Project Brief

The main building of Baiyun Chemical Fiber Company was completed and put into use in November 1995. It is a 7-story frame and brick-concrete structure house with cast-in-place floors and roofs. The design strength grade of structural concrete is C25, and the foundation is concrete raft foundation. On the afternoon of November 28, 2008, a fire broke out in this building. The fire was located between the 6th and 7th axis of the 8.00 m floor, which caused the 8.00 m, 11.62 m floor slab and the 3-story frame column to be over fired and damaged. Since the production raw materials are flammable and fluid substances, they flow to the second floor through the reserved holes in the floor slab, causing damage to the surface of the 2-story column, 2-story and 3-story floor slab.

The damaged floor slab is mainly manifested by the looseness of the two-layer slab surface, which reduces the actual slab thickness. 3. The four-layer boards were all over-fired and damaged, with serious cracks, local concrete bursts missing, the depth of concrete carbonization greatly exceeded 6.0 mm, and the lowest estimated concrete strength value was 15.6-17.0 MPa. The damaged frame column is the 2-story column was over-fired and damaged, and the column body was severely cracked. The three-story column is seriously damaged, the concrete body is densely cracked, and the local concrete is loose. The carbonization depth of the column concrete greatly exceeds 6.0 mm, and the estimated value of the concrete strength is 13.3 to 14.5 MPa. As the building is closely related to the company's production activities, it was decided to adopt feasible methods for structural strengthening as soon as possible.

Strengthening method

The purpose of maintenance and strengthening is to ensure and improve the reliability of the structure, make it reach the reliability level specified by the current national standards, and meet the requirements of the building's use function. The method of maintenance and strengthening should be selected through a comprehensive analysis of specific conditions, and the maintenance and strengthening of the building should be as far as possible to achieve comprehensive benefits.

The fire-damaged components are mainly externally bonded steel plate, and at the same time, the method of externally pasting unidirectional carbon fiber is used for strengthening treatment(mainly write new materials: carbon fiber strengthening). Different parts and types of components are reinforced and treated. 

Externally bonded unidirectional carbon fiber strengthening method

1 Carbon fiber strengthening materials

The 8.0 m floor frame beams of this project, all secondary beams that should be reinforced, and the B/A axis and B/c axis frame columns are reinforced with the unidirectional carbon fiber method. This method has high resistance to chemical corrosion and protection of the reinforced structure, and can improve the durability of the structure. Secondly, because of the high strength of the unidirectional carbon fiber material, the amount of external strengthening is small, the thickness is small, the load increase is minimal, and the shape and size of the original structure are hardly changed. Moreover, the construction period is short and the operation is simple. The carbon fiber adopts high-strength carbon fiber reinforced sheet with a specification of 200 g/m. , The calculated thickness is 0.111 mm, the standard value of tensile strength is ≥3 500 MPa, the modulus of elasticity is ≥2.3×10 MPa, and the elongation is ≥1.5

2 Carbon fiber strengthening method

11.62m secondary beam is pasted with a layer of 200 mm wide carbon fiber at the bottom of the beam, and four U-shaped pastes with a width of 100 mm and a spacing of 250 mm are pasted on both ends of the beam, and a 100 mm wide carbon fiber layering is covered. 8.00 m times the beam is pasted with two layers of 200 mm wide carbon fiber at the bottom of the beam, and four 100 mm wide U-shaped pastes are pasted at each end of the beam, and 100 mm wide carbon fiber layering is overlaid. The bottom of the main beam of the 8.00 m frame is pasted with two layers of 200 mm wide carbon fiber. The beam is pasted near the B axis with three 100 mm wide U-shaped pasting. Paste four U-shaped pastes with a width of 100 mm near the A axis and the C axis, and are covered with a 100 mm wide carbon fiber layer.

The frame columns of B/A and B/C axes are reinforced with a full-length column body and one layer of unidirectional carbon fiber.

3 Key points of carbon fiber strengthening construction

Remove spalling, looseness, honeycomb, corrosion and other deteriorated concrete on the surface of beam and column components, and repair and level it. The sticking surface should be polished until the new concrete surface is exposed. Wipe it with acetone to make it clean and free of dirt. Apply glue evenly on the sticking surface and the surface of the carbon fiber, then stick it, and use a roller to roll it along the fiber direction several times to squeeze out the bubbles and fully wet the carbon fiber. The fiber direction of the beam bottom paste and bead is parallel to the beam axis, and the u-shaped paste direction is perpendicular to the beam axis. The beam angle of the u-shaped paste is polished into a rounded corner, and the radius of curvature is not less than 20 mm. The direction of the column outer carbon fiber is perpendicular to the column axis. The column angle is polished to a rounded corner, and the radius of curvature is not less than 20mm. Carbon fiber sheets shall not be overlapped and pasted. The total effective bonding area of carbon fiber sheets shall not be less than 95. After drying, the surface carbon fiber glue is used as a protective layer, and a layer of clean river sand is sprinkled.

In conclusion 

(1) In the fire-damaged structure, the outer steel method is used to strengthen the frame beams and columns, and the external carbon fiber method is used to strengthen the secondary beams and part of the frame beams and columns.

(2) The correct construction method can ensure the successful application of structural strengthening measures.

(3) After strengthening treatment, under normal maintenance and use, it can meet the use requirements of the 50-year base period.