Repair treatment of concrete cracks
Whether cast-in-place or prefabricated reinforced concrete structures and structural members, the characteristics and causes of common cracks will be very different according to their structural types, mechanical characteristics, environment and uses. In order to facilitate the analysis, this paper briefly introduces the characteristics, causes and treatment methods of some common concrete structural cracks in buildings.
1. Reinforced concrete beam
(1) Vertical cracks and horizontal cracks on the side of reinforced concrete beams
This kind of crack is gradually produced after the formwork is removed, and its performance characteristics are as follows: water-like turtle crack: this kind of crack mostly appears at the upper and lower edges of the beam, which is unevenly distributed along the whole length of the beam, and the crack is shallow, reaching the stirrup surface at the largest; Vertical crack: This kind of crack usually appears every other section along the length of the beam, and its height will reach the height of the whole beam section. Sometimes the shape is wide in the middle and narrow at both ends, with different shades, generally around 20mm, close to the main reinforcement. The reasons for this kind of crack are usually: water crack is mainly due to the fact that the template is too dry and wet, especially when the wooden template is not soaked, it is more likely to produce this kind of crack; Vertical cracks are mainly caused by too little water for concrete curing, especially after formwork removal, which is not watered in time or the weather is hot. In the case of sunlight, it is easy to produce similar cracks, which are caused by plastic shrinkage and dry shrinkage cracks of concrete. Preventive measures are to strengthen the number of sprinklers and cover riot prevention.
Slight water ripples and vertical cracks can be left untreated, and serious cracks can be repaired with epoxy cement; If the water ripple joint is serious, the concrete strength is not high, or even peeling, the loose part will be chiseled clean, and then sealed with high-strength cement mortar.
(2) Cracks along the main reinforcement in reinforced concrete beams
This kind of crack usually appears after the project is put into use for a period of time, and it will develop with time at the lower side or the main reinforcement at the bottom of the beam. The reason is that the iron oxide formed when the steel bars begin to rust expands, which is usually called "rust first and then crack". The main reasons for the corrosion of steel bars are: the concrete protective layer is too thin; Use additives containing chlorine salts; The corrosive gas or liquid in the use environment is immersed in concrete; Precautionary measures: fix the steel skeleton during construction, and accurately control the thickness of protective layer at the lower part of the main reinforcement in advance; Try to make adaptability analysis before admixture is added; When external conditions permit, carry out anti-corrosion treatment on concrete members; The removal of formwork must be determined according to the requirements of construction specifications and the compressive strength of specimens with the same strength. This kind of crack will reduce the bonding force between steel bar and concrete. In severe cases, it will endanger the safe use function of the structure. Therefore, attention must be paid to joint filling or reinforcement. After the joints are widened, they are caulked with epoxy mortar, and then reinforcement measures are taken, such as outsourcing steel bars, outsourcing reinforced concrete sleeves, adding stress water to tie rods, and sticking steel plates to reinforce them.
(3) Vertical cracks or oblique cracks in reinforced concrete beams
This kind of crack often appears in the construction stage and in use, and belongs to load crack. Vertical cracks often appear in the middle of the beam, mainly due to the lack of crack resistance and bending strength of the normal section; In-plane oblique cracks often occur at both ends of the beam because the anti-crack and shear strength of the inclined section are insufficient. If it is judged that these two kinds of cracks are caused by design, it is mainly due to improper selection of section size, less tensile reinforcement in normal section and less transverse reinforcement in inclined section. Construction quality control, that is, the actual strength of concrete can not meet the design strength, the displacement of protective layer of tensile main reinforcement is too large or less, the transverse reinforcement is less, and the construction load is too large; If it is caused by use, it is mainly overload.
(4) Oblique crack of reinforced concrete beam under concentrated load.
This kind of crack mostly occurs in the primary and secondary beam structure system, which is characterized by oblique cracks on both sides of the lower part of the secondary beam at the connection between the secondary beam and the main beam, which is caused by excessive load. The main reasons are: the strength of concrete is too low, the configuration of steel bars or hanging bars is not enough, and the hanging bars are displaced. Preventive measures are: configure transverse reinforcement according to the requirements of construction drawings and design specifications to ensure that the concrete quality meets the requirements during construction and the protective layer of reinforcement is not accurately displaced. This kind of crack needs to be reinforced, and the simple treatment method is to adopt the reinforcement method of sticking steel plates.
(5) Cracks in reinforced concrete continuous beams
This kind of crack mostly occurs at the negative bending moment of continuous beam support. The main reasons are: the number of negative bending moment steel bars is insufficient and the position is offset. When processing, methods such as sticking steel plates, outsourcing steel bars and outsourcing reinforced concrete can be adopted.
(6) Cracks at both ends of reinforced concrete beams
This kind of crack mostly occurs after the project is put into use, and its characteristic is that the crack is obliquely distributed at both ends of the main beam, with a large upper opening and a small lower opening. The main reasons for this crack are: the binding force at both ends of the beam is large, such as the rigid skylight frame at both ends of the thin web beam, and the steel frame of the skylight is welded firmly with the embedded parts at both ends of the thin web beam. When the thin web beam is deformed under load, large bending moment and shear force will be generated at both ends of the beam, resulting in cracks at the beam ends. Preventive measures are for prevention.
(7) Cracks in reinforced concrete frame beams, ring beams and foundation beams
Most of these cracks are oblique cracks, which appear in the middle of the beam and also at the end of the beam, and run through the whole beam section. The main cause of cracks is uneven settlement of chess, so the direction of cracks is consistent with the direction of uneven settlement of foundation. To deal with cracks, we must first find out the causes of foundation subsidence, take effective measures to stop foundation subsidence, and then reinforce concrete beams. The reinforcement method is indium.
2. Reinforced concrete columns
(1) Column cracks along the reinforcement direction
Most of these cracks appear gradually in the process of use, which are cracks along the longitudinal main reinforcement of the column. The length and width of cracks tend to increase with time, and the depth generally reaches the position of main reinforcement. Most of these cracks belong to corrosion and expansion of steel bars. The causes and preventive measures of cracks are similar to those of the above-mentioned reinforced concrete dyed main reinforcement cracks.
(2) Horizontal cracks and watery cracks of reinforced concrete columns
This kind of crack can only be found when or after formwork removal. Its characteristics are that water-like cracks often appear along the four corners of the column, which are irregular cracks; In severe cases, a transverse crack appears every (300- 1200mm) along the column height. Cracks vary in width, which can be carefully observed by naked eyes, and the severe ones are 2-3mm, and the depth is mostly less than 30 mm. These cracks are dry shrinkage cracks with plastic shrinkage and drying too fast. The reason is that the formwork is too dry and the weather is hot, so no protective measures are taken. Covering and moisturizing is poor. The preventive measures are to fully wet the template and strengthen the covering and moisturizing protection. The treatment method is: the strength grade of concrete can meet the needs of the structure, the cracks should be turned into holes, and sealed with high-strength cement mortar or epoxy mastic; If the strength grade of concrete can't meet the design requirements, in addition to plugging the cracks, it is necessary to reinforce the columns with steel plates or concrete protective layers.
(3) longitudinal splitting cracks of reinforced concrete columns
This kind of crack may occur in the construction stage or in the use stage. It is characterized by longitudinal splitting in the middle of the column, and sometimes splitting at the end of the column. The causes of splitting cracks are: design errors; The strength of concrete is too low; Overload in construction stage and use stage. This kind of crack is a stress crack under load, which endangers the safety of the structure and should be reinforced immediately after it is found. The general treatment methods are as follows: if the cracks are light, they should be reinforced by wrapped steel or prestressed poles; If the crack is serious, it should be reinforced by wrapping reinforced concrete casing.
(4) X-shaped cracks in reinforced concrete columns
This kind of crack appears after the earthquake or after the column is severely hit, which belongs to the shear type crack caused by severe vibration. Its cracks endanger the safety of the structure, and the severity of the cracks should be carefully checked. The vertical bearing capacity of columns is strengthened by wrapping steel or reinforced concrete sleeves.
(5) Horizontal cracks on one side of reinforced concrete column
This kind of crack is caused by eccentric compression column and belongs to the crack caused by load stress. The main reasons are: the load value is too large to exceed the design value; The strength of concrete is too low; The diameter of steel bars is too small and the quantity is insufficient. This kind of crack will endanger the safe use of the structure, so it must be reinforced. The methods include: pasting steel plates, wrapping steel bars and wrapping reinforced concrete sheaths for reinforcement.
3. Prestressed concrete hollow wrench
(1) Turtle cracks appear on the plate surface.
It is characterized by irregular surface cracks and belongs to dry shrinkage cracks. The main reason is that the water-cement ratio of concrete is too high, or it may be caused by poor covering and moisturizing. Preventive measures are: adjusting water consumption as much as possible and controlling water cement ratio; Strengthen coverage and moisturizing. Slight cracks will generally not affect the strength and safety in use, but if the cracks are large and serious, surface treatment must be carried out, and the crack surface and loose parts should be chiseled away, cleaned up and caulked with high-strength cement mortar.
(2) the board surface spalling and bursting
Generally, it occurs after the concrete is hardened, which is caused by the unsatisfactory stability of cement or the reaction of using active aggregate. Preventive measures are: strengthen the inspection, acceptance and re-inspection system of cement entering the site, put an end to unqualified cement entering the site, and strengthen the experiment to select good quality raw materials (sand aggregate).
(3) transverse cracks appear on the surface of the plate.
Transverse cracks mostly occur after the final setting of concrete and during curing. Its characteristic is that there is a transverse crack on the board at regular intervals, and the width of the crack varies from small to large, and the serious width can reach 2-3mm, but the depth will not exceed the thickness of the upper flange of the board. There are many reasons for the transverse cracks on the board surface: one may be the plastic shrinkage caused by the failure to take sun protection and wind protection measures in time after pouring the perforated board, the evaporation is too fast, and the moisture preservation is not done well; Another possibility is that the prestressed steel wire is overstretched. However, this kind of over-tensile stress crack is mostly distributed in the middle of the plate, and it is rarely found at both ends, and the surface width is smaller than the plastic shrinkage crack. The preventive measures are: strengthening the covering and moisture curing of concrete to prevent violent explosion and rapid water loss due to high wind speed; Control the tensile stress of prestressed steel wire, and never overstretch; If cracks caused by the above reasons have appeared, epoxy mortar or high-strength cement mortar should be used for caulking.
(4) longitudinal cracks on the surface of the plate
Longitudinal cracks mostly occur on the porous plate produced by wire drawing die, and often occur when the steel pipe is pulled out. The cracks are located above the holes in the hollow slab and distributed along the longitudinal direction of the slab, belonging to plastic slump cracks. The main reasons are: the water-cement ratio of concrete is slightly higher; When the core tube is drawn, the outer wall of the tube is not smooth and jumps up and down, and the drawing speed is uneven, resulting in eccentric force. Preventive measures include: the water-cement ratio of concrete used in prestressed hollow slab must be small; Do not pull the core tube immediately after vibration screeding; The speed of core-pulling pipe is uniform, avoiding eccentric stress, controlling the pipe to be straight and preventing jumping; For cracks caused by plastic sinking after core-pulling pipe, after stopping sinking, fine stone high-grade concrete should be used for repair and light treatment immediately.
(5) transverse cracks at the bottom of the plate
Cracks at the bottom of the plate often appear after the plate is hoisted, transported or hoisted in place. The cracks hang over the plate span, and the height of the cracks is narrow, which will not exceed 2/3 of the plate height. The reasons for this kind of crack are: the strength of concrete is too low when lifting, and the adhesion at the bottom is too large; Improper fulcrum or severe walking vibration when hoisting in the car; Excessive local overload during construction; The strength of concrete is too low or the quality is questionable. This kind of crack belongs to stress crack caused by load, and the preventive measures are as follows: (1) the strength of prestressed slab must be sampled and qualified; Before lifting, it should be separated from the base and not adsorbed; When transporting, support the fulcrum and choose the road conditions to prevent vibration and cracking; Construction is not local overload; The pouring quality of concrete slab must be guaranteed. If cracks have appeared due to the above reasons, they will be banned in serious cases, and epoxy mortar can be used for reinforcement in minor cases.
(6) longitudinal cracks at the bottom of the plate
Longitudinal cracks usually appear in the period of concrete hardening to several years, which is characterized by cracks distributed along longitudinal steel (reinforcement) wires and may develop with time. These cracks are mostly caused by corrosion and cracking of steel bars, too thin concrete protective layer or improper selection of additives. The preventive measures are: strictly control the thickness of concrete protective layer to prevent the steel bar from shifting during construction; Choose additives with good performance and large dosage, and it is best to use additives with rust resistance of steel bars. Longitudinal cracks are caused by the expansion of rust, which affects the bonding force between concrete and steel bars. With the passage of time, the concrete protective layer will gradually fall off, and the strength and durability of the plate will also be lost. Plates with serious defects shall not be used in the future. If it is found to be installed on the floor, it must be replaced.
(7) Joint cracks at the bottom of plate
This kind of crack mostly occurs after use, and it also appears after several years of use. The causes of cracks are: if it appears at the bottom of the floor, it is caused by poor joint grouting; If it happens at the bottom of the house, it is due to the poor thermal insulation performance of the roof insulation layer, resulting in temperature difference deformation of the roof panel. The preventive measures are as follows: when the prestressed hollow wrench is used as the floor slab, the joint width of the control panel is about 30 mm, and when the hollow slab is used as the roof slab, the joint treatment of the slab is the same as that of the floor slab, but the roof insulation layer should be ensured on the basis of energy saving. At the same time, the construction quality must be guaranteed to reduce the deformation of roof panels in winter and summer. If cracks appear at the joints, only the joints can be reprocessed and the thermal insulation layer can be constructed.
(8) Cracks at plate end supports
This kind of crack appeared after the project was put into use for a period of time. The general characteristics are: if the hollow slab support is a rectangular beam, cracks will appear along the length of the beam; For example, when a hollow slab is supported by a basket-carrying beam, there will be two cracks along the length of the beam, which will develop gradually with the passage of time. The reason for the cracks is that almost all floors are simply supported at present, and local reinforcement treatment is generally not taken at the support. Therefore, when the floor bears a large load, the downward deflection of the middle part of the floor will cause a large tensile stress at the support, thus causing cracks at the end support of the floor. Precautions are as follows: the joint treatment must be carefully layered. Steel mesh is placed at the end support of the floor along the length of the beam to resist the negative bending moment at the support. Although the crack at the beam end will not endanger the structural safety, it is easy to cause leakage, and condensation will occur in winter in the north, which will accelerate the end damage. At the same time, it also affects the use function and visual quality, and it should be strengthened. The treatment method is to widen the cracks, clean up all loose parts, caulk with fine stone concrete, coat the surface with cement mortar, strengthen maintenance and prevent cracking.
4. Prestressed concrete large inner panel
(1) transverse cracks on the plate surface
This kind of crack often occurs after the final setting of concrete or during curing. The characteristics, causes and preventive measures of this crack are similar to the transverse cracks of prestressed hollow slab.
(2) Cracks in longitudinal ribs of plates
The characteristics of rib cracks are: (1) joints often appear after the prestressed roof panels are installed in place; Cracks crack at both ends of the longitudinal rib at an angle of about 450. The reasons are as follows: (1) The large roof plate is designed as a simply supported plate, and the bearing is welded at three points in actual installation and application, which leads to a certain embedded restraint effect of the bearing, so the plate end is not seriously stressed; When the design standard of roof insulation layer is low or the construction quality control is not strict, the roof panel will also be deformed by temperature (fluctuation), which will cause oblique cracks on the local stress surface at the end of the panel. The preventive measures are: adding φ 12- 14 diagonal steel bars at the ribs perpendicular to the direction of oblique cracks at the plate end, one end of which is welded with the embedded parts, and the other end is bent upwards and anchored at the upper wing of the plate.
This kind of crack may not be treated if it is slight, but it may depend on the degree if it is serious. Generally, epoxy cement is embedded or reinforced with steel plates.
(3) Angle crack of transverse rib at plate end
Corner cracks usually appear at the cross section of the transverse rib at the end of the plate, which is a 45-degree oblique crack. This kind of crack usually has one at each end rib, and in severe cases, four angles may appear at the same time. The reasons are as follows: due to the large and uneven adsorption force of the template during demoulding and demoulding, the components can not be demoulded at the same time, and the corners are easy to crack after demoulding; Cracks are easy to occur at the angle of large temperature difference between inside and outside before the component leaves the pool; Sudden stress concentration at the end of transverse rib. Preventive measures are as follows: changing the folding angle of the abrupt section into an arc angle to reduce stress concentration; Adding φ6-φ8 structural ribs with a length of 300nm in the crack-prone area can improve its crack resistance and limit the cracking of cracks.
summary
Cracks in concrete structures and members are usually normal. Many cracks are caused by inadequate quality control during construction. There are also some cracks that appear during concrete curing; In addition, when the strength of components is not enough, the structure and components crack due to local overload in the process of formwork removal, hoisting, transportation, installation and construction, as well as excessive load and corrosion in use. The characteristics and causes of cracks are different. The above analysis introduces the characteristics, causes, preventive measures and treatment methods of the most common cracks in reinforced concrete beams, columns, prestressed hollow slabs and prestressed roof slabs. This kind of crack is common in engineering, and the quality problems of these treatment methods in the construction process.
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