What are the causes of cracks in reinforced concrete members? What are the reasons that affect the crack width?
For all kinds of cracks, we must first find out their nature and reasons, and then determine the specific repair methods. Cracked foundation of reinforced concrete structure
Its causes can be divided into load cracks, temperature cracks, shrinkage cracks, corrosion cracks, settlement cracks and so on.
1 Causes of various cracks
1 1 1 load crack
Cracks caused by excessive deformation of the structure under load. Generally, it appears in the tension zone, shear zone or violent vibration of the component.
Location. The main reasons are structural design, construction error, insufficient bearing capacity and uneven settlement of foundation.
Reinforced concrete structure is the ultimate bearing capacity shared by concrete and steel bars, and structural designers need to carry out static and dynamic analysis according to foundation conditions.
Load, environmental factors and structural durability control load cracks. According to the relevant codes at home and abroad, cracks are caused by structural deformation.
There are two schools of thought: first, the design code is very flexible, and there is no clear provision for checking cracks, but designers can handle them freely. in addition
One is that the design code clearly stipulates the calculation formula of loaded cracks and strict allowable width restrictions, such as China's "Concrete Structure"
In the Code for Design (GB500 10-2002), engineers' poor consideration of structural deformation and crack control is the main reason for the excessive occurrence of structural load cracks.
Because.
1 12 temperature crack
Due to the change of atmospheric temperature, the influence of high ambient temperature and the hydration heat generated during mass concrete construction, the water in cement
The hydration heat is 165 ~ 250 J/g, and the adiabatic temperature can reach 50℃ ~ 80℃ with the increase of concrete cement content. Research shows that when the internal and external temperatures of concrete
When the difference is 10℃, the cold shrinkage value εc
= Δ tα = 0101%.If the temperature difference is 20℃ ~ 30℃, its cold shrinkage value is 0 102% ~ 0 103%, which is greater than that during condensation.
When the ultimate tensile value of soil is reached, concrete will crack.
1 13 shrinkage crack
This kind of crack is caused by material defects. The research shows that the absolute volume of cement decreases and the capillary phenomenon intensifies after hardening with water.
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The escape of water in cracks produces capillary pressure, which causes capillary shrinkage of concrete, so that the dry shrinkage value of cement mortar is 0 1 1% ~ 0 12%.
The shrinkage value of soil is 0 104% ~ 0 106%, while the ultimate tensile value of concrete is only 01010/02%, resulting in shrinkage cracks.
1 14 settlement cracks
Uneven settlement of cast-in-place members due to excessive foundation or masonry; Insufficient formwork rigidity, large support spacing, loose support, premature formwork removal, etc. , can be guided.
Leading to settlement cracks.
1 15 corrosion crack
Due to the harmful ions Cl-, SO4
2-, Mg2+, etc. Invade into the concrete, resulting in corrosion of steel bars and expansion cracks in the later stage of concrete.
2 reinforced concrete structure crack control measures
According to the foreign design codes and the current Code for Design of Concrete Structures in China (GB500 10-2002) and related test data, concrete is the most
The approximate control standard of large crack width: (1) no corrosive medium and no seepage control requirement of 013 ~ 014 mm; (2) The requirements for slight erosion and no seepage control are as follows
0 12 ~ 0 13mm； (3) The erosion is serious, and the anti-seepage requirement is 0 1 1 ~ 0 12mm. In order to achieve this standard, it is necessary to adopt facies for various cracks.
It should be a control measure.
2 1 1 load crack
In terms of structural design, structural designers must strictly abide by Article 8 1 1 in Code for Design of Concrete Structures (GB 50012002).
Check the crack control situation, and adopt corresponding reasonable reinforcement according to different structural parts.
2 12 temperature crack
In order to prevent the huge difference between the concrete itself and the outside temperature in the high temperature environment, heat insulation measures should be taken to strengthen the maintenance, especially in
In high temperature, windy and dry climate conditions, water should be sprayed as soon as possible. For mass concrete, cracks should be controlled, and mass concrete projects are scattered.
Cold shrinkage caused by thermal shrinkage is more likely to cause cracking than dry shrinkage, and conventional temperature control measures are complicated and expensive.
2 13 shrinkage crack
First, it can be controlled by improving the performance of materials, such as shrinkage compensating concrete used in engineering to control such cracks.
Very effective. Compensating concrete is a kind of moderately expansive concrete. According to the technical requirements of compensating concrete at home and abroad, during wet curing,
Under the test condition of reinforcement ratio ρ = 0 18%, the ultimate expansion rate is 0 102% ~ 0 103%, and the preloading stress is established in concrete.
It is 0 12 ~ 0 17 MPa, and this preloading stress can offset all or most of the stresses that cause concrete cracking. At the same time, the concrete has been delayed.
Shrinkage process is the anti-crack principle of compensating concrete.
2 14 settlement crack
Necessary compaction and reinforcement of soft soil foundation; Precast site should be tamped and compacted before use; Cast-in-place and prefabricated formwork shall be supported.
Support firmly, ensure its strength and stiffness, and dismantle it according to the specified time; Prevent rainwater and construction water from soaking the foundation.
2 15 corrosion crack
Ensure the compactness of concrete and prevent the invasion of corrosive medium, water and oxygen; Apply a protective layer on the surface of the component.
The analysis of quality problems in construction engineering is the premise of correctly formulating the treatment scheme of quality accidents and the basis of clarifying the responsibility of quality accidents. Therefore, the analysis of quality problems requires comprehensive, accurate and objective; The nature, harm, cause and responsibility of the accident cannot be omitted. There must be scientific argumentation and judgment; Very reasonable: only when the theory is well founded can we achieve the goal of unified understanding.
The analysis of quality problems in construction engineering is the premise of correctly formulating the treatment scheme of quality accidents and the basis of clarifying the responsibility of quality accidents. Therefore, the analysis of quality problems requires comprehensive, accurate and objective; The nature, harm, cause and responsibility of the accident cannot be omitted. There must be scientific argumentation and judgment; Very reasonable: only when the theory is well founded can we achieve the goal of unified understanding.
First, the wall crack analysis
(A) Analysis of wall cracks caused by uneven settlement of foundation
All the loads of the house are finally transferred to the foundation through the foundation, and the stress of the foundation spreads with the depth under the load. The greater the depth, the greater the diffusion and the smaller the stress. At the same depth, it is always the largest in the middle and gradually decreases at both ends. It is precisely because of the diffusion of soil stress that even if the foundation stratum is very uniform, the stress distribution of the building foundation is still uneven, which leads to the uneven settlement of the building foundation, that is, there is more settlement in the middle of the building and less settlement at both ends, forming a slightly concave basin-shaped surface settlement distribution. When the geology is good and uniform, and the ratio of length to height of the building is small, the difference of uneven settlement of the building foundation is relatively small, which generally will not have much impact on the safe use of the building. However, when the house is built on muddy soil or soft plastic cohesive soil, the absolute settlement and relatively uneven settlement of the house may be relatively large because of the low strength and high compressibility of the soil. If the length and height of the building are relatively large, the overall stiffness is poor, and the foundation is not strengthened, then serious cracks may appear in the wall. Cracks appear symmetrically at both ends of the longitudinal wall, inclined to the direction of large settlement, about 45% along the door and window openings. It is a regular figure of eight, with small cracks in the upper part of the house and large cracks in the lower part. This kind of crack must be caused by uneven settlement of foundation caused by additional stress of foundation.
When the soil layer of building foundation is unevenly distributed and the soil quality is quite different, obvious uneven settlement often occurs at the junction of different soil layers or at different thicknesses of the same soil layer, which leads to cracks in the wall, and the cracks are large and small, and tilt in the direction of soft soil or thick soil layer.
In the case of large height difference or load difference of buildings, when there is no settlement joint, large uneven settlement cracks are easy to occur at the junction of high and low weights. At this time, the crack is located in the part with less layers and light load, and inclines upward to the part with more layers and heavy load.
When the compressibility of the soil at both ends of the house is large and the middle part is small, the settlement distribution curve will be convex. At this time, in addition to the cracks inclined outward at both ends of the longitudinal wall, vertical cracks often appear at the top of the longitudinal wall.
In a multi-storey house, when the bottom windowsill is too wide, the uneven settlement of the foundation is often caused by the concentrated transfer of wall load between windows, which leads to the reverse bending of the windowsill under the action of foundation reaction, resulting in vertical cracks in the middle of the windowsill.
In addition, if the foundation of the new building is located below the original building, the ratio of height difference H to clear distance L at the bottom of the new and old foundation should be less than 0.5~ 1. Otherwise, the foundation settlement caused by the new house load will cause cracks in the original house and wall. Similarly, in the construction of adjacent high-rise and low-rise buildings, the construction should be organized according to the principle of high before heavy, low before light; Otherwise, if the low-rise building is built first and then the high-rise building, it will also cause cracks in the low-rise wall.
From the above analysis, we can see that the distribution of cracks is closely related to the height-width ratio of the wall. Houses with large ratio of length to height have poor rigidity and poor deformation resistance, and are prone to cracks. Because the length-height ratio of the longitudinal wall is greater than that of the transverse wall, most cracks occur on the longitudinal wall. The distribution of cracks is closely related to the distribution curve of foundation settlement. When the settlement distribution curve is concave, cracks mostly occur in the lower part of the house, and the width of cracks is large or small. When the settlement distribution curve is convex, cracks often appear in the upper part of the house, and the width of cracks is large or small. The distribution of cracks is closely related to the mechanical properties of the wall. Due to stress concentration, cracks often occur at doors and windows, plane turning points and height changes. Because the wall was destroyed by shear, its principal tensile stress was 45. So the crack is also inclined at 45 degrees.
In order to prevent wall cracking caused by uneven settlement of foundation, soft soil foundation and uneven foundation should be treated first, but when drawing up foundation reinforcement treatment scheme, foundation treatment and superstructure treatment should be combined to make them work together. We can't just start with foundation treatment, otherwise it will not only cost a lot; And the effect is also poor. In the treatment of superstructure, there are: changing the shape of the building; Simplify the building plane; Reasonable settlement joint; Strengthen the overall stiffness of the house (such as increasing transverse walls, increasing ring beams, adopting mat foundation and box foundation, etc.). ); Adopt light structure, flexible structure and so on.
(2) Analysis of wall cracks caused by temperature stress.
General materials have the property of thermal expansion and cold contraction, and the deformation of building structure due to the change of surrounding temperature is called temperature deformation. If the structure is free from any constraints and can deform freely when the temperature changes, then no additional stress will be generated in the structure. If the structure is constrained and cannot deform freely, additional stress or temperature stress will be generated in the structure. Structural expansion value caused by temperature stress.
Because the linear expansion coefficient of reinforced concrete is a =1.08x1c, while the linear expansion coefficient of ordinary brick masonry is 0.5x1c, the elongation of reinforced concrete structure is about twice that of brick masonry at the same temperature difference. Therefore, in the mixed structure, when the temperature changes, reinforced concrete roofs, floors, ring beams and so on. Moreover, the expansion and contraction of brick walls are different, which will inevitably contain each other and produce temperature stress, which will crack and destroy the building structure.
Wall cracks caused by temperature stress generally have the following situations:
1. splay crack
As shown in Figure 4-6, when the outside temperature rises, the external wall itself will elongate along the length direction, but the elongation value of the roof part (especially the reinforced concrete roof directly exposed to the atmosphere) is much larger. From the cutting of the joint between the roof and the wall, it can be seen that the extension of the roof will produce additional horizontal thrust on the wall, so that the wall will be pushed by the roof to produce shear stress, and shear stress and tensile stress will also cause principal tensile stress. When the principal tensile stress is too large, it will produce splayed cracks on the wall. Because the distribution of shear stress is generally zero in the middle and the maximum at both ends, splayed cracks mostly occur at both ends of the wall, generally occupying two or three bays and occurring on the top wall.
2. Horizontal cracks and angular cracks
In flat-topped houses, sometimes there are longitudinal horizontal cracks and corner cracks along the top of the external wall at the bottom of the roof panel or near the top ring beam, which are caused by the outward or inward pulling force caused by the elongation or shortening of the roof. Corner crack is actually a form of horizontal crack, which is connected by horizontal crack of external transverse wall and horizontal crack of vertical wall. In this case, there are generally no open cracks below. Sometimes, horizontal cracks in the outer longitudinal wall will also appear at the top window sill.
3, parapet root and vertical cracks
Due to the extension or shortening of the roof, the parapet root is pushed or pulled outward or inward, and the parapet outside the parapet root masonry is inclined outward, forming horizontal cracks. Sometimes, due to the shrinkage of reinforced concrete roof, the parapet may be in an eccentric compression state, which leads to the vertical cracking of the upper part of the parapet.
In addition, local vertical cracks are easy to appear in the walls on both sides of the stairwell or at staggered floors, which is caused by the large tension caused by floor shrinkage.
There are many and complicated reasons that affect the expansion and contraction cracks of houses. These are just some common situations. In order to reduce the influence of temperature stress, reasonable expansion joints can be adopted; Avoid dislocation of floor slab and expansion joint; Strengthen roof insulation; Use linoleum and talcum powder or iron sheet to isolate the roof panel from the wall, and leave a certain gap at the root of the parapet, so that it can expand freely and have expansion space; Planting roofs in the water storage roof area; The daughter has a constructional column on the wall; Technical measures such as strengthening the weak links of the structure and improving its tensile strength.
Second, the collapse analysis of cantilever structure
There are many examples of cantilever structure collapse, one is the whole overturning collapse; Second, it collapses along the roots of cantilever beams and plates. The main reasons are:
1. The stability moment is less than the overturning moment.
The stability of cantilever structure is maintained by weight or external tension, and the overturning safety factor is required to be not less than 1.5. If the stability moment is less than the overturning moment, it will inevitably lose stability, overturn and collapse. Such as awning and cantilever beam, when the weight (brick height) on the beam can not meet the stability requirements, the support and formwork will be removed, leading to collapse accidents.
2. Improper formwork support scheme
The stress at the root of cantilever structure is the largest. After pouring concrete, the strength is not enough, the formwork support will settle, and the concrete at the root will crack immediately. After the formwork is removed, it will break and collapse from the root. If the cantilever structure is variable cross-section, the formwork will be made into equal cross-section shape during construction, which will reduce the cross-section of the root and cause collapse accident after formwork removal.
3. Dislocation and deformation of steel bars
The negative bending moment at the root of cantilever structure is the largest, and the main reinforcement should be arranged on the upper part of beam slab. If the reinforcement is placed at the lower part during construction, or the downward deformation is too large when it is trampled, or the anchorage length is not enough, the root will collapse after formwork removal.
4. Building overload
The bending moment at the fixed end of cantilever structure is proportional to the applied load. If the construction load exceeds the design load, cracks will appear at the root of the formwork when it sinks. Especially when pouring concrete from the root outward, with the increase of load; Template deformation, but also easy to produce cracks in the root, leading to fracture after ripping.
5. Early form removal
Many collapse accidents of cantilever structures are caused by premature formwork removal and insufficient concrete strength. Therefore, the specification stipulates that the concrete formwork removal strength of cantilever beam slab with span less than 2m should be greater than or equal to 70%; For a cantilever beam slab with a span of more than 2m, the concrete formwork removal strength is 100%.
Three, reinforced concrete column hoisting fracture accident analysis
(1) accident overview
Column C of an engineering project is a column with equal cross section, with a length of l2m and a cross section of 40 omm * 6 omm. Symmetrical reinforcement is adopted, with 4 industries 16 on each side and 2 industries12 on structural reinforcement. The strength grade of concrete is C20, and it has reached 100% strength during hoisting. The column is prefabricated horizontally and hoisted at one point; The lifting point is 2m away from the top of the column; When just lifted off the ground, cracks appeared between the lifting point about 4.8m away from the column foot and the column foot. The cracks ran through both sides along the bottom surface, and the maximum width was 1.3mm, which led to the fracture of the column.
(2) Cause analysis of the accident
The main reasons for this accident are: when the column is prefabricated and hoisted horizontally, the stress on the lifting point is different from that when it is used; The choice of lifting point is unreasonable, the lifting torque is too large, and its bending strength and crack resistance can not meet the requirements. The analysis and calculation are as follows:
The selection of 1. lifting point does not conform to the principle of minimum lifting torque MDm.
The lifting moment of the column is closely related to the position of the lifting point, so it is damaged. The principle of selecting lifting point is that the lifting bending distance must be minimum. Therefore, when lifting a column with equal cross section at one point, it should make |Mmx|=| MD|, that is, the absolute value of the maximum positive bending distance in the span is equal to the negative bending distance at the lifting point. Accordingly, the lifting point position is 0 293L(L from the top of the column (L is the length of the column). When l is 12m, the distance between the lifting point and the top of the column should be 0.293x12 = 3.5m. The original lifting point is 2m away from the top of the column, which does not conform to the principle of minimum lifting point torque. When lifting, the absolute value of the maximum bending moment of mid-span must be greater than the absolute value of the negative bending moment at the lifting point, so the crack occurs at the section of the maximum positive bending moment of mid-span.
2. The bending strength of the column during hoisting is not enough.
Now, according to the minimum lifting bending moment, if precast horizontally, the bending strength of the column can not meet the requirements. The inspection results are as follows:
(1) calculation load g
If the gravity density of reinforced concrete is 25,000 N/m', the dead weight is 0.4x0.6x25,000 = 6,000 N/m; The dynamic load coefficient is 1.3~ 1.5. If 1.5 is taken, the calculated load is q =1.5x6000 = 9000 n/m. ..
(2) Calculation diagram
According to the principle of minimum bending during hoisting, the hoisting point is 3.5m away from the top of the column, and the column foot does not leave the ground when hoisting. The column just hoisted off the ground is similar to a cantilever simply supported beam.
3. The crack resistance of the column is not enough when it is hoisted.
According to the code for construction acceptance, the crack width in the tensile area of reinforced concrete members during hoisting is not more than 0.2~0.3mm, and the crack width is related to the tensile stress of steel bars. The greater the tensile stress of steel bars, the greater the crack width. Therefore, the tensile stress of steel bars is often used to control the crack width in column hoisting. As long as the tensile stress of steel bar meets the requirements of the following formula, it shows that the crack width is within the allowable range and can meet the requirements of crack resistance. It shows that the crack resistance can not meet the requirements.
(3) Experience and lessons
The following lessons should be learned from the above accidents:
(1) Because the lifting force of the column is different from the service force, the lifting calculation must be carried out.
(2) When the lifting force is different from the use force, the selection of lifting points should conform to the principle of minimum lifting torque to avoid damage due to excessive lifting torque. For example, in this case, according to the principle of minimum lifting moment, when the distance between the lifting point and the top of the column is 3 5m, the absolute value of the positive bending moment in the span is equal to the absolute value of the negative bending moment at the lifting point, both of which are 55. 125XlO. However, when the distance between the original lifting point and the top of the column is 2m, the maximum bending moment in the span is103.68x1o. N mm, the maximum bending moment section is 4.8m away from the column foot. It can be seen that the original mid-span bending moment of the lifting point is 1.88 times larger than that determined by the principle of minimum lifting point bending moment. There is a big crack in the column 4.8m away from the column foot, which leads to fracture, which also proves that the lifting moment of this section is the largest.
(3) When the lifting force is consistent with the use force, the selection of lifting points should meet the requirements of the use force as much as possible, such as the two lifting points of a simply supported beam should be close to both ends of the beam; The two lifting points of the cantilever beam should be on the two fulcrums of the beam.
(4) When the bending strength and crack resistance can't be satisfied after hoisting calculation, turn over and hoist first. For example, if a body crane is used in this example, bending strength and crack resistance can be satisfied. If the passing crane is still not satisfied, the lifting point can be increased from one point to two points to reduce the lifting point torque or take temporary measures.
In addition, in order to facilitate positioning and centering, and ensure the safety of hoisting, the center line of the hook should be aligned with the center of gravity of the component when binding; When horizontal members are hoisted and bound at two points, two lifting ropes should be used respectively; For equivalent section members, it is also required that the two lifting points are symmetrical left and right, and the two lifting ropes have the same length; The horizontal included angle of the lifting rope should be greater than or equal to 60. Not less than 45. ; It is forbidden for the crane to tilt and drive with load.
There are many reasons for cracks in cast-in-place reinforced concrete slabs, but the most important thing is that the tensile stress in concrete exceeds the tensile strength of concrete. If we find the reason, we can find a way to avoid it.
1. Cracks caused by drying shrinkage of cement appear on the surface of plates, which are relatively small. Cement is a kind of hydraulic material, which has dry shrinkage. If the water content is insufficient, cracks may appear in the early stage of hardening. The way to avoid it is to strengthen maintenance, cover and water regularly.
2. Cracks caused by temperature difference changes generally appear in environments with large temperature difference changes and components with large area or length. The solution is to leave expansion joints at appropriate positions.
3. Cracks caused by stress concentration generally appear in the yin-yang angle or support of the plate. This is caused by insufficient steel bars on the board or excessive spacing between steel bars. The way to avoid it is to add steel mesh or reduce the spacing of steel bars on the board.
4. Cracks caused by premature loading. It is because the formwork is removed too early, the strength of concrete can not meet the design requirements, so that the components are overloaded and cracks appear at the bottom of the slab. The way to avoid it is to strictly control the time of form removal and not to load it in advance (even if the form is not removed, it is not allowed to accumulate too much on the board).
5. In addition, there may be other reasons, such as the vibration or displacement of the formwork at the early stage of concrete hardening; Improper handling of construction joints may lead to cracks in the slab. As long as these are avoided in the construction.
If it is reinforced concrete, the problem is serious. Please check the reason from the design (not to mention here).
If it is plain concrete, cracks usually belong to cracks produced during the shrinkage of concrete. Please see below:
1. Because the lower part is a steel plate, the surface is usually bright, which makes the concrete poured on it have little grip on the joint surface and cannot resist the development of shrinkage and deformation caused by temperature change during concrete hardening.
2. Cement and mix proportion used in concrete are also important factors affecting concrete cracking. Under normal circumstances, the greater the hydration heat, the greater the slump, and the easier it is to cause cracking.
3. It is related to the design size of pouring layout. If the slenderness ratio of the plane size is too large, it is easy to crack because of the large difference in shrinkage stress between the long direction and the short direction.
4. The settlement and deformation of the building structure should also be checked in the design calculation.
5. On the basis of the above main reasons, whether the surface of the base is clean and free of oil pollution, the number of times of concrete compaction and leveling, concrete curing and other conventional construction technologies are also the construction technology problems that cause cracks.
About how to prevent:
1. Because the surface of steel plate is usually shiny, steel plate can be selected or treated to make the surface as rough as possible. At the same time, steel wire mesh or high-strength steel wire mesh can be added to concrete, and chopped high-strength fibers can also be added to concrete to resist shrinkage deformation.
2. Choose cement with low hydration heat and pour hard concrete. During the hardening of concrete, do a good job of temperature control, increase the surface temperature (such as covering and heat radiation), reduce the temperature difference between the inside and outside of concrete, and at the same time, receive light on the surface for at least three times, with an interval of 65438 0 ~ 2 hours each time, so as to avoid micro-cracks on the surface and reduce the deformation of concrete itself.
3. For concrete slabs with large length-width ratio, frame joints can be reserved in advance according to the layout to interrupt shrinkage stress and avoid stress concentration and cracking.
4. The settlement and deformation of the building need to be designed to provide parameters and checked.
5. Other construction technical issues are not discussed in detail, but are included in the regular construction specifications.
About how to deal with:
1, rework.
2. Long cracks are filled with epoxy resin.
3. Supplementary frame joints.