g PSQg 2
2w
2 2w 2
The order of feeding the ingredients into the mixer is as follows:
About 25 per cent of water required for mixing is first introduced into the mixer drum to
prevent any sticking of cement on the blades and bottom of the drum. Then the ingredients are
discharged through the skip. In the skip the sequence of loading should be to add first half the
coarse aggregate then half the fine aggregate and over this total cement and then the balance
aggregates. After discharging the ingredients into the drum the balance water is introduced.
The mixing time is counted from the instant complete water is fed into the mixer.
The speed of the mixers is generally 15 to 20 rpm. For proper mixing, the number of
revolutions per minute required by the drum are 25 to 30. Time of mixing also depends on
capacity of mixer and is given in Table 10.3.
2IH Q 22w g2
2w
w2 2
2@
A
@A x2
w
2
2 2Q P P6S
P I6S P
2I I6PS I6S
A poor quality of concrete is obtained if the mixing time is reduced. On the other hand if the
mixing time is increased it is uneconomical. However, it is found that if the mixing time is
increased to 2 minutes the compressive strength of concrete produced is enhanced and beyond
this time the improvement in compressive strength is insignificant. A prolonged mixing may
cause segregation. Also, due to longer mixing periods the water may get absorbed by the
aggregates or evaporate resulting in loss of workability and strength.
Concrete should be transported to the place of deposition at the earliest without the loss of
homogeneity obtained at the time of mixing. A maximum of 2 hours from the time of mixing
is permitted if trucks with agitator and 1 hour if trucks without agitators are used for transporting
concrete. Also it should be ensured that segregation does not take place during transportation
and placement. The methods adopted for transporting concrete depend upon the size and
importance of the job, the distance of the deposition place from the mixing place, and the
nature of the terrain. Some of the methods of transporting concrete are as below:
w 2 X This is the most common method of transporting concrete. This is labour intensive
method wherein the pans are passed from hand to hand and is slow and expensive method.
Since pan conveys small quantity of concrete, more and more concrete area is exposed to
atmosphere during transportation. This may lead to evaporation of water from concrete
particularly in hot weather and under conditions of low humidity. In this method of transporting
concrete can be deposited with equal ease on above or below the ground level without
segregation.
2 f X 2 Wheel barrows (Fig. 10.8) are used for transporting concrete to be placed at
ground level. These are used for concreting rigid payments. For long hauls due to uneven
ground surface segregation may take place. To check this, the barrows are provided with