Speakers
Description
Fibre-reinforced polymer (FRP) has, over the past years, become an appealing choice as a reinforcement for concrete structures, primarily because of their good resistance to chemicals and weathering. Basalt fibre reinforced polymer (BFRP) is the latest FRP material considered as reinforcement for concrete although it is not currently included as a design option in European design standards (the Eurocodes). At Reykjavik University Structural Research Center, SEL, multiple research projects have been undertaken to demonstrate and verify the advantages of BFRP as a structural material. This research has partly focused on increasing the understanding of BFRP as a potential prestressing reinforcing material for concrete. By prestressing concrete, it is possible to reduce or eliminate tensile stresses caused by applied loads, resulting in crack-free concrete at the serviceability stage. Prestressing also increases the effective stiffness of the concrete element and thereby helps to reduce displacements. The prestressing of concrete elements is usually implemented through tendons, which are stretched in the formwork before casting the concrete. After the concrete has cured, the tension in the tendons is transferred to the concrete as compression by releasing the tendons. The emphasis of this paper is on the potential relaxation of BFRP tendons with time, as understanding the relaxation process is essential for structural design. Experimental work was carried out with three prestressed BFRP tendons in an indoor open-air environment over several years to estimate the long-term relaxation of the material. The three tendons were pre-stressed to 50% of ultimate tensile strength. Strain gauges were glued on the surface of the BFRP rebars, and accelerometers were installed on the rebars to monitor their eigenfrequencies. Regular measurements were conducted over an initial period of 2000 hours. Further measurements were then taken on more irregular bases over an eight-year period. A logarithmic curve has been fitted to the data to estimate a typical lifetime relaxation of the tendons. Based on the relation found, the relaxation of the tension force in the tendons is estimated to be about 7% over 50 years.
| Topics | Structural materials, Structural Engineering, Strucural robustness |
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| Keywords | Concrete, Basalt fiber, BFRP, tendons, prestress, relaxation. |
