The real question is...
Is it alkalai resistant? Me...I have no idea. How a bout the rest of you folks?

see this article; section 1.12

It seems that there are ups and downs. Here's a quote re. synthetic fibres,

"Most of fibres fall in the categories of low modulus of
elasticity, such as polypropylene, polyester, polyethylene, and nylon. The main
advantages of these fibres are alkali resistance, high melting point (up to 165oC) and
low cost of the raw material. Disadvantages are poor fire resistance, poor bond with
cement matrix and sensitive with sunlight and oxygen."

The fibres shown are designed with this end use; the monofilament fibre, for example, has a crinkly, wavy shape. From this perspective I suppose that the home brew fibres might lack some of the benefit due to the "poor bond with cement matrix". He speculated. I dunno, sounds like an avenue to check out.

hth

Here is a link that might be useful: short fibres in cement article; pictures too

YO STRAW DOG: Appreciate the link to that engineers study on fiber reinforcement. Very interesting thou a bit deep in some areas. What I did get out of it was that the shape of the fiber has a lot to do with its worth as far as strength goes. Also sounds as if the poly fibers from the rope would be ok otherwise. Have a couple of ideas to change their shape that may add to their ability to do what I want. Am going to lay some out on my welding table and smack the dooey out of them with a hammer and see what changes I can make in their shape. Also will try to spread them out and "brush" them lightly with a propane torch ??? This should do something to change their shape, whatta ya think ?
Packrat

YO STRAW DOG: Appreciate the link to that engineers study on fiber reinforcement. Very interesting thou a bit deep in some areas. What I did get out of it was that the shape of the fiber has a lot to do with its worth as far as strength goes. Also sounds as if the poly fibers from the rope would be ok otherwise. Have a couple of ideas to change their shape that may add to their ability to do what I want. Am going to lay some out on my welding table and smack the dooey out of them with a hammer and see what changes I can make in their shape. Also will try to spread them out and "brush" them lightly with a propane torch ??? This should do something to change their shape, whatta ya think ?
Packrat

Hi Packrat,

just passing on what I've digging up.

Brushing them with the torch sounds like a good idea alright, the way that stuff beads up. You'll have to let us know. I'm very much a beginner to the world of concrete.

But, yeah, it sounds good to me,

cheers,

SD

packrat, What are you making that you need fibers? A lot of people hear about the fibers and think they need them. Some times it's just adding another expense and more work to your project.
Someone on this forum was cutting up rope to add to their mix but I don't remember who. Billie

Hi Billie Ann: I use fibers in all my 'crete work; bowls, didhes, leaves yard art, etc. It was me cutting up the rope. The strands are stronger than the stuff I got from the cement company plus I can adjust the length of the strands based on what I'm "creating". As to why, it adds a great deal of strength to the "creation".
Packrat

Hi! I was just cruising the forum to find some answers, and I think this discussion is pretty related to my problem.

Last year, my husband and I made our first hypertufa planter. We used the basic recipe in the "Creating & Planting Garden Troughs" book, and it seemed to go fine. The one thing we left out was the fibers, mostly because we couldn't find them in any place convenient to our home.

The planter seemed to stand the test of winter here in Pittsburgh with no problems. However, when I emptied it out and turned it over to look on the bottom, I kind of let it drop back down instead of lowering it slowly. Bam! Crack right across the middle of it. Now I have 2 half planters.

So here's my list of questions. Is this common? Is it a flaw in our workmanship or just the nature of HT? Would it have still happened if we had used the reinforcing fibers? Does this indicate that too many years of freeze/thawing would have done the same thing, or do I just need to be more gentle?

We have a nice, sunny porch on our house that doesn't have any railings. We were all gung ho to make a bunch of HT planters in lieu of railings, since it would be a perfect place for my herbs and veggies, right outside the kitchen. But I don't want to spend multiple weekends making these things only to find they're going to fall apart on me.

Any suggestions? Guidance?

Cheers,
Susan M.

Just for the (technical) record...fibers were developed for use in concrete specifically to counteract early onset cracking (caused by improper hydration/curing that leads to shrinkage)...and not to add "strength" (which is traditionally measured as the compressive load concrete will bear). To my knowledge, the only "fibers" that even claim to add any strength post cure are metallic and poly/steel fiber blends. And quite a few experts are still out on that issue.

There is however, one case in which they have been proven to help resist failure under stress by adding flexibility. It involves the very remarkable "bendable concrete" developed by Professor Victor Li at the University of Michigan (technically...Engineered Cemenetitious Composite or ECC). I have spoken with him (by Email) and the official explanation for its unique properties goes like this...
"under excessive strain, the ECC concrete bends because the distinctively coated matrix of fibers in the cement is allowed to slide within the cement." Google for the MPEG video...being 500 times more resistant to cracking & 40% lighter it really is amazing stuff. But that's flexibility...not "strength" per se.

Me?...I add fibers to just about all my base coat layers on sculptural work. As much for the mechanical "cling" the material demonstrates as its' crack reduction. However, I do not expect the mix to be any stronger once it is cured.

Just thought I'd pass these thoughts along.

All My Best --- Tango

PS...But if anyone has any data to the contrary, I would be very interested in seeing it.

Don't get discouraged... just glue those two half-planters back together with Goop (HD) or something like that... done it; works fine. It may have broken because, well, you dropped it :) Solid concrete will break, too. Perhaps you didn't tamp your planter tightly enough when you made it, and water/ice freezing and thawing got in the spaces and weakened it. Had a couple of planters break and that was my fault for not compressing them tightly enough during construction. I think this is a really important step.

The hypertufa recipe takes a little fooling around with and adjusting to come up with one that works right for you -- we all have a different touch when working with our hands, and the ingredients always have varying moisture contents at the outset.

Just some ideas. Keep at it; you'll get the hang of it.

Thanks to Tango & Debra! Good point on the fibers, and also on the tamping. I just packed it into a form by hand so I may not have been as firm towards the end as I should have been.

I'll also try to be more careful in the future.

Cheers,
Susan

'strength' is a commonly misused term, some people mean toughness (tyre rubber is fairly tough), some people mean tensile strength (like superman's hair). I can't see than anything with a lower yield strength (breaking point) can contribute anything to tensile strength, as nothing happens until the concrete breaks, at which point the fibre will fail too. Similarly, a low modulus of elasticity (stretchiness) will result in the fibre taking none of the load until the concrete fails due to brittleness.

Fibreglass is one of the few additives, along with steel, which can impart extra tensile strength to concrete. Glass has one of the highest tensile strengths, and highest modulus of elasticity (stiffness against pulling) out there and so can considerably strengthen concretes.

Poly fibres, with the possible exception of aramid type fibres and ultramegasuperchain PE are too 'soft' or stretchy to help with anything but early onset crack (as tango said). And generally poly fibres have poor bonding with cement.