Impact of Internal CFRP strips on the flexural behavior of heat-damaged reinforced concrete beams

Recent theoretical developments revealed that reinforced concrete (RC) structures are susceptible to deterioration risk upon exposure to high temperatures where the mechanical properties of their constituents are affected and therefore require upgrading their overall performance. However, the overall behavior could be improved by strengthing the RC beams using the well-known carbon fiber-reinforcement polymers (CFRP) materials where its efficiency is highly limited by detachment, and debonding problems appear as a result of the weakness in the bond between the concrete surface and the strengthening material or upon the stress concentration induced by the various anchoring systems. The CFRP sheets have been integrated as internal reinforcement in the maximum bending zone within the thermally damaged beams, a new technology used in this study. The suggested method was the first of its kind and did not need an adhesive to be applied where debonding problem is eliminated. In contrast to conventionally reinforced steel, CFRP composite materials are fully compatible with flexural steel and constrained concrete. A total of 40 RC with (150 × 200) mm² and an overall length of 1100 mm concrete beams were cast, and the studied parameters were the CFRP length, position, and exposure temperature. The internal strengthening technique has been found to ensure the full utilization of the strengthening material where the externally-strengthened beams fail preceding the CFRP strain reached, and this was confirmed using the linear weighted sum method where the internal strengthening has the highest ranking based on the mechanical characteristics comparisons. Moreover, the internal CFRP reinforcement improves RC beam performance, strength, stiffness, toughness, and serviceability more than exterior CFRP sheets. However, the enhancement percentages are twice as much for internal strengthening as the external one. It has also been found that the reinforcement's location substantially impacted the number and length of flexural cracks and its failure mode. In addition, for every 1% reduction in concrete compressive strength in heat damage, the average ultimate load was reduced by 0.8%. The CFRP profitability indexes decrease as sheet number and temperature increase; the average toughness decrease at 150 °C, 250 °C, and 500 °C is 12%, 21%, and 47%, respectively.