Engineering background

The concrete double T slabs of a factory that had excessive deflection. The factory building is a single-story frame structure, the beams and columns are cast-in-place frame members, and the slabs are prefabricated prestressed concrete double T slabs. Slab size: 29980mm*2390mm. T-beam bottom width is 110mm, web height: 710-1085mm.

Analysis of current situation and causes of double T slab diseases

 On-site diseases:

 ① There are many cracks on the T beam of the double T slab (the crack spacing within 10 meters of the mid-span is 1m, and the crack spacing within 10 meters of the beam end is 1.5m). The cracks penetrate the bottom slab and extend 30-35cm toward the web. The crack width is 1-2mm wide.

 ② The double T-slab sinks as a whole, the mid-span deflection of the cracked T-beam is about 30cm, and the mid-span deflection of the uncracked T-beam is about 25cm. (Note: The blue line in Figure 5 is the level stretched from the bottom of the beam at both ends, and the deflection is clearly visible)

Disease cause analysis:

 ① The actual load of the double T slab is much greater than the design load (the design load is 150kg/m2, the actual load is about 400kg/m2)

 ② The prestressed steel strand may have excessive prestress loss.

 ③ The strength of concrete is insufficient. (Concrete that is not dense and pitted is more common)

Reinforcement plan and construction technology

 Maintenance and reinforcement plan:

 As a prestressed concrete structure, this prefabricated double T slab has many transverse cracks, with a maximum crack width of 2mm and a maximum deflection of 30cm. According to the shape, distribution and development of cracks, the double T slab shows insufficient bearing capacity. In order to improve the durability and safety performance of the double T-slab, the 30m-span T-beam is reinforced with prestressed carbon fiber plates.

 ① For the 110mm wide T-beam, use a 50mm wide and 3mm thick carbon plate.

 ② Based on the span of 30m and the distribution of cracks, a 25m-long carbon plate is used.

 ③ In view of the on-site disease, the tension value of the prestressed carbon plate is designed to be 1000MPa (15t).

Construction technology:

 ① Set up a steel pipe frame under the double T-slab T beam.

 ② Drill holes at the anchorage position of the prestressed carbon plate.

 ③ Installation of prestressed carbon plate anchors.

 ④ Installation of prestressed carbon fiber plate.

 ⑤ Prepare glue and apply structural glue.

 ⑥ Prestressed tensioning.