1. Transit mixed concrete
It is also called dry batched concrete because all the basic ingredients including water are charged directly into the truck
mixer. The mixer drum is revolved fast at charging speed during the loading of the material and after that it continues rotating at a normal agitating speed. In this type of ready mix concrete, also three types of variations are possible as given below:
• Concrete mixed at job site:
While being transported towards the destination, the drum is revolved at a slow or agitating speed of 2 rpm, but after reaching the site just before discharging the material, it is revolved at maximum speed of 12 to 15 rpm for nearly 70 to 100 revolution
for ensuring homogeneous mixing.
• Concrete mixed in transit:
The drum speed is kept medium during the transit time, i.e. approximately 8 rpm for about 70 revolutions. After 70 revolutions, it is slowed down to agitating speed of 2 rpm till discharging the concrete.
• Concrete mixed in the yard:
The drum is turned at high speed or 12-15 rpm for 50 revolutions. This allows a quick check of the batch. The concrete is then agitated slowly while driving to the job site.
Concrete mixed in transit:
The drum is turned at medium speed or about 8 rpm for 70 revolutions while driving to the job site. The drum is then slowed to agitating speed.
2. Shrink mixed concrete
The concrete is partially mixed in the plant mixer and then balance mixing is done in the truck mounted drum mixer during transit time. The amount of mixing in transit mixer depends
upon the extent of mixing done in the central mixing plant. Tests should be conducted to establish the requirement of mixing the drum mixer
The concrete is partially mixed in the plant mixer and then balance mixing is done in the truck mounted drum mixer during transit time. The amount of mixing in transit mixer depends upon the extent of mixing done in the central mixing plant. Tests should be conducted to establish the requirement of mixing the drum mixer.
3. Central mixed concrete
Central-mixing concrete Batch plants include a stationary, plant-mounted mixer that mixes the concrete before it is discharged into a truck mixer. Central-mix plants are sometimes referred to as wet batch or pre-mix plants. The truck mixer is used primarily as an agitating haul unit at a central mix operation. Dump trucks or other non-agitat-ing unit’s are sometimes
be used for low slump and mass concrete pourssupplied by central mix plants. About 20% of the concrete plantains the USuse a central mixer.
Principal advantages include:
•Faster production capability than a transit-mix plant
•Improved concrete quality control and consistency and
•Reduced wear on the truck mixer drums. There are several types of plant mixers, including:
•Tilt drum mixer
•Horizontal shaft paddle mixer
•Dual shaft paddle mixer
The tilting drum mixer is the most common American central mixing unit. Many central-mix drums can accommodate up to 12 yd3 and can mix in excess of 200 yd3 per hour. They are
fas-tand efficient, but can be maintenance-intensive since they include several moving parts that are subjected to a heavy load. Horizontal shaft mixers have a stationary shell and rotating
central shaft with blades or paddles. They have either one or two mix-ing shafts that impart significantly higher horsepower inmixing than the typical drum mixer. The intensity of the mix-ing action is somewhat greater than that of the tilt drum mixer. This high energy is reported to produce higher strength concrete vi-ato thoroughly blending the ingredients and more uniformly coating the aggregate particles with cement paste. Because of the horsepower required to mix and the short mixing cyclerequired to complete mixing, many of these mixers are 4 or 5yd3 units and two batches may be needed to load a stand-ard truck or agitator. Pan mixers are generally lower capacity mixers at about 4 to 5yd3 and are used at precast concrete plants.