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Its not enough that a concrete mix correctlydesigned, batched, mixed and transported, its ofutmost importance that the concrete must be placedin systematic manner to yield optimum results.

The precautions and methods of placing concrete invarious conditions are as follows

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Eg.Foundation concrete for a wall or column

Before placing concrete in foundation, the bed isprepared by removing loose earth, roots of trees

passing through it, when bed is dry ,it is damped sothat water is not absorbed by bed from the concreteand in case of wet bed or rain-soaked ,water andslush must be removed completely to expose firmbed.

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Eg.Road slab and Airfield slab The groung surface must be free from loose earth,pool

of water and other organic matterslike grass, roots,leaves etc.

The earth must be properly compacted and madesufficiently damp to prevent absorption of water fromconcrete by polyethylene sheets.

Concrete must be placed in required thickness with

suitable joints

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Eg. Construction of dam ,abutment

When concrete is laid in great thickness, it is placed inlayers .

Thickness of layer is limited by method of compactionand concrete of 15 to 30cm is generally adopted and incase of mass concrete, 35 to 45 cm is adopted.

Several such layers may be placed in succession to formone lift.

Before placing concrete, surface of previous lift iscleaned and roughed by water jet, wire brush, sandblasting.etc.

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Under this condition, concrete is placed as follows 1.Bottom bucket method Concrete is taken through water in water-tight box or

bucket and on reaching the final place of deposition the ebottom is made to open by some mechanism and the

whole concrete is dumped slowly. 2.Dry or semi-dry mixture of cement, fine and coarse

aggregate are filled in cement bags and such bags aredeposited on bed below the water.

3.Tremie pipe method

A tremie pipe is of 20cm diameter with funnel fitted at topand bottom end is closed with plug, so no water enter thepipe.When pipe is filled wirh slump concrete of 15 to 0cm,tremie pipe is lifted up and a slight jerk is given by aawinch and a pully arrangement.

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So, due to weight of concrete, plug falls and concretegets discharged. Care is taken that end of piperemains inside concrete, so that no water enters into

pipe from bottom.

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Formwork should not be removed until the concrete

has developed strength of at least twice the stress towhich concrete may be subjected at the time ofremoval of formwork.

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In this method,