Subject: Re: Shotcrete at shear walls boundary elements
From: "Laurence B. Oeth III" <viacalx(--nospam--at)europa.com>
Date: Sun, 27 Jun 1999 18:58:19 -0500
> 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?
I haven't used shotcrete in a situation as you describe. I do know that
wire-wrapped, pre-stressed concrete tanks have had problems w/ wire
corrosion. Apparently water collects in voids between the wires and
inner corewall due to lack of infill by the shotcrete. I will be
personally investigating one of these structures later this summer, and
hope NOT to find such a problem.
You absolutely did the right thing with the mockup test. Heck, even
architects ask for mockups of brick panels to check materials and
Laurence Oeth, P.E.