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Re: Shotcrete at shear walls boundary elements

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In a message dated 6/27/99 8:34:28 PM EST, lhoward(--nospam--at)silcom.com writes:

<< 
 I guess my questions is;  Have any of the Engineers who subscribe to the
 list required these mockup tests on their shotcrete projects?  If you
 have, what were the results?  Are some of these shotcrete people
 actually able to get good density inside a wall boundary element with
 very large rebar and tight tie spacing?
 
 Lynn
  >>

I believe most of the larger design offices doing retrofit work in Los 
Angeles require mock ups, partly because the City of Los Angeles Department 
of Building and Safety requires this for bars larger than #5 and spaced 
closer than 12 bar diameters on center for the front curtain and 6 bar 
diameters for the back curtain of a two curtain wall.  There are also 
additional restrictions.  The mockup panel is used to demonstrate that it is 
possible to achieve the required density of concrete on a job by job basis.

We have had good luck with the mockups.  How the backboard of the mockup is 
constructed can impact the results of the test panel.  Plywood is typically 
used for the forms and will have more bounce and give than when the shotcrete 
is actually shot against concrete.  This is something which should be 
considered when making the mockup panels.  It would be best to place the 
mockup panel against the existing building if possible in seismic retrofit to 
help eliminate the bounce and flexure of the form board.

Mockup panels are a necessity in order to assure that the company doing the 
work can meet the specification.  The better shotcrete firms are more than 
willing to do this since it eliminates those which are not qualified. 

Our office has used full scale mockups for boundary elements with coupling 
beams (13 feet tall x 4 foot wide boundary portion of wall with a 36 inch 
deep coupling beam at the midheight of the wall) and also when we were 
jacketing existing concrete columns.  This can get quite expensive (in the 
range of $5,000 to $10,000 per test mockup).  I have seen mockups that were 
approximately 10 feet tall by 4 feet wide by 5 feet deep with #11 vertical 
reinforcing space at 4 to 6 inches on center with #4 ties and hairpins at 3" 
on center vertically and #4 hairpins at 4-6" on center horizontally across 
the surface.  The sawcut demonstrated that the nozzleman was able to fill all 
the voids.

One other thing you might want to consider is that does cast-in-place 
concrete assure you that you will have fewer voids than a shotcrete 
application?  Cast-in-place will require good vibration to assure that rock 
pockets, etc., do not occur, and afterwards you may still have some void 
spaces.

Michael Cochran