After arrival at the job site, the ready-mixed concrete should be placed as near as possible to its final position, Belt conveyors, truck-mounted chutes, and mobile concrete pumps are among the most commonly used today for concrete placement. To minimize segregation, concrete should not be moved over too long a distance during the placement into forms. In general, the concrete mixtures is deposited in horizontal layers of uniform thickness, and each layer is thoroughly compacted before the next is placed. The rate of placement is kept rapid enough so that the layer immediately below is still plastic when a new layer is deposited. This prevents cold joints, flow lines, and planes of weakness that occur when fresh concrete is placed on hardened concrete.
Consolidation or compaction is the process of molding concrete within the forms and around embedded items and reinforcing steel to eliminate pockets of empty space and entrapped air. This operation can be carried out by hand rodding and tampling. However, now it is carried out by mechanical methods such as power tampers and vibrators that make it possible to place stiff mixtures with low water-cement ratio or high content of coarse aggregate. High-consistency mixtures should be consolidated with care because they are likely to segregate when intensely worked. Vibrators should only be used to compact concrete and not to move it horizontally, as this wold cause segregation.
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