Carbonation, non-ferrous shotcrete, etc - looking for more info. 1

[Nafnlaus]

Hi all. I recently purchased some land with intent to build an "unusual" home on it - the intent is for it to appear to be like a cave, with lots of windows on the one or two sides that are exposed (no actual boring into the ground, it's to be built in a natural depression and then filled over). Concrete seems like the obvious structural material, and in particular due to the curved surfaces needed to make the place cavelike, shotcrete. But there's another issue at hand, which is I'd like to avoid steel rebar.

I hate the thought of my place, 40-80 years down the road, breaking up because steel rebar has rusted from within (versus Roman concrete structures which are still around today). And the more I read, the less I like the thought of steel in my concrete. For example, I was reading up on concrete degradation mechanisms and read about carbonation - how carbonation actually makes the concrete *stronger*, turning it into limestone (like a normal cave would be made of!), but it must be avoided because it lowers the pH to a more neutral level which facilitates rebar rust.

I know there's a wide range of non-ferrous reinforcement options available nowadays, but they're just not what people are used to using in home construction. I even found one company that sells a blend of synthetic fibers specifically designed for rebarless underground shotcrete construction:

http://www.fibermesh.com/application/shotcrete.html

I imagine there's other companies out there too.

Now, I just know that when I meet with my architect and especially his concrete guy, they're going to automatically want to go for your standard steel-reinforced poured concrete solutions. If I want to have a chance at avoiding that, I need to know as much as possible.

1. So, first off, is my information above correct and my goal plausible / reasonable?
2. I only know the basics of shotcrete - a structure is built to spray against, a mixture is prepared (either dry or wet), a person with a sprayer sprays it at high velocities at the target, where it not only sticks but is automatically well compacted. But I don't know any of the details. For example:
2a: What steps / important information have I left out?
2b: What sort of target structure is typically built to spray against, and how?
2c: Am I correct in assuming that the construction of the targets is generally grunt work while the spraying is a task that requires a professional with specific shotcrete experience to do well?
2d: What is the texture of shotcrete like without smoothing/leveling? Aka, is it "natural" looking or not, is it sharp/abrasive, etc?
2d: I've read that the cost of shotcrete is roughly the same as poured - is this correct?
3. Do you think professionals would generally be willing to use a product like the fibermesh example above, or would they balk because it's not "traditional"?
4. Do you know of any other non-ferrous reinforcement solutions I should look into?
5. How much would such a non-ferrous reinforcement affect the overall cost of the concrete?
6. Given that the goal is normally to discourage carbonation, what can be done contrariwise to actually encourage carbonation?
6a: In particular, what can be done to encourage early carbonation (before the concrete is fully set)? Any changes in material over time, even if they're into a stronger substance, would typically promote cracking, correct?
6b: Would professionals balk at doing it?
6c: How would it affect the overall cost?
6d: Does carbonation have any affect on the macroscale appearance of the concrete? (I know at the microscale, for example, you can see calcite crystals)?
7. Given that I live in Iceland, do you think shipping of whatever is necessary to import in order to produce a desireable concrete might complicate things, or are suppliers generally fine with shipping internationally / with reasonable prices?
8. Would the above meet code in the EU? (Iceland's building regulations are generally similar to or identical to EU regulations) If you don't know that, would it meet code in (Your-Location-Here)?
9. What else should I know?

 

[Ron]

This forum is for engineers to discuss with other engineers. It is not for DIYer's. HOWEVER! You are proposing something that is potentially dangerous so I feel I must respond to some of your issues.

I'll take No. 9 first....you need to know a lot more than your post implies that you know. Shotcrete is a good material and can be used in structural applications; however, there is still a need to follow the basic premises of reinforced concrete (concrete can be very strong in compression, but is noticeably weak in tension...you are proposing a structure that will have large area of concrete in tension (flexure), thus reinforcing is necessary.

Polypropylene, glass and steel fibers are available to enhance the properties of concrete. THEY ARE NOT AND CANNOT BE SUBSTITUTED FOR ADEQUATE TENSILE REINFORCEMENT IN CONCRETE!!! You can read the literature from Fibermesh, FortaFiber, Dramix, Mitchell Fibercon or any of the other producers of fibers and to some degree they are misleading, but most will tell you that their fibers cannot be used to take the place of flexural reinforcement in critical structural elements such as beams, columns, flat plates, curved plates and elevated floor or roof slabs.

Carbonation in concrete is not a good thing. We try to avoid carbonation in reinforced concrete as the resulting property changes in the concrete are deleterious to reinforcing steel and to the long term durability of concrete.

Carbonation is an indicator of lowering pH in the concrete. When the pH of concrete drops below about 10, the reinforcing steel is less protected and can corrode at a relatively fast rate, particularly if exposed to water. Carbonation in properly densified concrete, usually does not extend more than about a quarter of an inch into the concrete. That's ok. You just don't want it to go further.

For properly designed reinforced concrete, if properly densified and adequate cover is provided over the rebar, you can expect a very, very long life of the structure. Keep in mind that the Romans and others used large masses, short spans and took advantage of arches, arching effects in masonry and cut stone to achieve their structural stability.

No...a professional would not do what you are proposing. What you want to accomplish can be done, but not without adequate structural engineering input and design.

 

[Nafnlaus]

*Sigh*... this is what I hate about asking questions on the internet. You ask for information about something and people immediately lash out at you for not being an expert in the very subject you're trying to learn about. Ugh...

Second, at no point did I say "DIY". I don't know why you'd presume that a person who comes trying to learn more about the possible materials for building a custom house out of plans to try to DIY their whole house. No, I don't plan to in any way shape or form try to DIY the concrete for my home. I want to know about what my *options* are so I can be an informed customer. I want to *learn*.

I do apologize if this is not the sort of forum to ask in. I spent about half an hour trying to find an appropriate place to ask to try to learn more and didn't find much. There is, for example, an "Ask Dr. Concrete" site, but it's simply full of spam posts. If you do know of an appropriate place for me to ask concrete questions in the future - preferably one where I won't be treated like an idiot for not knowing everything about concrete - please let me know.

I'm not sure why you spent half your post talking about the dangers to structural steel when my questions were about non-ferrous concrete. At least you gave me information about fibers versus rebar, although I would have appreciate it if you had addressed some of the non-ferrous rebar products out there.

And even as a layperson, your explanation for the durability of Roman concrete doesn't pass the simple observation test, given that I can drive all over Iceland and see old concrete structures in little to no load condition that have still completely broken apart up from their rebar rusting through. Starting with on my neighbors' property. Concrete has long heavily been used here, esp. in the postwar period, as wood is scarce and weather conditions can be harsh, so there's no shortage of examples of old concrete structures.

Again, I'm sorry if this was the wrong site - I sincerely am. But please, don't treat me like an idiot for trying to learn.

P.S. - Are you basically saying that Fibermesh is a scam? Because they explicitly do say, over and over, no rebar, all of their illustrations show no rebar, etc. "No steel to shoot around so there are no voids or shadows; no rebar or wire mesh corrosion." / "unlike traditional steel reinforcement where more concrete is needed to cover wire mesh.", etc, in their product whose category is "Shotcrete and Underground", with the applications list, "Structural rehabilitation, Lining large underground cavities, Slope stabilization, Aqueduct rehabilitation, Channel linings, Ground support and stabilization, Mining, Marine structure repair and rehabilitation, Tanks, Seismic retrofits, Protection against fire spalling, Retaining walls and soil nailing, Pools"

 

[Nafnlaus]

(Because I can't edit a post, and want to be clear):

I'm not disputing that Roman concrete was dramatically weaker than modern concrete due to its lack of rebar. I mean, the walls of the pantheon were *6,4 meters* thick at the base. But, just saying, if they had built the pantheon with steel rebar-reinforced concrete, the pantheon would not exist today. It would have been gone just a couple generations after it was built.

I also need to clarify that I have any interest in specifically having a rebar-free concrete home. I simply have an interest in a *steel-free* concrete home. I've run into literally dozens of non-ferrous reinforcement products, both non-ferrous rebar and fibers, and I've read many articles about them - not from their manufacturers - talking about actual real-world projects they've been used in, their pros and cons, etc. I have a hard time believing that it's all just some elaborate prank and that the only thing one can build out of is steel rebar, as you make it sound. Is ACI 440.1R-03 some kind of joke?

Anyway, I just wanted to be clear on those two things.

 

[Ron]

Nafnlaus....my apologies if you thought I was castigating you for asking the questions. I'm not. We get a lot of inquiries here from those who are attempting to create their own designs by reading various bits and pieces of information on the internet and then stringing those pieces together to reach an erroneous conclusion. I was simply trying to prevent that.

Structural design, particularly of a curved shell as you described, is not an easy task. It requires a reasonable amount of specialized education and experience. You have combined a difficult geometric analysis with a material (shotcrete) that do not follow classical approaches to structural analysis and design.

Yes, there are non-ferrous reinforcements available. Most are in some stage of research and none have received a great deal of use outside the experimental stages, with the exception of some specific applications in harsh chemical environments where they are perhaps the only workable solution. I agree that rebar will likely corrode at some point. The key to longevity is good, responsive design to the exposure conditions followed by good construction practices to achieve the desired goal, then followed by appropriate maintenance.

You can research fiberglass reinforcement (most common is fiber reinforced plastic or FRP) for shotcrete applications. This likely will come about as close to what you want as anything. You still must have someone analyze the structure, determine the loads (I assume you have quite a high snow load), and design a section that is responsive and safe considering those loads.

No, Fibermesh and others are not scams. The uses they describe are legitimate; however, please note that they are applying shotcrete to competent structures...they are not promoting the shotcrete as a stand-alone structural entity. They are using it somewhat like a thick stucco application. The tunnels will stand without it. The shotcrete will protect ancillary movement and particles falling. It is good as a canal or other water-body lining.

I've been involved with design and research of fiber enhanced concrete. I was a committee member of the American Concrete Institute's Committee 547 on Fiber for several years in the early development of fiber enhancement. I have experience with structural shotcrete and non-structural shotcrete. There are appropriate applications for both.

ACI 440.1R is certainly a valid document and is, and should be, used by those contemplating the use of FRP rebar. It is specialized and expensive.

Good luck and I hope there are no hard feelings. Just trying to give you the benefit of some experience. You'll likely get other answers here over the next few days.

 

[Nafnlaus]

Thank you, Ron. I greatly appreciate the information.

Concerning FRP price, I saw one article which mentioned that it's about 3x the cost of steel but about 1/2 the cost of stainless. Does this sound correct to you? And that's just concerning the price of the rebar, correct? I'm more curious as to how it would affect the per cubic meter cost (or actually, more importantly, the square meter cost, in case it has an effect on the required thickness).

 

[Ron]

That's about right on the cost. Expensive.

Assuming an average of 1 percent reinforcement (by area), the cost would be in the same proportions per cubic meter; however, the FRP reinforcement will require a higher volume so the price will increase a bit....not a great deal over what you've researched, but a bit.

 

[StoneCold]

You might be interested in googleing "concrete dome". There are lots of people creating structures similar to what you are interested in. Some above and below ground.
Hope that helps. It may lead you to a more thoroughly explored solution.

Regards
StoneCold

 

[Nafnlaus]

Thanks, StoneCold, I'll do that.