First of all, we should know what the strength grade of concrete is. The strength grade of concrete is artificially divided according to the compressive strength value of concrete cube, which is the characteristic strength of concrete. According to the current relevant standards and specifications, the compressive strength of concrete cube is a cube specimen with a side length of 150㎜ made according to the standard method, and the compressive strength of concrete cube is measured in a standard curing room with a temperature of 20℃ and a humidity greater than 95% according to the standard test method. According to relevant standards, the strength grade of building materials should be expressed by the name of the material plus its strength standard value. Therefore, the strength grade of concrete is divided into C 10, C 15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75 and C80 according to the symbol C (abbreviation of concrete) and the cube compressive strength standard values behind it.

Because the load on concrete in different parts of general engineering is different, some are large and some are small, so it is impossible to completely use one strength grade of concrete. High-strength concrete should be used for the parts with high pressure, and low-strength concrete should be used for the parts with low pressure. Therefore, concrete is divided into different strength grades to meet the needs of different projects, and it is selected through design and calculation.

Why is the 28d strength of concrete specified as the standard strength?

Concrete gradually hardens through cement cementation, and its compressive strength is improved. Because the hardening of cement is not completed at once, it is gradually improved with the increase of time. Under normal curing conditions, the compressive strength increased rapidly in the first 7 days, slightly slower between 7 days and14 days, and even slower after 28 days. That is to say, the compressive strength after 28d is the standard strength, which is used as the standard of quality inspection in design and construction. Obviously, if the strength less than 28d is taken as the standard strength, the performance of concrete cannot be fully exerted. If the strength greater than 28d is taken as the standard strength, although the performance of concrete can be fully exerted, it takes too long to reach the standard strength, which affects the construction progress.

Why does concrete have requirements for water quality?

Industrial wastewater containing fat, vegetable oil, sugar, acid, etc. Can't be used to mix concrete. Because these impurities will reduce the cohesive force of cement and the strength of concrete, mineral water cannot be used, because mineral water contains a lot of salt, which makes cement unable to resist water erosion well. For the chemical composition of mineral water, it must reach the index specified by the family, or make a comparative test with ordinary drinking fresh water to see if the intensity is reduced before using it.

As for ordinary tap water and drinking water, it can be used to mix concrete. See standard GBJ63-89 for details.

Why are concrete test blocks in groups of three?

Concrete test block is the standard to measure the strength of concrete members, that is, the strength of test block under compression can be used as the strength of members. Therefore, in addition to making test blocks as components, there must be a certain number of test blocks. Because although we try our best to make the concrete of the test block the same as that of the component, there are still some differences after all. It is difficult to be reliable if only one test block is used to evaluate the strength of components. Therefore, in the case of a group of three, the average strength of three test blocks is taken as the strength of this group of test blocks (some values need to be removed in special cases), that is, it is taken as the strength of the component.

What does the durability of concrete mean?

In addition to proper strength, concrete should also include frost resistance, water seepage resistance, chloride ion permeability resistance, shrinkage, carbonation, steel corrosion resistance, sulfate corrosion resistance, compressive fatigue deformation resistance, alkali aggregate reactivity and so on. , collectively referred to as durability.

Impermeability: refers to the ability of concrete to resist the penetration of liquid and gas. Because there are interconnected pores and capillaries in concrete, and honeycomb and holes are generated due to vibration undercompaction, liquid and gas can penetrate into concrete. The invasion of moisture and air will corrode steel bars, and the invasion of harmful liquids and gases will deteriorate concrete, affecting its quality and long-term safe use. The impermeability of concrete is represented by the impermeability label P. For example, P4 means that under the corresponding water pressure of 0.4N/㎜2, the impermeability test is carried out with six cylindrical or conical test blocks with specified size, and four test blocks remain impermeable. The impermeability marks of concrete are generally divided into P4, P4, P6, P8, P 10 and P 12.

Frost resistance: refers to the ability of concrete to resist freezing. Concrete often freezes in cold areas, especially in the environment where it freezes when it comes into contact with water. This is because the water infiltrated into concrete will expand by 9% after freezing, which puts considerable pressure on the pores and capillaries inside the concrete. If the temperature rises and freezes and melts, the concrete will eventually be destroyed. The frost resistance of concrete is indicated by the frost resistance mark F. If the strength of the frozen-thawed test block is less than 25% lower than that of the unfrozen test block, the frost resistance is considered to be qualified. The frost resistance label is expressed by the maximum number of repeated freeze-thaw cycles that the test block can withstand. According to the number of freeze-thaw cycles, the frost resistance marks of concrete are generally divided into: F 15, F25, F50, F 100, F 150 and F200.

Corrosion resistance: refers to the corrosion resistance of concrete in various corrosive liquids and gases. The media that corrode concrete mainly include sulfate solution, acidic water, active and/or soft water with hydraulic pressure, seawater and concentrated alkali solution.

Heat resistance: refers to the property that concrete has certain chemical stability under the action of high temperature, its internal structure is not destroyed and its strength is not obviously lost.

Why do concrete have natural curing and steam curing?

Natural curing refers to the curing of concrete under natural conditions (temperature not lower than +5℃ and humidity 90 ~ 100%). As mentioned above, at the natural curing temperature, the strength increases very slowly, and only 30~70% of the concrete strength after 28 days can be obtained at the age of 7 days. It takes a long time to ensure the strength of formwork removal and factory strength. This will prolong the cycle time of the whole production process, at the same time, it needs a lot of template equipment and occupies a lot of production area, increasing capital investment. In order to accelerate the growth of concrete strength, steam curing can be adopted, and the concrete can be heated by steam, so that the concrete can be quickly hardened under the conditions of high temperature (70~90℃) and high humidity (about 90% or more). However, it is still suitable for nature conservation in areas with brighter and warmer climate. This can save fuel and a corresponding set of equipment investment, and reduce costs.

What is the workability of concrete?

(1) Concrete workability refers to whether the composition of concrete mixture can be kept uniform and whether it is easy to be poured and vibrated during production and operation.

(2) The workability of concrete is a comprehensive index. It includes fluidity, cohesiveness and water retention of concrete.

(3) The fluidity of concrete refers to the property that concrete mixture is easy to flow, transport and fill concrete formwork under the action of self-weight or mechanical vibration force.

(4) The cohesiveness of concrete means that in the production process, there is no delamination or segregation between the materials that make up the concrete mixture, and there is a certain cohesiveness.

(5) The water retention of concrete means that in the production process, the concrete mixture will not produce serious bleeding phenomenon, and has a certain water retention capacity.

How to measure workability of concrete?

At present, there is no comprehensive method to measure the workability of concrete mixture. Usually, its fluidity is measured, and then its cohesiveness and water retention are judged by experience.

Slump method is the most commonly used method to measure the fluidity of concrete. When measuring, the concrete mixture is divided into three layers and put into a standard size cone slump cone. Each layer is inserted vertically and evenly from the outside to the inside for 25 times with a rammer with a diameter of 16㎜. After three layers of compaction, level the mixture at the mouth of the cylinder, then lift the cylinder vertically and put it aside. At this time, the mixture collapses due to its own weight, and the downward collapse size (㎜) is measured. The greater the slump, the greater the fluidity of concrete.

After slump test, the cohesiveness and water retention of concrete can be observed at the same time. If there is not too much water on the concrete surface, it means that the water retention is good. Moreover, the mixture can be gently tapped from the side with a tamping rod, and the concrete with good cohesiveness will not collapse loosely under the tapping.

Slump test is only applicable to plastic concrete and low plastic concrete; For hard concrete, the method of measuring workability is often used. It is necessary to measure its working degree with Weber consistency meter.

Key operation points:

(1) The concrete mixture is filled in three layers, and each layer is compacted for 25 times. After tamping, lift the slump cone vertically and smoothly.

(2) Turn the transparent disc to the top of the concrete specimen and drop it gently to make it contact with the top surface of the concrete.

(3) Start the vibrating table and stopwatch at the same time, and record the time required for the bottom surface of the transparent disc to be covered with cement slurry. For the classification of concrete mixtures, please refer to:

Classification slump (㎜) dry hardness (s)

Hardness 060 ~ 120

Semi-hardness 030 ~ 60

Low plasticity 10 ~ 50 15 ~ 30

Plasticity 50 ~ 1505 ~ 15

(4) fluidity > 150- the choice of slump or workability of concrete depends on the molding method, section size and steel bar density.

What is ordinary concrete? What are its characteristics?

(1) Cement is used as cementing material, sand and stone are used as coarse and fine aggregates, and the concrete made by mixing with water is ordinary concrete. In the whole concrete family, ordinary concrete is the earliest and most widely used, and many future concrete varieties are developed on its basis.

② Ordinary concrete, or concrete for short. Its main features are as follows:

(3) The compressive strength of concrete is high, generally 20 ~ 50mpa, and it can bear a large load.

(4) Concrete has good plasticity before setting, and can be made into structures and members of various shapes and sizes as required.

(5) It has good durability and can withstand dry and wet, cold and hot, freeze-thaw changes in the air for a long time without damage.

(6) Under dry conditions, the thermal conductivity of concrete is1.3 kcal/m h℃, which is relatively large, but only one fortieth of that of steel, and it has certain thermal insulation performance.

(7) The bulk density of concrete is 2400㎏/m3. Compared with steel, when this kind of concrete is used as structural material, in order to bear the same load, it needs to choose a larger cross-sectional size, so its self-weight is greater.

(8) The compressive strength of concrete is very low. The ratio of compressive strength to tensile strength is called brittleness coefficient 10, so when concrete is damaged, brittle materials will suddenly be destroyed.

Why should additives be used in concrete?

The application of (1) admixture is increasingly extensive, which is the objective requirement of the development of construction engineering structure and technology. In recent years, high-rise, long-span and various new structural systems have appeared in integral cast-in-place concrete structures, and many new structural types such as large-scale and thin-walled precast concrete members have appeared. In concrete construction, new construction technologies such as mass concrete, shotcrete, vacuum water-absorbing concrete and slip-form construction concrete have emerged, so it is proposed that concrete has the characteristics of high fluidity, early strength, high strength, rapid setting, slow setting, low hydration heat and frost resistance.

(2) The performance of concrete is determined by the proportion of cement, sand, stone and water. In order to improve some properties of concrete, the proportion of raw materials can be adjusted. But this will often cause losses on the other hand. For example, in order to increase the fluidity of concrete, water consumption can be increased, but this will reduce the strength of concrete. In order to improve the early strength of concrete, the dosage of cement can be increased, but this may increase the shrinkage and creep of concrete besides increasing the cost. However, the use of admixture can avoid the above defects, and it can greatly improve a certain performance of concrete when it has little influence on other properties of concrete. For example, as long as 0.2% ~ 0.3% calcium lignosulfonate water reducer is added to concrete, the slump of concrete can be more than doubled without increasing water consumption; As long as 2% ~ 4% sodium sulfate, sugar and calcium (NC) composite agent are mixed into concrete, the early strength of concrete can be improved by 60% ~ 70% and the later strength of concrete can also be improved without increasing the dosage of cement.

Is there a mixed ratio?

The so-called "concrete mix ratio"-the ratio of cement, sand, stone and water.

Various properties of concrete, such as strength, durability and various properties of concrete mixture, are directly affected by its composition ratio; With the change of mix proportion, various properties of concrete also change; And the amount of cement will also affect the cost of concrete. In order to meet the workability and fluidity required by certain working conditions, to ensure the strength and durability of the proposed concrete in the project, to save more cement, to reduce costs, and to achieve economic and reasonable purposes, it is necessary to choose appropriate concrete mix proportion.

Concrete mixture ratio must be calculated by weight?

(1) The accuracy of concrete component batching (weighing) is an important condition to ensure the quality of concrete engineering and save raw materials. Generally speaking, the weighing error of cement, water, additives and other gelled substances shall not exceed 2% by weight. The weighing error of aggregate shall not exceed 3%.

(2) Because the humidity and density of sand, stone, cement and other materials are different, the weight of materials with the same volume often has great correlation. Therefore, the concrete mixture ratio is controlled by weight calculation. This mixing ratio calculation method is more accurate than the volume method.

What are the causes of cracks in concrete after hardening?

There are many reasons for concrete cracking after hardening, which are generally caused by more than two reasons. Usually, the concrete structure is overloaded, resulting in tensile stress. If the tensile strength of concrete does not reach the tensile stress, cracks will occur. The causes of concrete deformation include drying shrinkage, temperature change, chemical action and structural factors. At this time, it is necessary to minimize shrinkage from the aspects of materials, proportion and maintenance, and on the other hand, try to make up for various situations of congenital cracking in construction and design.

What is the reason for frost on the surface of concrete after hardening?

After the concrete is hardened and dried, the rainwater, groundwater and healthy water from outside penetrate into the hardened body through pores, so that the aqueous solution of hydraulic colloid or inorganic salt flows out, and then reacts with carbon dioxide in the air to generate sulfur-containing substances, and the water will be attached to the surface of the hardened body after evaporation. This phenomenon is called a hundred flowers, also known as "wall cancer". Flowers can be washed with soap at first. If it is carbonated, it can be washed with dilute hydrochloric acid and then washed off with water. However, flowers often flow out again after being wiped. To prevent flowers from blooming, we must use evenly poured and dense concrete to reduce the porosity, then appropriately reduce the water cement ratio, reduce the silt content in the aggregate, use clean mixed water, high-quality cement, add water reducer and maintain it properly. Only by doing our best to prevent rainwater from invading the site can problems be prevented.

Adaptability of concrete admixture to cement?

(1) The stability of cement ore leads to the stability of mineral composition, thus affecting the adaptability of concrete admixture to cement.

(2) Cement production processes, such as vertical kiln and rotary kiln, how to control the quenching measures in the cooling system and the temperature during gypsum grinding. The mineral composition, crystal phase and gypsum morphology in cement are changed, which affects the adaptability of concrete admixture to cement.

(3) Ability to adsorb additives in cement: C3AC 4A FC 3C2S, and the hydration rate of cement is directly related to mineral composition.

(4) After cement is stored for a period of time, the temperature drops, which improves the high-temperature adaptability of concrete admixture. Moreover, f-CaO absorbs moisture in the air and transforms it into Ca(OH)2, and absorbs CO2 in the air and transforms it into CaCO3, thus reducing Mwo, improving workability of concrete, slowing slump loss of fresh concrete and slightly prolonging setting time of concrete.

(5) The water demand of ordinary portland cement is slightly higher than that of slag cement, which has good water retention, but the slump loss is also faster.

(6) Cement with high 6)C3A content has fast slump loss and good water retention.

(7) The hydrophilic admixture in cement has good water retention; Pozzolanic cement has poor water retention and is prone to bleeding.

(8) Temperature and humidity directly affect the adaptability of concrete admixture to cement.

(9) The gradation of sand and stone and the proportion of sand, stone, water and glue in the mixture ratio also affect the adaptability of concrete admixture to cement.

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