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Re: A few questions for those who design concrete pools.

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Dennis (and Mark),

I fear that Mark Geoghegan's response to your post on gunite for swimming
pools (see below) includes mis-statements that could get you (and he, unless
he just mis-typed) into trouble.  Specifically, gunite and shotcrete are NOT
the same thing, and NEITHER is the same as conventionally placed concrete.

The following language is excerpted from my firm's standard general notes
for pneumatic concrete, which were developed based on relevant ACI
standards, including ACI 506 (Recommended Practice for Shotcreting) and the
City of Los Angeles Department of Building and Safety's Shotcrete and Gunite
rules:

A. THE TERMS SHOTCRETE AND GUNITE IN THESE STRUCTURAL DRAWINGS REFER TO
CONCRETE WHICH IS PNEUMATICALLY APPLIED TO THE WORK BY A SUITABLE MECHANISM
DESIGNED FOR THAT PURPOSE.

 1. SHOTCRETE IS MADE BY THE WET MIX METHOD, IN WHICH SUBSTANTIALLY ALL OF
THE WATER IS ADDED BEFORE THE MATERIAL IS DELIVERED TO THE NOZZLE.
SHOTCRETE IS A MIXTURE OF CEMENT, WATER, AND FINE AGGREGATE WITH OR WITHOUT
COARSE AGGREGATE.

 2. GUNITE IS MADE BY THE DRY MIX METHOD, IN WHICH SUBSTANTIALLY ALL OF THE
WATER IS ADDED AT THE NOZZLE.  GUNITE IS A MIXTURE OF FINE AGGREGATE AND
CEMENT TO WHICH WATER IS ADDED IMMEDIATELY PRIOR TO DISCHARGE.

 3. GUNITE, SHOTCRETE AND CONVENTIONALLY PLACED CONCRETE ARE DIFFERENT
MATERIALS WHICH MAY HAVE SIGNIFICANTLY DIFFERENT STRUCTURAL PROPERTIES.  THE
CONTRACTOR SHALL NOT SUBSTITUTE ANY ONE FOR ANOTHER WITHOUT THE WRITTEN
APPROVAL OF Owner, Architect, DANA AND Bldg Dept.

While gunite (as opposed to shotcrete) has been used successfully in
structural applications, it is generally considered a non-structural
material, primarily because the water-cement ratio (and therefore the
strength of the material) is controlled by the nozzleman, who regulates the
amount of water added to the dry materials at the nozzle.  This allows the
operator to alter the texture (slump) of the material as appropriate to
shooting conditions, and in the hands of a skilled nozzleperson will result
in a lower risk of such defects as sand pockets, voids or rebar "shadows".
In addition, gunite does not typically contain coase aggregate, which means
among other things that it may be subject to much higher volumetric
shrinkage during hydration.

Shotcrete is the variety of pneumatic concrete more commonly used for
structural applications.  While Mark may be right in principle to say that
design is no different for shotcrete than for cast concrete, the same can
certainly NOT be said for specification and detailing.  In addition to
significant restrictions on the minimum spacing and configuration of rebar
(e.g., contact lap splices of large bars are not permitted), there are a
myriad of special quality control and quality assurance requirements,
including pre-qualification of each nozzleperson/helper team, restrictions
on re-use of rebound material, and field sampling that the structural
designer needs to be aware of if he or she is going to specify or detail
shotcrete.

My advice to you would be to do your homework before you try and specify
pneumatic concrete.

Drew A. Norman, S.E.
Drew A. Norman and Associates
Pasadena, California

626-568-9322

----- Original Message -----
From: "Mark Geoghegan" <mgeoghegan(--nospam--at)structural-tech.com>
To: "Seaint" <seaint(--nospam--at)seaint.org>
Sent: Friday, September 07, 2001 1:05 PM
Subject: Re: A few questions for those who design concrete pools.


>
> dennis,
>
> i cannot assist you too much on the 7 question you have, however, the
> structural "design" of shotcrete (gunite) is really no different from any
> concrete design, BUT the detailing and mix design (specifications), and
> EXPERIENCED applicators is very important for successful shotcrete ...

<SNIP>


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