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Compaction Of Concrete

Presented to : Engineer Mohammad Shoaib

Prepared & Presented By:Usama Javaid 2014-UETR-CE-04M.Abaid Ullah 2014-UETR-CE-06Sheeraz Ahmad 2014-UETR-CE-19Abdul Haseeb 2014-UETR-CE-26Tayab Shokat 2014-UETR-CE-39

University of Engineering & Technology , Lahore

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Contents of Presentation:

1. Introduction2. Necessity of Compaction3. Methods of Compaction a) Manual Compaction b) Mechanical Compaction4. Effects on Hardened Properties5. Under-Vibration & Over-Vibration6. Re-vibration7. Segregation8. Summary

31 . Introduction“Compaction is the process which expels entrapped air from freshly placed concrete and packs the aggregate particles together so as to increase the density of concrete”.

It increases significantly the ultimate strength of concrete and enhances the bond with reinforcement. It also increases the abrasion resistance and general durability of the concrete, decreases the permeability and helps to minimise its shrinkage-and-creep characteristics. Proper compaction also ensures that the formwork is completely filled – ie there are no pockets of honeycombed material – and that the required finish is obtained on vertical surfaces. When first placed in the form, normal concretes, excluding those with very low or very high slumps, will contain between 5% and 20% by volume of entrapped air.Compaction of concrete is, therefore, a two-stage process. First the aggregate particles are set in motion and slump to fill the form giving a level top surface. In the second stage, entrapped air is expelled.

42. Why is compaction of concrete necessary ?

Air voids reduce the strength of the concrete. For every 1% of entrapped air, the strength falls by somewhere between 5 and 7%. This means that concrete containing a mere 5% air voids due to incomplete compaction can lose as much as one third of its strength.

Air voids increase concrete's permeability. That in turn reduces its durability. If the concrete is not dense and impermeable, it will not be watertight. It will be less able to withstand aggressive lquids and its exposed surfaces will weather badly.

5Moisture and air are more likely to penetrate to the reinforcement causing it to rust.

Air voids impair contact between the mix and reinforcement (and, indeed, any other embedded metals). The required bond will not be achieved and the reinforced member will not be as strong as it should be.

Air voids produce blemishes on struck surfaces. For instance, blowholes and honey combing might occur.

Summing up, fully compacted concrete is dense, strong and durable; badly compacted concrete will be porous, weak and prone to rapid deterioration. Sooner or later it will have to be repaired or replaced.

63. Methods of CompactionTo compact concrete we apply energy to it so that the mix becomes more fluid. Air trapped in it can then rise to the top and escape. As a result, the concrete becomes consolidated, and you are left with a good dense material that will, after proper curing, develop its full strength and durability.Compaction can be done Manually and Mechanically.

a) Manual / Hand Compaction Hand compaction is used for ordinary and unimportant structures. Workability should be decided in such a way that the chances of honeycombing should be minimum. The various methods of hand compaction are as given below.

1. RoddingIt is a method of poking with 2m long, 16 mm dia rod at sharp corners and edges. The thickness of layers for rodding should be 15 to 20 cm.

72. RammingIt is generally used for compaction on ground in plain concrete. It is not used either in RCC or on upper floors.

3. TampingIt is a method in which the top surface is beaten by wooden cross beam of cross section 10 cm x 10 cm. both compaction and leveling are achieved simultaneously. It is mainly used for roof slabs and road pavements.

8b) Mechanical CompactionVibration is imparted to the concrete by mechanical means. It causes temporary liquefaction so that air bubbles come on to the top and expelled ultimately. Mechanical vibration can be of various types as given under.

1. Immersion/Internal VibrationFrequently referred to as ‘poker’ or ‘needle ’ vibrators, immersion vibrators consist essentially of a tubular housing which contains a rotating eccentric weight. The out-of-balance rotating weight causes the casing to vibrate and, when immersed in concrete, the concrete itself. Depending on the diameter of the casing or head, and on the frequency and the amplitude of the vibration, an immersion vibrator may have a radius of action between 100 and 600 mm

The effectiveness of an immersion vibrator is dependent on its frequency and amplitude, the latter being dependent on the size of the head, the eccentric moment and the head weight – the larger the head, the larger the amplitude

92. External Vibration / Form vibratorsThis is adopted where internal vibration can’t be used due to either thin sections or heavy reinforcement. External vibration is less effective and it consumes more power as compared to the internal vibration. The form work also has to be made extra strong when external vibration is used.

3. Table VibrationIt is mainly used for laboratories where concrete is put on the table.

104. Platform VibrationIt is similar to table vibrators but these are generally used on a very large scale.(Figure 1)

5. Surface VibrationThese are also called screed board vibrators. The action is similar to that of tamping. The vibrator is placed on screed board and vibration is given on the surface. It is mainly used for roof slabs, road pavements etc., but it is not effective beyond 15 cm depth.(Figure 2)

(Figure 2)(Figure 1)

11POINTS TO CONSIDER WHILE COMPACTING CONCRETE USING VIBRATORS:When concrete is compacted with vibrators the following precautions may be taken to produce a concrete of good quality.1) The formwork should be strong and tight so that there is no leakage of mortar. The leakage

of mortar results in honey combing.2) The concrete should be laid in layers of thickness 15 cm to 60 cm. in order to avoid

trapping of air in the concrete the thickness should be as less as practicable.3) To regulate the degree of compaction in all portions of the concrete the vibrators should be

inserted vertically.4) Over vibration of concrete, particularly if the slump exceeds 100 mm should be avoided as

it causes settling of the coarse aggregate at the bottom and the accumulation of the mortar or water at the top.

5) The vibrators should not be used to move the concrete laterally in the forms as it leads to segregation.

6) The vibrator should not touch the surface of the formwork so that it may not be damaged.7) The vibrator should be withdrawn slowly at each location and should be operated

continuously while being withdrawn so that no hole will remain in the concrete.

124. Effects on Hardened Properties

As may be seen from Figure , the effect of compaction on compressive strength is dramatic. For example, the strength of concrete containing 10% of entrapped air (air voids) may be as little as 50% that of the concrete when fully compacted.Permeability may be similarly affected since compaction, in addition to expelling entrapped air, promotes a more even distribution of pores within the concrete, causing them to become discontinuous. This reduces the permeability of the concrete and hence improves its durability.The abrasion resistance of concrete surfaces is normally improved by adequate compaction. However, excessive vibration, or excessive working of the surface, can cause an excessive amount of mortar (and moisture) to collect at the surface, thereby reducing its potential abrasion resistance. In flatwork therefore, a careful balance is required to expel entrapped air without bringing excessive amounts of mortar to the surface of the concrete.

13RELATIVE STRENGTH (%)

AIR VOIDS (%)

0

20

40

60

80

0

15 20 25 30

100

5 1 0

Loss of strength through incomplete compaction

145. What is Under-Vibration & Over-Vibration

Normal-weight concretes that are well proportioned are not readily susceptible to defects caused by over-vibration. These result from segregation and are characterised by an excessive thickness of mortar at the surface of the concrete. The surface may also have a frothy appearance. over-vibration may cause problems when grossly oversized equipment is operated for an excessive length of time, but is more likely to cause problems with poorly-proportioned mixes or those to which excessive amounts of water have been added.When signs of over-vibration are detected, the initial reaction may be to reduce the amount of vibration. The most appropriate solution may be to investigate if suitable changes to mix design are warranted.Under-vibration is far more common than over-vibration and, when it occurs, can cause serious defects. Invariably, the concrete is incompletely compacted which reduces its strength , its durability and will possibly affect its surface finish.

156. What is Re-vibration

Re-vibration of concrete is the intentional systematic vibration of concrete which has been compacted some time earlier. It should not be confused with the double vibration which sometimes occurs with the haphazard use of immersion vibrators or multiple passes of a vibrating-beam screed.It is generally agreed that re-vibration of concrete can be beneficial to its strength, its bond to reinforcement and its surface finish, the practice is not widely used, partly due to the difficulty of knowing just how late it can be applied. a good rule of thumb is that re-vibration may be used as long as the vibrator is capable of liquefying the concrete and sinking into it under its own weight.

Situations in which re-vibration may be beneficial include:1. To bond layers of concrete into those preceding them. In elements such as walls, deep beams and columns2. To close plastic shrinkage and settlement cracks3. To improve the surface finish at the tops of columns and walls 4. To improve the wear resistance of floors

167. What is Segregation

“Segregation” as – Separation or isolation of one thing from another and place in a group apart from others.In concrete technology, segregation can be of two types. Firstly the coarse aggregate may separate from the main mass of concrete in its plastic state. Secondly, the grout (cement+paste) may separate from the mix. The second one is generally occurs if a concrete is too wet.

17WHAT CAUSES SEGREGATION IN CONCRETE

Concrete is a mixture of aggregate, cement, water & admixture. The sizes and their specific gravities are different from each other. For example the specific gravity of most of the aggregates fall in the range between 2.6 to 2.7 and that of most of the cement is in between 3.1 to 3.2. Because of this they show a tendency to separate from each other.

The separation can be of two types as mentioned below.

1. Coarse aggregate separating out or settling down from rest of the matrix (Type-1 segregation)

2. The paste separating away from coarse aggregate (Type-2 segregation)

18Causes of Segregation of Concrete:

1) Use of high water-cement ratio. This general happens in case of concrete mixed at site by unskilled workers.

2) Excessive vibration of concrete with mechanical needle vibrators makes heavier particles settle at bottom and lighter cement sand paste comes on top.

3) When concreting is done from height in case of underground foundations and rafts, which causes concrete to segregate.

We should Take under consideration above mentioned causes To avoid Segregation.

19Summary

1) Definition and Introduction of Compaction of Concrete.2) Why compaction of Concrete is Necessary?3) Different Methods of Compaction of Concrete.4) Effect of Compaction on Concrete Properties.5) What is under Vibration & Over Vibration?6) What is Re-Vibration?7) What is Segregation?

In Our Presentation We have Discussed The Following sub topics: