[PSUBS-MAILIST] concrete

[Marc de Piolenc]

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
>
>
>
>
>
>
>
>
>
>
>
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> --
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>
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