How to prevent temperature deformation of bulk concrete?

Bulk concrete.

The main reason for deformation is the heat of hydration, and the measure to prevent deformation and cracking is to leave construction joints.

Or take corresponding measures in the pouring process The deformation and cracking of the concrete temperature is mainly caused by the excessive temperature difference between the inside and outside of the concrete, and the temperature difference between the inside and outside of the concrete should be controlled below 25 degrees, and the heat of hydration will be generated after the concrete is poured, and the internal temperature is generally very high, you need to control it before pouring the concrete, such as adding ice water when stirring, reducing the internal temperature of the template when entering the mold, using cement with low hydration heat such as slag Portland cement, and controlling the pouring speed when pouring, etc. Then the maintenance should be kept warm and moisturized for oxidation, and the temperature measuring hole can be set up during construction to grasp the changes in the internal and external temperature difference in time, and steam curing can be used in winter. The curing of large-volume concrete should not only meet the needs of strength growth, but also prevent the cracking of concrete caused by temperature deformation through artificial temperature control.

Temperature control is to artificially control the pouring temperature of concrete and the maximum temperature inside the concrete. curing in concrete.

The temperature control of the stage should follow the following points: the difference between the core temperature and the surface temperature of the concrete, and between the surface temperature of the concrete and the outdoor minimum air temperature should be less than 20; When the structural concrete has sufficient crack resistance, it is not greater than 25 30. The internal cooling method is used to reduce the temperature difference between the inside and outside of the concrete.

The internal cooling method is to pre-bury water pipes inside the concrete and pass in cooling water.

Lowers the maximum temperature inside the concrete. Cooling starts when the concrete is just poured, and there is also the common rubble method, which can effectively control the cracking of concrete caused by the temperature difference between the inside and outside of the concrete. The insulation method is to cover the exposed concrete surface of the structure and the outside of the formwork with insulation materials (such as straw bags, sawn wood, green sand, etc.), so that the concrete can obtain the necessary strength in the slow heat dissipation process, so as to control the temperature difference between the inside and outside of the concrete is less than 20.

The surface layer of the concrete is laid with anti-crack reinforcement mesh to prevent dry cracking when the concrete shrinks. Large-scale concrete with a minimum geometric size of not less than 1m for concrete structures, or it is expected to be affected by the cementitious material in the concrete.

Concrete produced by temperature changes and shrinkage caused by hydration that cause harmful cracks is called bulk concrete. Modern buildings often involve large-volume concrete construction, such as high-rise building foundations, large-scale equipment foundations, water conservancy dams, etc. Its main feature is its large size, and the minimum size of the general entity is greater than or equal to 1m

Its surface coefficient is relatively small, the heat release of cement hydration is relatively concentrated, and the internal heating is relatively fast.

When the temperature difference between the inside and outside of the concrete is large, the concrete will produce temperature cracks, which will affect the structural safety and normal use. Therefore, it must be fundamentally analyzed to ensure the quality of construction. The American Concrete Institute (ACI) states:

The size of any large volume of concrete, which is poured in place, must require the resolution of the heat of hydration and the consequent volumetric deformation to minimize cracking." Large-volume concrete is generally common in hydraulic buildings, similar to concrete gravity dams. Large-volume concrete applications: hydraulic buildings; Considering the characteristics of water heating in construction engineering, the structure is thick, and the amount of concrete is large, and the project is complex, and the material characteristics are thick, the amount of concrete is large, the engineering conditions are complex (generally the underground cast-in-place reinforced concrete structure), the construction technology requirements are high, and the cement hydration heat is large (expected to exceed 25 degrees), which is easy to deform the temperature of the structure.

In addition to the minimum section and internal and external temperatures, there are also certain restrictions on the plane size of large-volume concrete. Because the plane size is too large, the temperature force generated by the restraint action is also greater, and if the temperature control measures are not taken properly, the temperature stress exceeds the tensile limit value that the concrete can bear, it is easy to produce cracks.

Causes of cracksThe cracks in large-volume concrete can be divided into three types: through cracks, deep cracks and surface cracks according to different depths. Penetrating cracks are the development of cracks on the surface of concrete into deep cracks, and eventually through cracks. 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; The deep cracks partially cut off the structural section, which is also harmful. Surface cracks are generally less harmful.

The temperature cracks generated during the construction stage of large-volume concrete are, on the one hand, the internal factors of the concrete: due to the temperature difference between the inside and outside; On the other hand, it is the external factor of concrete: the external constraints of the structure and the constraints between the various particles of the concrete prevent the shrinkage and deformation of the concrete, the compressive strength of the concrete is large, but relatively speaking, the tensile strength of the concrete is very small, so once the temperature stress exceeds the tensile strength that the concrete can bear, cracks will appear.

The width of such cracks is within the allowable limit, which generally does not affect the strength of the structure, but has an impact on the durability of the structure, so it must be paid attention to and controlled.

After the concrete is poured, it begins to heat up about four hours later.

Thermal insulation and curing effect:

1. Reduce the thermal diffusion of the concrete surface, reduce the temperature gradient of the concrete surface, and prevent surface cracks.

2. Extend the heat dissipation time and give full play to the potential of concrete and the relaxation characteristics of the material. The tensile stress generated by the average total temperature difference of the concrete is less than the tensile strength of the concrete, so as to prevent the formation of penetrating cracks.

The role of moisturizing and conditioning:

1. The concrete that has just been poured is still in the stage of solidification and hardening, and the speed of hydration is relatively fast, and the suitable humid conditions can prevent the dehydration of the concrete surface and produce dry shrinkage cracks.

2. Under wet conditions, concrete can make the hydration of cement go smoothly and improve the ultimate tensile strength of concrete.

The curing of waterproof concrete is crucial. After pouring, if the concrete is not cured in time, the moisture in the concrete will evaporate rapidly, so that the cement hydration is incomplete. The evaporation of water causes the capillary networks to communicate with each other, forming a seepage channel. At the same time, the shrinkage of the concrete increases, cracks appear, and the impermeability of the concrete decreases sharply, or even loses the impermeability completely.

If the curing is timely, the waterproof concrete hardens in a humid environment or in water, which can make the free water in the concrete evaporate slowly, the cement hydration is sufficient, and the cement hydration products block the capillary pores, thus forming inconnected capillaries, and improving the impermeability of the concrete.

According to Article 601 of the Code for the Construction of Bulk Concrete:

The test of surface temperature difference, cooling rate, ambient temperature and temperature strain in large-volume concrete pouring body should not be less than 4 times per day and night after concrete pouring; The measurement of the mold temperature should be no less than 2 times per shift.

Generally, the interval between the early heating stages of large-volume concrete construction is once every 2 hours, and the interval time can be determined according to the actual situation after the peak to the stage of complete removal of the insulation overlay, but it needs to meet the standard requirements.

It should be uncontroversial that the temperature measurement of bulk concrete should start after the concrete has been poured. As for when it will end, although it is not explicitly stated in the specification, there is an indicator that can be referred to, that is, when the maximum temperature difference between the surface temperature of the concrete and the environment is less than 20 °C, the cover can be removed.

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 (mixing, molding, curing). 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 temperature change is usually a rise before a change, and the specific time is related to the size of the structure. Usually the concrete hydration process basically reaches the highest level, about 3-5 days.

The change in temperature generally rises to a certain temperature and then decreases.

The temperature change is usually first rising and then rising again, and the specific time is related to the size of the structure and the external environment. Usually the concrete hydration process basically reaches the highest level, about 15 days.

Temperature control measures for bulk concrete (materials):

Choose medium and low heat cement varieties to make full use of the later strength of commercial concrete.

Add admixtures.

Coarse and fine aggregate selection.

Control the machine temperature and pouring temperature of commercial concrete.

Temperature control measures for large-volume concrete works (construction):

1. The temperature rise value of the concrete pouring body on the basis of the mold temperature should not be greater than 50.

2. The temperature difference between the surface of the concrete pouring block (excluding the equivalent temperature of concrete shrinkage) should not be greater than 25.

3. The cooling rate of the concrete pouring body should not be greater than.

4. The temperature difference between the surface of the concrete pouring body and the general atmosphere should not be greater than 20.

5. Before the construction of large-volume concrete, various pre-construction preparations should be made, and contact the local meteorological stations and stations to grasp the recent meteorological conditions. If necessary, the corresponding technical measures should be added, and in the winter construction, it should also meet the current national standards for concrete winter construction.

According to Article 1 of the "Construction Standard for Bulk Concrete" (GB 50496-2018).

The temperature control index of concrete construction with large nuclear failure volume shall comply with the following provisions:

1. The temperature rise value of the concrete pouring body on the basis of the mold temperature should not be changed, and it should be greater than 50 °C in Gaoqing;

2. The temperature difference between the surface of the concrete pouring body (excluding the shrinkage equivalent temperature of the concrete) should not be greater than 25°C;

3. The cooling rate of the concrete pouring body should not be large 2 0°c/d;

4. When removing the insulation cover, the air temperature difference between the surface of the concrete pouring body and the atmosphere should not be greater than 20°C.

Control measures to prevent the generation of cracks in bulk concrete:

1. Priority is given to the use of slag cement with low heat of hydration to mix concrete, and the appropriate use of retarding water reducer.

2. On the premise of ensuring the design strength level of concrete, appropriately reduce the water-cement ratio and reduce the amount of cement.

3. Reduce the mold temperature of concrete, and control the temperature difference between the inside and outside of concrete (when the design is not required, it should be controlled within 25 degrees).

4. Cover the concrete with thermal insulation and moisturizing materials in time.

5. The cooling water pipe can be embedded in the foundation to pass into the circulating water to force the temperature generated by the heat of hydration of the concrete.

Cement produces a lot of heat in the hydration process, which is the main heat inside the bulk concrete. Due to the large cross-sectional thickness of large-volume concrete, the heat of hydration is not easy to dissipate inside the structure, so that the temperature inside the concrete increases.

The maximum temperature inside the concrete, most of which occurs in 3 5d after pouring, when the temperature difference between the inside of the concrete and the surface is too large, temperature stress and temperature deformation will occur. The temperature stress is proportional to the temperature difference, and the larger the temperature difference, the greater the temperature stress. When the tensile strength of concrete is not enough to resist this temperature stress, temperature cracks begin to occur.

This is the main reason why large-volume concrete is prone to cracks.