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Compaction of Concrete |- it's Requirement, Purpose & Method

What is the compaction of concrete?

We can define the compaction of concrete in simple words in this way

Compaction is the process of moulding concrete in the desired shape formwork and obtain homogeneous concrete without voids so that compaction of concrete improves quality of concrete

The amount of air trapped in the concrete depends on the workability or hardness of the mixture

The Workability inversely proportional to the voids (air) available in the concrete mix i.e. If lower the Workability than the amount of air present in the mix concrete is higher

For example, the stiff concrete mixture has a high amount of air voids and therefore, it should need higher compacting work otherwise chances of segregation or honeycombs

The density, strength and durability of concrete highly depend on the compaction quality

    A requirement of compaction of concrete

    Necessary of Compaction is become useful like

    Condensation helps reduce internal friction between concrete material and between reinforcement and concrete and between formwork and concrete

    The internal friction of particle available in the concrete mixture makes it difficult to form concrete

    Friction also prevents the concrete from coming in close contact with the reinforcement, which becomes a cause of a poor bond between the reinforcement and the surrounding concrete

    The air inside the concrete mix reduces the strength of concrete greatly

    If I give  an example in data than only 5 percent Voids presence in hardened concrete left due to incomplete/improper compaction of concrete may decrease the strength of concrete by about 35 percent and 10 percent voids reduce the strength by about 50 percent

    Therefore, 100 Percent compaction of concrete is of the topmost importance in the operation of compaction

    Air voids available in the hardened concrete makes the concrete honeycombed and increase the permeability of concrete which is not allowable

    Hence fully compaction is the first requirement and also an answer to why compaction of concrete is necessary

    Purpose of Compaction of Concrete

    The main importance of compaction of concrete is to achieve a dense concrete without any air voids, to obtain the concrete to surround all reinforcement bars and to fill all conner of formworks

    Poor compaction of concrete give poor structural strength and durability

    • The best result achieved by the vibrator compaction method
    • Rodding, spading and tamping is also an alternative way of removing voids from concrete and to compact, however, the best and effective method is a vibration but at some condition, another method becomes useful due to some reason and we have done our work

    Methods of Compaction of Concrete

    The list of compaction of concrete methods is described bellow

    1. Hand compaction method
    2. Compaction by vibration method
    3. Compaction by pressure and jolting method
    4. Compaction by a spinning method

    The choice of a particular method of compaction of green concrete depends upon the various because some are useful for one condition but in the same condition, others are inadequate compaction of concrete results in bad effect

    So The choice of a particular method of compaction depends on varies factors like

    • Structure element type
    • Different properties of concrete mixture, mainly it's w/c ratio
    • The desired properties of hardened concrete like strength, durability, water tightness etc
    • Production process duration and rate of output in case of precast concrete elements
    Hand compaction of concrete mix can be done in mainly three ways
    1. Rodding
    2. Ramming
    3. Tamping 

    Compaction of concrete by hand is generally adopted for small and unimportant works and also adopted in a condition where a large quantity of reinforcement is used so that you cannot be compacted by mechanical methods

    a. Rodding

    • Rodding is nothing but concrete to be beaten with rods of about 2 meters in length and 16 mm in diameter
    • A small piece of bamboo can also be used to stick/rodding concrete
    • Concrete is packed between reinforcement and sharp corners and edges by rodding
    • The thickness of the concrete layer is limited to about 15 cm to 20 cm

    b. Ramming

    • Ramming is typically done in unreinforced foundation concrete or surface/ground construction
    • In the case of reinforced concrete or any upper floor construction, it should not be permitted
    • To compact dry mix concrete, the concrete surface is rammed with the help of a heavy flat-bottomed rammer, until a small quantity of cement paste is visible on the surface, indicating that the concrete voids Are filled

    c. Tamping

    • Tamping is adopted in compacting roofs or floors or roads pavements where the thickness of the concrete casting is comparatively small and the surface is smooth and flat
    • A wooden beam that is used for tamping which cross-section 10cm * 10cm
    Compaction by hand, give satisfactory results but the strength of concrete will be lower due to the higher w / c ratio required for complete condensation

    2. Compaction by vibration

    Construction work where high strength is required, it is necessary to use hardened concrete with a low W/C ratio. To compact the hardened concrete, mechanically operated vibrators are used

    Compaction done by vibration has many advantages compared to hand compaction which is as follows

    • A concrete of stiff consistency with the slump of around 4 cm can be placed in forms which containing closely spaced reinforcement structure design, while hand compaction required much higher consistency about maybe 12 cm slump 
    • If you use the vibrator compaction method than allows less water usage. Hence concrete of higher strength and better quality can be achieved with a given cement content because less mixing water can be used
    • Lower w/c ratio and lower cement content can produce equally strong concrete more economically than by hand compaction 
    • In the case of congested reinforcement or inaccessibility of the concrete in the formwork, hand compaction methods are not practicable. In such situations concrete can be compacted by a vibration more effectively
    • The increased bond strength between reinforcement and concrete
    • Decreased time of compaction and subsequent savings in labour cost etc

    Types of vibrator

    1. Internal vibrator
    2. External vibrator
    3. Table vibrator
    4. Surface vibrator

    3. Compaction by pressure and jolting

    • In this method, stiff concrete is subjected to external pressure from the top and vibration from the bottom along with a series of jolting actions
    • Excess water added during mixing exits due to large pressure
    • By using great pressure, a concrete of which has a very low w/c ratio can be compacted to yield very high strength
    • This method is used to compact hollow blocks, cavity blocks, and concrete blocks

    4. Compaction by Spinning

    • Compaction by Spinning is also called centrifugation
    • This method Compaction by Spinning is used in the manufacture of concrete pipes, concrete lamp posts etc. 
    • It consists of feeding concrete into a slow spinning horizontal mould
    • The spinning speed is increased after the pre-determined concrete is fed into the mould
    • Water is extracted from the mixture. At the end of the spinning process, the speed gradually decreases and the dry cement is sprinkled in small quantities
    • The initial w/c ratio of 0.35 to 0.40 reduced to about 0.30 after spinning

    Patented products such as spun pipes and Hume pipes are compacted by the spinning process

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