Construction joints are formed or placed into slabs to define stopping places or the extent of an individual concrete placement. There are different types of construction joints: butt-type with and without a keyway, doweled, and tied as shown in Figure 1. Dowels can be smooth round or square steel bars or plates.
Construction joints are different than contraction joints (also called control joints) but may be designed and constructed to open freely like contraction joints to help control unsightly, random slab cracking. If joints can open freely, tensile stresses caused by restrained dry and thermal concrete shrinkage are relieved. Relieving shrinkage stresses reduces the risk of random cracking. Therefore, construction joints that perform like contraction joints are desirable.
Only butt- and doweled-type construction joints can open freely or function like contraction joints. Tied construction joints cannot function like contraction joints. Also, if the slab reinforcement is continuous through a construction joint, the joint may function more like a tied joint than a contraction joint. Continuous reinforcement through joints can restrict joints from opening freely.
Butt-type construction joints are basically free slab edges that deflect under forklift, wheeled traffic or vertical loads. A butt-type construction joint will not transfer loads or maintain vertical slab alignment across the joint. Even keyed joints are not recommended where vertical load transfer or slab alignment is required because the two sides of the keyway lose contact as the joint opens. As the joint opens because of concrete shrinkage, the ability of the keyway to transfer loads across the joint and maintain vertical alignment are significantly reduced. Also, concrete cracking and failure above or below the keyway along the joint edge is likely. Only use butt-type construction joints with or without keyways where vertical load transfer and alignment are not a concern.
Historically, 3/4 inch x 14 inch and 1 inch x 16 inch smooth round dowels spaced at 12 inches on center have been used in construction joints for 5- to 6-inch and 7- to 8-inch-thick slabs (Ref. 1). When aligned carefully and only bonded to the concrete on one side of the joint, round dowels help carry vertical loads across the joint, maintain vertical alignment of the slabs and allow slab movements perpendicular to the joint. Typically, one-half of each dowel is greased or sheathed to prevent concrete bonding so dowels are free to slip and accommodate opening of the joint.
By allowing slab movements perpendicular to the joint, doweled construction joints can act like contraction joints and relieve shrinkage stresses perpendicular to joints. However, concrete shrinkage occurs in all directions causing slab movements both perpendicular and parallel to doweled joints. Traditional round dowels restrain or prevent slab movements parallel to joints creating tensile stresses that may cause random cracking to occur (Ref. 2).
Engineers first solved this problem by replacing smooth round dowels with square dowels cushioned on the vertical sides with a compressible material. Tops and bottoms of the square dowels are not cushioned so vertical loads are directly transferred from the concrete to the dowels and vertical alignment of the adjacent slabs is maintained. Using dowels with a compressible material on the vertical sides allows slab movements parallel to the joints. This reduces restrained shrinkage stresses parallel to joints and significantly reduces the risk of cracking.
Typically, 3/4 inch x 14 inch and 1 inch x 16 inch square dowels spaced at 14 inches on center have been used for 5- to 6-inch and 7- to 8-inch-thick slabs. Like round dowels, square dowels must be aligned properly and only bonded to concrete on one side of the joint in order to work. If not, joints will be mechanically restrained from opening, increasing the risk for random slab cracking. Installing and keeping round and square dowels aligned properly during the construction process can be difficult and many consider this issue a shortcoming of both round and square dowels.