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Jason,

I think that fire resistance between mixed occupancies is going to be more of 
a concern than the ACI minimum cover requirements which is based on corrosion 
protection of steel.

Remember that the concrete cover on reinforcing has to provide the same fire 
resistance as is required for occupancy separation.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Jason Kilgore wrote:

. > I have a project where they're renovating an old warehouse and converting 
. > it into a mix of retail (1st floor) and loft apartments.  The building is 
. > a cast-in-place concrete frame with 20'x20' bays.  The floor is a two-way
. > plate with drop panels and capitols.  The floors are 7" thick, with a 3"
. > topping slab in some areas.  The drop panels are an additional 4".
. > Column-band reinforcing is parallel to the grid lines.  Field reinforcing 
. > is at 45 deg., and is continuous over the entire floor.  As near as I can 
. > tell there is only one mat (bottom), but it is probably bent up at the 
. > columns. Over all, the building is in good shape.

. > For the first floor slab, the reinforcing was placed on chairs directly 
. > over a foam insulation.  This insulation was removed as part of the 
. > renovation, which is where the problem lies. During casting, the chairs 
. > sank into the insulation, and the bottom layer of bars in the bottom mat 
. > is exposed for about 75% of the floor. About half of the bar diameter is 
. > exposed over the majority of this area, with occasional spots with full 
. > exposure (you can see the second layer). There is full coverage at the
. > drop-panels.  All the bars are in good shape with no major corrosion.

. > The building was used as a warehouse with varying storage requirements for
. > several decades (from light boxes to 2-ton boxes of metal gears, with lift
. > traffic).  There is no obvious cracking.

. > The contractor wanted to leave is as is.  His reasoning was a) leave well
. > enough alone and b) it will add a huge unexpected cost to the owner to fix
. > it.

. > We specified meeting ACI-318 requirements of 3/4" min. cover.  The
. > contractor then wanted to do a single pass of shotcrete (about 1/2").  
. > We're still sticking to the 3/4", but the contractor and owner are 
. > screaming about the cost and time of installing the required mesh that 
. > 3/4" would require.