Concrete construction joints are used on placements when you won't pour the entire floor, slab, wall or driveway all at once. They are used in between pours and are designed to allow slab movement or transfer load from one pour to another.
The location of construction joints should be planned to minimize cracking in concrete slabs. It may be good practice to bond or continue reinforcement through a construction joint.
Concrete, when drying, has a natural tendency to shrink. For large pours or long narrow pours, like sidewalks, installing construction joints at preplanned locations will help minimize random cracking in the concrete.
Concrete construction joints are formed using some sort of wood, steel, or plastic bulkhead. In some cases the bulkheads are also used as screed rails for the next pour.
For light duty slabs a straight edged butt joint will suffice. For slabs with heavier traffic some sort of keyway or dowel must be used to transfer the load from one slab to the other.
Using keywayed construction joints was once the preferred method, but has since been found not to be as effective as dowels or diamond keyed joints. After the concrete dried, each slab shrinking away from each other opened the construction joint enough so the keywayed edge proved not to be very effective and more prone to fail.
For dowels to be effective they must be aligned properly. Dowels must be perpendicular to the joint both horizontally and vertically or joint deterioration can occur.
Smooth dowels work best. The concrete must be able to move freely along the dowel as it dries and shrinks. Drilling holes through the bulkhead and sliding the dowel into the fresh concrete is one method. Using dowel sleeves nailed to the inside edge of the bulkhead is another method used to install dowels.
Using diamond dowels for construction joints are a newer and very effective method for controlling load transfer and at the same time allowing the slab to shrink in two directions, eliminating all restraint.
Installing diamond dowels is as simple as nailing the plastic sleeve onto your form before you pour, then after you strip the form insert the metal diamond into the plastic sleeve.
Making sure the concrete is properly consolidated at the construction joint by using an internal vibrator will help insure the success of the dowel or diamond method used to transfer load between placements.
Concrete construction joints are just one way to help minimize random cracking in concrete floors. To help further reduce cracking in concrete floors read about concrete contraction joints and concrete expansion joints.
You can learn all about how to fix cracks in concrete floors by going to epoxy concrete repair.