Compaction Of Concrete:
Consolidation of plastic concrete is known as compaction. In the process of compaction, efforts are only directed to reduce the voids in the compacted concrete. Compaction of concrete can be done either by manually or mechanically. When it is done manually it is called hand compaction or tamping and in second case it is termed as machine compaction.
1. Hand Compaction:
Hand compaction is done with the help of steel tamping rods, or timber screeds. Narrow and deep members are compacted with tamping rods. Thin slabs and floors are tamped with the help of screeds. Compaction should be done in layers of 30 cm for mass concrete and 15 cm for reinforced concrete.
Compaction should be carried out for such a time that a layer of mortar starts appearing at the compacted surface. Excessive compaction and under compaction both are harmful to concrete. Due to excessive compaction, CA particles sink to the bottom cement and F.A mortars appear at the top. This makes concrete structure heterogeneous and hence affects strength.
2 Machine Compaction:
Machine or mechanical compaction is done by using vibrators. Vibrators produce vibrations which when transmitted to plastic concrete make it flow and affect compaction. The air bubbles are forced out of concrete due to vibrations. Over vibration should not be allowed otherwise C.A particles will concentrate at the lower layers and mortar will come to the surface. There are three types of vibrators in most common use:
a. Internal vibrator
b. External vibrator
c. Surface vibrator or screed vibrator.
This vibrator is also known as needle vibrator, immersion, or poker. It consists of a power unit and a long flexible tube at the end which a vibrating head is attached. This vibrator develops about 7000 vibrations per minute. Whenever compaction is to be done, the vibrating head is inserted in the concrete. This vibrator is very useful for compaction of mass concrete.
This vibrator is also known as form vibrator and clamped to the formwork and imparts vibrations to the concrete through formwork. This vibrator is used only if the use of internal vibrator is not practicable as in the case of thin and congested situations. It is also called external vibrator.
Surface or Screed Vibrator:
This vibrator is clamped to the screed. It imparts vibration to the concrete from the surface when screeding operation of the concrete is carried out. It is effective only for depths of about 20 cm and hence useful for thin horizontal surfaces such as pavements.