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Causes and control measures of concrete cracks
abstract:
In building engineering, there are cracks in concrete structures and components to varying degrees, which cause certain damage to the structure, seriously affect the strength and deformation of concrete, and even pose a certain threat to the safety of the structure. Serious may threaten people's lives and property safety, so it is particularly important to prevent and deal with concrete cracks.
Key words:
Shrinkage crack; Temperature crack; Subsidence cracks; measure
With the rapid development of the times, the scale of China's construction market is increasing day by day, and concrete plays an important role in modern construction projects. Concrete cracks not only affect the use function, but also affect the strength and deformation of concrete, which in turn causes the embroidering of steel bars and ultimately affects the durability and durability of the structure. Therefore, this paper discusses the causes, treatment measures and remedial methods of concrete cracks.
1, causes of concrete cracks
Concrete is a kind of heterogeneous brittle material, which consists of sand and gravel aggregate, cement, water and other additional materials. Due to the difference of composition materials, microstructure and external influences, the causes of concrete cracks are also varied:
1. 1 shrinkage crack
Concrete shrinkage is an inherent physical phenomenon of concrete materials. According to the test, the shrinkage value of concrete is generally (4-8) × l0-4, the tensile strength of concrete is generally 2-3 MPa, and the elastic modulus is generally (2-4) × L04 MPa. Formula 8= stare /E (where 8 is strain value, stare is concrete stress and E is concrete elastic modulus) shows that the allowable deformation range of concrete is only about one ten thousandth, while the actual shrinkage of concrete is (4-8) × l0 _ 4, which is greater than the allowable deformation range of concrete. Therefore, cracks in concrete are inevitable, and the key is to control the width of cracks.
The concrete surface may shrink due to water loss, and the main reason for plastic shrinkage is that the strength of concrete before condensation is very small or almost zero. It may also be that when the setting strength of concrete is very small, due to the influence of strong wind or high temperature, the surface water loses too quickly, resulting in a large negative pressure, which makes the volume of concrete shrink sharply, thus causing cracking.
1.2 temperature crack
Temperature cracks are caused by the internal constraints of concrete, which often occur during the construction of mass concrete, high-strength concrete or concrete in areas with large temperature changes. The trend of temperature cracks is usually irregular, and large-scale structural cracks are often criss-crossed. From pouring to mass concrete reaching design strength, that is, structural cracks during construction are mainly caused by temperature changes caused by hydration heat of cement. In mass concrete engineering, the amount of cement is large and the structural section is large. Therefore, after concrete pouring, the cement releases a lot of hydration heat, and the temperature of concrete rises. Due to the poor thermal conductivity of concrete, large volume and small relative heat dissipation, the hydration heat accumulated in concrete is difficult to radiate to the environment, which leads to a sharp rise in internal temperature, while the surface of concrete dissipates heat faster. In the heating stage, the surface temperature of concrete is always lower than the internal temperature. Due to the different degrees of thermal expansion and cold contraction, there is a certain tensile stress on the concrete surface. The expansion speed of concrete in the central part is faster than that in the surface concrete, and there is mutual constraint between the central part and the surface particles. The central part is constrained to expand and will not crack. The surface belongs to constrained shrinkage. When the surface tensile stress (T) exceeds the ultimate tensile strength of concrete, cracks will occur on the concrete surface. With the slowdown of cement hydration reaction and the continuous heat dissipation of concrete, the temperature of concrete changes from heating stage to cooling stage, and the temperature decreases and the volume shrinks. Because the heat inside the concrete radiates outward through the surface, there is still a difference between the surface temperature and the central temperature of the concrete during the cooling stage. If it is too large, surface cracks will appear as in the heating stage. In the process of cooling, the volume shrinkage of concrete, taking into account the boundary conditions and foundation constraints, belongs to constrained shrinkage.
1.3 structural cracks
Although the bearing capacity of the cast-in-place floor slab can meet the design requirements, the stiffness of the wall is relatively increased and the stiffness of the floor slab is relatively weakened after the prefabricated perforated slab is changed into the cast-in-place slab. Therefore, some structural cracks often occur in some weak parts and sudden changes in section. For example, 450 inclined cracks appear at the corner stress concentration, and tensile cracks appear on the surface of the plate where the negative bending distance of the plate end is large.
1.4 cracks caused by uneven settlement of structural foundation
When the foundation settlement of the structure is uneven, the structural members are deformed, which leads to diagonal tension and shear between the members in the structure, thus causing structural members to crack. With the further development of uneven settlement, cracks will further expand. The size, shape and direction of such cracks depend on the deformation of the foundation. The stress caused by foundation deformation is generally large, and the crack width is large, mostly 450, which is generally penetrating.
2, concrete crack prevention measures
Measures to control shrinkage cracks are as follows:
The main preventive measures for dry shrinkage cracks: choose medium and low heat cement and fly ash cement with small shrinkage, which will reduce the amount of cement. In addition, the water-cement ratio will affect the drying shrinkage of concrete, so the water-cement ratio should be well controlled in the concrete design and an appropriate amount of water reducing agent should be added. Water consumption should be strictly controlled in concrete mixing and proportioning. Set appropriate contraction joints in the structure. The main preventive measures of plastic shrinkage cracks: choose cement with low shrinkage strength, such as silicate or ordinary portland cement, and cover the surface with plastic film before the final setting of concrete to reduce the serious loss of water under strong wind and high temperature. Measures to control temperature cracks are as follows:
Adopt high strength cement with bottom hydration heat to reduce the hydration heat of cement and improve the crack resistance of concrete; Before use, the hydration heat of cement should be determined according to the current national standard "Experimental Method of Cement Hydration Heat (Direct Method)"; Improve aggregate gradation, adopt aggregate with good thermal conductivity, small linear expansion coefficient and reasonable gradation to reduce concrete temperature stress. In concrete construction, in order to improve the turnover rate of formwork, it is often required that the newly poured concrete be removed as soon as possible. When the concrete temperature is higher than the air temperature, the time of formwork removal should be properly considered to avoid early cracks on the concrete surface. Dismantling the formwork in the early stage of new pouring causes great surface tensile stress and "temperature shock" phenomenon. At the early stage of concrete pouring, due to the effect of hydration heat, considerable tensile stress is generated on the surface, and the surface temperature is also higher than the air temperature. At this time, when the mold is removed, the surface temperature drops suddenly, which will inevitably cause a temperature gradient, which will increase the surface tensile stress, which will be superimposed with the hydration thermal stress, and the surface tensile stress will reach a large value, which may lead to cracks. However, if the surface is covered with light thermal insulation materials, such as foam sponge, in time after formwork removal. Mixing mixed materials can reduce water consumption, save cement and reduce the crack resistance of concrete.
Adding additives to slow down the rate of hydration heat. In order to ensure the quality of concrete engineering, prevent cracking and improve the durability of concrete, the correct use of additives is also one of the measures to reduce cracking.
Measures to control subsidence cracks:
Deepen and strengthen the soft soil to ensure that the supporting formwork entering the frozen soil has enough hardness, and it must be firm and firm to support evenly and make it in a stable and even state.
3. Common remedial measures after cracks appear
3. 1 surface repair method. It is suitable for the treatment of surface cracks and deep cracks that have no influence on bearing capacity, and also suitable for the seepage prevention and leakage prevention of large-area small cracks. Coat the surface with cement mortar, epoxy mastic, paint and asphalt.
3.2 Internal repair of hair. Grouting pump pressed the cementing material into the cracks, which played a role in repairing the cracks because of its condensation and hardening, thus restoring the integrity of the structure.
3.3 Structural reinforcement methods. Strengthening the structure with materials such as anchor, steel plate and reinforced concrete can curb the further development of cracks and restore the integrity of the structure.
4. Overview
The causes of concrete cracks are discussed above. Shrinkage cracks and temperature cracks are common problems in concrete structures, but they can be completely avoided by applying the measures described in this paper and taking corresponding measures to effectively control the temperature. Through reasonable design, strict material control, systematic construction technology control and strict operating procedures, settlement cracks can be solved. In addition, in the case of mass concrete pouring, joints and blocks should be reasonably divided to avoid excessive height difference and long-term exposure of the side to ensure the engineering quality.
refer to
[1] Luo xianbing, causes and preventive measures of cracks in cast-in-place reinforced concrete floor slab [J]. western exploration project, 2006.
[2] China Ministry of Construction, Code for Design of Concrete Structures (GB500 10-20 10).
[3]. Beijing: China Building Industry Press, 20 10.
Expansion: Preventive Measures for Cracks in Concrete Face Slabs
It is difficult to completely avoid cracks in face slab concrete, and comprehensive measures must be taken to prevent them.
First, the prevention and control of structural cracks
The prevention and treatment of cracks in face slab structure mainly starts with the design of rockfill dam and the control of dam construction quality, and measures are taken to prevent the excessive uneven horizontal displacement and settlement of the supporting part of face slab from causing the void between cushion and face slab. In the design, the stability of panel support should be considered in the selection of dam slope and dam material compaction index, and the construction quality should be strictly controlled according to the design requirements, and the panel pouring can only be carried out after the dam settlement is stable.
1, treatment of foundation surface. In general, the foundation surface of concrete face rockfill dam is strictly treated in the range of about 1/3 dam thickness below the water surface. In other areas, if there is a thick gravel cover, it is generally not fully excavated, but only the density or porosity is tested, and the foundation can be used as the foundation surface after meeting the design requirements. It is necessary to clean up the vegetation and overburden on the dam slopes on both banks, cut the slopes at the steep or reverse slopes of local slopes, or fill the slopes with concrete and mortar to the designed slope ratio.
2. Control the dam filling quality. The control of rockfill deformation is mainly to control the filling density and rock mass strength. The control of rock mass strength is mainly to excavate and take materials from different filling areas according to the corresponding lithology during construction. Choosing a higher filling density is mainly achieved by controlling the gradation and rolling quality of filling materials, especially the gradation of filling materials. The compactness and mechanical properties of rockfill are closely related to gradation, and the rockfill with good gradation can obtain higher deformation modulus and shear strength after compaction.
In the process of dam filling, the construction is carried out in strict accordance with the design parameters, and the construction parameters and compaction effect are controlled by pit exploration method and mass addition method, so as to ensure the compaction quality of dam filling and avoid excessive settlement in the later stage of the dam. In order to reduce the uneven settlement of the dam, the whole section of the dam should rise as evenly as possible during the filling process. If it is necessary to fill temporary sections in flood season, it is also necessary to try to avoid large height difference between upstream and downstream.
3, panel joint and reasonable configuration of steel bar. According to the results of three-dimensional nonlinear finite element calculation, the face slab is in a three-dimensional complex stress state around the two abutments of the dam, most of which are in tension and the middle part of the face slab is in compression. According to the above mechanical characteristics, the tension zone panel should be a narrow plate with tension joints; Panels in compression areas shall be wide plates with compression joints. Joint separation materials should be arranged between compression joints to prevent the panel from lifting or tilting due to excessive compression stress. In addition, double-layer steel bars should be arranged in the tensile area, the parts with large compressive stress and the peripheral joints of the panel to improve the ability of the panel to adapt to deformation.
Second, the prevention and control of non-structural cracks
The prevention and treatment of non-structural cracks in the panel mainly take measures from the materials used in the panel itself and the construction methods of the panel. For cracks caused by improper construction and chemical reaction of materials, appropriate mix proportion should be adopted to meet the requirements of construction degree of panel concrete, and measures such as strengthening construction management and controlling construction quality of panel should be taken to avoid them. For aggregate with alkali activity, the total alkali content of unilateral concrete should be strictly controlled according to the specification requirements. The prevention and treatment of shrinkage cracks in panel is the most important and difficult work in the prevention and treatment of cracks in panel.
1, optimize the mix proportion of face slab concrete.
In the aspect of mix proportion of face slab concrete, high-quality raw materials should be selected to prepare face slab concrete.
1. 1 cement with low hydration heat is selected, and part of cement is replaced by fly ash, so as to reduce the temperature rise of hydration heat, thereby reducing surface cracks caused by internal and external temperature difference and penetrating shrinkage cracks caused by concrete shrinkage after hydration heat dissipates.
Choose 1.2 aggregate with small linear expansion coefficient to prepare concrete, so as to reduce the volume deformation of concrete caused by temperature change.
1.3 Select high-quality concrete water reducer, and reduce the unilateral water consumption of face slab concrete under the premise of meeting the slump of concrete construction, so as to reduce the drying shrinkage of concrete.
1.4 On the premise of meeting the design strength, reduce the dosage of concrete cementing materials as much as possible to reduce the shrinkage of concrete.
1.5 adopts fiber concrete. Adding proper amount of polypropylene fiber into concrete can inhibit the formation and development of early cracks; Reduce the elastic modulus of concrete and increase the ultimate tensile value of concrete; Improve the frost resistance grade of concrete, improve impermeability and durability. In addition, the concrete mixture should meet the requirements of panel construction, with good workability, fluidity and cohesiveness.
2, reduce the basic binding force
Cancel or reduce the insertion of the vertical reinforcement of the panel into the dam body, and spray emulsified asphalt on the foundation surface before pouring the panel, so that a small amount of relative displacement occurs between the panel and its foundation surface. At this time, the binding force of the base surface on the shrinkage deformation of the panel can be considered to be equal to the friction force between the panel and its base surface. Therefore, the binding force can be artificially controlled. If the friction resistance (sliding resistance) of the panel is less than the sliding force of the panel on the dam slope, it will not only greatly reduce the binding force of the panel cracking, but also increase the preloading stress of the horizontal section of the panel, which is conducive to improving the crack resistance of the panel.
2. 1 Strengthen the quality management of panel construction to avoid weak tensile surface.
2.2 Adopt the construction technology of panel secondary pressing. The secondary pressing technology of panel is helpful to eliminate surface cracks caused by sudden temperature drop and water loss.
2.3 Reduce the temperature of the face slab concrete silo to reduce the temperature difference of the foundation, thus reducing the shrinkage cracks. The ambient temperature for panel casting is generally 5 ~ 25℃.
2.4 During panel construction, do a good job in heat preservation, moisture preservation and windproof maintenance of the panel, so as to reduce the cold shrinkage and dry shrinkage of the panel concrete.
1 moisturizing. Long-term moisturizing and curing of panel is one of the main measures to prevent panel cracking. After the panel concrete is demoulded, it is plastered for the second time, the surface is covered with a thermal insulation quilt with plastic film, and water is sprayed regularly to keep moisture, so as to prevent the moisture on the concrete surface from evaporating too fast and forming dry shrinkage cracks;
② heat preservation. In the middle and later period, on the basis of covering the thermal insulation quilt with plastic film, sprinkling water is not stopped to achieve the purpose of thermal insulation, so as to avoid the temperature difference between the inside and outside of concrete caused by hydration heat or external temperature, thus causing temperature cracks;
③ windproof. Wind speed is an important cause of panel cracking. The increase of wind speed will increase the heat transfer coefficient of concrete, which will lead to the decrease of the surface temperature of the panel, the steep temperature gradient inside and outside the panel and the sharp increase of tensile stress, which will lead to cracks in the panel.
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