Typical Concrete Slab Construction Joint (pdf)
When you have large slab areas it is wise to use a construction joint.
The keyed joint prevents the slabs from rising or lowering. You may have experienced two concrete slabs that stick up or down from the adjacent slab. This is a tripping hazard.
You can use dowels and other methods to keep two slabs together; this generally is when you abut to an existing slab and there is not a key.
In a residential application a keyed joint is your best bet for new construction; if you are unsure you may email us to discuss.