The development and application of carbon fiber can be traced back to carbon fiber in 1850, and the research and application have been at a standstill since then. In 1950s, with the development of industrial technology and the requirements of military industry, the development and production of carbon fiber solved the problem of precursor selection and industrialized production process of high-temperature carbonization, which made the application of carbon fiber enter a new stage. First of all, it has been used in military fields such as aerospace, and gradually extended to advanced civil industries, and it has only been used for structural reinforcement of construction projects in recent ten years.
Carbon fiber and graphite fiber are collectively called carbon fiber, and the carbon content is above 95%. However, carbon element can only be melted at high temperature and high pressure, and it is impossible to prepare carbon fiber directly from carbon element. Theoretically, any organic fiber can be made into carbon fiber after carbonization. In fact, at present, the only precursors with industrial significance are polyacrylonitrile fiber and mesophase pitch, and carbon fiber is mainly produced by polyacrylonitrile fiber in various countries. After high-temperature carbonization and other special processes, superfine fiber filaments (diameter 5~ 10μm) are processed, which improves the strength of monofilaments, increases the surface area of a certain amount of fibers, and is more conducive to strengthening the combination with resin adhesive.
In the past, one of the factors restricting the application of carbon fiber reinforcement technology was the price of precursor and finished product. At the time of successful research and development, the price per kilogram of carbon fiber precursor was above 1000 USD, and now it has dropped below 30 USD. The countries with the highest carbon fiber precursor output are still Japan and the United States, while products from Japan, French, Swiss and Taiwan Province Province of China have entered the Chinese mainland market with carbon fiber fabrics. Domestic carbon fiber fabrics woven with imported precursors have also begun to enter the market, and the market competition is becoming increasingly fierce, and the price is gradually decreasing.
Uses: carbon fiber products are mainly used in civil engineering, such as fiber cloth, fiberboard, bar, profile, short fiber and so on. , each has a different scope of application. At present, carbon fiber cloth (sheet) is the most widely used in reinforcement engineering. The common specifications of carbon fiber cloth are 200g/m2 and 300g/m2, and the thickness is 0. 1 165438+ respectively. The thickness of carbon fiber composite board is generally 1.2~ 1.4mm, and it is made of 3~4 layers of carbon fiber cloth impregnated and cured by resin. It is mainly used to reinforce beams and slabs. Fiberboard reinforced structure has regular appearance and simple construction, but the unit price of raw materials is high, so it is not widely used in China.
The most important indexes of high performance carbon fiber cloth are still its strength, elastic modulus and elongation at break. Generally, the tensile strength is above 3500 Mpa, the elastic modulus is above 230000 Mpa, and the elongation is above 65438 0.4%. Structural reinforcement mainly uses the high tensile properties of carbon fiber, which is widely used to strengthen the joints of beams, slabs, columns and frames of reinforced concrete structures, and is also suitable for the maintenance, reinforcement, repair and restoration of ancient buildings or masonry structures. 1995 After Hanshin earthquake and Taiwan Province earthquake, the position of carbon fiber as seismic strengthening material and technology has been further developed and determined.
In recent years, many universities, scientific research departments and major design institutes in China have participated in the application and research of carbon fiber. At present, the country has not promulgated formal design and construction specifications. Among the research projects determined by the Ministry of Construction, there are topics such as "FRP used for strengthening and repairing urban bridges" and "new technology for strengthening concrete structures with carbon fiber materials", and the application research of carbon fiber has been paid attention by the state.
Structure: For flexural members, carbon fiber sheets are pasted in the tension area to make up for the shortage of steel bars, and carbon fiber sheets are pasted on both sides of the beam to improve the shear capacity of the beam. Wrapping around the outside of concrete columns limits the lateral deformation of concrete, which can improve the bearing capacity and axial compression ratio of columns. Winding around the joint of truss can strengthen the overall strength of the joint. The number of layers of carbon fiber cloth can be determined by calculation. Considering the same work coefficient, fatigue resistance and avoiding brittle failure of each layer, it is generally recommended not to exceed 5 layers. From the mechanical properties, single layer is better than multi-layer, and narrow width is better than wide width; When necessary, it can be longitudinally overlapped, and the overlapping length should not be less than100 mm. In order to ensure the reliable anchorage of carbon fiber ends, in addition to meeting the calculation requirements, necessary structural measures should be taken.
Bonding: the performance of adhesive and bonding process are the guarantee for the reinforced component to bear the same force as carbon fiber cloth. It is required that the adhesive has high bonding strength and strength to the bonded interface and carbon fiber cloth, and the tensile strength, compressive strength, especially bonding shear strength should be much higher than the corresponding strength of concrete. Adhesives should have good permeability and compatibility to the interface and carbon fiber cloth, and have excellent properties such as impact resistance, fatigue resistance and aging resistance, so special adhesives should be used.
Perfect construction technology is an important factor of reinforcement effect. The main requirements are: cleaning, repairing and leveling the pasted surface, which should be dry and moisture-proof; The carbon fiber cloth is coated with a high-viscosity adhesive with a special tool, and the adhesive is completely immersed in the monofilament after moderate rolling; Eliminate bubbles, and don't shell, disorderly line, bend or twist; Perfect technology and well-trained workers can make the effective bonding area reach more than 99%, so that the performance of carbon fiber can really be brought into play.
Excerpt from forum space