Background

Carbon fiber cloth reinforcement and repair structure technology is a new type of structural reinforcement technology. It uses resin bonding materials to paste carbon fiber cloth on the concrete surface to achieve the purpose of reinforcing structures and components. The research on the use of carbon fiber materials (CFRP) for reinforcement and repair of concrete structures began in the United States, Japan and other developed countries in the 1980s. my country started very late and has developed rapidly in recent years. , construction, electronics, sports equipment and many other fields have been widely used.
The use of carbon fiber cloth reinforcement has the following advantages: 1. High tensile strength, which is 10 times that of ordinary steel; 2. After reinforcement, the corrosion resistance and durability of the structure can be greatly improved; 3. Light weight (about 200g/m* ), the density is only 1/4 of that of ordinary steel; it basically does not increase the structure's self-weight and cross-sectional size; it is flexible, easy to cut, and has a wide range of applications, suitable for any shape; Wet work, easy to operate, good economy; 5, wide application range, short construction period.

Project overview

The residential building in front of the station in Siping City was built in the early 1990s. It has a frame shear wall structure at the bottom, a 2-story frame at the bottom, and a 4-story residential building at the top. In the original design, floors 1 and 2 were restaurants (with a live load of 2.5KN/m³), and the kitchen (with a live load of 4.0KN/m) was located on the first floor. It is now planned to renovate and open a KFC chain store, move the kitchen to the second floor, and set up a dining table at the original kitchen location on the first floor. Due to the increase in load due to the change of function, the original 2-story local beams will not meet the design requirements and need to be reinforced. After program demonstration and economic and technical feasibility comparison, it was decided to use carbon fiber composite materials for reinforcement.

Reinforcement design

Basic assumptions
The calculation of ultimate bearing capacity of concrete beams reinforced with carbon fiber materials adopts the following basic assumptions:
The effect of concrete in the tension zone is neglected;
After the beam is subjected to bending, the strains of concrete, steel bars and carbon fibers conform to the assumption of plane section;
(3) The linear elastic stress-strain relationship is adopted for carbon fiber materials: Ot=E×eet and e.<0.01.
3.2 Reinforcement design
According to the "Technical Regulations for Reinforcement and Repair of Concrete Structures with Carbon Fiber Sheets" (CECS146:2003), the limit design method based on probability theory is used for calculation. Considering that the live load acting on the reinforced structure is basically unloaded, the influence of the secondary force is not considered. Take the mid-span of the A-axis beam as an example
Beam section: b×h=350mm×650mm, C 30 concrete, original beam with Aao=2900mm
Design bending moment: M, =510Kn m
: According to the original design, the large size is calculated at 20mAa'9
A.o'=980mm²
Calculate the maximum bending moment design value according to the original design:
x=(f, A, one f, 'A,')/f.b=115mm
Mu=f.bx(ho-x/2)+fy'A.'(ho-a')
=14.3x350x115x(585-57.5)+300x980(585-40)=463.6kN m
In this design, 3 layers of 300mm wide carbon fiber cloth (A et=99.9mm*) are pasted on the bottom of the beam, calculated according to the formula
Carbon fiber sheet thickness reduction factor: k.=1-naE at a/420000=1-3x 2.3x 10°x 0.111/420000=0.818[ea] =k, ecx=0.017>0.01 take[ea] =0.01
Boundary relative height: n=0.8Xe. /(ea+[eg])=0.8×0.0033/((0.0033+0.01)=0.1985 The height x of the concrete compression zone and the tensile strain ec of the carbon fiber sheet on the tension surface should be calculated according to the following formula:
1), f. bx=f, A.-f, 'A, '+Eae: At
2) 、x=0.8ea/eat ea
Solution: No. cfb h=0.1985x 650=129<x~137mm
According to the specification formula 4.3.2-1, M=f.bx(ho two x/2)+fy'Aa'(ho one a') +Ee feet Aef(h one ho)=14.3x350x137x(585-68.5)+ 300x980(585-40)+2.3x10x0.0051x99.9x65=524kN·m From the calculation results, it can be seen that after strengthening, the design value of the maximum bending moment of this beam has increased by 13% compared with the original one. 3.3 Construction practices
In order to strengthen the anchorage of the carbon fiber cloth at the bottom of the beam and increase the shear resistance of the beam, one 160mm wide U-shaped hoop is pasted on each side of the beam end, and one 160mm wide U-shaped hoop is pasted on each side of the intersection of the primary and secondary beams.
The pasting height of U-shaped pasting should take the height of the component section. For the U-shaped pasting form, it is advisable to paste longitudinal carbon fiber sheet beading on the upper end. 2 Considering that the multi-layer paste of carbon fiber cloth is greatly affected by the construction site, it is recommended that the number of layers of carbon fiber cloth pasted by the bending member should not exceed 3 layers.

Application

The general process is unloading-base treatment-priming glue-leveling-pasting-protection.
Considering the special construction environment factors of this project, unloading should be carried out during reinforcement. Before reinforcement, remove the painted layer and sundries on the board surface.
During the reinforcement construction of this project, the protective layer should be cut to the base of the structural layer before the next step can be processed.
Basic construction steps:
(1) Base treatment:
① The deteriorating parts of the concrete surface, such as spalling, hollowing, honeycomb, corrosion, etc., should be chiseled off, and epoxy mortar should be used to repair the large area of inferior layers after chiseling off.
② Concrete angle grinder, sandpaper and other equipment to remove impurities such as laitance and oil stains on the concrete surface. The concrete on the base surface of the component should be polished flat, especially the raised parts on the surface should be smoothed, and the corners pasted should be chamfered and polished Into an arc shape (R ≥ 20mm).
③The fan cleans the concrete surface and keeps it dry.
(2) Primer
①According to the mixing ratio of main agent:curing agent=3:1, configure primer. Put the main agent and the curing agent in the container successively, measure with a spring balance, and stir evenly with an electric stirrer. Determine the dosage according to the actual temperature on site and strictly control the use time. Generally, it will be used up within 20-50mim.
②Apply the primer evenly on the surface of the concrete, and wait for the glue to cure (the curing time depends on the temperature on site, subject to finger dryness) before proceeding to the next step of construction. The general curing time is 6-24h.
(3) Leveling
① The sunken part of the concrete surface should be filled with repairing agent, and the parts with height difference such as formwork joints should be filled with repairing agent to minimize the height difference.
② The corners should also be repaired with a repair agent to form a smooth arc with a radius of not less than 20mm. The next step can be carried out after the surface of the repair agent is dry to the touch.
(4) paste
① Cut the carbon fiber cloth according to the size required by the design.
② Mix and stir the paste material adhesive (the method of use is the same as that of primer), and then apply it evenly on the parts to be pasted, and apply more on the lap joints, concrete corners and other parts. The thickness of the brushing should be slightly thicker than that of the primer. It is strictly forbidden to miss the brush, and pay special attention to the edge of the carbon fiber.
③Paste the carbon fiber cloth. After confirming that the pasted part is correct, use a special tool to repeatedly roll along the fiber direction to remove air bubbles, and make the adhesive fully soak the carbon fiber cloth; and use a scraper to scrape the surface of the carbon fiber cloth. uniform. The above steps should be repeated for multi-layer pasting, and the next layer of pasting can only be carried out after the surface of the carbon fiber cloth is dry to the touch.
④Apply adhesive glue evenly on the surface of the last layer of carbon fiber cloth. Use a tool to repeatedly roll along the fiber direction, and use a scraper to scrape the adhesive on the surface of the carbon fiber cloth to make it even.
⑤ The overlapping length of the carbon fiber cloth along the fiber direction shall not be less than 200mm.
(5) Protection
The surface of the reinforced carbon fiber cloth is protected by plastering or spraying fireproof paint.
(6) Quality requirements
①After each process is finished, it should be inspected according to the process requirements, and relevant acceptance records should be made. If there is any quality problem, it should be reworked immediately.
② On-site acceptance after construction is mainly to evaluate the bonding quality between carbon fiber cloth and concrete. Use small hammers and other tools to gently tap the surface of carbon fiber cloth to judge the bonding effect by echo. In case of empty drums and other irregularities in pasting, it should be remedied by injecting glue through the needle tube. If the bonding area is less than 90%, it is judged that the bonding is invalid and needs to be re-constructed.
③Strictly control the temperature and humidity of the construction site. The construction temperature is within the range of 5-35°C, and the relative humidity is not more than 70%.
5. Construction effect
It took less than two months for the project to be put into use from design to completion. It was constructed using carbon fiber reinforcement. It took only 10 days from preparation to completion. There was no quality problem, and the construction effect was good. Units, owners of praise.