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Concrete 15 Question-2

Thomas Huang posted this article in October 13, 2020

16. How long does it take for 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. Press lightly on the surface of the concrete without sticking to your hands. The surface of the concrete receives water and there is a shiny film for initial setting;
When the color of the concrete surface turns white and there is no mark by hand, 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.
17. What is early-strength concrete?
Ordinary concrete reaches 70% of the design strength in 7 days at room temperature, and 100% of the design strength in 28 days. Due to the construction progress or formwork turnover, measures are taken to make the concrete reach the design strength in about 15 days at room temperature, which is the early strength Concrete.
18. What is super early strength concrete?
Under normal temperature, the concrete that can make the concrete reach the design strength in about 7 days is called ultra-early concrete.
19. How to prepare super early strength concrete?
Generally, it can be formulated with super early-strength pumping agent, it can also be formulated with two concrete strength grade methods or with PO42.5R cement.
20. What should be paid attention to when designing mass concrete mix ratio?
Mass concrete must prevent structural cracks caused by the temperature difference between the inside and outside exceeding 25℃. Therefore, the following points should be paid attention to when designing the mix ratio:
Control the amount of cement and choose cement with lower heat of hydration and slow setting time. Such as dam cement, slag Portland cement, fly ash Portland cement, etc.
It is advisable to mix with fly ash and ground mineral powder to reduce the heat of hydration of concrete.
Slow-setting water reducer or high-efficiency water reducer should be used.
In order to reduce the temperature of concrete entering the mold, ground water that meets the standard for mixing water can be used, and part of ice can be mixed if necessary.
21. What is adiabatic temperature rise of concrete?
The cement in the concrete will release heat during the hydration process. The heat release is related to the type of cement used and the amount of cement. It is assumed that the concrete is in an adiabatic state where the upper and lower sides cannot emit heat. As the cement hydrates, the temperature inside the concrete will continue Rise, when it rises to the highest, the highest adiabatic temperature rise is reached.
22. How to calculate the adiabatic temperature rise?
The adiabatic temperature rise of cement hydration heat is calculated as follows:
Tmax=mq/cρ
Where: Tmax-cement hydration adiabatic temperature rise value (℃) m-cement consumption per cubic meter of concrete (kg), q-cement hydration heat, c-concrete specific heat (take 0.96j/K), ρ-concrete density ( 2400 kg/m3).
23. What is the heat of hydration of cement?
The heat of hydration of various cements can be taken from the table.
Heat of hydration Q per kilogram of cement (Kj/Kg)
24. What is the law of general concrete heating?
Generally, the mass concrete rises to the heat peak 2-3 days after pouring.
For example, the bottom plate of a project is 2 meters thick and is made of 360 kg/m3 of 42.5 grade cement. The temperature of the construction site is 32℃ in summer. The core temperature can be as high as about 80°C.
After 4 days, most of the cement hydration heat energy has been consumed, and the internal temperature gradually drops. This temperature may last from ten to thirty days. The hydration and heat dissipation process of some large-volume concrete in hydraulic engineering may be much longer.
Having mastered this law, the focus of mass concrete temperature control should be placed within a week after pouring, especially the first 2-3 days.
25. What effect does a large amount of fly ash have on suppressing the peak temperature of concrete?
The addition of fly ash is beneficial to reduce the temperature peak of mass concrete and delay the appearance of the temperature peak.
When the fly ash content is 10%-50% (percentage of the mass of the cement), the peak hydration value in 3 days will be reduced by 5.9%-35.1%, and the peak temperature will be delayed by 0.5-3.2 hours.
26. What matters should be paid attention to when entering coarse and fine aggregates?
Coarse aggregate should mainly control its particle size, gradation, grain shape, stone powder content, and mud content.
Each vehicle shall undergo a macro inspection, and shall not be unloaded if it is unqualified; in addition, various indicators shall be inspected in batches according to the requirements of the regulations.
Fine aggregate should be controlled fineness modulus, mud content and mud content. Each vehicle shall undergo a macro inspection, and the unqualified vehicle shall not be unloaded; it shall also be inspected in batches according to the specifications.
27. Why should the coarse aggregate particle size be controlled within 5-25mm?
The particle size of the coarse aggregate is restricted by the diameter of the concrete pumping pipeline and the pumping height. Generally, the maximum pumpable particle size decreases as the pumping height increases.
For example, when the pumping height is less than 50m, the ratio of the maximum particle size of the coarse aggregate to the delivery diameter is ≤1:3; when the pumping height is 100m, the ratio will drop to 1:5, otherwise the pipe will be easily blocked.
28. Why should the content of coarse aggregate needles and flakes be controlled for pumping concrete?
When its content is high, the flexural strength of the needle-shaped coarse aggregate is relatively low, and the bonding strength between the coarse aggregates decreases, which causes the concrete strength to decrease.
For ready-mixed concrete, the high content of needles and flakes will make the coarse aggregates poorly shaped, which will reduce the fluidity of the concrete. At the same time, the needles and flakes are easy to block at the pipeline, causing the pump to block or even burst pipes. .
Therefore, the content of needles ≤10% is required for pumped concrete, and the requirement for high-strength concrete is even higher.
29. What kind of sand is needed for ready-mixed concrete?
Ready-mixed concrete needs to use medium sand. In addition to the specification requirements for the sand gradation, mud content and mud content, it is also important to note that the sand passing through the 0.315mm sieve is not less than 15%.
This has a great influence on the pumpability of concrete. If this value is too low, the pump is easy to block, and the water retention of concrete is poor and bleeding is easy.
30. What impact will fine sand bring?
If the sand is too fine, the water demand of the concrete will increase. Moreover, the pumpability and plasticity retention of the concrete prepared with fine sand are extremely poor, the concrete strength will decrease and it is easy to crack.


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