Reinforce concrete beams with prestressed FRP

When FRP is used to reinforce a concrete member, although it bears a large load, the steel bars often yield and the concrete is crushed. In other words, FRP material has not exerted its tensile ability, and it is often withdrawn early due to premature opening of the glue. If prestress is applied to it, the stress of the steel bar and concrete will be reduced, and the deflection will also be reduced, enabling FRP to give full play to its potential.

Ordinary FRP reinforcement can only withstand the live load after reinforcement, and cannot withstand the dead load at that time, so it is called passive reinforcement. Reinforcing with pre-stressed FRP can not only improve the load-bearing capacity of the component, but also effectively control the number and width of cracks, and also avoid premature tearing off. However, due to its small ultimate tensile strain (about 0.15%), it consumes part of the deformation during tension, so the level of prestress applied is limited.

So far, the research on FRP prestress is mainly for the external bonding method. Under different prestressing systems, the performance of the reinforced components improves after reinforcement, the increase in bending resistance, the influence of temperature, and the failure mode. Prestressed FRP reinforcement can not only significantly increase the bearing capacity but also improve the fatigue performance.

The embedding method for prestressed FRP reinforcement can avoid the early cracking of FRP, increase the cracking moment and yield moment, and effectively control the crack width.

Prestress loss

The prestress loss of CFRP material reaches 35%. However, the strain gauge data shows that the strain loss of FRP slats is not large, the loss of the first section is 10%, and the closer to the middle, the smaller the loss, which can better transmit the prestress.

Crack condition

After the reinforcement of each group of beams, the crack layout at the time of failure is basically the same as that of the unreinforced beam. However, pre-stress is applied, which restricts the number and width of cracks. In the mid-span area, the cracks are bending cracks and “bend-shear” cracks. All experimental beams are roughly the same. The largest cracks all occurred within a range of about 1.0m near the middle of the span

Carrying capacity

The reference beam without any reinforcement has an ultimate bearing capacity of 255KN. The ultimate bearing capacity of beams strengthened by ordinary embedding method is 281KN, which is increased by 10%. The failure started from the opening of the glue, and the ultimate bearing capacity of the beam strengthened by the prestressed embedding method reached 293KN. The pre-stress levels are divided into 5%, 20% and 30% (respectively 5%, 20% and 30% of the ultimate strain), and their load-bearing capacity is increased by 11.5%, 14% and 14.5% respectively.

After prestressing the reinforced beam, its cracking load and steel bar yield load are significantly increased

In conclusion

The embedding method is used to prestress the CFRP to strengthen the concrete beam, which can increase the bearing capacity by 15%. When 30% prestress is applied to CFRP, it can effectively increase the yield load of the steel bar, which can be increased by 9% compared with strengthening the beam. The pre-stressed embedding method strengthens the beam, which obviously improves the performance of the construction and restrains the development and distribution of cracks. Compared with strengthening beams, the number of cracks is reduced by 5-30%, and the width of cracks is reduced by 22-52%. After applying prestress to CFRP, the tensile strength of the material can be fully utilized, and only about 60% of its maximum tensile force can be used without prestress. After adding prestress, its ultimate strain and ultimate tensile strength are fully exerted. CFRP is not broken by breaking. The ductility of beams strengthened with CFRP prestress is reduced.