Adding fiber

Snomam(Zone 3)September 19, 2005

I'll be making a rather large trough soon. Just got an order from, excellent service! I got nylon fiber and acrylic bonding admixture. Should I soak the nylon fibers in water before adding to the 'tufa mix? And should I use both nylon fibers AND the bonding mix, or just one of them?

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rcp1959(z4 N E Ia)

Yes put your fibers in with bonding mix and some water and stir it around to break it apart.

    Bookmark   September 19, 2005 at 9:33PM
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Sorry, but I strongly disagree. The best way to get polyfibers evenly distributed throughout the mix is to add them to your other dry ingredients and then mix them all thoroughly before adding any water. Whether you are hand mixing or using a powered mixer, you should always spend almost as much time pre-mixing the "dry" ingredients as you do after adding the "wet", regardless of what those ingredients might be. This old and established mason's practice (which is also employed in commercial conrete casting) will consistently yield better and stronger finished products. I always run my electric mixer for a full 10 minutes prior to introducing any wet ingredients and extend it to 15 minutes when using fibers.

    Bookmark   September 20, 2005 at 10:51AM
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rickharmer(z4British Colum)

I echo Tango's thoughts,even when using the "armstrong" method of mixing.I'd also read on this forum(I think-I'm old and I'll stick to that excuse!)where you separate the fibers before adding to the dry mix,so that clumping doesn't occur.I rub them between my palms to loosen them up,when I use them.I have some old(and,after moving them,beat up!)troughs that didn't get the palm treatment and dispite vigourous handmixing,didn't distribute as well as I'd hoped.
good luck on your ventures
Cheers from a still warm here!

    Bookmark   September 20, 2005 at 11:49AM
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I totally agree with Tango.
Dry mixing is extremely important.
That thin coating of portland on all other materials is one of the keys to a strong recipe.
Because it is such a dusty process I suspect it is often neglected and may be yet another cause of crumbling tufa.
It's hard work but I shake my dries together in sealed 5 gallon buckets. If it's a bigger batch I use my cement mixer to mix the dries even though I rarely use the mixer to mix wet. It just sucks for my light weight tufa and concrete recipes.

    Bookmark   September 20, 2005 at 12:48PM
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Snomam(Zone 3)

Dry is the only way. I tried a small batch of tufa mix with fibers I'd mixed in water first, they clumped up like wet cat hair. I pulled them out and tossed them. It will be a miracle if this large trough works out, what a disaster. A video would be hilarious, everything went wrong!

    Bookmark   September 20, 2005 at 4:07PM
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club_53(z5 MA)

I have to agree with you guys -- mix 'em dry. I once bought some fibers that required a lot of pulling and separating (definitely early cave), and in a flash of "brilliance" thought I could disburse them in water with a hand mixer lol. .. all I got was a mess that looked like cotton candy. Now I use the fibers that I get from the masonry supply house and just rub them between the palms of my hands and mix them in dry.

You're right Tufa Enough -- the cement mixer on small batches can be real savage amusement, although spinning some water in the barrel before starting it cuts down on the cussing. I have gone back to using my OddJob (which I know lots of your folks despise) for a small job and then dumping the tufa out into a big mortar tub for the final touches by hand...makes life a whole lot easier and only takes a few minutes.

    Bookmark   September 20, 2005 at 4:36PM
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Roger the dust problem. I finally cut a piece of corrugated plastic (a discarded gas station pole-sign) to fit over the front of my mixer while dry mixing. It's held on with four tiny spring clamps and has probably saved what little is left of my lungs. In fact I recently cut another small hole in the center and made a movable cover for it so that I could spray a little mist into the barrel while it's running without taking off the whole cover. It knocks down the dust and fibers enough that you can then open it up without creating a toxic cloud. Should'a done it years ago.

    Bookmark   September 20, 2005 at 9:31PM
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rcp1959(z4 N E Ia)

Hello Thanks alot for the info, I will try mixing the fibers with the portland cement from now on. This is the greatest site to get ideas from.Just casted some plastic sea shell plates with concrete pacther last night and they came out great.Now I want to panit them and put two shells together and use something for a pearl in the center but I don't know what use for a pearl.Again Thanks so much for the info. Randy

    Bookmark   September 20, 2005 at 9:32PM
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Not long ago I made a sand encrusted "seashore" base for a huge sponge that we have some small bromiliads in. Embedded a bunch of small shells along with various sized "pearls" from a $3 buck thrift store necklace. Tre's Kitch, but it actually turned out pretty neat and the fake pearls are what really make it. Check out your local junk shops for cheap costume jewelry...even their "rubies" are affordable.

    Bookmark   September 21, 2005 at 1:14AM
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rcp1959(z4 N E Ia)

Hi I didin't think of buying from a thrift store thanks for the info

    Bookmark   September 21, 2005 at 5:54PM
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Logic says to add the fibres to the dry mix, the little bag of fibres I just bought say to mix them in after the other materials have been wetted... I'm goin' with ye who know - dry just makes more sense. Good tip about rubbing them betwixt ones' palms.

BTW, I got mine from a local stucco supply place, a 3 oz. bag cost $2.40 CDN and will apparently do 300-350 lbs. of concrete mix! They are a product of Nycon, ( who have an informative web site. They claim that their nylon fibres will not give you hairy concrete. We'll see.

What length of fibres does everyone use?

    Bookmark   October 22, 2005 at 5:05PM
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rustinj(z7 AL)

Well, this is interesting. I had the completely opposite experience. I got sick of trying to break up the nylon fibers (from Butler's) by hand, and picking the fiber dust bunny clumps out of my dry ingredients. I now add the fibers to a small jug of water, along with other liquid additives, shake the heck out of it for 10 seconds, and then add it to my mixed dry ingredients. This gets mixed in and then the rest of the water is added. That completely eliminated my fiber clumping problems, and the lengthy amount of time it took to break them up. Maybe this only works for Butler's brand, but I'll never try mixing it with the dry stuff again.


    Bookmark   October 22, 2005 at 10:30PM
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leftwood(z4a MN)

An interesting concept. Kind of like moistening dry cornmeal before puting it in water to cook so it doesn't clump. My interest mounts as our experts reply . . .

    Bookmark   October 26, 2005 at 3:45PM
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coastal_concepts(Toronto CA)

Hi Guys,

I coulnd't resist jumping in on this thread. When trying to decide what is the best way to mix you should look to the pro's. Without fail the best mix from any day where we were sculpting concrete was the mix right after lunch. The concrete (provided its not too hot a day) would spin in the drum for 30-40 minutes before we got to it and it always produced the best for sculpting and bunching of the fibers was not an issue.

I think that the best method for home mixing is a shovel and a wheelbarrow. Even for smaller loads this really gives the leverage needed to get a consistant mix.

For anyone wondering if they are mixing enough, It would liklely be a safe bet to mix 3 times longer than you have been. Avoid overwatering at all costs as this dramatically compromises the strength of the concrete.

Spring for glass fibers if you can find/afford them instead of fiberglass and you will wonder how you ever dealt with the hassle of clumping fiberglass.

Hope this helps

    Bookmark   November 2, 2005 at 10:14PM
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What's the difference between glass fibers and fiberglass?

    Bookmark   November 6, 2005 at 4:24AM
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coastal_concepts(Toronto CA)

-Fiberglass is difficult to work with (clumping)
-Breathing fiberglass can be very hazerdous to you health
-Adds mild strenth to cured concrete
-Minimizes hairline cracking and fractures
-Creates a "hairy" effect that must be removed via heatgun
-Available in 2lb bags for $7-$10 CAD direct from concrete batch plants

Glass fiber has the consitency of the bristles on a hair brush so clumping is not an issue. Glass fiber adds FAR more overall strength to the mixture when compared to fiberglass. Glass Fiber is about 4-5 times more expensive and also very difficult to find. If you have access to speacalty concrete supply stores that would be the first place to try. They probably will not stock it, but may have access through their suppliers. The ease use use alone in comparison to fiberglass makes it a worthwhile investemnt. For the average hobbiest I would imagine a bag of fibers could last a very long time.


    Bookmark   November 10, 2005 at 8:34PM
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Hi Steve,

I'm still not sure what you mean by Fiberglass. I have a large bag of alkali-resistant glass fibre that I bought ten years ago, I'm wondering if it's the same thing as you're talking about? I didn't know there's a difference between fibreglass and glass fibre, have never come across anything but this kind of stuff. Is this what you mean by glass fibre?

    Bookmark   November 12, 2005 at 2:59AM
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I use this in different ways, but have never used it through the entire mix.
1. If I'm casting into a mould, I paint a thin skin of white cement/sand/SBR admixture into the mould, and allow to set slightly. Then I make a similar mix (half cement, half sand) and add the glass fibre. Pack that in, in a thin layer, cover the work and allow to cure for several days before demoulding. This give a very smooth fine surface which can be polished to look like marble.
2. Since I've started experimenting with hypertufa, I've been building the initial layer a bit thicker, using hypertufa, then packing in at the back with a very thin skin of cement/sand/glass fibre/SBR.
3. Working the other way round (building over a re-usable former, rather than casting into a mould) I make the initial layer from cement/sand/glass fibre/SBR, and then build up the hypertufa for a depth of an inch or so over that.

Here's a former I'm finishing at the moment. I want the flexibility of a former that can produce different shaped alpine troughs, depending on how I pile the hypertufa on.

I made this former using chunks of polystyrene insulation, glued roughly together. Then draped fabric soaked in Plaster of Paris over it. Finally filled in any undercuts using filler. (The red arrows are showing the undercuts that I need to fill.) I'll drape a sheet of plastic over the top, and build the trough on that. When the trough has set I can pull out the former, and re-use it.

I came across a link to a picture of your BBQ platform, and loved the effect you achieved by cutting into the wet hypertufa with a trowel. I'll be trying your methods out on the alpine troughs!

Here is a link that might be useful:

    Bookmark   November 12, 2005 at 3:25AM
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coastal_concepts(Toronto CA)

The bag of fibers shown above is fiberglass. From the looks of it, it could be automotive grade which when mixed with a resin apoxy becomes "bondo".

Fiberglass for concrete is white and has a fluffy cloud-like appearance.

Glass fiber for concrete looks similar to the fiberglass picture above except that glass fiber is generally white and is somewhat course to the touch.

If you want to learn more about glass fiber technology you may want to read up on the latest "translucent concrete" that is being made via embedding glass fiber strands uni-directionally in concrete allowing the fiber optic effect of light passing through the concrete. This technology allows light to pass through concrete up to 20 meters thick!

I should probably note that the home enthusiest will not experience light transmitting effects by simply throwing a bunch of glass fiber into the mix...

I should probably also note that most of the concrete technology that I refer to is unheard of in North America. A mix strength of 35 MpA is considered strong here and 100 MpA does not exist. In japan and other regions in the Asia-Pacific and Europe, concrete technology using special admixes and glass fiber technology are creating self supporting concrte with a strength nearing 250MpA. Locally a concrete batch plant would laugh at you if you requested anything higher than a 35-40 MpA mix.

Just to give some perspective.

sidewalks = 15 Mpa
average construction = 20 Mpa
Cocrete swimming pools = 30-35 Mpa
High Rise construction = 35-45 Mpa

The mix that I have revealed in earlier GW posts is engineered to a strength greater than 75 Mpa and can be achieved by the home user. This strength is comparible to the concrete used in spanning structures like bridges and has a great deal to do with the glass fiber reinforcement...

Ah I could just go on forever...


    Bookmark   November 12, 2005 at 4:00PM
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rickharmer(z4British Colum)

Thanks for you input and cheers from just north of ya!

    Bookmark   November 12, 2005 at 5:07PM
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Steve wrote:
"Ah I could just go on forever... "

Please do! I absorb and appreciate every bit! :)

Runner I'm curious about your mold - you're going to have tufa troughs with an island in the center? If you are covering the mold with tufa do you lay it on gently so as not to break the POP shell? (I'm trying to think how you are using the mold)

Do you have any pics of that which can be polished to look like marble?

    Bookmark   November 12, 2005 at 7:24PM
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Hi Steve - thanks for the response. I bought that fibre from Pilkingtons, alkali-resistant specifically for use with concrete. That's why I'm interested in your points, I'm wondering if I should have specified in a different way? I've never had any problems with it, though, and have a large quantity that I'll use up before buying anything else!

Chris - that plaster former is pretty strong, as there's a polystyrene base, covered with 5 or 6 layers of fabric soaked in plaster. So a bit like an egg-shell. I'll cover with plastic, and then just press on the hypertufa. Hope it doesn't form too strong a vacuum, or I won't be able to release it!! I'll probably put some kind of strong webbign strap across the former before putting on the plastic, so once the thing is finished I'll have a handle to give some leverage when removing the former. Hope that makes sense. Below is a link to a sculpture made with cement/sand/SBR/fibre polished to look like marble.

PS - we spell fibre and mould differently, I try to remember to spell your way but keep forgetting lol

Here is a link that might be useful:

    Bookmark   November 13, 2005 at 3:54AM
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Thanks Runner! I'm thinking you are somewhat gentle at packing on the tufa? I tend to cram it on and beat it into shape, would be nice to know I don't have to be so brutal to it :)

Now I remember that sculpture! it is the epitome of smooth, both in shape and texture!

as to your spelling - you just keep on typing it like you know it! as a Canadian I know some of our spelling seems odd to the Americans, but I still spell it as I learned it :)

    Bookmark   November 15, 2005 at 11:20PM
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Yeah, I just take handfuls and press it on quite gently.

    Bookmark   November 16, 2005 at 3:57AM
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