Compaction of Concrete: Voids and Methods | Concrete Technology

In this article we will discuss about:- 1. Introduction to Compaction of Concrete 2. Voids in Concrete 3. Methods 4. Types of Vibrators.

Introduction to Compaction of Concrete:

The process of compaction of concrete consists essentially of the elimination of entrapped air from the concrete. The oldest means of achieving this aim is by punning with a rod, or by ramming or by vibrating. Thus the object of compacting concrete is to obtain a dense mass of concrete without voids, to get the con­crete to surround all reinforcement and to fill all corners.

Compaction of concrete influences the strength, durability, impermeability of concrete to a great extent. Each one percent less compaction reduces the strength of concrete by about 5% on an average. During the process of manufacture of concrete a consi­derable amount of air is entrapped forming voids in it.

Voids in Concrete:

Voids present in concrete in the form of small pores reduce the strength and density of concrete.

Three kinds of voids may be present in the concrete as follows:

1. Water Voids:

These voids are present due to the water in excess of that needed for hydration.

2. Air Voids:

These are either due to insufficient compaction or due to deliberate entrainment of air in the mix. Concrete containing air voids left due to insufficient compaction of the freshly placed concrete lowers the strength very much. Experimental results have shown that 10% air voids left in concrete reduce its strength by more than 50%. Thus freshly laid concrete should be compacted sufficiently so that air voids are completely removed. Voids due to poor compaction can be seen readily when they are at the surface. The effect of air voids is shown in Fig. 11.1.

Fig. 11.1 (a) shows the position of fresh un-compacted concrete having about 10% air voids. In this condition its compressive strength is found 122.5 kg/cm² and weight 2166 kgm³. Fig. 11.1 (b) shows 5% air voids in the concrete, resulting its strength and weight as 190.5 kg/cm² and 2286 kg/m³; while 11.1 (c) shows a fully compacted concrete having its strength and weight as 272.0 kg/cm² and 2410 kg/m³ respectively. This shows the influence of air voids on the strength and weight of concrete. Honey-combed concrete does not develop good bond with reinforcement. Water may penetrate through these voids and corrode the steel.

Methods of Compaction for Concrete:

The method of compaction depends upon its workability, so that is can be fully compacted. In other words the required workability of concrete depends on how much it is to be compacted and by what method. It will also depend on the job condition.

Usually following methods may be adopted for concrete compaction:

I. Hand Compaction or hand tamping

II. Mechanical compaction

I. Hand Compaction:

This method is adopted for members having reinforcement, pavements, narrow and deep members etc. Rammers and iron rods are used for this purpose. Mass concrete should be compac­ted in layers not more than 30 cms in thickness with templates or light rammers. For compacting reinforced concrete works iron rods are used. In this case the thickness of concrete layers should not be more than 15 cms. The most satisfactory method of compacting concrete properly is to consolidate each layer separately so that its top surface becomes level and fairly smooth before the next layer is placed.

When the rod is inserted vertically into a mass of workable concrete and moved up and down, it causes the concrete to be set in motion similar to that of the rod. This action is known is tamping or punning. When the concrete is set in motion in this way, the air bubbles present in the concrete come to the surface.

Tamping should be carried out for each layer in such a way that concrete is filled in all the corners of the form work. While tamping is carried out care should be taken that the rod should penetrate the full depth of the last layer placed and to some extent into the under lying layer to ensure proper bond between them. Secondly the reinforcement and form work should not be disturbed from their positions.

Tamping of Road Slabs:

For compacting a concrete slab such as for roads, floors etc. a steel shod wooden tamping beam or screed is used as shown in Fig. 11.2. It can also ensure the correct level of the surface. The total length of the tamper should be 22.5 cms more than the width of the slab. For compacting the base coarse the tamper should be recessed near the ends by 2.5 cms as shown in Fig. 11.2.

The concrete is placed in the form, so that it is deeper than the side forms and the tamper is bumped up and down on the top surface of the concrete. By this actin concrete goes down to the level of the form. During tamping process, any low spots should be filled with concrete sprinkling from a shovel and tamped into the surface. A second run with the tamper is done with a to and fro motion to get the desired surface level.

Finishing Concrete Slabs:

The operations adopted for obtaining a true, and uniform concrete surface are called finishing operations. A tamper usually leaves a slightly ridged surface. Thus it needs finishing. Actually the choice of concrete finish depends upon the use of the completed work.

Finishing Operations:

For finishing floors and slabs following operations are adopted:

i. Screeding

ii. Floating

iii. Troweling

i. Screeding:

The levelling operation which removes bumps and hollows and gives a true and uniform concrete surface is known as screeding. For this purpose a straight edge is used. It is slightly longer than the section to be finished. The surface is struck off by moving the straight edge back and forth across the top of the surface. The straight edge is advanced forward a very little distance with each movement.

A small quantity of concrete mix should always be kept ahead of the straight edge to fill the voids and to maintain a plane surface. Screeds may be of roller type or vibrator type.

ii. Floating:

The operation of removing the irregu­larities from the surface of the concrete left after screen­ing is known as floating. It helps in leveling the surface and compacting the concrete. Floating is done with the help of 1.5 m long and 20 cms wide wooden floats. To give ease in handling the float, a handle is provided at its centre. Finishing is done by moving the float for­ward and backward.

iii. Troweling:

The final operation of finishing is known as troweling. It is done where smooth surface is desired. Troweling should be done after the evaporation of water from the concrete surface. Spreading of dry cement on wet surface to absorb excess water should not be allowed as it is not a good practice, and will develop cracks and blistering.

Types of Surface Finish:

According to the roughness of the surface, the surface finish can be classified as follows:

1. Brush Finish:

Brush finish can only be done after the surface has been floated or belted. It is done with a bamboo broom brushed lightly across the slab after the concrete has set slightly. If a rougher surface is desired, further brushing may be done till the aggregate becomes exposed.

2. Belt Finish:

A belt finish is produced by using a strip of canvas about 15 cms wide and about 1.5 m longer than the width of slab. The belt is laid across the surface and drawn to and fro with rather long strokes and moved forward slowly in the direction of concreting.

Suitability of Concrete Mix:

If concrete is to be compacted successfully by hand, then the concrete mix should be of proper propor­tions and required consistency.

Concrete to be hand compacted should have following slumps as shown in Table 11.1:

If the concrete is highly deficient in fine aggregate or highly under sanded it may not be possible to compact it, however wet it is made or however much it is tamped. Such a mix would be very harsh and the mortar would tend to separate out from the coarse aggregate causing honeycombing even though thorough tamping may be applied to it.

On the other hand if the mix is over sanded, to make it workable larger amount of water is needed. In such cases excessive maintenance would be formed on the surface after the concrete has been compacted. This laintence being a layer of slurry of cement and sand forms a weak and porous layer on the surface of the lift. Such layers often allow water to leak through construction joints of retaining walls and produce unsig­htly staining.

II. Mechanical Compaction:

Mixes which are too stiff to consolidate by hand, they can be easily compacted by mechanical com­paction. When a concrete mix is vibrated, it behaves as if it is very much wetter than it really is and will flow easily. This method of Compaction has proved very much effective than hand compaction.

In this case the concrete is compacted by vibration, during which the vibrator communicates rapid vib­rations to the particles, increasing the fluidity of concrete. Due to vibration, the particles occupy a more stable position and the concrete fills all the space and the air present is forced out to the surface, resulting in dense and durable concrete. The frequency, duration of vibration etc., depends on the conditions of vibration for compacting of the mix, which have been discussed below.

Advantages of Vibrated Concrete:

Following are the advantages of compaction by vibrations:

i. By the use of vibration, stiff mixes can be compacted easily.

ii. For a given cement/aggregate ratio a much stronger and dense concrete can be produced.

iii. Bond between reinforcement and concrete is improved.

iv. Creep and shrinkage is reduced.

v. Laintence is also reduced to some extent.

vi. Speed of placing concrete is increased.

vii. Better finish is obtained.

Types of Vibrators Used for Concrete Compaction:

Following three types of vibrators usually are used for concrete compaction: 

1. Internal Vibrator:

These vibrators are inserted into the concrete and withdrawn after the compaction has been completed.It is also known as immersion, poker or needle vibrator. Usually it is considered to be the most effective type of vibrator as it comes into intimate contact with the concrete.

Though there are different kinds of internal vibrators, but they essentially consist of the following components:

i. A power unit

ii. Flexible tube

iii. Vibrating head

i. A Power Unit:

It can be either a petrol engine or electric supply unit.

ii. Flexible Tube:

It is a rubber tube having power connecting device at one end and vibrator head on the other end.

iii. Vibrator Head:

It is a straight iron tube having eccentric weight fitted inside it. As the current passes through this tube, the vibratory action is caused by the rotation of the eccentrically weighted shaft at high speed, usually over 7000 r.p.m. which are transmitted to the concrete particles.

The needle diameter may vary from 2.0 cms to 7.5 cms and length from 25 cms to 90 cms. The frequ­ency of vibration may vary upto 12000 cycles per minute. The bigger needle is used in the construction of mass concrete works as dams.

The vibrating head is immersed in the fresh concrete and transmits vibrations to the concrete through the vibrator’s body. The vibrating head should be immersed vertically in the concrete and should be kept inside the concrete. While running the vibrator, the bearings should be kept cool and breakdowns should be avoided. As the concrete consolidates, its surface level falls and the surface becomes smooth. Air bubbles may be seen breaking at the surface. When these bubbles cease to appear on the surface, the concrete may be assumed as compacted sufficiently.

The vibrator should be lowered or inserted vertically in the concrete. After the concrete is compacted, the vibrator should be withdrawn slowly at the rate of 7.5 to 8 cm/sec. as this speed will fill the hole caused due to withdrawal of vibrator without any air being entrapped. Now the vibrator is inserted at another point at a distance of 50 cm to 100 cm or 45 cm to 75 cm. However this distance depends on the shape of the form work and amount of reinforcement. This distance should be kept uniform. For concrete of workability of 0.78 to 0.85 compacting factor the vibrator should be spaced at points 35 to 90 cms apart.

The internal vibrator neither should be allowed to tough the inside faces of the form work, as it may damage the surface nor the reinforcement as the displacement of reinforcement will upset all calculations.

The frequency of vibration varies from 3500 to 12000 cycles per minute. Frequency between 3500- 5000 cycles per minute has been suggested as a desirable minimum, with an acceleration of not less than 4 g, but recent studies found favour for frequencies from 4000-7000 cycles per minute.

Time of Vibration:

The time of compaction by the internal vibrator depends on the consistency of the mix. Usually from 5 to 30 seconds compaction is sufficient depending upon the consistency of the mix, but for some mixes upto 120 seconds compaction is required. The actual completion of compaction can be judged by the appearance of the surface of the concrete, which should neither contain excessive mortar nor it should be honey combed.

Some researchers have correlated the time of compaction by internal vibrators with the con­sistency of the mix and the frequency of the vibrator. For complete compaction of concrete having 1.25 cm slump required 90 seconds vibration at 4000 r.p.m., 45 seconds at 5000 r.p.m. and 25 seconds at 6000 r.p.m.

When concrete is placed in layers the vibrator should be immersed through the entire depth of the freshly deposited concrete and into the layer below it, if this is still plastic or can be brought again into a plastic condition. In this way a plane of weakness at the junction of the two layers can be avoided.

For layers more than 50 cms, the vibrator may not be fully effective in expelling air from the lower part of the layer. As stated above, the vibrator should remain vertical while in use. Fig. 11.5 shows a correct and incorrect position of the vibrators.

Precautions to be Observed While Using Internal Vibrator:

While using internal vibrator, following precautions should be taken:

1. In order to prevent any leakage of mortar, the form work should be as tight as possible.

2. In order to avoid air being trapped, the thickness of the concrete lift should not be less than 15 cm.

3. For lifts more than 50 cm, internal vibrators should not be used, but upto 45 cm lift, the vibrator should be allowed to penetrate into the previously compacted concrete below to obtain a proper bond between the two lifts or layers. If required the vibrating unit can be immersed completely upto the bottom of the form.

4. Vibrations should be controlled carefully by immersing the vibrator for 5 to 15 seconds at a point. The distance between two immersion points of the vibrator may vary from 45 to 75 cm or 50 to 100 cm.

5. The vibrator should be immersed vertically and should remain in vertical position throughout the operation at that point otherwise the degree of compaction cannot be controlled well.

6. Vibrator should not be used to push concrete laterally in the form works as it will cause segregation of the concrete.

7. The vibrator should be withdrawn slowly usually 7.5 cm to 8.0 cm per second as at this speed of withdrawal, the hole left by vibrator will close fully without any air being trapped.

8. The vibrator should be immersed through the full depth of freshly laid concrete and into the layer below it if the concrete is still plastic or it can be brought into plastic state.

9. The vibrator neither should touch the inside of form work nor the reinforcement.

10. The vibrator should never be kept running for long when it is not in the concrete.

11. The tube should never be bent at a very sharp angle as it will produce unnecessary heavy strain on the internal flexible drive and the outer tube. The tube should be kept as straight as possible.

Sharp bends should be avoided near the junction of the tube and the vibrating head. Internal vibrators are made in sizes down to 20 mm diameter so that they can be used for heavily rein­forced sections. These classes of vibrators are found comparatively more efficient as all the work is done directly on the concrete. Though internal vibrators are usually constructed very strong and are capable being used under very rough conditions, however they cannot be expected to work continuously for an in­definite period of time without wearing.

The bearings inside the vibrating head are subject to severe wearing conditions and are likely to be­come over heated. This can be prevented by immersing the full length of vibrating head into the concrete.

2. Form Vibrator or External Vibrators:

These vibrators are clamped on the side of the form work.These vibrators are used for compacting concrete where internal vibrators cannot be used such as for very heavily reinforced or very narrow sections or precast units etc. In such conditions vibration is applied through the form work by clamping these vibrators on the shuttering.

The vibrators are rigidly clamped to the form work resting on an elastic support, so that both the form and concrete are vibrated. In this case a considerable proportion of work done is consumed in vibrating the form work, resulting in low efficiency of the system. In order to check the leakage of cement mortar the form work should be strong and tight.

Principle of Working:

The principle of an external vibrator is same as that of an internal vibrator, but the frequency is usually between 3000 and 6000 cycles of vibration per minute. Bureau of reclamation has recommended at least 8000 cycles per minute.

When an external vibrator is used, concrete has to be placed in layers of suitable depth as air cannot be expelled through a great thickness of concrete. The position of vibrator has to be changed as concreting progresses. They should not be placed more than 90 cms apart in any direction.

The form vibrator consists of an electric motor with an unbalanced weight. This weight causes the unit to vibrate when it is rotated by the motor. These vib­rators should be fixed on the form work firmly and should not be placed more than 1 m apart.

Kango Hammer:

It is another form of external vibrator. It is an electric hammer used on the outside of the vertical shuttering. This type of vibrator is useful for vibrating those parts of shuttering where it would be diffi­cult to fix the ordinary clamp on the vibrator or where the vibrator is required to move continuously.

Vibrating Tables:

These are also external vibrators and are suitable for pre-cast concrete work. It provides a reliable means of compaction of precast concrete and has the advantage of offering uniform vibration.

3. Surface Vibrators:

These vibrators are placed on the top of the concrete surface and concrete compacted.These vibrators are used where a wide horizontal surface occurs such as in dams and very thick walls. Usually such vibrators are used for roads and floors. Though a large number of different types of surface vibrators exist, but pan type vibrator is generally used for floors and foundation works. It consists of a flat pan or tray with handles at each end and a vibrating unit mounted in the middle. The vibra­tory unit may have a petrol engine or electric motor or may be pneumatic type. This type of vibrator is moved across the surface of the concrete by the handles.

Vibrating Screeds:

External vibrator type unit is often used for surface vibration by mounting it on a screed or tamper. Vibrat­ing screeds may have more than one unit of such vibra­tors. These screeds can be used for compacting concrete layers not more than 20 cm in thickness.

Compaction by Pressure and Jolting:

This method has proved very effective for compacting very dry concrete. This method is used for com­pacting hollow blocks, cavity blocks and solid concrete blocks. The stiff concrete is vibrated, pressed and also jolted. With the combined effect of vibration, pressure and jolt, the stiff concrete gets compacted to a dense form to give good strength and volume stability. By applying great pressure, concrete of low water/cement ratio can be compacted to yield very high strength.

Compaction by Spinning:

This is one of recent methods of compaction of concrete. This method of compaction is employed for the production of concrete pipes. When plastic conc­rete is spun at a very high speed, it gets compacted by the action of centrifugal force. Patented pipes as “Hume pipes”, “Spun pipes” are compacted by this method.

Vibratory Rollers:

Compaction of very dry and lean concrete by vibratory rollers is one of the recent developments. Concrete compacted by this method is known as Roller Compaction Concrete. This method of concrete com­paction was developed in Japan and spread to USA and other countries mainly for the construction of dams and pavements.

For compaction of dry lean concrete, heavy rollers which vibrate while rolling are used. Such roller compacted M₁₀ grade concrete of 15 cms thickness has been successfully used as base course for Mumbai-Pune express highways and Delhi-Muthra highways.

Selection of Vibrators for Various Situations:

Actually the use of a certain type of vibrators dep­ends upon the situation of work.

Broadly following table 11.2 gives an indication of the use of the different vibrators:

Suitability of Mix for Compaction with Vibrator:

Concrete to be compacted by vibration, should be designed properly. The consistency of the concrete depends upon the conditions of placing, type of mix, and the efficiency of the vibrator. The slump of such concrete should not be more than 5 cm in any case otherwise segregation of concrete will take place, which should never be allowed to occur.

The value of slump and compacting factors for different works are shown in Table 11.3:

Comparison of Internal and External Vibrators:

1. Internal vibrators are more efficient as all the work is done directly on the concrete. They can be used for concrete layers upto 50 cm thickness.

2. Internal vibrators can be used even for heavily reinforced sections.

3. External vibrators are clamped rigidly to the form work hence a considerable portion of the work done is wasted in vibrating form work.

4. Form work used for external vibrators should be very rigid and strong. This increases the cost of the form work.

5. By the use of external vibrator, excessive cement mortar comes to the surface of the concrete work. This surface being comparatively weak and less pervious deteriorates very soon.

Output of Immersion Vibrators:

For light vibrators having a centrifugal force of about 200 kg, the output of compacted concrete may be taken as 3 to 5 m³ per hour depending upon the consistency of the mix and for heavy type vibrators having a centrifugal force of 450 kg, the output may be taken as 12 to 25 m³ per hour.