100+ Questions about Concrete

Concrete is one of the most important civil engineering materials in modern times. It is an artificial stone created through a process of proportioning, uniform mixing, compacting, and curing, using a combination of cementitious materials, granular aggregates, water, and, when necessary, additives and admixtures. The concrete is made by mixing these ingredients together in specific proportions to achieve the desired properties.

1. What is commercial concrete?

Answer: Commercial concrete refers to a mixture of cement, water, sand, stones, admixtures, and mineral admixtures that are proportionally mixed according to the requirements of the buyer, measured and mixed at a batching plant, and sold using a mixing transport vehicle to deliver to the buyer’s delivery location within a specified time.

2. What are considered general products?

Answer: Commercial concretes with a strength grade not exceeding C60, a slump not exceeding 180mm, and maximum stone size between 20-40mm, without any other special requirements.

3. What are considered special products?

Answer: Commercial concretes where any one indicator exceeds the scope of general product specifications or has special requirements. Examples include fine stone concrete, pile foundation concrete, impermeable concrete, anti-freeze concrete, etc.

4. What is the delivery location?

Answer: The delivery location is the point where the concrete is handed over between the supplier and the buyer, which is typically the construction site, as determined in the contract between the two parties.

5. What is factory inspection?

Answer: Factory inspection refers to the quality inspection carried out on commercial concrete before it leaves the factory. This includes sampling to test slump, making strength test blocks, and visually inspecting slump and workability.

6. What is delivery inspection?

Answer: Delivery inspection is the inspection of commercial concrete carried out at the delivery location by both the supplier and buyer, as well as supervision, which includes testing slump, making test blocks, and visually inspecting slump and workability.

7. What is cement? How many types of common cement are there?

Answer: Any hydraulic cementitious material made from a reasonable proportion of Portland cement clinker, blending materials, and an appropriate amount of gypsum ground into a fine powder is called cement. Commonly used cement includes Portland cement, ordinary Portland cement, and composite cement.

Portland cement is further divided into P·Ⅰ and P·Ⅱ types with strength grades of 52.5 and 62.5. Ordinary Portland cement is divided into ordinary and early strength types, with the code P·O, and strength grades of 42.5, 42.5R, 52.5, 52.5R. Composite cement has the code P·C with strength grades of 32.5, 32.5R, 42.5, 42.5R.

8. What are the items for cement re-inspection?

Answer: The items for cement re-inspection include setting time, soundness, strength, and fineness (specific surface area).

9. What is aggregate? How many types of aggregate are there?

Answer: In concrete, materials that provide the skeletal structure are called aggregate (also known as coarse aggregate). Stones are called coarse aggregate, and sand is called fine aggregate. Stones can be further divided into pebbles and crushed stone, and classified according to their lithology into limestone crushed stone, granite crushed stone, etc. Sand can be divided into river sand, sea sand, mountain sand, artificial manufactured sand, etc.

10. How many categories of sand are there based on its fineness modulus?

Answer: Sand can be divided into coarse sand (3.1-3.7), medium sand (2.3-3.0), and fine sand (1.6-2.2) based on its fineness modulus.

11. Why is medium sand preferred for pumping concrete?

Answer: Because medium sand has a reasonable gradation, with a particle content of below 0.3mm that can reach around 15%. These particles, in the mortar of the concrete, have very little frictional resistance against the pump tube wall, which is beneficial for pumping. Coarse sand has high frictional resistance, which is not conducive to pumping. Fine sand has a large surface area and requires more cement for the same strength, increasing the likelihood of cracking.

12. How many types of stone particle gradations are there?

Answer: There are two types: single-size and continuous gradation. Examples of single-size gradations include 10-20, 16-31.5, and 20-40. Examples of continuous gradations include 5-10, 5-16, 5-25, 5-31.5, and 5-40.

13. Why is continuous gradation stone preferred for pumping concrete?

Answer: Because, under the same maximum stone size conditions, the void ratio of single-size gradation is larger than that of continuous gradation, and these larger voids need to be filled with mortar. In the same conditions, for every cubic meter of concrete, the surface area of the aggregate is larger when using single-size gradation stones compared to continuous gradation stones, thus requiring more cement.

Additionally, concrete mixed with continuous gradation stones has better fluidity than that mixed with single-size gradation stones, which is beneficial for pumping construction. Therefore, continuous gradation stones are preferred.

14. What are the main quality requirements for good stones?

Answer: The main requirements are as follows:

1. The particle size meets the requirements, with a continuous gradation of particles;
2. There are no yellow patches (surface clay), mud balls, large mud clumps, or large particles;
3. The content of needle-like and flaky particles is as low as possible (the content of needle-like and flaky particles in pumped concrete should not exceed 10%);
4. The content of shale is as low as possible (because shale has lower strength and increases the content of flaky particles).

15. What are the quality requirements for good sand?

Answer: The requirements are as follows:

1. The particle size distribution meets the requirements for medium sand;
2. The particle size is medium sand, with few particles larger than 10mm, and at most no more than 10%;
3. There are no mud balls, large mud clumps, and the mud content does not exceed 3%;
4. There are no roots, grass roots, rotten mud, plastic bags or other debris.

16. What is fly ash?

Answer: Fly ash is a fine powder collected from the flue gas of coal-fired power plant furnaces by dust removal equipment.

17. How many grades are fly ash divided into?

Answer: There are three grades: Grade I, Grade II, and Grade III. Grade I is suitable for C40 and above concrete, Grade II is suitable for C15~C40 concrete, and Grade III is suitable as a blending material for cement.

18. Why is fly ash added to pumped concrete?

Answer: Because the microstructure of fly ash is spherical particles, which are finer than cement particles and have a certain activity. When mixed into cement, it can replace part of the cement and increase the fluidity of the concrete mixture, greatly improving the pumpability of pumped concrete while also improving the later strength and durability of the concrete.

19. What items should be inspected for fly ash entering the factory?

Answer: The fineness and water demand ratio of fly ash should be inspected, with the water demand ratio being the key item. If the fineness slightly exceeds the standard but the water demand ratio meets Grade II requirements, the ash can be used; otherwise, it should not be used.

20. What is concrete admixture?

Answer: Additives used to improve concrete performance are called concrete admixtures.

21. What is the difference between water reducer and pumping aid?

Answer: Water reducer is an admixture that reduces the amount of water used in concrete mixtures while maintaining the same workability, thereby increasing the strength of the concrete.

On the other hand, pumping aid is primarily composed of water reducer, with the addition of a small amount of plasticizing and slump-retaining components to facilitate concrete pumping during construction.

22. What are the different types of concrete admixtures?

Answer: There are water reducers, pumping aids, retarders, accelerators, early strength enhancers, anti-freezing agents, anti-freezing pumping aids, expanding agents, and waterproofing agents.

23. What should be tested when pumping aid is received at the plant?

Answer: The following tests must be conducted: water reduction rate, setting time, 1-hour slump change over time, and compressive strength ratio. Routine testing can be carried out using a C30 theoretical mix ratio for production purposes. Slump flow testing cannot be used as a substitute for concrete mix testing, as the results vary significantly and are not conducive to guiding production.

24. What does good pumping aid look like when tested?

Answer: When good pumping aid is tested in a mix ratio test, it has a water reduction rate of not less than 20%, and the concrete mixture has good workability, with the mortar wrapping around the stones and flowing easily, with a slump loss of less than 50mm over time.

Conversely, if some stones have no mortar on their surface, have poor flowability, and have a slump loss greater than 80mm, it indicates that the batch of pumping aid is of poor quality.

25. What types of water should be used for concrete mixing?

Answer: Freshwater, tap water, groundwater, potable water, and recycled water (partially used) should be used. Industrial wastewater, domestic wastewater, and seawater should not be used.

26. What does the word “concrete” mean and how is it pronounced?

Answer: It means artificial stone and represents concrete. The dictionary pronunciation is “tóng,” which can also be pronounced as “hùn níng tu.”

27. What is the purpose of expanding agent?

Answer: When expanding agent is added to cement, it reacts chemically with cement hydrates to form ettringite and other compounds, increasing the density of concrete while generating micro-expansion, reducing concrete shrinkage, and providing crack resistance and impermeability.

28. What are the strength grades of concrete?

Answer: There are 13 grades of concrete based on strength: C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C70, and C80. The grades are classified based on the strength of standard concrete test blocks after 28 days of standard curing, with each grade separated by 5MPa or 10MPa. “C” represents concrete, with the subscript number indicating the strength grade.

29. How are concretes classified based on performance?

Answer: They are classified as ordinary concrete, high-strength concrete (greater than C50), impermeable concrete, pumping concrete, anti-freezing concrete, frost-resistant concrete, mass concrete, non-vibration concrete (self-leveling concrete), dry hard concrete, and high-performance concrete.

30. What unit does MPa represent, and how is it converted?

Answer: MPa is a unit of strength: 1MPa = 10^6 Pa, 1Pa = 1N/m^2, and 1MPa = 1N/mm^2. 1MPa ≈ 10kg/cm^2.

31. What are the main properties of concrete mix that need to be tested?

Answer: The main properties of concrete mix that need to be tested are workability, slump, and air content. Workability is indicated by cohesion, water retention, and fluidity.

(1) Cohesion – Observed by eye, good cohesion is when the test specimen collapses evenly around the slump cone. Poor cohesion is when the specimen collapses on one side or some stones leak out, and the surface is not sticky with mortar.

(2) Water retention – When the concrete mix is placed in the slump cone and compacted, good water retention is indicated when there is little or no watery mortar at the bottom of the cone. If the concrete mix is poured onto an iron plate and there is water or watery cement mortar on the surface, it indicates poor water retention (i.e., segregation).

(3) Fluidity – Good fluidity is when the mixed material flows outward immediately after being piled up, while poor fluidity is the opposite.

32. What is concrete slump?

Answer: Concrete slump is measured by placing a slump cone on an iron plate, filling it with concrete mix and compacting it, then vertically lifting the slump cone. The concrete specimen collapses downward, and the distance between the top of the cone and the highest point of the specimen is measured as the slump. The slump of ordinary concrete is in the range of 30~90mm, while pumped concrete has a slump of 100~230mm. The higher the pumping height, the greater the slump, and vice versa.

33. What is the spread of concrete?

Answer: When the slump of concrete is greater than 220mm, the maximum and minimum diameters of the concrete mix after spreading are measured using a steel ruler, and the average value is taken as the spread. The greater the spread, the better the fluidity. Self-leveling concrete should measure its spread.

34. What does segregation of concrete mix look like?

Answer: If there is watery cement mortar flowing out from the edges of the concrete mix pile, and after a few minutes, sand and gravel sink to the bottom, forming a grabbing phenomenon, it is segregation of the concrete mix. Segregation is an indication of poor workability of the concrete and can easily cause pipe blockage during pumping, as well as reduce the strength of the concrete. The common cause of segregation is excessive admixture content, which can be reduced by 0.2% for retesting.

35. Why do stones not stick to mortar in concrete?

Answer: The appearance of some stones not sticky with mortar is due to less viscosity-increasing components added in the pumping agent. The dough-like appearance is due to incompatibility between the pumping agent and some components in the cement.

36. What is a standard test block?

Answer: A test block with sides measuring 150mm×150mm×150mm is a standard test block. A test block with sides measuring 100mm×100mm×100mm is a non-standard test block, and its compressive strength results need to be multiplied by a conversion factor of 0.95 to convert it into standard test block strength.

37. How should the quality of concrete test molds be inspected?

Answer: The length, width, and height of the test mold should be measured using a vernier caliper, with an allowable error of ≤0.5mm. The flatness of the four inner sides should be measured using a caliper and a 0.02-1mm stainless steel feeler gauge, with an allowable error of ≤0.05mm (standard method is to measure at 100mm length).

When inspecting a 150mm×150mm×150mm test mold, it is preferable to clamp along the diagonal length of the inner side and measure the flatness using a feeler gauge, with an allowable error of <0.13mm. This method is more precise than the standard method. Even if the standard method passes for a 150mm×150mm×150mm test mold, there is a possibility that the side flatness of the test block produced may be unqualified.

If a feeler gauge is not available, a razor blade can be used instead. The test mold is qualified if the blade cannot fit into the gap formed between the caliper and the inner side of the mold. The angles between adjacent sides should be measured using a standard angle ruler, with an allowable error of ≤0.3°.

Alternatively, a standard drawing triangle can be used to clamp tightly from the inside on one side of two adjacent sides, and a feeler gauge can be used to measure the gap between the triangle and the end of the other side, with allowable errors of <0.5mm (for 100mm×100mm×100mm) and <0.8mm (for 150mm×150mm×150mm).

It should be noted that most cast iron test molds currently available on the market for 150mm×150mm×150mm are concave on the sides, while most plastic test molds are convex on the sides. Therefore, it is necessary to carefully check the side flatness before purchase.

38. What constitutes standard curing?

Answer: Standard curing refers to placing the formed concrete test block in an environment with a temperature of 20±2℃ and humidity greater than 95% for curing.

39. What is meant by “same condition curing”?

Answer: Same condition curing refers to curing the formed concrete test block under the same curing conditions as the construction site components.

40. How many common curing ages are there for concrete?

Answer: The common curing ages for concrete are 3 days, 7 days, 14 days, 28 days, 60 days, and 90 days. However, the national standard for evaluating the qualification of concrete strength is based on the strength of standard test blocks cured for 28 days. When constructing mass concrete, it is possible to specify the use of 60-day or 90-day strength as the evaluation basis in the “Construction Organization Design”.

41. What is the compressive strength of concrete?

Answer: The compressive strength of concrete, which is obtained by standard methods of production, curing, and pressure testing, is the load that can be withstood per unit area, measured in MPa.

42. What is the flexural strength of concrete?

Answer: The strength obtained by breaking a 150mm x 150mm x 550mm test block on a bending fixture after standard curing is called the flexural strength, with the unit being MPa. The pavement of medium and high-grade highways requires the detection of the flexural strength of concrete.

43. What is the impermeability of concrete?

Answer: It refers to the ability of concrete to resist the penetration of pressure water into its interior. Basements and reservoirs require good impermeability.

44. How many impermeability grades are commonly used for impermeable concrete?

Answer: There are four commonly used impermeability grades: S6, S8, S10, S12 (old notation) or P6, P8, P10, P12 (new notation).

45. What is the frost resistance of concrete?

Answer: It refers to the ability of concrete to resist repeated freezing and thawing. The frost resistance grade is F100, F150, F200, F indicates frost resistance, and the number in the corner indicates the number of cycles of freezing and thawing of concrete from -15℃ to room temperature.

46. What is the difference between frost resistance and freeze 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 freeze resistance of concrete refers to the ability of concrete mixture to prevent low-temperature freezing damage during the setting and hardening process.

47. What is the durability of concrete?

Answer: It refers to the sustained ability of concrete to resist weathering, water infiltration, and erosion from harmful substances. The stronger this ability, the better the durability.

48. What is steam curing of concrete?

Answer: In order to accelerate the growth rate of concrete strength, steam curing can be adopted, which uses steam to heat the concrete and rapidly harden it under high temperature (70-90 ℃) and high humidity (about 90% or more) conditions.

49. What is permeable concrete?

Answer: Permeable concrete is a type of porous concrete, and commonly used permeable concrete is divided into permeable cement concrete and permeable organic binder concrete according to the bonding materials used.

Permeable organic binder concrete can be divided into permeable asphalt concrete, permeable polymer concrete, and permeable rubber concrete, etc;

According to the type of pavement construction, permeable cement concrete pavement is divided into ordinary permeable cement concrete pavement, colored permeable cement concrete pavement, exposed aggregate permeable cement concrete pavement, etc.

50. Why can’t the delivery of pumped concrete be too much at the beginning?

Answer: Because the situation on the construction site varies greatly, some may not have finished inspecting the steel bars, or they may not be ready. Even if there are no such problems, there is still the issue of whether the concrete can be pumped well. First send a truck, and then send another truck 15-20 minutes later.

When the first truck arrives at the construction site, first understand the situation on the site. If everything goes smoothly and the pumping is normal, you can continue to send trucks; if the construction site is not ready, you should not send any trucks; if the performance of the pumped concrete is poor and it is not easy to pump, you should immediately notify the director of the laboratory and adjust the proportion before sending the truck.

51. Why does the concrete need to rotate slowly after loading?

Answer: The drum is required to maintain a slow rotation of 2-4 revolutions per minute during transportation. If rotation stops, it can lead to segregation of the concrete, resulting in concrete clumping at the corners of the mixing blades inside the drum.

52. What inspections are required for concrete mixer trucks?

Answer: The driver must stop the vehicle and observe the slump of the mixture at the loading chute. If the slump is too small, they should adjust it to meet the customer’s requirements by adding an appropriate amount of pumping agent according to the “Usage Method for Adding Pumping Agent”. If the mixture is too loose, they should immediately return to the station for adjustments.

53. Why is it not allowed to add water to the concrete mix?

Answer: This is because the proportion of various materials in the concrete that leaves the factory has already been fixed. If water is added to the mixture because the slump is too small, it will increase the water-cement ratio. However, the strength of concrete is inversely proportional to the water-cement ratio, meaning that the higher the water-cement ratio, the lower the strength.

Therefore, adding water to the concrete is strictly prohibited. Adding an appropriate amount of pumping agent to dilute the concrete mixture is allowed according to the national standard “Ready-mixed Concrete” GB/T14902-2003.

54. Why is it necessary to rotate the mixing drum quickly before discharging?

Answer: Only rapid rotation of the mixing drum can achieve proper mixing. The company instructs drivers to rotate the mixing drum at medium to high speed for 20-30 seconds before discharging, to ensure that the concrete mixture is evenly mixed before unloading.

55. Why quickly rotate the mixing drum before unloading?

Answer: Since the production of concrete mixtures involves the hydration reaction between cement and water, if the pouring time is too long, the hydrates produced in the concrete will lose their coagulating effect and reduce the strength of the concrete. Therefore, standards stipulate that in summer, the time for discharging at the construction site should not exceed 1 hour, in spring and autumn it should not exceed 1.5 hours, and in winter it should not exceed 2 hours.

56. Why does concrete need to maintain a certain height?

Answer: When there is too little mixture in the pump hopper, air can easily be sucked in when the piston moves to extract the mixture, and when air is sucked in, it forms an air lock that prevents the mixture in the pipe from moving forward. Therefore, it is necessary to discharge material into the pump hopper continuously and evenly, consistent with the pumping speed.

Before starting the pump, the pump operator must check whether the mixture in the pump hopper exceeds 2/3 of the hopper height. If not, they should not start the pump.

57. What is the approximate setting time of ready-mixed concrete?

Answer: Generally, the initial setting time is 8-12 hours (4-6 hours when the temperature is high), and the final setting time is 12-16 hours (5-7 hours when the temperature is high). During autumn-winter or winter-spring transitions, the final setting time may be between 18-24 hours. If it exceeds this limit, it indicates concrete retardation, and the mixture ratio should be adjusted immediately or the admixture plant should be notified to reduce the retarding components.

58. What are the initial and final setting of concrete?

Answer: The initial setting of concrete refers to when the mortar in the concrete mixture begins to lose plasticity, which is difficult to determine. The final setting of concrete is when the mortar in the concrete mixture completely loses plasticity, meaning when pressure is applied with a finger on the surface of the mortar and no indentation is left.

59. What is concrete retardation and what causes it?

Answer: Concrete retardation occurs when the concrete mixture does not reach final setting even after 24 hours of pouring. This is caused by an excess of retarding components in the pumping agent or sometimes occurs after a sudden drop in temperature. When the temperature drops suddenly by 5-10℃, it will definitely prolong the setting time, leading to retardation.

60. What is mass concrete?

Answer: Mass concrete refers to concrete structures where the minimum solid dimension is ≥1m or concrete that is expected to cause excessive internal and external temperature differences due to hydration heat from cement (greater than 25℃), leading to cracking.

61. What precautions should be taken for mass concrete?

Answer:
(1) Use less cement while ensuring strength, and incorporate more high-quality fly ash or slag.

(2) Use retarding pumping agents to extend the final setting time of concrete, lower the peak temperature, and control the temperature difference between the center and surface of concrete members within 25℃. If this limit is exceeded, cover the surface with insulation materials for thermal preservation. Formwork should not be removed too early, and must only be removed when the temperature difference between the inner and outer surfaces of the member is less than 20℃.

When the length of mass concrete exceeds 50m, expansion agent is not used, and the average daily temperature during the construction period is 20℃ or above, the base must be level. If the base is thick and there are many rocks embedded in the base, a flexible sliding layer should be laid on the base to allow slight displacement of mass concrete during hardening and shrinkage, preventing cracks from forming in the center. Use compressive strength at 60d or 90d age as a basis for evaluating whether the concrete is acceptable.

62. Why pump water and cement mortar to wet the pipeline?

Answer: Because if the pipe wall is dry, it can easily absorb moisture from the mortar, drying it out and causing blockage. If water is used to lubricate the pipe and concrete is pumped directly, the cement paste in the concrete mixture can bond to the pipe wall, causing the mixture at the front of the pipe to become sandy and stony, increasing resistance in the pipe and leading to blockage.

63. Methods to prevent cracks in cast-in-place concrete slabs?

Answer: The best method is to pour a section of the board surface, use a vibrator to compact the concrete, use a wooden washboard to flatten the board surface, and then apply plastic film from front to back. Use a trowel to compact the film with the concrete at the joint or end, ensuring a strong bond and preventing it from being blown up by the wind.

If it is urgent to lay out the line during the final setting of concrete, a paper cutter should be used to cut off the parts that hinder the laying out along the benchmark, and not all should be lifted.

64. Why should heavy objects not be placed on concrete after it has just set?

Answer: Because concrete has just finished setting and its strength is very low. If heavy objects such as steel bars are lifted and placed on the board at this time, an impact force will be generated at the moment of placing them on the board. The combination of gravity and impact force will cause significant deformation and cracking of the board. Lifting heavy objects must be carried out 24 hours after the final setting of the concrete or when the strength of the concrete reaches 1.2 MPa or above.

If there is an urgent need to rush the construction period, the stiffness of the formwork support can be increased to reduce the deformation of the formwork during load-bearing. And when lifting heavy objects, try to place them on small compartments, and place them along the walls where there are shear walls.

65. What is winter construction?

Answer: The average temperature of the day is below 5 ℃ for five consecutive days, and winter construction begins when the lowest temperature is below 0 ℃.

66. What measures should we take during winter construction?

Answer: The following measures should be taken: (in cold areas) insulation and covering measures should be taken for water tanks, pools, and additive tanks; Steel sand and gravel silos should be enclosed and insulated; When the minimum temperature is below -5 ℃, transfer the remaining sand from the sand bin outside the bin at the end of production to prevent freezing.

Before loading every day, the frozen sand blocks should be shoveled aside and stacked, and then the unfrozen sand should be used. The admixture adopts an antifreeze pumping type. The temperature of the concrete mix produced shall not be lower than 6 ℃. The transport vehicle can be wrapped in cotton felt.

67. What is a post pouring strip?

Answer: When the length of the project exceeds 45m, in order to prevent structural cracks, a reserved strip of 1m wide is left on the middle beam and slab of the structure in advance without pouring concrete. It is poured after the main body is completed.

68. When should the post pouring strip be poured?

Answer: It should be carried out when the main body is completed or close to completion, and when the average ambient temperature is below 10 ℃.

Because the settlement and shrinkage that should occur after the completion of the main body are basically completed, the time when the temperature is below 10 ℃ is approximately from November to March of the following year, during which the concrete structure is in a state of shrinkage. At such an opportunity, it is not easy for the pouring strip to crack again after pouring. Otherwise, the result is the opposite.

69. What are the reasons why cast-in-place slabs are prone to cracking?

Answer: There are three main reasons:

•  The surface of beams or wall panels is subjected to tensile stress, and the designed amount of negative reinforcement (octagonal reinforcement) is insufficient.
• The amount of reinforcement is sufficient, but during construction, the personnel pouring concrete step on the negative reinforcement to the middle and lower parts of the board, which cannot resist the tensile force on the board.
• The thickness of the cast-in-place slab is less than the design specifications or the concrete strength is too low.

70. Why are pumping pipelines prone to blockage？

Answer: When the depth of the deep foundation pit exceeds 5m, the concrete mixture in the inclined pipe section from the ground to the bottom of the pit is subjected to a significant downward force, causing the mixture to move downwards, and the gas in it rises to the turning part, forming an air blockage.

A 180 ° bend can be installed a few meters below the inclined pipe section to solve the problem of pipe blockage; An exhaust valve can also be installed at the upper end of the lower bend to solve the problem of pipe blockage.

71. What is the durability of concrete?

Answer: In addition to having appropriate strength, concrete should also meet special requirements for use, including resistance to freezing, water penetration, chloride ion penetration, shrinkage, carbonation, steel corrosion, sulfate resistance, compressive fatigue deformation, alkali aggregate reaction, etc., collectively referred to as durability.

72. What is the corrosion resistance of concrete?

Answer: Corrosion resistance refers to the ability of concrete to resist erosion in various corrosive liquids and gases. The media that corrode concrete mainly include sulfate solution, acidic water, active and/or pressurized soft water, seawater, and concentrated alkaline solutions.

73. What is the heat resistance of concrete?

Answer: Heat resistance refers to the performance of concrete that, under high temperature, its internal structure is not damaged, its strength is not significantly lost, and it has a certain degree of chemical stability.

74. What is natural curing of concrete?

Answer: Natural curing refers to the curing of concrete under natural conditions (temperature not less than+5 ℃, humidity 90-100%).

75. Why does concrete have requirements for water quality?

Answer: Industrial wastewater containing fats, vegetable oils, sugars, acids, etc. cannot be used for mixing concrete. Because these impurities in water can reduce the bonding strength of cement and decrease the strength of concrete, mineral water containing a large amount of salts cannot be used, which makes cement unable to resist water erosion well.

76. What is rebound detection?

Answer: Use a rebound hammer to impact the surface of a concrete structure. After the impact, the hammer inside the instrument bounces back and drives the indicator slider to move. Read the rebound value based on the scale line on the indicator slider.

At each location (measurement area), 16 points of rebound were hit, with 3 maximum and 3 minimum values removed. The average of the remaining 10 points was taken as the rebound value of the measurement area, and the carbonization depth of the structure was measured; Use the rebound value and carbonation depth value of the measurement area to refer to the “Concrete Strength Conversion Table” and determine the strength value of the concrete in the measurement area.

77. What is carbonization depth?

Answer: The process in which calcium hydroxide in the surface layer of concrete reacts with carbon dioxide in the air to form calcium carbonate is called carbonation. The depth of carbonation is achieved by drilling a small hole on the surface of the concrete, spraying alcohol phenolphthalein test solution, and turning the uncarbonized material red while the carbonized material does not change color. The depth of carbonation is measured using a carbonation depth gauge.

78. What is drilling core detection?

Answer: Use a concrete core drilling machine (with a hollow drill bit with inner diameters of 100mm, 89mm, and 75mm) to drill core samples on the component, cut and grind the compressed surface, and press the failure load on the pressure testing machine to calculate the concrete strength.

79. When is drilling core detection used?

Answer: When rebound method is not suitable for testing, drilling core method can be used for testing.

80. What is early strength concrete?

Answer: Ordinary concrete reaches about 70% of the design strength in 7 days at room temperature, and reaches 100% of the design strength in 28 days. Due to construction progress or formwork turnover needs, measures are taken to make the concrete reach the design strength in about 15 days at room temperature, which is called early strength concrete.

81. What is ultra early strength concrete?

Answer: Concrete that can reach its design strength in about 7 days at room temperature is called ultra early concrete.

82. How to prepare ultra early strength concrete?

Answer: Generally, ultra early strength pumping agents can be used for preparation, or two concrete strength grades can be increased or PO42.5R cement can be used for preparation.

83. How to select aggregates？

Answer: Coarse aggregate should mainly control its particle size, gradation, particle shape, stone powder content, and mud content. Each vehicle shall undergo a macroscopic inspection and shall not be unloaded if it is found to be unqualified; In addition, various indicators should be inspected in batches according to regulatory requirements.

Fine aggregates should control the fineness modulus, mud content, and mud block content. Perform a macroscopic inspection on each vehicle, and do not unload if it is found to be unqualified; Batch inspection should also be carried out according to regulatory requirements.

84. Why control the particle size of aggregates between 5-25mm?

Answer: The particle size of coarse aggregate is limited by the diameter of the concrete pumping pipeline and the pumping height. Generally, the maximum particle size that can be pumped decreases with the increase of pumping height.

When the pumping height is less than 50m, the ratio of the maximum particle size of coarse aggregate to the transported diameter is ≤ 1:3; When the pumping height is 100m, the ratio decreases to 1:5, otherwise it is easy to block the pipe.

85. Why control the needle like content of coarse aggregate?

Answer: When its content is high, the flexural strength of needle shaped coarse aggregate is relatively low, and the bonding strength between coarse aggregates decreases, resulting in a decrease in concrete strength.

For ready-mixed concrete, a high content of needle shaped aggregates can lead to poor particle shape of coarse aggregates, resulting in a decrease in the fluidity of the concrete. At the same time, needle shaped aggregates are easily blocked at the pipeline, causing pump blockage and even pipe burst. Therefore, pumping concrete requires a needle content of ≤ 10%, while high-strength concrete requires even higher requirements.

86. What kind of sand is needed for ready mixed concrete?

Answer: Pre mixed concrete requires medium sand. In addition to complying with the specifications for sand grading, mud content, and mud block content, it is also important to ensure that the sand passing through the 0.315mm sieve is not less than 15%.

This has a significant impact on the pumpability of concrete. If the value is too low, it can easily block the pump and result in poor water retention and easy bleeding of the concrete.

87. What kind of sand is required for ready mixed concrete?

Answer: Pre mixed concrete requires medium sand. In addition to complying with the specifications for sand grading, mud content, and mud block content, it is also important to ensure that the sand passing through the 0.315mm sieve is not less than 15%. This has a significant impact on the pumpability of concrete. If the value is too low, it can easily block the pump and result in poor water retention and easy bleeding of the concrete.

88. What impact will fine sand bring?

Answer: If the sand is too fine, the water demand for concrete increases, and the pumpability and plasticity of concrete prepared with fine sand are extremely poor, resulting in a decrease in concrete strength and susceptibility to cracking.

89. What impact does the sand content increase?

Answer: Sand has a high mud content, which results in a high water demand for concrete, poor plasticity, increased shrinkage, decreased concrete strength, and easy structural cracking. Therefore, it is necessary to control the sand mud content to be ≤ 3% (C30-C50), and the requirements for the mud content of high-strength concrete are higher.

90. What impact will mud blocks have on concrete?

Answer: Mud blocks in sand and gravel not only have the same impact as mud content, but also seriously affect the strength of concrete. For example, mud blocks can weaken the concrete cross-section; When pouring the ground, the mud blocks will float up and form pits and other defects on the surface after drying and shrinking.

91. Why should high-strength concrete use small-sized stones?

Answer: As the particle size of coarse aggregate increases, its bonding with cement slurry weakens, increasing the discontinuity of the internal structure of concrete materials and leading to a decrease in concrete strength. Coarse aggregates play a restraining role in cement shrinkage in concrete.

92. Pebble concrete strength lower than crushed stone concrete?

Answer: The surface roughness of coarse aggregate is due to the interface strength between cement slurry and aggregate.
On the one hand, concrete prepared with pebbles has a lower crushing index than crushed stones due to its high content of weathered stones, and its surface is smooth with low interface strength. Therefore, the strength of concrete prepared with pebbles will be 3-4 MPa lower than that of crushed stone concrete with the same ratio.

93. What is alkali aggregate reaction?

Answer: The alkali in the concrete reacts with the aggregate whose chemical composition is active silicon dioxide. After the alkali silicic acid gel is formed, it absorbs water and expands. The expansion stress makes the concrete crack. This process is called alkali aggregate reaction.

94. How to prevent alkali aggregate reaction?

Answer: If the local coarse aggregate contains active silica, the alkali content of concrete admixtures should be strictly limited, and the total alkali content of various materials in concrete should be ≤ 3% of the concrete mass.

95. Advantages and disadvantages of reinforced concrete structures?

Answer:

Advantages: Reasonable material utilization; Good modularity; Good durability and fire resistance, low maintenance costs; The overall integrity of cast-in-place concrete structures is good, and through appropriate reinforcement, good ductility, vibration resistance, and radiation resistance can be achieved; High stiffness and damping are beneficial for controlling structural deformation; Easy to obtain materials locally.

Disadvantage: Self significant; Poor crack resistance; Limited bearing capacity; The construction is complex, with multiple processes and a long construction period, and the construction is greatly affected by seasons and weather; Once a concrete structure is damaged, its repair, reinforcement, and reinforcement are relatively difficult.

96. What are the factors that affect the strength of concrete?

Answer: Cement strength grade and water cement ratio. The higher the cement strength grade, the higher the concrete strength; On the premise of ensuring dense molding, the lower the water cement ratio, the higher the strength.

Aggregate variety, particle size, gradation, impurities, etc. When using larger particle size, well graded and clean crushed stones and sand, the water cement ratio can be reduced, the interfacial bonding strength can be improved, and therefore the strength of concrete is high.

Curing temperature and humidity. The influence of temperature and humidity on the strength of concrete is achieved by affecting the hydration, setting, and hardening of cement. When the temperature is suitable and the humidity is high, the strength development is fast. Conversely, it is not conducive to the growth of concrete strength.

Age. The longer the curing time, the more thorough the hydration, the smaller the porosity, and the higher the strength of the concrete.

Construction methods. Mainly referring to the mixing and vibration molding process. The strength of concrete is higher when mechanical mixing and compaction have a strong effect.

97. What requirements should be considered when preparing concrete?

Answer: The preparation of concrete should consider the following four basic requirements:

1. Meet the strength level requirements of structural design;
2. Meet the workability requirements for concrete construction;
3. Meet the requirements of the engineering environment for concrete durability;
4. In line with economic principles, that is, saving cement to reduce the cost of concrete.

98. What should be noted when using air entraining agents?

Answer: To control the air intake, it is generally advisable to use 2% -4% (volume content), otherwise it may cause a decrease in concrete strength.

Do not choose inferior air entraining agents with poor quality, large bubble diameter and spacing, such as sodium dodecyl sulfonate, wood calcium, etc.

99. Can concrete mixed with fly ash extend the acceptance age?

Answer: Yes, as the addition of fly ash contributes to the later strength of concrete, it is recommended to take 28 days for above ground engineering; Ground engineering should take 28 or 60 days; Underground engineering should take 60 or 90 days; Large volume concrete should take 60 or 180 days.

100. Under what conditions is silica fume used?

Answer: Silicon ash is mainly used to prepare high-strength concrete, sulfate resistant concrete, chloride corrosion resistant concrete, underwater concrete, deicing salt concrete, and high wear-resistant pavement concrete.

101. When is zeolite powder used?

Answer: Zeolite powder can be used as an active admixture for cement. Zeolite powder can replace some of the cement in concrete, improve the water retention of concrete, especially when preparing high flow and ultra-high strength concrete with polycarboxylate high-efficiency water reducing agent. Adding zeolite powder in an appropriate amount can effectively improve the water retention of concrete and prevent concrete bleeding.

When preparing lightweight aggregate concrete, the addition of it will increase the structural viscosity of the cement slurry, which can greatly improve the problem of floating of lightweight aggregate during vibration molding. Zeolite powder concrete is also suitable for underwater concrete and concrete cured in humid underground environments, with good frost resistance and impermeability. When using zeolite powder to prepare high-performance concrete, it is better to replace 10% of cement.

102. How to prevent concrete deficiency?

Answer: Production controllers should pay attention to changes in water usage during the production process. When the water usage is 10kg or more less less than the set water usage for the production mix, the less water should be added to the sand.

The dispatcher should randomly check the factory apparent density (bulk density) of concrete for every construction site that produces 40m3 or more. If it differs from the design value by more than ± 10kg/m3, the control room should be notified to investigate the cause. It may also be due to inaccurate batching scales, which need to be verified immediately.

103. What should we do if there is a shortage of concrete?

Answer: Immediately notify the production manager and laboratory director to investigate the cause.

1. Check if there is a significant negative deviation in the production records of the microcomputer, and if there is only water reduction without sand addition;
2. Go to the pouring site to check if there is any phenomenon of mold expansion, excessive thickness of the cast-in-place slab, or scattering and waste;
3. We need to carefully calculate whether there are any errors in the planned quantity.

104. Is there a sequence for pouring concrete?

Answer: When pouring concrete, it is necessary to determine the pouring sequence according to the construction drawings, pouring from low to high, to ensure the uniformity and density of the concrete. It is also necessary to control the pouring speed to avoid the slump of the concrete not meeting the requirements due to being too fast or too slow, which will affect the quality of the concrete.

When pouring concrete, it is necessary to control the thickness of each layer of concrete to avoid the uniformity and density of the concrete not meeting the requirements due to excessive or insufficient thickness.

When pouring concrete, it is necessary to control the water cement ratio to ensure the quality of the concrete.

105. What is aerated concrete?

Answer： Aerated concrete is a type of porous silicate concrete, and its various physical and mechanical properties depend on the concrete structure after steam curing, including the composition of pore structure and pore walls.

Like ordinary silicate concrete, the composition of the pore walls in aerated concrete is determined by the types and quantities of hydration products generated during the hydrothermal treatment of calcium and silicon materials, and is also the reason why aerated concrete has certain physical and mechanical properties.

The pore structure of aerated concrete not only has the microporous structure similar to that of ordinary silicate concrete, but also the pores formed by aluminum powder, which have a great impact on the physical and mechanical properties of aerated concrete.