Lesa square dowels
Why encased square dowels?
The report by ACI Committee 223 "Standard Practice for the Use of Shrinkage-Compensating Concrete" (ACI 223-93) states that, 'Provision should be made to accommodate differential movement between adjacent slabs in the direction parallel to the joint between two slabs. Differential movement may be caused by expansion of the shrinkage-compensating concrete, shrinkage, or thermal expansion from heat hydration of the new slab. If provision is not made for this movement, cracking perpendicular to the joint may occur in one or both slabs.' LesaDowels'" or LesaDowel sleeves allow for this movement as illustrated above. Keyways, strip foundations and the aggregate interlock theory all present problems in achieving a reliable, long term load transfer mechanism.
Square dowels encased in a plastic sheath are increasingly considered to be the only acceptable form of shear dowel. Round dowels restrain the sideways movement perpendicular to the bars and parallel to the joints, which may cause slab cracking. Rectangular plastic sheaths over square dowels allow lateral movement of the slab over the dowels, avoiding the problems associated with round dowels and permitting dowels to be used in joints which are at right angles.
(Ref 'A Solution to Cracking and Stress Caused by Dowels and Tie Bars', By Ernest K Schrader, Concrete International July 1991, American Concrete Institute, Detroit)
LesaDowels
Square mild steel dowels, fully encased in plastic sleeve to allow horizontal movement. Factory assembled, ready to install on site. Ends sealed.
Sizes
20 x 20 x 450mm long.
16 x 16 x 420mm long.
Optional dowel available with sleeve encasing hall length only with end sealed. Ideal for extension to existing slab.
Spacing and location of dowels to be specified by engineer.
Dowels must be adequately supported by reinforces or other means to ensure they remain in the specified position of line and level.
(Available in New Zealand only.)
LesaDowel sleeves
Plastic dowel sleeves for square dowels (identical to shape used in LesaDowels) ready to fit over dowel on site. Open ends have foam inserts inside to seal against steel.
For construction joints, use 'half length sleeves, fitted over dowels after first pour completed.
For sawcut joints, use 'two thirds' cover (to ensure the sawcut is over the covered section) fitted to dowels before concrete placement.
Dowels must be horizontal.
Dowel sleeve lengths
Construction joints
20 x 20 - 250mm long
16 x 16 - 280mm long
Sawcut joints
20 x 20 - 300mm long
16 x 16 - 240mm long
Misaligned dowels and tensile stress
Misaligned dowels, ie. dowels that are not parallel with each other and the surface and at 90 degrees to the slab's edge can lock joints together resulting in very high tensile stresses which may cause concrete cracking at the ends of the dowels. The likelihood of cracking from dowels misaligned in construction joints can be dramatically reduced with the use of LesaDowel sleeves.
By using proper supports, dowels can be located
quite accurately in a horizontal position. However it is difficult for contractors to hold the alignment of dowels at 90 degrees to the joint during concrete placement.
The voids created by LesaDowel sleeves allow for substantial misalignment from the perpendicular without inhibiting the joint's ability to open.