Mechanical Compaction of Concrete

This presentation is about the Compaction it's stages, its importance as well as Different types of Vibrators like Surface Vibrator, Beam Screed Vibrator, Roller Screed Vibrator, Plate Vibrator, Table Vibrator, their use, why to vibrate concrete, whats the effect of over vibration using needle vibrator in RCC and conclusion.

1. Mechanical Compaction of Concrete By: Abhishek Shah

2. Content . . . .

3. INTRODUCTION TO Compaction of concrete 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. Fig. 1 Compaction is actually two stage process Fig. 2 Fig.1 Fresh Concrete Compaction Source : Google Images

4. Fig. 2 Compaction of Fresh Concrete Source : Google Images

5. Stages of compaction of concrete Stage 1: with vibration, initial consolidation of the concrete can often be achieved relatively quickly. The concrete liquefies and the surface levels, giving the impression that the concrete is compacted Stage 2: In the second stage, entrapped air is expelled. Entrapped air takes a little longer to rise to the surface. Compaction must therefore be prolonged until this is accomplished, ie until air bubbles no longer appear on the surface.

6. IMPORTANCE OF COMPACTION

7. It is important to compact the concrete fully because, 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 about 5% air voids due to incomplete compaction can lose as much as one third of its strength. Figure 3 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 liquids and its exposed surfaces will weather badly. Moisture 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.

8. 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 minimize its shrinkage-and-creep characteristics. Proper compaction also ensures that the formwork is completely filled i.e. there are no pockets of honeycombed material and that the required finish is obtained on vertical surfaces When first placed in the form, normal concrete, excluding those with very low or very high slumps, will contain between 5% and 20% by volume of entrapped air. The aggregate particles, although coated with mortar, tend to arch against one another and are prevented from slumping or consolidating by internal friction . Benefits O f Compaction

9. Mechanical Compaction of concrete Immersion Vibration : In immersion vibration a mechanical device termed as needle vibrator is broadly used by many firms Fig. 4 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 Fig. 5 Fig 4. Immersion Vibrator Source : Google Images

10. Detail sketch of needle vibrator Fig. 5 Immersion Vibrator Source : Google Images

11. The out-of-balance rotating weight causes the casing to vibrate. 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. Fig. 6 Working with needle vibrator Source : Google Images

12. Recommendation for use of poker vibrator Diameter of head (mm) Recommended Frequency (HZ) Average Amplitude (mm) Radius of Action () Rate of Concreting ( cmt /hour) 20–40 150–250 0.4–0.8 75–150 1–4 30–65 140–210 0.5–1.0 125–250 2–8 50–90 130–200 0.6–1.3 175–350 6–20 75–150 120–180 0.8–1.5 300–500 11–31 125–175 90–140 1.0–2.0 400–600 19–38 Adapted from Table 5.1 ACI Committee Report: Guide for Consolidation of Concrete 309R-05 ACI Manual of Concrete Practice 2006 Part 2.

13. Following care should be taken while using needle vibrator As a general rule, the radius of action of a given vibrator not only increases with the workability of the concrete (higher slump), but also with the diameter of the head Immersion vibrators should be inserted vertically into concrete, as quickly as possible, and then held stationary until air bubbles cease to rise to the surface, usually in about 15–20 seconds Fig. 7 vertical position of needle Source : Google Images

14. The vibrator should then be slowly withdrawn and reinserted vertically in a fresh position adjacent to the first. These movements should be repeated in a regular pattern until all the concrete has been compacted Fig. 7 Random insertions are likely to leave areas of the concrete uncompacted. One should try that minimum overlap of this vibration circle should allowed Fig. 8 The vibrator should not be used to cause concrete to flow horizontally in the forms, as this can lead to segregation Fig. 8 Overlap of vibration radius of vibrator Source : CCANZ Bulletin

15. the vibrator should not be dragged through the concrete as this leads to inadequate compaction and increases the risk of segregation. In deep sections such as walls, footings and large columns, the concrete should be placed in layers about 300 mm thick The vibrator should penetrate about 150 mm into the previous layer of fresh concrete to meld the two layers together and avoid ‘cold-pour’ lines on the finished surface The vibrator should not be allowed to touch the forms as this can cause ‘burn’ marks which will be reflected on the finished surface Similarly, the vibrator should not be held against the reinforcement as this may cause its displacement.

16. inclined forms are prone to trapping air. To minimize this tendency, the best technique is to place the concrete close to, but away from the side of the form and insert the immersion vibrator close to the leading edge of the concrete, forcing it to properly fill the corner Fig. 9 Void-formers are also prone to trapping air on their undersides if concrete is placed from both sides and then compacted. Concrete should be placed at one side and, maintaining a head, vibrated until it appears at the other side. Fig. 9 Pouring of Concrete Source : CCAA bulletin

17. Surface vibration: Surface vibrators are applied to the top surface of concrete and act downwards from there. They are very useful for compacting slabs, industrial floors, road pavements, and similar flat surfaces. They also aid in leveling and finishing the surface . Fig. 10 There are a number of types of surface vibrators including vibrating-roller screeds, vibrating-beam screeds The most common type is the single or double vibrating-beam screed. Or Roller screed Fig. 10 Typical surface vibrator Source : Google Images

18. Beam Screed vibrator A vibrating-beam screed consists of either one or two beams, made from aluminum, steel or timber, to which is attached a form of vibrating unit to allow the beams to impart adequate vibration to the concrete Fig. 11 This may be a single unit, mounted centrally, or may consist of a series of eccentric weights on a shaft driven from a motor on one end and supported on a trussed frame Fig. 11 Double Beam Screed vibrator Source : Google Images

19. In general, the centrally-mounted units have a maximum span of about 6 m, but the trussed units may span up to 20 m. The intensity of vibration, and hence the amount of compaction achieved, decreases with depth because surface vibrators act from the top down. Therefore, the slab thickness for which compaction by surface vibrators is effective will vary (from 100 to 200 mm) depending on the size and operation of the unit used Depth of compaction decreases in vicinity of edge forms Depth of compaction (maximum 200 mm) Immersion vibrators needed to supplement compaction here Surface vibrator Edge Form Fig. 12 Effect of Surface Vibrator Source : Google Images

20. With centrally-mounted vibration units, the degree of compaction achieved may vary across the width of the beam Fig. 12 It is generally desirable, therefore, to supplement vibrating-beam compaction by using immersion vibrators alongside edge forms. The effectiveness of vibration, and hence degree of compaction, increases with an increase in the beam weight, the amplitude and the frequency, As the forward speed of beam increases compaction decreases and vise versa Speed of screed should be limited to between 0.5 and 1.0 m/min. for getting batter output The lower speed should be used for thicker slabs and where reinforcement is close to the top face

21. Roller Screed vibrator : Roller screed vibrator are same of that beam screed vibrators Fig. 13 In this type of vibrator beam is replaced by long cylindrical roller Here roller is given vibration through internal rotating imbalanced weight Vibration is occurred throughout the cross section is same Fig. 13 Roller screed vibrator Source : Google Images

22. Plate Vibrator : Plate vibrator are generally used in laboratories Fig. 14 It is generally used for compaction of practical specification made in lab It is not preferred on large scale of concreting done on big sites Vibrating table techniques are usually restricted to recasting operations Also reflection of the pressure waves against the concrete surface will influence the amplitude distribution Table vibrators can give less consistent results even with careful operation. Fig. 14 Plate vibrator Source : Google Images

23. Form vibration: In form vibration a mechanical vibrating device is used and it is attached with the form work Fig.15 ,Fig. 16 Form vibrators are useful with complicated members or where the reinforcement is highly congested They are clamped to the outside of the formwork and vibrate it thus compacting the concrete in this type of vibration first vibration is transferred to the form work and then it is transferred to the concrete Fig. 15 Form vibrator Source : Google Images

24. Due to above reason it consumes more power then the ordinary vibrators The formwork will need to be specifically designed to resist the forces imposed on it . All joint of the form work should specially design to withstand against the rapid shocking movement Fig. 16 Form vibrator Source : Google Images

25. Conclusion In today’s rapid growing world Concrete is most essential material for construction. But the concrete properties like strength, durability, serviceability are the problem. But with complete compaction of concrete one can improve concrete property like strength, durability, serviceability with great extent.

26. Reference Concrete technology by M. S. Shetty www.google.com www.concretetools.co.nz www.concrete.net.au www.vibco.com