Joints in slabs

Concrete Construction, Feb, 2005 by Bill Palmer

Construction joints

Unless the entire slab is cast all at the same time, you'll have some construction joints where the pour is stopped--typically against a bulkhead or edge form. If possible, plan to have the construction joint at a spot where there would have been a contraction joint. If that's not possible, then you may need to tie the construction joint together with tie bars.

If the construction joint is going to also serve as a contraction joint, coat the first part with form release agent to keep the second part of the slab from bonding. But there's one problem--there is no support across the joint to transfer vertical loads. In a normal contraction joint, the cracked concrete transfers the load by aggregate interlock, the irregular broken concrete. At a smooth construction joint that can't happen, so we use dowels. The traditional way to transfer load was with a keyed joint, but most experts don't recommend keyed joints because they don't transfer load very well once the concrete shrinks and the joint opens. Dowels may also be specified at contraction joints, if aggregate interlock won't provide the necessary load transfer across the joint. There are many proprietary dowel systems that work better than a simple greased rebar stuck through the edge form. For a list of load transfer devices, see our Material Selection Guide in the April 2004 issue of CONCRETE CONSTRUCTION.

If you want to know more about joints in slabs, or anything to do with slab construction, the best resource is ACI 302.1-04, "Concrete Floor and Slab Construction." Besides what's discussed in this article, ACI 302.1 can teach you more about reinforcement and doweling, placing sequence (strips, not checkerboard), saw cutting, curing at joints (important), and joint filling and sealing (delay as long as possible to let the slab shrink).