The concept and hazards of deep cracks in bulk concrete

Cracks in concrete. If the crack width of ordinary reinforced concrete exceeds the crack (the main stressed structure is allowed not to be exceeded, and the secondary structure is allowed not to be exceeded, because under this limit, even if there is a crack, it will not go to the deep layer. If this limit is exceeded, it is a deep crack.

The harm of deep cracks is very great, originally an integral structure, in the design of the overall joint force, now because of cracks, the original design stress state has changed, the internal stress of the structure has been redistributed. Originally, the secondary components may become the main components, and the real main components do not play much role, so it is very dangerous to stress the structure and should be paid attention to.

The reasons for the cracks in bulk concrete are: 1) The cement generates a lot of heat during the hydration process. Due to the large cross-sectional thickness of the bulk concrete, the heat of hydration is not easy to dissipate inside the structure, which increases the temperature inside the concrete.

The maximum temperature inside the concrete mostly occurs 3 5d after pouring. Temperature is generated when the temperature difference between the inside of the concrete and the surface is too large.

Stress. When the tensile strength of concrete is not strong enough to resist this temperature stress, temperature cracks occur, which are the main cause of cracks in bulk concrete. 2) When the structure is deformed, it will be inhibited to a certain extent, which will hinder the deformation.

Large-volume concrete and foundation poured together must be constrained by the lower foundation, and the concrete is prone to cracks. 3) The change of outside temperature during construction also has an important impact on the cracks of large-volume concrete. The higher the outside temperature, the higher the pouring temperature of the concrete.

The drop in external temperature, especially the sudden drop, greatly increases the temperature gradient between the outer concrete and the inner concrete, resulting in temperature stress and cracks in the large volume of concrete.

The floor concrete has almost been poured, and some concrete has been cracked, what is going on?

It is mainly caused by the excessive temperature difference between the center of the concrete and the surface. Secondly, concrete shrinkage is also one of the reasons.

Change the answer]: b

This question is purely a test of the quality inspection and acceptance of large-volume concrete pouring construction. Cracks in bulk concrete are divided into three types: surface cracks, deep cracks and penetrating cracks according to different depths.

Answer]: B2022 version of the textbook p277

The test point of this question is the classification of cracks in large-volume concrete structures.

Cracks in bulk concrete are divided into three types: surface cracks, deep cracks and penetrating cracks according to different depths.

1) Surface cracks are mainly temperature cracks, which are generally less harmful but affect the appearance quality.

2) Deep cracks partially cut off the structural section, which has a certain harm to the durability of the structure.

3) Penetrating cracks are developed from cracks on the surface of concrete to deep cracks, and finally through cracks are formed. It cuts off the cross-section of the structure, which may destroy the integrity and stability of the structure, and its harm is more serious.

Answer]: B textbook p362

Knowledge points] quality inspection and acceptance of large-volume concrete pouring construction. The cracks in the first-grade soil of large-volume concrete can be divided into three types: surface cracks, deep cracks and through cracks according to the depth.

Answer]: The cracks in the large-volume concrete are divided into three types: through cracks, deep cracks and medium-sized surface cracks according to the different depths of socks and pants.

Hazards of cracks in bulk concrete:

The stiffness of the building is reduced, and penetrating cracks appear to affect the normal performance of the structural function; The durability of concrete is reduced, and the appearance of cracks makes the corrosive medium enter the concrete, resulting in concrete carbonization, steel corrosion, and concrete strength, which affects the durability of Qingshou; Loss of basement moisture and water resistance. When there is a leakage, it is difficult to deal with and the cost is huge, which reduces the use function of the structure.

Control methods for cracks in bulk concrete:

The main reason for the cracks of large-volume concrete is that the temperature difference between the inside and outside is too large, and the selection of raw materials can be controlled. Cement: Ordinary cement has a higher hydration heat, and the slag Portland cement with a low hydration heat is selected for the combustion number of large-volume concrete, which has a slow hydration rate, low heat generation, and a small temperature difference between inside and outside. In meeting the design requirements, controlling the amount of cement can also reduce the generation of heat of hydration, and the general amount of cement is controlled.