Shotcrete for new wall construction

[ajk1]

Has anyone done perimeter multi-storey basement retaining walls with shotcrete?
If so, what are your experiences with it in terms of getting it done right (no voids, no sand pockets, fully encapsulated rebar, etc.) as well as quality and appearance of surface finish?

 

[SwinnyGG]

Based on what I know of shotcrete, it has significant disadvantages compared to conventional poured walls for a basement application. Cost, labor, time, finish quality.

 

[ajk1]

Although I would tend to agree with you, we are being told by a developer that it is more economical to shotcrete a basement retaining wall than to form and pour. I assume he is talking about where the shoring wall forms one face, so there would be no formwork required if it is shotcrete. I am not in a position to dispute his costs. Has anyone had experience with actually doing a basement retaining wall by shotcrete? How did it turn out?

 

[jdgengineer]

We've done a few of them for single-story residential basements. I'm not aware of any issues with them. Just need to be done by a competent shotcrete contractor and waterproofing needs to be installed properly. I would for sure require test panels to certify nozzleman and would recommend a waterproofing specialist be engaged to make sure waterproofing is installed properly.

 

[ajk1]

To jdgengineer : Yes that is what I was wondering. What kind of waterproofing do they use? If it is blind side type membrane fastened to the shoring, I would expect that the shotblast would destroy the membrane, so they must use something else, but what?

 

[cvg]

my experience (with slopes, un-shored, not vertical, no waterproofing) is that cost, labor and time are all less with shotcrete than conventional concrete. I have not dealt with any finish other than gun finish and with coloring. I have seen shoring done with shotcrete facing, vertical but it was un-finished. that was for 3 levels below grade parking structure and was the cheapest option

 

[jdgengineer]

I'm not sure, we don't typically get involved with the waterproofing design, but in the areas we have worked on there has been a waterproofing consultant brought onboard. One of the jurisdictions we work with requires that all waterproofing for basements be special inspected which would also provide another level of assurance.

I'm not sure what products are common for multi-level commercial basements, but in the residential world Grace Construction Products are what we run across most commonly. Preprufe is what is normally used for basement waterproofing around here. It's very expensive, but a good system from what I understand (but again we never spec waterproofing).

It looks like their product is intended to be used as blind-side waterproofing for shotcrete.

https://gcpat.com/construction/en-us/waterproofing/Preprufe-SCS-Waterproofing-System-for-Shotcrete

 

[jdgengineer]

Regards to finish, I believe they can make it look as nice as you want if the contractor is well qualified. Most of the shotcrete we have done in basements is concealed by a furring wall so the finish is not critical. However, if you did a steel float you should be able to make it completely smooth. It will cost $$$ and care has to be done in placing the shotcrete to prevent delaminating, but it's possible. In my opinion, you need to make sure you have a highly qualified installer who has a record of installing with the chosen finish successfully in the past.

 

[sandman21]

I have always found shotcrete to be cheaper. It will depend on the project, I have a one-story they are using cast-in-place for now, but on multi-story basement I have not heard or seen cast-in-place being used in large applications. You can apply the shotcrete to the waterproofing membrane, they will have a drainage board then apply a bentonite waterproofing. They also have admix's likePenetron. Finish can be almost anything you want, with the limit you cant place anything that would block the shot. You dont mention where you are but the IBC Section 1910 has limits on bar size, spacing,preconstruction testing, etc. Even with test panels you can still get deficient concrete, all my projects had a full time deputy inspector observing the shots.

 

[ajk1]

Thank you all for the very interesting responses. I will have some follow up questions, if I may, over the weekend. Thanks again.

 

[ajk1]

I have some follow up questions:

1) Thank you jdgengineer for the link to Preprufe waterproofing video. There is no commentary to the "Preprufe" waterproofing video, so I am assuming this requires both a blindside membrane and a grout injection after the shotcrete wall is finished. My gut feel is that if a smooth finish is required for appearance sake on the shotcrete, and both a membrane AND grout injection is required to render it waterproof, that this is going to be an expensive system. So Sandman when you say that shotcrete is cheaper, my question is: Is that with this waterproofing system and with a smooth finish. I also have reservations about relying on the grout to spread out all over so that there is no location without grout.

2) If the perimeter wall has structural columns that project into the building (example say 12" thick wall with 24" thick columns which project 12" into the building) would that still be a candidate for shotcrete?

3) To Sandman21: Is it a typo where you say "...on multi-story basement I have not heard or seen cast-in-place being used in large applications"? Did you mean to say that you have not heard of shotcrete being used in large applications on multistorey basement walls?

 

[sandman21]

In my area, So. California, I have only seen shortcrete for multi-story basement walls. I have not seen or heard of anyone using a cast-in-place. For a three story basement building, and one story podium, the contractor requested a change to have all interior concrete walls shotcrete over cast-in-place. I honesty can't think of a reason why a cast-in-place wall in a basement application would have over shotcrete. The waterproofing system is required regardless of cast-in-place of shotcrete. My basements have been exposed parking, I dont know what finish the architect has and what it would compare to a smooth finish. As I said we have testing requirements for the shooter and helper, not qualified for the job cant work on the project, always have a full time inspector just for shotcrete. If they are shooting a panel the inspector is watching. I have had deficient concrete on both shotcrete and cast-in-place. Even if you used CIP you still would not see the backside of the wall.

 

[ajk1]

Hi Sandman - What you say is is very interesting but astounding to me (I am assuming that you are an independent engineer and not working in the shotcrete industry - right?). In my 53 years of consulting structural engineering practice here in Ontario, I cannot recall ever encountering a multistory basement wall that was shotcrete, although we recently became aware there may be a few buildings so constructed recently. Ditto for the engineers with our competitors with whom I have spoken on this issue. I honestly cannot see why anyone would elect to shotcrete rather than cast-in-place unless it can be shown that it was significantly more economical. In the shotcrete case, the waterproofing membrane would be damaged by the force of the shotcrete hitting it, so it seems that is why they add a grouting operation to render the wall waterproof, but this grouting is expensive and time consuming and of uncertain penetration.

Be interesting to hear from others on their feelings pro an con on shotcrete multistorey basement walls (for parking areas) versus cast-in-place, assuming the wall must be waterproofed.

I am also interested in answer to my question about columns that project from the wall:
2) If the perimeter wall has structural columns that project into the building (example say 12" thick wall with 24" thick columns which project 12" into the building) would that still be a candidate for shotcrete?

 

[sandman21]

No, I dont work in the shotcrete industry. Well if the contractor did not take the proper steps to protect the waterproofing membrane from damage, a properly selected one used for shotcrete applications, a number of products that can be applied to finished concrete to provide waterproofing. If your client is concerned about waterproofing the company I posted earlier makes admixtures to provide waterproofing in the shotcrete. A quick Google search shows a number of projects and companies who provide shotcrete services. Perhaps it is time to discuss your project with them.

 

[ajk1]

I have done shotcrete repairs, so I am aware of some of the issues. There are few if any companies in my area that specialize in shotcrete. The one that I was aware of that did do so, is no longer in business, as far as I am aware. The concern with using a crystalling admixture in the concrete is that if the concrete ends up with cracks exceeding hairline, it probably won't succeed in rendering it watertight. Yes I can discuss it further with shotcrete company, but first was looking for knowledgeable input from a disinterested source, and thought this forum was one possible source for that. The lack of extensive response so far, seems to perhaps indicate that shotcreting multistorey basement walls is rarely done.