[PSUBS-MAILIST] concrete

[Sean T. Stevenson]

I wasn't thinking about fine mesh as in the sort of backing you would
use as a shotcrete / gunnite form, but rather a large (e.g. 3-4") wire
mesh just to discourage cracking.  In any case, interesting about the
reinforcement actually working against you.  I wouldn't have expected
that, but then I know only enough about concrete to be dangerous. 
;-)    I had another thought - could you use two concentric steel
spheres of relatively light gauge, that could be spun without too much
regard for perfect roundness or shape control, as forms for your
concrete, that would remain in place after the pour - the outer one
serving as impact protection, and the inner one serving as a base to
weld your interior fixtures to?

Sean


On 2014-04-12 17:13, Marc de Piolenc wrote:
> Fiber-reinforced mortar is very difficult to apply to reinforcement
> consisting of fine mesh. It's been a subject of discussion on the
> Ferrocement forum for some time - people wanting the benefit of
> distributed fiber reinforcement while retaining the advantage of
> multiple layers of mesh - namely the ability to dispense completely
> with molds.
>
> As for rebar, it has only one function in a ferrocement structure,
> namely giving the bare armature enough stiffness and strength to hold
> the weight of the uncured mortar without distortion. In the final
> structure it actually causes stress concentrations. The US Navy
> compared FC structures with and without rebar years ago and the latter
> won hands down in terms of structural efficiency and durability.
>
> If you have a mold - male or female - you don't need the rebar. Plain
> mesh works just fine. Martin Iorns' laminated ferrocement technique
> also works just fine. In that technique, instead of forcing mortar
> into multiple layers of mesh, mesh and mortar layers are applied in
> alternation. And if you have fiber-reinforced mortar and some way to
> hold it in place while curing, you don't need mesh (though you can
> still use it, with Iorns' technique)!
>
> But with FC we're not talking about pouring the matrix - the panels
> are too thin to do that reliably, vibrator or no vibrator. Instead,
> plastering technique has to be used. This is not entirely a bad thing,
> since it allows half the mold to be dispensed with and gives the
> applicators a good view of their work, so that they can catch voids
> and sand pockets in the making and correct them.
>
> Marc
>
> On 4/12/2014 9:37 PM, Sean T. Stevenson wrote:
>> Build a geodesic dome out of rebar, cover it in mesh, and pour a sphere
>> of ultra high strength (no aggregate) fiber reinforced concrete around
>> it, vibrating the hell out of it to eliminate bubbles. The shape might
>> be a bit strange, given that your openings need be reinforced with a pad
>> sufficent to replace the material missing in the opening, and your
>> concrete shell is quite thick. You'd have to build a bunch of them,
>> strain gauge them all, and lower them over the side until failure.
>>
>> Sean
>>
>>
>>
>> On April 12, 2014 6:40:53 AM MDT, hank pronk <hanker_20032000 at yahoo.ca>
>> wrote:
>>
>>     Hi Sean,
>>     Thank you for the calculation. That figure is quite interesting,
>> considering it is only 30mpa.  I wonder if re-enforcement is
>> beneficial given the sphere shape.
>>     Imagine if you could figure out a safe way to make an opening in
>> the sphere for ports and hatch, you could build a hull for under
>> 1,000 dollars.  Amazing!
>>     Hank
>>    
>> ------------------------------------------------------------------------
>>
>>     On Fri, 4/11/14, Sean T. Stevenson <cast55 at telus.net> wrote:
>>
>>       Subject: Re: [PSUBS-MAILIST] concrete
>>       To: "Personal Submersibles General Discussion"
>> <personal_submersibles at psubs.org>
>>       Received: Friday, April 11, 2014, 10:34 PM
>>
>>
>>
>>
>>
>>
>>
>>           Hank,
>>
>>
>>
>>             There are countless varieties of concrete, all with
>>       different
>>             mechanical properties, so it is difficult to make an
>>       effective
>>             comparison, but just for fun, I ran your scenario
>>       (6' OD, 4in
>>
>>     thick shell) with average material properties for
>>       ordinary
>>             concrete, and it turns out it's good to over 1000
>>       fsw!  See below.
>>
>>
>>
>>             Sean