© 2009 Er. Jagvir Goyal
Note: This highly informative and elaborate paper was written by the Author many years back and holds valuable information. Readers should update themselves with the latest developments that might have occurred after the presentation of this paper.
India has bid good bye to manual concrete mixing, once so prevalent in all fields of construction. A switchover to machine mixing at small projects and concrete batching and mixing at big projects has been found both economical as well as qualitative. Even for small constructions such as individual housing, Ready Mixed Concrete (RMC) is preferred wherever available. Now, the project developers find it easier to place orders for supply of concrete of desired mix at their sites instead of setting up their own production units and hiring staff for their management and control. However the main equipment behind the successful use and popularity of RMC are the transit mixers that supply the concrete to desired locations in plastic stage. With the increased use of RMC, the demand for supply of efficient transit mixers has also increased. The purpose of producing well mixed concrete is lost if concrete loses its strength during transportation. Therefore, efficient transit mixers constitute an integral part of the RMC plants as well as concrete batching plant units.
Simple operation: Measured quantities of concrete gradients are fed by the dry batching plant into the transit mixers in dry form. While the transit mixer transports these materials to the site, required quantity of water is added by it to the ingredients to mix them and produce concrete. Spiral blades inside the drum push the concrete deep into it. The process is called charging the transit mixer. The concrete is unloaded at site by rotating the drum in reverse direction and at the same time, lowering the open end. This is called discharging the transit mixer.
Components of transit mixers: Main components of a transit mixer unit are the mixer drum, the water pump, mixer hopper, gear system, the oil cooler, control levers, chutes and the chassis for their mounting. Their desirable features are explained here.
Mixer drum: The transit mixers are made of sturdy, high wear resistance grade steel. Normally, Chrome and nickel content of steel should be more than normal. It will increase the wear resistance of the drum and its life. The thickness of drum should be more at the bottom than at the sides. Bottom thickness of 6 mm and shell thickness of 4 mm can be considered as normal. The mixing spiral in the drum should be of cast steel for full resistance to wear. For proper and homogeneous mixing of concrete ingredients, there should be more number of spirals per metre length. These days, sets of outer spirals and inner spirals are provided in the mixers for better mixing of ingredients. Additional protection on the upper edge of the spiral is a preferred feature. Angle of inclination of the drum plays important role in its easy filling. Therefore, increased angle of inclination is preferable. Supporting arms need to be stronger and designed corresponding to the strain curve. Drum is supposed to have low centre of gravity for increased stability and safety. It can be raised or lowered at one end even while revolving. Suitable access ladder with platform should also be available on the transit mixers.
Geometrical volume of a 4 cum capacity mixer is around 8000 litres and 10500 litres for a 6 cum capacity mixer. Therefore, a 5 cum capacity mixer should have about 8700 litres capacity. Filling volumes vary from 50% to 60%.
Water Pump: The mixer should have a three way pump for pumping water into the water tank, for pumping water into mixer drum for cleaning purpose and for pressure cleaning of outer surface of transit mixer and the truck. The water supply to the drum is metered so that measured quantity could be added to it and designed water cement ratio could be maintained. The capacity of the water tank varies with the capacity of the mixer. Water tanks are provided with water level indicators. The tank is located in between the mixer drum and the driver’s cabin.
Mixer Hopper: The mixer hopper should be lined with a hard lining made of chilled cast steel to make it wear resistant. The hopper size is kept large for easy charging and is provided with spring board effect. Drop angle of discharge hoppers is kept around 45 degrees for easy discharge of concrete. Their size is also being increased in latest design for easier cleaning of mixers.
Gear system: Special attention is paid to the gear system of transit mixers as the mixers need to work at full efficiency even while negotiating rough terrain and acute curves. The gear box with deep reduction drive has the gear kept submerged in oil for continuous lubrication and the bearings are designed to absorb severest forces.
Oil cooler: The transit mixers should have a separate oil cooler fitted on them to ensure that the hydraulic oil is not overheated and consequently, the hydraulic pump and motor are not over loaded. This should be given special attention as mostly, the hydraulic system is an imported one and its repair or replacement is costly. A thermostatic control is desirable for the hydraulic oil cooler. The oil requirements of the hydraulic system and its cooler should be low and the replacement period for hydraulic oil should be large, to the tune of 2500 hours of operation.
Control levers: Transit mixers should be provided with all control levers at one place itself, clearly indicating the purpose of each lever. This arrangement helps in easy operation of the mixer. The lever material should be corrosion proof. Ample linear and rotational adjustments should be available through them. The control panel should also be placed near the levers to provide a complete control mechanism to the operator.
Chutes: Chutes of varying lengths should be provided with the transit mixers for easy pouring. These are to be horizontally and vertically adjustable for smooth concrete discharge. Light weight, highly wear resistant, extendable chutes made of plastic help the mixer operator to work without getting fatigued. Now the manufacturers provide foldable chutes with an option to line them. The chutes can be locked at different positions.
Ladders: Foldable ladders are preferred for provision. These are designed to fold out at angle than vertically to allow easy movement of the driver.
Power supply for transit mixers: The power supply for the transit mixer is made available either from the truck engine or a separate engine is provided. Whenever the power supply is made from the truck engine, the mixer drum is kept lighter in weight. When a separate engine to drive the mixer is provided on the truck, the drum may be heavier.
Painting work: The mixer drum and other components should be well coated with primer and anti-corrosion paint coats. Shot blasting should be preferred over sand blasting. Polyurethane top coatings are now adopted for the mixers.
Dimensions: Obviously, dimensions of transit mixers shall vary with the capacity of mixers. For a 6 cum per hour capacity mixer, diameter of a mixer drum shall be about 2.3 metres. The length, breadth and height of mixer shall be about 6 metres, 2.5 metres and 2.6 metres respectively. Total weight of mixer unit excluding chassis varies from 4.0 tonnes for a 4 cum capacity mixer to 5.5 tonnes for a 6 cum capacity mixer. Truck chassis should have a minimum pay load capacity of 15 tonnes for 4 cum capacity mixer to 20 tonnes for 6 cum capacity mixer. The size of the truck chassis depends upon the size of the mixer drum. Generally, 3 axle chassis is preferred in transit mixers. However, for small capacity mixers, 2 axle chassis can be used.
Technical Data: While selecting transit mixers for adding to your fleet, the following technical data should be demanded from the manufacturer and studied:
- Capacity of the mixer ( in litres or cubic metres )
- Power Requirement ( in Kw or HP)
- Drum speed ( rpm)
- Weight of mixer (Tonne)
- Height of mixer frame (mm)
- Size or capacity of water tank ( litres)
- Engine capacity
- Type of drive ( Driven by a separate engine or vehicle engine)
- Type of water pump
- Type of water connection
- Geometric volume of the mixer
- Volume of the water line
- Dimensions of the mixer
- Accessories available with the mixer.
- Hydraulic oil requirements.
- Hydraulic Oil replacement period.
Advantages of transit mixers: Biggest advantage of transit mixers is that these do not allow segregation of concrete because of continuous rotation. Unloading of concrete batch can be done at will in full or partially. Drum of concrete mixer doesn’t allow any foreign material to enter the concrete load. Concrete can be unloaded to a conveyor belt, a concrete pump or into the chutes. As the capacity of available transit mixers varies from 4 cum to 12 cum, choice is available as per site requirements and concrete production rate.
Truck agitators: These are different from mixers as these don’t mix concrete ingredients while transporting them. These simply transport already mixed concrete by keeping it in plastic state through continuous agitation of the drum. These therefore don’t require water supply arrangement to the drum for mixing of concrete. Water supply arrangement for cleaning of drum on end of concreting operation may be there.
Pump coupled transit mixers: These transit mixers have concrete pumps and placer booms mounted on the same truck chassis. These are suitable for transportation and placement of small quantities of concrete. These mixers can place concrete up to 90 feet height. Such a transit mixer needs support from simple transit mixers for continuous feeding of concrete to the pump as pump output is more than single mixer’s capacity.
Precautions during use of transit mixers: Whenever concrete is transported through transit mixers, the workers should be advised not to add any additional water to the mixer. This tendency may result in lowering the quality of concrete. If required, suitable mix design with use of super plasticizers may be evolved and super plasticizers should be added to the concrete. This will take care of any stiffness that occurs during transportation and pourable concrete will be available at pouring point. The number of revolutions of the drum per hour should also be studied and fixed.
Manufacturers: Construction equipment giant Schwing Stetter entered India in 1999 with sales as major aim and a pilot production unit as a trial. Today, this German firm has separate production units for batching plants, concrete pumps and transit mixers with its sales rising from Rs. 9 crores in 1999 to Rs. 600 crores today. Producing most efficient transit mixers of varying capacities, it has supplied more than 2500 mixers to various sites. The mixers produced by it are of 4, 6, 7, 8, 9, 10 and 12 cubic metres capacity. Now it plans to set up workshops to create skilled manpower for the operation of equipment produced by it. while Kirloskar engines are used in the Schwing Stetter mixers, the drive to the mixer drum is from the truck engine.
AM6SHC, the 6 cum capacity transit mixers of Schwing Stetter are easy to operate, can be cleaned up fast, convenient to charge and discharge and have low service costs. Their one arm swivel chute with 2 extensions available allows the operator to work without getting tired. AM10SHAN, the semi trailer transit mixers, allows to reduce the transportation cost as more volume gets transported in one round. These mixers have independent engine and mixer can be operated independent of the truck operation.
Greaves India is producing highly efficient and sturdy transit mixers. It has recently opened its second unit in Chennai for production of over 200 transit mixers per month. Greaves claims to control 38 pecent of Indian market for transit mixers. Vectra offers transit mixers of 6 cum and 7 cum capacity. Their mixers built on Tatra T815 Chassis can be hydraulically or mechanically driven. Vectra also offers mixer superstructure with independent engine for mounting on 3 axle chassis other than that of Vectra. Shirke Structurals of Pune also has transit mixers in its equipment kitty. Ajax Fiori, the Indian company working in collaboration with Italian company Fiori offers Argo range of self loading mixers that can scoop, weigh, load, mix, transport and place concrete independently and need only one operator for all operations.
Cost component: A Schwing Stetter Make 6 cum per hour capacity transit mixer shall cost around Rs. 8 lacs plus taxes. As most components of these mixers are imported except the engine, cost of indigenously produced mixers may be much less. Greaves transit mixers of 6 cum capacity mounted on three axle Indian chassis with drum and spirals made of special grade, wear and corrosion resistant steel are quite popular. Both Schwing Stetter and Greaves have made available spares and after sales service shops available all over India. Shirke India produces transit mixers of 4 to 6 cum capacity each with drum speed of 0-14 rpm, 3 axis chassis and water tanks of 600 litres capacity.
Hiring Transit Mixers: Whenever transit mixers are required for short duration, Indian construction companies prefer to get these on rental basis. Equipment rental business is growing in India and the rents may see a downward trend in future with the increase in competition amongst equipment rental firms. Wherever, an RMC plant is to be set up, it is economical to have one’s own fleet of transit mixers for supply of concrete to different destinations. Infrastructure development groups also have their own batching plants and mixers for use at one site and shifting to new sites. Sometimes, the concrete supply to a particular site is to be supplemented to enable large sized pours. In such cases, hiring of transit mixers to carry additional volumes proves economical.