1. What is ready-mixed concrete?
Answer: Ready-mixed concrete is the cement, water, sand, stones, admixtures and mineral admixtures and other components in a certain proportion, after being measured and mixed at the mixing station, it is transported to the buyer's delivery site within the specified time concrete mix. Concrete means artificially made stone, representing concrete. The pronunciation in the dictionary is tóng, and it can also be pronounced hùn níng tu.
2. What are the grades of concrete strength?
Answer: According to the strength of the standard concrete test block after 28 days of standard curing, it can be divided into 13 grades: C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C70, and C80. C is the first capital letter of the English word for concrete, which means concrete, and the number in the corner indicates the strength level.
3. Which are general products and which are special products?
Answer: The strength grade is not greater than C60, the slump is not greater than 180mm, and there is no other required concrete. Any one of the indicators exceeds the specified range of general products or concrete with special requirements. Such as fine stone concrete, pile foundation concrete, impervious concrete, antifreeze concrete, etc.
4. What is delivery inspection?
Answer: The inspection of commercial concrete is carried out by the supply and demand parties and the supervisor at the delivery site. Such as testing slump, making test blocks, visually measuring slump, workability, etc.
5. What is factory inspection?
Answer: The quality inspection of commercial concrete before leaving the factory. Such as sampling to detect slump, making strength test blocks, visually measuring slump, workability, etc.
6. How many cements are commonly used in mixing plants, and what items are re-inspected?
Answer: Commonly used cements in mixing plants are: Portland cement, ordinary Portland cement and composite cement. Portland cement is divided into P·Ⅰ and P·Ⅱ types, and ordinary Portland cement is divided into ordinary type and early strength type, code-named P·O. The items that need to be re-inspected for cement are setting time, stability, strength, and fineness (specific surface area).
7. How many requirements are there for the quality of good sand?
Answer: There are the following items:
(1) The particle size distribution meets the requirements of medium sand;
(2) The particle size is medium sand, and the content of particles larger than 10mm is small, up to 10%;
(3) No mud eggs, large mud lumps, mud content not exceeding 3%;
(4) No tree roots, grass roots, mud, plastic bags and other sundries.
8. Why is medium sand preferred for pumping concrete?
Answer: Sand is divided into coarse sand (3.1-3.7), medium sand (2.3-3.0) and fine sand (1.6-2.2) according to its fineness modulus. The gradation of medium sand is reasonable, and the content of particles below 0.3mm can reach about 15%, which is conducive to pumping. Coarse sand has high frictional resistance, which is not conducive to pumping. The surface area of fine sand is large, and more cement is used under the same strength, and the probability of cracking is high.
9. What are the main requirements for the quality of good stones?
Answer: There are mainly the following items:
(1) The particle size meets the requirements, and the particle gradation is continuous;
(2) No yellow skin (surface sticky mud), mud eggs, large mud lumps, and large grains of stones;
(3) The less the needle-like and flake-like particles, the better (the needle-like and flake-like particles of the stones used for pumping concrete should not exceed 10%);
(4) The less shale content, the better (because the strength of shale is lower, and the content of flaky particles is increased)
10. Why should continuous grade stones be preferred for pumping concrete?
Answer: single grain grade and continuous grain grade. Single-grain grades are: 10-20, 16-31.5, 20-40. Continuous grades are: 5~10, 5~16, 5~25, 5~31.5, 5~40. In the case of the same maximum particle size of the stones, the porosity of the single-grain grade is larger than that of the continuous-grain grade, and the large void ratio should be filled with mortar. The fluidity of the concrete mixed with continuous grade stones is better than that of single grade stones, which is beneficial to pumping construction.
11. What are the inspection items for fly ash entering the factory?
Answer: Fly ash is the fine powder collected by dust removal equipment in the flue gas of pulverized coal furnace in power plants, and it is divided into three grades; grade I, grade II and grade III. The fineness and water demand ratio of fly ash should be checked, and the key item is the water demand ratio. If the fineness slightly exceeds the standard and the water demand ratio meets the requirements of Class II, the ash can be used, otherwise it cannot be used.
12. Which items should be inspected when the pumping agent enters the factory?
Answer: It must be inspected: water reducing rate, setting time, 1h slump change over time, compressive strength ratio. Routine testing can use the C30 theoretical mix test for production, and the clean slurry fluidity cannot be used to replace the concrete mix test (the results of the clean slurry and the mix test are too different, which is not conducive to guiding production). A good pumping agent usually has a reasonable water reducing rate, good fluidity of the concrete mixture, and good slump retention.
13. What are the main performances of concrete mixtures?
Answer: Mainly test the workability, slump, and gas content of the mixture, and the workability is expressed by cohesiveness, water retention, fluidity, and comprehensiveness;
(1) Cohesion. Visual observation shows that when the slump is measured, the test body slumps to the surroundings, indicating good cohesion, while the test body collapses to one side or some stones leak out, and the surface does not stick to the mortar, indicating poor cohesion.
(2) Water retention - after the concrete mixture is loaded into the slump cylinder and compacted, there is a little slurry or no slurry at the bottom of the cylinder, which means good water retention. If the concrete mixture is poured on an iron plate, there will be water or cement slurry on the surface Precipitation indicates poor water retention (that is, segregation phenomenon).
(3) Fluidity - the fluidity is good if the mixture is piled up and flows around immediately, otherwise it is poor fluidity.
14. What does segregation of a concrete mix look like?
Answer: If it is found that the cement slurry flows out from the edge of the pile of concrete mixture. After a few minutes, the sand and gravel will sink, and the bottom grabbing phenomenon will be formed, which is the segregation phenomenon of the concrete mixture. Segregation is a manifestation of poor workability of concrete, which is easy to block pipes during pumping and will reduce the strength of concrete. The common reason for the segregation phenomenon is that the amount of pumping agent is too large. You can reduce the amount by 0.2% and try again.
15. Sometimes some stones in the concrete mixture do not stick to the mortar, or are not sticky like bean curd residue. What is the reason?
Answer: Some stone non-stick mortars appear because the pumping agent contains less viscosity-increasing ingredients. The appearance of bean curd residue is the result of incompatibility between the pumping agent and certain components in the cement.
16. What is a standard test block?
Answer: The test block whose side length is 150mm×150mm×150mm is the standard test block. The test block with a side length of 100mm×100mm×100mm is a non-standard test block, and the result after compression should be multiplied by a conversion factor of 0.95 to convert it into a standard test block strength.
17. What is the standard maintenance, what is the same condition maintenance?
Answer: Putting the formed concrete test block into an environment with a temperature of 20±2°C and a humidity greater than 95% is the standard curing. The curing conditions of the formed concrete test blocks are the same as the curing conditions adopted by the construction site components, which is called the same condition curing.
18. What is the impermeability of concrete?
Answer: It refers to the performance of concrete itself to resist the penetration of pressure water into the interior. Both the basement and the reservoir are required to have good anti-seepage performance. There are four commonly used anti-seepage grades: S6, S8, S10, S12 (old notation) or P6, P8, P10, P12 (new notation), S is the first capital letter of the word "seepage", P is the first capital letter of the English word "impervious".
19. What is the frost resistance of concrete?
Answer: It refers to the ability of concrete to resist repeated freezing and thawing. Frost resistance grades are F100, F150, F200, F means frost resistance, and the number in the corner indicates the number of cycles of repeated freezing and thawing of concrete from -15°C to room temperature.
20. What is the difference between frost resistance and frost resistance of concrete?
Answer: The frost resistance of concrete refers to the ability of concrete to resist repeated freezing and thawing after reaching the design strength, while the antifreeze of concrete refers to the ability of concrete mixture to prevent low temperature freezing damage during the setting and hardening process.
21. What tasks should the dispatcher pay attention to?
answer:
(A) Issue task orders and organize production reasonably
(B) Dispatch vehicles reasonably, so that the construction site does not overwhelm the vehicles, keeps gears, and produces continuously;
(C) Open the transportation bill: the mix ratio number, weight ratio and factory slump are consistent with the data issued by the laboratory.
22. What should I do if some construction sites report concrete deficits?
Answer: Immediately notify the production manager and the director of the laboratory to find out the reason. First, check whether there is too much negative deviation in the production records of the computer, whether there is only water reduction without adding sand; second, check whether there is mold expansion at the pouring site Phenomenon, over-thickness of cast-in-place slabs or scattered waste; the third is to check in detail whether the planned quantities are wrong.
23. Why should the maximum water consumption be limited in the production process?
Answer: Because when the weather is very dry, the moisture content of sand may be less than 4%. At this time, the maximum water consumption per cubic meter of concrete can be increased by 10kg according to the production water consumption in dry days. If the slump requirement is not met, It may be caused by the water-reducing rate of the pumping agent is too small. At this time, the dosage of the pumping agent should be increased by 0.1% to 0.2%. The water consumption must not be increased blindly in order to meet the slump requirements. When the water consumption exceeds 5kg, it must be reported to the director of the laboratory for approval.
24. Can observing the stirring current predict the degree of dryness of the concrete mixture?
Answer: It can be predicted. Because the drier the mixture, the greater the stirring current; the thinner the mixture, the smaller the stirring current. The controller must pay attention to observe and record the stirring current of each mixer, and determine the range of stirring current corresponding to a suitable slump. When the current is less than the range, the water consumption should be reduced; when the current is greater than the range, the water consumption should be increased. When the current is too large or too small, the concrete slump should be checked visually on the tank truck, and if it does not meet the requirements, adjust it to pass.
25. What tasks should be paid attention to before production in the rainy season?
Answer: (1) First, go to the stockyard to visually measure the moisture content of sand and stone, and determine which line of mix ratio to use.
(2) Run the belt dry for more than ten seconds before feeding to remove the accumulated water on the belt. Then confirm the opening mix ratio and water consumption according to the requirements of the task list. Before unloading, it must be confirmed that the driver has reversed the tank to drain water, especially after rain, it must be drained before loading, and then unload into the tank. Within 10 seconds after ringing the bell, you must confirm whether the car has left the station to prevent the wrong car from being loaded.
26. Do I need to check when inputting the mix ratio into the computer?
Answer: It must be checked. One person enters and the other checks to prevent mistakes. In some stations, the amount of cement has been entered incorrectly; in some stations, mineral powder was used as cement, and in others, fly ash was used as cement, resulting in rework of the project.
27. Why does the concrete transport vehicle reverse the tank to drain the sewage before loading?
Answer: Before loading each car, the tank body must be reversed to check whether there is water. If there is water, it should be drained immediately, and if there is no water, then reload. Prevent the vehicle from washing the car without draining the clean water, resulting in the factory concrete being too thin to be used, or loading the vehicle without draining the rainwater after the rain, resulting in the return of materials on the construction site.
28. Why is no one allowed to add water to the concrete mixture at will?
Answer: Adding water to the mixture increases the water-binder ratio, and the strength of concrete is inversely proportional to the water-binder ratio. The larger the water-binder ratio, the lower the strength. The concrete mixture can be diluted by adding an appropriate amount of pumping agent, which is allowed by the national standard "Ready Mixed Concrete" GB/T14902-2012.
29. What is the initial setting and final setting of concrete?
Answer: The mortar in the concrete mixture begins to lose its plasticity and is defined as initial setting, which is not easy to determine at this time. Generally, the initial setting time is 8-12 hours (4-6 hours when the temperature is high). The final setting time is 12~16h (5~7h at high temperature). In autumn and winter or winter and spring, the final setting time may be between 18 and 24 hours. When the concrete mixture has not finalized 24 hours after pouring, it is retarded, and the retarded is caused by excessive retarding ingredients mixed in the pumping agent. Sometimes when the weather temperature drops sharply by 5-10°C, the setting time of concrete is prolonged, and retarded setting occurs.
30. What problems should be paid attention to in mass concrete?
Answer: When the minimum size of the concrete structure entity is ≥ 1m, or the concrete that is expected to cause cracks due to the excessive temperature difference between the inside and outside of the concrete (greater than 25°C) due to the hydration heat of cement is called mass concrete.
(1) Use as little cement as possible while ensuring the strength, and mix more high-quality fly ash or mineral powder.
(2) Use retarding pumping agent to prolong the final setting time of concrete and reduce the peak temperature. Control the temperature difference between the center and surface of the concrete member within 25°C. The side form cannot be removed too early, and the form must be removed only after the temperature difference between the inner and outer surfaces of the component is less than 20°C.
31. Why do you need to disperse the cloth when pumping the pipe mortar?
Answer: Because the strength of the pipe-running mortar is relatively low, if the pumps are concentrated in one place, the concrete strength of the components at that place may not meet the design requirements.
32. What is the best way to prevent cracks in cast-in-place concrete slabs in spring?
Answer: The best method is: after pouring a section of the slab surface, use a vibrator to vibrate the concrete, rub the slab surface with a wooden washboard, and then apply the thatched plastic film from front to back, and use a trowel to separate the film from the joint or the end. The concrete is compacted to make the bond firm and prevent it from being blown up by the wind. If it is urgent to set out the line when the concrete is finally setting, use a paper knife to cut off the part that hinders the line-out along the benchmark, and do not lift it all up.
33. Why is it not allowed to hang heavy objects on the concrete just after it has finally set?
Answer: The concrete has just finalized and its strength is very low. Hanging heavy objects such as steel bars will cause large deformation and cracks in the slab. Lifting heavy objects must be carried out 24 hours after the final setting of the concrete or when the strength of the concrete reaches 1.2MPa or more.
34. What is rebound detection?
Answer: Use a rebound hammer to bounce the surface of the concrete structure. After the bounce, the bounce hammer in the instrument rebounds to drive the indicator slider to move, and read the rebound value according to the scale line on the indicator slider. Each place (measurement area) bounces 16 points, removes 3 maximum values and 3 minimum values, takes the average value of the remaining 10 as the rebound value of the measurement area, and then measures the carbonization depth value of the structure; use The rebound value and carbonation depth value of the survey area can be checked in the "conversion table of concrete strength in the survey area" to determine the strength value of the concrete in the survey area.
35. What is the reason why the cast-in-place slab on the beam or on the top of the wall is prone to cracks?
Answer: There are three main reasons:
(1) The surface of the beam or wall panel is subjected to tensile stress, and the designed negative reinforcement (horizontal reinforcement) is insufficient.
(2) The amount of reinforcement is sufficient, but the personnel who are pouring concrete during construction step on the negative reinforcement to the middle and lower part of the slab, which cannot play the role of resisting the tensile force on the slab.
(3) In some one-story houses built by individuals, the thickness of the cast-in-place slab is less than the design specification requirements or the concrete strength is too low.
