Q.: We're building a floor for which the plans call for keyed construction joints. The plans also call for wire mesh that runs continuous through the joint. We don't think this is a good detail either for constructibility or for floor performance. How can we convince the engineer that there are better ways to do the jointing work?
A.: Keys are not effective load transfer devices at construction joints if the joints open up too much when shrinkage occurs. That's probably why the designer wants to run wire mesh through the joint to prevent it from opening too wide. But if the joint also is to serve as a control joint, the wire mesh defeats the purpose of the joint. It restricts joint movement and may cause random cracks at other locations.
The detail also is expensive to build because you'll have to split the wood bulkhead used to form the keyway to let the wire mesh pass through it. That will increase the chances that the floor won't be as flat as it could be at the bulkhead.
Here are two recommended ways of handling the construction joint that are better than a keyed joint with wire mesh running through it:
- If the construction joint does not have to serve as a control joint, use deformed steel bars to tie the two joint faces together and ensure shear transfer. If the engineer wants a keyed joint with steel, drill holes in the keyed bulkhead and run short deformed steel bars through the holes. Then run the wire mesh up to the bulkhead and start it up again on the other side.
- If the construction joint does have to serve as a control joint (permitting movement), then pass smooth steel dowels through the bulkhead, but don't run the mesh through the joint. If you use smooth steel dowels that are greased on one end, you don't need a keyway.