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De Lorenzis, L., A. Nanni, and A. La Tegola, "Strengthening of Reinforced Concrete Structures with Near Surface Mounted FRP Rods" , bibl. International Meeting on Composite Materials, PLAST 2000, Milan, Italy, May 9-11, 2000.

Strengthening of Reinforced Concrete Structures with Near Surface Mounted FRP Rods Laura De Lorenzis, Antonio Nanni, Antonio La Tegola

Abstract Near Surface Mounted (NSM) FRP rods are now emerging as a promising technique, in addition to externally bonded FRP laminates, for increasing flexural and shear strength of RC members. The overall objective of this research project was to investigate the effectiveness of NSM FRP rods as a strengthening system for RC structures. First, bond characterization of NSM rods was carried out on coupon-size specimens, varying type of FRP rebars, bonded length and groove size. Then, the structural performance of full-size simply supported RC beams externally strengthened with NSM FRP rods was investigated. Glass and Carbon FRP rods of different sizes were used for flexural strengthening. Performance of the tested beams and modes of failure are presented in this paper. The test results confirm that NSM FRP rods can be used to significantly increase the flexural capacity of RC elements.

Introduction The use of Near Surface Mounted (NSM) Fiber Reinforced Polymer (FRP) rods is a promising technology for increasing flexural and shear strength of deficient reinforced concrete (RC) and prestressed concrete (PC) members. Advantages of using NSM FRP rods with respect to externally bonded FRP laminates are the possibility of anchoring the rods into adjacent members, and minimal installation time (Alkhrdaji et al., 1999). Furthermore, this technique becomes particularly attractive for flexural strengthening in the negative moment regions of slabs and decks, where external reinforcement would be subjected to mechanical and environmental damage and would require protective cover which could interfere with the presence of floor finishes. The method used in applying the rods is described as follows. A groove is cut in the desired direction into the concrete surface. The groove is then filled half-way with epoxy paste, the FRP rod is placed in the groove and lightly pressed. This forces the paste to flow around the rod and fill completely between the rod and the sides of the groove. The groove is then filled with more paste and the surface is leveled. As this technology emerges, the structural behavior of RC elements strengthened with NSM FRP rods needs to be investigated. Tensile and bond testing of the rods for application as NSM reinforcement were carried out in order to obtain a characterization at the material and sub-system levels. Subsequently, the structural level was examined by testing full-size beams (De Lorenzis, 2000). Both shear and flexural strengthening were investigated. In this paper, after an overview of previous work on NSM rods, results of bond and flexural tests are presented.

Previous Work on NSM Rods Although the use of FRP rods for this application is very new, NSM steel rods have been used in Europe for strengthening of RC structures since the early 50's. The earliest reference that could be found in the literature dates back to 1948 (Asplund, 1949). An RC bridge in Sweden experienced an excessive settlement of the negative moment reinforcement during construction, so that the negative moment capacity

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needed to be increased. This was accomplished by grooving the surface, filling the grooves with cement mortar and embedding steel rebars in them. All the technological and design problems and considerations are reported by Asplund (1949). Nowadays, FRP rods can be used in place of steel and epoxy paste can replace cement mortar. The advantage is primarily the resistance of FRP to corrosion, a property that is particularly important in this case due to the position of the rods very close to the surface. Very limited literature is available to date on the use of NSM FRP rods for structural strengthening. Laboratory studies are reported in Warren (1998), Yan et al. (1999), Crasto et al. (1999), and De Lorenzis (2000). Nevertheless, some significant field applications of this technique have already been carried out in the United States during the past two years. A strengthening project was carried out to upgrade the structural floor of Myriad Convention Center, Oklahoma City, OK (USA) in 1997-1998 (Hogue et al., 1999). A combination of externally bonded steel plates, Carbon FRP (CFRP) sheets and NSM CFRP rods was adopted. NSM rods were used in this case for shear strengthening of one of the RC joists. Vertical grooves 1/2-in. wide and 3/4-in. deep with a total length of 20 in. were saw-cut along the side surfaces of the joist at such positions that existing stirrups were avoided (Figure 1-a). CFRP No. 3 rods were then inserted in the epoxyfilled grooves. NSM CFRP rods were used for strengthening of two RC circular structures in the United States in 1998 (Nanni, 1998). Longitudinal and transverse grooves 1/2-in. wide and 1/2-in. deep were cut on the surface of the structures and CFRP sandblasted rods with a nominal diameter of 5/16 in. were embedded in the epoxy-filled grooves. Pier 12 at the Naval Station San Diego, CA (USA) was strengthened in November 1998 to meet demand of higher vertical loads (Warren, 1998). NSM CFRP rods were used to increase the capacity of the deck slab in the negative moment regions. Slots were saw-cut in the deck in the range of 7/8-in. deep and 5/8-in. to 3/4-in. wide. CFRP pultruded No. 3 rods were placed in sequence into the epoxy-filled slots and pressed to the bottom (Figure 1-b). After the epoxy was cured, the surface was abrasive blasted and a UV protective layer was added to the top of the slot. After completion of the upgrade, some spans of the deck were tested. Strain gages were attached to the CFRP rods in order to monitor the performance of the strengthening system, which proved to be satisfactory. Bridge J-857, located on Route 72 in Phelps County, MO (USA), was strengthened in August of 1998 while in service (Alkhrdaji et al., 1999). One of the three solid RC decks was strengthened using NSM FRP rods. The NSM reinforcement consisted of CFRP sandblasted rods with 7/16-in diameter. Strengthening to approximately 30% of the nominal moment capacity was desirable in order to upgrade the bridge decks for HS20-modified truck loading. The design called for 20 NSM CFRP rods spaced at 15 in. on-center. The rods were embedded in 20-ft long, 3/4-in. deep, and 9/16-in. wide grooves cut onto the soffit of the bridge deck parallel to its longitudinal axis. Strain gages and fiber optics sensors were applied to concrete, steel and FRP reinforcement to monitor strain during testing. Each of the three decks was tested to failure by applying quasi-static load cycles. For the deck with NSM rods, failure was initiated by the rupture of some CFRP rods at the location of the widest crack. This deck exhibited the highest failure load, corresponding to an increase in moment capacity of 27% over the unstrengthened deck. Two columns were also strengthened with NSM

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CFRP rods to increase their flexural capacity (Figure 1-c). The rods were mounted on two opposite faces of the columns and anchored 15 in. into the footings. A strengthening and load-testing program at the decommissioned Malcolm Bliss Hospital in St. Louis, MO (USA) was conducted in 1999 (Tumialan et al., 1999). In the building, a five-story RC-frame addition built in 1964, static load tests up to failure were carried out in order to validate strengthening of masonry walls and RC joists using externally bonded FRP laminates and NSM FRP rods. The program on masonry walls strengthened with FRP composites included testing of unreinforced masonry walls subjected to out-of-plane loading and reinforced masonry walls under in-plane loading. Figure 1-d shows the installation of NSM FRP rods on a masonry wall to be strengthened for out-of-plane loading.

(a)

(b)

(c) (d) Figure 1. Examples of Previous Applications of NSM FRP Rods

Bond Tests Introduction The importance of bond is that it is the means for the transfer of stress between the concrete and the FRP reinforcement in order to develop composite action. Among the many different types of bond tests reported in the literature, the most common are the direct pull-out test and the beam pull-out test. A beam pull-out test was adopted for this project, based on previous work on bond between CFRP sheets and concrete (Miller, 1999). Details about specimen configuration, instrumentation and test procedure are reported elsewhere (De Lorenzis, 2000). The variables examined in the experimental test matrix were bonded length, diameter of the rod, type of FRP material, surface configuration of the rod, and size of the groove. Table 1 summarizes all specimens. Concrete strength and type of epoxy were not varied, although they are significant parameters. Concrete with 4000-psi

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nominal compressive strength and a commercially available epoxy paste were selected as representative systems. The mechanical properties of the epoxy, as specified by the manufacturer, were: 2000 psi tensile strength, 4% elongation at break, 8000 psi compressive yield strength and 400 ksi compressive modulus. Tensile strength and modulus of elasticity of the CFRP deformed rods were determined from laboratory testing. The average values resulted to be 272 ksi and 15.2 Msi, respectively. The manufacturer specified for the GFRP No. 4 rods a tensile strength of 116 ksi and a Young’s modulus of 6000 ksi. Properties of the CFRP sandblasted rods were: 225 ksi tensile strength and 23.9 Msi Young’s modulus, as reported in (Warren, 1998). Table 1. Specimens and Results of Bond Testing Specimen Code G4D6a G4D12a G4D12b G4D12c G4D18a G4D24c C3D6a C3D12a C3D12b C3D12c C3D18a C3D24b C3S6a C3S12a C3S12b C3S12c C3S18a C3S24a C4S6a C4S12a C4S18a C4S24a

Type of FRP/Rod Size (No.)

Glass/4

Surface Config.

Deform.

Bonded Length (No. of db)

Groove Size (in)

6

5/8 5/8 3/4 1 5/8 1 1/2 1/2 3/4 1 1/2 3/4 1/2 1/2 3/4 1 1/2 1/2

12 18 24 6

Carbon/3

Deform.

12 18 24 6

Carbon/3

Carbon/4

Sandbl.

Sandbl.

12 18 24 6 12 18 24

5/8

Ultimate Pull-Out Load (lbs) 5548 7778 8307 9628 9563 13918 3523 6006 6880 6472 9452 9880 2965 3927 3460 3931 5602 5025 5082 5839 6634 7934

Avg. Bond Strength (psi) 1177 825 881 1022 676 738 1329 1133 1298 1221 1189 932 1119 741 653 742 704 474 1078 620 469 421

Failure Mode SOE SOE SOE+C SOE+C SOE SOE+C SOE SOE SOE+C C SOE SOE+C SOE PO PO PO PO+SOE PO+SOE SOE PO+SOE PO+SOE PO+SOE

Note: SOE = Splitting of Epoxy; C = Concrete Cracking; PO = Pull-Out Rods No. 3: nominal diameter 0.375 in.; Rods No. 4: nominal diameter 0.500 in. Results Test results in terms of ultimate pull-out load in the rod, average bond strength and failure mode are summarized in Table 1. The expression “pull-out load” has been adopted to refer to the tensile load directly applied to the NSM rod. This load could be computed with accuracy from the value of the external applied load as a result of the specimen configuration. Among the two different rod surface conditions examined, deformed and sandblasted, the former appeared to be more efficient (higher bond strength) and to have a greater tendency to induce splitting failure, as expected.

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When failure was by splitting of the epoxy cover (Figure 2-a), increasing the groove size led to a higher bond strength. For specimens G4D12, the ultimate load increased 8% and 24% as the groove size increased from 5/8 in. to 3/4 in. and 1 in., respectively. As the groove size increases, the thickness of the epoxy cover increases, thereby offering a higher resistance to splitting. The ultimate load increases correspondingly, and failure may eventually shift from the epoxy to the surrounding concrete (Figure 2-b). For specimens C3S12, increasing the groove size did not influence the failure load, since pull-out was the controlling mechanism. The ultimate load increased, as expected, with the bonded length of the rod. However, the average bond strength was found to decrease as the bonded length increased, as a result of the uneven distribution of bond stresses. From the experimental results involving different groove sizes, the optimum groove sizes appear to be 3/4 in. and 1 in. for embedment of NSM rods No. 3 and No. 4, respectively. This conclusion is based on testing of specimens with deformed rods. The effect of the groove size for specimens with sandblasted rods and bonded lengths greater than 12 rod diameters needs to be investigated.

(a) (b) Figure 2. Bond Specimens after Failure

Flexural Tests Specimens Four full-scale RC beams with a T-shaped cross-section and a total length of 15 ft. were tested. All the beams had a flexural reinforcement of two steel rebars No. 7 (nominal diameter 0.875 in.) on the tension side and two rebars No. 4 (nominal diameter 0.500 in.) in compression. The shear reinforcement, designed to ensure that flexural failure would control, consisted of steel stirrups No. 3 (nominal diameter 0.375 in.) every 5 in. The dimensions of the beam cross-section are given in Figure 3. The average concrete strength, determined according to ASTM C39-97 on three 6-in. diameter by 12-in. concrete cylinders, was 5250 psi. The yield strengths of the steel tension and compression reinforcement, as determined from tensile test on three coupon specimens according to ASTM A370-97, were 71.7 ksi and 51.8 ksi, respectively. Epoxy paste and FRP rods were the same used for the bond tests. Beam BFV (no external strengthening) was used as a baseline comparison to evaluate the enhancement in strength provided by the NSM FRP rods. Beam BFC3 and BFC4 were strengthened with two CFRP sandblasted rods No. 3 and No. 4, respectively. Beam BFG4 had two NSM GFRP deformed No. 4 rods. All the grooves had square cross-section, with size of 3/4 in. for beam BFC3 and 1 in. for beams BFC4

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and BFG4. The method of application of the rods has been described in the introduction. The epoxy paste was allowed to cure for 15 days (full cure time at room temperature) prior to testing of the beams. 15″ 4″ 2 No. 4

Stirrups No. 3

16″ 12″

@5" 2 No. 7 2.3″

NSM Rods

4.5″

6″

4.5″

Figure 3. Cross-Section of the Beams Procedure The beams were loaded under four-point bending with a shear span of 72 in. Load was applied by means of a 400-kip hydraulic jack connected to an electric pump and recorded with a 200-kip load cell. Each beam was instrumented with four LVDTs placed at mid-span, at quarter-span and at each support to derive the net deflections. Two more LVDTs were placed horizontally at mid-span to measure the deformation of the cross-section on the tension and compression sides. Another LVDT monitored slip at the end of an NSM rod. Strain gages were also applied on concrete, steel rebars and FRP rods at various locations. Load was applied in cycles of loading and unloading, with the number of cycles depending on the maximum expected load. Results Load vs. mid-span deflection envelopes of the tested beams are reported in Figure 4. Beam BFV, with no external strengthening, failed at a load of 35.20 kips. The failure mode was concrete crushing after yielding of the steel tension reinforcement. Failure of Beam BFC3, strengthened with two CFRP sandblasted No. 3 rods, occurred under an applied load of 45.76 kips, corresponding to a 30% increase in capacity with respect to BFV. Beam BFC4, strengthened with two CFRP sandblasted No. 4 rods, failed at 50.79 kips, which indicated a 44.3% increase over BFV. Both beams failed by debonding of the NSM rods. A typical crackling noise during the test revealed the progressive cracking of the epoxy paste. Longitudinal splitting cracks developed in the epoxy cover and led to the loss of bond of the NSM reinforcement. The load dropped to a value close to the capacity of the virgin beam and deflection kept on increasing, until the test was stopped. This part of the load-deflection curve has not been reported.

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60

Load (kips)

50 40 30

BFV BFC3 BFC4 BFG4

20 10 0 0

500

1000

1500

2000

Mid-span Deflection (mil in) Figure 4. Load vs. Mid-span Deflection Envelopes of the Tested Beams Beam BFG4, with two GFRP deformed No. 4 rods, failed at a load of 44.25 kips, 25.7% higher than the capacity of beam BFV. Failure was again by debonding of the NSM rods. However, unlike in beams BFC3 and BFC4, the epoxy cover and part of the concrete cover of the internal steel reinforcement were split off in a catastrophic fashion (Figure 5). Also in this case, the load dropped to a value close to the capacity of the control beam, while the beam kept on deflecting until the test was stopped. The difference in failure mode is due to the greater tendency of deformed rebars to induce splitting forces in the surrounding material. This has been previously observed from results of the bond tests.

Figure 5. Beams after Failure

Conclusions NSM FRP rods are a promising technique to enhance the flexural capacity of RC beams. Among the tested beams, the strengthened ones showed an increase in capacity ranging from 25.7% to 44.3% over the control beam. A remarkable increase in stiffness was also obtained. However, it appears that bond is of critical importance for the

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effectiveness of this technique. When bond controls, increasing the amount of the NSM reinforcement does not produce a significant gain in capacity. This can be seen from testing of beams BFC3 and BFC4. BFC4 had twice the area of NSM reinforcement but only 11% more capacity than BFC3. The next step in the research will be the analysis of all the deflection and strain data collected. In order to predict the behavior of the strengthened beams, the bond test results will be analyzed to understand the mechanics of bond between NSM FRP rods and concrete.

Unit Conversion Table 1 in = 1 kip =

25.4 mm 4.45 kN

1 ft. 1 ksi

= =

304.8 mm 6.89 MPa

Acknowledgements The authors would like to acknowledge the University of Missouri - Rolla NSF Industry/University Cooperative Research Center on Repair of Building and Bridges with Composites (RB2C) for supporting this project.

References Alkhrdaji, T.; Nanni, A.; Chen, G.; and Barker, M. (1999), “Upgrading the Transportation Infrastructure: Solid RC Decks Strengthened with FRP,” Concrete International, American Concrete Institute, Vol. 21, No. 10, October, pp. 37-41. Asplund, S.O. (1949), "Strengthening Bridge Slabs with Grouted Reinforcement", Journal of the American Concrete Institute, Vol. 20, No. 6, January, pp. 397406. Crasto, A.; Kim, R.; and Ragland, W. (1999), Private Communication. De Lorenzis, L. (2000). “Strengthening of RC Structures with Near-Surface Mounted FRP Rods”, MSc Thesis, Department of Civil Engineering, The University of Missouri – Rolla, Rolla, MO, 175 pp. Hogue, T.; Cornforth, R.C.; and Nanni, A. (1999), “Myriad Convention Center Floor System Reinforcement”, Proceedings of the FRPRCS-4, C.W. Dolan, S. Rizkalla and A. Nanni, Editors, ACI, Baltimore, MD, pp. 1145-1161. Miller, B. (1999), "Bond Between Carbon Fiber Reinforced Polymer Sheets and Concrete", MSc. Thesis, Department of Civil Engineering, University of Missouri – Rolla, Rolla, MO, pp. 138. Nanni, A. (1998), Private Communication. Tumialan, G.; Tinazzi, D.; Myers, J.; and Nanni, A. (1999), "Field Evaluation of Masonry Walls Strengthened With FRP Composites at the Malcolm Bliss Hospital", Report CIES 99-8, University of Missouri-Rolla, Rolla, MO. Warren, G.E. (1998), Waterfront Repair and Upgrade, Advanced Technology Demonstration Site No. 2: Pier 12, NAVSTA San Diego, Site Specific Report SSR-2419-SHR, Naval Facilities Engineering Service Center, Port Hueneme, CA. Yan, X.; Miller, B.; Nanni, A.; and Bakis, C.E. (1999), “Characterization of CFRP Bars Used as Near-Surface Mounted Reinforcement”, Proceedings 8th International Structural Faults and Repair Conference, M.C. Forde, Ed., Engineering Technics Press, Edinburgh, Scotland, 1999, 10 pp., CD-ROM version.

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