Concrete Mixers

A concrete mixer is a device that combines cement, aggregate such as sand or gravel, and water to form concrete. A typical concrete mixer uses a REVOLVING DRUM to mix the components.

FACTORS AFFECTING SELECTION OF MIXER

• LOCATION of the construction site (distance from the batching plant) 
• AMOUNT of concrete needed  
• CONSTRUCTION SCHEDULE (volume of concrete needed per hour)  
• COST (The allocated budget) 
• QUALITY of the concrete produced

Types of Mixers

There are two main types of mixers used:

1.batch mixers and
2.continuous mixers.

Batch mixers are the most common.

Continuous Mixer

• Continuous type mixer produces concrete at a constant rate. In this type, the constituents are continuously entered at one end as the fresh concrete exits the other end.

• They are usually non-tilting drums with screw-type blades rotating in the middle of the drum.

Batch mixers

• Two main types of batch mixer can be distinguished by the orientation of the axis of rotation:

1.horizontal or
2.inclined (drum mixers) or vertical (pan mixers).

The drum mixers have a drum, with fixed blades, rotating around its axis, while the pan mixers may have either the blades or the pan rotating around the axis.


Types of Drum Mixers

• Non-tilting drum mixer.
• Reversing drum mixer. 
• Tilting drum mixer.

Non-tilting drum mixer

The non-tilting drum mixer implies that the orientation of the drum is fixed. The materials are added at one end and discharged at the other.

X-Section of a Non Tilting Mixer

Reversing drum mixer(truck mixer)

X-Section of a Truck Mixer

• First truck mixer was invented in 1916.
• The reversing drum is similar to the non-tilting mixer except that the same opening is used to add the constituents and to discharge concrete. 
• The drum rotates in one direction for mixing and in the opposite direction for discharging the concrete.
• Trucks weigh 20,000 pounds (9,100 kg) to 30,000 pounds (14,000 kg), 
• Can carry roughly 40,000 pounds (18,000 kg) of concrete although many varying sizes of Mixer Truck are currently in use. 
• Typically the speed for mixing is 1.57 rad/s (15 rpm), while the transport of pre-mixed concrete uses only 0.2 rad/s (2 rpm) to0.6 rad/s (6 rpm).

Types of Truck Mixers

• Rear discharge truck mixers
• Front discharge truck mixers

Rear discharge trucks

"Rear discharge" trucks require both a driver and a "chute man" to guide the truck and chute back and forth to place concrete in the manner suitable to the contractor.

Rear Discharge Truck

Front discharge trucks

Newer "front discharge" trucks have controls inside the cab of the truck to allow the driver to move the chute in all directions.

Front Discharge Truck

 Tilting mixer

• In this type of mixer axis of the drum does not remain horizontal.
• The position of drum axis for different operations i.e. feeding, mixing and discharge is different.
  

Cross Section of a Tilting Mixer

Pan Mixer

Components of a pan mixer are,

• Pan
• Shaft
• Scraper