Concreting – Bar Chairs

No we are not talking about seats in a pub.

Bar Chairs are used to make sure the steel reinforcement in concrete ‘sits’ in the right place.

Here is a typical plastic ‘chair”

And here is a different type.

Getting the steel reinforcement in place is very important in making sure you get the best quality slab. . . .and the only way to make sure is to ‘sit’ the reinforcement mesh, or bars, on these chairs.

The chairs need to be 800mm apart unless the bars in the mesh are greater than7mm diameter.

The reinforcement  in paths and driveways should be a minimum of 40mm from the top and bottom of the slab. (This typically means around the middle of the slab)

Some concreters, especially the one who give you a cheap price, will tell you you don’t need bar chairs as they can pull the reinforcement up to the right position with special hooks while they are pouring the concrete*.

This is completely wrong.

Below is a picture of the underside of a concrete slab that had to be dug up due to excessive cracking after using this money saving** ‘technique’.

You can clearly see the rusting reinforcement that was on the underside of the slab was doing nothing to stop the slab cracking.

To ensure you get the best slab make sure there are bar chairs in place while the concrete is being poured…………. I have been told its not unknown to put the chairs in place for a pre-concrete inspection and then take them out before the pour!

 

*Footnote 1: Many concretors disagree with me. . . but in America where they lay huge amounts of concrete roads they have developed special machines to pull the steel up into place. . . .In 300mm thick slabs they have found that the steel can be 75mm out of place (Federal Highways Administration).

So how can workers correctly “hook and pull” mesh reinforcement accuratly While Standing On The Reinforcement?

**Footnote 2: The actual cost of the chairs works out at around $1/m2 of concrete!

For more posts on on getting your paths and driveways correct see Concreting

 

Concrete – Reinforcement Mesh

Purpose of Steel Mesh Reinforcement

A lot of people believe that the steel mesh adds substantially to the strength of concrete paving.

This is a myth!

The truth is large areas of concrete paving will crack.

What well thought out reinforcement steel does is:

  • Limits cracking where you don’t want it (in the middle of slabs)
  • If some cracking does occur it holds the cracks tightly closed.
  • A reduced amount of steel across joints helps to make sure that cracks do occur at the joints (where you want it to crack).

Sizes and Types of Mesh Reinforcement

A single full sheet of mesh is 6m by 2.4m.

Many local suppliers will sell you half a sheet, or less, for small jobs if you plan on doing it yourself.

These are the common types of mesh that you can buy in Australia for paving work, with my opinions on what mesh to choose.

Type

Description

Comment

SL52

5mm dia bars @ 200mm spacing

For garden paths (max joint spacing 2m)

SL62

6mm dia bars @ 200mm spacing

For plain paths or patios (max joint spacing 3m)

SL72

6.75mm dia bars @ 200mm spacing

Caravan or Boat parking areas

SL82

7.6mm dia bars @ 200mm spacing

Front Driveway minimum for plain concrete

SL92

8.6mm dia bars @ 200mm spacing

Front Driveway best for patterned concrete

RL81

7.6mm dia bars @ 100mm spacing

Polished concrete floors

There are many more types of mesh but they are more specialised, the above will be fine for typical concrete slabs around your home.

Depth of Cover

The minimum cover from the top and bottom surfaces of the slab should be 30mm.

This will prevent the steel rusting once the concrete is placed.

To make sure you get this cover see this link: ‘Bar Chairs‘.

Overlap

When you need to join two pieces of mesh the overlap should be 2 squares.

Do Not overlap mesh across the site where a joint is going to be. That will stop the crack forming at the joint.

Rust

Don’t worry about a bit of surface rust on the mesh, although you should remove any rust flakes. The roughness of the corroded area help the concrete grip the reinforcement.

The alkali nature of the concrete will stop any further rust, as long as the 30mm cover is maintained.

 

See this link for advice on the thickness of concrete slabs: Concrete Thickness

For more posts on on getting your paths and driveways correct see Concreting