Florian Riebel
In the context of sustainable development, building insulation represents a major concern as a means of increasing energy efficiency. In order to comply with new norms such as the Passive House standard, locations where load-bearing components must penetrate the building's insulating envelope, such as when cantilevered slabs (e.g. balconies) are anchored to building walls, constitute a particular challenge. With conventional steel reinforcement, thermal bridges are inevitable and insulation is weakened. To overcome this deficiency, a multifunctional joint has been developed in which GFRP (glass-fiber reinforced polymer) elements performthe necessary structural functions without compromising the continuity of the building's insulating envelope. This is due to GFRP materials' low thermal conductivity, which is magnitudes smaller than that of concrete and steel, making them particularly suitable. The anchorage of balconies requires linkages that can resist shear as well as tensile and compression forces in the upper and lower parts of the slab. In a first step, a hybrid joint was created in which the lower compression steel reinforcement was replaced by a compression-shear (CS-)element made of GFRP. This consisted of a short pultruded profile with cap plates bonded to its cut ends. In addition to compression forces, the element was intended to bear parts of the shear load. The joint was investigated in various full-scale beam specimens each representing a section of the slab. Based on the results, the joint's structural behavior was modeled analytically and structural requirements for the CS-element were determined. It was seen that shear transfer through the element increased with increasing shear-to-moment ratios. In a second step, an all-GFRP joint was created in which, in addition to the element in the compression zone, a tension-shear (TS-)element replaced the remaining steel reinforcement. This element consisted of the same pultruded profile cut to longer sections and penetrating the concrete. To anchor the tensile forces in the concrete, ribs were bonded to its surface. This joint type was also investigated through full-scale beam experiments similar to the hybrid-joint beams. The load transfer through the joint and into the concrete was studied and modeled analytically. The behavior of the all-GFRP joint was as ductile as that of the hybrid joints. The TS-element bore the main portion of the shear load independently of the shear-to-moment ratio however. The CS-element used in both joint types was of special interest with regard to remaining strength and stiffness after long-term service life. Exposure to alkaline concrete-pore solution represents a particular threat to the polyester matrix and glass fibers inside the GFRP material. Therefore, CS-elements were immersed in alkaline liquids at different temperatures and their compression strength and stiffness were studied during a period of eighteen months. Material degradation was investigated by SEM-microscopic images and EDX-analysis. The observed loss of compressive strength was ascribed to moisture diffusion and chemical degradation of the fibers, matrix, and fiber-matrix interface. It was shown that the strength degradation rate at different temperatures followed the Arrhenius rate law and that remaining strength after long time spans could therefore be projected by extrapolating the measurements. Remaining strength after 70 years of service life was found to be sufficient for the element never to become the critical failure location and classic concrete theory can be applied to verify the joint. Since stiffness at high temperatures was observed to already remain constant after short exposure times, this value could directly be extrapolated to apply to a 70-year service lifespan.
EPFL2006
