Subject: Re: Shotcrete at shear walls boundary elements
Date: Sun, 27 Jun 1999 23:06:21 EDT
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?
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
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