100 Questions about Ready-Mix Concrete (61-80)

Why are there three lines for each production mix ratio, and how should they be used?

Answer: The first line is the theoretical mix ratio (i.e., the moisture content of sand and stone is zero), used by some plants. The second line is the production mix ratio used on dry days. The third line is the mix ratio used on rainy days and after rain.

Can the mix ratio used for pavement or pile foundations be used for other parts?

Answer: No. Because the mix ratio for pavement or pile foundations is specifically designed for its technical requirements and is not suitable for other parts.

No.

Why is the maximum water content limited?

Answer: Because when the weather is very dry, the moisture content of the sand may be less than 4%, at this time, the maximum water content per cubic meter of concrete can be increased by 10kg according to the dry day production water content. If the slump requirement is still not met, it may be caused by the too small water reduction rate of the pump agent, in this case, the amount of pump agent should be increased by 0.1%~0.2%. Never increase the water content blindly just to achieve the slump requirement.

Under what circumstances should a narrow wall mix ratio be used?

Answer: When encountering projects with shear wall structures, a narrow wall mix ratio should be used when pouring shear walls or wall beams together. Because the shear wall is relatively narrow (mostly 200mm thick), subtracting four layers of wall bars and three beam bars, the net distance between the steel bars is approximately 3~100px, and slightly dry concrete is not easy to pour. The narrow wall mix ratio increases some fine aggregate particles, making it easier to pour.

Why should the mix ratio with mineral powder be prioritized?

Answer: Because adding an appropriate amount of mineral powder to concrete has little effect on the performance of concrete, it can replace cement in equal amounts, reducing production costs. Internal mixing of 20%~25% is better; excessive mixing will cause slow setting and increase the shrinkage of concrete. It is best to use less or no mineral powder during winter construction.

What are the regulations for the production controller to adjust the water content of concrete?

Answer: It is allowed to increase the water content by 5kg according to the production mix ratio, and there is no limit to reducing the production water content (as long as the slump of the concrete leaving the factory meets the requirements of the construction site). When the water content exceeds 5kg, it must be approved by the laboratory director.

Can the mixing current be observed to predict the dryness and wetness of the concrete mixture?

Answer: Yes. Because the drier the mixture, the greater the mixing current; the wetter the mixture, the smaller the mixing current. The controller must pay attention to observing and recording the mixing current of each mixer to determine a suitable slump corresponding to the mixing current range. When the current is less than this range, the water content should be reduced; when the current is greater than this range, the water content should be increased. When the current is too large or too small, the slump of the concrete should be visually inspected on the tank truck, and adjustments should be made until it is qualified if it does not meet the requirements.

What work should be done before production in the rainy season?

Answer: (1) First, visually check the moisture content of sand and stone at the material yard to determine which line of mix ratio to use.

(2) Before loading, run the belt for a dozen seconds to remove accumulated water. Then, confirm the opening mix ratio and water content according to the task order. Before unloading, be sure to confirm that the driver has reversed the drum to drain the water, especially after the rain, the driver must drain the water before loading, and then unload the material into the drum. Within 10 seconds after the bell rings, confirm whether the vehicle has left the station to prevent loading the wrong vehicle.

What records should the controller keep?

Answer: (1) Production operation records;

(2) Mix ratio adjustment records (records of changes made to the mix ratio);

(3) Identification relocation records (materials changed model, variety, and bin number, etc.).

Should the mix ratio be checked when inputting it into the computer?

Answer: It must be checked. One person inputs, and another person checks to prevent errors. Some plants have had incidents of incorrect cement input; some have used mineral powder as cement, and others have used fly ash as cement, resulting in project rework.

What work should be done when cleaning the inside of the main unit?

(1) Cut off the three-level power supply, lock the power distribution box with a personal lock, and keep the key with you.

(2) Wear work clothes, gloves, thick-soled shoes, and a safety helmet.

(3) Carry a pneumatic pick into the main unit, and the lighting fixture must have a protective net. The power cables of these two electrical appliances must have a leakage protection device. Another colleague should be present to supervise and assist with cleaning. This is to prevent accidental switching and injuries.

What kind of quality awareness should pump truck and tank truck drivers have?

Answer: First of all, they should establish the awareness that they are responsible for the quality of ready-mixed concrete leaving the factory, resolutely abandoning the narrow-minded idea that "I only transport and pump, and the quality of concrete has nothing to do with me." Actively check the delivery quality and adjust the concrete mixture with too small slump at any time.

Why should concrete transport trucks reverse the tank to drain wastewater before loading?

Answer: Because some vehicles have not drained the water after washing the car, resulting in the ready-mixed concrete being too thin to use, and some have not drained the rainwater after the rain before loading, resulting in the construction site returning the materials. Because the concrete we produce meets the requirements of the construction site, if there is extra water in the tank, it will definitely cause the concrete to be too thin and scrapped. Therefore, in order to prevent the above accidents, the company stipulates that each vehicle must reverse the tank to check for water before loading. If there is water, drain it immediately, and then load it if there is no water.

Why should the tank truck rotate slowly after loading?

Answer: The tank is required to rotate slowly at 2~4 r/min during transportation. If it stops, the concrete will segregate, causing the concrete to accumulate at the mixing blade angle in the tank.

What checks must a 75-tank truck driver perform before arriving at the delivery location?

Answer: The driver must stop and climb to the loading hopper to observe the slump of the mixture. If the slump is too small, an appropriate amount of pumping agent should be added according to the "Pumping Agent Addition Method" to adjust it to meet the customer's requirements. If the mixture is too thin, return to the station immediately to adjust it.

Why is it not allowed for anyone to arbitrarily add water to the concrete mixture?

Answer: Because the proportions of various materials in the factory-produced concrete are fixed. If water is added to the mixture due to a small slump, the water-cement ratio will increase, and the strength of the concrete is inversely proportional to the water-cement ratio. The larger the water-cement ratio, the lower the strength. Therefore, adding water to the concrete is strictly prohibited. An appropriate amount of pumping agent can be added to thin the concrete mixture, which is allowed by the national standard "Premixed Concrete" GB/T14902—2003.

Why is it necessary to quickly rotate the mixing drum before unloading?

Answer: Because only by quickly rotating the mixing drum can the mixing effect be achieved. The company stipulates that the driver should rotate the mixing drum at medium and high speeds for 20-30 seconds before unloading to ensure that the concrete mixture is evenly mixed before unloading.

Why is it necessary to limit the unloading time of the concrete mixture?

Answer: Because from the time the concrete mixture is produced and water is added, the cement and water begin to hydrate. If the pouring time is too long, the hydrates already produced in the concrete will lose their binding effect, reducing the strength of the concrete. Therefore, the standard stipulates that in summer, the unloading time 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.

Why must the concrete mixture be kept above the height mark in the hopper before pumping?

Answer: Because when there is too little mixture in the hopper, the piston movement will easily suck in air when sucking the mixture, and the air will form an air lock, making it difficult for the mixture in the pipe to move forward. Therefore, the material should be unloaded continuously and evenly into the hopper, consistent with the pumping speed. Before starting the pump, the pump truck operator must first check whether the mixture in the hopper exceeds 2/3 of the hopper height. If not, do not start the pump.

Why can't the driver put their head into the tank opening when checking the material?

Answer: Because it is easy to get injured. A driver from a certain company had their head cut off by the rotating tank opening because they put their head too low.