Why does concrete crack after pouring, and what is the reason why cast in place concrete cracks soon

There are many reasons for concrete cracks, the main ones are as follows:

1) In the process of hardening concrete, the crack that occurs when the volume deformation caused by dry shrinkage is constrained, the width of this crack is sometimes very large, and even runs through the whole structure.

2) A large amount of hydration heat generated during the hydration of large-volume concrete cannot be dissipated, resulting in a large temperature difference between the inside and outside of the concrete, which makes the deformation of the concrete exceed the limit and cause cracks.

3) Cracks caused by the plastic collapse of concrete in thicker components.

4) When there is constraint, the volume expansion and contraction of concrete thermal expansion and contraction, because of the limitation of binding force, temperature stress is generated inside, and due to the low tensile strength of concrete, it is easy to be torn and cracked by tensile stress caused by temperature, resulting in temperature cracks. Cracks due to sun exposure are also the most common phenomenon in engineering.

5) After the concrete is mixed with water, the alkaline substance in the cement reacts with the activated silicon oxide in the active aggregate, and the precipitated colloidal alkali-silica gel absorbs water from the surrounding medium and expands, and the volume increases by three times, so that the concrete cracks and cracks.

6) In hot and windy weather, the evaporation of the concrete surface is too fast, resulting in too high hydration heat in the concrete, and it is still in a plastic state after pouring concrete for several hours, which is easy to produce plastic shrinkage cracks.

7) Cracks caused by component overload, for example: the component produces internal force bending moment under the action of exceeding the designed uniform load or concentrated load, and cracks perpendicular to the longitudinal axis of the component appear, and the component produces oblique cracks under the action of larger shear force, and extends upward and downward.

8) When the foundation of the structure is unevenly subsided, cracks may occur, and with the further development of subsidence, the cracks will further expand.

9) When the reinforced concrete is in an unfavorable environment, such as: erosive water, due to the limited thickness of the concrete protective layer, especially when the concrete is not compacted, the chloride ions in the environment and the oxygen dissolved in the water will make the steel bars in the concrete rust and generate iron oxide, the volume of iron oxide is much larger than the volume of the original metal, and the volume of rust expands, squeezing the surrounding concrete, making the concrete expand and crack.

There is more sand and less rocks, too much water is used, and the shaking time is not enough.

Analysis of the causes of cracks in concrete:

Plastic cracks mostly appear on the upper surface of the newly poured concrete component exposed to air, and plastic shrinkage refers to the shrinkage of the surface of the concrete due to the rapid loss of water before it sets.

Plastic shrinkage cracks generally appear in hot or windy weather, and the cracks are mostly wide in the middle, thin at both ends and different in length, and incoherent with each other, the shorter cracks are generally 20 30 cm long, and the longer cracks can reach 2 3 m and 1 5 mm wide.

The main reasons for the occurrence of plastic cracks are: the concrete has almost no strength or very little strength before the final setting, or the concrete has just been finalized and the strength is very small, and the water loss on the surface of the concrete is too fast under the influence of high temperature or large wind, resulting in a large negative pressure in the capillary tube and the volume of the concrete shrinks sharply.

Concrete Action:

In concrete, sand and stone play the role of skeleton and are called aggregates; Cement forms a cement slurry with water, which wraps around the surface of the aggregate and fills its voids. Before hardening, the cement slurry plays a lubricating role, giving the mixture a certain workability and facilitating construction. After the cement slurry hardens, the aggregate is cemented into a solid whole.

The technical properties of concrete are largely determined by the properties of the raw material and its relative content. At the same time, it is also related to the construction process. Therefore, it is necessary to understand the nature, function and quality requirements of its raw materials, and reasonably select raw materials, so as to ensure the quality of concrete.

The water heat of the cement is too high; The gradation of concrete sand and gravel is not very good. It depends on how deep and large the cracking lines are. It can be watered and cured at the end of the setting.

Whether there is a foundation settlement or not, fundamentally speaking, the tensile force must be greater than that of concrete. Can you see if the structure of the place where the concrete is poured is unreasonable, and the concrete is stretched?

The moisture content is too high.

The vibration is not compact.

The template is deformed.

1. The temperature changes greatly, which produces tensile stress inside the concrete and temperature expansion and contraction cracks. 2. Uneven humidity causes the surface to dry and shrink, and cracks are opened. 3. The alkali aggregate mixed with admixtures in concrete reacts, resulting in internal stress expansion and cracking.

4. Cracks caused by plastic shrinkage of concrete surface. 5. In the pouring process, insufficient or excessive vibration will cause the concrete to produce segregation and water leakage, and form a cement slurry layer with more cement content on the surface, and the water will evaporate, and it is easy to form shrinkage cracks. 6. The concrete mix ratio is poor, the glue-aggregate is too large and the sand neutralization rate is too large, and the concrete itself lacks the skeleton effect of temperature resistance and dry shrinkage deformation, which is easy to crack.

7. Improper maintenance is the main reason for cast-in-place concrete cracks, premature curing will affect the cementing ability of concrete, too late curing, the evaporation of free water on the surface of the concrete is too fast, and the cement lacks the necessary hydration water, resulting in sharp volume shrinkage and cracks. 8. In the construction, the concrete does not reach the specified strength and is dismantled prematurely; The concrete is loaded before the final setting time, the protection of the number of steel bars is not paid attention to during construction, the steel bar is displaced, and the post-pouring belt is not constructed according to the requirements of the design specification.

There are five main reasons:

1. The ratio of cement lime is incorrect. In the process of construction, some staff may add water privately for the convenience of construction, thereby improving the fluidity of concrete, but increasing the proportion of water and cement, resulting in irregular cracks when hardening in the later stage.

2. It's too long. As a result, the water evaporates relatively quickly, and when it comes to on-site construction, the strength of the concrete does not meet the requirements, and cracks will occur.

3. The construction speed is relatively fast. If the pouring is more frequent, the fluidity will be relatively low, resulting in insufficient concrete settlement, and cracks are most likely to occur in the later pouring process, especially at the joints.

4. Unreasonable maintenance. In the later stage, there is no watering, and regular wetting is carried out, so that the surface dries relatively quickly, and once the cement loses water, it will shrink sharply. If the strength is reduced, cracking will occur in the later stage.

5. It is also related to the environment. For example, if the air is relatively dry, the temperature is relatively high, and the wind blows, the evaporation rate of the surface of the water concrete will be accelerated, and cracking will occur in the later stage. Especially in summer or winter, temperature difference cracks are most likely to occur.

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1. Structural design issues. Cracking due to uneven settlement of the foundation; Structural design defects, local uneven force occurs. 2. After the concrete is poured, the curing is not timely, which causes surface cracking.

This problem is common, but generally does not affect security. 3. When the mold is removed too early, it will cause cracking due to premature stress on the incompletely solidified concrete. 4. There is a problem with the concrete mix ratio, and the strength cannot reach the design strength, which causes the safety problem of stress.

5. The amount of cement is too much (it is also a mix ratio problem), when the concrete begins to solidify, the surface cracking is caused by excessive hydration heat and difficult to control the curing. 6. When the concrete floor slab is over-dried and calendered, the cement slurry layer with high water content will appear on the surface of the floor slab, and it will react chemically with the air, resulting in cracks in the concrete floor slab.

1. The proportion of cement lime is incorrect, and some staff may add water privately in the process of construction, for the convenience of construction, so as to improve the fluidity of concrete, but the proportion of water and cement is increased, resulting in irregular cracks when hardening in the later stage.

Second, the time is too long, resulting in the evaporation of water is relatively fast, and when it comes to on-site construction, the strength of concrete does not meet the requirements, and cracks will occur.

Third, the construction speed is relatively fast, the pouring is more frequent, and the fluidity will be relatively low, resulting in insufficient concrete settlement, and in the process of pouring in the later stage, especially at the joints, cracks are most likely to appear.

Fourth, the maintenance is unreasonable, there is no glue for regular wetting in the later stage, so that the surface is dry, relatively fast, and once the cement is wet, it will shrink sharply, reducing the problem of cracking in the later stage.

5. It is also related to the environment, such as the air is relatively dry, the temperature is relatively high, and the wind blows, at this time, the evaporation rate of the surface of the water concrete will be accelerated, and it is easy to crack in the later stage, especially in the summer or winter construction, it is easy to appear temperature difference cracks.

There are many reasons for this, such as not meeting the corresponding quality requirements with the label, etc.

The main causes of concrete cracking are as follows:

1. Shrinkage of concrete. Shrinkage is one of the main characteristics of concrete, which has a great impact on the performance of concrete. Microscopic cracks caused by shrinkage may cause cracking, deformation and even destruction of the mechanism once they develop.

2. Temperature stress. The cement in the concrete emits a lot of heat in the hydration reaction, which heats up the concrete, and forms a certain temperature difference with the external air temperature, resulting in temperature stress in the co-base. Its size is related to the temperature difference, and directly affects the cracking and crack width of the concrete.

3. Insufficient reinforcement. The concrete structure with large reinforcement spacing and small reinforcement ratio has more cracks, and the unreinforced concrete has more cracks than the reinforced concrete. The position of the steel bar should be correct, and the protective layer that is too large or too small may lead to concrete cracking.

4. Concrete material and mix ratio. Improper mix design directly affects the tensile strength of concrete, which is the cause of concrete cracking that cannot be ignored. Improper mix ratio refers to excessive cement dosage, large water-cement ratio, inappropriate sand content, etc., these factors are interrelated.

5. Maintenance conditions. Curing is an important means to harden concrete normally. Curing conditions have a key impact on the appearance of cracks. Under standard curing conditions, the concrete hardens normally and will not crack, but it is only suitable for the production of test blocks or prefabricated parts in the factory.

The most common problems with cracks in concrete engineering are shrinkage cracks caused by constrained shrinkage deformation and load (force) cracks caused by external loads. It is difficult to avoid the appearance of cracks in pure concrete, but they can be prevented and treated. After the crack appears, the crack is a problem that needs to be solved.

Extension: concrete, referred to as "concrete (tóng)": refers to the general term of engineering composite materials that are cemented into a whole by cementitious materials.

The word concrete usually refers to the use of cement as cementitious material, sand and stone as aggregate; Cement concrete, also known as ordinary concrete, is widely used in civil engineering engineering when it is mixed with water (which can contain admixtures and admixtures) in a certain proportion and is mixed to obtain cement concrete.

Concrete can also be divided into low-strength concrete (compressive strength less than 30MPa), medium-strength concrete (compressive strength 30-60MPa) and high-strength concrete (compressive strength greater than or equal to 60MPa); According to the amount of cement per cubic meter, it can be divided into: poor concrete (cement consumption does not exceed 170kg) and rich concrete (cement consumption is not less than 230kg).

The calculation of the water-cement ratio of high-strength concrete can not use the formula of the strength of ordinary concrete, and the relationship between the strength of concrete and the water-cement ratio should be proposed according to the test data, and then used as a diagram or calculation method to find the water-cement ratio corresponding to the strength of the concrete preparation. When a number of different mixtures are used for the strength test of the registered concrete, one of them should be the benchmark mixture, and the water-cement ratio of the other mixtures should be increased and decreased respectively compared with the benchmark mixture.