Rebar - Standees Question

[SteveBrady]

I have 36" slab with 2 mats of #6 @ 12" oc/ew. The problem is that the bottom mat is 9" off the ground. Does ACI or CSI say anything about this? My detailer is telling me that it's not standard to have standees directly on the earth. I don't believe that it is standard to use 9" continuous high chairs! ANY HELP???

 

[msucog]

use concrete bricks or get chairs made to on soil...concrete bricks are by far the most stable and cost effective solution if the rebar needs to be 2"-3" off the ground.

 

[civilperson]

Use 3" bricks on top of the piles.

 

[sdz]

You can get plastic tipped wire chairs up to 360mm high & metal trays for them to stand on so they won't sink into the ground. Or maybe use concrete bricks plus smaller chairs?

 

[hokie66]

Why would you want the bottom mat 9" off the ground? That is at 1/4 depth of the slab.

 

[boo1]

Would you place the rebar at the min cover?

 

[Epolkowski]

The detail sounds as if you have a 6' pile embed with 3" clr on top. I agree with civilperson's response with a 3" bloc on piles. Check the detail the detailer is referring to as to the validity of the 9" clr bottom. If no piles, than a HC with plate. Standees will sink into subbase unless on mudmat.

 

[beton1]

We generally pour a mud slab as there is often seepage
or water issues. Using concrete bricks as mentioned by a
a few other respondents provides a stable support for reinforcing steel. It also ensures proper cover, rebar embeddment and bonding. Dirt/mud and ensuing sinking or movement of rebar during pour is eliminated.

 

[lawayosh]

What jurisdiction governs? Building codes usually require 3" minimum clearance to reinforcing if concrete is to be placed against earth. Where is the bottom mat in the design? Nine inches seems excessive.

Standees, as stated by others, have a problem sitting on soil/sands. Dobies (concrete blocks w/tie wire embedded) are usually better for bottom mats. They are cheap and readily available.

There is also a potential for corrosion to travel up steel standees right to the rebar. The increased cross section necessary for tall standees would add to this problem. In wet or corrosive soils some specs call for plastic, coated steel or stainless steel standees if the designer elects not to allow dobies.