Introduction.
To achieve quality concrete it should be placed with utmost care securing the homogeneity achieved during mixing and the avoidance of segregation in transporting.
Research has shown that a delayed placing of concrete results in a gain in ultimate compressive strength provided the concrete can be adequately compacted. For dry mixes in hot weather delay of half to one hour is allowed whereas for wet mixes in cold weather it may be several hours.
The various situations in which concrete is placed are discussed below.
1. Foundations.
Concrete foundations for walls and columns are provided below the ground surface. Before placing the concrete in the foundation all the loose earth, roots of trees etc., are removed. If the surface is found dry it is made wet so that earth does not absorb water from concrete. On the other hand if the foundation bed is wet the water and mud is removed and cement is sprinkled before placing concrete.
2. Beams, Columns and Slabs.
Before placing the concrete, the forms must be examined for correct alignment. They should be adequately rigid to withstand the weight of concrete and construction loads without undue deformation. Forms should be light enough to avoid any loss of mortar resulting in honeycombed concrete.
The insides of the forms should be cleaned and oiled before use to avoid any sticking of concrete with the forms and making their stripping off difficult. Concrete should not be dropped but placed in position to prevent segregation. It should be dropped vertically from as small height as possible. It should be placed at one point in the formwork and allowed to flow side ways to take care of honeycombing. Laitance formation should be avoided. It can be checked by restricting thickness of layer of concrete by 150–300 mm for R.C.C work.
Laitance, however, if formed must be removed before placing the next layer of concrete. Several such layers form a lift, provided they follow one another quickly enough to avoid cold joints. The surface of the previous lift is kept rough and all the laitance removed before placing the next lift. The reinforcement should be checked for tightness and clean surface.
The loose rust or scales if any, are removed by wire brush. Paint, oil or grease if found should be removed. The minimum cover for reinforcement should be checked before concreting.
3. Mass concreting.
When the concrete is to be laid in mass as for raft foundation, dam, bridge, pier etc., concrete is placed in layers of 350–450 mm thickness. Several such layers placed in quick succession form a lift.
Before placing the concrete in the next lift, the surface of the previous lift is cleaned thoroughly with water jets and scrubbing by wire brush. In case of dams, sand blasting is done. The laitance and loose materials are removed and cement slurry is applied. When the concrete is subjected to lateral thrust, bond bars or bond stones are provided to form a key between different layers.
4. Concreting Highways and Runways.
Concrete is laid in bays for highway, runway, or floor slabs. First the ground on which concrete is to be laid is prepared and all the loose materials and grass etc., are removed. The earth is wetted and compacted. The subgrades over which concrete is to be laid should be properly compacted and damped to avoid any loss of moisture from concrete. Concrete is then laid in alternate bays.
This allows the concrete to undergo sufficient shrinkage and cracks do not develop afterwards. Concrete is not placed in heap at one place and then dragged, instead it is placed in uniform thickness.
5. Concreting Underwater.
Concrete may be placed underwater with the help of bottom dump buckets. The concrete is taken through the water in water- tight bucket. On reaching the place of deposition the bottom of the bucket is made to open and the concrete is dumped. In this process certain amount of cement is washed away causing a reduction in strength of concrete.
Another way of concreting underwater is by filling cement bag with dry or semi-dry mix of cement and aggregates and lowering them to the place of deposition. The draw back of this method is that the concrete will be full of voids interspersed with purticible gunny bags.
The best method of placing concrete underwater is by the use of termie pipe. The concrete is poured into it through funnel. The bottom end of the pipe is closed with a thick ploythene sheet, with the bottom end of the pipe at the place of deposition.
The concrete (slump 150–200 mm) is poured into funnel till the whole pipe is filled with concrete. The pipe is slightly lifted and given a jerk, the polythene sheet cover falls and concrete discharged. It should be ensured that the end of pipe remains inside the concrete so that water does not enter the pipe.
The pipe is again filled with concrete through funnel and the process repeated till the concrete level comes above the water level.
No compaction is required for underwater concrete as it gets compacted by the hydrostatic pressure of water. Concrete can also be placed underwater with the help of pipes and pumps.
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