In the structure strengthening,  pasting carbon fiber fabric and paste steel plate in the concrete structure is two frequently-used method. In the selection of the reinforcement scheme, the two methods often appear at the same time. From the point of view of the action mechanism, the two reinforcement methods are formed by the structural adhesive and the concrete base material, and the two are similar in the construction process, which often causes some obstacles to the selection of the methods.

In fact, the similar reinforcement mechanism does not mean that the two party can choose at will. There are differences in mechanical properties between CFRP sheets and steel sheets, resulting in two different performances in reinforcement.

Beam reinforcement as an example

1.Crack development

In the process of increasing load, the concrete will crack in the tensile zone of the bottom of the beam, and when the base material has old cracks, the cracking is easy to occur at the old crack. Because of the limitation of the number of carbon fiber sheets, the thickness will not exceed 0.7mm, and the steel plate constructed by manual coating can reach 5mm thick, and the section area of the steel plate is much larger than the carbon fiber section area under the same wide amplitude. Therefore, the beam strengthened by the stick steel is much higher than that of the beam strengthened by the carbon cloth.

From the number of the main cracks, with the increase of load, the reinforcement of the bonded steel is more than that of the CFRP, and the crack expansion and extension of the carbon cloth are slow. CFRP is better in limiting cracks, and CFRP is more suitable for crack resistance.

2.Deflection

The deflection is mainly related to the bending stiffness of beams. In structural reinforcement, sticking carbon cloth or sticking steel plate can enhance rigidity through two aspects. First, the elastic modulus of carbon fiber cloth is close to that of steel plate, so the improvement of stiffness depends mainly on the sectional area. In the contrast of the crack, it can be seen that the cross section of the carbon fiber cloth is far less than the section area of the steel plate, so the rigidity of the steel plate is much higher than that of the carbon fiber cloth.

On the other hand, the original stiffness of beams can be maintained from the perspective of restricting the development of cracks. In this regard, CFRP reinforcement has a higher restriction on cracks, so that the rigidity of beams can be better maintained. However, in the two aspects, it is much better to improve the stiffness effect by raising the section area to improve the stiffness of the crack. The concrete performance is that the deflection of the beams strengthened in the bonded steel is slower than that of the carbon bonded cloth, and the damage load of the bonded steel reinforcement is higher. That is to say, it is more effective to enhance the rigidity and flexural capacity of beams and strengthen with bonded steel plates.

3.Durability

Durability affects the long-term reinforcement effect of the structure. From the point of view of material, the durability of the steel plate is poor, easy to rust, and in the structure reinforcement, the thickness of the steel plate is thinner, when the rust is rusty, it will weaken the cross section or cause the peeling damage. The durability of CFRP is much better than that of steel plate, but direct exposure to sunlight and harmful media should be avoided. In addition, structural adhesives have effects on the durability of the two reinforcement methods. In general, the durability of CFRP reinforcement is better.

From the above comparison, we can find that although the two methods are similar to CFRP reinforcement and glued steel reinforcement, the applicability is different in different strengthening tendencies. In the actual scheme selection, appropriate construction technology should be selected according to the actual reinforcement requirements.