g PSIApart from mixing the ingredients, they also produce kneading and crushing action. A pan
type mixer is shown in Fig. 10.6. These are also known as dual drum mixers and are used for
mixing concrete for roads or pavements. The drum comprises of two hemispherical halves,
rotating on a horizontal shaft. One half of the drum is attached to the end of the shaft and the
other half is free to slide over the shaft allowing the drum to open at the centre. A rubber joint
and adjustable closing pressure ensures a perfect sealing when the drum is closed. Due to
difference in circumferential speeds because of the design of blades cross flow is created which
forces the concrete towards the centre causing turbulence and produces a perfect homogeneous
mix. The rotating sliding drum-half can be retracted for discharge of concrete in few seconds.
The concrete produced by this type of mixers show no segregation.
2w
X Truck mounted mixers also know as transit mixers (Fig. 10.7(a)) are very
popular and have replaced the dumpers and agitator cars used earlier to transport fresh
concrete from the batching plant to the site. Transit mixers of capacity 4 to 12 cum mounted on
truck chesis are available. These mixers are not very efficient and need long mixing duration
depending on the extent of dryness in the mix. Their function is mainly to keep the mix in an
agitated condition. With the development of twin fin process mixer (Fig. 10.7(b)), the transit
mixers have become more efficient. These mixers in addition to the outer spirals have two
opposed inner spirals. The outer spirals convey the mix materials towards the bottom of the
drum while the opposed mixing spirals push the mix towards the feed opening. The repeated
counter current mixing process is carried out within the mixer drum. A number of special
nozzles provided on the lower side of inner mixing spirals, precisely and uniformly spray
water on the mix under pressure along the entire length of the drum. This results in through
mixing of the entire drum contents within a short time. Hence, production of high quality
structural concrete in the transit mixer is possible at a faster rate.
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