carbon fiber structural reinforcement
Concrete is the most widely used building material in the world due to its high cost-effectiveness, abundant raw materials, easy manufacturing, and excellent physical and mechanical properties.
However, concrete also has limitations, which may affect its durability and service life. Mainly due to its low flexural and tensile strength, as well as brittleness, the structure may fail without warning under tensile and shear conditions.
Therefore, structural concrete usually needs reinforcement to improve the structural integrity of buildings. Although steel fiber is the main reinforcement element used globally, there are also some issues with its use, mainly in terms of its long-term performance and sustainability.
Why use carbon fiber?
In recent years, the application of carbon fiber in the construction industry has become increasingly widespread, mainly used for curtain walls, partition walls, and wall frames. Carbon fiber reinforced concrete (CFRC) was first used as a potential building material in the 1970s and has been proven to have some suitable properties.
Early research on CFRC made from PAN fibers significantly improved the impact strength, tensile strength, and elastic modulus of concrete. In 1980, a research report on the use of asphalt carbon fiber in Japan stated that significant improvement was achieved by using only 0.2% by volume of carbon fiber.
Further research has confirmed that even adding a very small amount of carbon fiber can significantly improve the mechanical properties of concrete, with carbon fiber performing better than steel or glass fiber. This is due to its excellent durability, heat resistance, weather resistance, and processability under harsh conditions.
However, one drawback of carbon fiber is its cost. So far, this has hindered its large-scale application in structural components, but researchers believe that the initial cost of carbon fiber is worth it compared to the cost of retrofitting damaged structures with traditional steel fiber-reinforced concrete.
Carbon fibers contain over 92 wt% carbon arranged in a crystal structure parallel to the fiber's long axis. According to the manufacturing process, the structure can be amorphous, partially crystalline, or fully crystalline, and can exist in the form of short or long fibers.
Carbon fiber is classified as a high-performance material due to its fundamental characteristics. These characteristics include low specific gravity, ability to withstand stress under dynamic and quasi-static load conditions, high thermal conductivity, excellent creep resistance, high elastic modulus, non corrosiveness, and chemical stability.
In addition, carbon fiber has excellent aesthetic properties and is attractive for projects that require high-quality finished products. However, carbon fiber also has some limitations that may hinder its use.
Fibers are prone to oxidation under heating or alkaline conditions. In addition, fibers also have disadvantages such as axial and transverse anisotropy and low fracture strain. The main obstacle to the use of CFRC is the poor ductility of carbon fiber, which is lower than alternatives such as glass, Kevlar, and SiO2.
TDS of carbon fiber structural reinforcement(12K and 24K)
Specific reinforcement methods
There are many commonly used reinforcement methods at present, such as increasing the cross-section method, wrapping steel reinforcement method, bonding steel reinforcement method, carbon fiber reinforcement method, etc. Carbon fiber reinforcement and repair structure technology is another new type of structural reinforcement technology after increasing the concrete section and bonding steel.
Scope of application
The carbon fiber reinforcement method can be used for flexural and shear reinforcement of concrete structures, and is widely used for seismic, crack, and anti-corrosion reinforcement of various industrial and civil buildings and structures. Reinforcement of beams, columns, and panels in concrete structures, bridges, and buildings. Reinforcement of tunnels, harbor facilities, chimneys, warehouses, and factories. Protection and reinforcement of concrete, bridges, and river structures damaged by salt.
Process principle
Pre impregnating carbon fibers with high tensile strength into composite reinforcement materials (unidirectional continuous fibers) using epoxy resin; Use epoxy resin adhesive to stick along the tensile direction or perpendicular to the crack direction on the structure to be reinforced, forming a new composite, so that the reinforced adhesive material and the original reinforced concrete are jointly stressed to increase the crack resistance or shear resistance of the structure, and improve the strength, stiffness, crack resistance, and elongation of the structure.
technological process
Construction preparation → Concrete surface treatment → Apply primer → Repair damaged surface of components → Paste carbon fiber → Surface curing → Leveling material preparation → Bottom layer or resin preparation → Impregnation resin preparation
Operation points
1, Concrete surface treatment
1. Clean up the damaged and broken parts on the surface of the concrete components.
2. Repair and restore the damaged parts of the components that have been chiseled, cleaned, and exposed.
3. Crack repair: For cracks with a width less than 0.2mm, apply epoxy resin for surface coating and sealing; Cracks larger than 0.2mm should be filled with epoxy resin.
4. Polishing: Polish the protruding parts of the component surface (such as the junction of concrete components and the joints of formwork) flat, and make the repaired component surface as smooth as possible.
5. Clean and polish the surface of the components, and allow them to dry thoroughly.
2, Apply primer to the defective area
Apply the prepared base adhesive (bonding agent) evenly onto the surface of the defective area of the concrete.
3, Repair of surface defects on components
The use of epoxy putty for repairing damaged surfaces on components is a crucial step, with the following key points:
1. Accurately weigh the putty main agent, curing accelerator, and curing agent according to the specified ratio, put them into a container, and stir evenly with a mixer.
2. Fill the concave parts on the surface of the component with epoxy putty and repair them to a smooth surface. When using epoxy putty in incomplete repairs, construction should be carried out under conditions where the temperature is above -5 ℃ and the relative humidity is less than 85%. After applying putty, any unevenness or roughness that still exists on the surface should be smoothed with sandpaper.
4, Apply primer
Apply the prepared adhesive (bonding agent) evenly onto the surface of the concrete to be bonded using a roller.
5, Paste carbon fiber
Before pasting carbon fiber materials, it is necessary to first confirm that the pasting surface is dry. When the temperature is above -10 ℃ and the relative humidity RH is greater than 85%, construction shall not be carried out without effective measures. To prevent damage to carbon fiber during transportation, storage, cutting, and pasting of carbon fiber materials. Cut the carbon fiber material with a steel ruler and wallpaper knife according to the specified size, and the length of each section should generally not exceed 6m. To prevent damage to materials during storage, the cutting quantity of materials should be based on the daily usage. The longitudinal joints of carbon fiber must overlap by more than 20cm. This area should be coated with more resin, and carbon fiber does not need to be overlapped horizontally.
The key points of its construction process are as follows:
1. The main agent, curing accelerator, and curing agent for pasting resin should be accurately weighed according to the prescribed ratio, loaded into a container, and stirred evenly with a mixer. The initial blending amount should be used up within the usable time.
2. When pasting, try not to have air between the carbon fiber and resin. Use a roller (special tool) to roll the carbon fiber material along the fiber direction multiple times, allowing the resin to penetrate into the carbon fiber.
6, Maintenance
After pasting the carbon fiber material, it needs to be naturally cured for 1-2 hours to achieve initial curing, and it should be ensured that there is no external interference or collision during the curing period.
7, Painting
If fire prevention is required for reinforced components, a fire-resistant coating can be applied after the resin has cured. Painting should be carried out after the initial curing of the resin and should comply with the relevant standards and construction regulations of the coating used.
Quality requirement
1. All incoming materials, including carbon fiber materials and bonding materials, must comply with quality standards and have factory product certificates, meeting the requirements of engineering reinforcement design.
2. Carbon fiber materials should not be compressed during transportation and storage to prevent damage to the carbon fibers, nor should they be directly exposed to sunlight and rain. Adhesive materials should be stored in a cool and sealed manner.
3. The construction quality of each process shall be guided and supervised by technical personnel. After each process is completed, it shall be submitted to the technician for inspection and approval before proceeding to the next process.
4. Apply bottom coat paint
The coating should be evenly applied and not missed. It is strictly prohibited to apply under unsuitable temperature conditions. The coating diluted with solvents should be used up within the specified time.
When the temperature is below 7 ℃, the relative humidity RH is less than 85%, the surface moisture content of concrete is below 8%, and there is no condensation phenomenon, modified epoxy resin must be used.
5. Surface damage repair of components
Small holes and interior corners on the surface of the structure must be repaired and leveled with epoxy putty. Any unevenness or roughness that still exists on the surface after applying putty should be smoothed with sandpaper.
6. Paste carbon fiber
① When pasting carbon fiber materials, the following conditions must be met:
a, Carbon fiber materials should be cut according to regulations.
b, The temperature, humidity, and surface moisture content of the components meet the requirements.
c, The bottom coating and putty have reached finger touch drying (the resin surface has solidified and hardened).
d, The type of bonding resin should be suitable for the temperature during construction. The main agent, curing agent, and curing accelerator should be accurately weighed according to the specified ratio, loaded into a container, and stirred evenly with a mixer. The amount of each mixing should be used up within the specified time.
② Quality inspection standards for carbon fiber bonding.
a, The bottom coating and top coating are well immersed in carbon fiber bundles.
b, Carbon fiber is tightly bonded.
c, For hollow drums with a diameter of 10mm to 30mm, less than 10 per square meter can be considered qualified; If there are more than 10 per square meter, it is considered unqualified and needs to be repaired. For hollow drums with a diameter of over 30mm, any occurrence is considered unqualified and requires repair.
d, The overlapping length along the fiber direction shall not be less than 20cm, and the overlapping parts between each layer shall not be located on the same straight line. The layers must be staggered by at least 50cm.
e, The specifications of carbon fiber material, including pasting position, length, width, fiber direction, and number of layers, comply with regulations
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