Ok everyone, it's that time of the year again: time to talk about concrete

So before we get into the nitty gritty, a bit of backgroud:
I have about 3 years and change of an engineering degree (graduated with a different degree though), during which I took several materials classes, including a few where concrete figured in significantly. I did a project on Ultra-High Performance Concrete, and another on concrete mix design. I also took a fluids class (incompressible) and a materials class (mostly focused around metals, but some time on ceramics).

With that said, I think it's time for a ferrocrete renaissance. Or, more accurately, a glass-fiber reinforced reactive powder concrete renaissance.

Consider the following:
Sailing in the US is an aging demographic that has followed a stratified trend in terms of boat cost. My wife and I are two college educated working people, and as far as I'm aware, there is no sail boat that is available to purchase new that can fit my family for less than the price of a house. We could (and most likely will, at some point) buy a used boat, but this is, I think, besides the point. The sport has shifted towards a floor that is above most of the middle class in the US. Rather than discussing the ins and outs of why (I'm happy to do that as well), let's discuss what could be done about it.

Why Concrete:
Despite apparent reputation to the contrary, concrete design has come a long way since the 70's, both in terms of the mixes that can be made, and in terms of the reinforcement. For example, it used to be the case that fiberglass was no good as reinforcement for concrete because the silica would react with the cement, as if it was sand. In a similar vein, it used to be that concrete was, for all intents and purposes, permeable to salt (and various other ions) that made it almost inevitable that steel reinforcement would corrode over long periods of time. Finally, concrete was brittle.

Fast forward to today, and there are both glass and basalt fibers, much stronger pound for pound in steel that do not react with portland cement. There is a much better understanding of pozzolons, which allow for concrete that is many times stronger (pound for pound), significantly more ductile, and impermeable to various ionic reagents.

Add to this that the materials for a few cubic yards of this concrete are measured in the hundreds, not thousands, of dollars. Add to this that concrete can be sprayed, potentially saving days worth of labor versus fiberglass.

"But the hull is only one part of the cost of the boat"

This is true. But consider a build budget of $50000 (or $20000). If I spend $15000 of that on the hull, I have $35000 left for rigging and interior. If I spend $5000, then I have an extra $10000 for interior and rigging.

Or perhaps, more appropriately: how much boat can I build for $20000 in concrete? How much in plywood? How much in glass?

Links
Types of concrete - Wikipedia https://en.wikipedia.org/wiki/Types_of_concrete#Ultra-high-performance_concrete
FISHSTONE Bottle-Pozz, 40lb bag - Fishstone - Concrete Countertop Supplies http://www.concretecountertopsupply.com/Item/GlassBottlePozz40
Supercizer 7 Super Plasticizer - Fishstone - Concrete Countertop Supplies http://www.concretecountertopsupply.com/Item/Supercizer7
Basalt Mesh Reinforcement for Concrete | Concrete Exchange http://store.concreteexchange.com/Basalt-Reinforcing-Mesh-and-Scrim-for-Concrete_2
AR Glass Net (Scrim) - Fishstone - Concrete Countertop Supplies http://www.concretecountertopsupply.com/Item/ARScrimNetByFoot

Link to self healing concrete paper:
http://www.drfixitinstitute.com/download/rebuild_2010/Rebuild vol 4 no 4 Oct-Nov 2010/Rebuild Vol. 4 No. 4 Oct-Dec 3rd.pdf

I'll try and find my UHPC presentation.

Found the answers to the questions, if not the poster itself.


EDIT: Didn't find that yet, but here's the Florida design paper:
http://canoe.slc.engr.wisc.edu/Design Papers/2013 - Florida.pdf

21.7 ft canoe
160 lbs.
Designed for at least two (but I think four) paddlers (load of ~600lbs).

Build time is 1800 man hours, but that's for two canoes (a main canoe, and a practice canoe) as well as the test canoes (several scale models).

EDIT EDIT:
"Put your money where your mouth is!" Jokes on you, I don't have any money!

Ok, but in all seriousness, I'm considering doing a relatively small scale build (16 or 17 ft, because that's the length of my garage) catamaran. I'll try and do something resembling a fat Nacra 4.5, and stick an appropriately sized rig for the righting moment on it (or the rig I can find that's complete and cheap). It probably won't be before this summer (I have a half-finished basement and three little monkeys), so if anyone beats me to it, I won't have hurt feelings

 

It doesn't really matter what the hull shell is made from. On the average sailing cruiser, say 30' - 35', which are the most common, the hull shell itself accounts for a fairly small percentage, of the total project outlay for this scale project, making the hull shell material choices nearly moot, unless you'd like to use paper mache. 1,800 for two canoes is a ridiculously long build time, in spite of a few models.

 

The savings, or lack thereof, of labour time, has to be considered as part of any project. On that basis, cheap materials are only advisable if they come with reduced, or similar labour input.

 

Two issues I see, are cost of the mold, even a cheap mold is going to seriously eat into that budget and weight, it's much lighter than ordinary concrete but it's still not as light as foam core panels.

 

You've painted some pretty broad strokes there: maybe you could give some more concrete (no pun intended) reasoning? You say "it doesn't matter" then follow up with comparative words like "small" and "long". What, to you is a "small" percentage? 10%?

I'm not sure I take your meaning by "outlay". If you mean overall cost: the cost of epoxy is hardly "small", nor is the cost of plywood. Each is on the order of thousands of dollars (at least). An order of magnitude of material savings ought not be dismissed out of hand.

As far as the man hours your point as far as it being "ridiculously long" is misleading at best. Given the amount of testing and one off systems built (many of which were build simply to be destroyed), I think your estimation is wholly inappropriate. Building three models, plus building forms, plus building the curing tent, plus the test batches, etc. are likely the lion's share of the time. The spraying of the shotcrete itself is likely a very small part of this. Moreover, the number isn't a direct comparison between different material build times. In any case, spraying concrete is a lot easier than brushing epoxy.

Finally, and perhaps most significantly, if an order of magnitude cost reduction can be achieved on the hull, perhaps there are other areas that are "small" where similar reductions could be achieved.

 

I think you're right about mold making, but again, I think there are some possibilities that make for a potentially much cheaper build than a traditional plywood form. The two that come immediately to mind are using a fiberglass scaffold to build on (which if I'm remembering correctly, is how the original ferrocrete boats were done --except with steel instead of fiberglass). The other is to make the mold out of sheets of polystyrene hot glued together. Really fast, really light, really cheap. Since you're only laying 1/4 inch (hell, call it an inch) of concrete, the weight is easily supported by the foam.

No, it's not as light as foam core, but it's 1/10th the cost. You can have a 20ft hull out of foam core or a 40 ft hull out of GFRC (or, you can have a 30 ft hull you can actually afford).

 

I have worked with gfrc in residential construction, counters and furniture, I don't believe 1/4 is enough ... 1/2 maybe with special frames ... 3/4 with frames is more likely, but 3/4 weighs soooo much more than foam core and one of the most important criteria of sailboat design is Keep It Light. If you figure out the weight, I think you will find it to be an overly heavy hull.

 

I think that's a good point, but I'd offer two caveats: If the savings are enough to offset the labor difference (e.g. if I can pay someone $15 an hour to do the labor for me) and still have some left over, then that's still a win. And secondly, I think that we should clarify what we mean by "cheap". Concrete is less expensive, by a wide margin, but with tradeoffs versus some other materials. Obviously, compared to carbon fiber that's being autoclaved, it's inferior in nearly every way except cost. But compared to some other "budget" materials, I think the tradeoffs are less cut and dry. It's WAY less maintenance than wood, probably even wood that's been glassed over. It's also probably lighter, for similar structural properties, than wood (not glassed). It's also going to be way easier to do certain kinds of curves in concrete than in plywood. It's way less toxic to deal with (barring inhaling dust) than epoxy. It also has much more flexible cure times.

And to be clear, I'm not, by any stretch of the imagination, suggesting that all boats be made of fiberglass reinforced concrete. Simply that there is a niche for the material that is currently being under-explored.

 

Curing on a mold for 16 days would never work for mass production.
Have any of these boats sat in the water for a year or two?
How do they hold up to running into things and general use and abuse?

 

The new concretes cure to full strength in less than 7 days but I still think they are not usable for small sailing craft.

 

I don't think you can base all these claims on something that hasn't been much tested. Like I said, it hasn't sat in the weather/water long, it's never been used abused in a regular manner for years and the shape, at least of the canoe in the paper, is more or less like a self supporting tube. Have you made any big flat panels? Have you devised a system for supporting big flat panels?

Or maybe this isn't a mass production idea, but an individual boat idea, a one at a time hand crafted thing?

 

I think that's somewhat missing the point. Show me a foam core hull that I can build for the same cost, and you have my vote. But that's not where we are. Can you build me a 40 ft foam core hull for $1000? I don't think so. But when you compare it to similarly inexpensive materials, the difference in mass isn't so great, especially when you factor in the total mass of the boat.

Also, the canoes are 3/8 inch I believe, so that's where the number came from. Also, what kind of admixtures were you working with? The glass fiber versus steel saves weight (and is less reactive), but doesn't add any compressive strength or ductility. What adds strength and ductility is the combination of adding pozzolons (e.g. fly ash) and superplasticizers (and also curing at high temperature and humidity). The former gives you a much stronger, more ductile and less permeable concrete, and the latter allows you to use a lot less water, which adds strength no matter what kind of concrete you're using. I honestly think the thickness will need to be determined experimentally. You can estimate based on calculations, but until you have a good handle on the material, you'll overbuild.

 

Construction concrete has a steep cure curve for about 28 days and then levels off. They cured the stuff for the canoe in the paper for 16 days.

 

A flat panel made from the material should suffice to test it for mechanical properties. It is hard to imagine it would stack up, pound for pound, against similar panels made from traditional materials. Weight would appear to be the handicap.

 

Actually these concretes have been used in marine environments for years. In the case of reactive powder, 2000 years (Roman concrete is believed to be made with volcanic ash and use a pozzolonic reaction). Their properties, I would argue, are far better documented and understood than just about any other material. The reason they haven't been used in boat building (I would argue) is that the marine industry (like most industries) is extremely conservative when it comes to new ideas. Also, in the case of personal sailing boats, there's just not a lot of money to be made in cheap hulls. For mass produced stuff generic thermoplastics are cheaper. So, from a certain standpoint, yes, I think it's best suited to small volume production (either an individual, or a little mom and pop builder), occupying a similar place to plywood, with the main advantages of cost, form, and low maintenance being the selling points.