Compaction of Fresh Concrete​: Adaptable General Terminology of  Compaction by Internal Vibrators

Compaction of fresh concrete:

Fresh concrete can contain 5% to 20% confined air which may reduce the concrete strength and density. It’ll help to increase the permeability of concrete which will decrease the durability of the concrete. According to studies, the strength of concrete can be reduced up to 5% if only 1% of air void presents at fresh concrete, So, Compaction of fresh concrete is the process which will eliminate this air void from freshly placed concrete and help increasing density of concrete by binding the particles all together. It helps to increase the ultimate strength of the concrete and enhances the bond with reinforcements.

^{Methods of compaction of concrete:}

• Hand Compaction
  • ^Tamping 
  • ^Ramming 
  • ^Rodding 
• ^{Rolling Compaction} 
• ^{Jolting Compaction} 
• ^{Spinning Compaction} 
• ^{Vibrating Compaction}
  • ^{Needle Vibrator/Immersion Vibrators/Poker Vibrator} 
  • ^{Surface Vibrators} 
  • ^{Formwork Vibrator} 
  • ^{Table Vibrator} 

_{General Terminology  of  Compaction by Internal Vibrators  (Needle Vibrator/Immersion Vibrators/Poker Vibrators):} 

Amplitude: The amplitude is the maximum distance a point on the vibrating head moves from its position of rest. Engineers expressed it in mm.

Frequency_: Frequency is the number of complete cycles of vibration per second of vibrating needle. It is Expressed in Hz. One hertz simply means one cycle per second. Internal vibrators chosen for most jobs have a frequency of 12,000 to 17,000 vibration per  minute (vpm) in air.  

_{Size of the vibrator:} it shall be denoted by the nominal outside diameter of the vibrating needle expressed in mm. Standard sizes available: 25, 35, 40, 50, 60, 75 and 90 mm 

Length of the vibrating needle: Standard sizes available: 300,  325, 350, 375, 400, 425, 450, 475, 500, 525, 550, 575, 600, 625 and 650 mm

Vibration is a two-part process. Figure shows it to be a two – part process. A vibrator creates pressure waves that separate aggregate particles, reducing friction between them. Piles of concrete flatten as the concrete flows around reinforcing steel and up to the form face. Large voids (honeycomb) disappear

But making the concrete flowable doesn’t finish the compaction process. Almost simultaneously, a second stage starts to occur as entrapped air bubbles rise to the surface. This deaeration process continues after the concrete has flattened out

Until completing the both vibration stages, the concrete isn’t fully consolidated. If site people remove vibrators too soon, some of the smaller bubbles won’t have time to rise to the surface. Vibration must continue until it removes most of the air entrapped during placement.

 

Site people should inserts immersion vibrators vertically into concrete, as quickly as possible, and then helds stationary until air bubbles cease to rise to the surface, usually in about 15–20 seconds, and a thin film of mortar (sheen) should appear on the surface. 

Characteristics and applications of immersion vibrators for compaction of fresh concrete

 

 

Spacing and Number of Insertion Position​:

To be fully effective, site people should immerse the needle of vibrator completely in concrete. As a general rule, the depth of the concrete laye should be in the range of 2/3 L to  L where ‘L’ is length of the vibrator needle.

Common Mistakes​ during concrete vibrating:

 

Site people should never use a vibrator to move concrete laterally. They should deposit concrete carefully in layers as close as possible to its final position in the form. Placing the concrete as each layer, they should insert the vibrator vertically.​

In deep sections such as walls, footings and large columns, they can place the concrete 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​

Site people should be careful about not allow the vibrators to touch the forms as this can cause ‘burn’ marks which will reflects on the finished surface. Generally, they should keep the vibrators about 50 mm clear of the form face. Similarly, the vibrator should not be held against the reinforcement as this may cause its displacement.​

Before vibration, percent consolidation for field concrete is typically at 80% to 95%. After vibration it’s usually between 95% and 100%.

Concrete Vibrating Tips

● Do not over vibrate the concrete as it lets the mixture break and separate. Also, do not under vibrate the concrete because air might still be trapped in the lower layers of the cement pour, which leads to a weak concrete structure that easily breaks with minimum pressure.

● The most common technique with an internal vibrator is to withdraw the vibrator slowly, at a rate of around 25mm inch per second. If bubbles are still emerging as the concrete vibrator is withdrawn, more vibration is necessary.

● To prevent cold joints from occurring to concrete layers as these are poured over with a new layer of cement mixture, make sure to penetrate the pour thoroughly with a vibrator and mixing in the previously poured layer with then newly placed one.

● For maximum efficiency, use only vertical movements when vibrating to have an even mixture of aggregate and concrete. Horizontal motion can lead to uneven distribution and increases the possibility of the vibrator tangling in the rebar or reinforcing steel.

● Avoid using the vibrator as a concrete placement tool because it will not produce a consistent surface and it might even provide more problems in structural strength and form.

● Use the appropriate type of concrete vibrator depending on the structure being built or the size of the working area. Consider using a compact lightweight concrete vibrator for smaller jobs for manageability and efficiency.

● Consider having a back-up concrete vibrator on hand to have a second replacement tool instantly if the first one encounters any problems while on the job.