PT Slab Rebar

[Andy J]

Looked at a PT Slab and there is a no 4 rebar tied around the edge of the slab behind the cable anchors. They told me they do that to keep the anchors from popping out of the slab but it seems like it would limit the cable pull to just where the anchors are, but maybe there isn't enough bond between the cables and the rebar to keep the cable from being pulled.

 

[Ingenuity]

These are 2#4 anchorage backup bars placed adjacent to the bearing face of the anchorages.

When there are multiple monostrand anchorages then additional reinforcing is required, consisting of hair-pin rebar to minimize horizontal spliting of the slab due to the increased concentration of applied force.

I do not understand what you mean when you state "limit the cable pull"?

 

[Andy J]

Was thinking it might interfere with the cable movement when they are stressed, after the concrete is poured, but the more I think about it they probably don't.

 

[Brad805]

Useful link -----> PT End Anchorage

 

[centondollar]

The purpose of rebar behind the anchor block is to take the vertical and horizontal tension forces originating from the concentrated anchor block compression. Saint Venant's principle applies only at a certain distance into the slab; close to the edge, the compressive stresses disperse at an angle (stress distribution depends on block location and amount of blocks), leading (with one block in the mid-height of the slab or beam) to the formation of two compression diagonals (often idealized as 45 degrees, measured clockwise and anticlockwise from the beam N.A.) and a tension diagonal.

The primary issue is not the anchors "popping out from the slab", but rather a bursting of the slab and a subsequent "pulling in of the anchor block into the slab" and catastrophic failure.

PS. The anchorage rebar is usually placed before the pour, and thus, they should not interfere with the tendon ducts and tendon stressing. The area adjacent to and behind the anchorage block is concreted after the tendons have been tensioned and locked.