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Retrofitting damaged unreinforced masonry using external shear strengthening techniques
Journal Pre-proof Retrofitting damaged unreinforced masonry using external shear strengthening techniques Enea Mustafaraj, Yavuz Yardim PII:
S2352-7102(19)30746-6
DOI:
https://doi.org/10.1016/j.jobe.2019.100913
Reference:
JOBE 100913
To appear in:
Journal of Building Engineering
Received Date: 7 May 2019 Revised Date:
1 August 2019
Accepted Date: 2 August 2019
Please cite this article as: E. Mustafaraj, Y. Yardim, Retrofitting damaged unreinforced masonry using external shear strengthening techniques, Journal of Building Engineering (2019), doi: https:// doi.org/10.1016/j.jobe.2019.100913. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
RETROFITTING DAMAGED UNREINFORCED MASONRY USING EXTERNAL SHEAR STRENGTHENING TECHNIQUES Enea Mustafaraj1*, Yavuz Yardim2 1
Department of Civil Engineering, Epoka University, Rr. Tirane-Rinas, Km12, 1039, Vore, Albania
*corresponding author: [email protected] 2
School of Engineering, The University of Edinburgh, Edinburgh EH9 3FB, UK
ABSTRACT The in-plane shear behavior of unreinforced masonry panels retrofitted with three different techniques was investigated, and in particular the effectiveness of reinforcing methods to restrain the diagonal cracking failure mode was studied. Fifteen standard sized panels of 1.2 x 1.2 x 0.25 m were subjected to diagonal compression testing and then repaired using polypropylene fiber reinforced mortar, ferrocement jacketing, or carbon fiber reinforced polymer (CFRP) wraps applied externally on both sides. The panels were made of two different types of mortar; type N representing modern masonry buildings and type O mortars for older buildings of the 1950s and earlier. Several parameters such as mode of failure, shear stress-strain behavior, shear strength, shear modulus, elastic modulus, and ultimate drift were determined. At the end of 30 diagonal compression tests, plain panels exhibited sudden brittle failure mainly characterized by step-like cracks along the diagonal, whereas retrofitted panels had a more ductile behavior, with an increment of shear strength by 161-454% and of ultimate drift by 863-1486%.
Keywords: diagonal compression test; retrofitting; shear strengthening; ferrocement jacketing; damaged masonry;
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1. Introduction Masonry is one of the most used construction types in Southern Europe and Mediterranean basin. These constructions built of stone or brick masonry load bearing walls were designed to resist only gravitational loads, or even worse, have not been designed at all, but simply realized by the rules of common practice [1]. Seismic activity in this region is characterized by medium-to-high intensity earthquakes, some of which have occurred in the last decades in Italy (Ischia Island in 2017, Umbria Marche in 2016 and 1997 and L’Aquila in 2009 and 2017) and Greece (Kos and Lesbos in 2017, Crete in 2009 and Athens in 1999) and have pointed out the vulnerability of masonry structures against this phenomenon. During seismic shaking, load bearing walls are mainly subjected to two types of possible failure mechanisms such as in-plane shear and out-of-plane bending [2]. The out-of-plane failure can be restrained by providing externally strengthening elements such as proper wall-diaphragm connection that supplies adequate stiffness [3]. It is the in-plane capacity to safely transfer lateral forces from the load bearing walls to foundations that governs the overall seismic performance of the unreinforced masonry (URM) buildings [4]. The principal in-plane mechanisms are generally characterized by the following failure modes: i) shear failure, ii) sliding failure, iii) rocking failure, iv) toe crushing failure. Among them, shear and sliding failure are the most common modes of failure observed in URM buildings. [5]. The rocking failure is the most ductile failure and the least harmful; sliding shear failure is the least frequent, whereas the diagonal shear failure is the worst type of wall failure, since it is very brittle and sudden. Diagonal shear failure could be observed after small or moderate magnitude earthquakes. The level of severity depends upon many factors such as: soil condition, natural frequency of the building, brick types, wall thickness, diaphragm slab rigidity, mortar quality, age and architectural features of buildings. In time, after every low and moderate seismic event, long-term settlements [6], temperature variations or other cyclic loads, masonry walls lose some of their stiffness. It can be observed mostly as different-sized diagonal cracks on the walls. In most of the cases, these cracks are covered with plaster to avoid minor consequences such as aesthetics, water leakage and thermal effects. However, the load bearing walls still remain vulnerable against these actions and there is a need to assess mechanical properties to predict their behavior. In the absence of real-scale experiments and dynamic loading, some of the most important parameters of URM can be simulated experimentally using two types of tests: diagonal compression test, ASTM E519-07 [7] or the shear compression test. The first one is used by many researchers [1-2, 4, 8-34] as the strength values obtained by diagonal compression test tend to be more conservative than shearcompression test [1].
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The structural performance of stone masonry has been subject of many researchers [1, 4, 8-11, 20-23, 29, 32-33]. Calderini et al., 2010, proposed a simplified procedure for prediction of the diagonal cracking failure mode in masonry piers by means of diagonal compression tests. To assess the reliability of this procedure, non-linear numerical simulations were performed [29]. Milosevic et al., 2013, studied structural behavior of newly built rubble stone masonry simulating old buildings of Portugal [32]. The same type of masonry was investigated by Borri et al., 2015, where rubble stone masonry panels were subjected to monotonic diagonalcompression and shear-compression tests. The results emphasized the selection of the testing method plays a crucial role in determining the correct values of these parameters [23]. Investigation of structural behavior of brick masonry panels has been performed through several experimental campaigns during the last decades [2, 12-15, 17-18, 24-28, 30, 34]. Ismail et al., 2011, investigated the in-plane shear behavior of URM wallets and suggested different schemes of application of twisted steel bars to restrain diagonal cracking failure mode [14]. Kalali and Kabir, 2012, proposed various configurations of GFRP aiming at improvement of mechanical properties of masonry of old structures in Iran [2]. The need to improve structural performance of URM structures has led to development of several strengthening techniques, as the so-called traditional ones, which consist of applying the reinforcement in form of: (i) filling cracks by grouting; ii) stitching of large cracks with metallic or brick elements; iii) external or internal post-tensioning with steel ties; iv) shotcrete jacketing; v) ferrocement and vi) center core etc. [2, 29]. Successful application of ferrocement jacketing technique has reported in [34, 37-42,] where it was seen that ferrocement provided considerable increase in ductility, improvement of crack resistance, increased stiffness, load carrying capacity as well as increase of in-plane resistance [18, 24, 34, 42]. Traditional techniques, despite their success, have some limitations such as: time consuming, reduction of available space, occupancy disturbance and affecting the aesthetics of the existing wall. To overcome these drawbacks, modern strengthening techniques were developed, which mainly consist of application of new innovative materials inside the plaster mix, or as attachments to the existing load bearing walls. Textile reinforced mortar (TRM) is a technique where the composite action is achieved through the mechanical interlock textile grids made of long fiber roving of carbon, glass or aramid arranged in two orthogonal directions externally embedded in cement or lime-based mortars instead of polymer resins [26]. Fiber reinforced polymer (FRP) made of carbon (CFRP), glass (GFRP) or aramid (AFRP) fibers bonded together in an inorganic polymeric matrix such as putty fillers, saturates and adhesives like epoxy is another very useful technique that offers high strength and stiffness in the direction of the fibers, immunity to
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corrosion, low weight, availability in various forms as laminates, fabrics and tendons of unlimited lengths, exceptional durability in many environments and cost effectiveness [27]. Most of the research found in literature was focused on analyzing either plain URM, or strengthened ones, without focusing on retrofitting of pre-cracked specimens. There were found only a few studies where damaged masonry had been repaired with basalt fiber reinforced polymer (BFRP) [26] and carbon fiber reinforced polymer (CFRP) [27]. In both cases those techniques were proven to be effective. In this paper, it is presented the usage of traditional (ferrocement jacketing) and modern (polypropylene reinforced plaster coating, and carbon fiber reinforced polymer) strengthening techniques for repair of damaged masonry wall panels made of two types of mortar.
2. Materials and Methods The main aim of the testing program was to investigate the in-plane capacity of URM panels and the enhancement of their mechanical parameters by using three retrofitting techniques (polypropylene reinforced plaster, ferrocement jacketing and carbon fiber reinforced strengthening). Fifteen unreinforced panels (three specimens for every case) were subjected to diagonal compression, and after failure, were repaired using the abovementioned techniques. In order to be more representative, two different types of mortar have been considered, ASTM type N and type O, replicating the mortars used in existing new buildings and existing old buildings, respectively. The testing procedures are the ones defined in American Society for Testing and Materials (ASTM) where are defined all the steps to be followed. These standards have been used by many researchers who have experimented with unreinforced clay brick masonry all over the world [2, 4,6,13-15,17-18, 24-31, 33-34]. The panels built from Type N mortar were grouped in Series 1 (S1), (W1-W6), and the panels built from Type O mortar are grouped in Series 2 (S2) (W7-W15). The solid clay bricks used for experiments were manufactured on site using the quarries of the clay nearby a 70-year-old factory in the city of Fier, Albania, were fired at the same time, and have similar to identical characteristics. For this set of tests, about 1700 brick were used. The remaining materials were obtained from Fushe-Kruje, Albania, a place well-known for cement and lime production. Cement was selected CEM II/B-L 32.5 R, suitable for lower water demand and improved workability, delivered in 50 kg bags. The full testing configuration is seen in Table 1, where the naming of the specimens is done in the following manner: “W” is designated for the standard sized wall panel and “R” represents the initially cracked and repaired wall panel, whereas FC, PP and CFRP represent plastering method of the specimens with 4
ferrocement, polypropylene and carbon fiber reinforced polymers, respectively. For example, W7(R)FC represents a pre-cracked panel, more specifically, W7, which is then repaired with ferrocement jacketing. The selected strengthening techniques were chosen the ones which are easily available and were selected from the extensive literature survey that other researchers had obtained satisfactory results. Another important consideration related to strengthening is that in all cases, all the reinforcing layers were applied on both sides. Single-sided strengthening was proven to be inefficient and would not provide satisfactory results [1, 42]. Determination of bricks’ [43] and mortar’s [44] mechanical properties as well as masonry prism [45] properties was of a fundamental importance before testing the wall panels in diagonal compression.
2.1 Unreinforced panels All the wall panels were built in the Civil Engineering Laboratory at Epoka University by experienced masons, using two-leaf, English bond (the prevalent bond of URM buildings in the world and Albania) with new clay bricks with typical nominal dimensions of 243 mm x 119 mm x 57 mm (Figure 1). For S1 panels, 15-mm-thick mortar joints made of hydraulic cement mortar with a volumetric mix ratio of cement: lime: sand, 1:1:6 (Type N) were used. On the other hand, for S2 panels, the hydraulic cement mortar had a volumetric mix ratio of cement: lime: sand, 1:2:9 (Type O). The panels were left to cure for 28 days. Prior to testing, material’s mechanical characteristics were determined. As it is seen in Table 2, the average compressive strength of the bricks, fb, was 24.03 MPa, and the average compressive strength of mortars after 28 days (as of ASTM C109), fj, was 5.678 MPa (Type N) and 2.816 MPa (Type O).
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Table 1. Specimen and types of reinforcement.
Series 1 (Type "N" mortar)
Specimen type
Wall Panel
Reinforcement type
W1
-
W2
-
W3
-
W4
-
W5
-
W6
-
W1(R)PP
repaired with polypropylene
W2(R)PP W3(R)PP W4(R)FC
repaired with ferrocement
W5(R)FC
Series 2 (Type "O" mortar)
W6(R)FC
W7
-
W8
-
W9
-
W10
-
W11
-
W12
-
W13
-
W14
-
W15
-
W7(R)PP
repaired with polypropylene
W8(R)PP W9(R)PP W10(R)FC
repaired with ferrocement
W11(R)FC W12(R)FC W13(R)CFRP
repaired with carbon FRP
W14(R)CFRP W15(R)CFRP
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Figure 1. Construction process of plain walls.
Table 2. Mechanical properties of masonry constituents. Specimen name
Compressive Strength (MPa)
CoV(%) Flexural Strength (MPa)
CoV(%)
brick
24.03
8.5
4.53
10.78
mortar N
5.68
4.66
0.551
3.65
mortar O masonry prism
2.82 10.68
6.93 12.45
0.27 -
7.34 -
2.2 Repaired panels All the fifteen unreinforced panels, after testing were repaired using three different strengthening techniques as follows:
- Polypropylene fiber reinforced mortar Repair “(R)PP”: consists on application of a 25-mm thick layer of fiber reinforced mortar on both sides of the cracked wall (Figure 2). The mortar mix is composed of sand and cement ratio of 1:1 and adding 1.5% polypropylene fibers in volume and a water/cement ratio of 0.5. The average compressive and flexural strength of this type of mix were 45 MPa and 6.3 MPa, respectively. Generally, the fibers improve cracking and shear capacity, as well as toughness of the wall. However, they do not have a considerable effect of the compressive strength of the matrix (mortar) [46]. The fibers’ technical specifications, obtained from the manufacturer, are summarized in Table 3. Preparation of the mix 7
consists on dry mixing the fibers with the sand and the cement, and addition of the water at the end, producing a plater mix with medium workability (Figure 2).
Table 3. Technical specifications of polypropylene fibers. Chemical base Specific gravity Fibre length Fibre diameter Melt point Ignition point Thermal conductivity Electrical conductivity Specific surface area of fibre Acid resistance Alkali resistance Tensile strength Modulus of elasticity
100% polypropylene fibre 0.91g / cm³ 12mm 18 micron‐nominal 160°C 365°C Low Low 250m² / kg High 100% 300 ‐ 400 N / mm² ~ 4000 N / mm²
Figure 2. Plastering process with polypropylene fibers (PP). - Ferrocement jacketing Repair “(R)FC”: consists of attaching a double-layered galvanized steel mesh (with the same dimensions as the panel) on both sides of the cracked wall, embedded in an inorganic matrix of mortar coating of 25-mm thickness (Figure 3). The technical specifications, obtained by the manufacturer, of the mesh are presented in Table 4. The repair of the cracked panel is done by attaching an extra layer along the crack diagonals on both sides. Its main function is to restrain crack propagation and “stitch” the diagonal crack, in order to make the panel behave as a whole. The mesh is fixed using anchors (threaded bolts of diameter 8 mm and length 70 mm with washers, mounted on previously drilled holes, having 10-mm wall plugs on the bricks every 300 mm). The spacing of the connections was slightly changed depending on the brick arrangements, ensuring that the connection was
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done on the brick and not on the mortar joint. The process of mounting the mesh on the faces of the wall was done carefully and a 5-10 mm allowance between mesh and the bricks for plaster mortar. The mortar mix is prepared using cement: sand 1:4, by volume and water/cement ratio of 0.4. The average compressive and flexural strength of the mortar mix were 18.24 MPa and 2.63 MPa, respectively.
Figure 3. Retrofitting with ferrocement jacketing. Table 4. Technical specifications for ferrocement mesh. Mesh type Mesh size (mm) Diameter (mm) Weight (kg/m2) Modulus of Elasticity, E (GPa) Yield strength, σy Ultimate strength, σu
galvanized welded 12 x 12 1 0.3 170 200 550
- Carbon fiber reinforced polymer (CFRP) wraps Repair “(R)CFRP”: consists of application of a 300mm-wide carbon reinforced polymeric wrap which is attached to a previously smoothly grinded surface using epoxy along the non-compressed diagonal (Figure 4). In order to improve connection, glass fiber anchorages are applied every 350 mm on previously drilled holes passing through both sides of the wall panel. Installation of CFRP is done after drilling the holes for anchorage and smoothing the diagonal surface, a layer of epoxy is applied on the wall diagonal using Sikadur 330. Then the CFRP wrap is attached to the wall specimens and another layer of epoxy is applied over the wrap ensuring that the anchorages are properly attached. Additionally, parallel to the main wrap, smaller sized wraps are bonded perpendicular to the cracked diagonal, “stitching” the main diagonal crack. The technical specifications, obtained from the manufacturer, of CFRP and epoxy are presented in Table 5.
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Table 5. Technical specification for CFRP and epoxy. Fiber Type
Mid strength carbon fibers.
Areal Weight Fabric Design Thickness Fiber Density Dry Fiber Properties Tensile strength Tensile E-modulus
230 g/m2 + 10 g/m2 0.131 mm (based on fiber content). 1.76 g/cm3
Laminate Properties Laminate thickness: Ultimate load: Tensile E-modulus:
4300 N/mm2 (nominal). 238000 N/mm2 (nominal).
1.0 mm per layer (impregnated with Sikadur®330). 350 kN/m width per layer (at typical laminate thickness of 1.0 mm). 28.0 kN/mm2 (based on typical laminate thickness of 1.0 mm).
Adhesive material Tensile Strength
2-Part Epoxy Impregnation Resin 30 N/mm2 (7 days at +23°C)
Tensile elastic modulus
4500 N/mm2 (7 days at +23°C)
Figure 4. Application of CFRP wrap.
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2.3 Test procedure All panels were built in laboratory and were tested in their own place. The system was designed in such a way that no disturbance would be caused to the walls from the transportation. ASTM E 519-07 [7] is a test method used to determine the diagonal tensile or shear strength of 1.2 by 1.2 m masonry assemblages by loading them in compression along one diagonal, thus causing a diagonal tension failure with the specimen splitting apart parallel to the direction of load. According to this procedure, the test should be carried out on at least three like specimens constructed with the same size and type of masonry units, mortar, and workmanship. It requires rotation of the tested specimen by 45° and vertical loading along one of the wall’s diagonals. However, due to low masonry bond strength of the wall, as well as the risk of disturbing the overall state of stresses, the test set-up was modified such that the wall specimen remained vertical on its original position and the loading mechanism was rotated as in the Figure 5. The movable test set-up consists of two loading shoes (with a loading area each of 330 mm x 250 mm) placed on two diagonally opposite corners of the panel connected by four high strength steel rods positioned along the compressed diagonal. The 50-tonne-capacity hydraulic jack was incorporated between the top loading shoe and a metallic plate connected to the steel rods, which when loaded, developed tension forces on the four steel rods connecting the loading shoes, compressing the wall diagonally, providing the desired failure mode; diagonal cracking and/or bed joint sliding failure. The applied load was gradually increased until failure. Deformations of the wall specimen (compression and elongation of diagonals) were recorded by two diagonally positioned displacement gauges attached on each side wall panel over a gauge length of 1000 mm that were oriented parallel and perpendicular to the loading direction. The calculation procedure is as follows:
=
.
(1)
where: Ss – shear stress (MPa); P – load exerted along the compression diagonal (N); An – net area of the specimen (mm2); =
∙
(2)
where: w – width of specimen (mm); h – height of specimen (mm); t – total thickness of specimen (mm); n percent of the gross area of the unit that is solid, expressed as a decimal. =
∆ ∆
(3)
where: γ - shear strain (mm/mm); ∆V– vertical shortening(mm); ∆H – horizontal extension; g – vertical gage length;
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=
(4)
where: G - modulus of rigidity, MPa. The shear modulus of the panel was calculated using the secant modulus of 0.05 and 0.75 of the stress-strain response curve. The stiffness of a wall specimen can be quantified by the Modulus of Elasticity, E, which is related to shear modulus by Equation 5, where = 0.25 adopted by [47]:
= 2" ⋅ (1 + )
(5)
Ductility of the tested wall panels, a drift ratio was defined as: () =
∆) *
(6)
where ∆+, is the diagonal displacement corresponding to the ultimate strength and H is the height of the wall panel.
Figure 5. Diagonal compression test set-up.
3. Results and discussion At the end of the testing diagonal compression testing program structural performance of the panels and the effectiveness of the retrofitting techniques was investigated; in particular the in-plane behavior, diagonal 12
shear cracking and/or bed joint sliding mode of failure, shear strength, elastic modulus, shear modulus and deformation expressed as drift. The diagonal compression testing as of ASTM E 519-07 [7] continued until the wall panels failed; after diagonal cracking and sudden drop of the ultimate load (it reached to zero up to a few tones). From the experimental results it was seen that all specimens presented a similar failure mode, mainly characterized by a step-like crack along one of the diagonals. However, crack propagation, maximum deformation as well as ultimate load carrying capacity of the panel was observed to be closely dependent upon the mortar type and retrofitting technique. In all the cases, these methods were found to be effective.
3.1 Mode of failure The plain panels of S1 and S2 failed in a similar way; cracking along the compressed diagonal, predominantly through the mortar joints in a diagonal step-like pattern (Figure 6, 7). However, in W8 and W15, sliding along the mortar bed joints, following by diagonally extended cracks was observed (Figure 7).
Figure 6. Failure mode of Plain walls of S1.
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Figure 7. Failure mode of Plain walls of S2.
In panels repaired with polypropylene fibers, no visible cracks or sign of failure was observed until 85% of the ultimate resistance was reached. They exhibited a typical diagonal tension failure; a crack along the compressed diagonal, followed in some cases with some smaller parallel cracks (Figure 8). The failure was sudden and apart from cracking, debonding of the reinforcing plaster layer was observed.
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Figure 8. Failure mode of polypropylene reinforced mortar repaired wall panels.
The failure of repaired panels with ferrocement jacketing was more ductile. The first visible crack was observed at 60% of the ultimate load and their behavior remained elastic until 80% of the ultimate load, followed by multiple hair-like cracks on the jacketing layer. The failure the panels occurred after the ferrocement layer yielded and when thicker cracks were developed. It was accompanied by debonding of the reinforcement layer due to the failure of mechanical anchorage, when bolts lost connection with the bricks (Figure 9).
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Figure 9. Failure mode of ferrocement reinforced mortar repaired wall panels of Series 1. Although repair of damaged walls was done locally (only around the diagonal cracks), the overall behavior of the CFRP retrofitted panels was similar to the other repaired walls. Their behavior was linear until 90% of the ultimate load. In some places near the main diagonal, loss of adherence of the wrap with the brick caused debonding of CFRP that became loose before failure. The failure occurred along the initially cracked location just after rupture of CFRP wrap (Figure 10). It was anticipated by a loud noise of breakage of the fibers. On the other hand, glass fiber anchorage remained intact. Rupture of the CFRP is a good indicator that the mechanical properties of the wraps were fully utilized.
Figure 10. Failure mode of CFRP reinforced mortar repaired wall panels of Series 1.
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3.2 Shear stress-strain response The shear stress-strain response of all specimens is presented in Figure 11. The curve starts with a steep slope increasing linearly, then after cracks become visible, the plastic phase (almost horizontal) with degraded stiffness can be observed. The plain wall panels of both series are very brittle, and the stress-strain response is very short. This similar behavior was also observed in other studies [8,11, 14, 22, 30]. The curves are plotted to various scales of maximum strain, , , in order to give a better representation of the stress-strain response for all the considered cases. On the other hand, in Figure 12, it is presented the summary of all tested panels of both series with a fixed maximum strain of ,-./ = 0.015. For polypropylene repaired panels of S1, in W1, little increase is seen on both shear stress and shear strain. In W2 both stress and strain have a considerable increase, whereas in W3, only deformation capacity is improved, whereas the shear stress has remained almost the same. This fact is closely related to the failure mode of the unreinforced panel. On the other hand, repaired panels of S2, there is a considerable improvement in all the specimens (W7, W8, W9) for both shear stress and shear strain. For ferrocement jacketing repaired panels and CFRP repaired panels increase of shear stress and shear strain is observed in all the specimens.
3.3 Shear Strength, Stiffness and Ultimate drift Maximum shear stress values, shear modulus, elastic modulus and ultimate drift are listed in Table 6 and Table 7. The plain panels of S1, exhibited an average shear strength of 0.360 MPa and ultimate drift of 0.085%. On the other hand, for plain panels of S2, as expected the shear resistance was lower, 0.160 MPa, but they happen to be more ductile with an ultimate drift of 0.35%. Variation in the mechanical properties is attributed to the non-homogeneity of unreinforced masonry. From the results, it is observed that all the three repair methods were effective. In S1, shear resistance was increased from 110% (in W1) up to 207% in (W6). The biggest effect was observed in the ultimate drift where it was improved from 316% (in W1) to 1486% (in W5). From all the repaired panels of this group, ferrocement jacketing technique achieved the best results in terms of ductility and keeping its integrity at the end stage of the test. In S2, the increase in shear resistance was seen at a range of 250% (in W9) up to 450% (in W10). The ultimate drift had an increase from 195% (in W8) to 1023% (in W10). The results show that the increase in shear resistance is higher in the panels of Series 2, as they are made with a weaker mortar. On the other hand, the increase in ultimate drift is observed in the panels of Series 1, as they were much stiffer and less ductile. 17
Table 6. Summary of mechanical parameters of plain and repaired panels of Series 1. Wall panel W1 W1(R)PP W(R)/W(U) W2 W2(R)PP W(R)/W(U) W3 W3(R)PP W(R)/W(U) W4 W4(R)FC W(R)/W(U) W5 W5(R)FC W(R)/W(U) W6 W6(R)FC W(R)/W(U)
Pmax (kN) 199.280 219.208 139.496 269.028 149.460 179.352 179.352 288.956 119.568 209.244 129.532 269.028
νmax (MPa) 0.470 0.517 1.100 0.329 0.634 1.929 0.352 0.423 1.200 0.423 0.681 1.611 0.282 0.493 1.750 0.305 0.634 2.077
δ (%) 0.024 0.076 3.167 0.093 0.305 3.280 0.083 0.401 4.831 0.082 1.075 13.110 0.150 2.229 14.860 0.078 0.794 10.179
G (MPa) 4434 1249
E (MPa) 11085 3123
4364 2695
10910 6738
1185 572
2963 1430
1170 1616
2925 4040
847 215
2118 538
2544 1422
6360 3555
Pmax -is the ultimate diagonal load; νmax - the maximum shear strength; ( – ultimate drift; G- modulus of rigidity (shear modulus); E – modulus of elasticity, W(R)/W(U)- the ratio of the property of repaired panel over the plain panel (for shear strength and ultimate drift).
Table 7. Summary of mechanical parameters of plain and repaired panels of Series 2. Wall panel
Pmax (kN)
νmax (MPa)
δ (%)
G (MPa)
E (MPa)
W7 W7(R)PP W(R)/W(U) W8 W8(R)PP W(R)/W(U) W9 W9(R)PP W(R)/W(U) W10 W10(R)FC W(R)/W(U) W11 W11(R)FC W(R)/W(U) W12 W12(R)FC W(R)/W(U) W13 W13(R)CFRP W(R)/W(U) W14 W14(R)CFRP W(R)/W(U) W15 W15(R)CFRP W(R)/W(U)
34.874 149.460
0.082 0.352 4.296 0.117 0.446 3.814 0.129 0.329 2.548 0.129 0.587 4.540 0.117 0.470 4.014 0.153 0.634 4.144 0.117 0.447 3.816 0.141 0.470 3.327 0.189 0.611 3.241
0.876 1.820 2.078 0.384 0.750 1.953 0.784 1.630 2.079 0.343 3.510 10.233 0.091 0.785 8.626 0.281 2.514 8.947 0.193 1.673 8.668 0.131 1.177 8.985 0.145 1.034 7.131
355 1008
888 2520
1512 763
3780 1908
474 1175
1185 2938
742 249
1855 623
676 233
1690 583
690 112
1725 280
665 57
1663 143
328 142
820 355
407 220
1018 550
49.820 189.316 54.802 139.496 58.802 249.100 49.820 199.280 64.766 269.028 49.820 189.316 59.784 199.280 79.712 259.064
Pmax -is the ultimate diagonal load; νmax - the maximum shear strength; ( – ultimate drift; G- modulus of rigidity (shear modulus); E – modulus of elasticity, W(R)/W(U)- the ratio of the property of repaired panel over the plain panel (for shear strength and ultimate drift).
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4. Conclusions The in-plane shear behavior of plain and retrofitted pre-cracked solid clay brick masonry panels with three different techniques under diagonal compression loading. The effectiveness of retrofitting techniques to restrain diagonal cracking failure mode was determined. The experimental campaign considered two-leaf English bond un-cracked and pre-cracked masonry repaired with externally applied polypropylene reinforced mortar coating, ferrocement jacketing and CFRP wrap reinforcement. The mechanical parameters such as shear strength, ultimate drift, shear modulus and elastic modulus were determined. The results were compared to their corresponding plain masonry panels. At the end of 30 diagonal compression tests, the following conclusions can be drawn: -
Plain panels of series S1 and S2 exhibit similar failure modes: sudden and brittle with cracking along the compressed diagonal, predominantly through the mortar joints in a diagonal step-like pattern. The shear strength is strongly dependent upon the mortar type (mortar strength) and the values obtained range 0.282 – 0.470 MPa (S1) and 0.082 – 0.189 MPa (S2). This variability in the results is attributed to the heterogeneous characteristics of masonry.
-
All the three techniques showed a considerable improvement in the mechanical parameters when compared to the plain panels. The improvement of shear strength (i.e., ν/ν0) ranged from 110% to 201% for S1 and 255% to 455% for S2, whereas the ultimate drift ranged from 310% to 1480% for S1 and 190% to 1020% for S2.
-
The panels retrofitted with ferrocement jacketing performed the best in terms of both shear strength and deformation capacity. The mesh restrained diagonal cracking and reduced the post-peak drop in load, increasing ductility.
-
All the repair methods were found to be more influential when applied to low strength mortar. It is attributed to the high resistive capacity of the plaster layer. After its failure, overall failure of the panel is observed.
-
Important issues to be considered when applying these techniques are CFRP wraps debonding and the proper anchorage of the ferrocement mesh. The failure of those panels was associated with either debonding of the wrap or the pullout of the mesh. The main problem that resulted when applying ferrocement jacketing was the anchorage of the mesh to the wall panels. When loading, the pullout of the mesh was observed.
19
Acknowledgements The authors would like to thank Epoka University for funding this study, and students Malvino Turku and Denis Saliko for their help during experimental phase.
Figure 11. Stress-strain response of all panels. 20
Figure 12. Stress-strain response summary of S1 and S2. 21
5. References [1] Faella C, Martinelli E, Nigro E, Paciello S. Shear capacity of masonry walls externally strengthened by a cement-based composite material: An experimental campaign. Constr Build Mater 2010; 24:84–93. [2] Kalali A, Kabir MZ. Experimental response of double-wythe masonry panels strengthened with glass fiber reinforced polymers subjected to diagonal compression tests. Eng Struct 2012; 39:24–37. [3] Wood WA. Load distribution of masonry walls subject to concentrated loads over openings. University of Pittsburgh, Pittsburgh, USA, 1998. [4] Gattesco N, Boem I. Experimental and analytical study to evaluate the effectiveness of an in-plane reinforcement for masonry walls using GFRP meshes. Constr Build Mater 2015; 88:94-104. [5] Magenes G, Calvi MG. In-plane seismic response of brick masonry walls. Earthq Eng Struct Dynam 1997; 26(11):1091–1112. [6] Yardim Y, Mustafaraj E. Effects of soil settlement and deformed geometry on a historical structure. Nat. Hazards Earth Syst. Sci. 2015; 15:1051-1059. [7] ASTM. ASTM E 519-07. Standard test method for diagonal tension (shear) in masonry assemblages. USA. ASTM International; 2007. [8] Corradi M, Borri A, Vignoli A. Strengthening techniques tested on masonry structures struck by the Umbria–Marche earthquake of 1997–1998. Constr Build Mater 2002; 16(4):229–239. [9] Corradi M, Borri A, Castori G, Sisti R. Shear strengthening of wall panels through jacketing with cement mortar reinforced by GFRP grids. Composites Part B 2014; 64:33-42. [10] Milosevic J, Gago AS, Lopes M, Bento R. Experimental assessment of shear strength parameters on rubble stone masonry specimens. Constr Build Mater 2013; 47:1372-1380. [11] Borri A, Castori G, Corradi M, Speranzini E. Shear behavior of unreinforced and reinforced masonry panels subjected to in situ diagonal compression tests. Constr Build Mater 2011; 25:4403–4414. [12] Alecci V, Fagone M,Rotunno T, Stefano MD. Shear strength of brick masonry walls assembled with different types of mortar. Constr Build Mater 2013; 40:1038-1045. [13] Dizhur D, Ingham JM. Diagonal tension strength of vintage unreinforced clay brick masonry wall panels. Constr Build Mater 2013; 43:418–427. [14] Ismail N, Petersen RB, Masia MJ, Ingham JM. Diagonal shear behavior of unreinforced masonry wallettes strengthened using twisted steel bars. Constr Build Mater 2011; 25:4386-4393. [15] Lin YW, Wotherspoon L, Scott A, Ingham JM. In-plane strengthening of clay brick unreinforced masonry wallettes. Eng Struct 2014; 66: 57-65. [16] Gattesco N, Dudine A. Effectiveness of a masonry strengthening technique made with a GFRP-meshreinforced mortar coating. Proc 8th International Masonry Conference, Dresden, Germany, 2010. [17] Najafgholipour MA, Maheri MR, Lourenço PB. Capacity interaction in brick masonry under simultaneous in-plane and out-of-plane loads. Constr Build Mater 2013; 38:619–626.
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[18] Yardim Y, Lalaj O. Shear strengthening of unreinforced masonry wall with different fiber reinforced mortar jacketing. Constr Build Mater 2016; 102: 149-154. [19] Gattesco N, Boem I. Experimental and analytical study to evaluate the effectiveness of an in-plane reinforcement for masonry walls using GFRP meshes. Constr Build Mater 2015; 88:94-104. [20] Corradi M, Tedeschi C, Binda L, Borri A. Experimental evaluation of shear and compression strength of masonry wall before and after reinforcement: Deep repointing. Constr Build Mater 2008; 22(4): 463-472. [21] Prota A, Marcari G, Fabbrocino G, Manfredi G, Aldea C. Experimental in-plane behavior of tuff masonry strengthened with cementitious matrix–grid composites. J Compos in Construct 2006; 10(3):223233. [22] Valluzzi MR, Tinazzi D, Modena C. Shear behavior of masonry panels strengthened by FRP laminates. Constr Build Mater 2002; 16:409–416. [23] Borri A, Castori G, Corradi M. Determination of shear strength of masonry panels through different tests. Int J Archit Herit 2015; 9:913–927. [24] Mustafaraj E, Yardim Y. External shear strengthening of unreinforced masonry panels using ferrocement jacketing. In: XVI International Scientific Conference VSU, 9–10. June, Sofia, Bulgaria. 2016. [25] Mustafaraj E, Yardim Y. In-plane shear strengthening of unreinforced masonry walls using GFRP jacketing. Period Polytech-Civ 2018; 62(2):330-336. [26] Zhou D, Lei Z, Wang J. In-plane behavior of seismically damaged masonry walls repaired with external BFRP. Compos Struct 2013; 102:9-19. [27] Santa-Maria H, Alcaino P. Repair of in-plane shear damaged masonry walls with external FRP. Constr Build Mater 2011; 25(3):1172–1180. [28] Kadam SB, Singh Y, Li B. Strengthening of unreinforced masonry using welded wire mesh and microconcrete – Behaviour under in-plane action. Constr Build Mater 2014; 54:247-257. [29] Calderini C, Cattari S, Lagomarsino S. The use of the diagonal compression test to identify the shear mechanical parameters of masonry. Constr Build Mater 2010; 24:677-685. [30] Mahmood H, Ingham JM. Diagonal compression testing of FRP-retrofitted unreinforced clay brick masonry wallettes. J Compos in Construct 2011; 15:810-820. [31] Bui TL, Larbi AS, Reboul N, Ferrier E. Shear behaviour of masonry walls strengthened by external bonded FRP and TRC. Compos Struct 2015; 132:923-932. [32] Milosevic J, Lopes M, Gago AS, Bento R. Testing and modeling the diagonal tension strength of rubble stone masonry panels. Eng Str 2013; 52:581-591. [33] Roca P, Araiza G. Shear response of brick masonry small assemblages strengthened with bonded FRP laminates for in-plane reinforcement. Constr Build Mater 2010; 24:1372-1384. 23
[34] Mustafaraj E, Yardim Y. Usage of ferrocement jacketing for strengthening of damaged unreinforced masonry (URM) walls. In: 3rd International Balkans Conference on Challenges of Civil Engineering. pp. 153–163. 19–21 May, Tirana, Albania. 2016. [35] Papanicolaou C, Triantafillou T, Lekka M. Externally bonded grids as strengthening and seismic retrofitting materials for masonry panels. Constr Build Mater 2001; 25(2):504–514. [36] Triantafillou TC. Strengthening of masonry structures using epoxy-bonded FRP laminates. J Compos Constr 1998; 2(2):96–104. [37] Rosenthal, I. Precast ferrocement columns. J of Ferrocement 1986; 16:273–284. [38] Winokur A, Rosenthal I. Ferrocement in centrally loaded compression elements. J of Ferrocement 1982; 12:357–364. [39] Razvi S, Saatcioglu M. Confinement of reinforced concrete columns with welded wire fabric. ACI Struct J 1989; 86(5):615–623. [40] Mourad S, Shannag MJ. Repair and strengthening of reinforced concrete square columns using ferrocement jackets. Cement Concrete Comp 2012; 34(2):288–294. [41] Kaushik S, Prakash A, Singh A. Inelastic buckling of ferrocement encased columns. In: Ferrocement: Proceedings of the Fifth International Symposium on Ferrocement UMIST, Manchester, 6–9 Sept. 1994. (Nedwell, P. J., Swamy, R. N. (Eds.)). Taylor & Francis, 1994. e-book. [42] Ahmed T, Ali S, Choudhury J. Experimental study of ferrocement as a retrofit material for masonry columns. In: Ferrocement: Proceedings of the Fifth International Symposium on Ferrocement UMIST, Manchester, 6–9 Sept. 1994. (Nedwell, P. J., Swamy, R. N. (Eds.)). Taylor & Francis, 1994. e-book. [43] ASTM. Standard Test Methods for Sampling and Testing Brick and Structural Clay Tile. ASTM C6703a. Pennsylvania (United States): ASTM International; 2003. [44] ASTM. Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or 50-mm Cube Specimens) C 109/ C109M - 16a. Pennsylvania (United States): ASTM International; 2003. [45] ASTM. Standard Test Method for Compressive Strength of Masonry Prisms. C1314-04. Pennsylvania (United States): ASTM International; 2004. [46] Johnston, CD. Fiber-Reinforced Cements and Concretes. Gordon and Breach: Sydney, Australia, 2001. [47] Pande G, Middleton J, Krajl B.Computer Methods in structural masonry. E & FN Spon, London, UK: 1998.
24
•
30 diagonal compression tests (as of ASTM E519-07) of 15 unreinforced and then, repaired damaged masonry panels
•
Application of three different strengthening techniques: ferrocement jacketing, polypropylene reinforced mortar and carbon fiber reinforcement.
•
Testing of 2 different groups of panels; S1 – walls made of type “N” mortar and S2 walls made of type “O” mortar
•
Comparison of improvement in terms of shear strength and ultimate drift
S2352-7102(19)30746-6
DOI:
https://doi.org/10.1016/j.jobe.2019.100913
Reference:
JOBE 100913
To appear in:
Journal of Building Engineering
Received Date: 7 May 2019 Revised Date:
1 August 2019
Accepted Date: 2 August 2019
Please cite this article as: E. Mustafaraj, Y. Yardim, Retrofitting damaged unreinforced masonry using external shear strengthening techniques, Journal of Building Engineering (2019), doi: https:// doi.org/10.1016/j.jobe.2019.100913. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
RETROFITTING DAMAGED UNREINFORCED MASONRY USING EXTERNAL SHEAR STRENGTHENING TECHNIQUES Enea Mustafaraj1*, Yavuz Yardim2 1
Department of Civil Engineering, Epoka University, Rr. Tirane-Rinas, Km12, 1039, Vore, Albania
*corresponding author: [email protected] 2
School of Engineering, The University of Edinburgh, Edinburgh EH9 3FB, UK
ABSTRACT The in-plane shear behavior of unreinforced masonry panels retrofitted with three different techniques was investigated, and in particular the effectiveness of reinforcing methods to restrain the diagonal cracking failure mode was studied. Fifteen standard sized panels of 1.2 x 1.2 x 0.25 m were subjected to diagonal compression testing and then repaired using polypropylene fiber reinforced mortar, ferrocement jacketing, or carbon fiber reinforced polymer (CFRP) wraps applied externally on both sides. The panels were made of two different types of mortar; type N representing modern masonry buildings and type O mortars for older buildings of the 1950s and earlier. Several parameters such as mode of failure, shear stress-strain behavior, shear strength, shear modulus, elastic modulus, and ultimate drift were determined. At the end of 30 diagonal compression tests, plain panels exhibited sudden brittle failure mainly characterized by step-like cracks along the diagonal, whereas retrofitted panels had a more ductile behavior, with an increment of shear strength by 161-454% and of ultimate drift by 863-1486%.
Keywords: diagonal compression test; retrofitting; shear strengthening; ferrocement jacketing; damaged masonry;
1
1. Introduction Masonry is one of the most used construction types in Southern Europe and Mediterranean basin. These constructions built of stone or brick masonry load bearing walls were designed to resist only gravitational loads, or even worse, have not been designed at all, but simply realized by the rules of common practice [1]. Seismic activity in this region is characterized by medium-to-high intensity earthquakes, some of which have occurred in the last decades in Italy (Ischia Island in 2017, Umbria Marche in 2016 and 1997 and L’Aquila in 2009 and 2017) and Greece (Kos and Lesbos in 2017, Crete in 2009 and Athens in 1999) and have pointed out the vulnerability of masonry structures against this phenomenon. During seismic shaking, load bearing walls are mainly subjected to two types of possible failure mechanisms such as in-plane shear and out-of-plane bending [2]. The out-of-plane failure can be restrained by providing externally strengthening elements such as proper wall-diaphragm connection that supplies adequate stiffness [3]. It is the in-plane capacity to safely transfer lateral forces from the load bearing walls to foundations that governs the overall seismic performance of the unreinforced masonry (URM) buildings [4]. The principal in-plane mechanisms are generally characterized by the following failure modes: i) shear failure, ii) sliding failure, iii) rocking failure, iv) toe crushing failure. Among them, shear and sliding failure are the most common modes of failure observed in URM buildings. [5]. The rocking failure is the most ductile failure and the least harmful; sliding shear failure is the least frequent, whereas the diagonal shear failure is the worst type of wall failure, since it is very brittle and sudden. Diagonal shear failure could be observed after small or moderate magnitude earthquakes. The level of severity depends upon many factors such as: soil condition, natural frequency of the building, brick types, wall thickness, diaphragm slab rigidity, mortar quality, age and architectural features of buildings. In time, after every low and moderate seismic event, long-term settlements [6], temperature variations or other cyclic loads, masonry walls lose some of their stiffness. It can be observed mostly as different-sized diagonal cracks on the walls. In most of the cases, these cracks are covered with plaster to avoid minor consequences such as aesthetics, water leakage and thermal effects. However, the load bearing walls still remain vulnerable against these actions and there is a need to assess mechanical properties to predict their behavior. In the absence of real-scale experiments and dynamic loading, some of the most important parameters of URM can be simulated experimentally using two types of tests: diagonal compression test, ASTM E519-07 [7] or the shear compression test. The first one is used by many researchers [1-2, 4, 8-34] as the strength values obtained by diagonal compression test tend to be more conservative than shearcompression test [1].
2
The structural performance of stone masonry has been subject of many researchers [1, 4, 8-11, 20-23, 29, 32-33]. Calderini et al., 2010, proposed a simplified procedure for prediction of the diagonal cracking failure mode in masonry piers by means of diagonal compression tests. To assess the reliability of this procedure, non-linear numerical simulations were performed [29]. Milosevic et al., 2013, studied structural behavior of newly built rubble stone masonry simulating old buildings of Portugal [32]. The same type of masonry was investigated by Borri et al., 2015, where rubble stone masonry panels were subjected to monotonic diagonalcompression and shear-compression tests. The results emphasized the selection of the testing method plays a crucial role in determining the correct values of these parameters [23]. Investigation of structural behavior of brick masonry panels has been performed through several experimental campaigns during the last decades [2, 12-15, 17-18, 24-28, 30, 34]. Ismail et al., 2011, investigated the in-plane shear behavior of URM wallets and suggested different schemes of application of twisted steel bars to restrain diagonal cracking failure mode [14]. Kalali and Kabir, 2012, proposed various configurations of GFRP aiming at improvement of mechanical properties of masonry of old structures in Iran [2]. The need to improve structural performance of URM structures has led to development of several strengthening techniques, as the so-called traditional ones, which consist of applying the reinforcement in form of: (i) filling cracks by grouting; ii) stitching of large cracks with metallic or brick elements; iii) external or internal post-tensioning with steel ties; iv) shotcrete jacketing; v) ferrocement and vi) center core etc. [2, 29]. Successful application of ferrocement jacketing technique has reported in [34, 37-42,] where it was seen that ferrocement provided considerable increase in ductility, improvement of crack resistance, increased stiffness, load carrying capacity as well as increase of in-plane resistance [18, 24, 34, 42]. Traditional techniques, despite their success, have some limitations such as: time consuming, reduction of available space, occupancy disturbance and affecting the aesthetics of the existing wall. To overcome these drawbacks, modern strengthening techniques were developed, which mainly consist of application of new innovative materials inside the plaster mix, or as attachments to the existing load bearing walls. Textile reinforced mortar (TRM) is a technique where the composite action is achieved through the mechanical interlock textile grids made of long fiber roving of carbon, glass or aramid arranged in two orthogonal directions externally embedded in cement or lime-based mortars instead of polymer resins [26]. Fiber reinforced polymer (FRP) made of carbon (CFRP), glass (GFRP) or aramid (AFRP) fibers bonded together in an inorganic polymeric matrix such as putty fillers, saturates and adhesives like epoxy is another very useful technique that offers high strength and stiffness in the direction of the fibers, immunity to
3
corrosion, low weight, availability in various forms as laminates, fabrics and tendons of unlimited lengths, exceptional durability in many environments and cost effectiveness [27]. Most of the research found in literature was focused on analyzing either plain URM, or strengthened ones, without focusing on retrofitting of pre-cracked specimens. There were found only a few studies where damaged masonry had been repaired with basalt fiber reinforced polymer (BFRP) [26] and carbon fiber reinforced polymer (CFRP) [27]. In both cases those techniques were proven to be effective. In this paper, it is presented the usage of traditional (ferrocement jacketing) and modern (polypropylene reinforced plaster coating, and carbon fiber reinforced polymer) strengthening techniques for repair of damaged masonry wall panels made of two types of mortar.
2. Materials and Methods The main aim of the testing program was to investigate the in-plane capacity of URM panels and the enhancement of their mechanical parameters by using three retrofitting techniques (polypropylene reinforced plaster, ferrocement jacketing and carbon fiber reinforced strengthening). Fifteen unreinforced panels (three specimens for every case) were subjected to diagonal compression, and after failure, were repaired using the abovementioned techniques. In order to be more representative, two different types of mortar have been considered, ASTM type N and type O, replicating the mortars used in existing new buildings and existing old buildings, respectively. The testing procedures are the ones defined in American Society for Testing and Materials (ASTM) where are defined all the steps to be followed. These standards have been used by many researchers who have experimented with unreinforced clay brick masonry all over the world [2, 4,6,13-15,17-18, 24-31, 33-34]. The panels built from Type N mortar were grouped in Series 1 (S1), (W1-W6), and the panels built from Type O mortar are grouped in Series 2 (S2) (W7-W15). The solid clay bricks used for experiments were manufactured on site using the quarries of the clay nearby a 70-year-old factory in the city of Fier, Albania, were fired at the same time, and have similar to identical characteristics. For this set of tests, about 1700 brick were used. The remaining materials were obtained from Fushe-Kruje, Albania, a place well-known for cement and lime production. Cement was selected CEM II/B-L 32.5 R, suitable for lower water demand and improved workability, delivered in 50 kg bags. The full testing configuration is seen in Table 1, where the naming of the specimens is done in the following manner: “W” is designated for the standard sized wall panel and “R” represents the initially cracked and repaired wall panel, whereas FC, PP and CFRP represent plastering method of the specimens with 4
ferrocement, polypropylene and carbon fiber reinforced polymers, respectively. For example, W7(R)FC represents a pre-cracked panel, more specifically, W7, which is then repaired with ferrocement jacketing. The selected strengthening techniques were chosen the ones which are easily available and were selected from the extensive literature survey that other researchers had obtained satisfactory results. Another important consideration related to strengthening is that in all cases, all the reinforcing layers were applied on both sides. Single-sided strengthening was proven to be inefficient and would not provide satisfactory results [1, 42]. Determination of bricks’ [43] and mortar’s [44] mechanical properties as well as masonry prism [45] properties was of a fundamental importance before testing the wall panels in diagonal compression.
2.1 Unreinforced panels All the wall panels were built in the Civil Engineering Laboratory at Epoka University by experienced masons, using two-leaf, English bond (the prevalent bond of URM buildings in the world and Albania) with new clay bricks with typical nominal dimensions of 243 mm x 119 mm x 57 mm (Figure 1). For S1 panels, 15-mm-thick mortar joints made of hydraulic cement mortar with a volumetric mix ratio of cement: lime: sand, 1:1:6 (Type N) were used. On the other hand, for S2 panels, the hydraulic cement mortar had a volumetric mix ratio of cement: lime: sand, 1:2:9 (Type O). The panels were left to cure for 28 days. Prior to testing, material’s mechanical characteristics were determined. As it is seen in Table 2, the average compressive strength of the bricks, fb, was 24.03 MPa, and the average compressive strength of mortars after 28 days (as of ASTM C109), fj, was 5.678 MPa (Type N) and 2.816 MPa (Type O).
5
Table 1. Specimen and types of reinforcement.
Series 1 (Type "N" mortar)
Specimen type
Wall Panel
Reinforcement type
W1
-
W2
-
W3
-
W4
-
W5
-
W6
-
W1(R)PP
repaired with polypropylene
W2(R)PP W3(R)PP W4(R)FC
repaired with ferrocement
W5(R)FC
Series 2 (Type "O" mortar)
W6(R)FC
W7
-
W8
-
W9
-
W10
-
W11
-
W12
-
W13
-
W14
-
W15
-
W7(R)PP
repaired with polypropylene
W8(R)PP W9(R)PP W10(R)FC
repaired with ferrocement
W11(R)FC W12(R)FC W13(R)CFRP
repaired with carbon FRP
W14(R)CFRP W15(R)CFRP
6
Figure 1. Construction process of plain walls.
Table 2. Mechanical properties of masonry constituents. Specimen name
Compressive Strength (MPa)
CoV(%) Flexural Strength (MPa)
CoV(%)
brick
24.03
8.5
4.53
10.78
mortar N
5.68
4.66
0.551
3.65
mortar O masonry prism
2.82 10.68
6.93 12.45
0.27 -
7.34 -
2.2 Repaired panels All the fifteen unreinforced panels, after testing were repaired using three different strengthening techniques as follows:
- Polypropylene fiber reinforced mortar Repair “(R)PP”: consists on application of a 25-mm thick layer of fiber reinforced mortar on both sides of the cracked wall (Figure 2). The mortar mix is composed of sand and cement ratio of 1:1 and adding 1.5% polypropylene fibers in volume and a water/cement ratio of 0.5. The average compressive and flexural strength of this type of mix were 45 MPa and 6.3 MPa, respectively. Generally, the fibers improve cracking and shear capacity, as well as toughness of the wall. However, they do not have a considerable effect of the compressive strength of the matrix (mortar) [46]. The fibers’ technical specifications, obtained from the manufacturer, are summarized in Table 3. Preparation of the mix 7
consists on dry mixing the fibers with the sand and the cement, and addition of the water at the end, producing a plater mix with medium workability (Figure 2).
Table 3. Technical specifications of polypropylene fibers. Chemical base Specific gravity Fibre length Fibre diameter Melt point Ignition point Thermal conductivity Electrical conductivity Specific surface area of fibre Acid resistance Alkali resistance Tensile strength Modulus of elasticity
100% polypropylene fibre 0.91g / cm³ 12mm 18 micron‐nominal 160°C 365°C Low Low 250m² / kg High 100% 300 ‐ 400 N / mm² ~ 4000 N / mm²
Figure 2. Plastering process with polypropylene fibers (PP). - Ferrocement jacketing Repair “(R)FC”: consists of attaching a double-layered galvanized steel mesh (with the same dimensions as the panel) on both sides of the cracked wall, embedded in an inorganic matrix of mortar coating of 25-mm thickness (Figure 3). The technical specifications, obtained by the manufacturer, of the mesh are presented in Table 4. The repair of the cracked panel is done by attaching an extra layer along the crack diagonals on both sides. Its main function is to restrain crack propagation and “stitch” the diagonal crack, in order to make the panel behave as a whole. The mesh is fixed using anchors (threaded bolts of diameter 8 mm and length 70 mm with washers, mounted on previously drilled holes, having 10-mm wall plugs on the bricks every 300 mm). The spacing of the connections was slightly changed depending on the brick arrangements, ensuring that the connection was
8
done on the brick and not on the mortar joint. The process of mounting the mesh on the faces of the wall was done carefully and a 5-10 mm allowance between mesh and the bricks for plaster mortar. The mortar mix is prepared using cement: sand 1:4, by volume and water/cement ratio of 0.4. The average compressive and flexural strength of the mortar mix were 18.24 MPa and 2.63 MPa, respectively.
Figure 3. Retrofitting with ferrocement jacketing. Table 4. Technical specifications for ferrocement mesh. Mesh type Mesh size (mm) Diameter (mm) Weight (kg/m2) Modulus of Elasticity, E (GPa) Yield strength, σy Ultimate strength, σu
galvanized welded 12 x 12 1 0.3 170 200 550
- Carbon fiber reinforced polymer (CFRP) wraps Repair “(R)CFRP”: consists of application of a 300mm-wide carbon reinforced polymeric wrap which is attached to a previously smoothly grinded surface using epoxy along the non-compressed diagonal (Figure 4). In order to improve connection, glass fiber anchorages are applied every 350 mm on previously drilled holes passing through both sides of the wall panel. Installation of CFRP is done after drilling the holes for anchorage and smoothing the diagonal surface, a layer of epoxy is applied on the wall diagonal using Sikadur 330. Then the CFRP wrap is attached to the wall specimens and another layer of epoxy is applied over the wrap ensuring that the anchorages are properly attached. Additionally, parallel to the main wrap, smaller sized wraps are bonded perpendicular to the cracked diagonal, “stitching” the main diagonal crack. The technical specifications, obtained from the manufacturer, of CFRP and epoxy are presented in Table 5.
9
Table 5. Technical specification for CFRP and epoxy. Fiber Type
Mid strength carbon fibers.
Areal Weight Fabric Design Thickness Fiber Density Dry Fiber Properties Tensile strength Tensile E-modulus
230 g/m2 + 10 g/m2 0.131 mm (based on fiber content). 1.76 g/cm3
Laminate Properties Laminate thickness: Ultimate load: Tensile E-modulus:
4300 N/mm2 (nominal). 238000 N/mm2 (nominal).
1.0 mm per layer (impregnated with Sikadur®330). 350 kN/m width per layer (at typical laminate thickness of 1.0 mm). 28.0 kN/mm2 (based on typical laminate thickness of 1.0 mm).
Adhesive material Tensile Strength
2-Part Epoxy Impregnation Resin 30 N/mm2 (7 days at +23°C)
Tensile elastic modulus
4500 N/mm2 (7 days at +23°C)
Figure 4. Application of CFRP wrap.
10
2.3 Test procedure All panels were built in laboratory and were tested in their own place. The system was designed in such a way that no disturbance would be caused to the walls from the transportation. ASTM E 519-07 [7] is a test method used to determine the diagonal tensile or shear strength of 1.2 by 1.2 m masonry assemblages by loading them in compression along one diagonal, thus causing a diagonal tension failure with the specimen splitting apart parallel to the direction of load. According to this procedure, the test should be carried out on at least three like specimens constructed with the same size and type of masonry units, mortar, and workmanship. It requires rotation of the tested specimen by 45° and vertical loading along one of the wall’s diagonals. However, due to low masonry bond strength of the wall, as well as the risk of disturbing the overall state of stresses, the test set-up was modified such that the wall specimen remained vertical on its original position and the loading mechanism was rotated as in the Figure 5. The movable test set-up consists of two loading shoes (with a loading area each of 330 mm x 250 mm) placed on two diagonally opposite corners of the panel connected by four high strength steel rods positioned along the compressed diagonal. The 50-tonne-capacity hydraulic jack was incorporated between the top loading shoe and a metallic plate connected to the steel rods, which when loaded, developed tension forces on the four steel rods connecting the loading shoes, compressing the wall diagonally, providing the desired failure mode; diagonal cracking and/or bed joint sliding failure. The applied load was gradually increased until failure. Deformations of the wall specimen (compression and elongation of diagonals) were recorded by two diagonally positioned displacement gauges attached on each side wall panel over a gauge length of 1000 mm that were oriented parallel and perpendicular to the loading direction. The calculation procedure is as follows:
=
.
(1)
where: Ss – shear stress (MPa); P – load exerted along the compression diagonal (N); An – net area of the specimen (mm2); =
∙
(2)
where: w – width of specimen (mm); h – height of specimen (mm); t – total thickness of specimen (mm); n percent of the gross area of the unit that is solid, expressed as a decimal. =
∆ ∆
(3)
where: γ - shear strain (mm/mm); ∆V– vertical shortening(mm); ∆H – horizontal extension; g – vertical gage length;
11
=
(4)
where: G - modulus of rigidity, MPa. The shear modulus of the panel was calculated using the secant modulus of 0.05 and 0.75 of the stress-strain response curve. The stiffness of a wall specimen can be quantified by the Modulus of Elasticity, E, which is related to shear modulus by Equation 5, where = 0.25 adopted by [47]:
= 2" ⋅ (1 + )
(5)
Ductility of the tested wall panels, a drift ratio was defined as: () =
∆) *
(6)
where ∆+, is the diagonal displacement corresponding to the ultimate strength and H is the height of the wall panel.
Figure 5. Diagonal compression test set-up.
3. Results and discussion At the end of the testing diagonal compression testing program structural performance of the panels and the effectiveness of the retrofitting techniques was investigated; in particular the in-plane behavior, diagonal 12
shear cracking and/or bed joint sliding mode of failure, shear strength, elastic modulus, shear modulus and deformation expressed as drift. The diagonal compression testing as of ASTM E 519-07 [7] continued until the wall panels failed; after diagonal cracking and sudden drop of the ultimate load (it reached to zero up to a few tones). From the experimental results it was seen that all specimens presented a similar failure mode, mainly characterized by a step-like crack along one of the diagonals. However, crack propagation, maximum deformation as well as ultimate load carrying capacity of the panel was observed to be closely dependent upon the mortar type and retrofitting technique. In all the cases, these methods were found to be effective.
3.1 Mode of failure The plain panels of S1 and S2 failed in a similar way; cracking along the compressed diagonal, predominantly through the mortar joints in a diagonal step-like pattern (Figure 6, 7). However, in W8 and W15, sliding along the mortar bed joints, following by diagonally extended cracks was observed (Figure 7).
Figure 6. Failure mode of Plain walls of S1.
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Figure 7. Failure mode of Plain walls of S2.
In panels repaired with polypropylene fibers, no visible cracks or sign of failure was observed until 85% of the ultimate resistance was reached. They exhibited a typical diagonal tension failure; a crack along the compressed diagonal, followed in some cases with some smaller parallel cracks (Figure 8). The failure was sudden and apart from cracking, debonding of the reinforcing plaster layer was observed.
14
Figure 8. Failure mode of polypropylene reinforced mortar repaired wall panels.
The failure of repaired panels with ferrocement jacketing was more ductile. The first visible crack was observed at 60% of the ultimate load and their behavior remained elastic until 80% of the ultimate load, followed by multiple hair-like cracks on the jacketing layer. The failure the panels occurred after the ferrocement layer yielded and when thicker cracks were developed. It was accompanied by debonding of the reinforcement layer due to the failure of mechanical anchorage, when bolts lost connection with the bricks (Figure 9).
15
Figure 9. Failure mode of ferrocement reinforced mortar repaired wall panels of Series 1. Although repair of damaged walls was done locally (only around the diagonal cracks), the overall behavior of the CFRP retrofitted panels was similar to the other repaired walls. Their behavior was linear until 90% of the ultimate load. In some places near the main diagonal, loss of adherence of the wrap with the brick caused debonding of CFRP that became loose before failure. The failure occurred along the initially cracked location just after rupture of CFRP wrap (Figure 10). It was anticipated by a loud noise of breakage of the fibers. On the other hand, glass fiber anchorage remained intact. Rupture of the CFRP is a good indicator that the mechanical properties of the wraps were fully utilized.
Figure 10. Failure mode of CFRP reinforced mortar repaired wall panels of Series 1.
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3.2 Shear stress-strain response The shear stress-strain response of all specimens is presented in Figure 11. The curve starts with a steep slope increasing linearly, then after cracks become visible, the plastic phase (almost horizontal) with degraded stiffness can be observed. The plain wall panels of both series are very brittle, and the stress-strain response is very short. This similar behavior was also observed in other studies [8,11, 14, 22, 30]. The curves are plotted to various scales of maximum strain, , , in order to give a better representation of the stress-strain response for all the considered cases. On the other hand, in Figure 12, it is presented the summary of all tested panels of both series with a fixed maximum strain of ,-./ = 0.015. For polypropylene repaired panels of S1, in W1, little increase is seen on both shear stress and shear strain. In W2 both stress and strain have a considerable increase, whereas in W3, only deformation capacity is improved, whereas the shear stress has remained almost the same. This fact is closely related to the failure mode of the unreinforced panel. On the other hand, repaired panels of S2, there is a considerable improvement in all the specimens (W7, W8, W9) for both shear stress and shear strain. For ferrocement jacketing repaired panels and CFRP repaired panels increase of shear stress and shear strain is observed in all the specimens.
3.3 Shear Strength, Stiffness and Ultimate drift Maximum shear stress values, shear modulus, elastic modulus and ultimate drift are listed in Table 6 and Table 7. The plain panels of S1, exhibited an average shear strength of 0.360 MPa and ultimate drift of 0.085%. On the other hand, for plain panels of S2, as expected the shear resistance was lower, 0.160 MPa, but they happen to be more ductile with an ultimate drift of 0.35%. Variation in the mechanical properties is attributed to the non-homogeneity of unreinforced masonry. From the results, it is observed that all the three repair methods were effective. In S1, shear resistance was increased from 110% (in W1) up to 207% in (W6). The biggest effect was observed in the ultimate drift where it was improved from 316% (in W1) to 1486% (in W5). From all the repaired panels of this group, ferrocement jacketing technique achieved the best results in terms of ductility and keeping its integrity at the end stage of the test. In S2, the increase in shear resistance was seen at a range of 250% (in W9) up to 450% (in W10). The ultimate drift had an increase from 195% (in W8) to 1023% (in W10). The results show that the increase in shear resistance is higher in the panels of Series 2, as they are made with a weaker mortar. On the other hand, the increase in ultimate drift is observed in the panels of Series 1, as they were much stiffer and less ductile. 17
Table 6. Summary of mechanical parameters of plain and repaired panels of Series 1. Wall panel W1 W1(R)PP W(R)/W(U) W2 W2(R)PP W(R)/W(U) W3 W3(R)PP W(R)/W(U) W4 W4(R)FC W(R)/W(U) W5 W5(R)FC W(R)/W(U) W6 W6(R)FC W(R)/W(U)
Pmax (kN) 199.280 219.208 139.496 269.028 149.460 179.352 179.352 288.956 119.568 209.244 129.532 269.028
νmax (MPa) 0.470 0.517 1.100 0.329 0.634 1.929 0.352 0.423 1.200 0.423 0.681 1.611 0.282 0.493 1.750 0.305 0.634 2.077
δ (%) 0.024 0.076 3.167 0.093 0.305 3.280 0.083 0.401 4.831 0.082 1.075 13.110 0.150 2.229 14.860 0.078 0.794 10.179
G (MPa) 4434 1249
E (MPa) 11085 3123
4364 2695
10910 6738
1185 572
2963 1430
1170 1616
2925 4040
847 215
2118 538
2544 1422
6360 3555
Pmax -is the ultimate diagonal load; νmax - the maximum shear strength; ( – ultimate drift; G- modulus of rigidity (shear modulus); E – modulus of elasticity, W(R)/W(U)- the ratio of the property of repaired panel over the plain panel (for shear strength and ultimate drift).
Table 7. Summary of mechanical parameters of plain and repaired panels of Series 2. Wall panel
Pmax (kN)
νmax (MPa)
δ (%)
G (MPa)
E (MPa)
W7 W7(R)PP W(R)/W(U) W8 W8(R)PP W(R)/W(U) W9 W9(R)PP W(R)/W(U) W10 W10(R)FC W(R)/W(U) W11 W11(R)FC W(R)/W(U) W12 W12(R)FC W(R)/W(U) W13 W13(R)CFRP W(R)/W(U) W14 W14(R)CFRP W(R)/W(U) W15 W15(R)CFRP W(R)/W(U)
34.874 149.460
0.082 0.352 4.296 0.117 0.446 3.814 0.129 0.329 2.548 0.129 0.587 4.540 0.117 0.470 4.014 0.153 0.634 4.144 0.117 0.447 3.816 0.141 0.470 3.327 0.189 0.611 3.241
0.876 1.820 2.078 0.384 0.750 1.953 0.784 1.630 2.079 0.343 3.510 10.233 0.091 0.785 8.626 0.281 2.514 8.947 0.193 1.673 8.668 0.131 1.177 8.985 0.145 1.034 7.131
355 1008
888 2520
1512 763
3780 1908
474 1175
1185 2938
742 249
1855 623
676 233
1690 583
690 112
1725 280
665 57
1663 143
328 142
820 355
407 220
1018 550
49.820 189.316 54.802 139.496 58.802 249.100 49.820 199.280 64.766 269.028 49.820 189.316 59.784 199.280 79.712 259.064
Pmax -is the ultimate diagonal load; νmax - the maximum shear strength; ( – ultimate drift; G- modulus of rigidity (shear modulus); E – modulus of elasticity, W(R)/W(U)- the ratio of the property of repaired panel over the plain panel (for shear strength and ultimate drift).
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4. Conclusions The in-plane shear behavior of plain and retrofitted pre-cracked solid clay brick masonry panels with three different techniques under diagonal compression loading. The effectiveness of retrofitting techniques to restrain diagonal cracking failure mode was determined. The experimental campaign considered two-leaf English bond un-cracked and pre-cracked masonry repaired with externally applied polypropylene reinforced mortar coating, ferrocement jacketing and CFRP wrap reinforcement. The mechanical parameters such as shear strength, ultimate drift, shear modulus and elastic modulus were determined. The results were compared to their corresponding plain masonry panels. At the end of 30 diagonal compression tests, the following conclusions can be drawn: -
Plain panels of series S1 and S2 exhibit similar failure modes: sudden and brittle with cracking along the compressed diagonal, predominantly through the mortar joints in a diagonal step-like pattern. The shear strength is strongly dependent upon the mortar type (mortar strength) and the values obtained range 0.282 – 0.470 MPa (S1) and 0.082 – 0.189 MPa (S2). This variability in the results is attributed to the heterogeneous characteristics of masonry.
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All the three techniques showed a considerable improvement in the mechanical parameters when compared to the plain panels. The improvement of shear strength (i.e., ν/ν0) ranged from 110% to 201% for S1 and 255% to 455% for S2, whereas the ultimate drift ranged from 310% to 1480% for S1 and 190% to 1020% for S2.
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The panels retrofitted with ferrocement jacketing performed the best in terms of both shear strength and deformation capacity. The mesh restrained diagonal cracking and reduced the post-peak drop in load, increasing ductility.
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All the repair methods were found to be more influential when applied to low strength mortar. It is attributed to the high resistive capacity of the plaster layer. After its failure, overall failure of the panel is observed.
-
Important issues to be considered when applying these techniques are CFRP wraps debonding and the proper anchorage of the ferrocement mesh. The failure of those panels was associated with either debonding of the wrap or the pullout of the mesh. The main problem that resulted when applying ferrocement jacketing was the anchorage of the mesh to the wall panels. When loading, the pullout of the mesh was observed.
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Acknowledgements The authors would like to thank Epoka University for funding this study, and students Malvino Turku and Denis Saliko for their help during experimental phase.
Figure 11. Stress-strain response of all panels. 20
Figure 12. Stress-strain response summary of S1 and S2. 21
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•
30 diagonal compression tests (as of ASTM E519-07) of 15 unreinforced and then, repaired damaged masonry panels
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Application of three different strengthening techniques: ferrocement jacketing, polypropylene reinforced mortar and carbon fiber reinforcement.
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Testing of 2 different groups of panels; S1 – walls made of type “N” mortar and S2 walls made of type “O” mortar
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Comparison of improvement in terms of shear strength and ultimate drift