Types and causes of cracks in concrete bridges
In fact, the causes of cracks in concrete structures are complex and varied, even many factors interact with each other, but each crack has one or several main reasons. The types and causes of cracks in concrete bridges can be roughly divided into the following categories:
I. Cracks caused by load
Cracks in concrete bridges under conventional static and dynamic loads and secondary stresses are called load cracks, which can be classified into two types: direct stress cracks and secondary stress cracks.
Direct stress crack refers to the crack caused by direct stress caused by external load. The causes of cracks are:
1. In the design calculation stage, the structural calculation is not calculated or partially omitted; The calculation model is unreasonable; The assumption of structural stress is inconsistent with the actual stress; Insufficient or omitted load calculation; Internal force and reinforcement calculation error; The structural safety factor is not enough. The possibility of construction is not considered in the structural design; Insufficient design section; Steel bar setting is too small or wrong; Insufficient structural stiffness; Improper structural treatment; Unclear design drawings, etc.
2. During the construction stage, the stacking of construction tools and materials is not restricted; Do not understand the mechanical characteristics of the assembled structure, flip, hoist, transport and install at will; Failing to construct according to the design drawings, changing the construction sequence of the structure and changing the stress mode of the structure without authorization; Do not check the fatigue strength of the structure under the vibration of the machine.
3. In the use stage, heavy vehicles exceeding the design load cross the bridge; Contact and collision between vehicles and ships; Strong wind, heavy snow, earthquake, explosion, etc.
Secondary stress crack refers to the secondary stress crack caused by external load. The causes of cracks are:
1. Under the design external load, the actual working state of the structure is different from the conventional calculation or the calculation is not considered, which leads to secondary stress in some parts and leads to structural cracking. For example, in the arch foot design of double-hinged arch bridge, hinges are often designed by arranging "X"-shaped steel bars and reducing the section size. Theoretically, there will be no bending moment in this place, but in practice, the hinge can still resist bending, and even cracks appear, resulting in corrosion of steel bars.
2. In bridge structure, it is often necessary to gouge, open holes and set supports. In conventional calculation, it is difficult to simulate the calculation with accurate schema, and the reinforcement is usually set according to experience. The research shows that the force flow will produce diffraction phenomenon after the stress component is dug out, and it will be dense near the hole, resulting in huge stress concentration. In a long-span prestressed continuous beam, the steel beam is often cut off and the anchor head is set according to the internal force of the section, but cracks can often be seen near the anchorage section. Therefore, if it is not handled properly, cracks are likely to appear at the corners of these structures or at the abrupt changes in the shape of components and at the cut-off of steel bars.
In practical engineering, secondary stress cracking is the most common cause of load cracking. The secondary stress cracks are mostly tensile, splitting and shearing. The secondary stress crack is also caused by load, which is generally not calculated according to the conventional method, but with the continuous improvement of modern calculation methods, the secondary stress crack can also be checked reasonably. For example, many finite element programs of plane bar system can correctly calculate the secondary stress caused by prestress and creep, but it was difficult 40 years ago. Attention should be paid to avoid structural mutation (or cross-section mutation) in design. When it is inevitable, local treatment should be done, such as fillet at the corner and gradual change at the abrupt change. At the same time, structural reinforcement should be strengthened, diagonal reinforcement should be added at the corner, and angle steel should be set around larger holes when conditions permit.
The characteristics of load cracks vary with different loads, showing different characteristics. This kind of crack often appears in tension area, shear area or serious vibration area. However, it must be pointed out that peeling or short cracks in the compression zone along the compression direction are often a sign that the structure reaches the limit of bearing capacity and a precursor to structural failure, and the reason is often that the cross-section size is too small. According to the different stress modes of the structure, the characteristics of cracks are as follows:
1, central nervous. Cracks run through the cross section of the member, with approximately equal spacing and perpendicular to the stress direction. When threaded steel bars are used, secondary cracks near the steel bars appear between cracks.
2. The center is under pressure. Short and dense parallel cracks parallel to the stress direction appear along the member.
Step 3 bend. Cracks perpendicular to the tensile direction began to appear at the edge of the tensile zone near the section with the maximum bending moment, and gradually developed to the neutral axis. When using threaded steel bars, short secondary cracks can be seen between cracks. When the structural reinforcement is less, the cracks are few and wide, and the structure may be brittle.
4. Large eccentric compression. Small eccentric compression members with large eccentric compression and less reinforcement in tension area are similar to flexural members.
5, small eccentric compression. Large eccentric compression members with small eccentric compression and more reinforcement in tension area are similar to central compression members.
6. cut. When the stirrup is too dense, the oblique compression failure occurs, and oblique cracks appear along the beam end and abdomen greater than 45; When the stirrup is suitable, shear and compression failure occurs, and oblique cracks parallel to each other in the direction of about 45 appear along the middle and lower part of the beam end.
Autogenous contraction. Self-shrinkage is the hydration reaction between cement and water during the hardening of concrete. This shrinkage has nothing to do with external humidity, and it can be positive (i.e. shrinkage, such as ordinary portland cement concrete) or negative (i.e. expansion, such as slag cement concrete and fly ash cement concrete).
Carbonization shrinkage. Shrinkage deformation caused by chemical reaction between carbon dioxide in the atmosphere and cement hydrate. Carbonization shrinkage can only occur when the humidity is about 50%, and it accelerates with the increase of carbon dioxide concentration. Carbonization shrinkage is generally not calculated.
Concrete shrinkage cracks are characterized by surface cracks, narrow crack width, crisscross cracks and irregular shapes.
The research shows that the main factors affecting the shrinkage cracks of concrete are:
1, cement variety, label and dosage. Slag cement, quick-hardening cement and low-heat cement concrete have higher shrinkage, while ordinary cement, pozzolanic cement and bauxite cement concrete have lower shrinkage. In addition, the lower the cement grade, the greater the dosage per unit volume, the greater the grinding fineness, the greater the shrinkage of concrete and the longer the shrinkage takes place. For example, in order to improve the strength of concrete, the method of forcibly increasing the amount of cement is often used in construction, and the shrinkage stress is obviously increased.
2, aggregate varieties. Timely, limestone, dolomite, granite and feldspar in aggregate have low water absorption and low shrinkage; However, sandstone, slate and amphibole have high water absorption and high shrinkage. In addition, the larger the aggregate particle size, the smaller the shrinkage, and the greater the water content, the greater the shrinkage.
3. Water cement ratio. The greater the water consumption, the higher the water-cement ratio and the greater the shrinkage of concrete.
4. External additives. The better the water retention of admixture, the smaller the shrinkage of concrete.
5. Maintenance methods. Good curing can accelerate the hydration reaction of concrete and obtain higher concrete strength. The higher the humidity, the lower the temperature, the longer the curing time and the smaller the shrinkage of concrete. Steam curing method has smaller shrinkage of concrete than natural curing method.
6. External environment. When the humidity in the atmosphere is low, the air is dry, the temperature is high and the wind speed is high, the water in concrete evaporates quickly and the concrete shrinks faster.
7, vibrating mode and time. The shrinkage of mechanically vibrated concrete is smaller than that of manually rammed concrete. Vibrating time should be determined according to mechanical properties, and generally it is appropriate to use 5~ 15s/ time. Time is too short, vibrating is not dense, resulting in insufficient or uneven concrete strength; Too long time causes delamination, coarse aggregate sinks into the bottom layer, fine aggregate remains in the upper layer, and the strength is uneven, and the upper layer is prone to shrinkage cracks.
For cracks caused by temperature and shrinkage, adding structural reinforcement can obviously improve the crack resistance of concrete, especially for thin-walled structures (wall thickness 20~60cm). Structural reinforcement should be mainly composed of small diameter steel bars (φ8~φ 14) and small spacing arrangement (@ 10~@ 15cm). The reinforcement ratio of the whole cross-section structure should not be less than 0.3%, and generally 0.3%~0.5% can be adopted.
Four, cracks caused by foundation deformation
Due to the vertical uneven settlement or horizontal displacement of foundation, additional stress is generated in the structure, which exceeds the tensile capacity of concrete structure and leads to structural cracking. The main reasons for uneven settlement of foundation are:
1, the accuracy of geological survey is not enough, and the test data is inaccurate. The design and construction without full understanding of geological conditions are the main reasons for uneven settlement of foundation. For example, for bridges in hills or mountainous areas, the distance between drilling holes is too far, and the bedrock surface fluctuates greatly, so the survey report can not fully reflect the actual geological situation.
2. The foundation geology is too different. For the bridge built in the valley, the geology of the valley and hillside changes greatly, even there is a soft foundation in the valley, and the foundation soil causes uneven settlement due to different compressibility.
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