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Bar Placement in Slab with Laser Screed

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I was watching the tail end of the pour of a slab on ground this morning and was discouraged with the placement of steel.  What was supposed to be top bars in the slab for shrinkage control ended up as middle or bottom bars.  The bars were up on chairs, but the placement method pushed them down.

 

The slab was screeded with a laser screed.  It’s called a 240 screed because it is 12 ft. wide and has a 20 ft. reach, making 240 sf per pass.  The screed pulls the concrete towards itself.  The tires on the screed push the rebar and any chairs down into the ground.  So when the concrete is pulled up to the screed it is holding down the rebar.  Now when they move the screed they attempt to pull the bars up, but in a 6-8” thick slab you can’t pull bars up from the bottom to the top.  It is impossible.

 

Isn’t there some method to use the laser screed and still get top bars?  I asked the operator and he said this is how it is done all the time.  I’m wondering why hasn’t this been perfected yet?  If it was truly critical that I get top bars for a strength reason there is no way I could count on it.

 

Has anybody developed a placement specification and method to cover this situation?

 

Rich Lewis

Lewis Engineering