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 
