That Vital Component
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, writes Er. Jagvir
India has bid goodbye to manual concrete mixing, a procedure
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 to be 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 the supply of concrete of desired mix
at their sites instead of setting up their own production
units and hiring staff for management and control of these
units. However, the main equipment behind the successful use
and popularity of RMC are the transit mixers that supply the
concrete to desired locations in the 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
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,
the 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
Components of mixers
The main components of a transit mixer unit are the mixer
drum, water pump, mixer hopper, gear system, oil cooler, control
levers, chutes and the chassis for their mounting. Their desirable
features are explained here.
The transit mixers are made of sturdy, high wear resistance
grade steel. Normally, the 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. A bottom thickness of
6 mm and shell thickness of 4 mm can be considered 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
an 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.
The drum is supposed to have a 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.
The geometrical volume of a 4 cum capacity mixer is around
8,000 litre and 10,500 litre for a 6 cum capacity mixer. Therefore,
a 5 cum capacity mixer should have about 8,700 litre capacity.
Filling volumes vary from 50 per cent to 60 per cent.
The mixer should have a three-way pump for pumping water into
the water tank, into the mixer drum for cleaning purposes,
and for pressure cleaning of the outer surface of the transit
mixer and the truck. The water supply to the drum is metered
so that measured quantity can be added 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.
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 a springboard
effect. The drop angle of discharge hoppers is kept around
45 degrees for easy discharge of concrete. Their size is also
being increased in the latest design for easier cleaning of
Special attention should be 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.
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 overloaded.
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 2,500 hours of operation.
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 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. Lightweight,
highly wear resistant, extendable chutes made of plastic help
the mixer operator work without getting fatigued. Now the
manufacturers provide foldable chutes with an option to line
them. The chutes can be locked at different positions.
Foldable ladders, designed to fold out at angle than vertically
to allow easy movement of the driver, are preferred for provision.
Power supply for 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
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.
Obviously, dimensions of transit mixers will vary with the
capacity of mixers. For a 6 cum per hour capacity mixer, diameter
of a mixer drum should be about 2.3 m. The length, breadth
and height of mixer shall be about 6 m, 2.5 m and 2.6 m, respectively.
The total weight of the mixer unit excluding chassis varies
from 4 tonne for a 4 cum capacity mixer to 5.5 tonne for a
6 cum capacity mixer. Truck chassis should have a minimum
pay load capacity of 15 tonne for 4 cum capacity mixer to
20 tonne 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.
These are different from mixers as they don't mix concrete
ingredients while transporting them, they simply transport
already mixed concrete by keeping it in a plastic state through
continuous agitation of the drum. These, therefore, don't
require a water supply arrangement to the drum for the mixing
of concrete. A water supply arrangement for cleaning of the
drum at the end of concreting operation may be there.
Pump coupled mixers
These transit mixers have concrete pumps and placer booms
mounted on the same truck chassis. They 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 the continuous
feeding of concrete to the pump as the pump output is more
than a single mixer's capacity.
Precautions during use
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, a suitable mix design with use of
super plasticisers may be evolved and super plasticisers 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.
Construction equipment giant Schwing Stetter entered India
in 1999 with sales as their major aim, with a pilot production
unit as trial. Today, this German firm has separate production
units for batching plants, concrete pumps and transit mixers
with its sales rising from Rs 9 crore in 1999 to Rs 600 crore,
today. Producing most efficient transit mixers of varying
capacities, it has supplied more than 2,500 mixers to various
sites. The mixers produced by it are of 4, 6, 7, 8, 9, 10
and 12 cubic m 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
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 two extensions available, allows the operator
to work without getting tired. AM10SHAN, the semi trailer
transit mixers, allows reduced transportation cost as more
volume gets transported in one round. These mixers have an
independent engine and mixer which can be operated independent
of the truck operation.
Greaves India is producing highly efficient and sturdy transit
mixers. It recently opened its second unit in Chennai for
the production of over 200 transit mixers per month. Greaves
claims to control 38 per cent of the 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
the Argo range of self-loading mixers that can scoop, weigh,
load, mix, transport and place concrete independently, needing
only one operator for all operations.
A Schwing Stetter 6 cum per hour capacity transit mixer will
cost around Rs. 8 lakh plus taxes. As most of the components
of these mixers are imported except the engine, the 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 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.
Whenever transit mixers are required for short duration, Indian
construction companies prefer to get these on a rental basis.
The equipment rental business is growing in India and the
rents may see a downward trend in the future with increased
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 the supply of concrete to different
destinations. Infrastructure development groups also have
their own batching plants and mixers for use at one site and
for shifting to new sites. Sometimes, the concrete supply
to a particular site may have to be supplemented to enable
large-sized pours. In such cases, hiring of transit mixers
to carry additional volumes proves economical.
Jagvir Goyal is the author of several technical books, columnist
in technical journals, Deputy Chief Engineer Civil, Punjab,
and can be contacted at firstname.lastname@example.org.
While selecting transit mixers for adding to your fleet, the
following technical data should be demanded from the manufacturer
Capacity of the mixer (in litres or cubic metres).
Power requirement (in Kw or HP).
Drum speed (rpm).
Weight of mixer (tonnes).
Height of mixer frame (mm).
Size or capacity of water tank (litres).
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 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