Concrete – Importance of Vibration

honeycomb concrete
Have you ever seen concrete that looks like this on a vertical face?

Its called ‘Honey combed’

Sometimes its in large areas like this, sometimes in smaller patches.

This concrete is obviously weaker and more porous than dense concrete.

The odd small hole isn’t a problem but when it gets like this it is a sign of poor laying practices.

What Went Wrong

As concrete comes out of the mixing truck/pump it traps bubbles of air within it.

Similarly as the concrete is placed against vertical forms air is trapped between the form and the wet concrete.

This trapped air prevents the cement paste flowing completly around the aggregate in the mix

Some dodgy concretors will add water to the concrete to make it easier to lay and finish without vibration, but that will make the risk of ‘honey combing’ greater.

Vibration

US Naval Mobile Construction Unit 4

To get a strong dense mix it is necessary to get the air out of the mix and allow the cement paste to fill the voids between the stone.

On site the normal method is to use a vibrating poker as in this photograph.

This should be slowly lowered into the concrete and then lifted out.

Each insertion is typically around 300 – 400mm apart.

Extra care is needed against forms with one method being to hold the vibrating poker against the outside of the form.

 

Precaution

If you are having some concreting done make sure that you tell the concretor you will expect to see proper poker vibration of the concrete to full depth.