Compaction of Concrete is an operation in which fresh concrete is compacted in forms and make it encircle reinforcements and other embedded objects such as tubes in the mold.
There are various problems that might arise if compaction of concrete is not carried out properly such as honeycomb and trapped inside concrete paste. Moreover, poor compaction of concrete could to permeability problems and therefore steel corrosion and decreasing ultimate capacity of hardened concrete.
Figure-1: Poor Compaction of Concrete results to Honeycomb and Rock Pockets
Methods of Compaction of Concrete
Concrete consolidation can be carried out either by hand or machines. There are many factors that should be considered to choose compaction methods such as reinforcement quantities and spacing, concrete paste consistency and formwork complexity.
Hand Compaction Method
Reasonably workable and flowable concrete mixtures are consolidated by hand employing a rod. The bar should adequately reach the bottom of the form work and rode diameter need to compact concrete between reinforcement spacing and formworks.
The concrete is tamped by the rod tool repeatedly to consolidate it. Mixtures with low slump value could be consolidated by hand if superplasticizers are added to decrease slump and make the concrete workable.
Furthermore, tools such as spade is used to provide good surface appearance and hitting formwork sides make way to repel entrapped air out of the concrete.
Mechanical consolidation is not recommended to use if the mixture is designed to compact by hand to avoid segregation.
Mechanical Compaction Method
The mechanical consolidation method is capable and suitable for concrete mixtures with great amount of coarse aggregate content and low water to cement ratio in heavily reinforced structural elements. Different types of mechanical compaction methods are explained in the following paragraphs:
1. Shock or drop table
Shock or drop table is used to consolidate extremely stiff low slump concrete in making precast units.
It is employed for compacting moderate to high slump mixtures in constructing poles, pipes, and piles.
3. Vibration Method of Concrete Compaction
Vibration method is probably the most extensively used technique for compacting concrete. In this method, internal friction between aggregate particles is eliminated for a short time and concrete mixtures behave like liquid and gravitational force will come into effect. This lead to move entrapped air to move upward and the mixture will settle in the formworks.
When the vibration is stopped, the internal friction is immediately developed again. Either number of vibrations per minute (vpm) or vibrations per second (Hertz) are used to express vibration.
Compaction of concrete with vibrators is divided into the following types:
a) Internal Vibrators
Internal vibrators which sometimes called spude or poker vibrators are usually applied to compact concrete in beams, walls, columns, and slabs. Not only does vibrators performance is influenced by concrete workability but also by frequency, amplitude, and head dimensions of vibrators.
Generally, vibrator head diameter is between 2-18 cm and the shape of the head is cylindrical. As the head diameter of vibrators is increased the effective action area is rose for instance radius action of vibrators with 4 cm head diameter is 15 cm while 45 cm is the radius action of vibrators with 8 cm head diameter.
Moreover, it is considerably significant to utilize internal vibrators correctly to achieve the best compaction. Horizontal movement of vibrators should be avoided to prevent concrete segregation and lowering head of vibrators to the bottom of considered concrete layer and it should over lab previous layer by about 15 cm. Compacted concrete layer thickness is about the head length or 50 cm.
Regarding using vibrators for compacting slabs, the vibrator must be kept submerged in concrete and this can be done by using it horizontally or at specific angle in addition to use 1.5 times the radius action as the distance to guarantee overlap previously adjacent vibrated layer. Not only does the vibrator should be held steady but also keep in specific station for 5-15 seconds to obtain desirable consolidation.
The length of the time that the vibrator employed at the same station is based on concrete workability, force of the vibrator, and the nature of the element that is being compacted. Adequate internal vibration is decided by concrete surface modifications for example changes in coarse aggregate particles, general batch leveling and top surface appearance of thin film of mortar.
In situations where vibrating concrete internally is not possible, it could be beneficial to vibrate the exposed part of reinforcement because it leads to remove entrapped air and water under reinforcement and increase the bond between concrete and steel. Figure 2 show internal vibration operation.
Figure-2: Internal Vibrators for Concrete Compaction
b) External Vibrators
There are two major types of external vibrators which are
- Table or surface vibrator, and
- Form vibrator.
The Table vibrator is widely suitable for compacting concrete surfaces such as floors and slabs. It adequately consolidates slabs up to 20 cm thickness; internal vibration is required for higher slab thicknesses.
The form vibrator is attached to the exterior face of mold or forms properly otherwise energy will be lost because of improper attachments. Moreover, form vibrator is the proper choice for compacting concrete in thin and heavily congested forms, consolidating stiff mixtures, and supplementing external vibrators.
Additionally, form vibrators can be advantageous for constructing pipes, masonry units, and other types of precast concrete. However, it is not recommended to employ form vibration at the top of vertical forms such as columns because it could cause gaps between concrete the molds as a consequent of in and out movements, so internal vibration is better to use in this case.
Form vibrators should be spaced properly to make uniform distribution of intensity above the form. Figure 3 shows form vibrator utilization for compacting fresh concrete.
Figure-3: Compaction of Concrete using Vibration Method
Read More: Types of Vibrators for Concrete Compaction
Results of Improper Vibrations of Concrete
There are various problems and defects that could arise when concrete is not vibrated adequately.
- Sand streaks
- Cold joints
- Excessive amount of entrapped air voids that most of the times called bug holes.
- Subsidence cracking
- Placement lines
Figure-4: Honeycomb in Concrete due to poor Compaction or Vibration
Figure-5: Cold Joints due to poor Compaction of Concrete