Reduce Random Cracking with Plate Dowels

by Kim Basham, PhD PE FACI
Created: December 31, 2008

Use plate dowels at construction joints to reduce the risk of random cracking.

Steel plate dowels carry loads across a construction joint, maintain vertical alignment between adjacent slabs and allow concrete shrinkage both perpendicular and parallel to the joint.
Steel plate dowels carry loads across a construction joint, maintain vertical alignment between adjacent slabs and allow concrete shrinkage both perpendicular and parallel to the joint.
Figure 1. Types of Construction Joints
Figure 1. Types of Construction Joints
With all plate dowel systems, it is important that workers properly place and consolidate concrete around and especially under the leave-in-place pocket formers and plate dowels using internal...
With all plate dowel systems, it is important that workers properly place and consolidate concrete around and especially under the leave-in-place pocket formers and plate dowels using internal vibrators. Otherwise, plate dowels can break out of the concrete from the top or bottom of the slab.
Figure 2. How Plate Dowels Accommodate Slab Movement
Figure 2. How Plate Dowels Accommodate Slab Movement
Using plate dowels that allow horizontal slab movements both perpendicular and parallel to joints lowers the risk of random cracking.
Using plate dowels that allow horizontal slab movements both perpendicular and parallel to joints lowers the risk of random cracking.

Plate dowels

Diamond- and rectangular-shaped plate dowels are becoming more popular than round or square dowels at construction joints because plate dowels are more cost-effective, easier to achieve proper alignment and allow movements parallel to the joints. Plate dowel sizes typically vary from about 4- to 6-inch diamonds or rectangles with thicknesses varying from 1/4- to 3/4-inch and are typically spaced along the joint from 18 to 30 inches on center. Of course, the plate spacing depends on the plate size, loading and slab details.

Research by Walker and Holland shows diamond and rectangular-shaped plate dowels are more effective than round or square dowels because plate dowels place more steel closer to the joint where the bearing, shear and bending stresses caused by vertical loads are the highest (Ref. 3). They also showed the stresses due to vertical loads are significantly reduced beyond the first inch of the dowel past the joint and say an embedment length longer than 4 inches does not significantly increase dowel performance. Because of the plate dowel's load transfer efficiency, plate spacing along the joint is larger than typical values used for round or square dowels. Walker and Holland calculated that a 1/4-inch-thick x 4 1/2-inch diamond dowel spaced at 18 inches on center is equivalent to a 3/4-inch round dowel spaced at 12 inches on center.

Plate dowels can accommodate horizontal slab movements parallel to joints by creating spaces along the vertical sides of the plates. Spaces allow adjacent slabs to move relative to each other in a direction parallel to the construction joint and significantly reduce the risk of random cracking caused by restrained concrete shrinkage. The spaces along the vertical sides of the plate dowels are created by: using a compressible material on the vertical sides of the plate dowels, using leave-in-place pocket formers that are slightly wider than the plate dowels, and by letting slab contraction perpendicular to the joint withdraw a diamond-shaped plate dowel from the leave-in-place pocket former as shown in Figure 2.

For the first two methods, the space is either created by the compressible material or an oversized leave-in-place pocket former. However, the third method utilizes the geometry of a diamond-shaped plate and concrete shrinkage to create spaces along the vertical edges. As the joint opens due to concrete shrinkage, the plate dowel is withdrawn from the cavity in the pocket former creating a space on each side of the plate due to the taper of the iamond-shaped plate.

Workers install plate dowels by inserting them into either a precut slot in the concrete forms or cavities in leave-in-place pocket formers that were attached to the forms and cast into the first slab. For the precut slot in the form system, plate dowels are held in place by the forms and the first side of the plate is cast directly into the first slab. With this system, there are no leave-in-place pocket formers. For the other two systems, leave-in-place pocket formers are first nailed to the forms and cast into the first slab. After stripping the forms, workers insert the plate dowels into the cavity of the leave-in-place pocket formers. When the second slab is placed, the second side of the plate dowels for all three systems becomes incased in concrete.

Because of the geometry, size of the plates and installation procedures, it is easy to achieve and maintain proper dowel alignment thus reducing the potential of mechanically locking the joint from opening. With all plate dowel systems, it is important that workers properly place and consolidate concrete around and especially under the leave-in-place pocket formers and plate dowels using internal vibrators. Otherwise, plate dowels can break out of the concrete from the top or bottom of the slab.