Compaction of Concrete Good Practice

Today I am suppose to write a post on earthwork but because I had a bad experience on site which I am presently supervising, I decided to write a post on compaction of concrete good practice. We did our first concrete pour yesterday on a raft foundation with a bunch of inexperience concreters.

The general view of main contractors or builders is that the compaction of concrete is a simple procedure that any unskilled construction worker can do.
Placing of concrete if not done properly will affect the integrity of the structure such as

a) Honeycomb

Honeycomb refers to voids left in concrete due to failure of the mortar to effectively fill the spaces among coarse-aggregate particles. It is a defect in the hardened concrete consisting of many air pockets or voids due to which concrete deteriorates by entrapment of moisture that causes corrosion of reinforcement bars resulting in failure of structural members over a period of time if not rectified at the earliest.

b) Segregation

Segregation can be defined as the separation of the constituent materials of concrete. A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture. There are considerable differences in the sizes and specific gravities of the constituent ingredients of concrete. Therefore, it is natural that the materials show a tendency to fall apart.

Vibration of concrete is one of the important methods of compaction. It should be remembered that only comparatively dry mix should be vibrated. It too wet a mix is excessively vibrated; it is likely that the concrete gets segregated. It should also be remembered that vibration is continued just for required time for optimum results. If the vibration is continued for a long time, particularly, in too wet a mix, it is likely to result in segregation of concrete due to settlement of coarse aggregate in matrix.

Concrete Compaction is the process adopted for expelling the entrapped air from the concrete. In the process of mixing, transporting and placing of concrete air is likely to get entrapped in the concrete. The lower the workability, higher is the amount of air entrapped. In other words, stiff concrete mix has high percentage of entrapped air and, therefore, would need higher compacting efforts than high workable mixes.

The next question is how to do it properly?

Immediately after the concrete is discharged either from a concrete pump hose or a concrete hopper and place in the area to be concreted, the concrete must be compacted with a vibrating poker. Vibration should be done in a poking manner [please see attached video for this]. What is important is when the poker is withdrawn from the concrete; the process of withdrawing must be done slowly so as to give enough time for the trap air bubble to surface. Quick withdrawal of the poker does not allow enough time for the concrete mixture to fill the space occupied by the withdrawing poker, thereby causing a void or honeycomb. One rule of the thumb guide to tell whether the concrete is well compacted is when the concrete surface stops popping out air bubbles.

In situation where compaction of concrete is done in high walls, lift walls, deep trenches or very thick foundations, the placing of concrete should be done in layers of 400 to 700 mm at a time. When concreting is being done for the next level, the vibrating poker should not be driven to the full depth of the pervious layer but driven only 100 to 150 mm down from the surface of the previous layer so as to have a homogeneous joint.

Below please video clips on compaction of concrete.

You know what? I already feel better writing this post after my bad experience from the supervision a bunch of unskilled so called concreters yesterday.

Feeling good!

Benson