Concrete
1. Definition:
Concrete is a kind of artificial stone made of cement, coarse and fine aggregates, and water, prepared in a certain proportion, stirred, compacted, cured, and hardened. Commonly referred to as concrete refers to cement concrete.
Ready-mixed concrete refers to cement, aggregates, water, admixtures, mineral admixtures, and other components that are added according to the needs according to a certain proportion. Concrete mix delivered to the place of use.
2. Composition:
Cementing materials, coarse and fine aggregates, and water, adding admixtures or admixtures if necessary.
(1) Cementing materials: the function is to cement loose aggregates into a whole, such as cement, gypsum, asphalt, etc.
(2) Coarse and fine aggregates: Mainly act as a skeleton to withstand external forces, coarse aggregates such as crushed stones, pebbles, gravel, etc.; fine aggregates such as river sand, sea sand, mountain sand, and artificial sand.
(3) Water: generally tap water, which must meet the "Concrete Mixing Water Standard" (JGJ63).
(4) Admixtures and admixtures: mainly used to improve concrete performance and save materials, admixtures such as water reducer, retarder, expansion agent, etc.; admixtures such as fly ash, slag, silicon powder, etc.
3. Classification:
It can be roughly divided into two types: ordinary concrete (cement concrete) and special concrete (such as waterproof concrete, heat-resistant concrete, hydraulic concrete, etc.)
4. Various properties of concrete
(1) Workability: It is an important index to express the construction performance of concrete, generally expressed by a slump or work degree. If the workability is good, it is easy to vibrate and compact, and the quality of the formed concrete is also good.
(2) Slump: refers to the value of free slump of concrete under certain test methods. Slump is an indicator of the fluidity of concrete. A large slump indicates high fluidity. The requirement for a slump is high in pumped concrete. The slump is generally in mm, accurate to 5mm. The slump of concrete produced by the company is generally 80-220mm. Fluid concrete refers to the concrete whose slump of the mixture is 100-150mm, and high fluidity concrete refers to the concrete whose slump of the mixture is ≥160mm.
(3) Bleeding property: Indicates the performance of the precipitation of water from the concrete during the transportation and pouring of the concrete. Concrete mixture with large bleeding will have an adverse effect on the quality of concrete, which will lead to a decrease in the strength of concrete.
(4) Water retention: As opposed to bleeding, good water retention means poor bleeding.
(5) Layered segregation: Indicates the nature of the separation of the constituent materials in the concrete mixture during the transportation and pouring of the concrete mixture. Concrete with serious stratification and segregation tends to be that the coarse aggregate in the mixture sinks and the cement mortar floats up, which affects the homogeneity of the concrete and reduces the strength of the concrete.
(6) Cohesion: as opposed to segregation.
(7) Strength: It is divided into compressive strength, tensile strength, flexural strength, etc. Among them, compressive strength is the main index indicating the strength of concrete, and the unit is Mpa. The strength grade is represented by the symbol C and the standard value. The grade can be divided into C10,...C30, C35,...C60, etc. High-strength concrete refers to concrete with a strength grade ≥ C60, and pump concrete refers to concrete with a slump of not less than 100mm and which is constructed by pumping.
(8) Impermeability: The performance of concrete to resist the pressure of water, oil, and other liquids. The impermeability is indicated by the impermeability label, which is divided into P6, P8, P10, and P12 (indicating the maximum water pressure when four of the six test pieces in each group do not see water seepage).
(9) In addition, there is frost resistance, shrinkage, creep, etc., which will not be introduced here.
Cement
1. Definition:
Cement is one of the most commonly used hydraulic binders. After the cement is mixed with water, it becomes a plastic paste, which can harden in air and in water.
2. Classification:
The six general-purpose types of cement we call: Portland cement (P.I, P.II), ordinary Portland cement (P.O), pozzolanic Portland cement (P.P), and slag portland cement (P.S), fly ash Portland cement (P.F) and composite Portland cement (P.C). Our company usually uses Portland cement and ordinary Portland cement.
(1) Portland cement: It can be divided into two types: P.Ⅰ--Portland cement without mixed materials; P.Ⅱ-mixed with limestone or granulated blast furnace slag mixed materials not exceeding 5% of cement weight Portland cement. Strength grades are 42.5, 42.5R, 52.5, 52.5R, 62.5 62.5R.
(2) Ordinary Portland cement: code-named P.O and its strength grades are: 42.5, 42.5R, 52.5, 52.5R.
3. Cement properties:
(1) Cement strength grade and strength: strength is an index to determine the strength grade of cement. At present, it is divided by 28-day strength at home and abroad.
(2) Cement mortar strength: Determine whether the cement strength is qualified according to the two indicators of cement mortar flexural and compressive strength.
(3) Water consumption of cement standard consistency: not only can directly understand the water demand of cement, but also can accurately test the setting time and stability performance.
(4) Cement setting time: The speed directly affects the construction. Generally, the initial setting time should not be earlier than 45 minutes, and the final setting time should not be later than 10 hours.
(5) Stability: refers to whether the volume change of cement is uniform during the hardening process. It is an important index to evaluate the quality of cement, and the use of cement with unqualified stability is strictly prohibited.
(6) In addition to this, there are fineness, specific gravity, cement mortar fluidity, etc.
4. According to various technical indicators of cement, cement can be divided into qualified products and unqualified products.
(1) Qualified products: All technical indicators of cement are in line with the provisions of the national standard GB175-2007.
(2) Unqualified products: Portland cement and ordinary Portland cement, where fineness, setting time, insoluble matter, stability, chloride ion, magnesium oxide content, sulfur trioxide content, and loss on ignition are not Comply with the provisions of GB175-2007 or the blending amount of mixed materials exceeds the maximum limit and the strength is lower than the index specified in the product label. The cement type, label, factory name, and factory serial number in the cement packaging mark are incomplete.
5. Storage of cement:
(1) The cement in storage should be stacked separately according to the type, label, and date of delivery, and should be clearly marked. First come first served, and prevent mixed-use, so when pouring cement into the warehouse, we must be careful to avoid wrong filling.
(2) The storage time of cement should not be too long, so as to avoid agglomeration and reduce the strength. The strength of commonly used cement will be reduced by 10-20% if it is stored in a normal environment for three months. Expired (cement storage time exceeds three months from the date of delivery) cement should be inspected before use.
Aggregate
(1) Aggregate, also known as aggregate, is one of the main components of concrete and acts as a skeleton.
(2) Classification: Coarse aggregate with a particle size above 5mm, and fine aggregate with a particle size below 5mm.
1. Fine aggregate
(1)Classification:
According to the production source, it can be divided into sea sand, river sand, and mountain sand;
According to the fineness modulus, it can be divided into coarse, medium, fine, and extra-fine sand;
(2) Technical requirements for concrete sand:
1) Particle gradation: the degree of thickness ratio must meet the standard requirements.
2) Mud content, mud content: For example, the sand mud content of concrete above C30 should not exceed 3%, and the mud content has a great influence on the performance of concrete, especially high-strength concrete.
3) Harmful substance content: such as mica, organic matter, etc.
4) Density, firmness, etc.
2. Coarse aggregate
(1). Classification:
According to the source of production, it can be divided into pebbles and crushed stones;
According to the specifications, it can be divided into single-grain grade and continuous-grain grade;
(2) Technical requirements for stones used in concrete:
1) Particle gradation: must meet the standard requirements.
2) Needle and flaky particle content: when preparing concrete larger than C30, it should not exceed 15%.
3) Mud content: when preparing concrete greater than C60, it should not exceed 1%
4) Strength: It directly affects the strength of concrete. Generally, it is required to be higher than the strength of concrete. The strength indicators mainly include cubic compressive strength and crushing index. Our company uses crushing index to express it.
(3) Matters needing attention:
1) The content of needles and flaky particles (too long and too thin particles) has a great influence on the workability and strength of concrete, so it should be strictly controlled.
2) There should not be too much mud, stone powder, and sundries in the gravel.
Impurities cannot be mixed during stacking, and stacked separately according to the place of origin, type, and specification.
Additives
1. Definition:
Refers to the material whose dosage is not more than 5% of the weight of cement and which can change the properties of concrete as required.
2. Classification:
1) Water-reducing agent: (ordinary, high-efficiency) reduces water consumption, increases concrete strength, or improves workability.
2) Air-entraining agent: increases air content, reduces bleeding and segregation, and improves workability.
3) Coagulation regulator: (retarding, early strength, rapid setting) to adjust the setting time.
4) Water repellant, antifreeze, etc.
5) Expansion agent: to expand the volume of concrete and improve impermeability.
Different types of admixtures have different main functions and scopes of application.
3. Key points for use:
1) The amount of admixture is expressed as a percentage of cement weight, and the amount of admixture should be strictly controlled. Improper amounts will affect the quality of concrete.
2) The use of admixtures has a compatibility problem with cement, so the cement adaptability test should be done before use, and it can be used only after passing the test.
3) The storage of admixtures should be marked clearly.
Admixtures
1. Definition of admixture:
Including fly ash, blast furnace slag, silica fume, etc. Adding admixtures is beneficial to save cement, improve concrete performance and adjust concrete strength level.
2. Fly ash:
(1) It is a fine powder collected from boiler flue gas of pulverized coal-fired power plant.
(2) Its quality indicators include loss on ignition, water content, sulfur trioxide, fineness, and water demand ratio. These indicators are divided into three grades: Ⅰ, Ⅱ, Ⅲ level.
(3) The use of fly ash can save cement and have a more significant economic effect. It can improve and enhance the technical performance of concrete: such as reducing the heat of hydration, it is the main admixture of mass concrete.
(4) There are technical requirements for the dosage and the ratio of replacing cement. For example, if Yinyang P·Ⅱ cement is used in ordinary reinforced concrete, the cement replacement ratio of fly ash should not exceed 30%.
3. Grinding slag:
(1) Ground slag is the product of granular blast furnace slag that has been dried and ground to a specified fineness.
(2) According to its quality indicators, specific surface area, activity index, water demand ratio, etc., it is divided into grades I, II, and III.
Production control and transportation of concrete
1. Concrete production process flow chart
2. Production plan requirements:
The production method of commercial concrete is different from other commodities, and its characteristic is that production is determined by sales and has a strong time limit. Once the concrete is produced, it is advisable to start unloading within 90 minutes and finish unloading within half of the initial setting time, so as to better ensure the quality of the concrete. Therefore, the producer and the user must coordinate to make the feeding speed consistent with the material consumption speed, so that the concrete supply on the construction site is neither out of joint nor piled up.3. Stirring requirements:
(1) Before mixing the concrete, add water and run it idle for a few minutes, pour out the accumulated water, and make the mixing drum fully wet.
(2) Pay attention to observe whether the feeding is uniform, and prevent the mixing of two trays of materials.
(3) Clean the mixer frequently to prevent low mixing efficiency caused by the mixing blades adhering to more concrete.
4. Ratio of materials:
(1) Measuring equipment should be calibrated regularly to maintain its accuracy. Before the formal weighing of each work shift, the measuring equipment should be checked at zero points. The allowable deviation of the measurement results of each raw material per tray: ±2% for cement, admixtures, water, and admixtures; ±3% for coarse and fine aggregates. The allowable deviation of the cumulative measurement results of each raw material: ±1% for cement, admixtures, water, and admixtures; ±2% for coarse and fine aggregates.
(2) Always pay attention to the change in water content of sand and stone, and adjust the amount of water added according to the measured water content, so as to control the water-cement ratio of concrete. During the mixing process, attention should always be paid to the slump of the mixed concrete and strictly controlled.
5. Stirring time:
Concrete should be stirred until the various constituent materials are mixed evenly and the color is consistent. For high-grade concrete, the mixing time should be extended by 10-30 seconds.
6. Transportation of concrete
(1) The delivery time of concrete refers to the time when the concrete is unloaded from the mixer into the transport vehicle until the transport vehicle starts to unload. The transportation time should meet the requirements of the contract. When the contract does not stipulate, the concrete transported by the mixer truck should be unloaded within 1.5 hours. When the highest temperature is lower than 25°C, the transportation time can be extended by 0.5 hours.
(2) During transportation, the most important thing is to maintain the uniformity of the concrete mixture, avoid stratified segregation, cement slurry loss, large changes in a slump, and initial setting, and keep the tank speed at 6-8 rev/min.
(3) Under hot, cold, or windy weather conditions, effective heat protection, heat preservation, wind protection, and rain protection measures should be taken.
(4) When transporting by vehicle, the road should be smooth and the driving should be stable to avoid serious stratification and segregation. If stratification and segregation occur, secondary stirring should be carried out before unloading.
(5) The temperature when it is transported to the construction site should not exceed 350C at the highest, and the minimum temperature should not be lower than 50C.
(6) It is strictly forbidden to add water to the concrete in the transport vehicle arbitrarily. If the slump loss of the concrete arriving at the construction site is too large due to various reasons, and it really cannot meet the construction requirements of the construction site, the admixture must be added reasonably according to the guidelines formulated by the relevant technical personnel, and the admixture should be stirred three times to make it even before unloading construction.
(7) When using a concrete pump, the concrete is required to have good workability and fluidity. The concrete mixture suitable for pumping has a slump of ≥100mm. When pumping, there should be enough concrete in the receiving hopper to ensure the continuous operation of the concrete pump. When the pumping interval exceeds 45 minutes or the concrete segregates, the concrete in the pipe should be flushed with pressure water or other methods immediately to prevent the pipe from being blocked.
Factors Affecting the Workability of Concrete
1. Adaptability of admixtures and cement:
Different types of cement and different types of admixtures have different adaptability, and the adaptability of the two has a great influence on concrete. Good adaptability between cement and admixture means good water reduction effect, good fluidity and water retention of concrete, less bleeding and segregation, less slump loss, normal setting time, and good workability.
2. Concrete waiting time:
Since the concrete has been hydrated since it was mixed, the slump of the concrete will be lost over time. The longer the waiting time for concrete, the greater the loss of slump, and it is easy to cause the performance of the concrete to fail to meet the requirements of the construction site when it is delivered.
3. Moisture content of sand and stone:
Their changes directly affect the slump of concrete. A 1% fluctuation in the moisture content of the sand will increase or decrease the slump of the concrete by 2-3cm.
4. Cement varieties:
Different cement varieties have great differences in strength, fineness, water demand, bleeding, heat of hydration, and setting time, all of which will affect the workability of concrete. For example, in terms of bleeding performance, concrete mixed with slag is easy to bleed. In addition, slag cement > ordinary Portland cement > Portland cement.
5. Mud content, mud content, particle size, and gradation of sand and stone:
If the mud content or mud content in the sand is too high, the water consumption of the concrete will increase, the concrete will be loose in appearance, and the fluidity will be poor; the non-continuously graded sand and stones with large particle sizes will make the concrete appear more stones, the viscosity is not good; the same small particle size stones are easy to segregate the concrete and affect the strength of the concrete.
6. Fly ash:
The fineness of coal ash, the ratio of water demand, and the amount of concrete will all affect the workability of concrete. Appropriately increasing the amount of coal ash can effectively improve workability, improve fluidity and inhibit bleeding.
7. Raw material measurement:
1% water measurement error is equivalent to 2-3 kg/m3 water, which can increase or decrease the concrete slump by 1-2cm. The measurement error and abnormality of the admixture will cause the abnormality of the concrete; the measurement of cement and coal ash will also affect the strength of the concrete, which will cause serious quality accidents.
8. Weather effects:
The weather is hot and the temperature is high, the loss of concrete slump will be large, and the influence of rain on rainy days cannot be ignored.
9. Other effects:
The factors affecting the workability of concrete are very complex, and even slight changes in conditions will have an impact on it. For example, the structure of the concrete mixer truck and the speed on the way will have different effects on the slump of the concrete. When the concrete with the same slump arrives at the construction site, the slump loss of the imported new car is smaller than that of the original old car.
10. Types of concrete and corresponding engineering parts:
Concrete is used in different engineering parts, and the corresponding concrete labels are also quite different. The wrong use of the label may lead to great engineering quality accidents, such as low-grade concrete C15 being pumped into the wall column. Common important engineering parts on the construction site include vertical structures such as core tubes, columns, hole piles, underwater piles, shear walls, and municipal roads with flexural requirements, all of which are key parts that require high strength. These parts must be taken seriously.
Concrete performance description:
(1) The setting time of concrete is about 6-12 hours, which varies according to the strength grade and concrete type;
(2) The general loss of concrete is 10-30mm/h, and corresponding measures should be taken in time to adjust if it exceeds;
(3) Strictly control the factory slump, and adjust the sand rate in time according to the material situation.
