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I have a problem involving shotcrete in a renovation project, and the
required cover for bars.  Are the minimum cover requirements different for
shotcrete than for cast-in-place?  If so what are they?  Also, we'll be
doing more projects like this in the future, so what is a good
shotcrete/gunite reference to buy?

Following is a description of the project and problem:

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

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.

Does anyone out there have any experience in this?

Jason W. Kilgore, P.E.
Leigh & O'Kane, L.L.C.
(816) 444-3144