Batching and mixing involved in concreting

The following are the process of manufacturing good concrete. They are

1. Batching
2. Mixing
3. Transporting
4. Placing
5. Compacting
6. Curing
7. Finishing

Batching 

Batching involves volume batching in terms 35 litres and weight batching in terms of 50 kg. Automatic batching plants - suitable for ready mix plants.

Mixing

Mixing involves both hand mixing and machine mixing. Mixers involved are tilting, non tilting and reversing.

Transporting concrete

1. Mortar plan
2. Wheel barrow, hand cart
3. Crane, bucket and rope way
4. Truck mixer and dumpers
5. Belt conveyors
6. Chute
7. Skip and hoist
8. Pump and pipe line
9. Helicopter

     Concrete production is the process of mixing together the various ingredients water, aggregate, cement and additives to produce concrete. Nowadays concrete production takes place in a large type of industrial facility called concrete plant. In US 70% of the concrete is produced in concrete plants where as in India only 25% is produced in concrete plants. Concrete plants come in two main types namely ready mix plants and central mix plants.

    Ready mix or truck mix plants load out all the ingredients of the concrete mixture into a mixer truck, without water and the truck mixes with water to form the concrete. Central mix plant mixes all the ingredients including water. Central mix plant offers more accurate control of the concrete quality through better measurements of the amount of water added, but must be placed closed to the work site where the concrete will be used, since hydration begins at the plant.

     Concrete plant can have a variety of parts and accessories including but not limited to large storage hoppers for various reactive ingredients like cement, storage for bulk ingredients like aggregate and water, cement batchers, aggregate batchers, radial stackers, machinery to accurately weigh, conveyors to move,  mixers, heaters, chillers, batch control plants and dust collectors and facilities to dispense the mixed concrete often to a concrete mixer truck.

     Each of these types of facilities can be either portable or stationery. Portable facility will generally build onto the truck trailers to facilitate transportation on public roadways. The structure of a portable facility is almost always comprised of an open framework. Stationery facility, the structure can be either a solid building or an open framework.

Storage arrangement for sand and aggregates

1. Boom scraper plant that is open storage with retaining walls.
2. Ground storage hopper for smaller plants
3. Ground storage hopper with weighing facility at belt conveyor 
4. overhead storage hopper indicating total storage capacity required for aggregates and sand and number of aggregate compartments required.

Storage arrangement for cement

1. Cement silo, indicating its capacity and number of compartments.

Weighing arrangement

1. Single weighing system for aggregates and cement
2.  Independent weighing arrangement for cement as well as for all aggregates together
3. Independent weighing arrangement for each type of aggregates
4. More than one weighing arrangement for cement to cater to the different types of cement
5. Arrangement for measurement of water through flowmeter or through weighment
6. Addition arrangements for weighment of ice flakes
7. Number of admixtures and whether weighing arrangement is required for any of them

Type of mixer

1. Non tilting reversible drum concrete mixer
2. Double conical tilting mixer
3. Pan mixer
4. Turbo pan mixer
5. Single shaft or twin shaft compulsory mixer or plug mill

Batching plant and equipment

 

  

Batching equipment is to accurately determine and control the prescribed quantities of various constituent materials of concrete. Proportioning of materials is done in the batching plant by weight, each type of material being weighed separately. Batchers are of three types namely

• Manual
• Semi automatic
• Automatic

Manual batcher

     Manual batcher will be charged by devices which are actuated manually. Accuracy of batching operation is dependent on the operators visual observation of a scale or volumetric indicator. Various elements of the batching plant may be actuated by hand or by pneumatic hydraulic cylinders.

Semi automatic batcher

     Charged by devices which are separately actuated manually to allow the material to be weighed but valves closes automatically when the designated weight of each material is reached.

Automatic batcher

     Charged by devices which when actuated by a single starter switch, will automatically start the weighing operation of each material and stop automatically when the designated weight of each material has been reached.

Aggregate batcher

     Each aggregate bin will have a discharge gate operated by a pneumatic cylinder. Cylinder receives air from a double acting solenoid valve that operates from an electrical signal from the batch control to open and close the discharge gate. To achieve greater efficiency the weighing cut off will be in two stages i.e. initial and final with necessary jogging action for treble feed as the final weight approaches. Aggregate batchers are equipped with a pneumatic vibrator for smooth flow of aggregate. 

Cement batching

 

  

 For  plants upto 30m^3/hr capacity, a cement hopper of suitable capacity with screw conveyor or manual arrangement is provided. For plants above 30m^3/hr, cement weigh batcher is provided separately with independent structure and the cement compartment is water tight and  provided with necessary air vent, aeration fittings for proper flow of cement and emergency cement cut of gate. If cement is used in bulk, a separate scale for cement is included. The cement from the bulk stock may be weighed separately from the aggregate and water shall be measured by volume.

     For plants of capacity 20m^3/hr  and above, cement shall be weighed separately. However for plants of capacity below 30m^3/hr, cement and aggregate can be weighed one weigh batcher. In such case, a separate compartment in the weigh batcher is provided so that the cement does not come into contact with aggregate or sand.

Liquid batching

     Reservoir tank complete with water proofing, float valves and other fittings for direct deliver to mixers without coming in contact with cement or the aggregate prior to the mixing operations. Water can be fed through a water meter of suitable size or by weigh hatcher. The quantity of each admixture is determined by volumetric measurement for each batch. Batching of water is by weight but may be a volume in case of plants upto and including 30m^3/hr capacity. In addition to the water measuring device, for plants above 30m^3/hr arrangement for measuring and introducing small increments of water into mixer for final tempering of the concrete.

Concrete mixers

 

   

Concrete mixers are capable of combining the aggregates, cement and water into a thoroughly mixed and uniform mass within the specific mixing period, and of discharging the mixture, without segregation. Concrete mixers are classified as batch mixers and continuous mixers. Batch mixers are classified further into

• Tilting type
• Non tilting type
• Reversible type
• Pan type mixers
• Twin shaft mixers

    Non tilting, pan and twin shaft mixers are commonly used in concrete premix plants. Reversible mixers are used in the concrete transit trucks. Pan type mixer is a forced action batch mixer, different from drum mixers which relay on free fall of concrete inside the drum. Twin shaft mixers are ideally suited to precast and ready mix concrete production . The mixer trough is compact,, with a large horizontal discharge gate and two counter rotating mixing shafts with mixing paddles.