|
|
Research Scope: This research project currently being conducted at Penn State seeks to advance the knowledge of interfacial bond behavior between epoxy bonded FRP laminates and concrete. By characterizing the interfacial bond failure, a reliable design procedure can be developed for structural elements strengthened by externally bonded FRP laminates. Valuable information in the area of fracture mechanics will be gained by evaluating the energy dissipated during the interfacial crack growth. In particular, a new test set-up is being designed to obtain a fracture parameter. This research program will be conducted in four phases combining experimental tests and analytical modeling as well as numerical simulation:
Introduction: Fiber Reinforced Polymeric (FRP) laminates, which have been used in the aerospace industry for several decades, are becoming increasingly popular in the construction industry for strengthening purposes. These laminates, particularly carbon fiber reinforced (CFRP), offer the advantages of composite materials such as immunity to corrosion, a low volume to weight ratio, a high strength to weight ratio, and unlimited delivery length (in sheet form), thus eliminating the need for joints [ACI 440, 2001]. Regarding flexural strengthening, test results have shown that beside the classical failure modes, such as steel fracture, concrete crushing or vertical shear failure, bond failures can also occur at the interface between the externally bonded Carbon Fiber Reinforced Polymeric (CFRP) laminate and the concrete element [Naaman, Lopez, et. al., 1999]. This interfacial bond failure, if present, occurs before the full strength capacity of the laminate can be achieved. In order to develop a reliable design procedure for structural elements strengthened by externally bonded FRP laminates, a full understanding of all types of failure mechanisms, including the failure at the interface level, is needed. The research project described here is part of a CAREER award whose main objective is to foment innovative teaching and research in civil engineering. The combined research and academic activities will provide a fundamental contribution on the use of non-traditional materials for new structures as well as for state-of-the-art repair and retrofit systems. PHASE I - CHARACTERIZATION OF THE INTERFACIAL CRACK PROPAGATION
Material Characterization, Sample preparation: The composite systems tested in this experimental program are comprised of a two component epoxy matrix reinforced with a custom fabric bonded to a concrete substrate. In order to obtain the material properties, samples of different laminate systems and their constituents are being prepared and tested under tension and shear. ASTM test procedures: ASTM D3039, D3518, D638, D5379.
Characterization of the Interface: The bond strength at the concrete-laminate interface is being studied in order to better explain the mechanism of crack propagation at this level. The effects of particular parameters such as: laminate and epoxy stiffness/toughness, sustained loading, concrete resistance, aggregate size, and environmental conditions, are being study both at the analytical and experimental level. In particular, a series of bond tests are being designed to provide needed information. One of the designed test set-ups is shown below.
Interfacial Failure Prediction: Results from bond tests are being used in order to predict the interfacial failure at the plate end of FRP strengthened beams. Information needed for the prediction model being developed includes: crack spacing at failure load, material properties, and interface strength.
Effects of Freeze-thaw cycles and sustained loading on FRP-Concrete Bond: The research intends to study the effects of freeze-thaw cycles and sustained loading on FRP-Concrete bond. Two different freeze-thaw regimes are to be followed. The first one according to ASTM Standard C666. The second regime will be representative of the freeze-thaw pattern observed in Pittsburg, PA, during the last 20 years. Sustained loading will be applied to these bond specimens as a percentage of the predicted failure load. In addition, effect of composite type on bond strength will be studied. Modeling the confinement effect of FRP wraps: The objective of this study is to provide an improved model of the behavior of concrete confined with FRP wraps. The initial phase of this study comprises a literature search where models proposed by research groups and/or adopted by codes (ACI, CEB-FIP) are being selected. A database of experimental tests is also being compiled. In the second phase of this study, critical parameters will be identified and a parametric analysis will be developed.
Parametric analysis: Among other analyzes, the effect of the different parameters controlling the concrete crack propagation, softening and shear transfer between closed cracks have been analyzed by comparing the experimental and predicted load deflection response.
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
