Placing of Concrete

Mixing, transporting, and handling of concrete should be carefully coordinated with placing and finishing operations. Concrete should not be deposited more rapidly than it can be spread, struck off, consolidated, and bull floated. Concrete should be deposited continuously as near as possible to its final position. In slab construction, placing should be started along the perimeter at one end of the work with each batch placed against previously dispatched concrete. Concrete should not be dumped in separate piles and then leveled and worked together; nor should the concrete be deposited in large piles and moved horizontally into final position.

In the use of concrete pumps, the Contractor shall ensure that following is complied with:

a) Pipes shall not be secured to formwork or reinforcement which may be displaced by the pulsating movement of the pipe.

b) Pumping shall commence first with blowing of water through the pipeline, followed by cement mortar, both of sufficient quantities and all these shall be discharged away from the area to be concreted.

c) When extra sections of pipe are to be added, each length of pipe shall be thoroughly wetted inside but no free water shall remain in the pipe.

d) Where the pipeline is exposed to the sun, the line shall be protected against absorption of heat by covering with damp sacking or other approved material during pumping.

Before proceeding to place the concrete, the formwork shall be re-aligned if necessary and water and rubbish therein shall be removed. Immediately prior to placing concrete, the formwork shall be well wetted and inspection openings shall be closed.

Raised gangways for workmen placing and vibrating concrete shall be provided wherever possible and shall be so constructed as to avoid damaging and displacing the reinforcement.

Concrete shall be placed in the formwork by shovels or other approved equipment and shall not be dropped from a height nor handled in a manner which will cause segregation. Accumulations of set concrete on the reinforcement shall be avoided. Concrete shall be placed directly in the permanent position and shall not be worked along the formwork to that position.

For beams and similar members. concrete shall be placed in a single operations to the full depth. In walls, columns and similar members, the concrete shall be placed in horizontal layers each not exceeding 1 m deep. Placing of concrete shall be continuous and temporary interruption shall not normally exceed 30 minutes. At the completion of a specified part, a construction joint shall be subjected to the approval of the contractor.

No concrete shall be placed unless the entire area of pour is covered, or other measures are taken to protect the wet concrete, to the satisfaction and subject to the approval of the contractor.

Placing of Concrete

Consolidation

In some types of construction, the concrete is placed in forms, then consolidated. Consolidation compacts fresh concrete to mold it within the forms and around embedded items and reinforcement and to eliminate stone pockets, honeycomb, and entrapped air. It should not remove significant amounts of intentionally entrained air. Vibration, either internal or external, is the most widely used method for consolidating concrete. When concrete is vibrated, the internal friction between the aggregate particles is temporarily destroyed and the concrete behaves like a liquid; it settles in the forms under the action of gravity and the large entrapped air voids rise more easily to the surface. Internal friction is reestablished as soon as vibration stops.

Finishing

Concrete slabs can be finished in many ways, depending on the intended service use. Options include various colors and textures, such as exposed aggregate or a patterned-stamped surface. Some surfaces may require only strike-off and screeding to proper contour and elevation, while for other surfaces a broomed, floated, or troweled finish may be specified. In slab construction, screeding or strikeoff is the process of cutting off excess concrete to bring the top surface of the slab to proper grade. A straight edge is moved across the concrete with a sawing motion and advanced forward a short distance with each movement.

Jointing is required to eliminate unsightly random cracks. Contraction joints are made with a hand groover or by inserting strips of plastic, wood, metal, or preformed joint material into the unhardened concrete. Sawcut joints can be made after the concrete is sufficiently hard or strong enough to prevent raveling. After the concrete has been jointed, it should be floated with a wood or metal hand float or with a finishing machine using float blades. This embeds aggregate particles just beneath the surface; removes slight imperfections, humps, and voids; and compacts the mortar at the surface in preparation for additional finishing operations. Where a smooth, hard, dense surface is desired, floating should be followed by steel troweling. Troweling should not be done on a surface that has not been floated; troweling after only bullfloating is not an adequate finish procedure. A slip-resistant surface can be produced by brooming before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression.

Figure 2: Finishing the concrete surface

All concrete shall be placed in daylight, unless an adequate system of lighting is provided and approval for night work is given by the Clerk-of-Works and all relevant authorities prior to commencement of the concreting.All concrete shall be placed in daylight, unless an adequate system of lighting is provided and approval for night work is given by the Clerk-of-Works and all relevant authorities prior to commencement of the concreting.

References:

http://www.portcement.org/cb/concretebasics_placing.asp