Beams Strengthening

The office building of a property management company was originally a two-story brick-concrete structure with double-sided inner corridors. The offices are generally double or three rooms, with a span of 5.4m, an open space of 3.6m, a floor height of 3.6m, and a building area of 2000m square. Due to the needs of the work, it is necessary to expand and build an additional floor. Empirically, the original roof beam oblique section bearing capacity meets the requirements, and the flexural bearing capacity of the normal section has a large gap. The roof beams and slabs need to be reinforced. In order to reduce the amount of dismantling and speed up the construction speed, after program demonstration and comprehensive comparison of economic and technical feasibility, it is determined to use unidirectional carbon fiber materials to strengthen the beams and slabs. 

Beam reinforcement design

It can be seen from the calculation results that the design value of the maximum bending moment of the beam after carbon fiber strengthening reinforcement is increased by 40% compared to the original.

In order to strengthen the anchoring of the one-way carbon fiber cloth at the bottom of the beam and increase the shear resistance of the beam, three 100mm wide U-shaped hoops were pasted on each side of the beam end, with a clear distance of 100. The height of the U-shaped hoop is to the bottom of the prefabricated board, and a strip of carbon fiber sheet material with a width of 100mm in the longitudinal direction should be pasted on the upper end. Taking into account the factors that are greatly affected by the construction site when the carbon fiber cloth is pasted in multiple layers, it is recommended that the number of layers of carbon fiber cloth pasted on the bending member should not exceed 3 layers.

Structural reinforcement materials

This project uses Horse unidirectional carbon fiber cloth made of carbon fiber yarn imported from Japan, weighing 200g/square meter.

Considering the special construction environment factors of this project, unloading should be carried out first in the reinforcement. Remove the waterproof layer and insulation layer on the roof of the roof panel before reinforcement. During the reinforcement construction, the protective layer should be cut to reach the base of the structure layer before proceeding to the next step. The basic construction steps are as follows:

• Unloading

• Substrate treatment

• Primer

• Leveling

• Apply carbon fiber adhesive

• Paste one-way carbon fiber cloth

• Apply carbon fiber glue again

• Protection

The site acceptance after the completion of construction is mainly to evaluate the bonding quality between carbon fiber cloth and concrete. Tap the surface of the carbon fiber cloth with a hammer or other tools to judge the bonding effect by echo. If there is a phenomenon that the paste is not dense, such as a hollow drum, the needle should be injected with adhesive glue to remedy it. If the bonding area is less than 90%, the bonding is judged to be invalid and re-construction is required. Strictly control the temperature and humidity of the construction site. The construction temperature is within the range of 5～35℃, and the relative humidity is not more than 70%.

The test showed that the unidirectional carbon fiber wrap reinforced components worked well under the new load, and no abnormal phenomena were found. It shows that the one-way carbon fiber reinforced concrete beam is reliable and successful.