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Experimental performance of FRCM retrofit on out-of-plane behaviour of clay brick walls
Accepted Manuscript Experimental performance of FRCM retrofit on out-of-plane behaviour of clay brick walls Claudio D'Ambra, Gian Piero Lignola, Andrea Prota, Elio Sacco, Francesco Fabbrocino PII:
S1359-8368(18)30135-5
DOI:
10.1016/j.compositesb.2018.04.062
Reference:
JCOMB 5670
To appear in:
Composites Part B
Received Date: 12 January 2018 Revised Date:
8 April 2018
Accepted Date: 27 April 2018
Please cite this article as: D'Ambra C, Lignola GP, Prota A, Sacco E, Fabbrocino F, Experimental performance of FRCM retrofit on out-of-plane behaviour of clay brick walls, Composites Part B (2018), doi: 10.1016/j.compositesb.2018.04.062. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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EXPERIMENTAL PERFORMANCE OF FRCM RETROFIT ON
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OUT-OF-PLANE BEHAVIOUR OF CLAY BRICK WALLS Claudio D’Ambra1, Gian Piero Lignola1, Andrea Prota1, Elio Sacco1, and
Department of Structures for Engineering and Architecture, University of Naples Federico II Via Claudio 21, 80125 Naples, Italy e-mail: [email protected], [email protected], [email protected], [email protected] 2
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Francesco Fabbrocino2
Department of Engineering, Telematic University Pegaso Piazza Trieste e Trento 48, 80132 Napoli, Italy e-mail: [email protected]
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Keywords: Masonry wall, clay brick, out-of-plane, repair, FRCM. Abstract. In this paper the capacity of an innovative composite basalt grid with inorganic matrix (FRCM) has been evaluated both in terms of repairing pre-damaged
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and strengthening clay brick walls under out-of-plane loads. Experimental tests have been performed on full scale clay brick walls subjected to out-of-plane loads. A wall,
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damaged after a test, has been repaired by means of basalt FRCM. A similar wall has been tested directly, without pre-damage, after strengthening by means of FRCM. This allowed to remark the effect of retrofitting pre-damaged and new walls. To simulate a non-uniform out-of-plane behaviour of the wall, two adjacent edges of the wall have been constrained and the other two were left free while a pointwise normal force has been applied at the free opposite corner of the wall. The purpose of this work was to assess the potentiality of FRCM to recover the capacity of a wall after significant
ACCEPTED MANUSCRIPT damage and to increase the global response of strengthened wall not previously damaged. The experimental results demonstrated that the externally bonded strengthening was able to prevent a brittle failure and it was not affected by debonding;
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ultimate load of the retrofitted wall almost doubled with respect to the unreinforced configuration, despite complex stress state, and that the failure was governed by shear
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sliding at higher displacement levels.
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Introduction Clay brick masonry walls are frequently used as infills in reinforced concrete frames;
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generally, they are non-structural elements and their seismic capacity is neglected in the
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evaluation of vulnerability. The masonry infill walls play a fundamental role in the global
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response of RC buildings [1], mainly with their in-plane behaviour. Recent earthquakes
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confirmed the vulnerability of masonry infill walls to seismic loads (Fig.1), mainly due to
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their reduced out-of-plane capacity. Consequently, masonry walls subjected to out-of-plane
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loading represent a significant source of risk in terms of injuries and economic losses and
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damages, and this highlighted the need to consider their specific behaviour in the evaluation
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of seismic vulnerability.
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Fig. 1. A reinforced concrete structure damaged by recent 2016 earthquake, Visso (MC) Italy
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Moreover, the current studies [2-4], showed an increase of the out-of-plane vulnerability of
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masonry infill walls to the combined action of in-plane seismic load.
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An interesting and promising technique for the reinforcement and strengthening of masonry
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walls against injuries and damages due to the in-plane and out-of-plane mechanisms
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activation is the application of composite grids into inorganic mortar layers onto the surface
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of masonry walls.
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compatibility of the mortar with the masonry substrate, i.e. the matrix, a fiber grid has been
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embedded in cement or pozzolanic-based mortar (FRCM materials). This innovative solution
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represents an evolution of the traditional steel reinforced plaster usually adopted to improve
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performances of masonry walls, but it has the significant advantage of easy installation and of
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using thinner layers of plaster preventing, thus, relevant increments of mass and stiffness for
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the retrofitted wall. Numerical and experimental studies have evidenced that such a
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retrofitting technique can be very effective to increase capacities of masonry walls and vaults
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in terms of both strength and ductility [5-14] mainly subjected to uniaxial bending loads. The
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extensive range of performances exhibited by the retrofitted walls is due to the wide
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availability on the market of several types of FRCM systems, using different types of mortar
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and fibers (grids) [15-17]. Experimental evidences remarked that the overall behaviour of
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retrofitted walls is strongly influenced by the mortar layer used for embedding the grid, both
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in terms of stiffness and strength of the retrofitted wall, especially when walls are
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characterized by low thickness and low strength masonry [18]. On the other hand, the need to
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reduce as much as possible the thickness of the mortar layer, in order to minimize the
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‘impact’ of the intervention and not to change significantly mass and stiffness, can lead to
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technological problems in the application of the grid.
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In this paper the out-of-plane behaviour of clay brick walls has been studied and the efficacy
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of FRCM system as out-of-plane strengthening system has been evaluated performing an
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experimental program on full scale infill walls at the laboratory of the Department of
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Structures for Engineering and Architecture at the University of Naples “Federico II” [19].
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Fig. 2. Experimental test: specimen and setup
Specific boundary conditions have been adopted in the experimental tests to induce double
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bending in the wall; in fact, two consecutive edges were restrained at different degrees
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allowing to prevent the activation of the simple uniaxial (cylindrical) bending of the wall (Fig.
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2). This type of boundary condition aimed to simulate the complex stress state inside a
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concrete frame where the level of constraint is different among edges and presence of
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openings could impair a simple uniaxial flexural behaviour. In fact, in the common condition
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depicted in Fig. 1, the basis of the wall can be considered as simply supported on the floor
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while the top is scarcely constrained to the beam, so that it can be assumed here as free.
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Concerning the constraints on the lateral edge, one side is connected to another orthogonal
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wall, while the other side is free due to the presence of openings and can be considered as
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free. The load due to the seismic excitation would be considered as distributed over the wall,
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but it is simulated by a more demanding concentrated force in the free corner for laboratory
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convenience. Moreover, such a setup is aimed at evaluating complex state of stresses in the
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out-of-plane behaviour of the masonry infill. In fact, the experimental outcomes aim to be
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benchmarks for future validation of numerical analyses where the stress state in the masonry
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wall is more complex than uniaxial bending and the mortar (bed and head) joints are involved
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in the load transfer in different ways.
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ACCEPTED MANUSCRIPT Three types of tests have been considered; initially, an unreinforced wall (URMW) has been
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tested to estimate its out-of-plane capacity and failure mode, applying an incremental load in
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displacement control till the wall collapse was reached. To repair the damage in the mortar
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joints produced after the first test, they have been repointed with inorganic matrix; then, a
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layer of basalt FRCM has been applied on the entire wall. The repaired masonry wall
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(RPMW) has been tested again. Finally, the same strengthening system has been applied on a
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new wall, denoted as reinforced masonry wall (RFMW), to assess any different behaviour
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compared to the RPMW.
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The paper is organized as follows: in Section 2 the experimental setup is illustrated, providing
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the material properties; Section 3 describes the results of the experimental tests; in Section 4,
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a discussion on the comparison of results obtained for unreinforced and reinforced walls is
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reported. Finally, in Section 5 some conclusive remarks on the experimental program are
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given.
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2
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The experimental investigation consisted on out-of-plane tests on full scale unreinforced and
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FRCM reinforced clay brick masonry walls. In particular, the overall dimensions of the
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masonry walls are approximately 1515 mm × 1755 mm × 120 mm. Each wall is made of
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twenty seven rows, each with six bricks having size 55 mm × 120 mm × 250 mm. The mortar
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for the bed joints is about 10 mm thick and it is composed by 75% of sand, 22.5% of Portland
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cement and 2.5 % of calcium hydroxide.
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ACCEPTED MANUSCRIPT 2.1 Material properties
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Mechanical properties of mortar for joints and mortar as matrix of the grids were determined
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by means of experimental tests according to EN 1015-11 [20] standard. Tensile and
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compressive strengths were evaluated by means of bending and compressive tests.
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2.1.1 Mortar for joints
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9 mortar prisms with dimensions 40 mm × 40 mm × 160 mm were tested in flexure with
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three-point bending; then, 18 blocks, obtained from failed mortar specimens in flexure, were
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subjected to compression tests. The 28-day tensile average strength obtained from the flexural
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tests was equal to 2.32 MPa with coefficient of variation (CoV) 12.14%; while the
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compressive average strength was 11.76 MPa with CoV=6.39%.
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2.1.2 Mortar for matrix
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A premixed bi-component pozzolanic based grout made also of hydraulic natural lime, sand,
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special additives, polymers, and short glass fibers spread in the matrix has been used as
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matrix. The tensile and compressive average strengths after 28-day were 6.60 MPa with
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CoV=1.10% and 14.5 with CoV=8.80%, respectively.
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2.1.3 Clay brick
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The mechanical properties of the clay bricks are taken directly from technical data of the
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producer. The average compressive strength is equal to 30 MPa, while the tensile strength is 6
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MPa.
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2.1.4 Basalt grid
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The basalt grid has a square mesh having dimension 6 mm × 6 mm made of basalt fibers,
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whose equivalent thickness of dry fabric is 0.039 mm. The elastic modulus of the dry fibers is
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89 GPa and the nominal tensile strength is 1542 MPa.
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2.2 Experimental setup Unlike other experimental researches as [21-,23], where a single uniaxial bending stress state
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has been considered, in this experimental program a specific boundary condition has been
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adopted to induce a complex stress state characterized by a double bending to evaluated the
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behaviour of the FRCM system for a bidirectional stress state. The experimental setup
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consisted of two steel profiles to provide the lateral restraints and a steel square plate to
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spread the point load, limiting the localization effects. Two UPN profiles on two consecutive
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edges provide to restrained the wall; the basis of the wall was supported on the floor by a
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UPN 280, while the lateral edge was constrained by a UPN 180 fixed in three points to a rigid
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steel frame and having length of 1800 mm (see Fig. 3). To connect the masonry wall and the
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steel profiles, a mortar layer has been used simulating a simple support at the base, while on
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the lateral side the constraint was more similar to a clamped one. The load perpendicular to
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the plane of the wall was applied on the left top corner, by means of a steel plate of
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dimensions 300 mm × 300 mm with a thickness of 10 mm to prevent premature failure of the
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wall corner due to the load localization. The jack pushed the wall from its rear side. To
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prevent local cracking along the fixed edge, the free space between UPN profiles and masonry
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was filled by mortar (Fig. 4).
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ACCEPTED MANUSCRIPT UPN 180
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Steel Plate
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LVDT D
LVDT G
LVDT E
LVDT F
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Point Load
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LVDT L
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LVDT A
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Fig. 1. Experimental Setup for out-of-plane test: a) Rear view; b) Front view
It is worth noting that a double bending in vertical and horizontal directions developed due to
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the asymmetric lateral constraints and the behaviour was significantly biaxial. To avoid more
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complex test setup and reduce uncertainties, no symmetry constraints were applied at the free
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edges, so the wall does not strictly represent a quarter of a wider wall. However, this does not
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limit the validity of the test because the setup is able to induce a complex shear and bending
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stress state in two planes, providing a very demanding state to the wall, to check the
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suitability of FRCM to retrofit out-of-plane a wall even under so demanding stress state.
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Moreover, as discussed in the Introduction, the proposed setup can simulate the response of
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the wall represented, for instance, in Fig. 1.
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Mortar
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UPN 280 200
Mortar
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(b)
Fig. 4. UPN profiles: (a) Base constraint, (b) Lateral constraint
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In particular, the experimental test has been performed on: -
a unreinforced wall (URMW);
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-
a reinforced wall obtained by repairing the previously damaged unreinforced URMW
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one (RPMW); -
a strengthened wall obtained by applying the reinforcement system on an undamaged unreinforced one (RFMW).
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The reinforcements for the second and third test have been accomplished applying the
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innovative system made of inorganic matrix and basalt grid, FRCM, on one surface of the
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wall. In particular, the RPMW test has the objective to evaluate the recovery capacity of the
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system when FRCM is applied on the pre-damaged wall URMW, after the mortar joints of the
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damaged wall have been repaired (deep skiving, about 20 mm, of the damaged joints followed
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by joints repointing with inorganic matrix). The RFMW test is performed for evaluating the
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improvement of the mechanical response of the masonry wall obtained by applying the
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FRCM system bonded on an undamaged wall.
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The installation procedure involved the following steps:
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a layer of inorganic matrix was applied on a side of the wall to fill the masonry superficial defects;
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•
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one ply of basalt grid was applied on the first layer of inorganic matrix, while it was
still wet (with an overlapping of 500 mm between two grids);
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a further layer of inorganic matrix was, then, applied to complete the composite system (see Fig.5).
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Due to the specific constraint and loading condition, monotonic up to failure, that have been
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adopted for the experimental setup, the double bending state leads to a complete surface
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which is subjected to tensile state in both the directions. Thus, the FRCM system was applied
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only on one side, the tensile side of the wall in terms of the bending behaviour, because it is
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usually assumed that FRCM does not provide contribution in compression. Overlapping
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First layer of inorganic matrix
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Basalt grid
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Second layer of inorganic matrix
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In the developed experimental program, two phases have been considered in each test: 1. initially, a low intensity loading-unloading has been performed for settling and
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Fig. 5. Installation procedure Rear View
estimating the initial stiffness of the walls;
2. then, monotonic increasing load was applied up to failure.
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The tests were carried out under displacement control with a slow displacement rate provided
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by a manually operated jack (Fig. 6) and were stopped at the complete failure of the walls.
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Seven linear displacement transducers (LVDTs) were installed on two alignments on the
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walls; in particular, four LVDTs (F,G,D,E) were applied orthogonally to the plane of the wall
ACCEPTED MANUSCRIPT in order to estimate the out-of-plane displacements in different locations, while two LVDTs
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(M,L) were applied parallel to the wall to monitor the rigid base rotation at the vertical axis of
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the point load.
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3
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For all the tests, a non-uniform behaviour has been observed due to the particular loading and
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boundary conditions. A biaxial flexure with a double curvature is testified by the
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displacements at failure, shown in Fig. 7. In fact, it can be noted that the behaviour of LVDTs
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E and D (vertical line at 740 mm from the lateral constraint) is completely different from the
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behaviour obtained by LVDTs F and G (vertical line at 1420 mm from the lateral constraint).
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The comparison (i.e. different displacements of the vertical line) of LVDTs F and G with the
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relative rigid rotation line shows the effective bending displacement of the wall. The
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displacement line due to rigid rotation, ∆rot, has been derived from the vertical displacements
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LVDTL and LVDTM, recorded respectively by LVDTs L and M, as follows:
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Experimental results
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Fig. 6. Spreader plate to apply the point load at the free corner.
ACCEPTED MANUSCRIPT LVDTL − LVDTM d
∆ rot = H arctan
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where d represents the distance between LVDTs L and M.
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For the repaired (RPMW) and reinforced wall (RFMW), the displacements recorded by the
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LVDTs demonstrated a different failure mode with respect to unreinforced wall (URMW);
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indeed, the higher displacements recorded by LVDT G with respect to LVDT D indicate a
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rotation of the portion of the wall around the vertical constraint between the upper and lower
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parts of the wall. In particular a sliding of a part of the wall has been recorded already at
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lower load values by LVDT E and LVDT G (Fig. 7 (a,c,e)), as showed by failure mode
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observed on front view of the wall (Fig. 7 (d,f)).
LVDT D
LVDT G
height (mm)
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LVDT E
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(URMW)
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Rigid Rotation Line (F - G) Line (E - D)
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Rigid Rotation Line (F - G) Line (E - D)
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LVDT G
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LVDT E
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Rigid Rotation Line (F - G) Line (E - D)
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Fig. 7. Experimental results: (a) LVDT displacement at failure (URMW); (b) Failure Mode rear view and side view (note that cracked joints were removed intentionally before the strengthening intervention) (URMW); (c) LVDT displacement at failure (RPMW); (d) Failure Mode: Front View, and side view (RPMW); (e) LVDT displacement at failure (RFMW); (f) Failure Mode: Front View, and side view (RFMW).
The cracks of unreinforced wall (URMW) have been localized in mortar joints; indeed, a
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diagonal crack has been found on the rear (tensile) side of the wall from the eighteenth to the
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twenty-fourth row of bricks; moreover, a horizontal crack has been found on front
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(compressed) side from the eighteenth line of bricks, as shown in Fig. 7 (b).
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In the repaired wall (RPMW), micro-cracks have been observed on the strengthening system,
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while two diagonal cracks, on the mortar joints, have been localized on the front side (Fig. 7
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(d)). An ascending diagonal crack also aroused from twenty-third line of bricks along the
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mortar joints characterized the failure mode of the reinforced wall (RFMW) (Fig. 7 (f)).
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For both repaired (RPMW) and reinforced wall (RFMW), the sliding recorded at the end of
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the test has generated a detachment of the reinforcement system localized on the diagonal
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crack.
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4
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In this section the results obtained by the tests are compared to investigate on the effect of the
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FRCM system.
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Comparison of results
ACCEPTED MANUSCRIPT Different failure modes have been found in the three tests. Indeed, on the unreinforced wall
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(URMW) a diagonal crack situated on the rear side of the wall, from the eighteenth to the
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twenty-fourth line of bricks, determined the collapse of the system; on the repaired (RPMW)
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and reinforced wall (RFMW) three different crack systems, on the front side of the wall
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(opposite to FRCM), for the three branches of the force-displacement curve occurred. A
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different diagonal crack system developed for each of the three transition load values. For the
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repaired wall (RPMW), at a load of about 4.5 kN a diagonal crack from the twentieth to the
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eighth row of bricks (black line in Fig. 8 (b)) determined a partition of the wall with a first
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change of stiffness of the system. At the peak load, a clear sliding of the upper region started
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and caused a diagonal crack from the third to the nineteenth row of bricks (blue line Fig. 8
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(b)). For the reinforced wall (RFMW) the cracks recorded, on front side, at different force
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values determined a different partition of the wall.
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Both the tests were stopped while an almost frictional behaviour was ongoing in large
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displacements, due to exhaustion of displacement capacity of the testing system, which was
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barely compatible with the vertical stability of the wall. It is noted that the sliding occurred
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not in a single row but along diagonal cracks being the sliding displacements larger at
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locations farther from the lateral constraint.
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ACCEPTED MANUSCRIPT Fig. 8. Failure Modes: (a) rear view of Unreinforced Wall (URMW), (b) front view of Repaired Wall (RPMW) (opposite to FRCM); (c) front view of Reinforced wall (RFMW)
The displacements recorded by LVDTs for all tests are compared in Fig. 9, the unreinforced
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wall (URMW) showed a brittle behaviour, a slightly nonlinear initial branch is followed by a
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sudden load drop. An almost continuous reduction of stiffness has been recorded during the
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test, as illustrated in Fig. 9 (a,c,e,g). A significant displacement capacity has been observed
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for the repaired wall (RPMW); in fact, three branches with different stiffnesses have been
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recorded during the test, at all monitored locations (Fig. 9 (a,c,e,g)). The first elastic branch is
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followed by a less stiff branch, while in the last horizontal branch only a sliding of the upper
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portion of the wall has been recorded. A behaviour similar to the repaired wall (RPMW) has
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been shown by reinforced wall (RFMW), but a lower stiffness of the first branch has been
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recorded. Also in this test it is possible to individuate three branches, the first two involving
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flexural behaviour and the last one is an horizontal plateau governed by sliding of the upper
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portion of the wall (Fig. 9 (a,c,e,g)).
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Force (kN)
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6.0 5.0 4.0 3.0
LVDT G
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7.0
Force (kN)
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URMW RPMW RFMW
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10
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30 40 50 60 displacement (mm) (b)
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30 40 50 60 displacement (mm) (f)
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LVDT E
20 30 40 50 displacement (mm) (g)
LVDT F
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20 30 40 50 displacement (mm) (e)
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URMW RPMW RFMW
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30 40 50 60 displacement (mm) (d)
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Force (kN)
Force (kN)
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URMW RPMW RFMW
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LVDT D
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URMW RPMW RFMW
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Force (kN)
Force (kN)
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3.0
LVDT E 2.0
URMW RPMW RFMW
60
4.0
URMW RPMW RFMW
1.0 0.0 70
0
10
20
30 40 50 60 displacement (mm) (h)
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Fig. 9. Experimental results: (a) Curve force vs LVDT G displacements; (b) Trilinear curve force vs LVDT G displacements; (c) Curve force vs LVDT D displacements; (d) Trilinear curve force vs LVDT D displacements; (e) Curve force vs LVDT F displacements; (f) Trilinear curve force vs LVDT F displacements; (g) Curve force vs LVDT E displacements; (h) Trilinear curve force vs LVDT E displacements .
ACCEPTED MANUSCRIPT In Fig. 9 (b,d,f,h) the (approximated) trilinear curves in terms of force vs LVDTs
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displacements are compared reporting the variation of the stiffness recorded during the tests.
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Two branches for unreinforced wall (URMW) and three branches for repaired (RPMW) and
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reinforced wall (RFMW) were found; in particular, the first branches represent the elastic
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behaviour of the whole wall, exceeding the capacity of the mortar joints determined a
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partition of the wall with a reduction of the global stiffness (second branch).
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For all tests the cracks have been localized mainly on mortar joints, the application of the
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FRCM system allowed to obtain an increase of the strength and of the displacements of the
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walls. In Fig.9 and in Table 1 the obtained increments in terms of force and displacement are
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compared.
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The sliding of the upper region of the wall is shown by the third branch of the curve, where
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high displacements without increases of force have been recorded. The significant values of
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the three branches are summarized in Table 1: for each branch the stiffness, the maximum
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force and the maximum displacements are reported. As it can be observed, similar stiffness
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for the second branch has been found for each test, the vertical internal constraint provided by
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the FRCM system allowed to get forces and displacements of the second branches. In general,
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the effect of the internal axial stresses induced by FRCM system (even in all the vertically
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unloaded infills) influenced mainly the mortar joints. In fact, all the figures illustrating the
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failure modes remarked the significant role of the mortar joints and, mainly, of the mortar to
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brick interfaces on the wall behaviour.
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First branch
Test
URMW
RPMW
LVDT G D F E G D
K1 N/mm 460 808 825 1487 471 819
F1 kN 2,7
3,9
Second branch d1 mm 5,9 3,4 3,3 1,8 8,2 4,7
K2 N/mm 51,3 114,8 114,0 252,3 96,9 187,6
F2 kN 2,9
6,0
d2 mm 9,9 5,2 5,1 2,7 31,5 16,8
Third branch K3 N/mm n.a. n.a. n.a. n.a. 0 0
F3 kN n.a. n.a. n.a. n.a. 6,0
d3 mm n.a. n.a. n.a. n.a. 62,0 31,7
∆(K1-K2) % -89% -86% -86% -83% -79% -77%
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RFMW
F E G D F E
861 1522 273 549 594 1052
2,2
4,5 2,5 8,1 4,0 3,7 2,1
171,0 208,2 83,9 172,2 174,3 264,6
5,3
17,7 13,1 46,0 22,6 22,1 14,2
0 0 0 0 0 0
5,3
29,7 23,8 67,7 33,4 31,6 20,7
-80% -86% -69% -69% -71% -75%
Table 1 Experimental results.
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The unreinforced wall (URMW) and repaired wall (RPMW) showed a similar initial stiffness,
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but the application of the strengthening system allowed to attain higher values of lateral force,
248
Fmax, and displacement, dmax. The lower stiffness values recorded during the reinforced wall
249
(RFMW) test are due to the natural variability of masonry performance.
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The damage of the unreinforced wall (URMW) is expected to influence (i.e. to reduce slope
251
of) the first branch of the repaired wall (RPMW), but the contribution of joints repointing
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with the mortar of the matrix and the FRCM system allowed to recover the same stiffness
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recorded for the first branch of unreinforced wall (Table 1). For this reason, it is expected that
254
the reinforced wall (RFMW) would have a higher stiffness, because there is no initial
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reduction of stiffness due to pre-damage; however, experimental evidences found that the
256
initial stiffness of RFMW was lower than the one of URMW. In addition, it is expected that
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the FRCM system does not reduce the initial stiffness of the wall. Thus, this reduced stiffness
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of the RFMW and both URMW and RPMW reasonably can be justified by the previously
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mentioned natural variability of the wall response. Indeed, the activation of the FRCM system
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is expected to occur mainly after exceeding the unreinforced masonry wall capacity, i.e. in
261
correspondence of the second branch of the mechanical response of the wall.
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Although different displacements and behaviours have been observed for the three types of
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tests at failure it can be in any case declared that the strengthening system allowed to obtain a
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ductile response compared to the unreinforced wall.
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In Fig. 10 the displacements recorded on line E – D (vertical line at 740 mm from the lateral
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boundary) and on line F – G (on the vertical line of the point load) were compared. In
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ACCEPTED MANUSCRIPT particular, in Fig. 10 (a) the displacement at the failure of the tests are compared,
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demonstrating that the repaired (RPMW) and reinforced (RFMW) walls show the same
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deformed shape, i.e. the double curvature caused by the lateral constraints determined the
270
lower displacements of the line D – E. However, the different behaviour of the tests is due to
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different locations of LVDTs with respect to the position of the main cracks. In the
272
unreinforced wall (URMW) (Fig. 10 (b,c)) the upper part monitored by LVDTs G and D is
273
sliding with respect to the lower part. Conversely, in the case of repaired wall (RPMW), the
274
middle portion connected in the corner with the loaded point and characterized by inclined
275
cracks due to particular constraints layout (Fig. 10 (b)), is subjected to sliding, so that LVDT
276
D shows a similar displacement as LVDT E and much smaller than LVDT G. A similar
277
behaviour in terms of displacements has been showed also by the reinforced wall (RFMW). In
278
fact, it is noted that shear deformability yields mainly to linear displacement profiles, while
279
flexural deformability to more complex displacement profiles, but the vertical profiles are
280
influenced also by rigid rotations due to particular hinge-like constraint evolving with the
281
increase of load.
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2000
2000
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LVDT G
1500
LVDT F
z y
x
height (mm)
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LVDT E
1000
1000 LVDT E
500
URMW RPMW RFMW
LVDT F
500
URMW RPMW RFMW
0
0 0 (a)
LVDT G
LVDT D
height (mm)
LVDT D
20 40 60 80 displacement (mm) (b)
0
20 40 60 80 displacement (mm) (c)
Fig. 10. LVDT displacement: (a) 3d View, (b) Line E-D, (c) Line F-G
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Conclusions
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Aim of this paper was the evaluation, by means of experimental tests, of the effectiveness of
284
FRCM systems in improving the out-of-plane capacity of a pre-damaged and a new masonry
285
wall. Three different tests on two masonry walls were considered, an unreinforced wall has
286
been tested till the collapse; then, the damaged wall has been repaired and the strengthening
287
system has been applied on rear side of the repaired wall. A second, similar, wall has been
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tested without pre-damage and directly strengthened by means of FRCM. This allowed to
289
remark the effect of FRCM pre-damaged and new walls. The setup was conceived to provide
290
a biaxial bending coupled with shear, hence a non-uniform complex stress state in the wall.
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The experimental tests showed a progressive stiffness reduction of the unreinforced masonry
292
wall demonstrating the progressive damage of the joints, the cracks localized in the mortar,
293
the other elements of the wall, i.e. the bricks, remained substantially in their elastic state. A
294
simple restoration of the most cracked joints, intentionally removed and filled with the same
295
mortar of the FRCM matrix, allowed to restore the stiffness of the repaired masonry wall. The
296
axial stresses induced by the strengthening system improved the mechanical characteristics of
297
the interface between clay brick and mortar joints, as they were found to be the elements
298
governing the failure mode. A trilinear curve force vs displacement describes the typical
299
behaviour of the repaired and reinforced walls, where the variation of the stiffness between
300
the first and second branch represents the partition of the wall. Even if the two walls were
301
built as practically identical, natural variability was recorded on the initial stiffness of the two
302
walls. The former wall, tested in unreinforced configuration, had a stiffness higher than the
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latter, reinforced without any pre-damage. Tests at failure were monotonic and FRCM was
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applied on the tensile side only; in real applications the retrofit system can be applied to both
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sides, but it can be considered effective on the tensile side only, while in compression it can
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ACCEPTED MANUSCRIPT be assumed to be not effective due to its slenderness and susceptibility to buckling in
307
compression. To prevent buckling, mechanical connectors can be applied improving the
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capacity in tension too, but it is not considered yet in this phase of the experimental program.
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In conclusion, the FRCM system proved to be able to almost double the out-of-plane strength
310
of the wall and its lateral displacement capacity despite the complex shear coupled to bending
311
biaxial stress state, not impacting significantly on mass and stiffness of the wall. It can be
312
remarked that the strengthening system significantly changed the failure mechanism of the
313
wall, due to the complex state of stress arising because of the double bending due to the
314
specific considered loading condition that could be very close to real situations.
315
ACKNOWLEDGEMENTS
316
The authors would like to thank Giovanbattista Borretti and Antimo Fiorillo for their support
317
in test execution.
318
This research has been possible thanks to the financial support of the ReLUIS project from
319
the Italian Department of the Civil Protection.
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The FRCM strengthening of the walls was supported by MAPEI Spa., Milan, Italy.
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S1359-8368(18)30135-5
DOI:
10.1016/j.compositesb.2018.04.062
Reference:
JCOMB 5670
To appear in:
Composites Part B
Received Date: 12 January 2018 Revised Date:
8 April 2018
Accepted Date: 27 April 2018
Please cite this article as: D'Ambra C, Lignola GP, Prota A, Sacco E, Fabbrocino F, Experimental performance of FRCM retrofit on out-of-plane behaviour of clay brick walls, Composites Part B (2018), doi: 10.1016/j.compositesb.2018.04.062. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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EXPERIMENTAL PERFORMANCE OF FRCM RETROFIT ON
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OUT-OF-PLANE BEHAVIOUR OF CLAY BRICK WALLS Claudio D’Ambra1, Gian Piero Lignola1, Andrea Prota1, Elio Sacco1, and
Department of Structures for Engineering and Architecture, University of Naples Federico II Via Claudio 21, 80125 Naples, Italy e-mail: [email protected], [email protected], [email protected], [email protected] 2
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Francesco Fabbrocino2
Department of Engineering, Telematic University Pegaso Piazza Trieste e Trento 48, 80132 Napoli, Italy e-mail: [email protected]
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Keywords: Masonry wall, clay brick, out-of-plane, repair, FRCM. Abstract. In this paper the capacity of an innovative composite basalt grid with inorganic matrix (FRCM) has been evaluated both in terms of repairing pre-damaged
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and strengthening clay brick walls under out-of-plane loads. Experimental tests have been performed on full scale clay brick walls subjected to out-of-plane loads. A wall,
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damaged after a test, has been repaired by means of basalt FRCM. A similar wall has been tested directly, without pre-damage, after strengthening by means of FRCM. This allowed to remark the effect of retrofitting pre-damaged and new walls. To simulate a non-uniform out-of-plane behaviour of the wall, two adjacent edges of the wall have been constrained and the other two were left free while a pointwise normal force has been applied at the free opposite corner of the wall. The purpose of this work was to assess the potentiality of FRCM to recover the capacity of a wall after significant
ACCEPTED MANUSCRIPT damage and to increase the global response of strengthened wall not previously damaged. The experimental results demonstrated that the externally bonded strengthening was able to prevent a brittle failure and it was not affected by debonding;
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ultimate load of the retrofitted wall almost doubled with respect to the unreinforced configuration, despite complex stress state, and that the failure was governed by shear
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sliding at higher displacement levels.
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Introduction Clay brick masonry walls are frequently used as infills in reinforced concrete frames;
3
generally, they are non-structural elements and their seismic capacity is neglected in the
4
evaluation of vulnerability. The masonry infill walls play a fundamental role in the global
5
response of RC buildings [1], mainly with their in-plane behaviour. Recent earthquakes
6
confirmed the vulnerability of masonry infill walls to seismic loads (Fig.1), mainly due to
7
their reduced out-of-plane capacity. Consequently, masonry walls subjected to out-of-plane
8
loading represent a significant source of risk in terms of injuries and economic losses and
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damages, and this highlighted the need to consider their specific behaviour in the evaluation
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of seismic vulnerability.
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Fig. 1. A reinforced concrete structure damaged by recent 2016 earthquake, Visso (MC) Italy
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Moreover, the current studies [2-4], showed an increase of the out-of-plane vulnerability of
12
masonry infill walls to the combined action of in-plane seismic load.
13
An interesting and promising technique for the reinforcement and strengthening of masonry
14
walls against injuries and damages due to the in-plane and out-of-plane mechanisms
15
activation is the application of composite grids into inorganic mortar layers onto the surface
16
of masonry walls.
ACCEPTED MANUSCRIPT To combine the advantages of high performance materials, i.e. the grids, with the good
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compatibility of the mortar with the masonry substrate, i.e. the matrix, a fiber grid has been
19
embedded in cement or pozzolanic-based mortar (FRCM materials). This innovative solution
20
represents an evolution of the traditional steel reinforced plaster usually adopted to improve
21
performances of masonry walls, but it has the significant advantage of easy installation and of
22
using thinner layers of plaster preventing, thus, relevant increments of mass and stiffness for
23
the retrofitted wall. Numerical and experimental studies have evidenced that such a
24
retrofitting technique can be very effective to increase capacities of masonry walls and vaults
25
in terms of both strength and ductility [5-14] mainly subjected to uniaxial bending loads. The
26
extensive range of performances exhibited by the retrofitted walls is due to the wide
27
availability on the market of several types of FRCM systems, using different types of mortar
28
and fibers (grids) [15-17]. Experimental evidences remarked that the overall behaviour of
29
retrofitted walls is strongly influenced by the mortar layer used for embedding the grid, both
30
in terms of stiffness and strength of the retrofitted wall, especially when walls are
31
characterized by low thickness and low strength masonry [18]. On the other hand, the need to
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reduce as much as possible the thickness of the mortar layer, in order to minimize the
33
‘impact’ of the intervention and not to change significantly mass and stiffness, can lead to
34
technological problems in the application of the grid.
35
In this paper the out-of-plane behaviour of clay brick walls has been studied and the efficacy
36
of FRCM system as out-of-plane strengthening system has been evaluated performing an
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experimental program on full scale infill walls at the laboratory of the Department of
38
Structures for Engineering and Architecture at the University of Naples “Federico II” [19].
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Fig. 2. Experimental test: specimen and setup
Specific boundary conditions have been adopted in the experimental tests to induce double
40
bending in the wall; in fact, two consecutive edges were restrained at different degrees
41
allowing to prevent the activation of the simple uniaxial (cylindrical) bending of the wall (Fig.
42
2). This type of boundary condition aimed to simulate the complex stress state inside a
43
concrete frame where the level of constraint is different among edges and presence of
44
openings could impair a simple uniaxial flexural behaviour. In fact, in the common condition
45
depicted in Fig. 1, the basis of the wall can be considered as simply supported on the floor
46
while the top is scarcely constrained to the beam, so that it can be assumed here as free.
47
Concerning the constraints on the lateral edge, one side is connected to another orthogonal
48
wall, while the other side is free due to the presence of openings and can be considered as
49
free. The load due to the seismic excitation would be considered as distributed over the wall,
50
but it is simulated by a more demanding concentrated force in the free corner for laboratory
51
convenience. Moreover, such a setup is aimed at evaluating complex state of stresses in the
52
out-of-plane behaviour of the masonry infill. In fact, the experimental outcomes aim to be
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benchmarks for future validation of numerical analyses where the stress state in the masonry
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wall is more complex than uniaxial bending and the mortar (bed and head) joints are involved
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in the load transfer in different ways.
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tested to estimate its out-of-plane capacity and failure mode, applying an incremental load in
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displacement control till the wall collapse was reached. To repair the damage in the mortar
59
joints produced after the first test, they have been repointed with inorganic matrix; then, a
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layer of basalt FRCM has been applied on the entire wall. The repaired masonry wall
61
(RPMW) has been tested again. Finally, the same strengthening system has been applied on a
62
new wall, denoted as reinforced masonry wall (RFMW), to assess any different behaviour
63
compared to the RPMW.
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The paper is organized as follows: in Section 2 the experimental setup is illustrated, providing
65
the material properties; Section 3 describes the results of the experimental tests; in Section 4,
66
a discussion on the comparison of results obtained for unreinforced and reinforced walls is
67
reported. Finally, in Section 5 some conclusive remarks on the experimental program are
68
given.
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2
70
The experimental investigation consisted on out-of-plane tests on full scale unreinforced and
71
FRCM reinforced clay brick masonry walls. In particular, the overall dimensions of the
72
masonry walls are approximately 1515 mm × 1755 mm × 120 mm. Each wall is made of
73
twenty seven rows, each with six bricks having size 55 mm × 120 mm × 250 mm. The mortar
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for the bed joints is about 10 mm thick and it is composed by 75% of sand, 22.5% of Portland
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cement and 2.5 % of calcium hydroxide.
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Mechanical properties of mortar for joints and mortar as matrix of the grids were determined
78
by means of experimental tests according to EN 1015-11 [20] standard. Tensile and
79
compressive strengths were evaluated by means of bending and compressive tests.
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2.1.1 Mortar for joints
81
9 mortar prisms with dimensions 40 mm × 40 mm × 160 mm were tested in flexure with
82
three-point bending; then, 18 blocks, obtained from failed mortar specimens in flexure, were
83
subjected to compression tests. The 28-day tensile average strength obtained from the flexural
84
tests was equal to 2.32 MPa with coefficient of variation (CoV) 12.14%; while the
85
compressive average strength was 11.76 MPa with CoV=6.39%.
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2.1.2 Mortar for matrix
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A premixed bi-component pozzolanic based grout made also of hydraulic natural lime, sand,
88
special additives, polymers, and short glass fibers spread in the matrix has been used as
89
matrix. The tensile and compressive average strengths after 28-day were 6.60 MPa with
90
CoV=1.10% and 14.5 with CoV=8.80%, respectively.
91
2.1.3 Clay brick
92
The mechanical properties of the clay bricks are taken directly from technical data of the
93
producer. The average compressive strength is equal to 30 MPa, while the tensile strength is 6
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MPa.
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2.1.4 Basalt grid
96
The basalt grid has a square mesh having dimension 6 mm × 6 mm made of basalt fibers,
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whose equivalent thickness of dry fabric is 0.039 mm. The elastic modulus of the dry fibers is
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89 GPa and the nominal tensile strength is 1542 MPa.
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2.2 Experimental setup Unlike other experimental researches as [21-,23], where a single uniaxial bending stress state
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has been considered, in this experimental program a specific boundary condition has been
102
adopted to induce a complex stress state characterized by a double bending to evaluated the
103
behaviour of the FRCM system for a bidirectional stress state. The experimental setup
104
consisted of two steel profiles to provide the lateral restraints and a steel square plate to
105
spread the point load, limiting the localization effects. Two UPN profiles on two consecutive
106
edges provide to restrained the wall; the basis of the wall was supported on the floor by a
107
UPN 280, while the lateral edge was constrained by a UPN 180 fixed in three points to a rigid
108
steel frame and having length of 1800 mm (see Fig. 3). To connect the masonry wall and the
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steel profiles, a mortar layer has been used simulating a simple support at the base, while on
110
the lateral side the constraint was more similar to a clamped one. The load perpendicular to
111
the plane of the wall was applied on the left top corner, by means of a steel plate of
112
dimensions 300 mm × 300 mm with a thickness of 10 mm to prevent premature failure of the
113
wall corner due to the load localization. The jack pushed the wall from its rear side. To
114
prevent local cracking along the fixed edge, the free space between UPN profiles and masonry
115
was filled by mortar (Fig. 4).
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ACCEPTED MANUSCRIPT UPN 180
1515
Steel Plate
1420 740
300
LVDT D
LVDT G
LVDT E
LVDT F
300
Point Load
1755
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1670
890
LVDT L
LVDT M
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(a)
(b)
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Fig. 1. Experimental Setup for out-of-plane test: a) Rear view; b) Front view
It is worth noting that a double bending in vertical and horizontal directions developed due to
117
the asymmetric lateral constraints and the behaviour was significantly biaxial. To avoid more
118
complex test setup and reduce uncertainties, no symmetry constraints were applied at the free
119
edges, so the wall does not strictly represent a quarter of a wider wall. However, this does not
120
limit the validity of the test because the setup is able to induce a complex shear and bending
121
stress state in two planes, providing a very demanding state to the wall, to check the
122
suitability of FRCM to retrofit out-of-plane a wall even under so demanding stress state.
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Moreover, as discussed in the Introduction, the proposed setup can simulate the response of
124
the wall represented, for instance, in Fig. 1.
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Mortar
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UPN 180
Steel frame
UPN 180 Brick 200
UPN 280 200
Mortar
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(b)
Fig. 4. UPN profiles: (a) Base constraint, (b) Lateral constraint
126
In particular, the experimental test has been performed on: -
a unreinforced wall (URMW);
128
-
a reinforced wall obtained by repairing the previously damaged unreinforced URMW
129 130
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one (RPMW); -
a strengthened wall obtained by applying the reinforcement system on an undamaged unreinforced one (RFMW).
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The reinforcements for the second and third test have been accomplished applying the
133
innovative system made of inorganic matrix and basalt grid, FRCM, on one surface of the
134
wall. In particular, the RPMW test has the objective to evaluate the recovery capacity of the
135
system when FRCM is applied on the pre-damaged wall URMW, after the mortar joints of the
136
damaged wall have been repaired (deep skiving, about 20 mm, of the damaged joints followed
137
by joints repointing with inorganic matrix). The RFMW test is performed for evaluating the
138
improvement of the mechanical response of the masonry wall obtained by applying the
139
FRCM system bonded on an undamaged wall.
140
The installation procedure involved the following steps:
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a layer of inorganic matrix was applied on a side of the wall to fill the masonry superficial defects;
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one ply of basalt grid was applied on the first layer of inorganic matrix, while it was
still wet (with an overlapping of 500 mm between two grids);
•
a further layer of inorganic matrix was, then, applied to complete the composite system (see Fig.5).
147
Due to the specific constraint and loading condition, monotonic up to failure, that have been
148
adopted for the experimental setup, the double bending state leads to a complete surface
ACCEPTED MANUSCRIPT 149
which is subjected to tensile state in both the directions. Thus, the FRCM system was applied
150
only on one side, the tensile side of the wall in terms of the bending behaviour, because it is
151
usually assumed that FRCM does not provide contribution in compression. Overlapping
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Second layer of inorganic matrix
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153 154 155
In the developed experimental program, two phases have been considered in each test: 1. initially, a low intensity loading-unloading has been performed for settling and
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Fig. 5. Installation procedure Rear View
estimating the initial stiffness of the walls;
2. then, monotonic increasing load was applied up to failure.
156
The tests were carried out under displacement control with a slow displacement rate provided
157
by a manually operated jack (Fig. 6) and were stopped at the complete failure of the walls.
158
Seven linear displacement transducers (LVDTs) were installed on two alignments on the
159
walls; in particular, four LVDTs (F,G,D,E) were applied orthogonally to the plane of the wall
ACCEPTED MANUSCRIPT in order to estimate the out-of-plane displacements in different locations, while two LVDTs
161
(M,L) were applied parallel to the wall to monitor the rigid base rotation at the vertical axis of
162
the point load.
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3
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For all the tests, a non-uniform behaviour has been observed due to the particular loading and
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boundary conditions. A biaxial flexure with a double curvature is testified by the
166
displacements at failure, shown in Fig. 7. In fact, it can be noted that the behaviour of LVDTs
167
E and D (vertical line at 740 mm from the lateral constraint) is completely different from the
168
behaviour obtained by LVDTs F and G (vertical line at 1420 mm from the lateral constraint).
169
The comparison (i.e. different displacements of the vertical line) of LVDTs F and G with the
170
relative rigid rotation line shows the effective bending displacement of the wall. The
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displacement line due to rigid rotation, ∆rot, has been derived from the vertical displacements
172
LVDTL and LVDTM, recorded respectively by LVDTs L and M, as follows:
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Fig. 6. Spreader plate to apply the point load at the free corner.
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∆ rot = H arctan
174
where d represents the distance between LVDTs L and M.
175
For the repaired (RPMW) and reinforced wall (RFMW), the displacements recorded by the
176
LVDTs demonstrated a different failure mode with respect to unreinforced wall (URMW);
177
indeed, the higher displacements recorded by LVDT G with respect to LVDT D indicate a
178
rotation of the portion of the wall around the vertical constraint between the upper and lower
179
parts of the wall. In particular a sliding of a part of the wall has been recorded already at
180
lower load values by LVDT E and LVDT G (Fig. 7 (a,c,e)), as showed by failure mode
181
observed on front view of the wall (Fig. 7 (d,f)).
LVDT D
LVDT G
height (mm)
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1000
LVDT E
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(URMW)
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Rigid Rotation Line (F - G) Line (E - D)
0
4 6 8 displacement (mm) (a)
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10 (b)
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LVDT G
height (mm)
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LVDT E
1000
LVDT F
500
Rigid Rotation Line (F - G) Line (E - D)
0 0
20 40 60 displacement (mm) (c)
80 (d)
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(RFMW)
LVDT G
height (mm)
1500
LVDT E
1000
500
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Rigid Rotation Line (F - G) Line (E - D)
0 20 40 60 displacement (mm) (e)
80
(f)
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Fig. 7. Experimental results: (a) LVDT displacement at failure (URMW); (b) Failure Mode rear view and side view (note that cracked joints were removed intentionally before the strengthening intervention) (URMW); (c) LVDT displacement at failure (RPMW); (d) Failure Mode: Front View, and side view (RPMW); (e) LVDT displacement at failure (RFMW); (f) Failure Mode: Front View, and side view (RFMW).
The cracks of unreinforced wall (URMW) have been localized in mortar joints; indeed, a
183
diagonal crack has been found on the rear (tensile) side of the wall from the eighteenth to the
184
twenty-fourth row of bricks; moreover, a horizontal crack has been found on front
185
(compressed) side from the eighteenth line of bricks, as shown in Fig. 7 (b).
186
In the repaired wall (RPMW), micro-cracks have been observed on the strengthening system,
187
while two diagonal cracks, on the mortar joints, have been localized on the front side (Fig. 7
188
(d)). An ascending diagonal crack also aroused from twenty-third line of bricks along the
189
mortar joints characterized the failure mode of the reinforced wall (RFMW) (Fig. 7 (f)).
190
For both repaired (RPMW) and reinforced wall (RFMW), the sliding recorded at the end of
191
the test has generated a detachment of the reinforcement system localized on the diagonal
192
crack.
193
4
194
In this section the results obtained by the tests are compared to investigate on the effect of the
195
FRCM system.
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Comparison of results
ACCEPTED MANUSCRIPT Different failure modes have been found in the three tests. Indeed, on the unreinforced wall
197
(URMW) a diagonal crack situated on the rear side of the wall, from the eighteenth to the
198
twenty-fourth line of bricks, determined the collapse of the system; on the repaired (RPMW)
199
and reinforced wall (RFMW) three different crack systems, on the front side of the wall
200
(opposite to FRCM), for the three branches of the force-displacement curve occurred. A
201
different diagonal crack system developed for each of the three transition load values. For the
202
repaired wall (RPMW), at a load of about 4.5 kN a diagonal crack from the twentieth to the
203
eighth row of bricks (black line in Fig. 8 (b)) determined a partition of the wall with a first
204
change of stiffness of the system. At the peak load, a clear sliding of the upper region started
205
and caused a diagonal crack from the third to the nineteenth row of bricks (blue line Fig. 8
206
(b)). For the reinforced wall (RFMW) the cracks recorded, on front side, at different force
207
values determined a different partition of the wall.
208
Both the tests were stopped while an almost frictional behaviour was ongoing in large
209
displacements, due to exhaustion of displacement capacity of the testing system, which was
210
barely compatible with the vertical stability of the wall. It is noted that the sliding occurred
211
not in a single row but along diagonal cracks being the sliding displacements larger at
212
locations farther from the lateral constraint.
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ACCEPTED MANUSCRIPT Fig. 8. Failure Modes: (a) rear view of Unreinforced Wall (URMW), (b) front view of Repaired Wall (RPMW) (opposite to FRCM); (c) front view of Reinforced wall (RFMW)
The displacements recorded by LVDTs for all tests are compared in Fig. 9, the unreinforced
214
wall (URMW) showed a brittle behaviour, a slightly nonlinear initial branch is followed by a
215
sudden load drop. An almost continuous reduction of stiffness has been recorded during the
216
test, as illustrated in Fig. 9 (a,c,e,g). A significant displacement capacity has been observed
217
for the repaired wall (RPMW); in fact, three branches with different stiffnesses have been
218
recorded during the test, at all monitored locations (Fig. 9 (a,c,e,g)). The first elastic branch is
219
followed by a less stiff branch, while in the last horizontal branch only a sliding of the upper
220
portion of the wall has been recorded. A behaviour similar to the repaired wall (RPMW) has
221
been shown by reinforced wall (RFMW), but a lower stiffness of the first branch has been
222
recorded. Also in this test it is possible to individuate three branches, the first two involving
223
flexural behaviour and the last one is an horizontal plateau governed by sliding of the upper
224
portion of the wall (Fig. 9 (a,c,e,g)).
6.0
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LVDT G
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Force (kN)
5.0
0
10
20
6.0 5.0 4.0 3.0
LVDT G
2.0
URMW RPMW RFMW
30 40 50 60 displacement (mm) (a)
7.0
Force (kN)
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URMW RPMW RFMW
1.0 0.0 70
0
10
20
30 40 50 60 displacement (mm) (b)
70
7.0
7.0
6.0
6.0
5.0
5.0
4.0 3.0
LVDT D
0.0 0
70
10
20
7.0
7.0
6.0
6.0
5.0
Force (kN)
4.0 3.0
LVDT F
2.0
10
7.0
5.0
0.0
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1.0
60
0
10
0
70
URMW RPMW RFMW
10
20
30 40 50 60 displacement (mm) (f)
70
7.0 6.0 5.0
LVDT E
20 30 40 50 displacement (mm) (g)
LVDT F
0.0
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20 30 40 50 displacement (mm) (e)
3.0
1.0
TE D
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2.0
70
4.0
2.0
URMW RPMW RFMW
0.0
3.0
30 40 50 60 displacement (mm) (d)
5.0
1.0
Force (kN)
60
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Force (kN)
Force (kN)
10
URMW RPMW RFMW
1.0
0.0 0
LVDT D
2.0
URMW RPMW RFMW
1.0
3.0
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4.0
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Force (kN)
Force (kN)
ACCEPTED MANUSCRIPT
3.0
LVDT E 2.0
URMW RPMW RFMW
60
4.0
URMW RPMW RFMW
1.0 0.0 70
0
10
20
30 40 50 60 displacement (mm) (h)
70
Fig. 9. Experimental results: (a) Curve force vs LVDT G displacements; (b) Trilinear curve force vs LVDT G displacements; (c) Curve force vs LVDT D displacements; (d) Trilinear curve force vs LVDT D displacements; (e) Curve force vs LVDT F displacements; (f) Trilinear curve force vs LVDT F displacements; (g) Curve force vs LVDT E displacements; (h) Trilinear curve force vs LVDT E displacements .
ACCEPTED MANUSCRIPT In Fig. 9 (b,d,f,h) the (approximated) trilinear curves in terms of force vs LVDTs
226
displacements are compared reporting the variation of the stiffness recorded during the tests.
227
Two branches for unreinforced wall (URMW) and three branches for repaired (RPMW) and
228
reinforced wall (RFMW) were found; in particular, the first branches represent the elastic
229
behaviour of the whole wall, exceeding the capacity of the mortar joints determined a
230
partition of the wall with a reduction of the global stiffness (second branch).
231
For all tests the cracks have been localized mainly on mortar joints, the application of the
232
FRCM system allowed to obtain an increase of the strength and of the displacements of the
233
walls. In Fig.9 and in Table 1 the obtained increments in terms of force and displacement are
234
compared.
235
The sliding of the upper region of the wall is shown by the third branch of the curve, where
236
high displacements without increases of force have been recorded. The significant values of
237
the three branches are summarized in Table 1: for each branch the stiffness, the maximum
238
force and the maximum displacements are reported. As it can be observed, similar stiffness
239
for the second branch has been found for each test, the vertical internal constraint provided by
240
the FRCM system allowed to get forces and displacements of the second branches. In general,
241
the effect of the internal axial stresses induced by FRCM system (even in all the vertically
242
unloaded infills) influenced mainly the mortar joints. In fact, all the figures illustrating the
243
failure modes remarked the significant role of the mortar joints and, mainly, of the mortar to
244
brick interfaces on the wall behaviour.
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First branch
Test
URMW
RPMW
LVDT G D F E G D
K1 N/mm 460 808 825 1487 471 819
F1 kN 2,7
3,9
Second branch d1 mm 5,9 3,4 3,3 1,8 8,2 4,7
K2 N/mm 51,3 114,8 114,0 252,3 96,9 187,6
F2 kN 2,9
6,0
d2 mm 9,9 5,2 5,1 2,7 31,5 16,8
Third branch K3 N/mm n.a. n.a. n.a. n.a. 0 0
F3 kN n.a. n.a. n.a. n.a. 6,0
d3 mm n.a. n.a. n.a. n.a. 62,0 31,7
∆(K1-K2) % -89% -86% -86% -83% -79% -77%
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RFMW
F E G D F E
861 1522 273 549 594 1052
2,2
4,5 2,5 8,1 4,0 3,7 2,1
171,0 208,2 83,9 172,2 174,3 264,6
5,3
17,7 13,1 46,0 22,6 22,1 14,2
0 0 0 0 0 0
5,3
29,7 23,8 67,7 33,4 31,6 20,7
-80% -86% -69% -69% -71% -75%
Table 1 Experimental results.
246
The unreinforced wall (URMW) and repaired wall (RPMW) showed a similar initial stiffness,
247
but the application of the strengthening system allowed to attain higher values of lateral force,
248
Fmax, and displacement, dmax. The lower stiffness values recorded during the reinforced wall
249
(RFMW) test are due to the natural variability of masonry performance.
250
The damage of the unreinforced wall (URMW) is expected to influence (i.e. to reduce slope
251
of) the first branch of the repaired wall (RPMW), but the contribution of joints repointing
252
with the mortar of the matrix and the FRCM system allowed to recover the same stiffness
253
recorded for the first branch of unreinforced wall (Table 1). For this reason, it is expected that
254
the reinforced wall (RFMW) would have a higher stiffness, because there is no initial
255
reduction of stiffness due to pre-damage; however, experimental evidences found that the
256
initial stiffness of RFMW was lower than the one of URMW. In addition, it is expected that
257
the FRCM system does not reduce the initial stiffness of the wall. Thus, this reduced stiffness
258
of the RFMW and both URMW and RPMW reasonably can be justified by the previously
259
mentioned natural variability of the wall response. Indeed, the activation of the FRCM system
260
is expected to occur mainly after exceeding the unreinforced masonry wall capacity, i.e. in
261
correspondence of the second branch of the mechanical response of the wall.
262
Although different displacements and behaviours have been observed for the three types of
263
tests at failure it can be in any case declared that the strengthening system allowed to obtain a
264
ductile response compared to the unreinforced wall.
265
In Fig. 10 the displacements recorded on line E – D (vertical line at 740 mm from the lateral
266
boundary) and on line F – G (on the vertical line of the point load) were compared. In
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ACCEPTED MANUSCRIPT particular, in Fig. 10 (a) the displacement at the failure of the tests are compared,
268
demonstrating that the repaired (RPMW) and reinforced (RFMW) walls show the same
269
deformed shape, i.e. the double curvature caused by the lateral constraints determined the
270
lower displacements of the line D – E. However, the different behaviour of the tests is due to
271
different locations of LVDTs with respect to the position of the main cracks. In the
272
unreinforced wall (URMW) (Fig. 10 (b,c)) the upper part monitored by LVDTs G and D is
273
sliding with respect to the lower part. Conversely, in the case of repaired wall (RPMW), the
274
middle portion connected in the corner with the loaded point and characterized by inclined
275
cracks due to particular constraints layout (Fig. 10 (b)), is subjected to sliding, so that LVDT
276
D shows a similar displacement as LVDT E and much smaller than LVDT G. A similar
277
behaviour in terms of displacements has been showed also by the reinforced wall (RFMW). In
278
fact, it is noted that shear deformability yields mainly to linear displacement profiles, while
279
flexural deformability to more complex displacement profiles, but the vertical profiles are
280
influenced also by rigid rotations due to particular hinge-like constraint evolving with the
281
increase of load.
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2000
2000
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1500
LVDT F
z y
x
height (mm)
AC C
1500
LVDT E
1000
1000 LVDT E
500
URMW RPMW RFMW
LVDT F
500
URMW RPMW RFMW
0
0 0 (a)
LVDT G
LVDT D
height (mm)
LVDT D
20 40 60 80 displacement (mm) (b)
0
20 40 60 80 displacement (mm) (c)
Fig. 10. LVDT displacement: (a) 3d View, (b) Line E-D, (c) Line F-G
ACCEPTED MANUSCRIPT 5
Conclusions
283
Aim of this paper was the evaluation, by means of experimental tests, of the effectiveness of
284
FRCM systems in improving the out-of-plane capacity of a pre-damaged and a new masonry
285
wall. Three different tests on two masonry walls were considered, an unreinforced wall has
286
been tested till the collapse; then, the damaged wall has been repaired and the strengthening
287
system has been applied on rear side of the repaired wall. A second, similar, wall has been
288
tested without pre-damage and directly strengthened by means of FRCM. This allowed to
289
remark the effect of FRCM pre-damaged and new walls. The setup was conceived to provide
290
a biaxial bending coupled with shear, hence a non-uniform complex stress state in the wall.
291
The experimental tests showed a progressive stiffness reduction of the unreinforced masonry
292
wall demonstrating the progressive damage of the joints, the cracks localized in the mortar,
293
the other elements of the wall, i.e. the bricks, remained substantially in their elastic state. A
294
simple restoration of the most cracked joints, intentionally removed and filled with the same
295
mortar of the FRCM matrix, allowed to restore the stiffness of the repaired masonry wall. The
296
axial stresses induced by the strengthening system improved the mechanical characteristics of
297
the interface between clay brick and mortar joints, as they were found to be the elements
298
governing the failure mode. A trilinear curve force vs displacement describes the typical
299
behaviour of the repaired and reinforced walls, where the variation of the stiffness between
300
the first and second branch represents the partition of the wall. Even if the two walls were
301
built as practically identical, natural variability was recorded on the initial stiffness of the two
302
walls. The former wall, tested in unreinforced configuration, had a stiffness higher than the
303
latter, reinforced without any pre-damage. Tests at failure were monotonic and FRCM was
304
applied on the tensile side only; in real applications the retrofit system can be applied to both
305
sides, but it can be considered effective on the tensile side only, while in compression it can
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ACCEPTED MANUSCRIPT be assumed to be not effective due to its slenderness and susceptibility to buckling in
307
compression. To prevent buckling, mechanical connectors can be applied improving the
308
capacity in tension too, but it is not considered yet in this phase of the experimental program.
309
In conclusion, the FRCM system proved to be able to almost double the out-of-plane strength
310
of the wall and its lateral displacement capacity despite the complex shear coupled to bending
311
biaxial stress state, not impacting significantly on mass and stiffness of the wall. It can be
312
remarked that the strengthening system significantly changed the failure mechanism of the
313
wall, due to the complex state of stress arising because of the double bending due to the
314
specific considered loading condition that could be very close to real situations.
315
ACKNOWLEDGEMENTS
316
The authors would like to thank Giovanbattista Borretti and Antimo Fiorillo for their support
317
in test execution.
318
This research has been possible thanks to the financial support of the ReLUIS project from
319
the Italian Department of the Civil Protection.
320
The FRCM strengthening of the walls was supported by MAPEI Spa., Milan, Italy.
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ACCEPTED MANUSCRIPT REFERENCES
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[10]
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