Cyclic flexural performance of fire-damaged reinforced concrete beams strengthened with carbon fiber-reinforced polymer plates

This paper presents the cyclic flexural performance of reinforced concrete (RC) beams after being exposed to a fire for 3 hours, air-cooled, and then strengthened with partial-length, adhesive-bonded carbon fiber-reinforced polymer (CFRP) plates. A total of 15 T-beams were tested under static four-point bending with periodic unloading and reloading at regular intervals until failure. Test variables included the level of exposure temperature (700 or 900°C [1292 or 1652°F]), sustained service loading (in addition to the beam self-weight) at the elevated temperature, and the CFRP plate length. The investigated flexural responses included the cyclic load-deflection relationship, CFRP strain-deflection relationship, and failure mode. No repair was conducted on beams exposed to 700°C (1292°F) before installation of the externally bonded CFRP plates. However, repair was necessary for beams exposed to 900°C (1652°F) due to excessive spalling of the concrete. For these beams, the concrete substrate was removed to a depth of 60 mm and replaced with the repair mortar before FRP strengthening. The externally bonded CFRP plates were found to reinstate the fire-damaged beams in terms of their flexural strength and stiffness at load levels beyond the cracking load of an undamaged beam. The level of strength enhancement was higher for the fire-damaged beams subjected to the simulated service loading than the counterpart beams exposed to fire with no load and the undamaged beams. Using mechanical end anchorages, the level of strength enhancement increased when the CFRP plate was extended closer to the supports. However, the addition of CFRP plates decreased the ductility of most strengthened beams. All CFRP-strengthened RC beams exhibited an intermediate crack-induced debonding failure. A comparison between the effective strain limits predicted with the ACI 440.2R-17 equation and the measured tensile strains in CFRP plates showed that the equation could be unconservative for the CFRP-strengthened RC beams under cyclic loading.