The Generation And Treatment Of Concrete Cracks

Because of its wide range of materials, low price, high compressive strength, concrete can be poured into various shapes, good fire resistance, not easy to weather, and low maintenance cost, it has become the most widely used building material in the building structure in the world today.

The main disadvantages of concrete are poor tensile strength, brittleness, and easy cracking. A lot of engineering practice and theoretical analysis show that almost all concrete components work with cracks, but some cracks are very fine and even invisible to the naked eye (<0.05mm). Generally, there is no major harm to the use of the structure. Exist; China's current design codes for construction, railway, highway, water conservancy and other departments all adopt the method of limiting the width of component cracks to ensure the normal use of concrete structures.

Some cracks are constantly generated and expanded under the action of load or external physical and chemical factors, causing concrete carbonization, protective layer peeling, steel reinforcement corrosion, weakening the strength and rigidity of concrete, reducing durability, and even collapse accidents in severe cases , The normal use of hazardous structures must be controlled.

Concrete cracking can be said to be "common disease" and "frequent disease", which often troubles engineers and technicians. In fact, if certain design and construction measures are taken, many cracks can be overcome and controlled

In fact, the causes of concrete cracks are complex and numerous, and even multiple factors affect each other, but each crack has one or several main reasons for its occurrence. This report makes a more comprehensive analysis of the types and causes of concrete cracks and proposes corresponding prevention measures for reference and discussion by colleagues and experts.

The types of concrete cracks can be roughly classified as follows:

Cracks caused by load

The cracks produced by concrete members under conventional static and dynamic loads and secondary stresses are called load cracks. In summary, there are mainly direct stress cracks and secondary stress cracks.

Direct stress cracks refer to cracks caused by direct stress caused by external loads. The causes of cracks are:

1. In the design and calculation stage, the calculation of the structure is not calculated or partially omitted; the calculation model is unreasonable; the structural force assumption does not match the actual force; the load is calculated or missed; the internal force and the reinforcement are calculated incorrectly; the structural safety factor is insufficient. The structure design does not consider the possibility of construction; the design section is insufficient (Ningbo Sea-crossing Bridge); the steel bar is set less or the layout is wrong; the structural rigidity is insufficient; the structure is not handled properly; the design drawings are unclear.

2. During the construction phase, stack construction equipment and materials without restrictions; do not understand the stress characteristics of the prefabricated structure, turn over, lift, transport, and install at will; do not construct according to the design drawings, change the construction sequence of the structure without authorization, and change the stress mode of the structure ; Do not check the fatigue strength of the structure under machine vibration, etc.

3. In the use phase, the design load is exceeded on the floor and walls of the building; the industrial plant is overloaded; strong wind, snow, earthquake, explosion, etc. occur.

Secondary stress cracks refer to cracks caused by secondary stress caused by external loads.

The causes of cracks are:

1. Under the design external load, because the actual working state of the structure is different from the conventional calculation or the calculation is not taken into consideration, it causes secondary stress in some parts and causes the structure to crack.

For example: the design of the arch foot of a two-hinged arch bridge often adopts the method of arranging "X"-shaped steel bars and reducing the section size at the same time to design the hinge. According to the theoretical calculation, there will be no bending moment at this point, but the hinge is still able to resist bending and even cracks. This leads to corrosion of steel bars.

2. In industrial buildings, grooving, opening, and setting of corbels are often required. It is difficult to simulate calculations with accurate diagrams in conventional calculations. Generally, the force-bearing steel bars are set according to experience. Research has shown that after the force-bearing member is digging a hole, the force flow will produce diffraction, which is dense near the hole, resulting in huge stress concentration.

In long-span prestressed continuous beams, the steel strands are often cut off according to the internal force of the section within the span, and anchor heads are set, and cracks are often seen near the anchored section. Therefore, if it is not handled properly, cracks are likely to appear at the corners of these structures, the sudden changes in the shape of the members, and the cutoffs of the stressed steel bars.

In actual engineering, secondary stress cracks are the most common cause of load cracks. Secondary stress cracks are mostly tensile, split and shear properties.

Secondary stress cracks are also caused by loads, and are not calculated according to conventional rules. However, with the continuous improvement of modern calculation methods, secondary stress cracks can also be reasonably checked. For example, many plane finite element programs can correctly calculate the secondary stress caused by prestress and creep, but it was more difficult 40 years ago.

In the design, attention should be paid to avoid structural mutation (or cross-sectional mutation). When it cannot be avoided, local treatment should be done, such as round corners and gradual transition at the abrupt changes. At the same time, strengthen the structural reinforcement and increase the slope at the corner. When conditions permit for larger holes, edge protection angles can be set around the reinforcement.

The characteristics of load cracks vary with different loads and present different characteristics. This kind of cracks mostly appear in tension zone, shear zone or severe vibration part. However, it must be pointed out that if there are skinning or short cracks along the compression direction in the compression zone, it is often a sign that the structure has reached the bearing capacity limit and a precursor to structural failure. The reason is often that the section size is too small.

According to the different stress modes of the structure, the crack characteristics are as follows:

² Central tension: The crack penetrates the cross section of the component, the spacing is roughly equal, and it is perpendicular to the direction of force.When using threaded steel bars, secondary cracks located near the steel bars appear between the cracks.

² Central compression: short and dense parallel cracks parallel to the force direction appear along the component.

² Bending: cracks perpendicular to the direction of tension appear from the edge of the tension zone near the maximum section of the bending moment, and gradually develop towards the neutral axis. When using rebar, shorter secondary cracks can be seen between the cracks. When the structure has less reinforcement, the cracks are few and wide, and the structure may undergo brittle failure.

² Large eccentric compression: Large eccentric compression and small eccentric compression members with less reinforcement in the tension zone are similar to bending members.

² Small eccentric compression: small eccentric compression and large eccentric compression components with more reinforcement in the tension zone are similar to central compression components.

² Shearing: when the stirrups are too dense, oblique compression failure will occur, and diagonal cracks greater than 45° will appear along the abdomen of the beam end; when the stirrups will undergo shear compression failure, there will be approximately 45° parallel to each other along the middle and lower part of the beam end. Diagonal cracks.

² Torsion: There are multiple oblique cracks at 45° in the abdomen on one side of the component, and they expand in a spiral direction to the adjacent surface.

² Punching: A slant fracture of about 45° occurs along the inner four sides of the stigma to form a punching surface.

² Local compression: multiple short cracks roughly parallel to the pressure direction appear in the local compression zone.

Cracks caused by temperature changes

Concrete has the properties of thermal expansion and contraction. When the external environment or the internal temperature of the structure changes, the concrete will deform. If the deformation is restricted, stress will be generated in the structure. When the stress exceeds the tensile strength of the concrete, temperature cracks will occur. In some long-span beams, the temperature stress can reach or exceed the live load stress. The main feature of temperature cracks that distinguishes other cracks is that they will expand or close together with temperature changes. The main factors that cause temperature changes are:

² Heat of Hydration

It appears in the construction process that after the mass concrete is poured, the internal temperature is very high due to the exothermic heat of cement hydration, and the temperature difference between the inside and the outside is too large, causing cracks on the surface.

According to the actual situation, the cement type (slag cement) with low heat of hydration should be selected as far as possible during the construction, the unit dosage of cement should be limited (using water reducing agent), the temperature of aggregate entering the mold should be reduced (ice water mixing), and the temperature difference between inside and outside should be reduced (through the surface Insulation), and slowly cool down. If necessary, a circulating cooling system (pre-buried) can be used for internal heat dissipation, or a thin layer of continuous pouring can be used to accelerate heat dissipation.

² Improper construction measures during steam curing or winter construction, the concrete is suddenly cold and hot, the temperature inside and outside is uneven, and cracks are easy to appear.

² Annual temperature difference

The temperature in the four seasons of the year changes constantly, but the change is relatively slow. The annual temperature difference in my country is generally based on the monthly average temperature in January and July. Considering the creep characteristics of concrete, the elastic modulus of concrete should be reduced when calculating the internal force of the annual temperature difference.

² Rizhao

After the roof and wall are exposed to the sun, the temperature is significantly higher than other parts, and the temperature gradient is non-linear. Due to its own restraint, the local tensile stress is relatively large and cracks appear. Sunshine and the sudden drop in temperature described below are the most common causes of structural temperature cracks.

² Sudden cooling:

Sudden heavy rain, cold air intrusion, sunset, etc. can cause a sudden drop in the temperature of the outer surface of the structure, but the temperature gradient is generated due to the relatively slow change of the internal temperature. The internal force of sunshine and sudden temperature drop can be calculated according to design specifications or actual data. The elastic modulus of concrete is not considered to be reduced.

When the steel embedded parts are connected with the steel bars or other steel parts, if the welding measures are not proper, the concrete near the iron parts is easy to burn and crack. When the prestressed component is stretched by the electric heating method, the temperature of the prestressed steel can be increased to 350℃, and the concrete component is also prone to cracking.

Experimental studies have shown that the strength of concrete with high temperature burns caused by fires and other reasons decreases significantly with the increase of temperature, and the bonding force between steel bars and concrete decreases. After the temperature of concrete reaches 300℃, the tensile strength decreases by 50%, and the compressive strength Decrease by 60%, the bonding force between the smooth steel bar and the concrete is decreased by 80%; due to heat, a large amount of free water in the concrete body can also evaporate and cause rapid shrinkage.

Cracks caused by shrinkage

In actual engineering, cracks caused by shrinkage of concrete are the most common. Among the types of concrete shrinkage, plastic shrinkage and dry shrinkage are the main causes of concrete volume deformation, in addition to autogenous shrinkage and carbonization shrinkage.

² Plastic shrinkage

Occurs during the construction process and about 4 to 5 hours after the concrete is poured. At this time, the cement hydration reaction is fierce, the molecular chain is gradually formed, bleeding and water evaporate rapidly, the concrete loses water and shrinks, and the aggregate sinks due to its own weight. When the concrete has not hardened, it is called plastic shrinkage.

The magnitude of plastic shrinkage is very large, up to about 1%. If the aggregate is blocked by steel bars during the sinking process, cracks along the direction of the steel bars will be formed. At the vertical variable cross-section of the component, such as the junction of the T-beam, box girder web and the top and bottom plates, cracks along the web direction of the surface will occur due to uneven settlement before hardening.

In order to reduce the plastic shrinkage of the concrete, the water-cement ratio should be controlled during the construction, and the mixing for too long should be avoided. The cutting should not be too fast, the vibrating should be dense, and the vertical variable cross-section should be layered.

² Dry shrinkage

After the concrete is hardened, as the surface moisture gradually evaporates, the humidity gradually decreases, and the volume of the concrete decreases, which is called dry shrinkage (shrinkage shrinkage). Because the surface layer of concrete loses water quickly and the internal loss is slow, uneven shrinkage occurs with large surface shrinkage and small internal shrinkage. The surface shrinkage deformation is restricted by the internal concrete, causing the surface concrete to bear the tensile force. When the surface concrete bears the tensile force exceeding its tensile strength When, shrinkage cracks occur. The shrinkage of concrete after hardening is mainly dry shrinkage. Such as components with a large reinforcement ratio (more than 3%), the restraint of the steel bar on the shrinkage of the concrete is obvious, and the concrete surface is prone to cracks.

² Spontaneous contraction

Autogenous shrinkage is the hydration reaction between cement and water during the hardening process of concrete. This shrinkage has nothing to do with the external humidity, and can be positive (ie shrinkage, such as ordinary Portland cement concrete) or negative (ie expansion , Such as expanded cement concrete mixed with expansion agent).

4. Carbonization shrinkage

The shrinkage deformation caused by the chemical reaction between carbon dioxide in the atmosphere and the hydrate of cement. Carbonization shrinkage can only occur when the humidity is about 50%, and it accelerates as the concentration of carbon dioxide increases. Carbonization shrinkage is generally not calculated.

 The characteristics of concrete shrinkage cracks are that most of them are surface cracks, the width of the cracks is relatively small, and the cracks are criss-crossed and formed into cracks with no regular shape.

But if the shrinkage value is too large, it will also cause through cracks (broken boards) in the concrete. For example, if the expansion joints are not cut in time for large areas of cement concrete floors, the slabs will inevitably break.

Research shows that the main factors affecting concrete shrinkage cracks are:

① Cement type, label and dosage

Slag cement, quick-hardening cement, and low-heat cement concrete have higher shrinkage, while ordinary cement, pozzolan cement, and bauxite cement concrete have lower shrinkage. In addition, the lower the cement label, the larger the unit volume, and the greater the grinding fineness, the greater the shrinkage of the concrete and the longer the shrinkage time. For example, in order to increase the strength of concrete, the practice of forcibly increasing the amount of cement is often used during construction, resulting in a significant increase in shrinkage stress.

② Variety of aggregate

Among the aggregates, quartz, limestone, dolomite, granite, and feldspar have low water absorption and low shrinkage; while sandstone, slate, and amphibolite have high water absorption and high shrinkage. In addition, the larger the aggregate particle size, the smaller the shrinkage, and the larger the water content, the larger the shrinkage.

③ The greater the water-cement ratio, the higher the water-cement ratio and the greater the shrinkage of concrete.

④ Admixtures The better the water retention of admixtures, the smaller the shrinkage of concrete.

⑤ External admixture The higher the fineness of the external admixture, the greater the shrinkage of the concrete. The greater the amount of external admixtures, the greater the shrinkage of concrete. Generally, commercial (pumped) concrete contains a large amount of fly ash (15%~30%), and the concrete shrinks greatly. Therefore, the project using commercial (pumped) concrete is easier to crack.

⑥ Conservation method

Good curing can accelerate the hydration reaction of concrete and obtain higher concrete strength. During curing, the higher the humidity, the lower the temperature, and the longer the curing time, the smaller the concrete shrinkage. The steam curing method has less concrete shrinkage than the natural curing method.

⑦ External environment

If the humidity in the atmosphere is low, the air is dry, the temperature is high, and the wind speed is high, the moisture in the concrete will evaporate faster and the concrete will shrink faster.

⑧ Vibration method and time

The mechanical vibrating method has less concrete shrinkage than the manual tamping method. The vibrating time should be determined according to the mechanical properties, generally 5~15s/time is appropriate. If the time is too short, the vibrating will not be compacted, resulting in insufficient or uneven concrete strength; too long, causing delamination, coarse aggregate sinks into the bottom layer, fine aggregate stays in the upper layer, uneven strength, and shrinkage cracks in the upper layer.

For cracks caused by temperature and shrinkage, adding structural steel bars can significantly improve the crack resistance of concrete, especially thin-walled structures (wall thickness 20~60cm). In the structure, the reinforcement should be given priority to using small diameter steel bars (φ8~φ14) and small spacing layout (@10~@15cm). The reinforcement ratio of the full section structure should not be less than 0.3%, generally 0.3%~0.5% can be used.

² Cracks caused by deformation of ground foundation

Due to the uneven vertical settlement or horizontal displacement of the foundation, additional stress is generated in the structure, which exceeds the tensile capacity of the concrete structure, resulting in structural cracking. The main causes of uneven foundation settlement are:

1. Insufficient accuracy of geological survey and inaccurate test data

Design and construction without fully grasping the geological conditions is the main cause of uneven foundation settlement. For example, for bridges in hilly areas or mountainous areas, the drill holes are too far apart during the survey, and the foundation rock surface is undulating and large, and the survey report cannot fully reflect the actual geological conditions.

2. The foundation geology is too different

For buildings built in mountain valleys, the geology and slopes of the river ditch vary greatly. There is even a weak foundation in the river ditch, and the foundation soil has uneven settlement due to different compressibility.

3. The structural load difference is too large

Under the conditions of relatively consistent geological conditions, when the foundation loads of each part differ too much, uneven settlement may be caused. For example, the main building of a high-rise building has a heavier load than the surrounding podium, and the settlement in the middle part is larger than that in the surrounding area.

4. The type of structural basis varies greatly

In the same building group, mix different foundations such as strip foundation and pile foundation, or adopt pile foundation at the same time