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Concrete 15 Question

Thomas Huang posted this article in October 13, 2020

1.What should I do when the raw materials cannot be stopped immediately and the supply of concrete cannot be interrupted?
It must be at the same time that the raw material supplier is in contact with a person to control the mixing water consumption and on-site concrete vulcanization monitoring to prevent the construction site from adding water.
When necessary, control the water-binder ratio unchanged, and appropriately increase the amount of cement to increase the fluidity of the concrete.
Generally, for every increase of 1cm of slump, the volume of cement slurry needs to be increased by 1.5%~2.5% for every 1M3 of concrete.
2. What kind of fly ash can be used in ready-mixed concrete?
According to its fineness, water requirement ratio, and carbon content, fly ash is classified into grades I, II, and III. Grade I and II fly ash can be used in ready-mixed concrete.
3. Under what circumstances should fly ash not be used?
In winter construction, especially when the temperature is -10℃ and below, because the early strength of fly ash concrete is low, it needs to be pre-cured at positive temperature for a long time to reach the critical strength of frost resistance, and the site insulation is poor, and it is easy to be frozen. Mixed with fly ash.
In addition, the concrete floor shall not be mixed with fly ash when the concrete floor is pressed.
4. Can concrete with fly ash extend the acceptance period?
Yes, because fly ash mixing contributes to the later strength of concrete, the age of the design strength grade of fly ash concrete is specified.
It is suitable for 28 days for ground works; 28 or 60 days for ground works; 60 or 90 days for underground works; 90 or 180 days for mass concrete.
5. Under what conditions is silica fume used?
Silica fume is mainly used to prepare high-strength concrete, anti-sulfate concrete, anti-chlorine salt corrosion concrete, underwater concrete, deicing salt concrete, high wear-resistant pavement concrete. Around 3000RMB/ton.
6. Under what circumstances should zeolite powder be used?
Zeolite powder can be used as an active admixture for cement.
Zeolite powder can replace part of the cement in concrete to improve the water retention of concrete, especially when polycarboxylate superplasticizer is used to prepare large-fluid, ultra-high-strength concrete, proper amount of zeolite powder can be added to effectively improve the water retention of concrete , Prevent concrete bleeding.
When preparing lightweight aggregate concrete, its mixing will increase the structural viscosity of the cement slurry, which can greatly improve the floating problem of lightweight aggregate in vibrating molding.
Zeolite powder concrete is also suitable for underwater concrete and concrete cured in underground humid environment, with good frost resistance and impermeability.
When using zeolite powder to prepare high-performance concrete, it is better to replace 10% of the cement.
7. What is the reason why there are so many air bubbles on the concrete surface after demoulding of concrete produced with naphthalene-based superplasticizer? What should I do?
Some admixture manufacturers purchase cheap crude naphthalenes (normal naphthalenes are white and yellow in appearance, and impurity naphthalenes are pink or red brick) and mixed naphthalenes.
These chemically impure naphthalenes have high phenol content and low purity, which will bring many bubbles in the superplasticizer (inferior naphthalenes will bring 3% bubbles in the concrete), and the bubbles are larger and the defoaming time is longer. Long, directly affect the strength of concrete.
At this time, you should contact the admixture factory in time to suspend the use to prevent affecting the concrete strength.
8. How to detect the gas content of the water reducing agent?
In addition to measuring the air content of the concrete by the air content tester to infer the air entrainment of the admixture, the following methods can also be used to test.
Take a 100ml colorimetric tube with stopper, add 4g powder sample, add water to 40ml, use a steel ruler to record the vertical height of the solution (h0), hold the colorimetric tube plug tightly with your hand, shake it vigorously for 20 times, and immediately count with a stopwatch At the same time, measure the height of the top of the foam (h1) until the foam disappears on the liquid surface and the liquid surface is exposed, and then record the time (s).
Control index: foaming height≤45mm, defoaming time≤50s.
9. What should I pay attention to when using air-entraining agent?
First of all, it is advisable to control the amount of air entrained to 2%-4% (volume content), otherwise, the strength of the concrete will decrease.
In addition, inferior air-entraining agents with poor quality, large bubble diameter and spacing, such as sodium dodecyl sulfonate and wood calcium, cannot be selected.
A slight mixing amount will cause the concrete strength to drop significantly. Before use, it is necessary to determine which variety to use and how much to mix.
10. If the concrete waits too long on site, what impact will it have on its quality?
The concrete is mixed with a retarder, but its retardation time is limited. After more than 2h, the cement begins to hydrate, the slump begins to decrease, and the concrete strength will lose part of it.
Some companies have done experiments in this area, and the test results prove that the concrete strength begins to decline after the on-site waiting time exceeds 2 hours.
For this reason, the waiting time of concrete on site should not exceed 2h.
11. What are the consequences of adding water to the concrete truck?
The normal water-binder ratio of concrete ensures that it has a certain degree of fluidity and strength. In addition, adding water afterwards will reduce the strength of concrete (the excess water will evaporate to form voids and weaken the concrete section). According to the test data of some concrete enterprises, every increase in concrete For 10 kilograms of water, its 28-day strength drops by 3.7Mpa.
Therefore, it is necessary to strictly control the construction site not to add water to the transport vehicle.
12. What should I do if the concrete sag on site is very small due to various reasons and it is difficult to pump?
Only secondary vulcanization of concrete vulcanizing agent can be used.
At present, most concretes use naphthalene-based pumping agents, and the vulcanizing agent is a naphthalene-based superplasticizer, either liquid or powder.
13. How to control the amount of vulcanizing agent?
The amount of vulcanizing agent should be adjusted according to the slump of the concrete in the truck.
When powder is used, it is generally 0.5 kg/m3-1 kg/m3;
When it is liquid, it is 1 kg/m3-2 kg/m3; after adding, the tank body can be vulcanized after 1-2 minutes of rapid rotation.
14. Due to various unforeseen reasons, after the first vulcanization failed to pump in time, the slump of the concrete has dropped, what should I do?
Several tests have proved that concrete can be vulcanized repeatedly, as long as no water is added and the stay at room temperature does not exceed 2 hours, its strength will generally not decrease.
15. Why not use pumping agent as vulcanizing agent?
Because there are components of retarder and air-entraining agent in the pumping agent, mixing into concrete may cause the concrete to retard or decrease its strength.

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