Adhesive Connections in Civil Engineering

The use and applications of composite structures made of FRP (Fibre Reinforced Polymers or Plastics) are increasing due to the FRP favorable properties like rapid on-site assembly, lightweight, high resistance to aggressive chemicals, superior fatigue life and electromagnetic neutrality. For FRP composite structures to be competitive with structures made of traditional materials, they must be safe, serviceable, durable and economical. Structural safety and serviceability depend on the structural members’, as well as on their joints’ or connections’, strength and stiffness. The connections must be able to safely resist reasonable levels of load compared to the members’ load-bearing capacity, and must possess sufficient stiffness to avoid excessive deformations. Currently, the connections in FRP structures are commonly made using bolted connections, akin to those used in steel structures. A recent European report [1] by Technical Committee 250 of CEN (Comité Européen de Normalization) stipulates that bonded connections should not be allowed for primary load bearing components, where failure of the connection could lead to progressive collapse or unacceptable risks and their use is permitted only in combination with or as a backup for bolted connections. At the same time, the report highlights that prohibition of bonded connections represents one of the key issues that impede the steadily increasing market for FRP profiles in the field of civil construction, which already utilizes 35% of the annual world-wide production of GFRP profiles. Theoretically, there are reasons to believe that bonded connections can be superior to bolted connections in FRP composite structures. For example, it is well known that the holes made in structural members with bolted connections cause stress concentration and increase the risk of moisture penetration in members. On the contrary, it has been observed in simple bonded lap joint connections that due to the absence of holes, the stresses are more uniformly distributed over the bonded surfaces, stress concentration and damage to the fibers caused by the holes are non-existent. The main reason for the prohibition of bonded connections is lack of knowledge about and experience with the performance of such connections. To fill this gap in knowledge, an experimental investigation on the behavior of full-scale bonded beam-to-column moment resisting connections was recently performed [2], with the ultimate goal of developing high performance bonded connections that can equal or surpass the performance of similar bolted connections in FRP structures. A single pultruded GFRP I-profile is used for the two members. In four of the specimens the beam and the column are connected by epoxy adhesive and GFRP seat angles, similar to the so-called “standard bolted connection”. In the remaining four specimens, the seat angles are supplemented by additional GFRP angles and stiffeners to strengthen the column flange and web. The beam-column assembly forms an inverted L-shape frame, with the column being fixed at the bottom and attached to the beam near the top. The beam, acting as a cantilever, is loaded by a point load near its free end, which subjects the connection to bending and shear. The current standard connection failed by debonding within the column flange while the improved/strengthened connection failed within the adhesive or at the adhesive-column flange interface. The test results reveal that both the standard and improved connection can have at least the same strength as the corresponding bolted connection, irrespective of whether GFRP or steel bolts are used to make the connection. Hence, the current restrictions against the use of adhesive beam-column connections in GFRP frame structures may be unjustified. Based on the authors’ knowledge, this is the first study of its kind and therefore its scope is limited to monotonically increasing static loading of the connection, without dealing with issues of creep or fatigue, which would be subjects of future investigations