Repair of Damaged Aluminum Truss Joints of Highway Overhead. Sign Structures Using FRP

Transcription

1 Repair of Damaged Aluminum Truss Joints of Highway Overhead Sign Structures Using FRP Amir Fam a*, Sarah Witt b and Sami Rizkalla c a Queen s University, Kingston, Ontario, Canada K7L 3N6 b Fyfe Co. LLC, San Diego, California, USA c North Carolina State University, Centennial Campus, Raleigh, NC, USA ABSTRACT An innovative repair technique is introduced for aluminum truss-type highway overhead sign structures, using fibre-reinforced polymer (FRP) sheets. The welded k-joints are typically subjected to excessive fatigue-induced cracking under the effects of wind and moving traffic. The repair technique proposed in this paper utilizes longitudinal FRP layers bonded to the diagonals and wrapped around the main chord to form alternating v-patterns, followed by additional circumferential layers for anchorage. Eight tests were conducted on four full-scale specimens. Weld lines at the junction between diagonals and main chord were ground to simulate a 90 percent loss of joint strength. After repair, diagonals were loaded to failure in tension. The study showed that full strength of the welded joints was restored using carbon-frp sheets. Only 70 percent of joint strength was restored when using glass-frp. The strengthening technique is particularly sensitive to quality control during installation. A field application using the proposed technique was successfully completed by the New York State Department of Transportation for a cracked aluminum truss over Route 88 in NY State. Keywords: Aluminum, joint, crack, FRP, repair * Corresponding author. Tel: ; Fax: ; fam@civil.queensu.ca 1

2 1. INTRODUCTION The use of fibre reinforced polymers (FRP) in restoring the strength of damaged or deteriorated concrete and masonry structures has been proven to be quite successful [1, 2]. On the other hand, for metallic structures, including both steel and aluminum, a recent state-of-the-art study [3] has pointed out that the research on using FRP materials for retrofit of metallic structures is still limited and field applications have not yet gained a wide potential. One of the promising retrofit applications of metallic structures, where FRP would be quite suitable, is the repair of truss-type highway overhead sign structures, shown in Fig. 1(a). These aluminum overhead sign structures have been in service for the past five decades. A large number of these structures suffer from fatigue cracking of the welded joints between the diagonal and chord members due to vibration under wind loading and the effect of moving traffic. The cracking in the weld could propagate and cause complete failure of the connection as shown in Fig. 1(b). The problem in these situations is that field welding of aluminum is not allowed because of the difficulties associated with providing the necessary quality control conditions in the field. On the other hand, replacing the whole structure could be very costly. The use of FRP flexible sheets to repair the cracked joints could be a quite promising technique. In addition to their ease of installation, light weight and durability, the FRP sheets offer additional unique advantage in this case. The diagonal and chord members are likely to have circular cross sections of different diameters and are also connected at an angle, which result in a complex and quite irregular surface geometry at the joint. Therefore, FRP sheets can be effectively used in this situation as they conform quite well to such a surface. Care should be taken when bonding carbon-frp (CFRP) materials to metals, especially aluminum. Although the CFRP material itself is non-corrosive, as the carbon fibers become in contact with aluminum, a galvanic corrosion process may be generated. The three requirements necessary for galvanic corrosion to occur are: electrolyte (such as salt water) bridging between the 2

3 two materials, electrical connection between the two materials, and a sustained cathodic reaction on the carbon. Karbhari and Shully [4] suggested that, using a hybrid of glass and carbon, where the glass fabric is sandwiched between the steel or aluminum surface and the subsequent CFRP layers adds durability to the system. 2. OBJECTIVES This study is intended to demonstrate the effectiveness of a proposed repair technique for damaged truss-type aluminum highway overhead sign structures using FRP wraps through laboratory tests and actual field application. The welded joints between the diagonal members and the main chords of this type of structures suffer excessive fatigue-induced cracking, yet welding of these cracked joints in the field is not feasible. The goal of this research program is to restore the strength of cracked joints, using FRP sheets. The evaluation is based on testing the joint under axial tension load applied to the diagonal member of the truss. The cracking in the welded joint has been simulated in the laboratory by grinding the weld line between the diagonal member and the main chord to simulate a 90 percent loss of joint strength. Both GFRP and CFRP wraps were investigated individually. While CFRP wraps were installed directly on the aluminum surface in this short term study, it is recommended to use a layer of GFRP as a barrier between the aluminum and the CFRP for field applications, where galvanic corrosion could take place over time. 3. EXPERIMENTAL PROGRAM The experimental program included eight tests conducted using four different specimens tested at the Constructed Facilities Laboratory (CFL) of North Carolina State University [5]. Each specimen consisted of a k-shape portion of the aluminum truss, including two diagonal members connected to one horizontal chord. For each specimen, two tests were conducted by loading each of the diagonal members independently in tension up to failure. 3

4 3.1 Configuration of the Test Specimens Four full scale k-shape aluminum joints, each including two diagonal members connected to one chord member, were taken from an actual structure and used in this experimental program. The joints are referred to as G1, C1, C2 and C3. Specimen G1 was repaired using GFRP sheets whereas specimens C1, C2 and C3 were repaired using CFRP sheets. Each specimen consisted of two diagonal members welded to the main chord and oriented at 45 degrees with respect to the main chord. The two diagonals consisted of round aluminum tubing of 50.8 mm outer diameter and 4.8 mm wall thickness. The main chord also consisted of round tubing of mm outer diameter and 8.1 mm wall thickness. The tubing material was aluminum alloy ASTM 6061-T6 with tensile yield strength of 240 MPa, ultimate tensile strength of 290 MPa and modulus of elasticity of 69.6 GPa [6] as shown in Table 1(a). The strength of the diagonal members is normally governed by the strength of the welded joint. The diagonals were connected to the main chord through shop welding using an inert gas arc-welding process. The allowable tensile stress in the diagonal near the weld is 82.7 MPa [6]. 3.2 Surface Preparation and FRP Installation In order to simulate the damage of the welded joint, which is typically induced by fatigue cracking, the weld between the diagonal members and the main chord was completely ground along the perimeter of the joint, except for two spots on opposite sides, 10 mm long each, left intact in order to hold the diagonal member in place in the appropriate configuration. This represents about 90 percent loss of the capacity of the joint. Fig. 2 shows the grinding process and the reduction of the weld length. A wire brush was then used to roughen the surfaces of the diagonals and main chord in the area surrounding the joint, where the FRP was to be applied. Two types of FRP sheets were used in this experimental program, namely GFRP and CFRP (commercially known as Tyfo SEH-51 and Tyfo SCH-35 respectively), both utilizing epoxy resin for 4

5 bonding. The GFRP was used to repair specimen G1, whereas CFRP was used to repair specimens C1, C2 and C3. The GFRP laminate has tensile strength and modulus of 575 MPa and 20.7 GPa respectively and a thickness of 1.3 mm as shown in Table 1(b), while the CFRP laminate has tensile strength and modulus of 991 MPa and 78.6 GPa respectively and a thickness of 0.89 mm as shown in Table 1(c). The repair scheme and process of installation was identical for all specimens. The surface of the aluminum tubes was wetted with epoxy and left till the epoxy became tacky. The dry glass (or carbon) fabrics were saturated with epoxy before being applied to the aluminum joints. Six FRP layers, 840x100 mm each, were wrapped around the joint in a v-shape pattern in an alternating fashion such that the fibers are parallel to the diagonal members as shown in Fig. 3(a). Those layers were notched 13 mm from both sides, at 305 mm from each end as also shown in Fig. 3(a), in order for the sheets to conform to the curved surface at the joint, where the diameter of the diagonal member is different from that of the main chord. Four additional 510x100 mm layers were wrapped transversely around the diagonal members, at two locations each, to confine and anchor the longitudinal FRP layers. One additional 1020x100 mm layer was also wrapped transversely around the main chord at the joint as shown in Fig. 3(b). The surface was finally finished using a thick paste of silica fume mixed with epoxy and left to cure in room temperature. 3.3 Test Setup and Procedure The main chord of the aluminum truss specimen was anchored to the strong floor of the laboratory as shown in Fig. 4(a). This was achieved using a solid steel shaft inserted inside the main chord and held down at each end using a stiff hollow square section (HSS). Each HSS was anchored to the strong floor using two anchor rods as shown in Fig. 4(c). One diagonal member of the specimen was connected to a 980 kn MTS hydraulic actuator. The actuator was anchored to the vertical reaction wall and positioned at 45 degrees angle to line up with the diagonal member of the specimen as 5

6 shown in Fig. 4(b). The actuator included a hinged connection at each end in order to insure axial tension loading in the diagonal members. In order to facilitate the connection between the diagonal members and the hydraulic actuator, aluminum end plates, were attached at the ends of the diagonals. Three types of connections were used, A, B and C as shown in Fig. 4(a). In connection A, the diagonal member was directly welded to a square aluminum plate. This connection was successfully used for specimen G1, however, it was inadequate for the specimens retrofitted with CFRP, due to the higher capacity of the specimens. In connection B, which was used for specimen C1, the end of the diagonal member was inserted into aluminum ring and welded from both sides. The ring was then positioned to bear on the square end plate through a grove cut into the plate as shown in Fig. 4(a) Joint B. One diagonal member of specimen C1 failed within the tube, while the other failed within the weld of connection B. It was, therefore, decided to use a stronger connection C for specimens C2 and C3. Connection C was similar to A, except that four additional gusset plates were welded between the end square plate and the diagonal member as shown in Fig. 4(a), Joint C. Axial tension load was applied using stroke control at a rate of loading ranging between 0.25 to 0.75 mm/min, until failure occurred. The specimen was then rotated 180 degrees and the second diagonal member was also tested to failure. The axial tension load and axial displacement at the loading end of the diagonal were continuously monitored. In specimen G1, the longitudinal strain in the diagonal member was also measured in the aluminum using electric resistance strain gages. 4. TEST RESULTS AND FAILURE MODES Specimens C1 and C3 (repaired with CFRP) failed by fracture of the aluminum tube, close to the loading end, near the weld, and hence, provided an experimental value of 114 kn for the axial tensile strength of the diagonal member. This value is based on the average strength of the two diagonals of each of specimens C1 and C3 of 115, 105, 125, 112, and 125 kn respectively. It should be noted that 6

7 this load corresponds to an induced stress of 164 MPa, which is double the maximum allowable stress of 82.7 MPa specified by the Aluminum Association Inc. [6], but also lower than the 290 MPa ultimate strength of aluminum, mainly due to the welding effect. Since the welded length at the retrofitted joint was reduced by 90 percent, the reduced strength of the member (after simulating the fatigue cracking) was predicted as 11.4 kn. Summary of the test results of all specimens is provided in Table 2. The axial load displacement behavior of specimen G1 (repaired with GFRP) is shown in Fig. 5. Both diagonals failed by fracture of the GFRP wraps at the joint as also shown in Fig. 5. The failure loads for both diagonals were 78.7 and 78.3 kn and the displacements at failure loads were 6.5 and 6.9 mm respectively. These loads are about 31 percent lower than the target strength of 114 kn (strength of a welded aluminum tube), which indicates that the GFRP repair scheme was inadequate to recover the full strength of the diagonal member. It, however, increased the strength of the artificially damaged specimen by about 6.9 times. The load strain behavior of Diagonal 1 is shown in Fig. 6. The strain was measured on the aluminum tube outside the FRP repair system. It is noted that failure occurred before yielding of the aluminum as indicated by the linear response. Based on the ultimate strain of and the elastic modulus of 69.6 GPa, the stress level developed in the diagonal member at failure is 123 MPa only, which is relatively comparable with the stress based on the measured ultimate loads (113 MPa). The load displacement behavior of specimen C1 (repaired with CFRP) is shown in Fig. 7. As indicated earlier, both diagonals failed at the loading end within the aluminum tubes, near or at the weld as also shown in Fig. 7. The ultimate loads and displacements were 115 and 105 kn and 11.4 and 11.8 mm respectively. The CFRP repair system was fully intact at failure and the full strength of the aluminum diagonal was achieved. The strength of the artificially damaged specimen was increased 9.6 times. 7

8 Although specimen C2 (repaired with CFRP) was quite similar to specimen C1, both diagonals of C2 failed by fracture of the CFRP wraps at the repaired joint as shown in Fig. 8. The ultimate loads and displacements of both diagonals were 102 and 104 kn and 14.5 and 19.4 mm respectively as also shown in Fig. 8. This indicates a 7 percent lower strength and 46 percent higher displacement in comparison with C1. This is, in fact, attributed to the slightly lower level of quality control encountered in specimen C2. It is believed that the FRP sheets were not wrapped quite tight around the joint, which led to some slack. This was reflected in the higher axial displacements measured in C2 compared to C1. The strength of the artificially damaged joint was however increased 9 times. While flexible FRP sheets are ideal for conforming to curved surfaces in general, it should be noted that the geometry of the curved surfaces at the joint of the aluminum truss in this case is quite complex due to the difference in diameter and the angle between the diagonal and main chord. As such, quality control is crucial for such application in order to produce consistent results, especially when the FRP is hand applied. Following the testing of specimen C2, it was decided to fabricate another identical specimen, C3, insuring a better quality control and conduct another testing to confirm the results of specimen C1. The load displacement behavior of specimen C3 (repaired with CFRP) is shown in Fig. 9. Both diagonals failed at the loading end within the aluminum tubes, near the weld as also shown in Fig. 9. The ultimate loads and displacements were 115 and 125 kn and 14.1 and 15.1 mm respectively. Similar to specimen C1, the CFRP repair system was fully intact at failure and the full strength of the aluminum diagonal was achieved. The strength of the artificially damaged joint was increased 10.5 times. This test has provided a rather consistent result with specimen C1 in terms of failure mode and a slightly higher strength. It should be noted that in all the eight tests conducted on the FRP repaired joints, no slip was observed between the FRP repair scheme and the diagonal aluminum tube, which suggests sufficient 8

9 bond strength. In all tests, failure occurred by fracture of the aluminum tube, out side the FRP repair patch or by fracture of the FRP wraps at the joint. 5. FIELD APPLICATION The New York State Department of Transportation (NYS DOT) has found in a recent study that 10 percent of the State s overhead sign structures had some form of structural damage. The most common damage found was cracking of the joint between internal diagonal trussing and the main chord. The NYS DOT was approached with the proposed innovative repair method described in this paper to repair the cracked truss joints. NYS DOT joined partners with the State of Utah DOT to research this solution to examine the feasibility as well as time and cost savings. After conducting the laboratory tests, it was concluded that the repair using this technique is promising and can restore the strength of a fully welded connection. The NYS DOT decided to conduct a field application using the proposed technique on a cracked aluminum overhead sign truss located over Route 88 (Westbound Direction) in NY State, about half a mile east of Exit 2. A Contract was awarded to EMI Guiderail. Tyfo Fibrwrap System was used in the repair of the cracked structure. Representatives from EMI Guiderail, Inc., R. J. Watson, Inc. (Local distributor of Tyfo Fibrwrap System), NYS DOT (Materials), and NYS DOT Region 9 were present during this unique retrofit application on September 10, The weather conditions were quite suitable for the installation. Access to the aluminum sign post was provided by EMI Guiderail, using a man-lift. Similar to the steps followed in the laboratory testing, the twocomponent epoxy matrix was mixed for approximately 5 minutes prior to saturating the dry fabric. The aluminum sign structure was prepared by grinding the surface area where the FRP was to be applied. A prime coat of epoxy was applied to the areas that have been prepared and left until the epoxy became tacky. The dry fabric strips were saturated with epoxy and applied in a pattern similar to that shown in Fig. 3. The repair system was then left to cure. 9

10 6. SUMMARY AND CONCLUSIONS An innovative retrofit technique using FRP sheets has been introduced for repair of truss-type aluminum highway overhead sign structures comprised of welded tubular members. The welded K- joints between diagonal members and main chords are typically cracked due to fatigue loading. The repair system utilizes narrow FRP longitudinal strips wrapped around the K-joint and attached to the diagonal members in an alternating fashion. The longitudinal layers are then wrapped with additional layers in the circumferential direction for anchorage. Both GFRP and CFRP materials have been investigated. The cracking at the joints was simulated by grinding the welded perimeter at the intersection between diagonals and main chord to simulate a 90 percent loss of weld. Based on eight tests conducted on four specimens, it is concluded that the full strength of the welded joint can indeed be restored when CFRP layers are applied using the proposed repair technique along with adequate level of quality control. When similar number of layers of GFRP were used, only 79 percent of the capacity of the joints was restored, compared to CFRP. While using additional layers of GFRP could have achieved the desired target strength, this was not investigated in this study. In all tests conducted, failure was either due to rupture of the FRP sheets at the joints or fracture of the aluminum tube near or at the weld at the loading end. In no case bond failure was observed between the FRP wraps and the aluminum tubes. It is also noted that failure of one diagonal at the repaired joint has insignificant effect on the strength of the second diagonal attached to the same joint, when tested later. Based on the test results, the New York State Department of Transportation has decided to implement this technology in the field. On September 2003, aluminum truss overhead sign structure with cracked joints has been successfully repaired using the FRP repair scheme and procedure described in this paper as shown in Fig. 10. The structure is located over Route 88 (Westbound Direction), about half a mile east of Exit 2. To date, no signs of unsatisfactory performance have been reported. 10

11 7. ACKNOWLEDGEMENTS The authors wish to acknowledge Fyfe Co. LLC for supplying the FRP materials, New York State Department of Transportation for providing the test specimens, Mr. Jerry Atkinson for his assistance during the experimental work, and the Constructed Facilities Laboratory of NC State University. 8. REFERENCES [1] Fam A, Musiker D, Kowalsky M, Rizkalla S. In plane testing of damaged masonry wall repaired with FRP. Advanced Composites Letters Journal 2002; 11(6): [2] Rizkalla S, Hassan T. Design recommendations for the use of FRP for reinforcement and strengthening of concrete structures. Journal of Progress in Structural Engineering and Materials 2003; 5(1): [3] Shaat A, Schnerch D, Fam A, Rizkalla S. Retrofit of steel structures using fiber-reinforced ploymers (FRP): state-of-the-art. Transportation Research Board (TRB) annual meeting, Washington, D.C., Jan. 2004, CD-ROM ( ). [4] Karbhari VM, Shulley SB. Use of composites for rehabilitation of steel structures determination of bond durability. Journal of Materials in Civil Engineering, ASCE 1995; 7(4): [5] Fam A and Rizkalla S. Experimental testing of aluminum truss joints repaired using FRP sheets. North Carolina State University-CFL, Report submitted to Fyfe Co. LLC [6] Aluminum Association Inc. Section 1: Specifications for Aluminum Structures, Aluminum construction manual, 5 th Edition, Washington DC [7] State of New York Department of Transportation (NYSDOT) Region 10 and Office of Engineering Structures Design and Construction Division. Overhead Sign Structures Inventory and Inspection Manual 1999 p

12 List of Tables Table 1 Table 2 Mechanical properties of the aluminum tubes and FRP wraps Summary of test results 12

13 List of Figures Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7 Fig. 8 Fig. 9 Fig. 10 Typical aluminum truss overhead sign structure and associated problem Reducing the length of weld at the joint through grinding Installation of FRP repair scheme Details of test setup Load-displacement behavior and failure mode of specimen G1 Load-axial strain behavior of Diagonal 1 of specimen G1 Load-displacement behavior and failure mode of specimen C1 Load-displacement behavior and failure mode of specimen C2 Load-displacement behavior and failure mode of specimen C3 Field repair of aluminum truss over Route 88 in New York State using CFRP sheets 13

14 Table 1 Mechanical properties of the aluminum tubes and FRP wraps (a) Aluminum alloy ASTM 6061-T6 Property Value Tensile yield strength 240 MPa Ultimate tensile strength 290 MPa Allowable tensile stress Compressive yield strength Modulus of elasticity (b) GFRP Tyfo SEH-51 Property Longitudinal tensile strength Longitudinal modulus Transverse tensile strength Laminate thickness 82.7 MPa 240 MPa 69.6 GPa Value 575 MPa 26 GPa Elongation at break 2.2% (c) CFRP Tyfo SCH-35 Property Longitudinal tensile strength Longitudinal modulus Transverse tensile strength Laminate thickness 20.7 MPa 1.3 mm Value 991 MPa 78.6 GPa Elongation at break 1.26% 0 MPa 0.89 mm

15 Table 2 Summary of test results Specimen I.D. Test No. Ultimate Load (kn) Displacement at ultimate (mm) Strength as % age of Damaged specimen Intact specimen G1 C1 C2 C

16 (a) Truss-type overhead sign structure Fractured weld (b) Damaged joint - adopted from reference [7] Fig. 1 Typical aluminum truss overhead sign structure and associated problem

17 10 mm Diagonal 10 mm Bottom chord Fig. 2 Reducing the length of weld at the joint through grinding

18 305 mm 13 mm deep 305 mm 100 mm 510 mm 100 mm 100 mm 840 mm 1020 mm (a) Applying longitudinal v-shape FRP layers (b) Complete repair after applying transverse FRP layers Fig. 3 Installation of FRP repair scheme

19 Hydraulic actuator Square plate weld Aluminum tube Load cell MTS Hinge Ring Joint A Aluminum tube Anchor bars HSS Test specimen FRP wraps Solid steel shaft Square plate Square plate Joint B 4 gusset plates Strong floor Aluminum tube Joint C (a) Schematic view of test setup and connections Hinge Hinge (b) General view of test setup (c) Close-up of test setup Fig. 4 Details of test setup

20 120 Full strength of the welded diagonal = 114 kn Load (kn) Diagonal 2 Diagonal 1 Reduced strength of diagonal = 11.4 kn Displacement (mm) Failure of Diagonal 1 by fracture of GFRP Failure of Diagonal 2 by fracture of GFRP Fig. 5 Load-displacement behavior and failure mode of specimen G1

21 Load (kn) Strain gage Strain x 10-6 Fig. 6 Load-axial strain behavior of Diagonal 1 of specimen G1

22 Load (kn) Full strength of the welded diagonal = 114 kn Diagonal 1 Diagonal 2 Reduced strength of diagonal = 11.4 kn Displacement (mm) Failure of Diagonal 1 in the weld at the loading end Failure of Diagonal 2 by fracture of aluminum tube Fig. 7 Load-displacement behavior and failure mode of specimen C1

23 Load (kn) 120 Full strength of the welded diagonal = 114 kn 100 Diagonal Diagonal Reduced strength of diagonal = 11.4 kn Displacement (mm) Failure of Diagonal 1 by fracture of CFRP Failure of Diagonal 2 by fracture of CFRP Fig. 8 Load-displacement behavior and failure mode of specimen C2

24 Full strength of the welded diagonal Diagonal 1 = 114 kn Load (kn) Diagonal 2 Reduced strength of diagonal = 11.4 kn Displacement (mm) Failure of Diagonal 1 by fracture of aluminum tube Failure of Diagonal 2 by fracture of aluminum tube Fig. 9 Load-displacement behavior and failure mode of specimen C3

25 (a) Aluminum overhead truss over Route 88 (c) Application of epoxy coating to the surface (e) Sheets applied in longitudinal and transverse directions (b) Cracked joint shown after surface preparation (d) Application of CFRP wetted sheets (f) Repaired structure Fig. 10 Field repair of aluminum truss over Route 88 in New York State using CFRP sheets