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RE: Shotcrete Loading

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John,

I don't have a reference for the shotcrete forces, but I have used
shotcrete on two projects and have made some observations.  The first
project was a one story seismic retrofit project and we constructed
shear walls out of shotcrete.  The shotcrete was sprayed up against some
plywood forms and one person (the "nozzleman") controlled the spray
without any problems.  I don't think the forces from the spray were very
big.  The only problem we had was rebound of the material inside the
facility.

The other project was a large tie-back wall constructed to stabilize a
slope.  I prepared the performance spec for the project and another very
capable firm did the actual design work for the wall (i.e., a specialty
foundation subcontractor).  This wall was very large and shotcrete was
used for the facing.  The engineer of record called for a 14" thick
facing wall with double reinforcing and fc' of 5000 psi.  My firm helped
the client with Construction Management, and I was concerned about
several things including: achievement of the design parameters;
potential for voids in the 14" thick wall; and application methods.  We
had the contractor prepare a rather large test panel and he successfully
got consistent cored concrete break strengths over 4,000 psi, but not
the 5,000 as designed.  A slight re-design of the wall (by the
subcontractor) was made in order to make everything work with the lower
compressive strength.  The concrete cores revealed very few voids in the
shotcrete, so that concern went away.  The application method was a bit
unusual because this contractor had extremely large nozzle forces.  A
single nozzleman could not control the hose (this was a larger diameter
hose with a greater flow capacity), and because of safety concerns the
contractor asked to use an attachment of the hose/nozzle to a bobcat.
We did not like this method because the nozzleman really controls the
quality of the shotcrete application.  How could a bobcat serve this
function?  To be short, we witnessed the proposed application method in
the field and the bobcat operator was very experienced.  All concrete
design parameters were being met, so we let the contractor go with his
idea.  The wall was successfully placed and was completed on a very
agressive schedule! 

My advice would be to find out how the contractor plans on applying the
shotcrete.  Will one person hold the hose?  If so, the forces probably
won't be excessive.  However, if bigger hoses are used to cover large
areas or place the material quickly, then larger forces will be placed
on the back-up material.

Hope this helped,

Michael Ritter, PE
Lockwood Greene Technologies

> -----Original Message-----
> From:	John Schenne [SMTP:jschenne(--nospam--at)localnet.com]
> Sent:	Thursday, June 24, 1999 9:05 AM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Shotcrete Loading
> 
> I am designing some bracing for a wall that is to be shotcreted.  Does
> anyone know of any referenced for the loads induced by shotcreting
> operations. Thanks 
>  
> John Schenne, PE