Subject: Shotcrete at shear walls boundary elements
From: Lynn <lhoward(--nospam--at)silcom.com>
Date: Sun, 27 Jun 1999 18:32:55 -0700
Michael Cochran wrote:
> Shotcrete has been used extensively in many of the concrete shear wall
> seismic retrofits of older buildings in the Los Angeles area. Getting 5000
> psi mixes for shotcrete does not appear to have been a problem either, but in
> most cases the design is often for 4000 psi. Due to the high congestion of
> rebar in boundary elements, mockup wall panels are typically required which
> are sawcut afterwards to verify that the nozzleman can get the concrete
> placed around all the reinforcing (vertical bars, ties at 3" o.c. spacing,
> diagonal bars from coupling beam reinforcing) leaving no voids.
Here in Santa Barbara we did a seismic retrofit of a parking structure
and used shotcrete extensively. In one area we had added a very large
shearwall (24" thick) with very large boundary elements and tight ties
spacing. For this particular wall we had specified cast-in-place
concrete because we did not believe the shotcrete could be used in an
area with such large bars congested together with the tight tie spacing.
The successful bidder had bid the project using shotcrete for this wall,
saying, as you had mentioned, that shotcrete shear walls such as this
are done in LA all the time. The shotcrete subcontractor claimed to
have done many walls like the one we had in LA.
I agreed to allow them to use the shotcrete so long as they could
"prove" that they could do the shotcrete where the congested rebar was
by doing a mock-up and saw cutting to look for "shadowing" behind the
bars. They tried twice, and failed twice. They said they used their
best nozzleman for the mockups. I was not impressed.
As a result of the test, I made them use cast-in-place concrete.
Needless to say the Contractor was upset. He stated that such tests
were unreasonable, (although we just followed the ACI standards, I think
it was ACI 501) and that I was being too tough on them. He further
stated that they are not routinely made to do such tests by LA
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?