How to join Concrete

To watch the video go to ‘How to join Concrete’.

When we place concrete there are 3 different types of joint that are commonly used, the Expansion Joint, the Control Joint and the Construction Joint.

Although they all look the same on the surface of the slab they are used for different reasons and to achieve different results.

Expansion Joint

The Expansion Joint is used where large amounts of concrete are poured together such as a large slab or footpath or where the concrete is to be placed against a different material that will restrict the movement of the concrete.

To form the joint a piece of compressible material such a purpose made foam or bitumen impregnated fibre is placed in between the two sections or the slab and other material.

The result of insufficient expansion joints can quite often be seen in footpaths where sections pushing against each other lift up causing an uneven surface.

Control Joint

The Control Joint is used to control where the slab will crack. It is almost certain that a concrete slab will crack at some stage so by using Control joints we can control where it will crack and improve the look of the finished product.

The Control Joint is traditionally formed by using a jointing tool to create a groove in the slab. The groove creates a weak point in the slab while hiding the resulting crack. More recently the Control Joints are being cut into the slab after it has been poured, generally within a few days, using a diamond blade.

The placement of these joints is not an exact science and comes from experience however the common places that a slab will crack are at high stress points such as internal corners and along the line where the reinforcement sheets join.

Construction Joint.

The Construction Joint is used where we want to do a pour a slab in sections but we still want the slab to look and act as if it one slab.

If we were to just pour the two slab sections against each other without joining them together there is a chance that the movement of the ground could lift one side of the join and not the other again creating an uneven surface and a trip point. 

There are two common methods of joining the sections, The Key Joint and the Dowel Joint.

The Key Joint

The Key Joint uses a profiled metal extrusion used as part of the formwork.

The extrusion creates a groove in the first section, when the second section is poured against it it fills the groove. 

The groove will allow the two sections of the slab to move up and down together while still allowing it to separate. The key joint can also incorporate a expansion joint.

The Dowel Joint

The Dowel Joint involves drilling a series of holes into the poured section.

These hole are lubricated and then a dowel, usually reinforcement rod, is inserted into the holes.

When the second section is poured it will lock onto the dowels tying the two section together while still allowing for expansion and contraction. The Dowel Joint can be used with the Key Joint and can also incorporate a expansion joint.