Ready Mix Concrete-Batching, Mixing, Transporting, and Handling
Quality control of ready mix concrete is important as it has to be mixed and transported to different locations. Batching, mixing, transportation and handling of ready mixed concrete is discussed. While Ordering Ready Mixed Concrete Following Points is to be specified:- Coarse aggregate size
- Slump
- Air content
- Basis for proportioning, select Options A ,B, or C.
Option A - Performance Based Ready Mixed Concrete
Purchaser specifies compressive strength of concrete required and the concrete producer selects mix proportions for the same.Option B—Prescription Based Ready Mixed Concrete
Purchaser specifies mixture proportions including:- Cement content
- Water content
- Admixture type and content
Option C - Mixed Option Based
Purchaser specifies compressive strength, cement content, admixture type and content for the ready mix concrete. Concrete producer selects mix proportions for the same.Batching and Mixing of Ready Mixed Concrete
Stationary Mixing of Concrete
Stationary Mixers are used onsite or Central Mix R/M plant. It is used for complete or shrink mixing. The concrete mixer can mix up to 9m3. It can have tilting or non-tilting drum, open top revolving blade or paddle. Typical complete mixing times of this machine is minimum of 1 min. for up to 1 m3 or less mixer capacity plus 15 seconds for each additional m3 or fraction thereof.Ready Mixed Concrete
There are two types of ready mixed concrete, shrink-mixed concrete in which concrete is mixed partially in a stationary mixer and completed in a truck mixer. Truck-mixed concrete in which concrete is mixed completely in a truck mixer.
Fig.2. Concrete can be mixed at the jobsite in a stationary mixer.
Fig. 3. Truck-mixed concrete is mixed completely in a truck mixer.
Central-mixed concrete
It has concrete mixed completely in a stationary mixer and delivered in a truck agitator. The truck has mixer operating at agitating speed. There can be even a non-agitating truck.
Fig. 4. Central mixing in a stationary mixer of the tilting drum type with delivery by a truck mixer operating at agitating speed.
Fig: 5. Ready Mix Plant
Truck Mixed Concrete
It requires 70 to 100 revolutions required for mixing. Mixing speed varies from 6 to 18 rpm. After mixing, drum revolves at agitating speed, 2 to 6 rpm. The concrete needs to be discharged before exceeding 300 drum revolutions and before 1½ hours.Mobile Batcher Mixer
It is used for intermittent production of concrete at jobsite, or small quantities. Its advantages include combined materials transport and batching and mixing system and requires only one-man operation.
Fig. 6. Mobile batcher measures materials by volume and continuously mixes concrete as the dry ingredients, water, and admixtures are fed into a mixing trough at the rear of the vehicle.
Non-Agitating Truck
It is used for transport concrete on short hauls over smooth roadways. Its advantages include cost of non-agitating equipment is lower than that of truck agitators or mixers. Watch for: Slump should be limited. Possibility of segregation. Height upon discharge is needed.
Agitator Trucks
It is used for transporting concrete for all uses. Haul distances must allow discharge of concrete within 1½ hours. Advantages: Operate usually from central mixing plants. Watch for: Timing of deliveries should suit job organization. Concrete crew and equipment must be ready onsite to handle concrete.
Fig .8. Truck agitators are also used with central-mix batch plants. Agitation mixing capabilities allow truck agitators to supply concrete to projects with slow rates of concrete placement and at distances greater than non-agitating trucks.
Remixing of Concrete
Water may be added at jobsite provided specified w/c-ratio is not exceeded, measured slump less than specified, allowable mixing and agitating time (or drum revolutions) are not exceeded. Concrete is remixed for minimum 30 revolutions at mixing speed until uniformity is within limits.Transporting and Handling of Ready Mixed Concrete
Wheelbarrows and Buggies
These are used for short flat hauls on all types of onsite concrete construction. its advantages: Versatile—ideal inside and on jobsites with changing placing conditions. This method is slow and labor intensive.
Fig. 9. Versatile power buggy can move all types of concrete over short distances.
Belt Conveyor
It is used for conveying concrete horizontally or higher/lower level. Advantages: Adjustable reach, traveling diverter, variable speed. End-discharge arrangements needed to prevent segregation and mortar on return belt. Belt cover needed in hot and windy weather.
Fig. 10. The conveyor belt is an efficient, portable method of handling concrete. A drop chute prevents concrete from segregating as it leaves the belt; a scraper prevents loss of mortar. Conveyor belts can be operated in series and on extendable booms of hydraulic cranes.
Truck Mounted Conveyors
It is used for conveying concrete horizontally or higher/lower level. Advantages: Conveyer arrives with concrete. Adjustable reach and variable speed. Watch for: End-discharge arrangements needed to prevent segregation and mortar on return belt. Belt cover needed in hot and windy weather.
Fig. 11. A conveyor belt mounted on a truck mixer places concrete up to about 12 meters (40 feet) without the need for additional handling equipment.
Buckets
It is used with cranes, cableways, and helicopters. Advantages: Enables full versatility of cranes, cableways, and helicopters to be exploited. Clean discharge. Watch for: Select bucket capacity to conform to size of the concrete batch and capacity of placing equipment
Fig. 12. Concrete is easily lifted to its final location by bucket and crane.
Fig. 13. In comparison to conventional rear-discharge trucks, front-discharge truck mixers provide the driver with more mobility and control for direct discharge into place.
Cranes and Buckets
It is used for work above ground level. Its advantages: Can handle concrete, reinforcing steel, formwork, and sundry items. Watch for: Has only one hook. Careful scheduling between trades and operations is needed to keep crane busy.
Fig. 14. The tower crane and bucket can easily handle concrete for tall-building construction
Pump
Pumps are used for conveying concrete from central discharge point to formwork. Its advantages: Pipelines take up little space and can be readily extended. Delivers concrete in continuous stream. Pump can move. Watch for: Constant supply of freshly-mixed concrete is needed without any tendency to segregate.
Fig.15. A truck-mounted pump and boom can conveniently move concrete vertically or horizontally to the desired location.
Flexible Hose at End of Pump’s Rigid Pipeline
Fig. 16. View of concrete discharging from flexible hose connected to rigid pipeline leading from the pump. Rigid pipe is used in pump booms and in pipelines to move concrete over relatively long distances. Up to 8 m (25 ft) of flexible hose may be attached to the end of a rigid line to increase placement mobility.
Screw Spreaders
These are used for spreading concrete over large flat areas. Advantages: Concrete can be quickly spread over a wide area to a uniform depth. Watch for: They are normally used as part of a paving train. They should be used for spreading before vibration is applied.
Fig. 16. The screw spreader quickly spreads concrete over a wide area to a uniform depth. Screw spreaders are used primarily in pavement construction.
Work Above Ground Level
It is especially applicable to tall buildings where tower cranes cannot be tied up with placing concrete. Concrete is supplied to the boom through a pipeline from a ground-level pump. Concrete can be pumped hundreds of meters (feet) vertically with these pumping methods. 