Deflection Calculation of Damaged RC Beams Strengthened with a BFRP Grid–Reinforced ECC Matrix

As an innovative high-performance composite material, basalt fiber–reinforced polymer (BFRP) grid–reinforced engineered cementitious composite (ECC) effectively leverages the superior mechanical properties of both BFRP grids and ECC. This composite material is characterized by its high durability, enhanced toughness, and exceptional crack control capabilities. A flexural performance test was conducted on 10 test beams to investigate the reinforcement efficacy of this novel composite material on damaged RC beams. The test results showed that the BFRP grid–reinforced ECC composite reinforcement method significantly improved the flexural performance of the damaged beams, with an increase in the ultimate bearing capacity ranging from 7% to 40% and a decrease in the midspan deflection ranging from 3.7% to 36.5%. The flexural capacity, stiffness, and ductility of the strengthened beam decreased as the damage degree increased. The initial deflection of the RC beam after unloading and the strain hysteresis effect of the strengthened layer after reinforcement were proportional to the damage, and both affected the mechanical properties of the strengthened beam. When the damage degree of the RC beam was significant and the BFRP grid was excessively thick, the tensile performance of the grid could not be fully optimized. The stiffness calculation formulas for undamaged strengthened beams and predamaged strengthened beams were derived and verified in this paper. The formula could serve as a reference for calculating the deflection of similar composite strengthened damaged beams.