The influence of the reinforcement member itself

For concrete cylinders, carbon fiber composite reinforcement materials can produce uniform lateral restraint effects after reinforcement. In recent years, after a lot of theoretical and experimental research, a relatively mature theoretical system has been formed. Most of the cross-sectional shapes of concrete columns in actual projects are rectangular. Due to the limitation of the cross-sectional shape, the lateral restraining pressure generated by the fiber composite material on the rectangular column cross-section is uneven, which easily leads to poor reinforcement effects.

The carbon fiber composite material has obvious effect on the shear reinforcement of beams with a small hoop ratio. When the stirrup ratio of the concrete beam is close to the minimum reinforcement ratio, the shear resistance of the oblique section of the beam after reinforcement can be increased by 30% to 40%, while the reinforcement effect is not obvious when the stirrup ratio is close to the maximum stirrup ratio.

The effect of carbon fiber cloth reinforcement method, spacing and number of layers

Reinforcement method

At present, the main forms of bonding of fiber cloth at home and abroad are: wrapping and pasting, closed winding and pasting, side pasting, and U-shaped pasting (including double L-board U-shaped pasting). The side paste mainly includes: X-shaped paste on the side, I-shaped paste on the side, full paste on the side, and parallel paste in the direction of the vertical diagonal crack.

U-shaped paste is better than I-shaped paste. Without end anchoring, sticking horizontal fiber cloth and fiber cloth along diagonal cracks on the side of the structure has no effect on the improvement of the shear resistance of the structure. Pasting multiple fiber cloths perpendicular to the diagonal cracks can obtain the same reinforcement effect as the U-shaped pasting. Enclosed package paste construction usually has to go through the floor or even the wall. Although good results can be obtained, it is difficult to achieve in actual engineering. In summary, U-shaped paste with end anchors is an ideal reinforcement method.

spacing

By reducing the bonding distance of the fiber cloth, the fiber cloth can better delay and prevent the formation of main cracks, limit the development of small cracks, increase the bite force of the shear surface aggregate, and significantly increase the shear bearing capacity. When the paste spacing is large, the reinforcement effect is relatively weak.

Paste layer number

The effect of the number of layers of carbon fiber cloth on the increase of the bearing capacity of the oblique section of the beam is not obvious. This is mainly because if one layer is pasted, part of the carbon fiber cloth on the side of the beam can be broken and the strength is fully utilized. As the number of layers increases, the required anchorage length increases accordingly, and the anchorage length that can be provided during shear reinforcement is limited, resulting in the failure of the carbon fiber cloth to bond, and its strength cannot be fully utilized. If effective measures can be taken to ensure the anchoring performance of the carbon fiber cloth, the reinforcement effect will be better as the number of layers increases. Although the increase in the number of layers does not significantly improve the shear capacity, it has a significant inhibitory effect on the cracking of the beam.

The effect of adhesive properties on the reinforcement effect

The adaptability of the adhesive and carbon fiber is obvious for fully exerting the tensile strength of carbon fiber materials, and the difference in the mechanical properties of adhesives will also affect the bond strength of the interface between CFRP and concrete, thereby affecting the ultimate strength of carbon fiber materials.

The properties such as the shear strength of the adhesive also have a greater impact on the reinforcement effect. The higher the shear strength, the more obvious the reinforcement effect.

Therefore, in actual engineering, the mechanical properties, working performance, long-term stability and compatibility with carbon fibers should be considered when selecting adhesives to ensure that the adhesive can fully penetrate into the gaps between the carbon fiber monofilaments, so that each carbon fiber monofilament work together.

The viscosity can be adjusted by heating, and it must not be diluted with organic solvents. The main agent and hardening agent must be used strictly after mixing, and the mixture that exceeds the available time must not be used.

Quality control during construction

Except for special conditions, the reinforcement construction of carbon fiber cloth should be organized in accordance with the requirements of the standard construction process. The standard construction process is as follows.

(1) Surface treatment of the structure, determine the construction surface range, remove the degraded layer, and polish the surface.

(2) Repair the construction surface to remove the bad peeling part, repair the cement, fill the cracks with resin, and trim the arc surface of the outer corner.

(3) Apply the bottom layer resin to prepare the bottom layer resin; apply with a brush; dry.

(4) Trim uneven surfaces and flatten the protruding parts; trim the inner corners smoothly; fill the recessed parts with leveling materials.

(5) Pasting carbon fiber cloth to prepare impregnating resin; roller brush coating (bottom coating); pasting carbon fiber cloth construction: plastic sheet is smoothed and infiltrated and defoamed; roller brush coating (upper coating); plastic sheet is smoothed and infiltrated.

(6) Maintenance.

(7) Surface protection. If cement or inorganic materials are used to fill the recessed part during construction, the order of items 3 and 4 in the above construction process shall be interchanged.

Temperature and humidity

Construction can be carried out only when the air temperature and the temperature of the structure are above 5℃, and heating treatment can be done if necessary to ensure the temperature requirements. However, the temperature difference between the structure temperature and the ambient temperature should not be too large to prevent condensation. If there is moisture or humidity above 85% on the construction surface, the construction should be stopped in rainy weather or weather prone to condensation. When there is water leakage, stop and conduct water treatment.

Construction surface flatness requirements

In order to ensure the paste effect, the unevenness of the construction surface should be trimmed to within 1 mm; the arc of the outer corner trimming surface must be R=10 mm or more (when using high modulus carbon fiber cloth, R=20 mm or more); The corners should be trimmed to be smooth.

Concluding remarks

To sum up, the technology of reinforcing reinforced concrete with carbon fiber composite material, as a new type of reinforcement technology, has a good reinforcement effect. With the continuous deepening of research on carbon fiber reinforcement technology and the continuous reduction of carbon fiber composite material cost, the application of carbon fiber reinforcement technology will certainly be more popular and concerned.