The idea of this system is to be capable of release energy, by the sliding of
masonry units along vertical joints. In this case the infill can withstand inter-storey drift
without damage, when traditional infills are already damaged. The out-of-plane behaviour
was improved by adding steel rebars in the face of masonry units, being connected at top and
bottom reinforced concrete beams. Therefore, the masonry infill has dry vertical joints and
mortared bed joints, for which a general-purpose M10 mortar is used.
(a) (b)
Fig. 2 - Masonry infill systems developed at University of Minho: (a) UMSystem1; (b) UMSystem2.
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The UMSystem 2 proposed at university of Minho, also called Térmico system, use the
concept of maintain the infill rigidly attached to the frame, using internal reinforcement and
connectors between the infill and frame. This system, is a single-leaf clay masonry wall made
with a commercial vertical perforated masonry unit produced in Portugal. The proposed
system uses a M10 mortar in the bed joints, and dry head joint with interlocking. To improve
the in-plane and out-of-plane performance of masonry infill walls, truss reinforcements was
used in the bed joints. Additionally, the walls are connected to the columns by metallic
connectors at each two rows where bed joint reinforcement is applied (see Figure 2(b)). The
masonry infill panel was built with 294x187x140mm bricks with vertical perforation, using
murfor RND 0.5 100 reinforcement and in each two rows, and murfor L +100 anchors to
connect the infill and RC frame at the same levels of reinforcements.
The idea of UMSystem2 is making the infill and the frame one system, increasing the initial
stiffness by using connectors and reinforcement, which not only helps to increase the
maximum load, as to control cracking and the out-of-plane collapse.
EXPERIMENTAL CHARACTERIZATION OF MATERIALS
Masonry Units
To produce the UMSystem1 and UMSystem2, two different units were used. For UMSystem1
was used a unit produced in Portugal, called Uniko (see Figure 3(a)). In the case of
UMSystem2 was used a unit called Térmico (see Figure 3(b)), that have better thermal
properties and beginning to be used in Portugal.
For characterization of masonry units, the dimensions of the units were measured based on
EN772-16:2000 [13] by taking two measurements near the edges of each specimen. The
information about the measurements of the dimensions of the units is summarized in Table 1.
After measurements, compression tests were performed according to EN 772-1:2011 [14]. For
Uniko unit, twelve units were tested, six for direction 1 (parallel to the holes) and six for
direction 2 (perpendicular to the holes), according Figure 3(a). For Térmico unit, eighteen
units were tested, six in each direction according to scheme of Figure 3(b).
To evaluate the flexural strength of the masonry walls, 18 tests were performed, according to EN1052-2: 1999 [17], 9 tests for UMSystem1, and 9 tests for UMSystem2. For each system, were built specimens for flexion parallel to bed joint and specimens for flexion perpendicular to bed joint. All specimens only use mortar in the bed joints, for head joints both system have interlocking joints. For UMSystem1 the specimens for flexure parallel to the horizontal joints were constructed with the dimensions of 1300x750mm, and the configuration is presented in Figure 8(a), and have 3 specimens without reinforcement and 3 specimens with reinforcement in the lateral groves of units, in the case of bending perpendicular to the horizontal joints, the test specimens have the dimension of 1000x750mm as shown in Figure 8(b). The specimens were air cured, for more than 28 days.
(a)
(b)
Fig. 8 - Specimens used for flexural tests of UMSystem 1. (a) parallel to bed joints; (b)
perpendicular to bed joints.
For UMSystem2, the configuration of specimens, for flexural tests parallel to the bed joint is
presented in Figure 9(a). The dimensions of these specimens are 1000x600mm. In case of
flexural tests perpendicular to bed joints, the configuration of specimens, is presented in
Figure 9(b). The dimensions of these specimens are 1200x800mm. In this case 3 specimens
were constructed without reinforcement and 3 specimens were constructed with reinforcement
in the first and third row.
(a)
(b)
Fig. 9 - Specimens used for flexural tests of UMSystem 2. (a) parallel to bed joints; (b)
perpendicular to bed joints.
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The tests were performed in displacement control with a velocity of 0.01 mm/s. The test setup
shown in Figure 10, is composed of a reaction structure where the specimen to be tested is
placed. This structure allows the adjustment of the supports to fit the different sizes of the test
pieces. The hydraulic actuator used in the test is anchored in the reaction wall, and at the end
of the actuator, a metal beam is placed with the supports that apply the load to the specimen.
Four lvdt’s were used to control de displacement of the specimen, two under the loading
sections (lvdt1 and 3) and two on the middle span (lvdt 2 and lvdt4) of the specimen (one on
each side). The force is measured using a loading cell at the end of hydraulic actuator.
Fig. 10 - Test setup for flexural tests.
The force displacement curves of the flexural strength are presented in Figure 11, for
UMSystem 1, and in Figure 12 for UMSystem 2. The results obtained in all tests for both
systems are presented in Table 4. From the results is possible to see that for UMSystem 1, the
flexural strength parallel to bed joint is higher than flexural strength perpendicular to bed
joint, even without the reinforcement. The addiction of reinforcement in the direction parallel
to the bed joint increase a lot the value of flexural strength that change from 0.317MPa to
0,809MPa. In the other direction perpendicular to bed joint the flexural strength is 0.157MPa.
In this case the flexural strength is only guarantee by the interlocking mechanism of masonry
units.
(a)
(b)
Fig. 11 - Force displacement curves from flexural tests in UMSystem1. (a) parallel to bed
joint; (b) perpendicular to bed joint.
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For UMSystem 2, the value of flexural strength, perpendicular to bed joints is higher than for
direction parallel to bed joints. In this case the value of flexural strength parallel to bed joint is
0,227MPa, and in direction perpendicular to bed joint is 0,486MPa, for the specimens without
reinforcement and 1,674MPa, for specimens with reinforcement. In this case for direction
perpendicular to bed joints, the addiction of reinforcement increase more than 300% the value
of flexural strength.
(a)
(b)
Fig. 12 - Force displacement curves from flexural tests in UMSystem2. (a) perpendicular to
bed joint; (b) parallel to bed joint.
Table 4 - Mechanical proprieties of uniko system for flexural tests.
Specimen Dimensions (mm) Fmax.
(N)
fx
(N/mm2) l1 l2 b tu
UMSystem1
without
reinforcement
ll1
1200 600 750 100
2433 0,292
0,317 ll2 --- ---
ll3 2845 0,341
with
reinforcement
ll4 6457 0,775
0,809 ll5 7661 0,919
ll6 6105 0,733
with mortar
joints
T1
900 400 750 100
1662 0,166
0,157 T2 1433 0,143
T3 1602 0,160
UMSystem2
without
reinforcement
T1
1050 400 800 140
6267 0,390
0,486 T2 9989 0,621
T3 7194 0,447
with
reinforcement
T4 27473 1,708
1,674 T5 26715 1,661
T6 26554 1,651
with mortar
joints
II1
900 300 600 140
3237 0,248
0,227 II2 2964 0,227
II3 2712 0,208
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Diagonal Compression
The shear strength of the walls was measured according to the American Standard ASTM
E519 [18]. For this purpose, the walls were subjected to diagonal compression. In Figure 13,
the test setup and the instrumentation used in these tests are presented. The tests were
controlled in displacement. The load was applied through a hydraulic jack with maximum
capacity of 500kN. In case of instrumentation, the deformations on the diagonals of wallets
were measured by lvdt’s. Four transducers were used to measure deformations on the
diagonals of the walls (two lvdt’s in each face).
(a)
(b)
Fig. 13 - Test setup for diagonal compression. (a) UMSystem1; (b) UMSystem2.
A total of 12 specimens were constructed (6 for each system). For both systems 3 specimens
were constructed without reinforcement and 3 with reinforcements, in case of UMSystem1
the reinforcements are in lateral grooves of unit, and for UMSystem2, the reinforcements are
in first and third mortar joints. The dimensions of the specimens take in to account
dimensions of the masonry units are: length=750mm, height=750mm, thickness=100mm, for
UMSystem1, whereas the dimensions of UMSystem2 specimens are: length=830mm,
height=830mm, thickness=140mm. The walls are constructed following the normal
procedure.
The results of these tests are presented in Table 5. As shown in results the presence of
reinforcement result in a higher shear strength. In terms of rigidity modulus this difference
isn’t so clear. For UMSystem 1, the average value of shear strength is 0,272MPa for the
specimens without reinforcement and 0,426MPa, for the specimens with reinforcement. For
the UMSystem2 the values are similar, for the specimens without reinforcement the average
value of shear strength is 0,299MPa, and 0,417MPa for the specimens with reinforcement, the
rigidity modulus is almost equal in both cases around 0,65MPa.
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Table 5 - Mechanical proprieties obtained from diagonal compression tests.
Specimen Fmax.
(N)
εv
(‰)
εh
(‰)
Ss
(MPa)
γ
(‰)
G
(Mpa)
UMSystem1
with
reinforcement
1 27850 -0,835 0,565 0,493
0,426
1,400 0,352
0,399 2 21780 -0,614 0,272 0,386 0,886 0,436
3 22470 -0,674 0,300 0,398 0,974 0,409
without
reinforcement
4 19810 -0,420 0,154 0,351
0,272
0,574 0,612
0,294 5 9670 -2,126 1,895 0,171 4,021 0,043
6 16640 -0,770 0,517 0,295 1,287 0,229
UMSystem2
without
reinforcement
1 28050 -0,253 0,317 0,398
0,299
0,570 0,698
0,649 2 19030 -0,237 0,153 0,270 0,390 0,693
3 16170 -0,184 0,228 0,229 0,412 0,555
with
reinforcement
4 28030 -0,350 0,662 0,395
0,417
1,012 0,391
0,674 5 27360 -0,314 0,337 0,389 0,651 0,597
6 32700 -0,306 0,144 0,466 0,450 1,036
Initial shear along joints
To characterize the interfaces between masonry units, 18 specimens (9 each system) were
tested. The mechanical proprieties (friction angle and cohesion) of these interfaces were
determined using the test procedure of EN 1052-3:2002 [19]. In the case of the UMSystem1,
the vertical interface was characterized, to model the behavior of continuous vertical joints.
For the UMSystem2 was characterized the horizontal mortared joint. The tests were
performed on triplets as it is possible to see in Figure 14. The specimens were air cured more
than 28 days before de test. Test setup used for these tests is shown in Figure 14, and meet the
specifications of the standard.
(a)
(b)
Fig. 14 - Test setup for initial shear test, (a) UMSystem1, (b) UMSystem2.
All tests were made in displacement control, with a pre-defined velocity of 0.01mm/s in the
vertical actuator. The horizontal pre-compressive force was maintained constant during the
test. Three different pre-compressive forces were used, to make possible plot the shear
strength versus pre-compressive stress, as shown in Figure 15.
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(a)
(b)
Fig. 15 - Relation between shear strength and pre-compressive stress for: (a) UMSystem1; (b) UMSystem2.
From results, plotted in the diagrams of Figure 15, using linear regression, is possible to get
the value of cohesion and friction angle of this interface. For UMSystem1 the friction angle is
43,84 degrees, and the value of cohesion in 0,0047MPa. In the case of UMSystem2 the value
of friction angle is 20,09 degrees, and the cohesion have a value of 0,3888MPa.
CONCLUSIONS
This paper presents and discuss the results obtained from a material characterization of two
new masonry infill system, to be used in new construction in Portugal. From the analysis of
the results the following conclusions were made: the real thickness of the bricks is different
with their nominal thickness reported by the manufacturer, representing significant
differences in same cases. This is a result of poor quality control in production phase. From
compressive tests in both masonry units, was possible verify that these units present
anisotropic behavior at different directions.
The addiction of reinforcement in uniaxial compression tests, do not affect significantly the
value of compressive strength, but change the elastic modulus to higher values. In the case of
Flexural tests and diagonal compression the addiction of reinforcement contributes to higher
values of flexural strength and shear strength, that in the case of UMSystem2 was more than
the double. It is also concluded that the flexural strength of the specimens in direction of
perpendicular to the bed joints are significantly higher than their flexural strength in direction
of parallel to the bed joints, for the case of UMSystem2, for UMSystem1 is different due to
the fact of masonry apparel are rotated 90 degrees.
Finally, in initial shear test, higher amount of confining stress in the specimens increased the
initial shear strength of the specimens. Based on different levels of confining stresses different
behaviors in terms of force-sliding curves were obtained. For UMSystem1, as expected the
value of friction angle was higher than for UMSystem2. The opposite happened in the value
of cohesion, where the UMSystem1 have almost 0 do to the inexistent of mortar in the
interface, and UMSystem2 presented a higher value.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the funding from the European Union’s Seventh
Framework Programme for research, technological development and demonstration under
grant agreement No 606229, which support this work.
This work was also supported by FCT (Portuguese Foundation for Science and Technology),
within ISISE, project UID/ECI/04029/2013, and through a doctoral scholarship reference
SFRH/BD/125094/2016.
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