Principal Investigators:
Antonio Nanni (Currently at Univ. Missouri-Rolla)
Charles E. Bakis (Engineering Science & Mechanics)
Barbara A. Shaw (Engineering Science & Mechanics)
Thomas E. Boothby (Architectural Engineering)
Structural repair of reinforced concrete (RC) structures is becoming an increasingly important option for all deteriorated constructed facilities in the United States. Externally bonded fiber reinforced plastic (FRP) reinforcement is the basis of a new repair technology that combines advanced composite material forms with an installation method originally introduced for steel plates approximately 30 years ago. Repair with externally bonded FRP reinforcement is attractive to owners, engineers, and contractors because of ease and speed of installation, corrosion resistance, low profile, and versatility (e.g., strength and stiffness can be oriented according to the need). In order to implement this technology, a quantitative engineering assessment of its value has to be conducted. It is to be demonstrated that it provides durable repair and that its strengthening/stiffening effects are quantifiable for use in analysis/design algorithms.
The objective of this project is to provide the engineering science necessary for the implementation of externally bonded FRP reinforcement for the repair of concrete structures, except for the case of members subjected to compression where enhancement of strength and ductility is the result of lateral confinement of concrete (e.g., column wrapping). The material forms of interest are highly flexible sheets made of dry fibers or prepregs (i.e., pre-impregnated fibers) that can be adhered to a concrete surface after minimum surface preparation (e.g., cleaning/sealing/priming) by means of a polymeric resin. The fibers are high-strength and high-modulus such as carbon and aramid. The methodology of the project consists of an experimental and analytical study based primarily on tensile RC specimens. Tests are conducted under quasi-static conditions after mechanical conditioning (repeated and sustained loading) and environmental conditioning (thermal cycling and exposure to aggressive environments). The experimental program is necessary to quantify the effects of externally bonded FRP reinforcement on concrete resistance to cracking (i.e., concrete tensile stiffening) and to assess long-term strengthening performance. A part of the durability study is to determine whether the presence of FRP reinforcement creates/alters degradation of steel reinforcement in concrete (i.e., galvanic corrosion when FRP is made of conductive fibers). Various material forms, condition at time of repair (e.g., virgin specimen, cracked concrete-elastic steel, cracked concrete-plastic steel), and preparation conditions (e.g., un-sealed cracks, sealed cracks) are investigated. Primary measurements include load, strain, interfacial delamination, crack opening, moire interferometry and photoelasticity.
A full field image of shear strain on the surface of a composite sheet applied to a rectangular prism of steel reinforced concrete loaded in tension is shown in Figure 1. The image was obtained by placing a thin birefringent (photoelastic) sheet of polymer on the surface of the FRP sheet and examining the surface using circularly polarized light. The regions of high color gradient correspond to high strain gradients in the sheet. The strain gradients are caused by cracks in the concrete. This type of data provides information on the local transfer of load in the vicinity of cracks, and can be used to predict debonding of the sheet in highly loaded situations. A full field image of contours of constant longitudinal displacement in a longitudinally loaded concrete prism with external sheets applied is shown in Figure 2. The view shown is through the thickness of the concrete. The black hash marks on the bottom denote tenths of an inch. The specimen was loaded in the horizontal direciton and the FRP sheet appears as a thin perturbed zone of fringes at the top of the specimen. This image was recorded by moire interferometry, which uses the interference of laser light with a several-micrometer thick corrugated aluminum coating on the edge of the specimen. In this case, the FRP extended to the edge of the specimen so that through-thickness displacements in the concrete, adhesive and FRP could be observed. Each fringe in this image corresponds to a longitudinal displacement increment of 417 nm. When differentiated with respect to spatial position, the displacement fringes provide strain data as well. Figure 3 shows a color-coded image of the longitudinal displacements in a specimen like that shown in Figure 2. A zone of high gradient of horizontal (longitudinal) displacement with respect to vertical (lateral) position exists near the pre-existing crack in the concrete, indicating high shear strain. We have observed that diagonal cracking due to this high shear strain is a common mode of damage propagation and eventual sheet debonding near large, pre-existing cracks in concrete repaired with FRP sheets.
The significance of this project is in providing a scientific assessment of the mechanisms of reinforcement and the long-term performance of a high potential technology for the repair of prestressed and non-prestressed concrete structures (including masonry). The project is conducted with the collaboration of manufacturing and construction industry in the intent of securing suitable materials and of using field applications as a test bed for laboratory findings and verification of modeling.
Nanni, A., Bakis, C. E., Boothby, T. E., Frigo, E. L., and Lee, Y.-J., "Tensile Reinforcement by FRP Sheets Applied to RC," Proc. Intl. Composites Expo '97, Soc. Plastics Engineers, New York, 1997, pp. 9C.1-9C8.
Nanni, A., Bakis, C. E., and Boothby, T. E., "Externally Bonded FRP Composites for Repair of RC Structures," Proc. FRPRCS-3, Third International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Vol. 1, Japan Concrete Institute, Tokyo, Japan, 1997, pp. 303-310.
Nanni, A. Boothby, T. E., Cetin, K. M., Shaw, B. A., and Bakis, C. E., "Environmental Degradation of Repaired Concrete Structure," Proc. FRPRCS-3, Third International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Vol. 2, Japan Concrete Institute, Tokyo, Japan, 1997, pp. 155-162.
Lee, Y.J., Tripi, J. M., Boothby, T. E., Bakis, C. E., and Nanni, A., "Tension Stiffening Model for FRP Sheets Bonded to Concrete," Proc. 2nd International Conference on Composites in Infrastructure, Vol. I, H. Saadatmanesh and M. R. Ehsani, Eds., 5-7 Jan. 1998, University of Arizona, Tucson, AZ, pp. 175-186.
Cetin, K. M., Shaw, B. A., Bakis, C., Nanni, A. and Boothby, T., "Environmental Degradation of Repaired Concrete Structures," Proc. 2nd International Conference on Composites in Infrastructure, Vol. II, H. Saadatmanesh and M. R. Ehsani, Eds., 5-7 Jan. 1998, University of Arizona, Tucson, AZ, pp. 488-500.
Lee, Y. J., Boothby, T. E., and Bakis, C. E., "Strain Transfer of Sheet-Bonded FRP to Concrete and Masonry," Proc. ICCM-12, 12th International Conference on Composite Materials, T. Massard, Ed., International Committee on Composite Materials, 1999, 9 pp. CD-ROM version.
Bakis, C. E., Boothby, T. E., Tripi, J. M., and Nanni, A., "An Optical Investigation of Damage and Load Transfer Mechanisms in Concrete with External FRP Sheet Reinforcement," Proc. 8th Intl. Structural Faults and Repair Conf., M.C. Forde, Ed., Engineering Technics Press, Edinburgh, Scotland, 1999, 10 pp. CD-ROM only.
Miller, B., Nanni, A., and Bakis, C. E., "Analytical Model for CFRP Sheets Bonded to Concrete," Proc. 8th Intl. Structural Faults and Repair Conf., M.C. Forde, Ed., Engineering Technics Press, Edinburgh, Scotland, 1999, 10 pp. CD-ROM only.
Lee, Y. J., Boothby, T. E., Bakis, C. E., and Nanni, A., "Slip Modulus of FRP Sheets Bonded to Concrete," J. Composites for Construction, 3(4):161-167 (1999).
Roko, K., Boothby, T. E., and Bakis, C. E., "Failure Modes of Sheet Bonded Fiber Reinforced Polymer Applied to Brick Masonry," Proc. 4th Intl. Symp. on Fiber Reinforced Polymer Reinforcement for Concrete Structures, (FRPRCS-4), C. W. Dolan, S. H. Rizkalla, and A. Nanni, Eds., SP-188, American Concerete Institute International, Farmington Hills, MI, 1999, pp. 305-311.
Tripi, J. M., Bakis, C. E., Boothby, T. E., and Nanni, A., "Deformation in Concrete with External CFRP Sheet Reinforcement," J. Composites for Construction, 4:85-94 (2000).
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To obtain more information on this research, please contact Prof. Thomas E. Boothby (email: tebarc@engr.psu.edu) of the Department of Architectural Engineering.
Last substantial update: 13 Nov 99. Copyright 1999, C. E. Bakis.