Abstract: At present, the most widely used material in building structures is reinforced concrete. However, cracks are still common in these reinforced concrete buildings, which have a great impact on the stability and safety of the whole building, and the hidden safety hazards directly affect people's personal safety. Therefore, the control and treatment of cracks in building structure design need to attract extensive attention of relevant personnel in the industry. This paper analyzes the causes and types of cracks in architectural structure design, and discusses the control measures of cracks in architectural structure design.
Keywords: building structure; Causes of cracks; Crack control measures;
1, Introduction
With the development of China's economy, the construction industry is developing better and better, and more and more construction projects are put into construction. At present, the most widely used material in building structures in China is concrete. Due to the design characteristics of concrete structure and the great influence of concrete materials from the outside world, cracks may appear in the building structure, which will have a great impact on the overall stability and safety of the building, and then affect people's personal and property safety. Therefore, relevant personnel must deal with these cracks to improve the safety and quality of the whole building. Next, this paper analyzes the types and causes of these cracks.
2. Types of cracks
2. 1 plastic settlement crack
In practice, building reinforcement, formwork and other factors will affect the settlement of concrete aggregate, and under this influence, plastic settlement cracks will be formed. In addition, the construction quality will also have an impact on the formation of plastic settlement cracks. If there is no normal and standardized binding operation of the formwork in the actual construction process, there is a certain probability of plastic settlement cracks in the building structure. Plastic settlement cracks are shuttle-shaped and wide in the middle and narrow at both ends, which often appear at the variable cross-section of the structure, the junction of beam and slab, and the junction of beam and slab and rib, and the depth of cracks usually reaches the surface of reinforcement. This kind of crack should control the water cement ratio, sand ratio and slump not to be too large; For members with large cross-section difference, they should be poured in the deep part first, let stand for 1~ 1.5h, and then poured together with the upper thin cross-section after settlement is stabilized. Finally, the thickness of the protective layer should not be too thin.
2.2 Plastic shrinkage crack
Plastic shrinkage cracks are generally formed during construction. In the process of pouring concrete, it is exposed and affected by external factors, such as high temperature or windy weather. And concrete materials tend to expand when heated and contract when cooled. When the concrete material is in a plastic state, the water in the concrete will be further reduced, the concrete material will harden, and plastic shrinkage cracks will be formed on the building surface. Plastic shrinkage cracks are irregular polygons or generally parallel to each other. The smallest distance between cracks is several centimeters, and the largest is more than ten centimeters. These cracks are shallow at first and will gradually develop into penetrating cracks. Preventive measures include strictly controlling the water-binder ratio of concrete, cement dosage and silt dosage; Measures should be taken to ensure the construction quality under high temperature, strong wind and dry weather.
2.3 temperature stress cracks
As the name implies, the main influencing factor of temperature stress crack is temperature. In the actual concrete pouring construction process, it often takes a long construction period. During this period, due to the large temperature difference between day and night, indoor and outdoor, the surface of concrete structure dissipates heat quickly, which will have an impact on pouring. When the surface tensile stress caused by temperature difference exceeds the tensile stress intensity that concrete can bear, it is easy to form temperature stress cracks. Although it is not obvious, its actual impact on the stability and safety of buildings cannot be underestimated. Thermal stress cracks are mainly surface cracks with different depths. It is necessary to prevent the temperature difference between the inside and outside of concrete and the gradient of concrete surface, or to prevent the concrete from being supercooled and the old concrete from being supercooled, so as to reduce the constraint between the new and old concrete.
2.4 Other cracks
These cracks may be related to the construction quality, construction technology or the quality and nature of the building materials themselves. If these aspects are not up to standard, it is easy to form corresponding cracks, which has a great impact on the quality of buildings.
3. Cause analysis of cracks
3. 1 load factor
After the reinforced concrete structure is completed, it is necessary to consider the load from the mechanism structure and the whole building system. These external loads will bring pressure to reinforced concrete structures. If the load value is higher than the design standard for a long time, load cracks will appear with the accumulation of time, which are mainly related to dynamic and static loads and secondary stresses.
3.2 Temperature factors
According to investigation, the linear expansion coefficient of concrete used in China is 1× 10-5/℃. When the temperature difference between the inside and outside of concrete is large, it will produce compressive stress and then tensile stress. When the tensile stress exceeds the compressive ultimate strength of concrete, cracks will occur. The following formula can explain the relationship between temperature stress and components: amax = 0.5tllyl The highest temperature in the middle of components is a, which means that the friction coefficient between components is t LLl, which means the length of components. The friction coefficient between groups is also affected by water, but with the increase of temperature, a lot of water inside the concrete structure will evaporate, which will accelerate the formation of plastic shrinkage cracks. If there is no supplementary water outside, the situation will get worse.
3.3 Construction quality factors
Construction quality factors also have a great influence on the formation of cracks. The main construction link involved is concrete pouring. Temperature and other factors should be considered in the process of concrete pouring, and the concrete structure needs to be replenished in time. On the other hand, the pouring quantity should be controlled according to the maximum load marked on the design drawing, and pouring should be carried out in strict accordance with the design requirements. On the other hand, when binding templates, standardization should be achieved. If the bond is not firm, it will also lead to cracks.
3.4 Quality factors of raw materials
The quality of raw materials is another key factor. The hydration value, viscosity and other properties of concrete materials will affect whether cracks will occur. Good quality raw materials can enhance their own hardness, and have better bearing effect on load and tensile stress. Therefore, the quality factors of raw materials have a great influence on the occurrence of cracks.
4, crack control measures
4. 1 Strict control design
When designing, we should consider all possible factors as a whole. On the other hand, the overall stiffness of building concrete structure should be considered to improve the bearing capacity of the whole building. In addition, the possible uneven settlement of buildings should be considered. When uneven settlement occurs, it will produce corresponding compressive stress and tensile stress, which will lead to the decline of the ability of building structure to resist temperature stress. Therefore, it is necessary to accurately calculate the bearing capacity of buildings and predict possible accidents and uncontrollable factors in building design, so as to increase the scientificity and rationality of design.
4.2 Strictly control the selection of materials
When selecting materials, the quality of concrete materials should be strictly controlled. Generally, fly ash cement or slag cement with low hydration value should be used for mass concrete. At the same time, in order to further improve the consistency of cement slurry and increase the tensile strength of concrete, additives such as salt water crack inhibitor can be added in a proper amount. In addition, aggregate should be selected and optimized in strict accordance with national standards and appropriate sand ratio. Relevant managers need to check the purchased materials to ensure the quality of the materials.
4.3 Strictly control the construction
The quality of construction links has a great influence on the quality of buildings. Therefore, it is necessary to improve the construction technology and supervise the construction norms and quality of construction personnel. In the process of concrete pouring, factors such as temperature and wind speed should also be considered. In the face of high temperature environment, construction workers should choose the method of layered pouring, and use the heat dissipation function of the pouring surface itself to weaken the influence of high temperature. In addition, water pipes can be buried, which can not only cool down physically, but also keep the moisture of concrete structure, killing two birds with one stone. In the process of pouring, the construction personnel should also consider the location of the reinforced structure to ensure that the reinforced structure is not affected by the concrete pouring work, thus improving the stability of the whole building. For example, building structures usually belong to mass concrete, and the concrete strength of beams and floors of building structures should be consistent, so the intermediate grade should be selected. When the concrete grade of the wall and column of the building structure is higher than that of the beam and slab, the concrete strength grade of the joint core area should be consistent with that of the column and wall. When the concrete strength grades of beams and columns are different, the joint practices are shown in Figure 1. The concrete strength grade of cast-in-place beam and floor slab should be consistent. When the concrete strength grade of columns and walls is greater than that of beams and slabs, the concrete strength grade of the joint core area should be the same as that of columns and walls. See figure 1 for the joint practices of beams and columns with different concrete strength grades.
4.4 Prestressing and Structural Design
According to the field situation in recent years, the design of prestressed structure should consider the size of building geometry, the number of prestressed tendons and the crack resistance requirements of prestressed structure. Generally, the beam length is designed as115. Under the existing design and construction level, it is generally118 ~1/20, which can not only reduce the steel consumption, but also reduce the structural weight. For example, the plane layout of the building structure should ensure the regularity of the plane layout of the building structure and avoid sudden changes in the shape of the plane layout. When there is a notch in the plane, a pull beam should be set at the edge of the notch, the floor around the notch should be properly thickened and reinforced, and the negative reinforcement of the floor should be pulled through. In addition, the length of the building structure should be controlled according to relevant codes and requirements. When the length of the building structure exceeds the value specified in relevant codes, post-pouring belt should be set in the underground part and expansion reinforcement belt should be set on the ground. Generally, the post-cast strip is set at the position of 1/3 beam and floor width, and the width should be within the range of 800 ~1000 mm. Generally, the width of the reinforcement strip is 2000mm, and dense-hole steel mesh is arranged on both sides of the strip to separate the concrete in the strip from the concrete outside the strip. The steel wire mesh is vertically arranged between the upper and lower layers (or the inner and outer layers) and reinforced with steel bars. In the expansion reinforcement zone, horizontal temperature steel bars of 15% are added, and the horizontal temperature steel bars are evenly distributed in the upper and lower floors. After adding 12% expansive concrete, the strength grade of concrete is improved by one grade. The setting of post-pouring belt and reinforcing belt should completely separate the beam, wall and plate, and the steel bars should still be continuously configured. When the house length exceeds the specified value, deformation joints should be set. When the height difference between the building and the main building is quite large, it is necessary to set settlement joints or post-pouring zones between the main building and the main building, which can effectively avoid or reduce cracks caused by foundation settlement.
5. Conclusion
Because buildings are closely related to people's production and life, the quality of buildings directly affects the safety of people's lives and property. Cracks in building structures are an important threat to building quality and safety, so corresponding measures should be taken to consider the factors that may cause cracks as a whole and control them. In the process of building structure design, improving the scientificity and rationality of design can better estimate some standard parameters. In addition, in the process of building construction, it is necessary to strengthen management, improve construction quality, and control material selection and construction technology. In a word, relevant construction units should adopt corresponding reasonable solutions according to different types of cracks, so as to minimize potential safety hazards and ensure people's personal safety.
refer to
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[2] Dong Chunling, Li Xingkai. Measures to control cracks in building structure design [J]. Engineering Science and Technology.
[3] Li Guang and. Cause analysis and preventive measures of cracks in cast-in-place concrete construction [J]. Theoretical research on urban construction, 20 1 1, 8.
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