Keeping concrete afloat -- a low tech and a higher tech approach


Greetings all,

Many people are nervous about the long-term prospects for keeping anything made of concrete afloat. I respect that wariness; concrete, left to itself, usually sinks like a stone!

There are ferro-cement ships afloat. There are also reinforced (and possibly also pre-stressed or {more likely} post-tensioned) barges that have been built, with design lifetimes of many decades – 80 years for

But that level of engineering is quite expensive! And I can understand folks still worrying about cracking (I still do, somewhat.)

Is there a lower-tech means to keep concrete structures afloat?
– even if they spring some tiny cracks?
(as concrete eventually tends to do.)

I think there may well be.

Many of you probably know about

Let’s explore that basic idea – first with a low-tech variation, as follows.

The basic idea underlying syntactic (put together) buoyancy foam is to enclose many tiny, pressure-resistant volumes of air, and attach them permanently to the floating body, so they can neither escape nor lose their air. Syntactic foams do that very well, although it’s costly.

Since most of us want to stay fairly near the surface, I first propose that we consider other containers that are less pressure resistant than microspheres used in most marine syntactic foams. Right now, I’m thinking about the problem of economically keeping a barge or similar reliably afloat on the surface.

What we really want are:

  1. many, separate, air-tight volumes,
  2. that are durable,
  3. are very low cost,
  4. easy to work with,
  5. and available all over the populated world.

So long as they’re durable, small enough for us to handle and simple to affix to our “barge,” I don’t think the size matters very much. If we have some other way of affixing them, we don’t need to put them directly into a matrix of polymer or concrete.

Consider the humble, too-often-littered, used plastic soda bottle – as used for the ship (though not quite the same way.) When capped tightly, they stay not just watertight, but pressure tight to a nominal 150 PSI (if memory serves.)

If one were to construct a concrete barge, with multiple buoyancy compartments, and fill those compartments with empty, capped soda bottles (instead of leaving them only containing air), then what we have done is trapped those bottles of air in that compartment – even if the compartment cracks and begins leaking! The air in the bottles keeps supplying bouyancy – so long as the crack isn’t big enough to let entire bottles pass through. (If you suddenly develop that big a crack – or cracks that suddenly span many compartments, then this approach won’t make you perfectly safe, but we can engineer for that, I suspect.)

So, a buoyancy compartment – even if it developed a leak and filled the entire compartment’s non-bottle volume with water – would preserve roughly half of the trapped air, and thus that much buoyancy. If 10% of the bottles are leaky, over and above this we can still tolerate it. We would need ballpark of twice the nominal buoyancy, if all compartments were to simultaneously spring leaks. I think that’s a worst case; please check my thinking. (And I note that soda bottles are not spheres, so my estimate of fill fraction is only that. See below about spheres.) Theoretically, uniform spheres can fill almost 75% of a volume: My guestimate for bottles was roughly based on that

Is this safety/engineering perfect? No! Is it safer than the same concrete-walled buoyancy compartments, sans bottles? I think so.

And this is something amenable to small, rather low-tech experimentation. If a buoyancy compartment springs a leak (due to concrete cracking), we loose some some – but not all – of the buoyancy. And if we want to inspect the chambers, we remove the bottles (probably with a big vacuum), inspect, repair if needed and replace.

Could we do better? I suspect we could. If we ever get past the bootstrap stage, we could have custom, sealed spheres (1 liter?) molded closed (no caps to fail.) Even better (for economy, ecology and image) have such spheres made from recycled plastic! So long as most stay floating, we’d be OK – and weeding out the early sinkers seems pretty straightforward.

In later stages, we might even mine floating ocean garbage patches – if there are any in the Indian ocean. (The Pacific is a long ways to go for plastic scrap.)

Improved safety, cheap and reuse all that plastic garbage.

It's free real estate

Foam cement and embedded plastic beads have some discussion threads. So have various larger plastic up to 50 gallons. But traditional cement of every version has little tensile strength, so if you build you must include some structure to pull it together and hold it tight. I think everyone is leaning towards basalt rebar. And using geopolymer instead of lime cement, for many reasons. Heating the geopolymer will prolly destroy the plastic air holders. But sure, buy a field of 40gal plastic barrels, lay in some basalt rebar correctly between them all, and cover up with a high-cement concrete, and you could make acres of real estate fairly cheaply. I bet it still cracks!


I’m saying not to cast the bottles into the concrete. Build with conventional reinforced concrete – basalt rebar or chopped fiber if you can get it. The compartments and walls remain inspect-able for cracks. (Offloading the bottles is a hassle, but not a show-stopper.)

If cracks develop, we only loose a fairly small fraction of that compartment’s buoyancy, not all of it, and probably not suddenly. I sure wouldn’t bet my life on it without testing, but (right now) I’d rather live on a barge with the bottles than with no other crack mitigations.


I understood. But by pouring between the 50gal barrels, as i described, you get 3ft tall trusses of basalt rebar, and with a 2x2 ft grid of such trusses, cracking will be minimised, and because of the basalt rebar instead of steel, minimally important. Those 50gal barrels are very strong, and i can’t imagine them ever needing replacement. But you could cast holes for each barrel if you wish, and insert the barrels (or bottles, or beads) one at a time later. You know, whatever floats your boat.


Hard to tell without trying, but the lower limit for curing Anenome’s geopolymer recipe is actually slightly below body temperature, which I doubt would post much danger to a soda bottle.


Personally I suspect that just building a cellular concrete structure without plastic vessels would be easier, but you might be correct. If you were to go with plastic air chambers though, I suspect Kat is correct in his suggestion for using the barrels. They’d be much cheaper and require less labour to get everything into position.


First, not all geopolymer needs added heat, to cure, it has to do with how well the base reacts with the silica.

Nice thing about Geopolymer is that it can be thoroughly inspected, using current oilfield technology (portable X-ray). Standard ferrocement is impervious to X-ray, making visual inspection the only means of examination.

Next, There is nothing wrong with using normal pour-in-place flotation foam, with geopolymer, just as they do with fiberglass boats.



I fully agree about building concrete barges and similar “floaters” in a cellular fashion!

Filling those cells with removable floats means that such a cell/chamber would retain ballpark of half its buoyancy, even if that compartment sprung a leak (e.g. from a crack in the concrete.) Removable floats also makes the compartments inspectable. Longer term, I think the ability to inspect compartments for cracks or leakage will be important for safety and understanding service life.

Personally, that would make me feel easier about living on such a floating structure! For eventual insurance and investors, non-technical impressions can still have big impacts. If repurposing a bunch of soda bottles lowers people’s worries, I’m all for it, so long as the technique actually works.

Figuring out whether it’s worth doing requires more thought and experimentation.


Fair enough. If you can work out a system of making them accessible then well and good.

(Larry G) #10

There’s a ton of discussion in the old forum about this. A lot of it comes down to discussing the longevity and the maintenance cycle.

Disposable plastic like soda bottles is made as cheaply as possible, and doesn’t do well in UV. You’re proposing to shelter it from that, so it’s less concern than it might be otherwise. The long term resistance to saltwater exposure on normal soda bottles is anecdotally accepted to be quite a while (one of the many complaints about them). But not really rigorously tested, as far as I know.

The perception of the longevity and stability of the platform is vitally important to attracting investors, vitally important to insurance companies, and almost as important to being legitimately accepted by marine authorities.

Individual belief in the soundness of the design varies immensely, of course. Ask a Cuban in Miami how safe something has to be before you can set to sea: you may get a different answer from the suits. Incentives and personal preferences matter.

There is no island of “great pacific garbage patch”. It’s an urban myth perpetuated by people who think activism has to be dramatic and that lying is ok if your intentions are good.



Point taken about the density – or lack thereof – of plastic garbage at sea.

I agree about the long-term stability issue of plastics in (or near) seawater. I fully agree about the need for testing, in phases. (And ditto for the basalt fiber – interesting, but I wouldn’t trust my life to it, until I see data I trust, about lifetime in the relevant conditions.)

On the other hand, I wouldn’t be above making use of people’s biases to do some micro-fund-raising:

In environmentally conscious places (for sake of argument, my birth state of California), if the used bottle concept works decently (and could be shown in a time-lapse video, via Youtube or similar), I submit that we might actually be able to get people to pay us a token fee per recycled bottle (a dollar?), to put their bottle into a second-stage prototype float of this type. And for $5, we’ll email you a photo of your very own, signed bottle, going into a buoyancy chamber :wink:

Didn’t the Plastiki project do something like that?

And say what you will, that ship sailed quite a long ways!


Yeah… I was a bit miffed about how misleading all the articles about that were back when I first encountered them. Also they’re usually accompanies with the same photographs of some trash filled harbour in south-east-asia.

(system) closed #13

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