[Ferro List] Reinforcement

[Bo Atkinson]

Janosh,

I think this is a central issue in ferrocement and should apply to  
concrete too, if scaled up proportionately. Besides Paul's and Chris's  
points, i'll mention an ENR  magazine cover photo (Oct 26, '89 issue)  
of the San Francisco Freeway after the big earthquake. (Still hanging  
on my wall)... It looks like 4 inch rebar brisling out of a shattered  
concrete support column, lots of steel but a relatively poor tie-in.  
By using the same total weight of smaller rebar, formed into 3D mesh,   
it might not have shattered that way, methinks.  Even expanded metal  
concentrates a weak shear plane IMO... I think it was Keith who sent  
an article to the FC group here, about research in South Africa. It  
pointed out something like finer woven mesh holding up better against  
catastrophic failure.  I've done some lath type FC demolition and saw  
large pieces of cement flake off of half inch mesh. That's why i got  
into wire more than lath. Nothing against lath here, it is just a big  
point of curiosity for me. I think Martin also used wire mesh (not  
lath) in the Navy boat tests, (i could be wrong though, anyone?).  I  
think the PVA fibers also bear a similar problem-- For some reason  
they like to cut fibers too short, IMO. I think fibers help spread  
stresses as well as shrinkage, (both big issues in themselves). Strong  
aggregate, strong sand with some flyash helps too, but brittleness is  
also a key for tending to shearing off bigger pieces. It's hard to  
find really good sand. The glacial sand type here includes soft stone  
as well as hard. I can break the larger sand particles with my finger  
nails! Yet well reputed ready mix plants use things stuff and claim  
4000 psi abilities. ~Bo


On Mar 24, 2008, at 2:00 PM, list-request at ferrocement.net wrote:
>
> Hi, I hope this isn't a boring question, but I just have to ask.
> The biggest problem, with large diameter rebar, or steel, in cement  
> is the
> localizing of stresses and delamination around the bar.  The biggest  
> problem
> with fibers is catastrophic failure.  It's possible that fiber  
> reinforced
> cement can take greater loads than rebar reinforced armatures, but  
> that
> failure is sudden when experienced compared to what happens to a  
> similar
> structure made up of a rebar web that can hold cement materials to  
> some
> degree even when crumbled while the fiber reinforced one can't.   
> This can
> save lives by giving occupants time to escape.  This might not help  
> at all
> in a boat, but may be an advantage on land.  I think most of us are  
> aware of
> the advantages laminated lath reinforcement which bypasses the  
> problem.
> I guess the best I can do is ask if fibers can make a stronger  
> matrix than
> lath or replace at least some of the lath and make it stronger.
> How beneficial is it to add fiber and at what cost?
> Janosh