14-11-2012, 06:16 PM
COMPACTION OF CONCRETE
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It must be borne in mind that 100% compaction is important not only from the point of view of strength but also from the point the durability in recent time durability become more important then strength Insufficient compaction increases the permeability of concrete resulting in easy entry for aggressive chemicals in solution which attack concrete and rain forces cement to reduce the durability of concrete there for 100% compaction of concrete is of paramount importance. In order to achieve full compaction and maximum density with reasonable compacting efforts available at site, it is necessary to use mix with adequate workability . it is also of common knowledge that the mix should not be too wet for easy compaction which also reduced the strength of concrete. for maximum strength driest possible concrete should be compacted 100%. The overall economy demand 100% compaction with a reasonable compacting effort available in the field
HAND COMPACTION:
Hand compaction of concrete is adopted on case of unimportant concrete work of small magnitude. Sometimes, this method is also applied in such situation, where a large quantity of reinforcement is used, which cannot be normally compacted by mechanical means. Hand compaction consists of rodding , ramming or tamping. When hand compaction is adopted, the consistency of concrete is maintained at higher level. The thickness of the layer of concrete is limited to about 15 to 20cm. Rodding is nothing but poking the concrete with about 2 metre long, 16mm diameter rod to pack the concrete between the reinforcement and sharp corners and edges. Rodding is done continuously over the complete the area to effectively pack the concrete and drive away entrapped air. Sometimes instead of iron rod, bamboos or cane is also used for rodding purpose.
Ramming should be done with care. Light ramming can be permitted in unreinforced foundation concrete or in ground floor construction. Ramming should not be permitted in case of reinforced concrete or in the upper floor construction, where concrete is placed in the formwork supported on struts. If ramming is adopted in the above case the position of reinforcement may be disturbed or the formwork may fail, particularly, if steel rammer is used.
Tamping is one of the usual methods in compacting roof or floor slab or road pavements where the thickness of concrete is comparatively less and the surface to be finished smooth and level. Tamping consists of beating the top surface by wooden cross beam of section about 10*10cm. Since the tamping bar is sufficiently long it not only compacts, but also levels the top surface across the entire width.
Formwork Vibrator:
Formwork vibrators are used for concreting columns, thin walls or in the casting of precast units. The machine is clamped on to the external wall surface of the formwork. The vibration is given to the formwork so that the concrete in the vicinity of the shutter gets vibrated. This method of vibrating concrete a particularly useful and adopted where reinforcement, lateral ties and spacers interfere too much with the internal vibrator. Use of formwork vibrator will produce a good finish to the concrete surface. since the vibration is given to the concrete indirectly through the formwork, they consume more power and the efficiency of external vibrator is lower than efficiency of internal vibrator.
Table vibrator:
this is the special case of formwork vibrator, where the vibrator is clamped to thye table. or table is mounted on the springd which are vibrated transferring the vibration to the table. They are commonly used for vibrating concrete cubes. any article kept on the table gets vibrated. This is adopted mostly in the laboratories and the making small but precise prefabricated R.C.C. members.
Platform vibrator:
platform vibrator is nothing but a table vibrator, but it is larger in size. this is used in the manufacture of large prefabricated concrete elements such as electric poles, railway sleepers, prefabricated roofing elements etc. Sometimes, the platform vibrator is also coupled with jerking and shock giving arrangements such that a through compaction is given to the concrete.
Compaction by pressure and jolting:
this is one of the effective methods compacting very dry concrete. This method is often used for compacting hollow blocks, cavity blocks and solid concrete blocks. The stiff concrete is vibrated, pressed and also given jolts. With the combined action of the jolts vibration and pressure, The stiff concrete gets compacted to or dense from to give good strength and volume stability by employing great pressure, a concrete a very low water cement ratio can be compacted to yield very high strength.
Compaction by spinning:
spinning is one of the recent methods of compaction of concrete. This method of compaction is adopted for the fabrication of concrete pipes the plastic concrete when spun at a very high speed, gets well compacted by centrifugal force patented product such a “Hume pipes”, “spun pipes” are compacted by spinning process.