Concrete Equipment for Ready Mixed Concrete

The concrete equipment of a Ready Mixed Concrete plant generally are: 

1. Material feed system 
2. Sand making machine 
3. Storage system i.e. hopper  and cement silo 
4. Weigh hatching  system and reversible type mixer 
5. Transit  mixer 
6. Pumping system 

Let us examine some of these in  more detail :

Cement Input and Delivery 

This consists of a cement silo. It receives cement by vertical screw movement. From silo, cement can be delivered by a slant screw  movement. On its way it passes through a weigh  hatching bin connected to a scale which ensures correct weight. 

Flow  Chart of Making Ready  Mixed  Concrete

Aggregate Storage

Depending upon the requirements, four compartments can be  provided to store the aggregate. Three of these compartments can store coarse aggregate of different  grading  after  they have passed the vibrating screens. If  good natural sand is available then  the same can be  used, otherwise a sand mill is incorporated to produce artificial or crushed stone sand conforming to IS: 383-1970. The aggregates pass through weighing bins  before reaching the mixer. 

The Batch Plant 

The batch  plants  are  available in three categories  namely  manual, semi-automatic  and  automatic. 

In  semi-automatic  plant,  the charging  and discharging of  batches are activated manually but are automatically  terminated.  In  a  fully  automatic  plant,  a single starter  switch  activates the batching sequence, the  weight  and  volumes of which have been  already  programmed into the system. Sometimes the plants have both manual  as well as fully automatic system. At Nave Sheva near Bombay the plant used  was  of this type and had  a capacity of 80 cm/hour. The central console was  air conditioned  and computers were located in it. The design mix program data is punched in the card and inserted in the computer. In the auto mode,  the exact quantities as per the  design for aggregates, cement,  water  and  admixtures  are  delivered to the mixer. The mixing time is around 40 seconds. The water  together with admixtures is atomized  and sprayed'on to the mix to ensure thorough  mixing. The concrete is then 

discharged by bottom slide gates of  plan  directly to the waiting transit mixers. The batch plant.could have capacity to mix 0.75 cum onward at a time in the pan. 

Transit Mixer  

The transit  mixers are available in several  sizes in international  and Indian markets  varying from 2 to 6 cum capacity. A minimum of 2 nos. are required for a batch  plant. The transit  mixer rotates  at a speed of about 13 revolutions  per  minute  and  keeps the green concrete in  a  agitated state. During travel, the transit  mixer has the  facility to mix water  of  measured quantity. The efficiency of the batch plant depends on the tale at which  the quantity  manufactured from the plant is drawn. If insufficient number of transit mixers are employed, then the hatch plant is forced to remain  idle for part of the time, thus  making the system uneconomical. In order to avoid it, the required number of transit mixers  should be procured. ASTMC 94 allows a maximum of 1.5 hour  or before  the drum  had 300 revolutions, whichever is earlier, form the introduction of water to placement of concrete into formwork The mixing in transit mixer usually results in stiffening of the mix resulting in reduction of slump by about 2 cms. ASTMC-94 allows the addition of water at job site to restore the slump initially then that required at job site, so that when it reaches the site, the slump is the required  one. In order for you to appreciate the different components of a transit mixer can be seen in Figure. 

Transit Mixer 

Placement of Ready Mixed Concrete (RMC) 

The RMC arrives at the site.by transit mixers. Now it has to be placed without segregation and before it has achieved an initial set into its final position in the  formwork. This placement can be done in different ways according to site conditions. For example, if the site conditions allow, the concrete could be discharged directly from the transit mixer to the  formwork. Another  method  is use of belt conveyors which are used when large quantities of concrete are involved. However, this system consumes considerable time in erection, maintenance and dismantling.  Another method involves use of buckets and hoppers which discharge concrete at its lowest practical  slump by sliding gates. A crane is  used together with it to place concrete at distances and heights which cannot be reached by transit mixer. However, concrete pump is a very popular mode of transporting concrete horizontally and vertically. These are available in both stationery and mobile models with capacities of 20 cumin to 60 cm/hr. Therefore, an efficient ready  mixed concrete facility must include one concrete pump.