Abstract
One of the main concerns of experimental and numerical investigations regarding the behavior of fiber-reinforced polymer reinforced concrete (FRP-RC) members is their fire resistance to elevated temperatures and structural performance at and after fire exposure. However, the data currently available on the behavior of fiber-reinforced polymer (FRP) reinforced members related to elevated temperatures are scarce, specifically relating to the strength capacity of beams after being subjected to elevated temperatures. This paper investigates the residual strength capacity of beams strengthened internally with various (FRP) reinforcement types after being subjected to high temperatures, reflecting the conditions of a fire. The testing was made for concrete beams reinforced with three different types of FRP bars: (i) basalt-FRP (BFRP), (ii) hybrid FRP with carbon and basalt fibers (HFRP) and (iii) nano-hybrid FRP (nHFRP), with modification of the epoxy matrix of the rebar. Tested beams were first loaded at 50% of their ultimate strength capacity, then unloaded before being heated in a furnace and allowed to cool, and finally reloaded flexurally until failure. The results show an atypical behavior observed for HFRP bars and nHFRP bars reinforced beams, where after a certain temperature threshold the deflection began to decrease. The authors suggest that this phenomenon is connected with the thermal expansion coefficient of the carbon fibers present in HFRP and nHFRP bars and therefore creep can appear in those fibers, which causes an effect of “prestressing” of the beams.
Highlights
For the last few decades, the use of fiber-reinforced polymers (FRP) composites for concrete structures has experienced explosive growth
As one of the main aims was to examine the influence of different reinforcement types, the testing was made for concrete beams reinforced with three different types of FRP bars, including newly developed hybrid FRP bars: (i) basalt-FRP (BFRP), (ii) hybrid FRP with carbon and basalt fibers (HFRP)
The performed tests provide a means to estimate the influence of elevated temperatures on beams reinforced with FRP and investigate their thermal behavior being subjected to those temperatures
Summary
For the last few decades, the use of fiber-reinforced polymers (FRP) composites for concrete structures has experienced explosive growth. Some authors suggest that such hybridization of the constituents of FRP bars can prevent changes in their behavior, making it semi-ductile instead of linear [8,9] Their use in the form of FRP bars as a replacement of conventional reinforcement for reinforced concrete (RC) structures has become attractive. The material properties and behavior of FRP composites at elevated temperatures is a critical topic that needs clear understanding in terms of their structural uses in civil engineering. The relatively non-corrosive behavior of the fibers prevents a significant reduction of concrete clear cover without compromising the material’s durability [28,29,30] This enables thinner structures to be built, whose construction requires less raw materials and is more sustainable, environmentally friendly, durable, and cost-efficient. The results of the investigations were discussed, with particular emphasis on the structural performance of the concrete members and on the types of FRP bars used
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