The placement methods of readymix concrete plays an important role as it affects the strength and durability of concrete structures. The time of delivery, quality checks and time of placements affects the ready mix concrete.
Ready mix concrete is a concrete which is manufactured as per required mix ratio in batching plant,and then is transported to construction site on continuously mixing trucks.
Ready Mix Concrete Placement Methods
Ready Mix Concrete placement methods include following basic principles:
1. When arriving to the site the concrete transport certificate must be checked for desired characteristics of ordered concrete (quantity, class, maximum aggregate size, slump, temperature, type cement etc.) and time duration of transport.
2. Concrete shall be delivered to the site and discharged from the truck completely and in the forms ready for vibration within 1-1/2 hours after batching.
3. Concrete shall be placed in maximum 15 minutes after its arrival to the site, and the finishing of placement will take place before the cement starts setting.
4. Concrete shall be stored / deposited as near as (physically and economically) possible to its final position, in crane hoisted buckets, concrete pumps, chutes etc.
5. The receptacles used for the transport and deposition of concrete shall be cleaned and washed out at the end of each day’s work and whenever concreting is interrupted for more than 30 minutes.
6. If the concrete, due to transport, is segregated. It should be mixed again on clean platforms, without adding water, if not possible the batch should be refused.
7. As placing begins the consistency of the delivered concrete should be checked with a slump cone for conformance with specifications and the required number of samples must be taken (1 sample = 3 specimens) according to the volume of concrete placed.
8. Concrete shall be placed to prevent segregation. The concrete shall not be permitted to strike against forms and ricochet on bars and form faces. The free fall of concrete should in no case exceed 1,5 m. For greater heights, as in walls or columns, metal or rubber hoppers, chutes or flexible hoses must be lowered into the formwork to control the rate of fall of the concrete, if not the concrete will flow rapidly entering the forms invariably resulting separation and honeycombing.
9. In some cases, when it may not be possible to use chutes inside the formwork, concrete may be deposited through openings in the side of the form, known as cleanout windows. They provide an outside pocket from which the concrete can flow into the form at a controlled rate, rather than allowing it to enter directly into the form at a high velocity.
10. When concrete is dumped from a cart or wheelbarrow, there is a tendency for the heavier particles to separate from the mass. To prevent this, the concrete should be discharged against a striking (baffle) board.
11. Concrete shall not be allowed or caused to flow horizontally or on a slope in the forms. Concrete placing on a slope shall begin at the lower end of the slope and progress upward. If necessary, boards forming the sloping surfaces may be placed as concreting progresses
11. When unloading concrete on the surface of a cast member, the direction deposition should be into the face of previously placed concrete and not away from it.
12. Concrete shall be placed as nearly as possible in its final position. It should not be placed in large quantities in one position or allowed to flow or be worked over more than 1,5 m. The mortar will tend to flow ahead of the coarser materials, thus causing stone pockets and sloping work planes.
13. The concrete should be spread and deposited in horizontal uniform thick layers, according to the type of vibrating equipment used (it should be usually between 200 – 400 mm thick).
14. Placing of concrete shall be so regulated that the pressures caused by the wet concrete shall not exceed those used in the design of forms.
15. Concreting should be as continuous as possible to avoid planes of weakness. When the concrete placement stops do to different reasons a construction joint will be made;
16. Efforts should be made to consolidate the concrete by using different means of vibration (manual or mechanical) according to type of member, site possibilities, volume of works etc.
When fresh Ready Mix Concrete is to be placed on hardened concrete surface
1. It is important to secure a good bond and a watertight joint.
2. The hardened concrete shall be level, rough, clean, and moist. Some of the aggregate particles must be exposed by cutting away part of the existing surface by sandblasting cleaning with hydrochloric acid, or by using a wire brush. Any laitance or soft layer of mortar must be removed from the surface.
3. When concrete is to be placed on a hardened concrete surface a layer of mortar must be placed on the hard surface first. This provides a cushion on which the new concrete can be placed and stops aggregate from bouncing on the hard surface and forming stone pockets.
4. The mortar should be approximately 50 mm deep leveling concrete.
5. The concrete shall be placed continuously from one side or end of the section to the other, using precaution to put the full load upon any given area of form as rapidly as possible.
6. The rate of delivery of concrete to the work shall be such as to insure continuity of placement. No partially completed surface shall be allowed to stand more than 45 minutes before continuing the placing of concrete thereon.
7. The mixing drums of transit-mix trucks shall be thoroughly washed after discharging each load to prevent the accumulation of adherent layers of concrete.
8. Concreting of any unit or section of the work shall be carried out in one continuous operation or until an authorized construction joint is reached.