Importance of compacting concrete
The most essential thing immediately after placing concrete is providing good compaction. Even though it is a simple, easy, and quick thing to do, it has ample effect on the performance of the concrete. Let’s see, how to select a suitable vibrator for concrete.
We compact the concrete for two purposes. those are improving the aggregate structure and remove entrapped air. During compaction, first, the aggregates settle down and prepare proper aggregate structure. Then the entrapped air in the concrete moves to the top and gets separate from the concrete. Typically concrete mix can contain 5% – 25% of air voids by volume in the noncompact state. Compaction will help to remove entrapped air and reduce voids in the concrete.
Advantages of compacting concrete
- Helpful in gaining strength: Before you compact the concrete, it behaves as lump, and aggregate may not very well stack. As a result of the vibrational force, the mix liquifies. This settles the aggregates in a well-stacked manner improving interlocking between particles.
- In another way, entrapped air is separated from concrete providing good bonding between aggregate and present reinforcement. This helps to improve the quality and performance of the concrete.
- Improve durability: Compacted concrete shows a greater lifespan than noncompact concrete. When compacted, surface quality increases. Poorly compacted or non-compacted concrete mostly contains honeycombs. This makes the concrete surface less wear resistance and more permeable. So, well-compacted concrete is better considering durability aspects such as less permeability and improved aggregate structure.
- Monolithic properties: With compaction, unevenly placed aggregate comes into a stacked position making evenly distributed. Concrete becomes monolithic material due to this.
- Improve finishing: Not only the technical advantage but also compacting will improve finishing quality. Vibration liquefies the concrete and helps to flow & fill according to the shape of the formwork. Poorly compacted concrete can contain honeycombs. These are hollow spaces of concrete with too little of cement paste.
The advantages of consolidating concrete by vibration are:
- Improve the density of the concrete
- Greater strength
- Improved bond with reinforcement steel
- Greater bond on construction joints
- Greater durability
- Improved product finish
- Reduction in the amount of required cement through the use of drier mixes
Available methods to compact concrete
Manual Compaction
The most primary method for compaction is rodding. This is an effective method for moderate slump concrete. A similar method is used in casting cube specimens and slump testing procedures. Simply use a steel rod (commonly 16 mm reinforcement bar) to compact the concrete manually.
Mechanical compaction
For larger-scale works, this manual compaction is not practical. Various types of mechanical vibrators are developed suitable for different types of construction. All the instruments consolidate the concrete with vibration energy. Never use vibration to flow the concrete over the concreting area.
External vibrators:
The common name for this type of vibrator are shutter vibrators because they are fixed to the formwork and vibrate it. These are the most suitable vibrator for concrete in pre-cast concrete construction and construction of thin walls. The recommended influencing depth for this type of vibrator can vary up to 750 mm. Rotary and reciprocating type external vibrators are available in the market. Keep in mind that forms can fail due to the vibration so, good support for the forms is essential to use these instruments.
Roller vibrators also can be categorized as external vibrators. These are using in roller compacted concrete as well as soil compaction. Read to get some idea about the concrete which compacted by roller vibrators.
Internal vibrators:
This is the most popular vibrator in the construction field. Poker, Immersion vibrator, and needle vibrator are the common names for it. These are available in various sizes and power options. The instrument consists of a vibrating steel rod that can insert into the concrete. An eccentric weight is attached to a motor. This makes the vibratory pulse which generates using electricity, fuel or pneumatic force.
Selecting a suitable vibrator
There are few things to consider when selecting a vibrator. first is the needle size. Of course for thin concrete sections and members with too close reinforcements, small diameter vibrators are preferable. The effectiveness of the vibrator is depending on its amplitude and frequency during vibration. When both are high in number, the instrument is more effective for the work. ACI 309R-05 summarizes the following table as a guideline to select a suitable vibrator for concrete.
| Diameter of the head (mm) | Recommended frequency in concrete (Hz) | Average amplitude (mm) | Influence radius (mm) | Rate of concrete placement (m3/hr per vibrator) | Application |
|---|---|---|---|---|---|
| 20 – 40 | 150 – 250 | 0.4 – 0.8 | 80 – 150 | 1 – 4 | High slump concrete in very thin members and confined places. |
| 30 – 60 | 140 – 210 | 0.5 – 1.0 | 130 – 250 | 2 – 8 | Concrete of 100–150 mm slump in thin walls, columns, beams, precast piles, thin slabs, and along construction joints. |
| 50 – 90 | 130 – 200 | 0.6 – 1.3 | 180 – 360 | 6 – 20 | Concrete (less than 80 mm slump) in normal construction. |
| 80 – 150 | 120 – 180 | 0.8 – 1.5 | 300 – 510 | 11 – 31 | Mass and structural concrete of 0 to 50 mm slump. |
| 130 – 150 | 90 – 140 | 1.0 – 2.0 | 400 – 610 | 19 – 38 | Mass concrete in gravity dams, large piers, massive walls, etc. |
When the diameter reduces the influencing circle radius decrease. You have to insert the vibrator in close distances to cover all the area completely. The influencing radius of the vibration generally calculated as 4 times the diameter of the needle. 2 different patterns are available to use immersion vibrator.
Square pattern
Here vibrator insert along square grid pattern so that 1/3 of the influence zones are overlapping
Offset pattern
A similar square grid is used with this pattern too. But vibrating points are located in a zig-zag manner.
Centrifugal vibration:
This method is useful when casting concrete pipes. The barrel shape form is filled with concrete and rotates rapidly. Then due to the centrifugal force, concrete is pressed to the form giving enough compaction.
Keep in mind when compacting…..
Few points to follow when vibrating with these instruments are as follows.
- Use the most suitable vibrator for concrete and construction type
- Always follow one vibratory pattern.
- For immersion vibrators, limit concrete layer thickness to the needle height
- Provide vibration for 15s to 20s time in each go
- Do not use vibration to flow the concrete
- Do not over vibrate, it can segregate concrete
- Always check forms for any mortar leakages
- Use enough number of vibrators corresponding to the placing rate
- When placing layers, insert the vibrator 10cm into the previous layer to improve interlayer bonding.
