Strength and Ductility of Concrete Cylinders Confined with Fiber Metal Laminate Composites


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Virginia Tech


Fiber reinforced polymer (FRP) is a composite material made of fibers that carry tensile loads embedded in a polymeric matrix. Externally bonded FRP retrofits of reinforced concrete elements provide an efficient, economical, and accepted method of mitigating deficiencies related to seismic and blast loads, as well as addressing corrosion-related issues. FRPs retrofits are widely regarded as cost effective as the cost associated with retrofit installation and facility down-time are usually less than similar retrofit systems. Besides issues of bond and anchorage between the FRP and the substrate, the main disadvantage of FRP materials is that they behave in a brittle, linear elastic manner. As a result, strengthening concrete structures with FRP may introduce new and undesirable behaviors that are mitigated by design codes through strict strain limits. Because FRP is designed for very low strain levels to prevent brittle rupture and unpredictable debonding, buildings and bridges are strengthened in such a way that restricts their energy dissipation capacity at the ultimate limit state. This runs counter to the structural design philosophy of new buildings where the design objective is to develop significant plastic deformation to dissipate energy.

An ideal composite material for infrastructure strengthening is one that combines the ease of application of FRP rehabilitation systems with the ability of ductile metals to yield under relatively large strains to provide energy dissipation and ensure ductile behavior. Known as a fiber metal laminate (FML), the aerospace industry has successfully developed a composite consisting of thin metal sheets alternatively bonded to epoxy saturated fiber fabric that is widely used to construct aircraft fuselages and wings. Unlike FRP, FML composites possess a well-defined yield point and exhibit inelastic behavior. However, aerospace grade FML composites cannot directly be applied to building and bridges because they: (i) were developed for low-stress fatigue resistance rather than performance near ultimate stress; (ii) are precisely manufactured to unnecessarily tight tolerances by civil construction standards; and (iii) are not economical compared with current FRP strengthening techniques. Therefore, developing a multifunctional civil engineering composite material based on FML theory would unlock opportunities related to plastic design, energy dissipation, and other mechanisms not currently possible with FRP.

This dissertation presents a comprehensive study on the use FML jackets to enhance the strength and ductility of concrete cylinders. The confinement effect and failure mechanisms of FML confined concrete were analyzed for a range of experimental parameters, including the effect of the number of layers, the fiber orientation, and fabric architecture of the FML jackets. The experimental program was divided into two phases. The first phase consisted of a series of uniaxial tension coupon tests to investigate how the stacking arrangement of various E-glass fabrics and aluminum sheets could be tuned to control the yield strength, post-yield stiffness, and ductility characteristics of the FML lay-ups. Mechanical roughening of aluminum sheets and the addition of a bond enhancement agent to the resin system was found to enhance the interlayer bonding and splice capacity of metal and fiber layers. The results demonstrated that FML coupons with [±45°] glass fabrics exhibited pseudo-elastic-plastic stress-strain response, while coupons with [0°] and [0°/90°] fabrics exhibited strain hardening after yielding of aluminum layers. Furthermore, the ratio of the relative contribution of composite layers to the total elastic stiffness of the FML composites was found to be a good indicator of the mechanical properties and shape of the uniaxial stress-strain response of the FML lay-ups. An analytical model based on the Rule of Mixtures (ROM) was used to predict the tensile behavior of the FML coupons.

The second phase consisted of axial compression testing of concrete cylinders confined by FML jackets to investigate the influence of various lay-up schemes on the strength and ductility of the confined concrete. Cylinders jacketed with FML showed a significant increase in their strength and ductility. The degree of strain-softening response, maximum strength, peak strain, ultimate deformation, and energy dissipation capacity of the FML confined concrete was found to be controlled by the pseudo-ductile stress-strain response of the FML jackets. FML lay-ups which exhibited strain hardening uniaxial behavior tended to produce greater enhancements in confined concrete strength and steeper strain softening response than FML lay-ups which exhibited pseudo-elastic-plastic uniaxial behavior. Furthermore, FML confined concrete showed improved performance, compared to FRP confined concrete, in terms of confined concrete behavior and failure mode. Finally, the project also demonstrated that an in-situ, hand lay-up preparation procedure for FML jackets provided a level of performance and construction tolerance suitable for use in civil infrastructure applications. Although the results of this study encourage the use of FML as a viable substitute to FRP for retrofitting deficient concrete members, further research is recommended on large-scale columns to verify the feasibility of this innovative retrofit technique.



Fiber Metal Laminate, FML, Concrete Confinement, FRP