Connection technology

There are three major types of connections that can be used to guarantee an effective shear transfer mechanism between the concrete and the FRP shape in a hybrid beam. Henceforth, the connection can be formed with a bonded joint, mechanical joint, or with a combination of the two. The selection of the joint type is usually determined by several factors such as the geometry of the members to be joined, the loads that need to be transferred, and the serviceability, fabrication and cost requirements, to name just a few.

2.3.2.1. Bonded joints

Briefly, bonded joints are generally realized with high-strength adhesive agents which are classified in function of their type, form, and curing process. The epoxy resins stand out as the most encountered solution for gluing FRP profiles to concrete as they provide strong joints and excellent creep properties. Furthermore, the epoxies have a suitable resistance to weathering agents, oils, chemical solvents, and elevated temperatures. They are essentially thermosetting resins which cure by polymerization, and come available either as two-part mixtures (resin plus hardener) or as a one-part resin, depending on the curing process involved. The limitations of the epoxy resins are embodied by the precise formulation requirements, exothermic reaction, and short pot life.

There are two possible application methods for bonding FRP and concrete, the first being the dry bond, where the adhesive is applied on a cured concrete surface, respectively the wet bond, where fresh concrete is cast on top of the adhesive agent before it has cured. Notwithstanding, there is also a third

variant where no adhesive is utilized in which concrete grout is poured around and/or inside the composite shape, connection known as pure bond.

Adhesive bonding technique is the most efficient way of achieving composite action between the FRP and concrete as it leads to connections with high strength and high stiffness. Because the load is distributed over a large interface area, there is a more uniform distribution of stresses and higher resistance to flexural, dynamic, and fatigue induced stresses [12]. Bonded joints are relatively inexpensive, are light and fast to apply, and are more appropriate for connecting irregular surfaces and obtaining esthetic forms. Lastly, they offer a good electrical and thermal insulation, and they act as a sealant, minimizing water ingression effects.

Nevertheless, bonded joints require special tools, materials and installation conditions, all which increase application costs; are difficult to inspect and disassemble, while time-dependent environmental factors (i.e., temperature, humidity, air composition, etc.) can possibly affect their properties and durability. Perhaps the most important drawback is that the failure in glued joints takes place suddenly, exhibiting a brittle behavior. It must be noted also that the load-bearing capacity is not proportional to the surfaces of the adhered components, and that the bonded connection takes a long time to develop strength.

The flexibility of bonded joints is affected by the thickness of the adhesive, its elastic properties, and the eventual local stiffeners disposed near the joined area. In addition, the connection is sensitive to the stiffness of the hybrid beam components and to the joint configuration.

2.3.2.2. Mechanical joints

Similar to the case of conventional composite members, mechanical joints in FRP-concrete hybrid beams can be realized with dowels, fasteners, bolts, threaded rods, or profiles of various shapes and sizes. The connectors are usually made from steel (galvanized or stainless, to prevent corrosion) or from fiber-reinforced polymers. Normally, one end of the mechanical connector is attached to the profile while the other is embedded into the concrete.

Because FRP composites are heterogeneous, anisotropic and brittle, every discontinuity of the fibers can reduce the pin bearing capacity of the element. Furthermore, the connection capacity is greatly influenced by the fiber orientation, thickness of the FRP, edge distance, hole clearance, and clamping force, among many other factors.

Mechanical connections are usually preferred over bonded joints due to the ease of inspection, installation and disassembly, due to the short time they take to fully develop their strength capacity, and to the ductile behavior they can possess. Moreover, no surface preparation of the base materials is required and the connection solution can turn out to be more economical when the cost of both shop and field labor work is taken into account [1]. Lastly, minor misfits generated by hole sizes or positions are easily correctable for mechanical connectors using simple hand tools.

The bolting technique produces high stress concentrations at the holes since FRP materials have a linear elastic behavior and no local plastic deformations are permitted. These important stress concentrations coupled with the anisotropy of the composite material lead in most situations to overly- conservative designs. Apart from this aspect, bolt tension can decrease over time due to strain relaxation, and shear stresses may not be distributed evenly to multiple rows of fasteners.

The use of FRP connectors can assure a thermal and electrical insulation of the joint, however, the resulting connection has a brittle failure more. On the other hand, metallic fasteners, although ductile, can lead to insulation and corrosion problems, and increase the weight of the structure. Other issues of mechanical connections are related to the time needed for realizing the assembly, the raw finished aspect of the joint, and the modest fatigue endurance. As a final point, because drilled holes in FRP profiles can provide a way for moisture and chemical agents to degrade the performance of the base material, the openings should be ideally sealed with resins.

The flexibility of bolted joints is notably influenced by the flexibility of the fasteners, slip, and bearing of fastener holes. As in the case of bonded joints, the flexibility is also susceptible to the mechanical properties of the constituent materials of the hybrid beam, and to the joint configuration.

2.3.2.3. Combined joints

Shear connectors may be added to bonded joints in order to deter the occurrence of brittle failure modes and to assure a backup solution for the initial connection system. The resulting combined joint is characterized by high strength and stiffness, and by potential post-elastic capacity. In addition, bolt connectors can provide support and pressure during assembly and curing of the adhesive, and can hinder the growth of bondline defects [12]. To emphasize, hybrid FRP-concrete beams with combined joints have a high degree of composite action and manifest little to no slip.

Regardless, combined joints are a costly constructive solution given their build complexity and the fact that the performance of the mechanical joint is only utilized after the adhesive’s capacity has been exhausted.

Various experimental tests on bonded, mechanical, and combined joints for FRP-concrete beams have been reported in ref. [30,31]. In the absence of standardized tests for characterizing the performance of pultruded FRP-concrete connections, Albiol Ibáñez [32] has recently studied the bond between GFRP laminates and concrete by analyzing the influence of a series of surface treatments applied to the composite’s surface and combined with mechanical fasteners, with the aim of identifying a suitable ductile connection for hybrid beams.