Construction joints

[irishengdave]

I practiced in the british isles for 10 years and in vertical walls, we always just scabbled (exposed the aggregates) at the joint carried the rebar through and assumed it has the same shear capacity as a monolithic joint.

I practice in N America (Canada) now and they always cast a shear key in the joint (expensive). Any insight as to why that is adopted would be appreciated.

 

[Shotzie]

It may be preferable to have a shear key instead of a roughened surface if the wall is unreinforced or minimally reinforced, in the case of residential foundation walls.

 

[JAE]

I'm in the US and we typically do what you do...call for a 1/4 in roughened surface and carry the reinforcement through the joint. I used keyways many years ago. Probably switched around late 1980's.

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[KootK]

I practice in Canada and agree, the keyway is the common detail. I don't care for it. I think that it's lower capacity than a plain roughened joint as you wind up depending on just 1/3-ish of the wall depth to resist shear at the joint on the female side.

at the joint carried the rebar through and assumed it has the same shear capacity as a monolithic joint.

I disagree with that. I would argue that you get the shear capacity associated with shear friction rather than full capacity. They'll be close though.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

A shear key can work well and is likely less expensive. It is quite common.

Dik

 

[ATSE]

For shear and moment continuity, a roughened joint (1/4" amplitude) with 100% reinforcing steel crossing the joint is superior to a key. The key is not necessary, and can be detrimental if the key aspect ratio is less than 3.
ACI 224.3 (report) has some useful discussion on this topic.
Keys are more appropriate for movement joints in lightly loaded slabs (not pavement).