Overview

Traditional strengthening of reinforced concrete structures mostly adopts the method of sticking steel or outsourcing concrete and reinforcing steel, but its construction method is complicated, the volume is huge and heavy, and the construction period is long. Carbon fiber strengthening is a new technology for strengthening of reinforced concrete structures. It is one of the crack management and structural strengthening technologies recently adopted at home and abroad. It will become a new trend for strengthening of concrete structures at home and abroad. The basic principles, application scope and construction examples of carbon fiber strengthening technology are described below.

Basic principles of carbon fiber strengthening technology

The carbon fiber with extremely high tensile strength is pre-impregnated with epoxy resin into a composite reinforced sheet (unidirectional continuous fiber sheet). Use epoxy resin adhesive to stick on the structure to be reinforced along the tensile direction or perpendicular to the crack direction to form a new composite body. Make the reinforced carbon sheet and the original reinforced concrete bear the force together, increase the tensile or shear resistance of the structure, and improve the strength, rigidity, crack resistance and ductility of the structure.

Application of carbon fiber sheet repairing and strengthening concrete structure technology in engineering

(1) Beams and slabs: strengthening in the tension zone of the bending member (the lower part of the middle span, the upper part of the continuous beam and slab support)-pasted in the whole length. Design according to needs to determine the number of layers. The cracks in the tension zone can be sealed with carbon fiber sheets after filling the joints to control the continued expansion of the cracks.

(2) Beam: Shear strengthening-For oblique cracks caused by the main tensile stress, carbon fiber sheets are pasted on the vertical axis of the beam side, and unidirectional fiber sheets can be cross-bonded if necessary.

(3) Columns and piers are under central compression, eccentric compression, and lateral force-bearing members. strengthening of central compression members-the middle of the column is wrapped around to strengthen the restraint on the concrete and improve the compressive strength. Eccentric compression members and seismic strengthening-the column is longitudinally pasted on all sides for bending strengthening. Flexural strengthening of the plastic hinge area of the pier column. Shear strengthening when the column is subjected to lateral force-the column end ring wraps (equal to stirrup encryption). Ductility strengthening of seismic column structure-combined bending and shear strengthening.

(4) Shear wall-carbon fiber sheets are pasted along the direction of shear strengthening on one or both sides.

(5) Sealing function-the carbon fiber sheet is pasted across the seam to make the carbon fiber and the concrete work together to extend the service life of the concrete component.

Advantages of carbon fiber repair and strengthening technology

(1) The strengthening is thin and light, and hardly increases the size of the original structure and the weight of the structure itself. The tensile strength of carbon fiber sheet is 7-10 times that of steel plate of the same cross-section, and the thickness of the finished sheet is only 0.111mm~0.167mm. It is difficult to achieve this with the strengthening method of outsourcing concrete, especially when used in places with limited storey heights.

(2) The construction is simple and fast. The carbon fiber sheet can be directly pasted and wrapped on the surface of the building, the process is simple, no formwork and large-scale hoisting equipment are required, the requirements for the construction site are low, and it is suitable for narrow space operations. The construction is quick and the construction period is short. It can also be constructed under the conditions of normal operation of production and facilities to avoid the difficulties and losses of production and shutdown.

(3) Anti-corrosion of acid, alkali and salt media-used in offshore, chemical industry, and structures exposed to corrosive media can be protected from protection. This is incomparable to the methods of bonding steel strengthening and outsourcing concrete strengthening.

(4) It can effectively close the cracks of the concrete structure and prolong the service life of the structure.

(5) The carbon fiber sheet can be pasted in multiple layers, suitable for structures of various shapes, and fully meet the requirements of repair and strengthening.

(6) It is easy to keep the structure intact (paste with the original shape). The outer concrete method increases the volume of the structure, while the stick steel strengthening method is more difficult to strengthen irregularly shaped structures, especially for special-shaped beams and columns (such as haunched beams, fish-belly beams, thin web I-shaped beams, and special-shaped beams). Column) and so on. The use of carbon fiber sheets for strengthening can maintain the structural characteristics of the original building to the utmost extent and make the appearance of the original structure unchanged. The surface of the carbon fiber sheet is also easy to paint, and it can be easily painted with various coatings or attached to various boards to adapt to the requirements of the surrounding environment and minimize the traces of repair.

Project Overview

A certain scientific research building was originally an eleven-story building with frame structure. Due to the needs of the owner, the function of the scientific research building needs to be changed. According to the requirements of the design drawings: epoxy resin grout is used to repair the cracks of the original structural slab; the related beams and slabs involved due to functional changes are used to reinforce the structure by pasting carbon fiber. The wall columns of the foundation section are reinforced by the outer steel strengthening method (the wall column strengthening method is omitted).

Construction method Pasting carbon fiber The construction requirements are: the concrete surface is treated, smoothed, cleaned up, and then the primer is painted on the concrete surface, carbon fiber cloth is manually dipped, and the carbon fiber cloth is pasted. And use a special tool to remove air and bubbles, and finally apply a layer of adhesive evenly on the surface.

(1) Overview of pasted carbon fiber design

The treatment of the concrete base surface with carbon fiber and the full impregnation of carbon fiber are the difficulty in the construction of the carbon fiber. It is also the key to whether the construction period can be completed on time. Therefore, the construction should be strengthened to ensure the quality and quantity are completed.

(2) Pasting carbon fiber construction process

The construction process of pasting carbon fiber includes: construction preparation, concrete surface treatment, leveling and repairing of unevenness with reinforced leveling mortar, bottom resin preparation and painting, pasting resin preparation, pasting carbon fiber, surface protection.

(3) Pasting carbon fiber construction method

①Concrete surface treatment: Locate the position where the design is pasted according to the design drawing;

Remove the concrete mortar surface layer to expose the concrete surface. To paste the board surface, use a marble cutter to cut grooves on the ground terrazzo, remove the mortar surface layer, and use an angle grinder to remove oil and other impurities until the new surface of the concrete structure is completely exposed, and the carbonized layer of the concrete is required to be ground;

If there are cracks, the cracks need to be grouted and sealed;

Use an angle grinder to trim the arc of R~>20mm on the external corners of the concrete components, and smooth the uneven parts of the concrete surface. If the concrete surface has large spalling, honeycomb, corrosion and other defects, it should be chiseled out ；

Use high-pressure air to remove dust and sand on the concrete surface, and use acetone to wipe the surface of the concrete structure;

Before applying the primer resin, the treated concrete surface must be dry and clean

Finally, use a 1m straight edge ruler to check the flatness difference of the treated surface. The maximum allowable difference in the lm range in any direction is ±5mm.

②Partial surface leveling treatment of concrete:

Prepare leveling, and mix according to the process requirements of the epoxy latex mud provided by the supplier;

The recessed part of the concrete surface shall be filled and leveled with a scraper, and there shall be no edges or corners, and the difference between the repaired surface and the concrete surface shall not exceed 1mm. Use leveling resin to repair the corners to a smooth arc with a radius of not less than 20mm;

After consolidation, the next step of construction shall be carried out as soon as possible after the surface of the leveling material is dry to the touch.

③Smear the base resin

Use the supporting bottom resin to put the main agent and the curing agent into the container according to a certain ratio, and mix them evenly to prepare the bottom resin. Determine each dosage according to the actual situation on site and strictly control the use time;

Use a roller brush to evenly coat the bottom layer of resin on the surface of the concrete with a thickness of not more than 0.4mm, and no leakage, flow, or air bubbles;

The carbon fiber sheet should be pasted immediately after the bottom resin finger is clean

④Adhesive resin preparation: use the matching adhesive resin provided by the supplier, and the ratio is A material: B material = 2:1.

⑤ Paste carbon fiber

Before sticking carbon fiber, it is necessary to calculate the size of the sticking range, surface shape, construction environment, construction time, construction personnel, etc. in a planned way;

The appropriate amount of resin must be blended to avoid exceeding the available time. After preparing the paste resin, immediately apply it evenly with a roller brush. The amount of the bottom layer is 500~600g/m;

After coating the resin, immediately paste the pre-cut carbon fiber on the construction site, and scrape it lightly with rubber or plastic for several times.

After bonding, use a roller to roll back and forth in the direction of the carbon fiber several times to let the resin soak and squeeze out bubbles;

After 30 minutes and within 3 hours, check whether the resin of the bottom layer is saturated or not, then use a roller to roll in the direction of the fiber, and then paint according to the standard of the upper layer. The upper layer coating amount is 300~400g/m. ；

When multi-layer pasting, repeat the above steps one by one, but the surface of the carbon fiber should be touched, and then wait 12 hours before applying the carbon fiber to paste the next layer; the surface of the last layer of carbon fiber should be evenly coated with a bonding agent.

⑥The order of pasting carbon fiber sheets according to the design requirements is:

When the carbon fiber bonded to the beam surface overlaps with the carbon fiber bonded to the board surface, the carbon fiber used for board strengthening is first pasted, and then the carbon fiber cloth used for strengthening of the beam surface is pasted. When the bottom of the beam is reinforced with carbon fiber, the beam is first reinforced with carbon fiber and then the fiber hoop is bonded.

⑦Surface protection: 1:3 mortar thickness 25mm is used to protect the carbon fiber pasted on the board surface. To ensure reliable bonding between the mortar and the carbon fiber sheet, sprinkle a layer of medium-coarse sand before the adhesive is cured. Apply the next layer immediately after drying, such as more than 60 minutes

Concluding remarks

Carbon fiber strengthening is a new treatment and strengthening technology that has emerged in recent years. The technology is characterized by high tensile strength, light weight, extremely thin veneer, corrosion resistance and fatigue resistance, as well as physical characteristics such as easy cutting, easy concealment, wide adaptability, and convenient construction. Carbon fiber strengthening shows excellent technical effects and economic benefits, and its development prospects are very broad. It is currently one of the ideal crack compensation measures.