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Out-of-plane behavior of surface-reinforced masonry walls
Construction and Building Materials 16 (2002) 341–351
Out-of-plane behavior of surface-reinforced masonry walls Sameer Hamousha,*, Mark McGinleya, Paul Mlakarb, Muhammad J. Terroc a
Architectural Engineering Department, North Carolina A&T State University, Greensboro, NC 27411, USA b Construction and Materials Division, Waterways Experiment Station, Vicksburg, MS, USA c Civil Engineering Department, Kuwait University, Safat 1300, Kuwait Received 15 February 2001; received in revised form 12 February 2002; accepted 17 April 2002
Abstract This paper presents the results of an experimental program designed to evaluate the out-of-plane shear strength of masonry wall system; and to evaluate of the influence of the area of externally bonded FRP composites on the shear strength of the system. Eighteen compact masonry wall panels (39=29=80, 900=600=200 mm) were tested for static out-of-plane loads. Nine panels were reinforced by one layer of WEB ‘S-Glass’ fiber-reinforcing system attached to the tension side of the wall, while the remaining nine were reinforced with two layers of composite overlay on the tension side. The influence of the overlay’s embedded length (the distance between the support and the overlay’s end) on the shear strength was also investigated. The variables evaluated included three layout configurations and two reinforcement ratios. Three different distances between the overlay end and the adjacent support were tested, 0, dy4 (d is the block unit thickness) and dy2. Both one and two layers of WEB fibers were used and three specimens were evaluated for each variable. An MTS machine was used to test each panel under four-point load conditions. The failure loads, mid-span deflection, fiber-end slippage and failure modes were recorded. Based on the results of the experimental program, it appears that the out-of-plane shear strength of the concrete masonry wall systems is constant over the range of variables tested. The measured shear strength of the masonry wall specimens evaluated in this program indicates that the code defined shear strengths may not be as conservative as assumed. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Shear strength; Out-of-plane; Concrete masonry; Fiber reinforced composites; Experimental testing
1. Introduction Retrofitting masonry walls with fiber-reinforced composites has been shown to increase the out-of-plane flexural strength of the system and minimize the damage due to failure by allowing for greater ductility of the wall and therefore greater energy absorption. However, recent research performed by Hamoush and McGinley w1x indicated that the out-of-plane failure of unreinforced masonry walls retrofitted by external fiber reinforcement might be controlled by the shear strength of the system at the supports. These test results also indicated that the shear strength at failure was inconsistent for all walls tested. It was concluded by this study that the dowel action of the retrofitting fibers might have been a contributing factor to the measured inconsistency in the shear strength at failure. *Corresponding author. Fax: q1-336-334-7126.
Other researchers such as Marshall et al. w2x have investigated the use of overlaying fiber-reinforced polymer (FRP) composite systems to strengthen un-reinforced masonry (URM) and brick wall panels. They investigated the out-of-plane strength of the retrofitted system. They concluded the FRP systems maintained the structural integrity of the repaired walls after failure. Ehasani et al. w3x and Ehsani et al. w4x conducted an experimental study on three half-scale un-reinforced brick walls retrofitted with graphite (FRP) vertical composite strips. The walls were tested under cyclic out-of-plane loading. They showed that the tested specimens were capable of supporting out-of-plane loads of a magnitude of up to 32 times the weight of the tested wall. At failure, the deflection of each wall was as much as 2% of the wall height. Although both the unrepaired brick walls and the retrofitted walls both exhibited brittle behavior, the two systems were capable of dissipating some energy. The study concluded that the GFRP FRP
0950-0618/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 0 2 . 0 0 0 2 4 - 7
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composite strips are a good system for retrofitting URM walls for lateral loads caused by seismic forces. Hamilton and Dolan w5x performed a series of tests on unreinforced concrete masonry walls strengthened with FRP composites. The composite system was attached to the tension side. The objective of their research work was to develop a general approach to provide design guidelines to strengthen concrete masonry walls with FRP. In their study, retrofitted masonry wall systems were tested for flexure under out-of-plane static loading. The test results indicated that the flexural strength of FRP retrofitted masonry systems may be controlled by either the fracture of the FRP, shear or tension in the masonry, or by delamination of the reinforcing FRP systems. Lee et al. w6x developed an analytical tension-stiffening model for fiber-reinforced polymer (FRP) sheets bonded to reinforced concrete. This model was based on equilibrium and the assumption of a linear relationship between bond stress and slip. They performed tests of both small and large-scale specimens comprised of concrete, CFRP sheets and epoxy. The study concluded that the experimentally measured crack opening deformations matched those predicted by the model with reasonable accuracy. Also, it was concluded that the interfacial slip modulus, for concreteyFRP combinations, was independent of the type of FRP sheet. Triantafillou w7x presented systematic numerical analysis procedures for predicting short term strength of masonry walls strengthened with externally bonded FRP laminates, under monotonic out-of-plane bending, inplane bending and in-plane shear. An experimental program including testing 12 small wall specimens was performed to validate the analysis and showed that strengthening of masonry wall with externally bonded FRP laminates appears to be an effective method for retrofitting. It was also noted that when out-of-plane bending response dominates, the increase in the bending capacity is quite high. Marshall et al. w8x evaluated techniques for seismic retrofitting un-reinforced masonry walls. The research focused on the applicability FRP composite materials systems for strengthening URM walls. Unreinforced masonry walls have shown poor performance in past earthquakes. In their study, new 49=49 double width brick wall panels with FRP composite reinforcing applied to one face were constructed and tested under out-of-plane loading. In addition, the shear performance of different widths and thicknesses of FRP composites applied across the mortar joints of brick triplets (three brick high prisms with the center brick offset by half an inch) were evaluated. It was concluded from the research that the seismic rehabilitation of URM brick walls with FRP composites shows great potential. It is also concluded that the strength of the joints is a function
of the width of the FRP composite overlay. With application of multiple layers of FRP, the shear strength of the mortar joints increased sufficiently to cause the failure to occur in the brick. Bizindavyi and Neale w9x investigated the development length needed to achieve full strength in the connection between concrete and FRP composite overlays. An experimental program and theoretical study were used to evaluate the bond between the concrete and FRP composites. They showed good agreement between their analytical model and measured behavior. They concluded that full development is achievable with a proper selection of the composite system and the surface preparation of the concrete. From the studies presented, it appears that using FRP composites to strengthen URM walls may be an effective way to enhance the wall’s flexural strength. However, the shear strength of the masonry wall system may limit the effectiveness of this retrofit technique. This paper presents the results of an experimental program designed to evaluate the out-of-plane shear strength of masonry wall systems reinforced with externally bonded FRP composites. 2. Testing program Eighteen, 39=29=80 (900=600=200 mm) hollow concrete masonry walls were fabricated under laboratory conditions and tested. Nine walls were reinforced externally by one layer of glass fiber composites on the tension side of the wall, and the remaining nine were similarly reinforced with two layers of external glass fiber reinforcement. Hollow, 80 (200 mm) concrete masonry units, complying with The American Society Of Testing and Materials (ASTM) Specification C90 w10x and ASTM C270 Type S mortar was used to construct the walls. 2.1. Materials Nine batches of mortar type ‘S’ were mixed during wall fabrication. Three standard 20 cubes were made from each of the batches and compression tested as described in ASTM C780 w11x. The average compressive strength of the mortar obtained from testing of the cubes was 1500 psi (10.34 MPa). 2.2. Fibers and bonding adhesives The un-reinforced masonry wall specimens were strengthened with a continuous WEB fabric (primary glass fabric) overlay and a Tyfo S Hi-Clear epoxy matrix. The WEB fibers were attached to the walls using Tyfo S epoxy resin. The Tyfo S epoxy resin consisted of two parts of Tyfo A and B which were
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Table 1 Material properties of the fiber used in the experimental program
2.3. Specimen preparation
Composite materials
WEB and Tyfo Hi-Clear epoxy
Ultimate tension strength Ultimate elongation Elastic modulus Design thickness
60 ksi (414 MPa) 2.0% 4000 ksi (27 580 MPa) 0.014 inch (0.4 mm) per layer
The surfaces of the masonry wall specimens were sand blasted using an air gun to clean and prepare the surface for the FRP overlays. Attention was focused on cleaning the joints and removing excessive mortar from the wall surface. A low-pressure water jet was applied to the sand blasted surface to remove the remaining dust. The water jet was applied at least 24 h prior to application of the composite overlay. Fig. 1a,b show the wall elevation and the cross-section with the actual dimensions. A painting roller was used to apply the epoxy to the prepared surface. The roller was saturated with epoxy, and then applied to the cleaned wall surface. The WEB
mixed in a ratio of 100:42 by volume, as required for the standard Tyfo S products. Two different fiber reinforcement areas were evaluated in the testing program and the mechanical properties of the two composite material configurations are shown in Table 1.
Fig. 1. (a) Wall specimen configuration. (b) Specimen cross-section.
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Fig. 2. Test set-up.
fiber mats were pre-cut to 240=280 (600=700 mm), 240=320 (600=800 mm) and 240=360 (600=900 mm) sizes. The pre-cut fabric mats were placed on the top of the rolled epoxy and then more epoxy was applied to the surface of the fabric. Attention was focused on clearing all bubbles between the fabric and wall surface. During the application of the epoxy, the change of fabric color from white to yellow indicted that saturation had been reached. 2.4. Test set-up Each of the wall specimens was tested in a simply supported beam configuration in an effort to evaluate the out-of-plane shear strength of the retrofitted masonry system. A clear span of 360 (900 mm) was used for all walls. Two point loads, spaced 90 (225 mm) from the supports and at 180 (450 mm) centers, were applied to the specimens through distributing beams and wood bearing strips. Loads were applied monotonically to failure. Fig. 2 shows the test apparatus with a specimen ready for test. Before placing the wall specimens in the supports, the fabric overlay was trimmed with a grinder to ensure that a uniform distance was maintained from the fabric end and the test supports. This trimming was conducted after the bonding epoxy had been cured. 2.5. Testing apparatus A servo-controlled hydraulic MTS 110 kip (490 kN) actuator was used to apply a load to the specimens. The walls were simply supported at the two sides with circular solid pipe supports. A fast setting gypsum was used to level each specimen on the two supporting pipes to insure a uniform distribution of the support loads along the width of the specimen and to prevent any force concentration due to imperfections in wall construction.
2.6. Instrumentation and data acquisition Four LVDTs and a load cell were used to monitor the response of the specimens to the applied transverse loads. Two LVDTs were placed so that they measured the interfacial movement between the masonry blocks and the fiber composite overlay at the end of the walls. The remaining two LVDTs measured the mid-span deflection on each side of the wall specimen. Readings of load, displacements and deflections were recorded by a computerized data acquisition system and stored in an Excel spreadsheet file. 2.7. Testing procedures The testing procedures were performed as described below: 1. A small initial load was applied to insure complete contact of the specimen with the two supports and to allow the gypsum to seat. 2. Once the wall had full contact, the load was released and an initial reading of loads and deflection was recorded. 3. The loading was then re-applied at a steady rate in increments of 500 pounds and held steady for a minimum of 1 min. 4. At each loading increment, the specimen was inspected visually for any distress and initiation of cracks. 3. Test results One of the principal focuses of this investigation was on the load deformation characteristics of the fiber reinforced concrete masonry wall systems. The testing program evaluated six different configurations of wall specimens, each set consisting of three walls. Set 1 consisted of Walls 1, 2 and 3 and investigated walls with two layers of fiber reinforcement overlay extended
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Fig. 3. Load vs. deflection of specimens 1, 2, 3, for two layers with 320=240=0.0140 fiber composites to within dy4 to support.
to a distance of dy4 from supports (d is the depth of the block wall). Set 2 consisted of Walls 4, 5 and 6, and investigated walls with two layers of fiber reinforcement overlay extended to a distance of dy2 from supports. Set 3 consisted of Walls 7, 8 and 9, and investigated walls with two layers of fiber reinforcement overlay extended to the supports. Set 4 consisted of Walls 10, 11 and 12, and investigated walls with one layer of fiber reinforcement overlay extended to a distance of dy4 from supports. Set 5 consisted of Walls 13, 14 and 15, and investigated walls with one layer of fiber reinforcement overlay extended to the supports. Finally, Set 6 consisted of Walls 16, 17 and 18, and investigated walls with one layer of fiber reinforcement overlay extended to a distance of dy2 from supports. The influence of the two experimental variables (reinforcement ratios and fiber length) on the wall failure loads and deformation are shown in Figs. 3–8. Fig. 3 shows the loads vs. the deflection for specimens of Set 1. The load deflection is shown to be linear elastic up to failure of the specimen, for all walls of Set 1, although Wall 2 supported more loads than either Wall 1 or Wall 3. The average failure load for the three walls was 5891 pounds (26.27 kN). It was noted that the amount of slippage at the fiber ends of the specimens was extremely small, and no significant interfacial slip was measured for the stress levels evaluated.
The load deflection curves shown in Fig. 4 (Set 2) are shown to be linear elastic up to failure for Walls 4 and 5. Wall 6 revealed more ductile behavior than either Wall 4 or Wall 5. The average failure load for the three walls was 5934 pounds (26.46 kN). The load deflection curves shown in Fig. 5 (Set 3) exhibit non-linear behavior with a continuously reducing slope. The average failure load for the three walls was 5298 pounds (23.62 kN). Fig. 6 shows the load vs. the deflection for the wall specimens of Set 4. The load deflection of this configuration is shown to be non-linear up to failure. Wall 11 supported more loads than either Walls 10 or 12, and the average failure load for the three walls was 5470 pounds (24.4 kN). The load deflection curves in Fig. 7 (Set 5) show the nonlinear behavior of Walls 13, 14 and 15. Wall 13 revealed more ductile behavior than either Wall 14 or Wall 15. The average failure load for the three walls was 5676 pounds (25.3 kN). The load deflection curves in Fig. 8 (Set 6) show that Walls 16, 17 and 18 exhibited essentially linear behavior up to failure. The average failure load for the three walls was 5888 pounds (26.26 kN). Table 2 summarizes the experimental results for the 18 specimens tested, including the maximum loads and the corresponding average mid-span deflection obtained
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Fig. 4. Load vs. deflection of specimens 4, 5, 6, for two layers with 280=240=0.0140 fiber composites to within dy2 to support.
by averaging the two LVDT reading at each side of each specimen. Table 3 summarizes the failure loads, the calculated shear stress at the measured maximum loads, and the observed mode of failure for all the wall specimens tested. It can be noted that three specimens failed in a typical diagonal shear failure (Type 1 failure, see Fig. 9). The remaining 15 specimens failed by opening of the mortar joint in the shear area, followed by the formation of a diagonal shear crack that extended from the vertical joint to the point of the concentrated load (Type 2 failure, see Fig. 10). Final failure occurred when the block web crack met the interface between the fiber and the masonry blocks and then propagated along the interface to the fiber composite end.
4. Discussion of experimental results A review of Table 2 shows that the average load of walls with 28-inch fiber reinforcement (distance to supportsdy2) is slightly higher than that of walls with 32-inch fiber reinforcement (distance to supportsdy4). Also, walls with 32-inch fiber reinforcement support slightly higher average loads than that with 36-inch fibers (distance to supports0). The load deflection behavior of walls with the shorter fibers varied significantly, for the three walls tested. The variation in behavior can be noted clearly in Fig. 4. Also, although the behavior of all three of the short fiber walls is basically linear elastic up to failure, Wall 6 shows some ductility prior to its failure.
Fig. 5. Load vs. deflection of specimens 7, 8, 9, for two layers with 360=240=0.0140 fiber composites extended to support.
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Fig. 6. Load vs. deflection of specimens 10, 11, 12, for one layer with 320=240=0.0140 fiber composites to within dy4 to support.
Fig. 3 shows that the load deflection curves of walls with longer FRP (fiber closer to the support) are more consistent and exhibit more ductility than that of the shorter overlays. Even greater consistency in behavior occurs with walls with 36-inch FRP overlays (Fig. 5). Also, ductility increases with the longer composite overlays. This behavior suggests that the two-layer overlay and longer embedment lengths allow the wall system to act more as an integrated system an the more uniform fibers dominate the behavior. With shorter overlays (larger distances to the support), the failure is controlled more by the more variable individual strength of the blocks located in the critical stress area. Table 3 shows that the average load of walls with 28inch single layer fiber reinforcement (distance to supportsdy2) is higher than that of walls with 32-inch and 36-inch single layer fiber reinforcement. However, con-
trary to the two layer trends, walls with 36-inch fiber reinforcement (distance to supports0) had a higher average load than those with 32-inch fibers (distance to supportsdy4). It appears that the load deflection behavior of walls with one layer of FRP composite is different than that of the walls with two layers of FRP composites. Walls with shorter single layer overlays exhibit behavior that is very consistent up to failure, for all three walls tested. However, with an increase in the overlay length, the variation in load deflection behavior can be clearly noted, see Fig. 7. This difference may be attributed to a lower contribution of the FRP to the overall behavior of the system with the single overlay layer. With shorter overlays, the strength is controlled basically by the strength of masonry blocks and the FRP overlay does not appear to significantly contribute to the shear strength of the assembly. The wall behavior shown in
Fig. 7. Load vs. deflection of specimens 13, 14, 15, for one layer with 360=240=0.0140 fiber composites extended to support.
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Fig. 8. Load vs. deflection of specimen 16, 17, 18, for one layer with 280=240=0.0140 fiber composites to within dy2 to support.
Figs. 6 and 8 suggest that FRP contribution increases as the length of the overlay increases. It can be noted that when the FRP overlay is extended to the supports, the wall specimens with the two layered overlay systems appear to the behave as a fully integrated system, while the wall specimen with a single layered overlay system does not have as uniform a behavior. In contrast, when shorter FRP overlay systems were used, the two-layered systems appear to contribute
slightly to the wall system behavior, while the one layered system does not show significant contribution to the system shear behavior.
Table 2 Results of the wall specimen tests
Table 3 Results of the shear calculations of the masonry wall systems
Spec. Reinf. Distance of fiber no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Maximum loads
layers to supports
2L 2L 2L 2L 2L 2L 2L 2L 2L 1L 1L 1L 1L 1L 1L 1L 1L 1L
dy4 dy4 dy4 dy2 dy2 dy2 0 0 0 dy4 dy4 dy4 0 0 0 dy2 dy2 dy2
Average deflection at max loads
(lb)
(kN) (in)
(mm)
14802.32 11504.11 9039.535 11204.85 12578.96 11821.66 9269.774 10450.48 12068.97 10605.11 11196.07 11014.12 10148.78 12682.37 11226.07 11600.45 13032.36 10696.28
65.84 51.17 40.21 49.84 55.95 52.59 41.23 46.49 53.69 47.17 49.80 48.99 45.14 56.41 49.94 51.60 57.97 47.58
2.47 2.10 1.75 3.33 2.71 4.49 2.69 3.22 3.53 2.87 3.76 3.25 4.05 2.60 3.05 2.75 3.23 2.76
d ‘wall depth’s7.625 ins194 mm.
0.097 0.0823 0.0688 0.131 0.106 0.177 0.106 0.127 0.139 0.113 0.148 0.128 0.159 0.102 0.120 0.108 0.127 0.109
5. Analytical evaluation The analytical evaluation of the strengthened masonry wall systems was focused on three aspects of behavior, the flexural strength of the composite system, the shear strength of the masonry blocks, and finally the contri-
Spec. no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Maximum shear force (V)
Shear stress
(lb)
(kN)
(psi)
(MPa)
7401 5752 4520 5602 6290 5911 4635 5225 6035 5301 5598 5507 5074 6341 5613 5800 6516 5348
32.92 25.59 20.11 24.92 27.98 26.29 20.62 23.24 26.85 23.58 24.90 24.50 22.57 28.21 24.97 25.80 28.98 23.79
208 162 127 158 177 166 130 147 170 149 158 155 143 178 158 163 183 151
1.43 1.12 0.88 1.09 1.22 1.14 0.90 1.01 1.17 1.03 1.09 1.07 0.99 1.23 1.09 1.12 1.26 1.04
Mode of failure
Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type
1 inchs25.4 mm, 1 pounds4.46 N, 1 psis6.896 kPa.
2 2 1 2 1 1 2 2 2 2 2 2 2 2 2 2 2 2
Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear
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causes compression failure of the masonry blocks was achieved at a loading level, V, of: Vs16 Kips (71.75 kN) for the one layer WEB configuration; and Vs30.5 Kips (136.75 kN), for two layers WEB configuration. An example of calculation of the nominal moment (Mn) of one layer is given below: the thickness of the WEB layers0.014 inch (0.356 mm) Afs(0.014)(24)s0.336 inch2 (217 mm2) Strain equilibrium: ´fs(0.003)(dyx)yx is assumed to be less than the ultimate elongation. The force equilibrium gives as1.02 inch (25.9 mm). Using as1.02 inches, the FRP fails before the concrete crushes. The ultimate force in the FRP is 20.16 Kips. The moment is calculated as Fig. 9. Typical type-1 shear failure.
bution of the overlays on the ultimate shear strength of the system. 5.1. The flexural strength of the composite systems A simplified analytical method was developed to predict the ultimate strength of the fiber reinforced masonry wall systems. The method was based on the following assumptions: (1) linear strain distribution through the full depth of the wall; (2) small deformations; (3) no tensile strength in the masonry blocks, (4) no slip between the fiber reinforced composites and the masonry wall, and (5) plane sections remained plane. The stress–strain relationship of the fiber reinforced composite systems was generally considered to be linear elastic up to failure, while the stress–strain behavior of the masonry block is modeled as and idealized uniform stress block at failure. The ultimate compression strain in the masonry blocks was assumed to be 0.003. From the internal force equilibrium and assuming a linear distribution for the section strains, the flexural strength capacity was determined (see Fig. 11). It should be noted that the compressive strength of the masonry assembly, ( f 9m) was 1200 psi (8.28 MPa), supplier specification based on the plant testing, and the elastic modulus of the FRP composite system was 4000 ksi (27.6 GPa). The ultimate strain of the fibers was assumed to be 0.02, while the ultimate stress is taken as 60 ksi (414 MPa). In the experimental program, each wall was simply supported at the two ends with a clear span of 3 feet (900 mm). The actual width of each specimen was 1.5 feet (450 mm). Based on the above assumptions, the ultimate strength of the composite wall systems that
Mns12.02 Kip feet (53.9 kN m) MnsV=9y12 Vs16 Kips (71.35 kN) It should be noted that the unreinforced walls would be expected to have a moment capacity between 0.7 and 2.0 kip ft depending on what is assumed to be the modulus of rupture of the masonry assembly. 5.2. The shear strength of the masonry walls The shear strength of the masonry wall systems is evaluated based on the maximum shear stress in the web of the blocks at failure using standard elastic theory. The shear stress was evaluated in the masonry blocks
Fig. 10. Typical type-2 shear failure.
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Fig. 11. The flexural strain and stress diagrams of the tested walls.
and not in the mortar joints, since all the observed cracks appeared to be initiated in the web of the masonry blocks near the support. It should be noted that the fiber reinforced composite overlays extended over all the mortar joints of each wall specimen. Since the block appeared to fail in shear, the contribution of the external reinforcement to the shear strength was ignored for this calculation. The calculated shear failure stresses of the walls are not uniform and varied as is common for masonry systems. Tables 3 and 4 show the evaluated shear stress Table 4 Comparison between shear stress to the ratio code allowable Spec. no.
Shear stress (psi) (MPa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
208 162 127 158 177 166 130 147 170 149 158 155 143 178 158 163 183 151
Average shear stress (psi) wMPax
SCOVa (%)
1.43 166 24.1 1.12 w1.14x 0.88 1.09 167 5.71 1.22 w1.15x 1.14 0.90 149 13.5 1.01 w1.03x 1.17 1.03 154 2.98 1.09 w1.06x 1.07 0.99 161 11.00 1.23 w1.11x 1.09 1.12 166 9.76 1.26 w1.14x 1.04 Overall averages160 psis1.10 MPa
Ratio to code allowableb (FVs37 psi) w0.255 MPax 5.62 4.38 3.43 4.27 4.78 4.89 3.51 3.97 4.595 4.03 4.27 4.19 3.87 4.81 4.27 4.41 4.946 4.1
1 inchs25.4 mm, 1 pounds4.46 N, 1 psis6.896 kPa. a COVscoefficient of variationsstandard deviationymean (as a %). b Building code requirements for masonry structures, ACI 530.199yACSE 6-99yTMS 602-99.
at failure for the 18 wall specimens. The ratios of shear stress at the measured failure load to the code allowable shear stress generally fall within the three to five range typically assumed for this ratio, although the ratios do tend to the higher side. It should be noted that the calculations summarized in Tables 3 and 4 are based on the moment of inertia of 665 inch4 (27679.4 cm4) for the entire wall and first a moment of area of 112 inch3 (1835 cm3) (see Fig. 1). It can be also be noted from Table 4 that the shear strength of the masonry walls at failure varies and that some of this variation may be attributed to dowel action of the external reinforcements. The shear calculation is performed based on the shear formula: tsVQyIŽByb. (from Fig. 1b), where tsshear stress in the member at the neutral axis, Vsshear force, bstotal width of the cells, Bstotal width of the wall, and Q is the first moment of the areas from the neutral axis of the solid portion of the blocks. Using the actual dimensions leads to bs(23.250y 60)s17.250 (440 mm), Bybs(23.25y17.25)s60 (150 mm). 6. Recommendations and conclusions This investigation involved conducting an experimental program designed to evaluate the out-of-plane shear strength of masonry wall system reinforced with externally bonded FRP composites. Eighteen compact masonry wall panels (39=29=80, 900=600=200 mm) were tested for out-of-plane static loads. The results of this investigation suggests the following conclusions: 1. Strengthening of un-reinforced masonry walls by externally bonded composite overlays contributes to the flexural performance of the walls. 2. Adding more than one layer of FRP overlay increases the structural integrity of the system and appears to reduce the variation in the behavior of the retrofitted
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walls, especially when the overlays are extended to the supports. 3. When a single layer overlay is used, the distance of the overlay from the support has only a minor influence on the behavior of the retrofitted system. 4. There appeared to be no significant effect of the reinforcement fiber area and the amount of fiber extension to the support on the shear strength of the wall assembly. However, the highly variable nature of the masonry shear strength may have hidden less pronounced influences. References w1x Hamoush S, McGinley M. Out-of-plane strengthening of masonry walls by reinforced composite, final report, no. 441156. North Carolina A&T State University, 1998. w2x Marshall OS, Sweeney SC, Trovillion JC. Army Corps of Engineers special publication. Illinois: Construction Engineering Research Laboratory (CERL), 1998. w3x Ehasni MR, Saddatmanesh H, Velazquez-Dimas JI. Behavior of retrofitted URM walls under simulated earthquake loading. J Composite Construct 1999;3(3):134 –42.
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w4x Ehsani MR, Saadatmanesh H, Al-Saidy A. Shear behavior of URM retrofitted with FRP overlays. J Composite Construct 1997;1(1):17 –25. w5x Hamilton HR, Dolan CW. Flexural capacity of glass FRP strengthened composite masonry walls. J Composite Construct 2001;5(3):170 –8. w6x Lee YJ, Boothby TE, Bakis CE, Nanni A. Slip modulus of FRP sheets bonded to concrete. J Composites Construct 1999;3(4):161 –7. w7x Triantafillou TC. Strengthening of masonry structures using epoxy bonded FRP laminated. J Composite Construct 1998;2(2):107 –15. w8x Marshall OS, Sweeney SC, Trovillion JC. Seismic rehabilitation of unreinforced masonry walls. Illinois: Construction Engineering Research Laboratory (CERL), 1998. w9x Bizindavyi L, Neale KW. Transfer lengths and bond strengths for composites bonded to concrete’. J Composite Construct 1999;3(4):153 –60. w10x The American Society for Testing and Materials. ASTM C90 standard specification for load bearing concrete masonry units, ASTM annual book of standards. West Conshohochen, PA: ASTM, 1999. w11x The American Society for Testing and Materials. ASTM C780 test method for prognostication and construction evaluation of mortars for plain and reinforced unit masonry, ASTM annual book of standards. West Conshohochen, PA: ASTM, 1999.
Out-of-plane behavior of surface-reinforced masonry walls Sameer Hamousha,*, Mark McGinleya, Paul Mlakarb, Muhammad J. Terroc a
Architectural Engineering Department, North Carolina A&T State University, Greensboro, NC 27411, USA b Construction and Materials Division, Waterways Experiment Station, Vicksburg, MS, USA c Civil Engineering Department, Kuwait University, Safat 1300, Kuwait Received 15 February 2001; received in revised form 12 February 2002; accepted 17 April 2002
Abstract This paper presents the results of an experimental program designed to evaluate the out-of-plane shear strength of masonry wall system; and to evaluate of the influence of the area of externally bonded FRP composites on the shear strength of the system. Eighteen compact masonry wall panels (39=29=80, 900=600=200 mm) were tested for static out-of-plane loads. Nine panels were reinforced by one layer of WEB ‘S-Glass’ fiber-reinforcing system attached to the tension side of the wall, while the remaining nine were reinforced with two layers of composite overlay on the tension side. The influence of the overlay’s embedded length (the distance between the support and the overlay’s end) on the shear strength was also investigated. The variables evaluated included three layout configurations and two reinforcement ratios. Three different distances between the overlay end and the adjacent support were tested, 0, dy4 (d is the block unit thickness) and dy2. Both one and two layers of WEB fibers were used and three specimens were evaluated for each variable. An MTS machine was used to test each panel under four-point load conditions. The failure loads, mid-span deflection, fiber-end slippage and failure modes were recorded. Based on the results of the experimental program, it appears that the out-of-plane shear strength of the concrete masonry wall systems is constant over the range of variables tested. The measured shear strength of the masonry wall specimens evaluated in this program indicates that the code defined shear strengths may not be as conservative as assumed. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Shear strength; Out-of-plane; Concrete masonry; Fiber reinforced composites; Experimental testing
1. Introduction Retrofitting masonry walls with fiber-reinforced composites has been shown to increase the out-of-plane flexural strength of the system and minimize the damage due to failure by allowing for greater ductility of the wall and therefore greater energy absorption. However, recent research performed by Hamoush and McGinley w1x indicated that the out-of-plane failure of unreinforced masonry walls retrofitted by external fiber reinforcement might be controlled by the shear strength of the system at the supports. These test results also indicated that the shear strength at failure was inconsistent for all walls tested. It was concluded by this study that the dowel action of the retrofitting fibers might have been a contributing factor to the measured inconsistency in the shear strength at failure. *Corresponding author. Fax: q1-336-334-7126.
Other researchers such as Marshall et al. w2x have investigated the use of overlaying fiber-reinforced polymer (FRP) composite systems to strengthen un-reinforced masonry (URM) and brick wall panels. They investigated the out-of-plane strength of the retrofitted system. They concluded the FRP systems maintained the structural integrity of the repaired walls after failure. Ehasani et al. w3x and Ehsani et al. w4x conducted an experimental study on three half-scale un-reinforced brick walls retrofitted with graphite (FRP) vertical composite strips. The walls were tested under cyclic out-of-plane loading. They showed that the tested specimens were capable of supporting out-of-plane loads of a magnitude of up to 32 times the weight of the tested wall. At failure, the deflection of each wall was as much as 2% of the wall height. Although both the unrepaired brick walls and the retrofitted walls both exhibited brittle behavior, the two systems were capable of dissipating some energy. The study concluded that the GFRP FRP
0950-0618/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 0 2 . 0 0 0 2 4 - 7
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composite strips are a good system for retrofitting URM walls for lateral loads caused by seismic forces. Hamilton and Dolan w5x performed a series of tests on unreinforced concrete masonry walls strengthened with FRP composites. The composite system was attached to the tension side. The objective of their research work was to develop a general approach to provide design guidelines to strengthen concrete masonry walls with FRP. In their study, retrofitted masonry wall systems were tested for flexure under out-of-plane static loading. The test results indicated that the flexural strength of FRP retrofitted masonry systems may be controlled by either the fracture of the FRP, shear or tension in the masonry, or by delamination of the reinforcing FRP systems. Lee et al. w6x developed an analytical tension-stiffening model for fiber-reinforced polymer (FRP) sheets bonded to reinforced concrete. This model was based on equilibrium and the assumption of a linear relationship between bond stress and slip. They performed tests of both small and large-scale specimens comprised of concrete, CFRP sheets and epoxy. The study concluded that the experimentally measured crack opening deformations matched those predicted by the model with reasonable accuracy. Also, it was concluded that the interfacial slip modulus, for concreteyFRP combinations, was independent of the type of FRP sheet. Triantafillou w7x presented systematic numerical analysis procedures for predicting short term strength of masonry walls strengthened with externally bonded FRP laminates, under monotonic out-of-plane bending, inplane bending and in-plane shear. An experimental program including testing 12 small wall specimens was performed to validate the analysis and showed that strengthening of masonry wall with externally bonded FRP laminates appears to be an effective method for retrofitting. It was also noted that when out-of-plane bending response dominates, the increase in the bending capacity is quite high. Marshall et al. w8x evaluated techniques for seismic retrofitting un-reinforced masonry walls. The research focused on the applicability FRP composite materials systems for strengthening URM walls. Unreinforced masonry walls have shown poor performance in past earthquakes. In their study, new 49=49 double width brick wall panels with FRP composite reinforcing applied to one face were constructed and tested under out-of-plane loading. In addition, the shear performance of different widths and thicknesses of FRP composites applied across the mortar joints of brick triplets (three brick high prisms with the center brick offset by half an inch) were evaluated. It was concluded from the research that the seismic rehabilitation of URM brick walls with FRP composites shows great potential. It is also concluded that the strength of the joints is a function
of the width of the FRP composite overlay. With application of multiple layers of FRP, the shear strength of the mortar joints increased sufficiently to cause the failure to occur in the brick. Bizindavyi and Neale w9x investigated the development length needed to achieve full strength in the connection between concrete and FRP composite overlays. An experimental program and theoretical study were used to evaluate the bond between the concrete and FRP composites. They showed good agreement between their analytical model and measured behavior. They concluded that full development is achievable with a proper selection of the composite system and the surface preparation of the concrete. From the studies presented, it appears that using FRP composites to strengthen URM walls may be an effective way to enhance the wall’s flexural strength. However, the shear strength of the masonry wall system may limit the effectiveness of this retrofit technique. This paper presents the results of an experimental program designed to evaluate the out-of-plane shear strength of masonry wall systems reinforced with externally bonded FRP composites. 2. Testing program Eighteen, 39=29=80 (900=600=200 mm) hollow concrete masonry walls were fabricated under laboratory conditions and tested. Nine walls were reinforced externally by one layer of glass fiber composites on the tension side of the wall, and the remaining nine were similarly reinforced with two layers of external glass fiber reinforcement. Hollow, 80 (200 mm) concrete masonry units, complying with The American Society Of Testing and Materials (ASTM) Specification C90 w10x and ASTM C270 Type S mortar was used to construct the walls. 2.1. Materials Nine batches of mortar type ‘S’ were mixed during wall fabrication. Three standard 20 cubes were made from each of the batches and compression tested as described in ASTM C780 w11x. The average compressive strength of the mortar obtained from testing of the cubes was 1500 psi (10.34 MPa). 2.2. Fibers and bonding adhesives The un-reinforced masonry wall specimens were strengthened with a continuous WEB fabric (primary glass fabric) overlay and a Tyfo S Hi-Clear epoxy matrix. The WEB fibers were attached to the walls using Tyfo S epoxy resin. The Tyfo S epoxy resin consisted of two parts of Tyfo A and B which were
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Table 1 Material properties of the fiber used in the experimental program
2.3. Specimen preparation
Composite materials
WEB and Tyfo Hi-Clear epoxy
Ultimate tension strength Ultimate elongation Elastic modulus Design thickness
60 ksi (414 MPa) 2.0% 4000 ksi (27 580 MPa) 0.014 inch (0.4 mm) per layer
The surfaces of the masonry wall specimens were sand blasted using an air gun to clean and prepare the surface for the FRP overlays. Attention was focused on cleaning the joints and removing excessive mortar from the wall surface. A low-pressure water jet was applied to the sand blasted surface to remove the remaining dust. The water jet was applied at least 24 h prior to application of the composite overlay. Fig. 1a,b show the wall elevation and the cross-section with the actual dimensions. A painting roller was used to apply the epoxy to the prepared surface. The roller was saturated with epoxy, and then applied to the cleaned wall surface. The WEB
mixed in a ratio of 100:42 by volume, as required for the standard Tyfo S products. Two different fiber reinforcement areas were evaluated in the testing program and the mechanical properties of the two composite material configurations are shown in Table 1.
Fig. 1. (a) Wall specimen configuration. (b) Specimen cross-section.
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Fig. 2. Test set-up.
fiber mats were pre-cut to 240=280 (600=700 mm), 240=320 (600=800 mm) and 240=360 (600=900 mm) sizes. The pre-cut fabric mats were placed on the top of the rolled epoxy and then more epoxy was applied to the surface of the fabric. Attention was focused on clearing all bubbles between the fabric and wall surface. During the application of the epoxy, the change of fabric color from white to yellow indicted that saturation had been reached. 2.4. Test set-up Each of the wall specimens was tested in a simply supported beam configuration in an effort to evaluate the out-of-plane shear strength of the retrofitted masonry system. A clear span of 360 (900 mm) was used for all walls. Two point loads, spaced 90 (225 mm) from the supports and at 180 (450 mm) centers, were applied to the specimens through distributing beams and wood bearing strips. Loads were applied monotonically to failure. Fig. 2 shows the test apparatus with a specimen ready for test. Before placing the wall specimens in the supports, the fabric overlay was trimmed with a grinder to ensure that a uniform distance was maintained from the fabric end and the test supports. This trimming was conducted after the bonding epoxy had been cured. 2.5. Testing apparatus A servo-controlled hydraulic MTS 110 kip (490 kN) actuator was used to apply a load to the specimens. The walls were simply supported at the two sides with circular solid pipe supports. A fast setting gypsum was used to level each specimen on the two supporting pipes to insure a uniform distribution of the support loads along the width of the specimen and to prevent any force concentration due to imperfections in wall construction.
2.6. Instrumentation and data acquisition Four LVDTs and a load cell were used to monitor the response of the specimens to the applied transverse loads. Two LVDTs were placed so that they measured the interfacial movement between the masonry blocks and the fiber composite overlay at the end of the walls. The remaining two LVDTs measured the mid-span deflection on each side of the wall specimen. Readings of load, displacements and deflections were recorded by a computerized data acquisition system and stored in an Excel spreadsheet file. 2.7. Testing procedures The testing procedures were performed as described below: 1. A small initial load was applied to insure complete contact of the specimen with the two supports and to allow the gypsum to seat. 2. Once the wall had full contact, the load was released and an initial reading of loads and deflection was recorded. 3. The loading was then re-applied at a steady rate in increments of 500 pounds and held steady for a minimum of 1 min. 4. At each loading increment, the specimen was inspected visually for any distress and initiation of cracks. 3. Test results One of the principal focuses of this investigation was on the load deformation characteristics of the fiber reinforced concrete masonry wall systems. The testing program evaluated six different configurations of wall specimens, each set consisting of three walls. Set 1 consisted of Walls 1, 2 and 3 and investigated walls with two layers of fiber reinforcement overlay extended
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Fig. 3. Load vs. deflection of specimens 1, 2, 3, for two layers with 320=240=0.0140 fiber composites to within dy4 to support.
to a distance of dy4 from supports (d is the depth of the block wall). Set 2 consisted of Walls 4, 5 and 6, and investigated walls with two layers of fiber reinforcement overlay extended to a distance of dy2 from supports. Set 3 consisted of Walls 7, 8 and 9, and investigated walls with two layers of fiber reinforcement overlay extended to the supports. Set 4 consisted of Walls 10, 11 and 12, and investigated walls with one layer of fiber reinforcement overlay extended to a distance of dy4 from supports. Set 5 consisted of Walls 13, 14 and 15, and investigated walls with one layer of fiber reinforcement overlay extended to the supports. Finally, Set 6 consisted of Walls 16, 17 and 18, and investigated walls with one layer of fiber reinforcement overlay extended to a distance of dy2 from supports. The influence of the two experimental variables (reinforcement ratios and fiber length) on the wall failure loads and deformation are shown in Figs. 3–8. Fig. 3 shows the loads vs. the deflection for specimens of Set 1. The load deflection is shown to be linear elastic up to failure of the specimen, for all walls of Set 1, although Wall 2 supported more loads than either Wall 1 or Wall 3. The average failure load for the three walls was 5891 pounds (26.27 kN). It was noted that the amount of slippage at the fiber ends of the specimens was extremely small, and no significant interfacial slip was measured for the stress levels evaluated.
The load deflection curves shown in Fig. 4 (Set 2) are shown to be linear elastic up to failure for Walls 4 and 5. Wall 6 revealed more ductile behavior than either Wall 4 or Wall 5. The average failure load for the three walls was 5934 pounds (26.46 kN). The load deflection curves shown in Fig. 5 (Set 3) exhibit non-linear behavior with a continuously reducing slope. The average failure load for the three walls was 5298 pounds (23.62 kN). Fig. 6 shows the load vs. the deflection for the wall specimens of Set 4. The load deflection of this configuration is shown to be non-linear up to failure. Wall 11 supported more loads than either Walls 10 or 12, and the average failure load for the three walls was 5470 pounds (24.4 kN). The load deflection curves in Fig. 7 (Set 5) show the nonlinear behavior of Walls 13, 14 and 15. Wall 13 revealed more ductile behavior than either Wall 14 or Wall 15. The average failure load for the three walls was 5676 pounds (25.3 kN). The load deflection curves in Fig. 8 (Set 6) show that Walls 16, 17 and 18 exhibited essentially linear behavior up to failure. The average failure load for the three walls was 5888 pounds (26.26 kN). Table 2 summarizes the experimental results for the 18 specimens tested, including the maximum loads and the corresponding average mid-span deflection obtained
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Fig. 4. Load vs. deflection of specimens 4, 5, 6, for two layers with 280=240=0.0140 fiber composites to within dy2 to support.
by averaging the two LVDT reading at each side of each specimen. Table 3 summarizes the failure loads, the calculated shear stress at the measured maximum loads, and the observed mode of failure for all the wall specimens tested. It can be noted that three specimens failed in a typical diagonal shear failure (Type 1 failure, see Fig. 9). The remaining 15 specimens failed by opening of the mortar joint in the shear area, followed by the formation of a diagonal shear crack that extended from the vertical joint to the point of the concentrated load (Type 2 failure, see Fig. 10). Final failure occurred when the block web crack met the interface between the fiber and the masonry blocks and then propagated along the interface to the fiber composite end.
4. Discussion of experimental results A review of Table 2 shows that the average load of walls with 28-inch fiber reinforcement (distance to supportsdy2) is slightly higher than that of walls with 32-inch fiber reinforcement (distance to supportsdy4). Also, walls with 32-inch fiber reinforcement support slightly higher average loads than that with 36-inch fibers (distance to supports0). The load deflection behavior of walls with the shorter fibers varied significantly, for the three walls tested. The variation in behavior can be noted clearly in Fig. 4. Also, although the behavior of all three of the short fiber walls is basically linear elastic up to failure, Wall 6 shows some ductility prior to its failure.
Fig. 5. Load vs. deflection of specimens 7, 8, 9, for two layers with 360=240=0.0140 fiber composites extended to support.
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Fig. 6. Load vs. deflection of specimens 10, 11, 12, for one layer with 320=240=0.0140 fiber composites to within dy4 to support.
Fig. 3 shows that the load deflection curves of walls with longer FRP (fiber closer to the support) are more consistent and exhibit more ductility than that of the shorter overlays. Even greater consistency in behavior occurs with walls with 36-inch FRP overlays (Fig. 5). Also, ductility increases with the longer composite overlays. This behavior suggests that the two-layer overlay and longer embedment lengths allow the wall system to act more as an integrated system an the more uniform fibers dominate the behavior. With shorter overlays (larger distances to the support), the failure is controlled more by the more variable individual strength of the blocks located in the critical stress area. Table 3 shows that the average load of walls with 28inch single layer fiber reinforcement (distance to supportsdy2) is higher than that of walls with 32-inch and 36-inch single layer fiber reinforcement. However, con-
trary to the two layer trends, walls with 36-inch fiber reinforcement (distance to supports0) had a higher average load than those with 32-inch fibers (distance to supportsdy4). It appears that the load deflection behavior of walls with one layer of FRP composite is different than that of the walls with two layers of FRP composites. Walls with shorter single layer overlays exhibit behavior that is very consistent up to failure, for all three walls tested. However, with an increase in the overlay length, the variation in load deflection behavior can be clearly noted, see Fig. 7. This difference may be attributed to a lower contribution of the FRP to the overall behavior of the system with the single overlay layer. With shorter overlays, the strength is controlled basically by the strength of masonry blocks and the FRP overlay does not appear to significantly contribute to the shear strength of the assembly. The wall behavior shown in
Fig. 7. Load vs. deflection of specimens 13, 14, 15, for one layer with 360=240=0.0140 fiber composites extended to support.
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Fig. 8. Load vs. deflection of specimen 16, 17, 18, for one layer with 280=240=0.0140 fiber composites to within dy2 to support.
Figs. 6 and 8 suggest that FRP contribution increases as the length of the overlay increases. It can be noted that when the FRP overlay is extended to the supports, the wall specimens with the two layered overlay systems appear to the behave as a fully integrated system, while the wall specimen with a single layered overlay system does not have as uniform a behavior. In contrast, when shorter FRP overlay systems were used, the two-layered systems appear to contribute
slightly to the wall system behavior, while the one layered system does not show significant contribution to the system shear behavior.
Table 2 Results of the wall specimen tests
Table 3 Results of the shear calculations of the masonry wall systems
Spec. Reinf. Distance of fiber no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Maximum loads
layers to supports
2L 2L 2L 2L 2L 2L 2L 2L 2L 1L 1L 1L 1L 1L 1L 1L 1L 1L
dy4 dy4 dy4 dy2 dy2 dy2 0 0 0 dy4 dy4 dy4 0 0 0 dy2 dy2 dy2
Average deflection at max loads
(lb)
(kN) (in)
(mm)
14802.32 11504.11 9039.535 11204.85 12578.96 11821.66 9269.774 10450.48 12068.97 10605.11 11196.07 11014.12 10148.78 12682.37 11226.07 11600.45 13032.36 10696.28
65.84 51.17 40.21 49.84 55.95 52.59 41.23 46.49 53.69 47.17 49.80 48.99 45.14 56.41 49.94 51.60 57.97 47.58
2.47 2.10 1.75 3.33 2.71 4.49 2.69 3.22 3.53 2.87 3.76 3.25 4.05 2.60 3.05 2.75 3.23 2.76
d ‘wall depth’s7.625 ins194 mm.
0.097 0.0823 0.0688 0.131 0.106 0.177 0.106 0.127 0.139 0.113 0.148 0.128 0.159 0.102 0.120 0.108 0.127 0.109
5. Analytical evaluation The analytical evaluation of the strengthened masonry wall systems was focused on three aspects of behavior, the flexural strength of the composite system, the shear strength of the masonry blocks, and finally the contri-
Spec. no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Maximum shear force (V)
Shear stress
(lb)
(kN)
(psi)
(MPa)
7401 5752 4520 5602 6290 5911 4635 5225 6035 5301 5598 5507 5074 6341 5613 5800 6516 5348
32.92 25.59 20.11 24.92 27.98 26.29 20.62 23.24 26.85 23.58 24.90 24.50 22.57 28.21 24.97 25.80 28.98 23.79
208 162 127 158 177 166 130 147 170 149 158 155 143 178 158 163 183 151
1.43 1.12 0.88 1.09 1.22 1.14 0.90 1.01 1.17 1.03 1.09 1.07 0.99 1.23 1.09 1.12 1.26 1.04
Mode of failure
Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type
1 inchs25.4 mm, 1 pounds4.46 N, 1 psis6.896 kPa.
2 2 1 2 1 1 2 2 2 2 2 2 2 2 2 2 2 2
Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear Shear
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causes compression failure of the masonry blocks was achieved at a loading level, V, of: Vs16 Kips (71.75 kN) for the one layer WEB configuration; and Vs30.5 Kips (136.75 kN), for two layers WEB configuration. An example of calculation of the nominal moment (Mn) of one layer is given below: the thickness of the WEB layers0.014 inch (0.356 mm) Afs(0.014)(24)s0.336 inch2 (217 mm2) Strain equilibrium: ´fs(0.003)(dyx)yx is assumed to be less than the ultimate elongation. The force equilibrium gives as1.02 inch (25.9 mm). Using as1.02 inches, the FRP fails before the concrete crushes. The ultimate force in the FRP is 20.16 Kips. The moment is calculated as Fig. 9. Typical type-1 shear failure.
bution of the overlays on the ultimate shear strength of the system. 5.1. The flexural strength of the composite systems A simplified analytical method was developed to predict the ultimate strength of the fiber reinforced masonry wall systems. The method was based on the following assumptions: (1) linear strain distribution through the full depth of the wall; (2) small deformations; (3) no tensile strength in the masonry blocks, (4) no slip between the fiber reinforced composites and the masonry wall, and (5) plane sections remained plane. The stress–strain relationship of the fiber reinforced composite systems was generally considered to be linear elastic up to failure, while the stress–strain behavior of the masonry block is modeled as and idealized uniform stress block at failure. The ultimate compression strain in the masonry blocks was assumed to be 0.003. From the internal force equilibrium and assuming a linear distribution for the section strains, the flexural strength capacity was determined (see Fig. 11). It should be noted that the compressive strength of the masonry assembly, ( f 9m) was 1200 psi (8.28 MPa), supplier specification based on the plant testing, and the elastic modulus of the FRP composite system was 4000 ksi (27.6 GPa). The ultimate strain of the fibers was assumed to be 0.02, while the ultimate stress is taken as 60 ksi (414 MPa). In the experimental program, each wall was simply supported at the two ends with a clear span of 3 feet (900 mm). The actual width of each specimen was 1.5 feet (450 mm). Based on the above assumptions, the ultimate strength of the composite wall systems that
Mns12.02 Kip feet (53.9 kN m) MnsV=9y12 Vs16 Kips (71.35 kN) It should be noted that the unreinforced walls would be expected to have a moment capacity between 0.7 and 2.0 kip ft depending on what is assumed to be the modulus of rupture of the masonry assembly. 5.2. The shear strength of the masonry walls The shear strength of the masonry wall systems is evaluated based on the maximum shear stress in the web of the blocks at failure using standard elastic theory. The shear stress was evaluated in the masonry blocks
Fig. 10. Typical type-2 shear failure.
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Fig. 11. The flexural strain and stress diagrams of the tested walls.
and not in the mortar joints, since all the observed cracks appeared to be initiated in the web of the masonry blocks near the support. It should be noted that the fiber reinforced composite overlays extended over all the mortar joints of each wall specimen. Since the block appeared to fail in shear, the contribution of the external reinforcement to the shear strength was ignored for this calculation. The calculated shear failure stresses of the walls are not uniform and varied as is common for masonry systems. Tables 3 and 4 show the evaluated shear stress Table 4 Comparison between shear stress to the ratio code allowable Spec. no.
Shear stress (psi) (MPa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
208 162 127 158 177 166 130 147 170 149 158 155 143 178 158 163 183 151
Average shear stress (psi) wMPax
SCOVa (%)
1.43 166 24.1 1.12 w1.14x 0.88 1.09 167 5.71 1.22 w1.15x 1.14 0.90 149 13.5 1.01 w1.03x 1.17 1.03 154 2.98 1.09 w1.06x 1.07 0.99 161 11.00 1.23 w1.11x 1.09 1.12 166 9.76 1.26 w1.14x 1.04 Overall averages160 psis1.10 MPa
Ratio to code allowableb (FVs37 psi) w0.255 MPax 5.62 4.38 3.43 4.27 4.78 4.89 3.51 3.97 4.595 4.03 4.27 4.19 3.87 4.81 4.27 4.41 4.946 4.1
1 inchs25.4 mm, 1 pounds4.46 N, 1 psis6.896 kPa. a COVscoefficient of variationsstandard deviationymean (as a %). b Building code requirements for masonry structures, ACI 530.199yACSE 6-99yTMS 602-99.
at failure for the 18 wall specimens. The ratios of shear stress at the measured failure load to the code allowable shear stress generally fall within the three to five range typically assumed for this ratio, although the ratios do tend to the higher side. It should be noted that the calculations summarized in Tables 3 and 4 are based on the moment of inertia of 665 inch4 (27679.4 cm4) for the entire wall and first a moment of area of 112 inch3 (1835 cm3) (see Fig. 1). It can be also be noted from Table 4 that the shear strength of the masonry walls at failure varies and that some of this variation may be attributed to dowel action of the external reinforcements. The shear calculation is performed based on the shear formula: tsVQyIŽByb. (from Fig. 1b), where tsshear stress in the member at the neutral axis, Vsshear force, bstotal width of the cells, Bstotal width of the wall, and Q is the first moment of the areas from the neutral axis of the solid portion of the blocks. Using the actual dimensions leads to bs(23.250y 60)s17.250 (440 mm), Bybs(23.25y17.25)s60 (150 mm). 6. Recommendations and conclusions This investigation involved conducting an experimental program designed to evaluate the out-of-plane shear strength of masonry wall system reinforced with externally bonded FRP composites. Eighteen compact masonry wall panels (39=29=80, 900=600=200 mm) were tested for out-of-plane static loads. The results of this investigation suggests the following conclusions: 1. Strengthening of un-reinforced masonry walls by externally bonded composite overlays contributes to the flexural performance of the walls. 2. Adding more than one layer of FRP overlay increases the structural integrity of the system and appears to reduce the variation in the behavior of the retrofitted
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walls, especially when the overlays are extended to the supports. 3. When a single layer overlay is used, the distance of the overlay from the support has only a minor influence on the behavior of the retrofitted system. 4. There appeared to be no significant effect of the reinforcement fiber area and the amount of fiber extension to the support on the shear strength of the wall assembly. However, the highly variable nature of the masonry shear strength may have hidden less pronounced influences. References w1x Hamoush S, McGinley M. Out-of-plane strengthening of masonry walls by reinforced composite, final report, no. 441156. North Carolina A&T State University, 1998. w2x Marshall OS, Sweeney SC, Trovillion JC. Army Corps of Engineers special publication. Illinois: Construction Engineering Research Laboratory (CERL), 1998. w3x Ehasni MR, Saddatmanesh H, Velazquez-Dimas JI. Behavior of retrofitted URM walls under simulated earthquake loading. J Composite Construct 1999;3(3):134 –42.
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w4x Ehsani MR, Saadatmanesh H, Al-Saidy A. Shear behavior of URM retrofitted with FRP overlays. J Composite Construct 1997;1(1):17 –25. w5x Hamilton HR, Dolan CW. Flexural capacity of glass FRP strengthened composite masonry walls. J Composite Construct 2001;5(3):170 –8. w6x Lee YJ, Boothby TE, Bakis CE, Nanni A. Slip modulus of FRP sheets bonded to concrete. J Composites Construct 1999;3(4):161 –7. w7x Triantafillou TC. Strengthening of masonry structures using epoxy bonded FRP laminated. J Composite Construct 1998;2(2):107 –15. w8x Marshall OS, Sweeney SC, Trovillion JC. Seismic rehabilitation of unreinforced masonry walls. Illinois: Construction Engineering Research Laboratory (CERL), 1998. w9x Bizindavyi L, Neale KW. Transfer lengths and bond strengths for composites bonded to concrete’. J Composite Construct 1999;3(4):153 –60. w10x The American Society for Testing and Materials. ASTM C90 standard specification for load bearing concrete masonry units, ASTM annual book of standards. West Conshohochen, PA: ASTM, 1999. w11x The American Society for Testing and Materials. ASTM C780 test method for prognostication and construction evaluation of mortars for plain and reinforced unit masonry, ASTM annual book of standards. West Conshohochen, PA: ASTM, 1999.