Thermal and structural performance of a new fiber-reinforced polymer thermal break for energy-efficient constructions

The thermal bridges constitute critical regions in building envelopes and they are created due to the interruption of the insulation layer. One of the many structural thermal bridges in building envelopes is created in balcony junctions because of the required structural continuity of the concrete slab. The detrimental effects of such thermal bridges are minimized, by using thermal breaks that interrupt the heat flow towards the external environment by adding an intermediate insulation layer between the internal floor and the balcony slab. For the majority of these thermal breaks, the load transfer from the balcony’s cantilever to the main structure occurs through stainless steel bars, which penetrate the insulation layer and still have a high thermal conductivity. The current research proposes a new thermal break composed of fiber-reinforced polymer composite materials (FRP) that have much better thermal properties than the conventional materials and investigates its short- and long-term structural and thermal behavior. The tensile force from the cantilever moment is transmitted by an aramid-FRP (ARFP) loop, whose thermal conductivity is about 170 times smaller than that of stainless steel. The compression component of the moment is transferred by a short glass-FRP (GFRP) element while the shear compression diagonal is transmitted by a FRP-PU hexagonal foam sandwich. The insulation layer is composed of a thin layer of aerogel granulate. All the components are assembled in a polymer box. The thermal performance of the above-mentioned thermal break was validated by estimating its impact on the energy balance of a traditional building, using three different building envelopes (a MINERGIE envelope, a MINERGIE-P envelope and an optimum envelope). Furthermore, 3d finite element steady-state thermal simulations were performed in order to define the true losses through the optimized thermal break. Finally, the thermal behavior of the thermal break was compared with that of traditional thermal breaks. The structural validation of the FRP thermal break included the mechanical characterization of the components through static, long-term and durability experiments. The static experiments of the components comprised tensile experiments (AFRP loop) and compression experiments (FRP-PU hexagonal foam sandwich and GFRP bar). The long-term behavior was evaluated through tensile and compression creep experiments, while its simulation and prediction were achieved by applying traditional creep methods and two new proposed models that were able to predict the secondary creep stage (Gradient regression and Gradient regression with NHPP). The durability was evaluated by their immersion in alkaline environment, simulating that of the concrete. Finally, the total behavior of the balcony junction was examined through full scale beam experiments that led to the analytical modelling of the system.

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