I found a better newspaper.
As for the crack width, please see the specification:
Code for Design of Reinforced Concrete and Prestressed Concrete Bridges and Culverts of Highway JTG D62-2004: Crack Calculation Block
Remember that the allowable crack width of partial prestress is 0. 1mm, the total prestress is not allowed to crack, and the steel reinforced concrete member is 0.2 mm
Concrete has become the most widely used building material in the world because of its wide range of materials, low price, high compressive strength, good fire resistance, low weather resistance and low maintenance cost. The main disadvantage of concrete is its poor tensile strength and easy cracking. A large number of engineering practice and theoretical analysis show that almost all concrete members work with cracks, but some cracks are so thin that they are even invisible to the naked eye (< 0.05mm), which are generally harmless to the use of the structure and can be allowed to exist; Under the action of service load or external physical and chemical factors, some cracks are constantly generated and expanded, causing concrete carbonization, protective layer peeling and steel bar corrosion, weakening the strength and stiffness of concrete, reducing its durability, and even causing collapse accidents in serious cases, endangering the normal use of the structure, which must be controlled. The current design codes of highway, railway, construction, water conservancy and other departments in China all adopt the method of limiting the crack width of components to ensure the normal use of concrete structures. What is discussed in this paper only refers to the latter crack.
In recent years, China's transportation infrastructure has developed rapidly, and a large number of concrete bridges have been built in various places. In the process of bridge construction and use, it is not uncommon to see reports that cracks affect the engineering quality and even lead to bridge collapse. Concrete cracking can be said to be "frequently-occurring disease" and "frequently-occurring disease", which often puzzles bridge engineers and technicians. In fact, many cracks can be overcome and controlled if certain design and construction measures are taken. In order to further strengthen the understanding of concrete bridge cracks and try to avoid the occurrence of harmful cracks in engineering, this paper makes a comprehensive analysis and summary of the types and causes of concrete bridge cracks as far as possible, so as to facilitate the design and construction to find feasible methods to control cracks and achieve the role of nip in the bud.
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 and 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. It is difficult to simulate the conventional calculation with accurate schema, and the reinforcement is generally 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 adding rounded corners at corners, gradual transition at abrupt changes, strengthening structural reinforcement at the same time, adding diagonal braces at corners, and setting edge protection angles 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.
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.
7. twist. At first, many inclined cracks in 45 direction appeared on one side of the member, which spread to the adjacent surface in a spiral direction.
8. punch. Draw a 45-degree inclined plane along the inner four sides of the stigma plate to form a punching plane.
9. Local compression. There are many short cracks in the local compression zone that are roughly parallel to the pressure direction.
Second, cracks caused by temperature changes.
Concrete has the characteristics of thermal expansion and cold contraction. When the external environment or the internal temperature of the structure changes, the concrete will be deformed. If the deformation is limited, stress will occur in the structure, and when the stress exceeds the tensile strength of concrete, temperature cracks will occur. In some long-span bridges, the temperature stress can reach or even exceed the live load stress. The most important feature that distinguishes temperature crack from other cracks is that it will expand or close with the change of temperature. The main factor causing temperature change are:
1, annual temperature difference. The temperature changes all the year round, but it changes slowly, which mainly leads to the longitudinal displacement of the bridge structure. Generally, it can be coordinated by structural measures such as bridge deck expansion joint, bearing displacement or flexible pier, and temperature cracks will only appear when the displacement of the structure is limited, such as arch bridge and rigid frame bridge. Generally speaking, the annual temperature difference in China is based on the monthly average temperature of 1 month and July. Considering the creep characteristics of concrete, the elastic modulus of concrete should be reduced when calculating the internal force of annual temperature difference.
2. Sunshine. After the bridge deck, main girder or pier side is exposed to the sun, the temperature is obviously higher than other parts, and the temperature gradient is nonlinear. Due to self-restraint, the local tensile stress is large and cracks appear. Sunlight and subsequent sudden cooling are the most common causes of structural temperature cracks.
3. Sudden cooling. Sudden rainstorm, cold air intrusion, sunset, etc. It can lead to a sudden drop in the temperature of the external surface of the structure, but a temperature gradient is generated because the internal temperature changes relatively slowly. When calculating the internal force of sunshine and quenching, design specifications or real bridge data can be used, without considering reducing the elastic modulus of concrete.
4. Hydration heat. Appear in the construction process, after mass concrete (thickness greater than 2.0m) is poured, the internal temperature is very high due to the exothermic hydration of cement, and the temperature difference between inside and outside is too large, resulting in cracks on the surface. During construction, cement varieties with low hydration heat should be selected as far as possible, the unit dosage of cement should be limited, the temperature of aggregate entering the mold should be reduced, the temperature difference between inside and outside should be reduced, and the temperature should be lowered slowly. If necessary, circulating cooling system can be used for internal heat dissipation, or thin-layer continuous casting can be used to speed up heat dissipation.
5, steam curing or improper winter construction measures, concrete quenching and heating, uneven internal and external temperature, prone to cracks.
6. When the partition between prefabricated T-beams is installed, when the embedded steel plate of the bearing is welded with the leveling steel plate, if the welding measures are improper, the concrete near the iron piece is easy to burn and crack. When prestressed members are tensioned by electric heating tensioning method, the temperature of prestressed steel bars can rise to 350℃, and concrete members are easy to crack. The experimental study shows that the strength of concrete after high temperature burning caused by fire and other reasons decreases obviously with the increase of temperature, and the bonding force between steel bar and concrete also decreases accordingly. When the temperature of concrete reaches 300℃, the tensile strength decreases by 50%, the compressive strength decreases by 60%, and the bonding force between round steel and concrete decreases by 80%. Due to heating, a large amount of free water in concrete evaporates and shrinks sharply.
Third, cracks caused by shrinkage.
In practical engineering, concrete cracks caused by shrinkage are the most common. Among the shrinkage types of concrete, plastic shrinkage and shrinkage (dry shrinkage) are the main reasons for the volume deformation of concrete, and autogenous shrinkage and carbonation shrinkage are also the main reasons.
Plastic shrinkage. It happens during the construction and about 4-5 hours after concrete pouring. At this time, the hydration reaction of cement is fierce, molecular chains are gradually formed, exudation and moisture evaporate rapidly, and concrete shrinks due to water loss. At the same time, the aggregate sinks due to its own weight, so the concrete has not hardened, which is called plastic shrinkage. The range of plastic shrinkage is very large, which can reach about 1%. If the aggregate is blocked by steel bars during sinking, cracks will form along the direction of steel bars. At the vertical variable cross-section of members, such as the joint of T-beam and box girder web with top plate and bottom plate, cracks will appear on the surface along the web direction due to uneven settlement before hardening. In order to reduce the plastic shrinkage of concrete, the water-cement ratio should be controlled during construction to avoid too long mixing time, too fast blanking, dense vibrating and vertical variable cross-section layered pouring.
Shrinkage shrinkage (dry shrinkage). After concrete hardens, with the gradual evaporation of surface moisture, the humidity gradually decreases and the volume of concrete decreases, which is called shrinkage (dry shrinkage). Because concrete loses water quickly on the surface and slowly on the inside, uneven shrinkage occurs with large surface shrinkage and small internal shrinkage. Surface shrinkage deformation is constrained by internal concrete, which makes the surface concrete bear tension. When the tensile force of surface concrete exceeds its tensile strength, shrinkage cracks will occur. The shrinkage of concrete after hardening is mainly shrinkage. For members with large reinforcement ratio (greater than 3%), the reinforcement has obvious restraint effect on concrete shrinkage, and cracks are prone to appear on the concrete surface.
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, cracking and irregular shape.
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 boreholes 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.
3, the structural load difference is too big. Under the same geological conditions, when the foundation load at different parts is too different, it may cause uneven settlement. For example, the load in the middle of a high-filled box culvert is greater than that on both sides, so the settlement in the middle is greater than that on both sides, and the box culvert may crack.
4. The types of structural foundations are quite different. In the same bridge, mixed use of different foundations, such as enlarged foundation and pile foundation, or pile foundation with large difference in pile diameter or pile length, or enlarged foundation with large difference in foundation elevation, may also cause uneven settlement of foundation.
5, the basis of phased construction. When a new bridge is built near the original bridge foundation, such as the left and right half bridges of expressway built by stages, the foundation soil will be consolidated again due to the load of the new bridge or foundation treatment, which may cause great settlement to the original bridge foundation.
6. Foundation frost heaving. Under the condition of below zero, the foundation soil with high water content expands due to freezing; Once the temperature rises, the frozen soil melts and the foundation sinks. Therefore, the freezing or melting of foundation will cause uneven settlement.
7. When the bridge foundation is placed in bad geology such as landslide, karst cave or active fault, it may cause uneven settlement.
8. After the completion of the bridge, the original foundation conditions have changed. After most natural and artificial foundations are immersed in water, especially special foundation soils such as plain fill, loess and expansive soil, the strength of soil decreases and the compressive deformation increases when it meets water. In soft soil foundation, due to artificial pumping or dry season, the groundwater level drops, and the foundation soil layer consolidates and sinks again. At the same time, the buoyancy on the foundation decreases, the negative friction increases and the load on the foundation increases. Some bridge foundations are buried too shallow, and the foundations may be displaced due to flood scouring and excavation. With the change of ground load conditions, if a large amount of waste and sand are accumulated near the bridge due to landslides and other reasons, the soil layer in the bridge site may be compressed and deformed again. Therefore, in the process of use, the change of the original foundation conditions may cause uneven settlement.
For arch bridges and other structures that generate horizontal thrust, the main reasons for horizontal displacement cracks are insufficient understanding of geological conditions and unreasonable design, which destroyed the original geological conditions during construction.
Five, steel corrosion caused by cracks
Due to the poor quality of concrete or insufficient thickness of protective layer, the protective layer of concrete is corroded and carbonized to the surface of steel bar by carbon dioxide, which reduces the alkalinity of concrete around steel bar, or because of the intervention of chloride ions, the oxide film on the surface of steel bar is destroyed, and iron ions in steel bar react with oxygen and moisture invading concrete, and the volume of iron hydroxide, a corrosive substance, is increased by about 2-4 times, thus generating expansion stress on the surrounding concrete, leading to cracking and peeling of protective layer concrete and cracks along the longitudinal direction of steel bar. Due to corrosion, the effective cross-sectional area of steel bars is reduced, the bonding force between steel bars and concrete is weakened, the bearing capacity of the structure is reduced, and other forms of cracks are induced, which intensifies the corrosion of steel bars and leads to structural damage.
In order to prevent steel corrosion, the crack width should be controlled according to the specification requirements, and the thickness of the protective layer should be sufficient (of course, the protective layer should not be too thick, otherwise the effective height of the component will decrease and the crack width will increase when stressed); During construction, we should control the water-cement ratio of concrete, strengthen vibration, ensure the compactness of concrete, prevent the invasion of oxygen, and strictly control the dosage of chlorine-containing additives, especially in coastal areas or other areas where air and groundwater are highly corrosive.
Six, cracks caused by frost heaving
When the atmospheric temperature is below zero, the water-saturated concrete freezes, and the free water turns into ice, which expands by 9%, thus the concrete generates expansion stress. At the same time, the migration and redistribution of supercooled water (freezing temperature below -78℃) in the gel pores of concrete causes osmotic pressure, which increases the expansive force in concrete, reduces the strength of concrete and leads to cracks. In particular, concrete is most seriously frozen during initial setting, and the strength loss of concrete after aging can reach 30%~50%. In winter construction, frost heaving cracks along the pipeline direction may occur if thermal insulation measures are not taken after grouting of prestressed tunnel.
The temperature is below zero and the concrete is saturated with water, which are necessary conditions for frost heaving damage. When there are many voids in the concrete aggregate
Strong water absorption; Aggregate contains too many impurities, such as soil; The water cement ratio of concrete is too large, and the vibration is not dense; Poor maintenance will lead to premature freezing of concrete, which may lead to frost heaving cracks in concrete. In winter construction, concrete can be cured at low temperature or negative temperature by using electric heating method, greenhouse method, underground heat storage method and steam heating method, and adding antifreeze into concrete mixing water (but chlorine salt is not applicable).
Seven, cracks caused by the quality of building materials
Concrete is mainly composed of cement, sand, aggregate, mixing water and additives. Unqualified materials used to prepare concrete may cause cracks in the structure.
1, cement
(1), the stability of cement is unqualified, and the content of free calcium oxide in cement exceeds the standard. The hydration of calcium oxide is very slow in the setting process, and it continues to hydrate after the cement concrete is set, which can destroy the hardened cement stone and reduce the tensile strength of concrete.
(2), cement factory strength is insufficient, cement be affected with damp or expired, may make the concrete strength is insufficient, leading to concrete cracking.
(3) When the alkali content of cement is high (such as above 0.6%) and aggregate with alkali activity is used at the same time, alkali-aggregate reaction may be caused.
2, sand, stone aggregate
(1), particle size, gradation and impurity content of sand and gravel.
Too small sand and gravel particle size, poor gradation and large void ratio will lead to an increase in water consumption for cement and mixing, affect the strength of concrete and increase the shrinkage of concrete. If ultra-fine sand is used in excess, the consequences will be more serious. High mica content in sand and gravel will weaken the bond between cement and aggregate and reduce the strength of concrete. The high silt content in sand and gravel will not only increase the dosage of cement and mixing water, but also reduce the strength, frost resistance and impermeability of concrete. Excessive organic matter and light substances in sand and gravel will delay the hardening process of cement and reduce the strength of concrete, especially the early strength. Sulfide in sandstone can react chemically with tricalcium aluminate in cement, and its volume expands by 2.5 times.
(2), alkali aggregate reaction.
There are three types of alkali-aggregate reaction:
(1), alkali silicic acid reaction. The aggregates involved in this reaction are rhyolite, andesite, tuff, opal, black silica, flint, tridymite, glassy volcanic rock, chalcedony, microcrystal or metamorphic age. Alkali reacts with microcrystalline silica, and its product silica gel expands when it meets water, which produces great internal stress in concrete, which can lead to sudden explosion of concrete. This reaction is the main form of alkali-aggregate reaction.
② Alkali-silicic acid reaction. Aggregates involved in this reaction include claystone, phyllite, hard sandstone, siltstone and so on. The characteristic of this reaction is that the expansion speed is very slow, and there is little gel that can ooze from the expansion to cracking of concrete.
③ Alkali carbonate rock reaction. Most carbonate rocks have no alkali activity. Only argillaceous fine-grained dolomite limestone and argillaceous fine-grained limestone dolomite with specific structures have alkaline activity to react with alkali, and they can only react and expand in an environment with high alkalinity and certain humidity.
The shape and distribution of alkali-aggregate reaction cracks are related to the constraint of steel bars. When the limiting force is small, cracks in the map often appear, and there are white or transparent extracts in the cracks. When the constraint is strong, cracks will appear along the reinforcement. In engineering practice, it is necessary to test the alkali activity of aggregate, use materials that are harmless to engineering and use cement varieties with low alkali content.
3. Mix water and additives
When the content of impurities such as chloride in mixed water or admixture is high, it has a great influence on the corrosion of steel bars. Mixing concrete with seawater or alkali-containing spring water or using alkali-containing additives may affect the alkali-aggregate reaction.
Eight, cracks caused by the quality of construction technology
In the process of concrete structure pouring, component fabrication, formwork hoisting, transportation, stacking, assembly and hoisting, if the construction technology is unreasonable and the construction quality is poor, it is easy to produce longitudinal, transverse, oblique, vertical, transverse, plane, deep and penetrating cracks, especially thin-walled structures. The location, direction and width of cracks vary due to reasons, and the typical ones are:
1, the concrete protective layer is too thick, or the upper bound steel bar is trampled indiscriminately, which makes the protective layer of the stressed steel bar thicker under the negative bending moment, which leads to the reduction of the effective height of the member and the formation of cracks perpendicular to the stressed steel bar.
2. The concrete vibrating is not compact and uneven, and there are honeycombs, pits and cavities, which lead to corrosion of steel bars or other load cracks.
3. The concrete is poured too fast, and the fluidity of concrete is low. Before hardening, the concrete is not compacted enough, and after hardening, the concrete is compacted too much, so it is easy to produce cracks after pouring for several hours, that is, plastic shrinkage cracks.
4, concrete mixing, transportation time is too long, make too much water evaporation, cause the concrete slump is too low, make the concrete volume irregular shrinkage cracks.
5. The concrete will dry sharply during the initial curing period, resulting in irregular shrinkage cracks on the concrete surface in contact with the atmosphere.
6. In pumping concrete construction, in order to ensure the fluidity of concrete, increase the water consumption and cement consumption, or increase the water-cement ratio for other reasons, which leads to the increase of shrinkage when concrete sets and hardens, resulting in irregular cracks in concrete volume.
7. When concrete is poured in layers or sections, the joints are not handled well, and cracks are easy to appear between new and old concrete and construction joints. For example, when concrete is poured in layers, due to power failure, rain and other reasons, it is impossible to pour concrete before initial setting, resulting in horizontal cracks between layers; When segmental cast-in-situ casting is adopted, the contact surface of the first poured concrete is not chiseled and cleaned, the cohesive force between the new and old concrete is small, or the post-poured concrete is not well maintained, which leads to concrete shrinkage and cracks.
8, concrete early freezing, make component surface cracks, or local peeling, or empty drum phenomenon after demoulding.
9. In the process of construction, the rigidity of the formwork is insufficient. When pouring concrete, the formwork is deformed due to lateral pressure, resulting in cracks consistent with the deformation of the formwork.
10, premature formwork removal during construction, insufficient concrete strength, resulting in cracks in components under self-weight or construction load.
1 1. Before construction, the support is not fully compacted or has insufficient rigidity, and the support sinks unevenly after pouring concrete, resulting in cracks in concrete.
12, prefabricated structure, when the components are transported and stacked, the supporting skids are not in a vertical line, or the cantilever is too long, or it is violently bumped during transportation; When hoisting, improper lifting point position, T-beam and other components with small lateral stiffness and no reliable lateral reinforcement measures may all cause cracks.
13, incorrect installation sequence, insufficient understanding of the consequences, resulting in cracks. For example, in the cast-in-place construction of reinforced concrete continuous beams and full-house supports, if the reinforced concrete wall guardrail and the main beam are poured at the same time, cracks will often appear after dismantling the frame; Cracks are not easy to occur when pouring guardrails after the frame is removed.
14, poor construction quality control. Random application of concrete mixture ratio leads to inaccurate measurement of water, sand and cement materials, resulting in insufficient strength of concrete, and other properties (workability and compactness) decline, leading to structural cracking.