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How to supply and transport concrete ?
The structural concrete shall be supplied to each part of the structure being concreted without unnecessary unloading and reloading. Distributing it on the structure is performed by using light conveyor feeders, vibrating feeders, chutes, skips, pumps, compressed air and vibrating troughs. The method of handling and conveying and the composition of the concrete shall be so adjusted among the various operations as to obviate segregation and shall be subject to the approval by the Engineer.
Buckets
The capacity of the buckets shall conform to the size of the concrete batch, or a multiple thereof, in order that there will be no splitting of batches in loading buckets. It shall be capable of prompt discharge of low-slump, lean-mix concrete. The dumping mechanism shall permit discharge of a relatively small portion of concrete in one place. Buckets shall be filled and discharged without noticeable separation of coarse aggregate.
Trucks
Trucks are convenient for conveying concrete from batching plant to structures in exceptional cases only. No free water shall be on the surface of the concrete as delivered, nor shall segregation of coarse aggregate occur.
Chutes
Use of chutes is to be avoided as far as reasonably possible but the operations if any must be controlled in a way that segregation and objectionable loss of slump will be avoided. Concrete which has been transported in non-agitating equipment and whose consistency has suffered from transport vibrations shall not be reworked by adding water prior to placing and shall thus not be used unless explicitly approved by the Engineer.
Belt Conveyors
Slump loss, in use of belt conveyor, is largely preventable by protecting the belt from the sun and wind. Segregation, which occurs chiefly at transfer points and at the end of the conveyor, shall be avoided by use of a suitable hopper and drop chutes.When belt conveyor carries concrete, scrapers and devices for holding the concrete together at the throw-off point shall be provided.Pumping
For conveying concrete by pumping, it shall not be permitted to use light alloy pipes. Pipelines for pumping concrete shall be laid in such a way that no interruption can occur in the flow of concrete in the pipe. Pump lines shall be as short as possible.
The concrete pumping plant shall be provided with a water pressure valve, connecting pipe with needle valve cleaning rods, outlets for drainage of water, and a high-pressure (30-35 kg/cm2) centrifugal pump for flushing out the concrete pipelines. Pipes for pumping concrete shall have a
minimum inner diameter of 15 cm. Pipe joints for concrete pumping shall permit a deviation in pipeline axis by at least 3 degrees. They shall be of the quick-connection type to permit easy disconnection and emptying if concrete gets stuck in the conveying pipeline and cannot be moved.
At the end of the pumping operation the conveying pipeline shall be immediately emptied and cleaned (blowing out).
Truck Mixers
Truck mixers consist of equipment mounted on trucks and capable of mixing concrete in conveying between the batching plant and the point of placing. There is often considerable slump loss in truck-mixed concrete, especially in warm weather. Such loss shall be kept to a minimum by stopping the initial mixing at 30 revolutions and/or to reduce the rotary speed to avoiding over mixing.
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