Cut and joints are put into concrete slabs to hopefully control where the concrete cracks as it shrinks.  Most people don’t think about the fact that when concrete is poured, it is very wet and almost sloppy.  And that as it dries and cures the water escapes and the concrete volume decreases.

There are 2 main measures that are used to control the occurance of cracks – Control cuts and Construction Joints.

A Contol cut is usually a thin saw cut and should be 1/3rd the depth of the slab.  In residential concrete, these are normally about 3 Metres apart.  in commercial slabs that are usually 5M apart.  These cuts are made to create a weak point, so the concrete can crack in a nice controlled way (it you look at the end of cuts, where they don’t reach the wall, you will often see the crack to the edge of the slab.

A Construction Joint is usually only used in larger Commercial and industrial floors. These break up bigger pours into “isolated” slabs.  These can be just when one pour ends and the next slab butt against it, or more common now days are Armoured joints – where the edges have steel plates protecting the edges.

While sometimes control cuts can open up (get wider) it is usually the Construction Joints that can open a long way.  Shrinkage can take as long as 18-24Months.  There can be continued movement due to earth movement (the ground swelling and shrinking when wet or dry) and also thermal expansion and contraction of the concrete itself.