Structural performance of GFRP reinforced balcony slab with thermal break

Thermal bridging in building envelopes can lead to heat exchange with the outside. Significant thermal bridging occurs through cantilevered balconies, because they pierce through the building envelope. A potential solution is to add a thermal barrier and use materials with low thermal conductivity, such as GFRP, to reinforce balconies. The central objective of this research was to compare the thermal and structural performance of three types of thermal breaks in GFRP reinforced cantilevered balcony slabs. This study is Phase II of a two-phase research project at the University of Manitoba. In Phase I the thermal and structural performance of specimens with ArmathermTM 500 break reinforced with carbon steel, stainless steel and GFRP reinforcement were investigated (Boila, 2018). For this study, nine segments of full-scale balcony slabs were constructed and tested. All specimens were reinforced with #15M GFRP rebars and included a thermal break midway along their length, creating an inside and outside slab separated by this thermal break. Specimen dimensions were 1600 mm by 500 mm by 190 mm. The three types of thermal breaks used in the nine specimens were ArmathermTM 500, DOW and UHMW, and each had a thickness of 13 mm. These breaks were chosen based on their thermal properties, strength and market availability. Six of the specimens (three pairs, each pair with the same type of thermal break) were tested in dual thermal chambers in which the cantilever end, representing the outside slab, was at about -30 °C. The floor end, representing the inside slab was kept at about +21 °C. The purpose was to measure the amount of heat exchange through the slab and its thermal break between the two thermal environments. Thermal breaks were included in the location of maximum moment. Concrete, which carries most of the flexural and shear load, is completely replaced by the thermal break at that location. To evaluate the strength of this connection, structural tests for all nine specimens were carried out to failure by applying a monotonic load at the tip of the cantilever. Strain gauges and PI gauges were installed on rebar and on concrete to measure strain, dilation between thermal break and concrete, as well as crack widths on concrete. An LVDT device measured deflection due to load applied at the cantilevered end. Thermal testing showed that ArmathermTM 500 is the most effective thermal break in decreasing thermal bridging through balcony slabs: In ArmathermTM 500 specimens the temperature difference across the thermal break was 27% and 72% greater than the temperature difference across the thermal breaks in DOW and UHMW specimens, respectively. Structural tests showed that at service load the largest deflection in ArmathermTM 500 slabs was 27% and 42% smaller than the largest deflections in UHMW and DOW slabs, respectively. The dilation between the ArmathermTM 500 and concrete was the smallest among the three thermal break types as well. In slabs with ArmathermTM 500 the dilation was 33% and 18% smaller than that in slabs with DOW and UHMW thermal breaks, respectively. In summary, at service load, the thermal and structural performance of slabs with ArmathermTM 500 thermal break was better than that of specimens with DOW and UHMW thermal breaks.

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