What Is Consolidation of Concrete?
Consolidation of Concrete is the process of reducing or eliminating the volume of voids, air pockets, and entrapped air from the fresh concrete mix, usually accomplished by applying mechanical energy.
Concrete Consolidation is also related to a process of reducing the gap between aggregate and aggregate; between aggregate and reinforcement; and between aggregate and forms. The main aim of the consolidation of concrete is to eliminate the air voids from concrete and increase its density of concrete.
As we know the higher density of concrete gives high compressive strength. Proper and close contact between reinforcement and concrete is ensured by proper consolidation. Consolidation of concrete is significantly the ultimate strength of concrete and enhances the bond with reinforcement.
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Concrete Consolidation Methods
The following are various methods of consolidation used for concrete,
- Hand Compaction
- By Rodding (ii ) By Ramming (iii ) By Tamping
- Compaction by Vibration
- Internal vibrator (Needle vibrator)
- Formwork vibrator (External vibrator)
- Table vibrator
- Platform vibrator
- Surface vibrator (Screed vibrator)
- Compaction by Pressure and Jolting
- Compaction by Spinning.
Let’s discuss each method in detail,
1. Hand Compaction
Hand compaction of concrete is generally employed where concrete quantity is small and for unimportant work.
Rodding:
This first method of hand compaction is Rodding. In this method, a rod of a minimum of 16 mm and a length of about 1m is used. Rodding is continuously inserted in concrete to compact concrete and remove air void from it.
Ramming:
The light ramming method is generally used for unreinforced foundation concrete or in-ground floor construction.
Tamping:
Tamping is mostly used in the case of pavement, and floor slabs where the thickness of concrete is comparatively less and the surface is finished smooth and level.
Tamping is done by continuously beating the concrete surface with the wooden crossbeam.
2. Vibration
In the case of high-strength concrete, the water-cement ratio is too low and the concrete is too stiff to compact by hand. To compact such types of concrete, mechanically operated vibratory equipment must be used.
By use of a vibrator, we can complete compact 4cm slum value concrete in a closely spaced reinforced concrete work, whereas, for hand, compaction can use for only up to 12 cm slum value concrete.
Vibration reduces the friction between concrete ingredients and gives them time to settle down in a compacted mass of concrete.
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(i) Internal Vibrator
The internal vibrator is widely used on construction sites. It is also called a “Poker Vibrator” or “Needle Vibrator”. It generally consists of a power unit, a flexible shaft, and a needle.
Internal vibrators are used to compact concrete in beams, walls, columns, and slabs. Many factors affect the performance of the internal vibrator-like frequency, amplitude, and head dimensions of vibrators.
Generally, the shape of the vibrator needle is cylindrical and has a needle diameter of around 2 to 12 cm.
As the needle diameter increases the effective actionable area of the needle also increases. For example, if the needle dia. is 4 cm then its effective area in concrete will be 15 cm, and if the dia. of the needle is 8cm then its effective area in the concrete is about 45 cm.
Moreover, the correct method of using an internal vibrator is also important to achieve the best compaction. To avoid segregation defects in the concrete horizontal movement of the vibrator must be avoided. Effective compaction depth is about the length of the needle or 50 cm.
While using the needle vibrator in the slab, try to keep it in concrete this can be done by laying the needle horizontally or at an angle in concrete to use an additional 1.5 times the radius action as the distance to guarantee overlap of the previously adjacent vibrated layer. The needle must be kept in the same position for at least for 5-15 seconds to obtain desirable consolidation.
The time for which the needle is kept submerged in concrete generally depends on workability, the force of the vibrator, and the nature of the element that is being compacted.
When aggregate from the surface of concrete goes down and cement slurry comes up, then we can say that the compaction of concrete is done properly.
(ii) Formwork Vibrator (External Vibrator)
Formwork vibrators are mostly used where the depth of concrete members is too high and needle vibrators may not reach full the depth of members such as columns, thin concrete walls, and precast concrete members. This vibrator machine is clamped to the external surface of the formwork.
Formwork vibrator must be properly attached to forms otherwise energy will be lost because of improper attachments.
This type of vibrator is proven best choice in cases compacting concrete in thin and heavily congested forms, consolidating stiff mixtures, and supplementing external vibrators.
Formwork vibrators are played an important role in the manufacturing of pipes, masonry units, and other types of precast concrete.
The spacing of the form vibrator is also important to apply the transfer same amount of vibration inside the concrete member.
(iii) Table Vibrator
This is one type of formwork vibrator that is clamped to the table. They’re generally used for compacting concrete cubes and small concrete precast members.
(iv) Platform Vibrator
A platform vibrator is similar to a table vibrator, but its size is larger than it. This type of vibrator is generally used to consolidate big prefabricated concrete elements such as electric poles, railway sleepers, prefabricated roofing elements, etc.
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(v) Surface Vibrator
Surface vibrators are called “Screed Board Vibrators”. A small size vibrator fixed on a screed board is called a surface vibrator. Screen Board vibrator is proven effective for compaction and leveling of thin concrete members, such as floor slabs, roof slabs, and road surfaces.
(vi) Vibratory Roller
Generally, for compaction of very dry and lean concrete vibratory rollers are used.
Effect of Improper Compaction
If compaction or consolidation of concrete is done properly, then many defects can develop in concrete such as,
- Honeycomb
- Sand streaks
- Cold joints
- Subsidence cracking
- Placement lines
Factors Affecting Concrete Consolidation:
Workability of concrete:
Workability refers to its ease of mixing the ingredients, compacting, placing, and consolidating the concrete and it plays a significant role in the consolidation process of concrete. Concrete with higher workability is generally easier to consolidate effectively.
Reinforcement and embedded item:
The presence of embedded items like electrical pipes, conduits, and reinforcement bars can obstruct the movement of the vibrator, so proper placement and positioning of this element is crucial for ensuring thorough consolidation.
Shape and Size of the Formwork:
The shape and size of the formwork used for casting the concrete element and it affect the consolidation process.
Thickness of Concrete Pour and Vertical Height:
The thickness of the concrete pour and the vertical height of the pour can influence the consolidation process; higher vertical height or thicker concrete section may require special attention.
Concrete mix proportion:
The mix proportion and the properties of the concrete like shape and size of aggregate, water-cement ratio, and admixture can impact its consolidation ability and workability.
Equipment and techniques used:
The selection of appropriate consolidation equipment and techniques it plays a vital role in achieving proper consolidation technique.
Different consolidation methods like external vibrators, internal vibrators or formwork may be more suitable for specific project requirements.
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