commpaction of concrete by ranchhod BY 9712068802 MECHANICAL COMPACTION OF CONCRETE: A GOVERENING FACTOR FOR DURABILITY AND SERVICEABILITY OF THE CONCRETE Ranchhod Matal, Prof. Jayeshkumar Pitroda2, Prof. J. J. Bhavsar3 IStudent of first year M. E (C. E & M), B. V. M Engineering College, Vallabh Vidyanagar 2Assistant Professor and Research Scholar, Civil Engineering Department, B. V. M. Engineering College, Vallabh Vidyanagar-GuJarat-lndia 3Associate Professor and PG Coordinator (M. E C E & M), Civil Engineering Department, B. V. M.
Engineering College, Vallabh Vidyanagar-GuJarat-India ranchodl 1 1 @gmail. com 2Jayesh. [email protected] ac. in [email protected] co. in ABSTRACT: Compaction is the governing factor for the strength, durability and serviceability of the concrete. During the placing of the concrete in the form air is likely to trap within the concrete body, hence the density of the concrete is decreasing; ultimately it affects the strength, durability and serviceability of the concrete body. Vibration is the best remedy for getting rid off the trapped air from the concrete.
At earlier stages when advanced vibrators were not found generally hand compaction method were dopted, but nowadays is a trend to use mechanical compaction method for compaction of the concrete. According to the condition we can use immersion vibration, surface vibration, or from vibration. We must use such vibration method with certain precaution to avoid any damages. KEYWORDS: Compaction, Vibration, strength, Durability, serviceability 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. The aggregate particles, although coated with mortar, tend to arch against one nother and are prevented from slumping or consolidating by internal friction. Compaction of concrete is, therefore, a two-stage process. In first stage with the 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, then after 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, i. e. until air bubbles no longer appear on the surface. Shown in Figure 1. 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. Even air-entrained concrete needs to be compacted to get rid of entrapped air voids. The difference between air voids and entrained air bubbles should be noted at this stage.
The air bubbles that are entrained are relatively small and spherical in shape, increase the workability of the mix, reduce bleeding, and increase frost resistance. Entrapped air on the other hand tends to be irregular in shape and is detrimental to the strength of the mix. It is to remove this air that the concrete must be properly compacted. There is little danger that compaction will remove the minute air bubbles that have been deliberately entrained, since they are so stable. IMORTANCE OF COMPACTION OF CONCRETE 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 2 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. Air voids produce blemishes on struck surfaces. For instance, blowholes and honeycombing might occur. Summing up, fully ompacted 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. It pays, therefore, to do the Job properly in the first place.
METHODS OF MECHANICAL COMPACTION IMMERSION VIBRATION Figure 3: Detail Sketch of Needle Vibrator In immersion vibration a mechanical device termed as needle vibrator is broadly used by many firms Frequently 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. 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. As the water cement ratio of concrete decreasing the higher compactive effort required so we should use the larger iameter head for such kind of work. Immersion vibrators may be driven by: a flexible shaft connected to a petrol, diesel, or electric motor; or an electric motor situated within the tubular casing; or compressed air.
But most commonly vibrators no the site are driven by a flexible shaft connected to a petrol, diesel, or electric motor as shown in Figure 3. Cash study for needle vibrator : TABLE 1: Diameter of head (mm) Recommended Frequency Average Amplitude Radius of Action Rate of Concreting (cmt/hour) 20-40 1 50-250 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 6-20 120-180 0. -1 . 5 300-500 11-31 125-175 90-140 1 . o-2. o 19-38 Source: Adapted from Table 5. 1 ACI Committee Report: Guide for Consolidation of Concrete 309R-05 ACI Manual of Concrete Practice 2006 Part 2.
Following care should be taken while using the immersion 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 The ibrator 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 Random insertions are likely to leave areas of the concrete uncompacted. The vibrator should not be used to cause concrete to flow horizontally in the forms, as this can lead to segregation 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, he 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 One should try overlap of this vibration circle should limited to allowed overlapping limits shown in figure.
Because it leads to over vibration at the overlapped portion of the vibrating circle as shown in fgure 4 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, he vibrator should not be held against the reinforcement as this may cause its displacement. 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, 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.
Figure 4 : Pattern of compaction 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 levelling and finishing the surface. 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. Or Plate vibrator as Shown below in Figure 6. Beam screed vibrator Roller screed vibrator
Plate vibrator Fig. 6: Different Surface vibrators Source : Google Images Beam Screed vibrator: A vibrating-beam screed consists of either one or two beams, made from aluminium, steel or timber, to which is attached a form of vibrating unit to allow the beams to impart adequate vibration to the concrete. 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 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. As shown in Figure 7. Figure 7: Surface Vibrator With centrally-mounted vibration units, the degree of compaction achieved may vary across the width of the beam. 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 Roller Screed vibrator: Roller screed vibrator are same of that beam screed vibrators in mechanism. In this type of vibrator beam is replaced by long cylindrical roller.
Here roller is given ibration through internal rotating imbalanced weight. Vibration is occurred throughout the cross section are is same Plate Vibrator: Plate vibrator are generally used in laboratories. 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.
Form Vibration: Figure 8: Form Vibration In form vibration an external mechanical vibrating device is used and it is attached with the form work. Shown in Figure 8. Form vibrators are useful with complicated members or where the reinforcement is highly congested, This types of vibrator must used with smooth surface form work so it can allow easy flow of concrete over the surface. 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..
Due to above reason it onsumes more power then the ordinary vibrators The formwork will need to be specially designed to resist the forces imposed on it. CONCLUSIONS: Todays rapid growing world Concrete is most essential material for construction. But the concrete property 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 ACKNOWLEDGMENT The Authors thankfully acknowledge to Dr. C. L. Patel, Chairman, Charutar Vidya Mandal, Er. V. M. Patel, Hon. Jt.
Secretary, Charutar Vidya Mandal, Mr. Yatinbhai Desai, Jay Maharaj construction, Dr. F. S. Umrigar, Principal, B. V. M. Engineering College, Dr. L. B. Zala, Head and Professor, Civil Engineering Department, Dr. A. K. Verma, Head and Professor, Structural Engineering Department, B. V. M. Engineering College, Vallabh Vidyanagar, Gujarat, India for their motivations and infrastructural support to carry out this research. REFERENCE: Concrete Technology by M. S. Shetty Cement & Concrete Association of New Zealand Bulletin Cement Concrete & Aggregate Australia Bulletin wrww. concrete. net. au www. wikipedia. org www. google. co. in