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Consolidation of concrete: Tools, methods, meaning, Internal and External vibration

The term consolidation of concrete is normally a mechanical process for compaction of concrete elements (like aggregate, reinforcement, cement ) and forms by application of mechanical energy. The main target of consolidation of concrete is to reduce the void ratio, air entrapped in bubble form in the fresh mixture, and thus to give maximum density to the concrete. Solid contact between concrete and reinforcement is achieved by proper consolidation.

Important for you: Cement Concrete piles |Cast-in-situ Concrete piles |Precast Concrete Pile

Why is a consolidation of concrete important?

Research data shows that the appearance of 5% of voids can reduce 30% the strength of concrete due to this reason consolidation of concrete is most important at the time of concrete casting.

Here  I am clear about the difference between voids and pores: the voids are the gaps between two individual particles and the pores are openings within the individual particles. The concrete consolidation is practiced either by hand or by means of vibrators I.e. mechanical process. Concrete consolidation tools are also discussed here.

The followings are the methods for consolidation of concrete :

  • Hand consolidation

(i)Rodding (ii) Ramming (iii) Tamping

  • Vibrators

(i) Internal or immersion vibrators (Needle vibrator)

(ii) Surface vibrators

(iii) Form or shutter vibrators (External Vibrator)

(iv)Plateform vibrator

(v)vibratory Roller

(vi) Vibrating tables.

  • Compaction by Pressure and Jolting
  • Compaction by Spinning.

1.Hand consolidation:

Hand methods of consolidation of concrete are done manually the hand which includes ramming, tamping, spading, and slicing by using suitable tools. It is normally used in unimportant works.

 The hand method is good for fairly wet concrete. It should however be remembered that as soon as possible the hand compaction should be applied because the use of a vibrator may cause the segregation of the aggregates and leakage of cement slurry. Due to these reasons, the concrete mixes which can be compacted by hand should not be compacted by the use of vibrators.

(2) Vibrators:

Vibrators are the mechanical devices that are used to compact concrete in the formwork work. The followings are advantages of mechanical vibrators compared to hand methods:

The use of vibrators can make a harsh and stiff concrete mix, with a workability of slump value of 40 mm or less.

 (i) Consolidation time is reduced by using vibrators. Hence the vibrators are used where the quick finish of work is of great importance.

Fig.2. Vibrating Table for Concrete

(iii) By the use of vibrators quality of concrete can be improved, less water will  be required or we can say, concreting work is economical by adopting a leaner mix when the vibrators are used.

(iv) Casting concrete in the congested area, small openings, or places where it will be difficult to deposit concrete by the hand methods, the vibrator is a suitable alternative.

 Following are the four types of vibrators:

(1) Internal or immersion vibrators

(2) Surface vibrators

(3) Form or shutter vibrators

(4) Vibrating tables.

Internal or immersion vibrators:

Internal vibrators are fitted with a steel tube that is inserted in fresh concrete. This steel tube also called poker is connected to an electric motor or a petrol engine through a flexible tube. They are available in different sizes ranging from 40 mm to 100 mm diameters and their size is decided by considering the spacing between reinforcing bars in concrete. The frequency of vibration is about 3000 r.p.m. to 6000 r.p.m.

In the internal vibrators, the poker vibrates when it is inserted and should be withdrawn slowly and they should be operated continuously while they are being withdrawn. Otherwise, the holes will be formed inside the concrete. The vibrator can be dropped in concrete vertically or at a slight inclination not exceeding 10° to the vertical to avoid the flow of concrete due to vibration into the mold and reduce segregation.

Hence, skilled and experienced men should be required to handle the internal vibrators. This type of vibrator is most commonly used because it is more efficient than other types of vibrators.

Surface vibrators:

Surface vibrators are mounted on platforms support or screeds. They are mostly used to finish and level concrete surfaces such as bridge floors, road slabs, station platforms, etc.

Plate vibrator and Table vibrator
Fig.3. Plate vibrator and Table vibrator

These vibrators are found to be more suitable for compacting very dry concrete mixes because the concrete is compacted in a confined zone by vibration acts in the same direction of gravity.

These vibrators finish the surface more because of the movement of fine material to the top. But the movement of excess fine material at the top will not be beneficial for plastic mixes as the wearing resistance of such fine material is very low.

Form or shutter vibrators:

Form vibrators are attached to the formwork and external centering of walls, columns, etc. The effect of vibrating action is transferred to concrete through the formwork. Some vibration energy is absorbed by the formwork during the transmission of vibrations.

Hence they are not generally used. But they are so useful and efficient for concrete sections which are too thin for the use of internal vibrators.       

One disadvantage of it is vibrators require more power by lossing of some power in vibrating the rigid shutters. They are also heavy and hence difficult to clamp at as many points as possible for uniform compaction of concrete.

Board Vibrator and Needle vibrator
Fig.4. Board Vibrator and Needle vibrator

The compaction by these vibrators is found to be suitable and economical only up to a distance of about 45cm (18 inches.)  from the face of the formwork.

(4) Vibrating tables

Vibrating tables:

Vibrating tables are in the form of a rigidly built steel platform mounted on the flexible springs and there is need of electromagnetic action or electric motors to operate it.

They can be very effective in compact stiff and harsh concrete mixes and hence they are mostly used in the preparation of precast structural products in factories like railway slippers and test specimens in laboratories.

Generally, the tables are vibrated by mechanically or by placing the springs under the supports of tables. The rate of vibration is in the range of 3000 to 7200 vibrations per minute.

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There are  two important parameters of vibrations are frequency and time and they are discussed here:

Frequency of Time

There is a relation that the frequency is inversely proportional to the time of vibration. In other words, the consolidation of concrete will be achieved in less time if the frequency is more, and vice versa.

Frequency α 1/Time

Factors which Influence Density

The density of concrete has a significant influence on many characteristics, such as workability, compressive strength, and permeability. Of all factors that influence density, the water-cement ratio is one of the most influential. Other factors influencing density are cement type, aggregates (size distribution), curing condition, and mix design method.

 For example, when using coarse aggregate in a proportion higher than 40%, density will be decreased. On contrary, using a fine aggregate in a proportion higher than 40% will increase the density of concrete.

 Also, using coarse aggregate with a high specific gravity in the proportion of more than 10% will increase the density of concrete; while using fine aggregate with low specific gravity in proportion of more than 10% will decrease the density of concrete.

In addition to these factors mentioned above, there are some other factors that also have an effect on density; for example temperature and porosity at the time of casting or freezing temperature and drying conditions during the hardening process, etc.

New Technology Consolidation of concrete

When you mix cement and water together, they create a strong bond. Over time, that bond gets stronger and creates what we call concrete. That process is called consolidation. When you take an ingredient from its natural state and apply pressure or heat, it will become denser or consolidated.

If that ingredient is a liquid, like water or oil for example, then it becomes less fluid over time through consolidation.

The best way to increase density in your concrete is to use heat because it helps mix chemicals properly and allows chemical reactions in concrete to complete more effectively.

 By adding heat, you can improve the strength and durability of your concrete product.

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