Sixty questions about concrete

1.What should we do when the raw materials cannot be used immediately and the supply of concrete cannot be interrupted?

While contacting the raw material suppliers, we must assign someone to control the amount of water used for mixing and monitor the on-site concrete vulcanization to prevent the site from adding water itself.

If necessary, control the water to glue ratio unchanged and increase the amount of cement to increase the fluidity of concrete.

Generally, for every 1cm increase in collapse, 1.5% to 2.5% of cement slurry volume should be added for every 1m3 of concrete.

2.What kind of fly ash can be used in ready-mixed concrete?

According to its fineness, water demand ratio and carbon content, fly ash is divided into Ⅰ, Ⅱ, Ⅲ, Ⅰ, Ⅱ fly ash can be used in ready-mixed concrete.

3.What are the conditions under which fly ash should not be used?

Winter construction, especially in -10 ℃ and below the temperature construction, due to the fly ash concrete early strength is low, to reach the critical strength of antifreeze need to be in the positive temperature pre-curing time, site insulation is poor, it is easy to freeze, should not be mixed with fly ash.

In addition, with the hitting of light concrete ground, there are de-icing salt requirements of concrete shall not be mixed with fly ash.

4.Is it possible to extend the acceptance period of concrete mixed with fly ash?

Yes, since the incorporation of fly ash contributes to the later strength of concrete, China’s “Technical Specification for the Application of Fly Ash Concrete” (GB J146) specifies the age of the design strength grade of fly ash concrete.

28 days for aboveground projects; 28 days or 60 days for ground projects; 60 days or 90 days for underground projects; 90 days or 180 days for mass concrete.

5.Under what conditions should silica fume be used?

Silica fume is mainly used for the preparation of high-strength concrete, anti-sulfate concrete, anti-chlorine salt corrosion concrete, underwater concrete, de-icing salt concrete, high wear-resistant road concrete, China’s more used when Aiken silica fume, the price is about 3000 yuan / ton.

6.What is the case of zeolite powder?

Zeolite powder can be used as an active admixture of cement.

Zeolite powder can replace part of the cement in concrete, improve the water retention of concrete, especially in the polycarboxylic acid efficient water-reducing agent to engage in the preparation of large-flow, ultra-high-strength concrete, the appropriate amount of zeolite powder mixed in, can effectively improve the water retention of concrete, to prevent concrete secretion.

The preparation of light aggregate concrete, as its incorporation will improve the structural viscosity of the cement paste, can make light aggregate in the vibratory vibration in the formation of the floating problems improved greatly.

Zeolite powder concrete is also suitable for underwater concrete and underground wet environment maintenance of concrete, frost resistance, good impermeability.

The use of zeolite powder preparation of high-performance concrete, instead of cement amount of 10% is better.

7.sometimes with naphthalene efficient water-reducing agent production of concrete demolition, concrete surface bubbles especially after demolition, what is the reason? What should we do?

Some admixture manufacturers purchase cheap crude nano (normal nano appearance color is white and yellow, impurity nano is pink and red brick color), mixed nano.

(All purchases from this WMS are given a bonus of 1-5% of the commodity price)

These chemically impure Nene, high phenol content and low purity, will bring many bubbles in the high efficiency water reducing agent (inferior Nene will bring 3% bubbles in the concrete), and the bubbles are larger and take longer to defoam, which directly affects the strength of the concrete.

At this point, you should promptly contact the admixture plant to suspend the use of additives to prevent the impact on the strength of concrete.

8.How to detect water-reducing agent gas content?

In addition to gas content meter to determine the gas content of concrete to infer the amount of air-entraining admixture, you can also use the following methods to test.

Take 100ml of color tube with plugs, add 4 grams of powder sample, add water to 40ml, use a steel ruler to write down the vertical height of the solution (h0), hold the color tube plug tightly, shake vigorously for 20 times, immediately use a stopwatch to time, while measuring the height of the foam tip (h1) to the liquid foam disappeared, and when the liquid surface is exposed, write down the time (s).

Control indicators: foam height ≤ 45mm, defoaming time ≤ 50s.

9.the use of air-entraining agent to pay attention to what?

First, it is necessary to control the amount of air-entraining generally 2% -4% (volume content) is appropriate, otherwise, it will result in a decrease in concrete strength.

In addition, not to choose poor quality, bubble diameter and spacing of poor quality air-entraining agent, such as sodium dodecyl sulfonate, calcium wood, etc..

Mixing slightly will cause a significant reduction in concrete strength. Before using the test to determine the use of that species, mixed with a small amount of more.

Sixty questions about concrete

10.concrete in the field waiting time is too long, what will affect its quality?

Concrete is mixed with a retarder, but its retardation time is limited, more than 2h after the cement begins to hydrate, the collapse also begins to decline, the strength of the concrete will lose part.

Some enterprises have done this test, the test results prove that the site waiting time of more than 2h, the strength of concrete began to decline.

Therefore, the waiting time for concrete at the site should not exceed 2h.

11.What are the consequences of adding water to concrete trucks?

The normal water to glue ratio of concrete to ensure that it has a certain degree of fluidity and strength, in addition, the addition of water will reduce the strength of concrete, (the evaporation of excess water to form voids, weakening the concrete section), according to some concrete enterprises test data show that every 10 kg of water in concrete, the strength of the 28-day drop of 3.7Mpa.

Therefore, we should strictly control the construction site shall not add water to the transport truck.

12.For various reasons, the collapse of concrete at the site is very small and difficult to pump how to do?

Only the secondary vulcanization of concrete vulcanizing agent can be used.

Most of the current concrete using naphthalene pumping agent, the vulcanizing agent is a highly efficient water-reducing agent, liquid agent, powder can be.

13. how to control the amount of vulcanizing agent?

Depending on the size of the concrete in the truck collapse to adjust the amount of curing agent.

When using powder, generally 0.5 kg / cube – 1 kg / cubic.

The liquid is 1kg/cubic-2kg/cubic; after adding, the tank can be vulcanized in about 1-2 minutes.

14.For various unforeseen reasons, the concrete slump may drop after the first vulcanization if the concrete is not pumped in time, what should I do?

Through many tests have proved that concrete can be repeatedly vulcanized, as long as the water is not added, stay at room temperature no more than 2h, generally its strength will not decrease.

15.Why not use pumping agent as a vulcanizing agent?

Because the pumping agent will have a slow-setting, air-entraining agent components, mixed into the concrete may cause the concrete slow-setting or strength reduction. much time after the initial concrete pouring, the final set? How can you tell?

(a) The initial setting time of concrete at room temperature is 6-8 hours; the initial setting time is when the hand presses lightly on the surface of the concrete, without sticking to the hand, and the concrete surface collects water and has a shiny film.

When the color of the concrete surface turns white, no seal by hand, that is, the final setting, the final setting time is about 8-10 hours, depending on the temperature in summer and winter, the initial final setting time will be shortened or extended.

17.What is early-strength concrete?

Ordinary concrete at room temperature for 7 days to reach about 70% of the design strength, 28 days to reach 100% of the design strength, due to the construction progress or the need for formwork turnover, to take measures to make the concrete at room temperature about 15 days to reach the design strength, that is, early-strength concrete.

18.What is super-early-strength concrete?

At room temperature, the concrete can reach the design strength of the concrete about 7 days is called super early concrete. to prepare ultra-early concrete?

Generally can be used to prepare ultra-early-strength pumping agent, can also be used to improve the strength of the two concrete strength level method or the use of PO42.5R cement to prepare.

20.large volume concrete ratio design should pay attention to what?

Bulk concrete is designed to prevent structural cracks caused by a temperature difference between inside and outside of it exceeding 25°C, so the following points should be taken into account when designing the proportion.

Control the amount of cement and use cement with lower heat of hydration and slower setting time. For example, dam cement, slag Portland cement, fly ash Portland cement.

It is advisable to mix with fly ash and finely ground mineral powder to reduce the heat of hydration of concrete.

Slow-setting water reducer or efficient water reducer should be used.

In order to reduce the temperature of concrete into the mold, can be used to meet the standards of groundwater mixing water, if necessary, can be mixed with part of the ice.

21. what is called concrete adiabatic temperature rise?

The cement in the concrete in the process of hydration will release heat, the heat release and the use of cement varieties, cement dosage, assuming that the concrete is in the upper and lower left and right can not emit heat in the adiabatic state, with the cement hydration, the temperature in the concrete will continue to rise, when rising to the highest, the highest adiabatic temperature rise.

Sixty questions about concrete

21.How to calculate the adiabatic temperature rise?

The adiabatic temperature rise of cement hydration is calculated by the following formula.

Tmax = mq / cρ

22.In the formula:Tmax- cement hydration adiabatic temperature rise (℃)m- cement consumption per cubic concrete (kg), q- cement hydration heat, c- concrete specific heat (0.96j/K), ρ-concrete Density (2400 kg/m3).

23.What is the heat of hydration of cement?

Heat of hydration of various cements can be taken from the table.

Heat of hydration per kg cement Q (Kj/Kg)

24.What is the law of temperature rise of bulk concrete?

Generally 2-3 days after the pouring of large-volume concrete to heat peak.

Such as: a project slab 2 m thick, using 360 kg / cubic 42.5 grade cement preparation, construction of the summer, into the mold temperature of 32 ℃, the central temperature can be as high as 80 ℃. The central temperature can be as high as 80 ℃.

4 days later, most of the cement hydration heat energy has been consumed, the internal temperature gradually fell, this temperature may extend for more than ten days to more than thirty days, some hydraulic mass concrete hydration heat dissipation process may also be much longer.

Mastered this law, the focus of temperature control of large concrete should be placed within a week after pouring, especially the first 2-3 days.

25.What is the role of heavily mixed fly ash in suppressing the peak concrete temperature?

Fly ash blending helps reduce the peak temperature of mass concrete and postpones the time of peak temperature.

When fly ash is mixed with 10%-50% (the percentage of the quality of the cement material), the peak hydration will be reduced by 5.9%-35.1% in 3 days, and the peak temperature will be postponed by 0.5-3.2 hours.

26.coarse and fine aggregates into the field to pay attention to what matters?

Coarse aggregate should mainly control its particle size, grading, particle shape, stone powder content, clay content.

Macroscopic inspection of each car, unloading is not allowed if unqualified; in addition, the indexes should be tested according to the specifications and by batch.

Fine aggregates should control the fineness modulus, mud content and mud content. Macroscopic inspection of each car, unloading unqualified cars; batch test should also be required by the same specifications.

27.why should the coarse aggregate particle size control in the 5-25mm?

Coarse aggregate size by the concrete pumping pipe diameter and pumping height constraints, generally the maximum size of the pumping can be reduced with the increase in pumping height.

Such as pumping height <50m, the maximum size of coarse aggregate and transport diameter ratio of ≤ 1:3; and pumping height of 100m, the ratio will be reduced to 1:5, or easy to block the pipe.

28.why should the pumping of concrete to control the needle-like content of coarse aggregate?

The content of high, needle-like coarse aggregate flexural strength is relatively low, and coarse aggregate bonding strength decreased, thus resulting in the decline in concrete strength.

For ready-mixed concrete, high needle-like content, will make the coarse aggregate particle shape is not good, thus decreasing the fluidity of concrete, and at the same time needle-like aggregate is easy to clog in the pipeline, resulting in plugging the pump, or even burst pipe.

Therefore, pumped concrete requires its needle content ≤ 10%, high strength concrete requirements are higher.

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

Ready-mixed concrete should be used in sand, in addition to the sand grading, mud content and clay content according to the specifications, but also pay attention to the 0.315mm sieve hole through the sand not less than 15%.

This has a great influence on the pumpability of concrete, and it is easy to block the pump if the value is too low, and the concrete is poor in water retention and easy to secrete water.

30.What will be the effect of fine sand?

Sand is too fine, concrete water demand rises, and the preparation of concrete with fine sand, its pumpability, plasticity are extremely poor, concrete strength will be reduced, easy to crack.

31.only fine sand how to do?

Such as sand source problems, available fine sand plus part of the mechanism of sand preparation pumping concrete, such as the available fineness modulus less than 2.0 of fine sand mixed with fineness modulus 3.0-3.2 of the mechanism of sand, about 6:4 ratio, observe its liquidity, pumpability, specific test to determine the ratio.

32.What are the consequences of a sand content conference?

Sand contains large amounts of mud, concrete water demand, poor plasticity, increased shrinkage, decreased concrete strength, the structure is easy to crack, the structure is easy to crack, so to control sand mud content ≤ 3% (C30-C50), high strength concrete mud content requirements are higher.

33.What is the effect of mud in sand and gravel on concrete?

In addition to having the same effect as mud in sand and gravel, mud lumps can also seriously affect the strength of concrete. For example, clods can weaken the concrete section.

When pouring the ground when the mud block floats, dry shrinkage will form pits and other defects on the surface.

34.why should the preparation of high-strength concrete should use smaller grains of stone?

As the coarse aggregate particle size increases, its bond with the cement slurry weakens, increasing the discontinuity of the internal structure of the concrete material, leading to a decrease in concrete strength.

The coarse aggregate plays a restraining role on the shrinkage of cement in concrete.

Due to the different elastic modulus of the coarse aggregate and cement slurry, it produces tensile stress within the concrete.

This internal stress increases with the increasing size of the coarse aggregate and leads to a decrease in the strength of the concrete.

As the coarse aggregate size increases, the directional arrangement of Ca (OH)2 crystals in the transition zone of the coarse aggregate interface increases, and the interface structure is weakened, thus reducing the strength of the concrete.

Tests show that.

The width of interfacial cracks around 15-25mm coarse aggregate in concrete is about 0.1mm, and the length of the cracks is 2/3 of the circumference of the grain size, and more interfacial cracks are connected with the cracks in the surrounding cement slurry: while in 5-10mm coarse aggregate concrete, the width of the interfacial cracks is more uniform, only 0.03mm, and the length of the cracks is only 1/6 of the circumference of the grain size.

Different sizes of coarse aggregate, concrete hardening in the lower part of the size of the formation of the water vesicle accumulation is also different, the lower part of the coarse aggregate large size and more water vesicles, water evaporation of water in the vesicles, the interface under the formation of interface joints must be wider than the small size, the interface strength is low.

35.Why is the same proportion of concrete, pebble concrete strength than crushed concrete 3-4MPa lower?

The coarse surface of the coarse aggregate is due to the interfacial strength of the cement paste and the aggregate.

According to the author’s experiments over the years, the concrete made of pebbles, on the one hand, contains more weathered stones, and its crushing index is lower than that of crushed stones, and on the other hand, the surface is smooth and the interfacial strength is low, so the strength of the concrete made of pebbles will be 3-4 MPa lower than that of the concrete made of crushed stones.

36.Why should the water content of sand and gravel be measured in every shift?

Sand and gravel in ready-mixed concrete each about 800-1100kg/m3 dosage, and every 1% of its water content will bring the impact of 8-11kg of water in concrete.

Especially sand, which is usually collected from the river, has a large variation of water content. If the water content is not checked regularly and the water consumption is not adjusted in time, the slump, pumpability and strength of the concrete will fluctuate greatly.

37.What is alkali aggregate reaction?

Alkali in concrete and the chemical composition of the active silica aggregate chemical reaction, resulting in alkali – silicate gel after water absorption and expansion, expansion stress to crack the concrete, this process is called alkali aggregate reaction.

38. how to prevent the reaction of alkali aggregates?

If the local aggregate contains active silica, it is necessary to strictly limit the alkali content of the concrete admixture, such as “Design Code for Concrete Structures” (GB50010-2002) stipulates that when using alkali-activated aggregates, the total amount of alkali in the concrete of all kinds of materials ≦ 3% of the concrete quality.

39.What items should be tested at the entrance of cement?

Every 500T of cement should be randomly sampled and tested for compressive resistance, flexural resistance (3d, 28d) standard consistency water volume, initial and final setting time, and stability.

40.What should be done before signing the contract?

Before signing the contract, the project manager should make a full investigation of the customer’s capital, credit, project profile, structure, construction area, concrete type, concrete quantity, construction unit, construction unit, etc.), and report to the relevant leaders if the investigation is lined. The general manager, sales manager, production manager, chief engineer meeting, to prevent the contract signed inadvertently, bring undue losses to the enterprise.

41.What are the main contents of the contract meeting review?

First of all, to see if the customer has funds, the past cooperation with the concrete enterprise is to keep the reputation, the implementation of the contract is in place. Enterprises should have a list of bad customers, the payment defaults, do not keep the contract customers, try not to sign the contract.

Secondly, we should look at the product unit price and payment methods, excessively reduce the unit price, not only low profits, difficult operation, but also will cause the operation of a vicious cycle. Generally according to the progress of the project (i.e., according to the layer) payment, the project should have binding terms before topping out, to prevent the concrete body topping out, the payment is difficult to receive.

Once again, it is necessary to review the user’s technical requirements, construction requirements companies have the ability to do, to prevent the start of the project is passive.

There is also an important question is whether the user is a legal entity, such as the contract chapter is x x project manager, such as future disputes, the prosecution is invalid, some companies have lost millions of dollars in payment.

42.What kinds of settlement methods are there for concrete?

The first one is to settle according to the actual amount of concrete supplied, referred to as car knot, customers can check if the amount of concrete is enough at any time, and if one is missing, ten will be punished, and customers can raise objections within 24 hours after the concrete is poured, and both parties will find out the reason and solve the problem in time. This settlement method is simple and easy, which is the preferred settlement method.

The second one is settlement according to the drawing, which is called “drawing” (hereinafter collectively called “drawing”) in the project and requires the construction unit to provide the drawing (jie Shi Diagram, Jian Shi Diagram) and design change in time, and both A and B will calculate the project volume in time.

With the construction progress and timely check the amount of concrete. Once the amount of difference exceeds 2%, to immediately find the cause and be resolved. To prevent the completion of the project and even wipe the dust after the account, at this time, all the concealed project is covered up, concrete super thick, super wide and other conditions can not be measured.

43.How to estimate the amount of concrete for the project?

The total amount of the project should be estimated before signing the contract to prevent the advance from exceeding or approaching the total amount of the project and causing difficulty in getting back the money.

The author collected statistics on the amount of concrete needed for various structures from a budgeting firm and included them in Table 1 for your reference.

44. Which works or parts are not suitable for drawing knots?

The brick and concrete structure generally uses only concrete for beam slab, staircase, circle beam, over beam and structure column; the villa project has small residence cross-section, and it is easy to scatter material and waste concrete more seriously during the pouring process; these two types of projects are not suitable for drawing knots.

In addition, underground projects such as bore piles, bedding, foundation slab, modeling project and basement are also not suitable.

45.Why do we need to add loss in the map junction project?

In some areas of China (such as Liaoning Province), the quota stipulates that there will be some material loss in the transportation and pumping of ready-mixed concrete, such as the concrete bonded to the inside of the tank, pumping truck pumping end of the pumping bucket, pumping pipe, there is a part of the remaining concrete to be discharged, which is about 2% of the total.

In addition, concrete into the mold, after tamping, its volume will be smaller than the original, after tamping volume divided by the loose volume known as the filling factor, this factor is generally 0.95.

In view of the above reasons, there should be about 2% loss in the project.

Sixty questions about concrete

46.How should ready-mixed concrete enterprises prevent the phenomenon of insufficient supply?

Ready-mixed concrete production enterprises should be annually verified by the statutory metrological department of the electronic scale of the mixing plant to a certain extent, the enterprise also monthly electronic scale of the mixing plant self-calibration.

The measurement error of sand and gravel should be ≤ ± 3%, admixture, cement, blending materials, water measurement error should be ≤ ± 2%.

If it exceeds the above requirements, we should stop production in time to find the reason and solve it.

In addition, the production enterprises should be weekly spot-checked self-weight concrete, when the weight of heavy trucks and empty weight of the difference divided by the corresponding volume, the apparent density of less than the design value, it is necessary to adjust the amount of ingredients to prevent the amount of insufficient.

There is also an easy to overlook is that the crushed stone pile density sometimes changes due to changes in the collection site and particle grading, pile density, if the stone under the same amount of material, may be prepared to reduce the volume of concrete, resulting in a shortage of concrete supply.

Therefore, the laboratory should often use the capacity of the cylinder to sample the apparent density of concrete, such as the apparent density of concrete conspiracy to become larger, the same weight of the volume of concrete should be small, then the amount of stones should be promptly adjusted up.

47. the salesperson signed the contract in the technical aspects of what to pay attention to?

Flooring project should know its length, width dimensions, any length exceeding 30M to notify the technical department in a timely manner to prevent irregular cracks in the ground.

Ground engineering to know whether the upper part of the decorative layer, whether to play with the calendered ground (that is, the surface of the ground after calendering no longer do any decoration), such as playing with the calendered ground must be indicated, and appropriately increase the selling price.

Northern region road and bridge project must ask the user whether the use of snow removal agent, who scattered snow removal agent of concrete may not add admixtures, but also add air-entraining agent, so the contract must indicate that this is a special concrete, must increase the price.

Northern areas of outdoor reservoirs (such as sewage treatment plant reservoirs), hydraulic engineering, the use of stage will have freeze-thaw requirements, must be prepared according to anti-freeze concrete, so the contract indicates, and increase the price.

Where the project has large span structures, pre-stressed structures, large volume structures, extra long structures (greater than 80cm in length), the technical department should be informed promptly after the contract is signed so that the technical staff can focus on monitoring the construction process of these structures.

Where the project has clear water concrete (i.e., concrete whose surface is no longer decorated in the future and which requires a smooth surface and consistent color) and self-compacting concrete, the technical department and the material supply department should be notified in advance. Due to the large amount of this kind of concrete binder, the material requirements are higher, so the unit price should also be increased.

48.What should we pay attention to when signing contracts for group projects?

There are dozens or even dozens of projects in a park, and the name of each individual project should be indicated when the contract is signed, and should be respectively controlled according to the contract refund, otherwise, the group project construction until the completion of all concrete main body and then pay the final payment, ready-mixed concrete enterprises will bear great economic pressure, too many arrears will bring difficulties to the operation of enterprises.

49.What is the difference between the completion of the main concrete structure, the completion of the main structure of the project and the completion of the project?

The main structure of the project includes concrete structure, masonry structure, steel structure, etc., while the completion of the whole project includes the main structure, waterproofing, plastering, plumbing, electrical, decorative, doors and windows, etc., and the quality of concrete is only related to the concrete structure in the main structure.

Therefore, when signing the contract, the final payment of the project can not wait for the main structure of the project or after the completion of the acceptance of all projects before payment.

50.When the underground project is settled according to the actual quantity supplied, what should be paid attention to when the above-ground project is settled?

Before the end of the underground work and the start of the above ground work, the underground work pouring should be verified and confirmed by both parties.

If the elevation of the pile foundation is in accordance with the design requirements, and the underground part of the stairs and equipment foundation is finished, it is often due to the low elevation of the top of the pile foundation or some parts of the basement are not finished pouring, resulting in part of the concrete being mixed with the concrete according to the actual amount of concrete supplied, causing a “loss” in the supply of ready-mixed concrete.

51. What should we pay attention to before starting the project?

Before the start of the project, the head of production and technical departments to personally go to the site to understand the road and plant ground conditions, the presence of backfill, high-voltage lines, the project is located in the region with or without truck traffic restrictions, to determine the location of the pump and tanker truck driving route, the development of safe construction measures to prevent backfill ground caused by trapped cars, overturned cars and high-voltage electric shock and other accidents.

In addition, but also to determine the receipt of each project authorized visas, in writing to save its signature wording, to prevent future disputes.

52.What to pay attention to during the implementation of the contract?

After the implementation of the contract, a special person should be sent to the site in time to check the supply of concrete, timed issuance of statements, when the supply of concrete close to the settlement of the contract, the user should be notified in advance to prepare payment, to prevent oversupply, backlog of payment, resulting in ready-mixed concrete enterprises low return rate and operational difficulties.

53.What does the signed contract require users to fill in the concrete production order before each pouring?

All the management of the enterprise should be recorded in writing, and although the users have notified the ready-mix concrete enterprise by telephone 24 hours before they need concrete, the notification of the mouth gate is often wrong, especially the concrete model is often miscommunication, misheard and misremembered, causing engineering accidents.

Therefore, it is necessary for the user to fill in the concrete production entrustment form, which includes the pouring part, model, quantity, technical requirements, pouring time and other information.

The entrustment form is an important document with legal effect, and should be kept properly.

54.What are the advantages and disadvantages of reinforced concrete structure?

Advantages: reasonable material utilization; good moldability; good durability and fire resistance; low maintenance cost; good integrity of cast-in-place concrete structure and good ductility, vibration and radiation resistance through suitable reinforcement; large stiffness and damping, which is conducive to deformation control of the structure; easy to use local materials.

Disadvantages: self-importance; poor crack resistance; limited load-bearing capacity; complex construction, many procedures (support mold, tie steel, pouring, maintenance, etc.), long construction period, construction is greatly affected by the season and weather; e) Once the concrete structure is destroyed, its repair, reinforcement, reinforcement is more difficult.

55.What are the effects of concrete cracking on the structure?

(a) Cracking affecting the structural integrity of the structure.

(a) Cracking can lead to durability problems.

Loss of service function due to cracking.

Cracking is aesthetically unacceptable.

56. What are the main factors affecting the strength of concrete? How does it affect?

The main factors affecting the compressive strength of concrete are.

Cement strength grade and water-cement ratio. The higher the strength grade of cement, the higher the strength of concrete; under the premise that it can guarantee the dense formation, the smaller the water-cement ratio, the higher the strength.

Aggregate species, particle size, grading, impurities, etc. When using gravel and sand with larger particle size, better grading and cleanliness, the water-cement ratio can be reduced and the interfacial bond strength can be improved, thus the strength of concrete is high.

Curing temperature and humidity. The effect of temperature and humidity on the strength of concrete is achieved by influencing the hydration and setting hardening of cement. When the temperature is appropriate and the humidity is high, the strength develops quickly, and vice versa, it is not conducive to the growth of concrete strength.

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

Construction method. Mainly refers to the mixing and vibratory tamping process. The strength of concrete is higher when the mechanical mixing and vibratory tamping compacting action is strong.

57.What are the effects of clay, silt, fine dust and other powdery impurities and clods in sand and stone on the properties of concrete?

The content of powdery impurities such as clay, silt and fine particles in sand and stone increases, which will increase the mixing water consumption (W) of concrete, i.e. W/C, in order to ensure the fluidity of the mix, and clay and other powdery materials also reduce the interfacial bond strength between cement and stone and sand and stone, thus leading to a decrease in the strength and durability of concrete and an increase in deformation.

If the strength is not reduced, the amount of cement must be increased, but this will increase the deformation of concrete.

58.What basic requirements should be considered in the preparation of concrete?

The following four basic requirements should be considered in formulating concrete, namely.

(a) Meet the strength rating requirements of the structural design.

(b) Satisfy the required ease of concrete construction.

(a) Meet the requirements of the environment in which the work is carried out for the durability of concrete.

In line with the economic principle, that is, to save cement to reduce the cost of concrete.

59.When pouring concrete at the site, it is forbidden to add water to the concrete mix at will, what is the theoretical harm of adding water to the concrete quality?

When pouring concrete on site, contractors add water to the concrete mix, which increases the amount of water used and improves fluidity, but will decrease the cohesiveness and water retention of the concrete mix.

In particular, the increase of water-cement ratio W/C increases the content of pores in the concrete, which reduces the strength and durability of concrete and increases the deformation of concrete, resulting in quality accidents.

Therefore, when pouring concrete on site, it is strictly forbidden to add water to the concrete mix at will.

60.What information should be provided by the customer?

(a) Structural construction drawings (drawings of beams, slabs, columns, stairs, walls, etc.) shall be provided by the contractor for the knot work.

Building construction drawings (including construction drawings of columns, beams, beams, canopies, gutters, drainage slopes, windows, bedding, floors, roof protection, levelling and balconies, molding, etc.), in addition to change orders or drawings during the construction process.