What is the importance of the compaction of concrete??

Short answer:

If you do not vibrate concrete you will have:

1. voids

2. heavier rocks will sink

3. reinforcement (steel) will not be well covered

That’s not good.

• Concrete shrinks when it sets (hardens) and cracks will develop between voids.

• If rocks are sunk and fine concrete is on top, the material have different strengths along its height.

• The voids do not behave well in fires (concrete is an excellent fireproofing material and makes concrete buildings much more resistant to fires than pure steel structures, like the Twin Towers).

• If there is no binding between reinforcements and concrete, you do not have reinforced concrete: what you have steel slippage and certain structural failure.

Long answer:

These days compaction is done mostly by vibration.

In times of yore you could actually compact the mixture, but today you use vibrators (really powerful, not recommended for other uses, unless… wink, wink).

Heavy duty concrete vibrator. Kinky.

Heavy duty concrete vibrator. Kinky.

The first thing you can have if you do not use one of those giant vibrators is segregation.

No, not that kind of segregation.

The other one: heavier rocks in the concrete will sink to the bottom.

Segregation of aggregates in concrete not vibrated: the rocks at the bottom of the batch and finer concrete on top

The second thing you can have is that rocks will interlock and leave voids between them.

It is called honeycombing (in English) and ant farms in Spanish (hormigueros).

Honeycombing in concrete: the sectional strength is compromised because instead of concrete you have holes (honeycombs). If stressed, cracks will join those “ant farms”. Notice that the mixture is more or less uniform.

On a side note, in Spanish the name for concrete is “hormigón” because in the beginning of times, concrete would be full of those “ant farms”.

The Spanish name roughly translates as “huge ant”.

When reinforced it is called “hormigon armado” which, even weirder, translates as “armed huge ant”.

Thirdly, the concrete slurry may not cover well or insert itself between steel rod reinforcements.

The water can attack the steel and rust it or you can have huge voids.

For example, sometimes you have the bars of steel close to the edges of the beam, wall or slab you are building.

Improperly vibrated concrete with edge steel reinforcement not covered: bad idea

Worse yet: reinforcing steel sometimes is quite dense, specially in new high strength concretes.

Reinforcement steel meshes sometimes are quite dense. As you can imagine, they act as stoppers of the concrete: this guy is using a new technique called self-compacting concrete: you not only vibrate the concrete but pushes it with a jet

On top of that, sometimes it is impossible to vibrate the concrete.

When this happens you use really, really huge jets to splash it into things, like in tunnels, for example.

This is called shotcrete (shot concrete).

Allow me to tell you something: when you are three kilometers deep into a tunnel where air does not enter and you are shooting concrete into the walls and there are diesel fumes of huge trucks retiring the excavated soil andyou are blasting the rocks you cannot breath.

Even with those huge fans I could not take more than one hour inside the Buenavista tunnel in Colombia: shotcrete is a biatch. Those things are 2.5 meters high.

… but I digress.

The idea, then is to vibrate that thing hard.

Vibrate, vibrate like there is no tomorrow!

Shake that thing, baby