huangyhg

slab on grade column blockout re-entrant corner reinforcemen
it seems to be widely accepted that slab on grade tends to crack at re-entrant corners. please see my attached image for my question.
--does everyone here put rebar at column blockouts below the sawcut jt at re-entrant corners to prevent full depth cracking?

--why or why not?
--please comment. even better, point me towards some reference material i can read.
thanks.
for the steel column detail, i see no need for extra rebar because the diagonal joint leads directly into the sawcut.
for the concrete column detail, i would locate joints on the face of the column instead of on the centerline. add diagonal rebar where there are no joints. another variation of this is the pinwheel arrangement (no additional rebar needed)where a joint comes off of each corner.
try to keep the rebar fairly close to the column to prevent the crack from forming. if it is located too far away, the slab will still crack.

agree with jike on the steel column. we show our slab joints coming to the corners of the column isolation block (column blockout).
for concrete columns, we go with cirular column blockouts.
the diamond shape for the column blockout is the best because it doesn't give you any reentrant corners so no stress concentrators. but often you end up with square columns especially at the edges and end up with at least one reentrant corner. however, if the distance between joints is less than about 4-5m then it won't give you any trouble especially since the reentrant corner is close to the corner of your panel. i think this fact is often overlooked and these reentrant corners are not as big a problem as we think.
carl bauer
at diamond shaped column block-outs, there's no need for additional reinf. at re-entrant corners in your slab perimeter, try to have a saw-cut joint starting from that corner. if you do that, you won't need additional reinf either. otherwise, place a couple #4 bars.
a variation of the concrete column detail is to cast the slab on ground before the columns, bearing directly on the footings, and without blockouts. in this case it is better to saw the joints away from the columns. this allows the slab to shrink toward the columns, and makes the sawing simple.

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