In the operation of installing prefabricated concrete parts such as concrete wall, concrete facing, concrete cornice and sometimes New Jersey concrete, etc., it is necessary to supply fresh concrete for the execution of the concrete or concreting behind the parts. Considering that these concretes are transported by a truck mixer to the place of installation of prefabricated concrete parts.
Mixer Track
The truck mixer is a container for transporting concrete, which rotates, mixes and prevents it from sticking to the final destination. These tanks can be installed on trolleys, trucks or even rail cars. Track mixers can also be used to complete the mixing operation. If the truck mixers are used as conveyors of concrete made by the concrete center, the truck mixer will only act as a mixing unit. If the dry materials are poured into the truck mixer and the mixing is done, the volume of concrete made is 63% of the nominal capacity of the truck mixer, and if the concrete made by the central concrete machine is transported by the truck mixer, it becomes 80% of the nominal capacity. p>
Truck mixers can perform 3 stages of loading, transfer from the place of production to the place of consumption and unloading. All three steps should be done without wasting any time due to the importance of time in concrete setting. To properly perform the mixing process in the mixer chamber, about 55% to 60% of the geometric volume of the tank is filled with concrete and the rest remains empty.
Track mixer tanks mainly have two different rotation speeds. The mixing speed of 10 to 14 revolutions per minute when pouring fresh concrete materials into the tank causes complete mixing of concrete components. The stirring speed of 3 to 5 revolutions per minute is used to transfer the concrete and prevents the concrete from sticking until it reaches the place of use. Concrete should be rotated about 70 to 100 times at mixing speed after all materials are poured into the mixer. Water must be present in the mixer before mixing. In order to add water after mixing the concrete into the tank, the mixer must perform at least 30 additional rotations at the mixing speed. Additional rotations of the mixer should be done at the speed of stirring and the total number of mixing rotations should not exceed 250 times.
One of the most important factors during mixing is the type and order of adding materials. If the materials are dry, mixed and entered into the mixer, the mixing time is greatly reduced. Concrete in truck mixers needs mixing time of about 8 to 15 minutes. It is necessary to avoid excessive mixing in order to prevent the increase of cracks and the temperature of the concrete mixture and the removal of useful air in the concrete mixture.
When using the mixer truck, the following should be observed:
1- From the time when water is added to the dry concrete mixture, the number of cycles of 70 to 100 cycles at high speed is enough for the initial mixing. If the ready concrete is transported inside the pot and we want to regain homogeneity during emptying, it is enough to stir the concrete 30 to 40 times at a slow speed. The maximum number of revolutions of the boiler is limited to 300 revolutions (including slow and fast revolutions), in this way, the duration of transportation in normal conditions (except for pouring concrete in hot and cold weather) from the time of loading to unloading is 1 to 5/ 1 hour is limited. However, out of 300 revolutions of the boiler, only 100 revolutions should be at the speed of mixing and the rest should be at the speed of stirring, because the long time of transportation or the number of revolutions of the boiler will reduce the slump, wear of the aggregates and the body of the boiler, and also reduce the strength and durability of concrete. .
2- In cases where the concrete transfer time is long or there is a possibility of heavy traffic, it is better to use the dry mix method and add the mixed water to other materials in the workshop. However, in this case, accurate control of the amount of water faces problems, and careful monitoring is needed to add the mixed water to the specified amount.
3- The use of a mixer truck is not recommended for the following concretes:
concrete with a slump of less than 40 mm
concrete with a maximum aggregate size of more than 50 mm
concrete with a water-cement ratio less than 0.4 (without the use of lubricant or super-lubricant additives) in the dry measuring method, microsilica should not be used in powder form. be done, because the distribution of microsilica particles in the mixture is not uniform.
4- If a mixer truck is used for initial mixing (dry measure), the volume of concrete made in it should be limited to two-thirds of the boiler's nominal capacity. For example, with a mixer truck with a nominal capacity of 6 cubic meters, it is possible to mix 4 cubic meters of concrete by dry measuring method.
