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Re: Shotcrete -- Rebars Larger than #5, Testing

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Approximately one year ago, the September 2001 issue of Concrete International (ACI) published an article 
by Rob Vonarb, VP of Superior Gunite.  This article will give you a good idea of the types of projects that 
can be done with shotcrete, including lap-spliced bars up to #11 size.   The state of the art in shotcrete is 
well ahead of the prescriptive code language.    The preconstruction testing required by the UBC along 
with the entire description of how to specify shotcrete, certify nozzlemen, and evaluate shotcrete is 
provided by ACI Committee 506 in several reports and guides(Available in ACI MCP):

Guide to Shotcrete (ACI 506R-90)
Specification for Shotcrete (ACI 506.2R-95)
Guide to Certification of Shotcrete Nozzlemen (ACI 506.3R-91)
Guide for the Evaluation of Shotcrete (ACI 506.4R-94)

The preconstruction test is critical in the selection of the nozzlemen and to verify the capability of the
nozzlemen to achieve the level of shotcrete performance needed for the structure.  The test should 
include a representation of the most rebar congested, most difficult area of the project for shotcrete
placement, and should include lap spliced bars, if they are part of the work.  Strength test cores and 
cores used for "grade" testing are used to verify the quality of the work.  Core grades are discussed
in the ACI documents and range from 1 to 5.  Usually a grade of 2 or better is used for certification of 
the nozzlemen and a grade of 3 is acceptable during the production work.  These grades relate to 
cores cut through the reinforced portion of the test panel and are a measure of how well the rebar is
encapsulated by the concrete, and how many voids may be present.  Strength tests are done for cores
taken through concrete sections without the reinforcing (i.e. between the bars).

Similar grade tests and strength tests should be required during the production phase, but may be done
on samples that represent the average work in a region.    In addition, you can take the initial 
preconstruction test and cut horizontal slices through the test sample to view how uniform the 
shotcrete placement was, and how well the concrete is consolidated around the bars.  

Superior Gunite was a subcontractor on a project that I worked on a few years ago.  
They came in as a contractor substitution for cast-in place basement walls for a multi-story,
below grade parking garage, close to a seismic fault with closely spaced lap spliced #7 vertical
bars at 6" o.c. and with column cages with #9 bars and ties.  Horizontal rebar was #5. Concrete 
strength was 5000 psi.   Shotcrete saved the contractor much time and formwork placement at 
the perimeter, enabling the interior of the garage to be placed more quickly.  I had some prior 
experience with shotcrete, but was very impressed with  the capability of this company and the 
current state of the art.  The preconstruction testing is crucial - as each nozzleman can vary in 
their abilities, and the quality control during placement is important to ensure the quality of the 
production placement.

- J. Diebold, SE - San Diego

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