Concrete Knowledge Summary-Who Had mastered this “concrete knowledge“, can be called an expert

1、Why should concrete divide the strength level?

Why the concrete should be divided into strength levels, first of all, you should know what the concrete strength level is. The level of concrete strength is divided according to the value of the concrete cube resistance strength value. It is the characteristic strength of the concrete. According to the current relevant standards and specifications, concrete cube resistance strength is a cube test piece with a standard size of 150㎜ based on the standard method. The temperature that is consistent with the ISO test method is 20℃, 20℃, and the humidity is more than 95%or more. Standard maintenance room, maintenance to 28D age, the concrete cubes measured according to the standard test method.

According to the relevant standards, the strength level of the building material should be expressed based on the name of the material plus its strength standard value. Therefore, the concrete strength level is divided into: C10, C15, C20, C30, C35, C45, C50, C50, C60 , C70, C75, C80.
Because the loads of concrete in different parts in general engineering are different, some are large, and some are small, which cannot completely use a intensity -level concrete. For parts with high pressure, high -strength level concrete must be used. For parts with low pressure, low -intensity levels must be used. Therefore, concrete is divided into different intensity grades to meet the needs of different projects. Essence

2、Why does concrete stipulate that the strength of 28D is the standard strength?

The concrete is based on the collagen of the cement, which gradually hardened, and improves the pressure resistance. Because the hard knot of the cement is not completed all of a sudden, it is gradually perfect over time. Under normal maintenance conditions, the pressure of compressive pressure in the first seven days increased rapidly, and the growth between 7D ~ 14D was slightly slower, while after 28D, the strength growth was relatively slow. In other words, after 28D, the pressure resistance strength is the standard strength, as a standard for design and construction inspection quality. Obviously, if the strength of less than 28D is used as the standard strength, the performance of the concrete will not be fully played. If the intensity greater than 28D is used as the standard strength, although the performance of the concrete can be fully played, it will affect the construction progress due to the long time when the standard strength is reached.

3、Why does concrete require water quality?

Industrial wastewater and sewage containing fat, vegetable oil, sugar, acid, etc. cannot be used for mixing and concrete. Because these water containing impurities will reduce the bonding force of the cement and reduce the strength of the concrete, it cannot be used in mineral water containing a large amount of salt, so that the cement cannot resist the erosion of the water. For the chemical composition of mineral water, it is necessary to meet the indicators stipulated by the family, or compare with ordinary drinking freshwater as a comparative test, which can only be used by depending on the intensity.
As for general tap water and water that can be used, it can be used to mix concrete. For details, see the standard GBJ63-89.

4、Why are concrete test blocks in one group?

The concrete test block is the standard for measuring the strength of the concrete component, that is, the intensity of compression and destruction of the test block, as the strength that can be used as a component. Therefore, in addition to the test block, the test block must be made in a certain amount. Because although we try to make the concrete of the test block the same as the concrete of the component, there is a certain difference after all. If it is based on a test block to identify the strength of the component, it may be reliable. Therefore, in three groups, the intensity of the intensity of three test blocks is used as the intensity of this group test block (in special circumstances to remove part of the value), that is, the strength of the component is determined by it.

5、What is the durability of concrete?

In addition to the proper strength, concrete should also be based on special requirements in use. It should include anti -freezing, water -resistant penetration, chlorine -resistant ion penetration, contraction, carbonization, steel rust, anti -sulfuric acid salt, compressive fatigue deformation, alkaline bone bone, alkali bone Material response, etc., collectively referred to as durability.

Anti -permeability: refers to the performance of concrete resistance to liquid and gas penetration. Because there are interconnected pores and capillary inside the concrete, and honeycomb and holes are generated due to vibration, the liquid and gas can penetrate the inside of the concrete. Make concrete deterioration, and the results will affect the quality and long -term safe use of concrete. The anti -penetration of the concrete is represented by the anti -seepage label P. For example, P4 indicates that under the corresponding 0.4N/㎜2 hydraulic effect, six cylindrical or cone test blocks used for anti -seepage tests still maintain 4 test blocks. The anti -seepage label of concrete is generally divided into P4, P4, P6, P8, P10, P12.

Anti -freezing: refers to the ability to resist freezing. The concrete is often frozen in cold areas, especially in the environment where water is exposed to water, and is frozen. This is because the water infiltrated in the concrete is frozen and frozen, and the volume expands 9%, which puts a lot of pressure on the pores and capillary in the concrete. been destroyed. The frost resistance of concrete is represented by frost anti -freezing. Compared with the intensity of unbourced test blocks, if the intensity of the freezing tested block is reduced by no more than 25%, it is considered to be qualified. The frozen bid is represented by the maximum repeated frozen loop that can be suffered by the test block. According to the number of frozen loops, concrete antifreeze marks are generally divided into: F15, F25, F50, F100, F150 and F200.

Anti -erosion: refers to the performance of concrete in various corrosive liquids and gas. The medium that erodes concrete is mainly sulfate solution, acidic water, activity, and soft water, seawater, and alkaline thick solution with water pressure.

Refers to heat resistance: refers to the performance of concrete under high temperature, internal structures, no significant loss of strength, and a performance with a certain chemical stability.

6、Why do concrete have natural maintenance and steam maintenance?

Natural maintenance means that under natural conditions (temperature is not lower than+5 ° C, and humidity 90 to 100%) is maintained. As mentioned earlier, the strength growth is slow at the natural maintenance temperature. The age of 7D can only get 28D (the strength of the concrete 28 days later) 30 ~ 70%. It takes a longer time to ensure the strength of the demolition and the strength of the factory. This will extend the cycle of the entire production process. At the same time, it is required to be equipped with a large number of template equipment and occupying a large number of production area to increase infrastructure investment. In order to accelerate the growth rate of concrete strength, steam can be used, and steam can be used to heat the concrete, so that the concrete can be quickly hardened at the conditions of higher temperature (70 ~ 90 ° C) and higher humidity (about 90%). However, in areas where the climate is relatively warm and warm, it is still suitable for natural maintenance. This can save fuel and a set of equipment investment and reduce costs.

7、What is the ease of concrete?

The concrete and ease of concrete refer to whether the component of the concrete mixture is uniform, and whether it is easy to irrigate and vibrate during the production operation.
Concrete harmony is a comprehensive indicator. It includes three contents: liquidity, adhesion and water retention of concrete.
The liquidity of concrete means that the concrete mixes are prone to flow, easy to transport, and easy to fill the concrete templates under the action of self -gravity or mechanical vibration.
The adhesion of concrete refers to the material between the materials that form the concrete mixture during the production process, which does not produce layers and out -of -analysis, and has a certain ability to adhere.
The reservation of concrete refers to the phenomenon of serious water secretion in the production process of concrete mixture during the production process, and has a certain ability to reserve water.

8、How to determine the ease of concrete?

At present, there is no way to comprehensively determine the concrete mixing material and easy method, usually to determine its liquidity, and then judge its adhesion and water retention with experience.
The most commonly used method for determining the liquidity of concrete is the slump method. During the measurement, the concrete mixing material is divided into three layers into the standard size of the cone tube of the standard size. Each layer is installed, and a pounding rod with a diameter of 16㎜ is perpendicular and uniformly. , Scrape the tube mouth mixing material, then lift the tube vertically, and place it aside. At this time, the mixture collapses due to self -weight. The greater the slump, the greater the liquidity of the concrete.
After the collapse test, the adhesion and water retention of concrete can be observed at the same time. If there is no too much moisture on the surface of the concrete, it means that water preservation is good. You can also gently tap the mixture from the side to use a pounding rod to stick to the concrete with good polymerization. It will not loosen and collapse under the knock.
The slump test is only applicable to plastic concrete and low plastic concrete; for dry hard concrete, the method of determining work is often used. It is necessary to use the Victoria consistency instrument to determine its work.
Operation points:
（1）The concrete mixes are loaded in three layers, and each layer is plugged in 25 times. After pounding, the slump tube was lifted vertically and smoothly.
（2）Turn the transparent disc to the concrete test and gently fall to contact with the concrete top surface.
（3）At the same time, turn on the vibration table and the stopwatch, write down the bottom surface of the transparent disc, which is covered with the time required by the cement slurry. See the classification of concrete compounds:
Classified slump dry hardness (s)

Dry hardness 060 ~ 120

Semi -dry hard 030 ～ 60

Low plasticity 10 ~ 5015 ～ 30

Plasticity 50 ～ 1505 ～ 15

Liquidity> 150-The choice of concrete slump or working degree should be determined according to the method of molding, cross-section size, and reinforcement density.

9、What is ordinary concrete? What are its characteristics?

Cement is used as a gel material, and the concrete made of sand and stones is used as fine, rough aggregate, and mixed with water and mixed with water, which is ordinary concrete. Throughout the concrete family, ordinary concrete uses the earliest and the most widely used. The concrete of many futures varieties is developed on its basis.
Ordinary concrete, referred to as concrete. Its main features are as follows:

1. The concrete has a high pressure resistance, generally 20 to 50MPa, which can withstand large loads.

2. Before the concrete is condensed, it has good plasticity, and can be made into a structure and component of various shapes and sizes as needed.

3. There is a good durability that can undergo changes in dry wet, hot and cold, and freezing in the air without damage.

4. In the case of drying, the thermal conductivity of the concrete is 1.3kcal/m · h · ℃. Although its value is large, it is only one -fixture of the steel, so it has a certain heat insulation performance.

5. The capacity of concrete is 2400㎏/m3. Compared with steel, this kind of concrete is used as a structural material. In order to withstand the same load, a larger section size needs to be selected, so the self -weight is large.

6. The pressure resistance of concrete is very low. The ratio of compressive strength and tensile strength is called the crispy factor of 10, so when the concrete is damaged, the characteristics of the sudden damage of crispy materials will occur.

10、Why use an external agent in concrete?

The increasingly commonly used external agent is an objective requirement for the development of construction engineering and technological development. In recent years, high -rise, large span and various new structural systems have appeared in the overall pouring concrete structure. Many new component types such as large and thin walls have appeared in prefabricated prefabricated concrete components. Various performance requirements such as seepage are used to improve the performance of concrete, improve the quality of the project, reduce the cost of engineering, and promote the requirements of new processes.
The performance of concrete is determined by the proportion of cement, sand, stones and water. In order to improve a certain performance of concrete, the proportion of raw materials can be adjusted. But this often causes another loss. For example, in order to increase the liquidity of concrete, water dosage can be increased, but this will reduce the strength of the concrete. In order to increase the early intensity of concrete, the amount of cement can be increased, but in addition to increasing costs, it may also increase the contraction and slow change of concrete. With external additives, the above -mentioned defects can be avoided. In the case of some other performance on the concrete, the use of concrete external agents can greatly improve a certain performance of concrete. For example, as long as 0.2%～ 0.3%of the calcium sulfonate hydramid reduction agent is mixed with 0.2%～ 0.3%of the concrete, the concrete slump can be increased by more than doubled without increasing the amount of water; ～ 4%of sodium sulfate and calcium (NC) complex agent can increase the early concrete strength of 60%to 70%without increasing the amount of cement, and it can also increase the later strength of the concrete.

11、Why is there a cooperative ratio?

The so -called “concrete coordination ratio” -the ratio between cement, sand, stones and water consumption.
Various properties of concrete, such as strength, durability, and various properties of concrete mixes are directly affected by its composition ratio; the cooperation ratio changes, and the various properties of the concrete are also changed; and the amount of cement dosage is How much will affect the cost of concrete. In order to meet the easeness and liquidity requirements required by certain operating conditions, and the various performances such as concrete strength and durability proposed in the project, as well as saving cement, reducing costs, and achieving economic reasonable purpose, we must choose to choose a reasonable purpose. A suitable concrete mix ratio.

12、The concrete cooperation ratio must be calculated at weight?

The accuracy of concrete composition materials (weighing) is an important condition for ensuring the quality of concrete engineering and saving raw materials. Generally, the weighing error of cement, water, mixture and other gel substances shall not exceed ± 2%in weight measurement. The measuring error error of aggregate must exceed ± 3%.

Due to the different humidity and density of materials such as sand, stone, cement, etc., the weight of the same volume of materials is often very large. Therefore, the concrete coordination ratio is controlled by weight calculation. Such a cooperation is more accurate than the volume method.

13、What is the cause of cracking after concrete hardening?

There are many causes of cracking after concrete hardening, which is generally caused by two or more reasons. Generally, the concrete structure is overloaded and the stress is generated. If the concrete tensile strength does not tagine the stress, it will cause cracks. The causes of concrete deformation include dry contraction, temperature change, chemical effects, and structural factors. At this time Various situations of cracked turtles.

14、What is the adaptability of concrete plus agents to cement?

1. Whether the cement ore stable causes whether the mineral component is stable, which affects the adaptability of the outer additive to the cement.
2. Cement production technology, such as Li Kiln and Capital Kiln, how the emergency cold measures in the cooling system are controlled, the temperature during the grinding of gypsum powder, etc., causing the mineral composition, crystal state, and the shape of the gypsum in the cement, which affects the shape of the gypsum form, which affects the affecting the shape of the gypsum. The adaptability of outer plus agent to cement.
3. Adsorbing external agent capabilities in cement: C3A> C4AF> C3S> C2S, cement hydration rate is directly related to mineral components.
4. After the cement is stored for a period of time, the temperature decreases, so that the high temperature adaptability of the outer additional agent is improved, and the F-CAO absorbs water in the air to CA (OH) 2, and the CO2 in the air will be transformed into CACO3, so that thereby The reduction of MWO has also improved the concrete and easyness, the loss of the collapse of the new concrete, and the condensation time of the concrete is slightly extended.
5. The amount of water demand for ordinary silicate cement is slightly greater than the slag cement. 6. C3A has a high content of cement.
6. The hydrophilicity in the cement is well -preserved; the volcanic gray cement has poor water retention and is easy to secrete water.

7. Temperature and humidity directly affect the adaptability of extrateripotic agents to cement.

8. The proportion of sand, stone grades, stone, stone, water, and glue in the cooperation ratio also affects the addition of concrete

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