1. What should be done when the raw materials cannot be stopped immediately and the concrete supply cannot be interrupted?

Must be in contact with the raw material supplier at the same time, send a person to control the amount of mixing water and on-site concrete sulfide monitoring, to prevent the site to add water. If necessary, control the water-cement ratio unchanged and increase the amount of cement appropriately to increase the fluidity of concrete. Generally every 1cm increase in slump, every 1m3 concrete needs to increase 1.5% to 2.5% of the volume of cement paste.

Concrete Basics 30 Questions

Crack concrete texture surface background.

  1. 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 and Ⅲ, Ⅰ and Ⅱ fly ash can be used in ready-mixed concrete.

  1. When should fly ash not be used?

Winter construction, especially in ~ 10 ℃ and the following temperature construction, due to the low early strength of fly ash concrete, to reach the critical strength of frost-resistant need in the positive temperature preheating time is long, site insulation is poor, easy to freeze, should not mix fly ash. In addition, with the playing of calendered concrete floor, there are de-icing salt requirements of the concrete shall not be mixed with fly ash.

  1. Is it possible to extend the acceptance age of fly ash concrete?

Yes, because fly ash blending contributes to the later strength of concrete, so china’s “Technical specification for the application of fly ash concrete” (gbj146) stipulates the age of fly ash concrete design strength grade. 28 days for above-ground works; 28 days or 60 days for ground works; 60 days or 90 days for underground works; 90 days or 180 days for bulk concrete.

  1. Under what conditions is silica fume used?

Silica fume is mainly used to formulate high strength concrete, sulfate resistant concrete, chloride salt corrosion resistant concrete, underwater concrete, de-icing salt concrete, high wear resistant road concrete.

  1. When is zeolite powder used?

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

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

When preparing light-aggregate concrete, as its admixture will improve the structural viscosity of cement paste, can make the light aggregate in the pounding and molding of the floating problem is greatly improved.

Zeolite powder concrete is also suitable for underwater concrete and underground wet environment maintenance of concrete, frost resistance, seepage resistance is good. When using zeolite powder to prepare high-performance concrete, replace the amount of cement 10% is good.

  1. Sometimes the concrete produced with naphthalene high efficiency water reducing agent after demoulding, what is the reason for the special bubbles on the concrete surface? What should be done?

Some admixture production plants purchase cheap crude nai (normal nai appearance color was white, yellow, impurities nai was pink, red brick color), mixed nai. These chemical composition impure nai, high phenol content, low purity, will bring many bubbles in the high efficiency water reducing agent (poor quality nai will bring in 3% of the bubbles in the concrete), and the bubbles are larger, longer defoaming time, directly affect the concrete strength. At this time, should be timely contact with the admixture factory to suspend the use of the concrete strength to prevent the impact.

  1. The use of air-entraining agent to pay attention to what?

First of all, to control the amount of air-entraining generally 2% to 4% (volume content) is appropriate, otherwise, it will cause a decrease in concrete strength. In addition, do not choose poor quality, bubble diameter and spacing of poor quality air-entraining agent, such as sodium dodecyl sulfate, calcium wood, etc.. The amount of mixture will cause a significant decrease in concrete strength. To determine the use of that variety through the test before use, how much to mix.

  1. What is the effect on the quality of concrete if it waits too long at the site?

Concrete is mixed with retarding agent, but its retarding time is limited, more than 2h after the cement began to hydrate, the slump also began to decline, the concrete strength will lose part. Some companies have done tests in this regard, the test results prove that the site waiting time of more than 2h, the concrete strength begins to decline. For this reason concrete should not wait more than 2h at the site.

  1. What will be the consequences of adding water to the concrete truck?

The normal water-cement ratio of concrete ensures that it has a certain fluidity and strength, in addition, adding water will reduce the strength of concrete (excess water evaporates to form voids, weakening the concrete section), according to some concrete enterprises test data show that for every 10 kg of water added to concrete, its 28-day strength drops 3.7 mpa. Therefore, we should strictly control the construction site should not add water to the transport truck.

  1. For various reasons, the slump of concrete to the site is very small, it is difficult to pump how to do?

Concrete admixture can only be used for secondary vulcanization. Depending on the size of the concrete slump in the truck to adjust the amount of admixture. When using powder, generally 0.5 kg / cube ~ 1 kg / cube; liquid agent for 1 kg / cube ~ 2 kg / cube; after adding, the tank fast turn 1 ~ 2 minutes or so can be vulcanized. Through a number of tests to prove that the concrete can be repeatedly vulcanized, as long as no water is added, stay at room temperature for no more than 2h, generally its strength will not decline.

Because the pumping agent will have slow-setting, air-entraining agent components, mixed into the concrete may cause concrete slow-setting or strength decline.

  1. How much time is the initial and final setting of concrete after pouring? How to judge?

The initial setting time of concrete at room temperature is 6~8 hours, when the concrete surface is lightly pressed by hand, it is not sticky, and the concrete surface receives water and has a shiny film as the initial setting; when the concrete surface becomes white in color and there is no mark by hand, it is the final setting, the final setting time is about 8~10 hours, depending on the temperature in summer and winter, the initial and final setting time will be shortened or extended.

  1. What is early-strength concrete?

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

Generally can be formulated using ultra-early strength pumping agent, but also available to improve the strength of the two concrete grade method or use po42.5r cement to formulate.

  1. What should i pay attention to when designing the ratio of large volume concrete?

Large volume concrete to prevent its internal and external temperature difference of more than 25 ℃ caused by structural cracks, so the ratio design should pay attention to the following points.

Control the amount of cement and use the cement with lower heat of hydration and slower setting time. Such as dam cement, slag silicate cement, fly ash silicate cement, etc.

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

Slow-setting water-reducing agent or high-efficiency water-reducing agent should be used.

In order to reduce the temperature of concrete into the mold, groundwater can be used to meet the standard of mixing water, if necessary, can be mixed with some ice.Concrete Basics 30 Questions

  1. What is the rule of bulk concrete warming?

Generally 2 to 3 days after pouring bulk concrete rises to the thermal peak. For example: A project base plate 2 meters thick, using 360 kg / cube of 42.5 grade cement preparation, construction at the summer, into the mold temperature 32 ℃. Its central temperature can be as high as about 80 ℃. 4 days after the cement hydration heat has been mostly consumed, the internal temperature gradually down, this temperature may continue for more than ten days to more than thirty days, some hydraulic concrete hydration heat dissipation process may also be much longer. Grasp this law, the bulk concrete temperature control focus should be placed on the week after pouring, especially the 2nd to 3rd day.

  1. What is the role of large amount of fly ash to inhibit the peak temperature of concrete?

Fly ash is conducive to reduce the peak temperature of bulk concrete and delay the time of the peak temperature. Fly ash 10% to 50% (as a percentage of the mass of cement), 3 days to reduce the peak hydration 5.9% to 35.1%, delaying the peak temperature of 0.5 to 3.2 hours.

  1. Coarse and fine aggregates into the field to pay attention to what matters?

Coarse aggregate should mainly control its grain size, gradation, grain shape, stone powder content and mud block content. Each car should be checked macroscopically, and no car should be unloaded if it is not qualified; in addition, the indicators should be tested according to the specifications and in accordance with the batch. Fine aggregates should control fineness modulus, mud content and clay content. Each car for macroscopic inspection, unqualified not unloaded; the same should be required by the specification batch inspection.

  1. Why should the coarse aggregate particle size be controlled at 5~25mm?

Coarse aggregate particle size by the concrete pumping pipe diameter and pumping height constraints, generally can pump the maximum particle size with the pumping height increases and decreases.

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

  1. Pumping concrete why should control the coarse aggregate flake content?

When its content is high, the needle-shaped coarse aggregate flexural strength is relatively low, and the bond strength between coarse aggregates decreased, resulting in a decrease in concrete strength. For ready-mixed concrete, the high needle flake content will make the coarse aggregate particle shape is not good, so that the concrete flow decreases, while the needle flake aggregate is easy to block at the pipe, resulting in pump blockage, or even burst pipe. Therefore, pumping concrete requires its needle flake content ≤ 10%, and high strength concrete requirements are even higher.

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

Ready-mixed concrete needs to use medium sand, in addition to the sand grading, mud content and clay content according to the specifications, but also pay attention to the sand through the 0.315mm sieve of not less than 15%. This has a great impact on the pumpability of the concrete, this value is too low easy to block the pump, and the poor water retention of concrete, easy to water.

  1. What is the effect of fine sand?

If the sand is too fine, the water requirement of concrete will rise, and the pumpability and plasticity of concrete prepared with fine sand will be extremely poor, and the strength of concrete will drop and crack easily.

  1. What if there is only fine sand?

If the sand source has a problem, fine sand plus part of the mechanism of sand to prepare pumping concrete, such as the available fineness modulus less than 2.0 fine sand mixed with fineness modulus 3.0 ~ 3.2 mechanism of sand, about 6:4 ratio, observe its fluidity, pumpability, the specific can be determined by test ratio.

  1. What are the consequences of sand containing mud?

Sand with high mud content, concrete water demand, poor plasticity, shrinkage increases, concrete strength decreases, the structure is easy to crack, the structure is easy to crack, therefore, to control the sand mud content ≤ 3% (c30 ~ c50), high strength concrete mud content requirements are higher.

  1. Why the preparation of high-strength concrete should be used when the particle size of a smaller stone?

As the coarse aggregate particle size increases, its bond with the cement paste weakened, increasing the discontinuity of the internal structure of the concrete material, resulting in a reduction in concrete strength. Coarse aggregate plays a restraining role in cement shrinkage in concrete. As the coarse aggregate and the cement paste have different modulus of elasticity, thus generating tensile stresses within the concrete. This internal stress increases with the increase in coarse aggregate size and can lead to a reduction in concrete strength. With the increase in coarse aggregate size, the degree of orientation of ca(oh)2 crystals in the transition zone of the coarse aggregate interface increases, which weakens the interface structure and thus reduces the strength of concrete.

The test shows that: Concrete particle size 15 ~ 25mm coarse aggregate around the interface crack width of about 0.1mm, crack length for the grain circumference of 2 / 3, interface cracks and cracks in the surrounding cement paste connected more: And 5 ~ 10mm grain size coarse aggregate concrete, interface crack width is more uniform, only 0.03mm, crack length is only the grain circumference of 1/6. Grain size of different coarse aggregate, concrete hardening in the lower part of the particle size of the formation of water capsule accumulation is also different, large particle size coarse aggregate lower water capsule is large and more, the water in the water capsule evaporated, the interface joints formed under the interface is bound to be wider than the small particle size, the interface strength is low.

  1. Why the same ratio of concrete, pebble concrete than crushed concrete strength is 3 to 4mpa lower?

The rough surface of the coarse aggregate, there are gains in the strength of the interface between the cement paste and the aggregate.

According to the author’s test for many years, the pebbles formulated concrete on the one hand because it contains more weathered stone, the crushing index itself is lower than the crushed stone, and the surface is smooth, the interface strength is low, so the strength of the concrete formulated by it will be lower than the same proportion of crushed concrete 3 to 4mpa.

  1. Why do we need to measure the moisture content of sand and gravel in every shift?

Sand and gravel in ready-mixed concrete are each about 800 ~ 1100kg/m3 dosage, and every 1% of its water content will bring 8 ~ 11kg of water consumption in concrete. Especially sand, usually collected from the river, has a large variation in moisture content. If the moisture content is not frequently detected and the amount of water used for mixing is adjusted in time, it will cause great fluctuations in the slump, pumpability and strength of each tray of concrete.

  1. What is the effect of concrete cracking on the structure?

Cracking affects the integrity of the structure.cracking causes durability problems.cracking causes loss of service function.cracking is unacceptable from an aesthetic point of view.

  1. 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 cement strength grade, the higher the strength of concrete; under the premise of ensuring dense molding, the smaller the water-cement ratio, the higher the strength.

Aggregate varieties, particle size, grading, impurities, etc.. The use of larger particle size, better gradation and clean gravel and sand, can reduce the water-cement ratio, improve the interface bond strength, and thus the strength of concrete.

Maintenance temperature, humidity. The effect of temperature and humidity on the strength of concrete is achieved by influencing the hydration of cement to set hard. When the temperature is suitable and the humidity is high, the strength develops quickly, and vice versa, it is not conducive to the growth of concrete strength.

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

Construction method. Mainly refers to the mixing, pounding molding process. Mechanical mixing and pounding dense action when the concrete strength is higher.

  1. Which basic requirements should be considered for the preparation of concrete?

The following four basic requirements should be considered for the preparation of concrete, namely

To meet the strength grade requirements of the structural design.

To meet the requirements of the construction of concrete and ease.

To meet the requirements of the environment in which the project is located on the durability of concrete.

Meet the economic principles, that is, save cement to reduce the cost of concrete.

  1. When pouring concrete on site, it is strictly forbidden to add water to the concrete mix at will, try to analyze theoretically the harm of adding water to the quality of concrete?

When pouring concrete on site, the construction workers add water to the concrete mix, although it increases the water consumption and improves the fluidity, but it will reduce the cohesion and water retention of the concrete mix. In particular, the increase in the water-cement ratio w/c increases the content of capillary pores inside the concrete, thus reducing the strength and durability of the concrete and increasing the deformation of the concrete, resulting in quality accidents.

Therefore, when pouring concrete on site, the construction personnel must be strictly prohibited from adding water to the concrete mix at will.