Dottorato in INGEGNERIA STRUTTURALE E GEOTECNICA

Titolo della tesi: Fiber-Reinforced Polymer Materials for Structural Applications: Experimental and Numerical Studies vs. Anchorage Systems

Despite the recent worldwide spread, Fiber Reinforced Polymers (FRPs) have relatively old origins. They were introduced in the second half of the last century starting from defence, aerospace and nautical fields. The first use of polymers in civil engineering dates back to 1957, when the House of the Future, entirely made of plastic elements, was built in the Disneyland Park at Anheim, California. This paradigmatic experience fueled interest among researchers on the use of innovative materials for constructions and it paved the way to further works. In this regard, FRPs drew attention fast due to the noticeable physical and mechanical properties, such as lightness, high stiffness- and strength-to-weight ratios and admirable resistance to corrosion. In the beginning, FRPs were introduced for retrofitting of existing structures, mainly reinforced concrete and masonry, and the success they achieved made them viable alternatives to traditional materials for new constructions. According to the knowledge of the author, the first documented FRP structure was a footbridge in Tel-Aviv, Israel, erected in 1972. It is no coincidence that, among civil structures, footbridges were the first to embrace FRPs: the lightness of material facilitates the quick fabrication of the structure. Recently, a pilot project started at the University of Salerno. It concerned the design of a cable-stayed footbridge entirely made of PGFRP (Pultruded Glass Fiber-Reinforced Polymer) elements, except for the cable stays, which were ϕ12 PCFRP (Pultruded Carbon Fiber-Reinforced Polymer) cables. Within this project, the design of the split wedge anchorage for the CFRP stays, performed in a previous work (Quadrino, 2020), requested particular efforts. This was due to the intrinsic orthotropy of FRPs, which exhibit both stiffness and strength along the transverse direction one or two orders of magnitude lower than the fiber direction. Hence, the uncontrolled combination of pressure and shear stress exerted on the FRP bar by the anchorage device could trigger its premature failure, which is typically sudden. Researchers agree upon shaping contact surfaces of the anchorage in order to mitigate the stress concentrations along the FRP cable. Inspired by the existing solutions of the technical literature, the split wedge system was conceived with double-angle wedges having (1) a constant slope of 3° for the 25% of the length and (2) the remaining 75% toward the loaded end of the cable tilted at 3.1°. Experimental tensile tests on anchorage specimens were carried out on two double-angle (DA) and three with a single-angle (SA) wedge profile. Tests once returned a failure load of 257 kN for the DA system, that is the cable average tensile capacity provided by the manufacturer. This anchorage prototype can be considered a novelty due to the carried load that, according to the literature, has never been reached for a one-cable anchorage. Such satisfactory results encouraged the author to pursue the investigation of the anchorage system from the numerical viewpoint, with the goal to validate the system performance, also including the fracturing damage induced by the anchorage parts to the FRP cable during loading. In fact, as far as split wedge systems are concerned, forces transferred to the cable by wedges may trigger the onset and growth of cracks on the bar surface and the consequent weakening of the system until the failure. The development of a DIC (Digital Image Correlation) software constituted the first stage of the present study. The elaborated experimental results were used to calibrate the contact parameters of finite element models of the anchorage prototypes. Numerical results highlighted that the DA configuration turned out to satisfactorily avoid stress peak superpositions on the cable, with a reduction of pressure in the loading end of the cable with respect to the SA model. However, finite element results can be considered reliable to evaluate stresses in the anchored bar under ideal conditions, assuming a linear elastic material behavior. In conjunction with the lack of adequate strength criteria, this stimulated the effort to implement specific models, namely microplane and discrete lattice models (DLMs), which could capture the effects of damage induced by fracture into the FRP. Additional experimental tests were performed on small specimens of epoxy resin and PCFRP cable, replicating the load conditions of the anchorage system. The experimental results allowed the calibration of the meso-models, subsequently used for evaluating the effects of the anchorage on the CFRP cable. According to the knowledge of the author, the present work was the first to include the description of the fracturing damage within the field of anchorage systems for FRP cables.