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1、Causes and control measures of concrete cracks study the problemKeywords: Causes prevention of concrete cracksAbstract: At present, paid close attention to the problem of concrete cracks, this crack in the concrete on the basis of classification, analysis of the causes of different cracks, and propo

2、sed measures to crack prevention and treatment.1.IntroductionIs the maximum amount of concrete as a building material, widely used in industrial and civil construction, agriculture and forestry with urban construction, water conservancy works in the harbor. However, many concrete structures occurs d

3、uring the construction and use of different degrees and different forms of fracture. This not only affects the appearance of the building, but also endanger the normal use of buildings and structures durability.Therefore, the cracks become people concerns. In recent years, with the ready-mixed concr

4、ete and vigorously promote the use and structure become increasingly large, complex, making the problem even more prominent.However, cracks in concrete structures is a fairly common phenomenon, large number of engineering practice and modern science on the concrete strength of micro studies show tha

5、t the structures of the crack is inevitable, which is a property of the material. Therefore, the scientific treatment of cracks in the crack problem is to classify on the basis, adopt effective measures to harmful levels of crack control to the extent permitted. This concrete structure will cause cr

6、acks in common, control measures and the repair method to analyze some light.2.Classification of concrete cracks 2.1 Divided by Crack According to the causes of concrete cracks, structural cracks and can be divided into two major categories of non-structural cracks. （1） Structural cracks Caused by a

7、 variety of external loads cracks, also called load cracks. It includes the external loads caused by the direct stress cracks and the structure under external loads caused by secondary stress cracks. （2） Non-structural cracks Deformation caused by the change from a variety of cracks. It includes tem

8、perature, shrinkage and swelling caused by factors such as differential settlement cracks. Such cracks in the structure when the deformation is restricted due to the stress caused. Research data from abroad and a large number of engineering practice, non-structural cracks in the works in the majorit

9、y, about 80%, which led to shrinkage cracks. 2.2Divided by the time the cracks （1）Cracks during construction Including plastic shrinkage cracks, settlement shrinkage cracking, drying shrinkage cracks, shrinkage cracks itself, the temperature cracks, the cracks were improper construction operations,

10、the role of early frost, and some irregular cracks caused by cracks. （2）Use of crack during Including the expansion of steel corrosion cracks generated, salt and acid erosion type liquid medium caused by cracks, the cracks caused by freezing and thawing, alkali aggregate reaction, and cracks caused

11、by cyclic loading cumulative damage caused by cracks.2.3Classification of fractured by cracks in the shape of the shape can be divided by:（1）Longitudinal cracks parallel to the bottom component, the distribution along tendons, mainly caused by the role of steel corrosion（2）Transverse cracks perpendi

12、cular to the bottom component mainly by the loading, temperature effects caused（3）Shear cracks due to displacement caused by vertical load or vibration（4）Diagonal cracks eight shaped or herringbone cracks, common in the wall of concrete beams, mainly due to the uneven foundation settlement, and ther

13、mal effects caused by（5）X-shaped cracks common in the framework of beams, columns and walls on the ends, due to the impact effect, or moment loads caused by earthquake（6）All kinds of irregular cracks such as repeated freezing and thawing, or fires caused by cracksIn addition, concrete mixing and tra

14、nsport time for long cracks due to mesh, square appears floor slab or plate surface radial cracks appear in the cross cracks and so on.2.4 The development of the state divided by cracksAccording to fracture the movement in which the state and development trends, can be divided into the following cat

15、egories:（1）Stable crack This crack does not affect the persistence of applications, including two types.One is in motion the process of self-healing of fractures could be common in a number of new water projects, this is because the crack of cement particles in the leakage of water further compounds

16、 the process, precipitate Ca （OH） 2 crystal and part of the Ca （ OH） 2 has dissolved in the water with CO2 carbonation reaction to form CaCO3 crystallization occurs, both the formation of cracks in the gel material will be glued closed, and thus stop the leakage, cracks to heal. The other is in a st

17、able movement of the cracks, such as the periodic load generated by the cyclical expansion and closure of cracks.（2）Unstable crack This will result in instability of crack extension, affecting the sustainable use of structures, should be considered part of its expansion, to take corresponding measur

18、es.3.Causes of cracks in concrete and control measures3.1 Shrinkage cracks Shrinkage cracks are caused by the humidity, it accounts for non-structural cracks in concrete in the main part. We know that concrete is a cement as the main cementing material to natural sand, stone aggregate mixing water,

19、after casting molding, hardens and the formation of artificial stone.In the construction, in order to ensure its workability, often adding cement hydration than water needed for 4 to 5 times more water. More of these water to free state exists, and the gradual evaporation of the hardening process, r

20、esulting in the formation of large pores inside the concrete, voids or holes, resulting in volume shrinkage of concrete. In addition, the hardening process of concrete hydration and carbonation of concrete volume will lead to shrinkage. According to the experimental determination of the ultimate shr

21、inkage of concrete is about 0104% 0106%.Shows that shrinkage is the inherent physical properties of concrete, in general, the larger water-cement ratio, the higher the concrete strength, aggregate less, the higher the temperature, surface water loss is larger, the larger the value of its contract, t

22、he more easily shrinkage cracks. According to the formation of shrinkage cracks and formation mechanism of the time, works in the common shrinkage cracks are mainly plastic shrinkage cracks, settlement shrinkage cracking and drying shrinkage cracks in three categories, in addition to their contract

23、（chemical shrinkage） cracks and carbonation shrinkage cracks.3.1.1Plastic shrinkage cracks Plastic shrinkage cracks in concrete plastic stage, before the final set. The cause of this is concrete paste and quickly evaporating water flow to the surface, with the increase in water loss, capillary negat

24、ive pressure generated by contraction of the concrete surface of the drastic volume shrinkage. Strength of concrete at a time has not yet formed, which resulted in cracking of the concrete surface.This multi-cracks in dry weather, hot and windy, the fracture shallow, intermediate width, both ends of

25、 the fine, of different lengths, and disconnected.3.1.2 Settlement shrinkage cracking Settlement shrinkage cracks in concrete pouring about half an hour after the occurrence and hardening stops. The cause of this is occurring after the slurry in the Pouring uneven sink, sinking of coarse aggregate,

26、cement grout float, when the settlement was inhibited （such as steel or embedded parts of the block） is due to shearing and cracking of the concrete. In addition, floating in the plasma layer formed on the surface will be a result of bleeding and cracking.This multi-cracks in the concrete surface, a

27、nd pass along the long direction of the reinforcement, or the stirrups the distribution width of both ends of the narrow middle, is a common early cracks, especially in the pump construction is more common.3.1.3 Drying shrinkage cracks Drying shrinkage cracks in the concrete curing only appeared aft

28、er completion. Its formation was mainly due to the concrete to harden, the water evaporation caused by shrinkage of the concrete surface, when the shrinkage deformation of concrete by internal constraint, have a greater tensile stress to crack the concrete surface is pulled.Shrinkage cracks on the s

29、urface generally produces very shallow location, multi-component along the short direction of distribution, were parallel, linear, or mesh, can be severe throughout the member section.3.1.4 Self-shrinkage cracks Shrinkage cracks itself has nothing to do with the outside humidity, but because of the

30、hydration reaction of cement clinker in the process, the reaction resultant of the average density of smaller volume shrinkage caused by system （called chemical shrinkage） due. Mainly due to hydration products of free water into a part of it39;s specific volume reduced by 1 / 4 （ie 0125cm3Pg）.Theref

31、ore, the chemical shrinkage of the size of the reduction depends on the chemical combination of cement hydration products in the amount of water. 3.1.5 Carbonation shrinkage cracking Carbonation carbonation shrinkage cracks are free ions generated by water evaporation, causing shrinkage in the slurr

32、y. Carbonation is atmospheric CO2 conditions in the water reacts with the hydration product of CaCO3, alumina, silica and water free state, this part of the volume shrinkage of concrete caused by water evaporation （known as carbonation shrinkage）, and its essence is the carbonate of the cement corrosion.General alkalinity of cement hydration products and the higher the concentration of CO2 in air and moderate humidity （50%）, the more prone to carbonation. Therefore, this crack propagation in alternating wet and dry environment, and dry or water saturated environment, there is not easy; and be