Post-strengthening with Externally Bonded Prestressed CFRP Strips

摘要

Since post-strengthening, structural repair and upgrading of reinforced concrete structures has increasingly become one of the main tasks in structural engineering, effective techniques and methods suitable for on-site application as well as high performance materials are needed to be able to handle this task with 1. a minimum of expenditure 2. a minimum of shut-downs or disruptions during repairs 3. a maximum extension of the member's life. The in-situ rehabilitation of reinforced concrete members using bonded CFRP strips which are applied by means of adhesive as a second level external flexural reinforcement has proven to be an effective and economic method of improving structural performance. So far, the CFRP strips are used unstressed at the time of bonding. The benefits of using prestressed externally bonded CFRP strips for the strengthening of concrete members have been investigated in laboratories and have already been recognised for many years. The advantages compared to unstressed CFRP strips can be summarised as follows: 1. The strain and stress capacity of this high-tensile material can be used to a greater degree 2. The strains on the internal reinforcement can be relieved 3. The strips participate in carrying dead load and permanent loads at the time of bonding 4. The strips participate in bearing the load of additional loadings without any slip-delay 5. At low temperatures (member shrinkage) the CFRP strip still remains a tension element 6. Cracked sections can be stressed/compressed back to the uncracked state, thus gaining in stiffness 7. Crack widths and bending deflection can be reduced, thus improving serviceability Therefore, a new post-tensioning and anchorage system for prestressed externally bonded CFRP strips, which is suitable for on-site application and can be directly anchored on the surface of the member to be strengthened, has been developed by Leonhardt, Andra and Partner. The system can be easily handled on-site and is suitable for both vertical and overhead application and provides a maximum prestress of about 1000 N/mm~2 (at a strain of 5.5 %o) to each laminate during the pot life of the adhesive.