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Why does hypertufa crack?

Posted by monomer none (My Page) on
Thu, Jan 23, 14 at 23:20

I've been making hypertufa pots and other objects for two years now and have noticed that some of them seem to suddenly develop cracks months after they've been made. I've done internet searches and can find very little on hypertufa pots cracking... surely this must be an issue within the hobby. I would venture that approx. 10-15% of our pots have small cracks... and nearly always in the same places. They all start at the top lip and proceed down, never any other places... most are barely visible and only go down an inch or two. On the square shaped pots its always mid-way and never found near a corner... almost always in a pair of cracks found on opposite sides across from either other and the other two perpendicular sides remain crack free... I think this is because the two cracks on opposing sides release the stresses. These cracks appear far less frequently on round shaped pots and have yet to appear on any solid projects I've made. Though these cracks can show up at anytime, more often they appear after several months, a day or two after a thorough watering. Other pots from the same batch will not have cracks which tends to have me believe its not so much recipe/ingredient related nor is it curing related. The way I see it is cracks are visual proof there are tensile forces at work at the top of these pots and since cement is known to have a deal of compressive strength but almost no tensile strength, it will crack under relatively little tensile stress regardless of the aggregate used. Sand and gravel only add to concrete's compressive strength and do nothing for its tensile and shear strength... that's why rebar or wire mesh is added, to provide the tensile and shear strength needed in structural applications where shear and tensile forces will be a factor. All perlite does is reduce compressive strength and adds nothing to tensile properties... and peat, like most organic matter added to cement, actually reduces all mechanical properties, even more so over time as it decomposes. In some of my batches I've used glass fibers (well mixed) to add some reinforcement and flexibility... I also add some nylon fibers to prevent plastic cracking (micro-fractures that can appear during the plastic state of cement curing), also tried adding a small amount of acrylic fortifier (which mainly enhances bonding adhesion). They don't seem to make a lot of difference prevent of the types of cracks I'm talking about.

Now I must mention that my pots are not thick proportion-wise... example an 8" pot might have a (very densely compressed) 3/4" wall thickness... I don't like large bulbous 'hobbit' shapes that use 3" walls leaving behind a tiny thimble's worth of potting soil remaining for the plant. So, short of adding wire mesh, does anyone have any proven ideas on how to prevent cracking at the lip permanently.

The most recent event that prompts my search for an answer are two (very wide square) pots that recently had cracks suddenly appeared the very next day after a thorough watering... they were "inside" pots, not exposed to the elements or freezing temps, though the air is quite dry indoors here in the winter. My guess as to the cause at this point is it must have something to do with the lower portion of the pot soaking in a lot of water and trying to expand in circumference at the bottom while the top lip is still dry and so can't respond by also expanding, thus a building up a tensile force at the lip which ultimately split (aka cracked) in order to finally release the tensile force and allow the bottom to expand.


Follow-Up Postings:

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RE: Why does hypertufa crack?

Been making pots for a long time and never had the cracks you have. I don't use any fibers, plasticizers, wire or acrylic fortifiers. I also don't have 3" walls unless it's a very large piece, 3' long or longer.
What recipe are you using? How are you curing your pieces?


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RE: Why does hypertufa crack?

NEVER??? Really?

I've used a myriad of recipes as I have now made well over a hundred pots and other hypertufa objects (its likely nearer to two hundred actually). For all my earlier pots I've used the standard 1/3 peat/perlite/cement mix (this has been the vast majority of my batches)... however I've also used coir, vermiculite, EPS (polystyrene beads), sand, diatomaceous earth, kitty litter, clay, pine needles, wood chips in various combinations at one time or other. Currently (in winter) I basically do a two-week moisture cure (meaning in a plastic bag in the house @ 70F) followed by another week to a month or more outside of the bag laying around the house before giving it a dilute vinegar wash and rinsing. Some are planted and some are still not... the only pots that have cracks are planted pots, NEVER while unplanted. During the summer the curing pots in their plastic bags are left outdoors in the backyard where they might get a little direct sun but mostly are in shade, then once out of the bag they were left in the sun for a few weeks before put into service or stored under an overhang.

Having made so darn many of these things, really there is no 'set' recipe and exact curing procedure that has been the standard for all of them. Even so the thing is there appears to be no pattern because I generally make a bunch of pots out of the same batch at the same time and they cure side by side yet only one or two from that batch might ever develop cracks months later. So I'm thoroughly convinced its neither recipe nor curing related... that's why I'm pursuing it as either design or environmentally caused.

BTW, there are others who have experienced these same cracks without solution (I've read those threads) however no one seems to have any real answers other than to say they've never had a pot crack but that's not much help to someone who has experienced it.

My current project is a highly unusual shaped growing container that is especially prone to cracking from the outside-in due to its wide flares... this is why I'm probing around to see if anyone understands what the cause is and has a solution before I continue to make any more of these. So far the only solutions I have in mind involve either increasing weight or cost or both... I'd prefer the extreme light-weight, breathability and ultra cheap cost of 'standard' hypertufa... with that in mind I'm open to suggestion to the cause and a solution.


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RE: Why does hypertufa crack?

BTW, I'd like to add that none of these are deep structural type cracks... in fact, my wife didn't even notice any of them until I pointed them out and now she thinks they add character to the pieces... go figure. I need my reading glasses to find most of them but as small as they are I am still bothered by them and would like to be able to prevent them from occurring.


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RE: Why does hypertufa crack?

how about a few photos.

I have experienced cracks but mostly after being outside for some time.
I can say that the biggest problem I had with cracks was when I first started and I was making them in the garage in the winter and they did not cure properly. Or if they cured too fast.
I am not making that many pots.
I use 1/3 potting soil, 1/3 Portland, 1/3 perlite. sometimes vermiculite, sometimes sand, occasionally use bonding agent, only use fibers for the really big pots.
Most cure 2 weeks in bag outside, wet all the time. in the shade - just because it is close to the hose. Wash off well and plant.
I usually do not start making until the outside temps are warmer. Maybe around 50 F

Here is a link that might be useful: photos of my hypertufa


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RE: Why does hypertufa crack?

Concrete is by its nature "cracky". Surface fractures are not unusual. I have found after making hundreds of pots that, for me, the best assurance against cracks is really hard compressing of the mixture while making the pot, also making sure to mash down on that top lip and seal it against water/rain seeping in. I do use the fiberglass fibers for insurance. It is said that the fibers inhibit the micro-cracks from joining the Concrete Cracking Brothers Union and making a larger crack, but I really don't know. As I sell my pots, I err on the side of caution so that no one returns a pot because it didn't hold up.

I prefer curing the pots in a water bath; they seem to cure faster and harder. After the initial cure, the pots come out of their molds and are immersed in a tub of water, or if you have a pond throw them in. Depending on my schedule, I just leave them in the water for two or three weeks, or longer if I forget about them.

Little pots can be made with thinner walls as these pots are not subjected to the stresses of the weight load of large amounts of soil, made heavier with water, expansion from freezing, etc.

Well, that's what I've learned; maybe it will help lol


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RE: Why does hypertufa crack?

"•Posted by monomer none (My Page) on
Sat, Jan 25, 14 at 1:59
"NEVER??? Really?" "
You asked for help and I get disbelief when I try to help. Been working with hypertufa since the 1970's. Other than a too wet mix which resulted in very thick bottom and strange walls, when I first started, I've never had a problem.
When I ask people to post exactly what mix they use, what mold, what release, how big/thick, how long & procedure for curing. It's to help try to find an answer to their problem. Most of the time it's one of those things and most of the time people do not tell you exactly what they did.
There is no "standard" hypertufa mix. There is the original mix and all the variations. You use certain mixes for different looks and functions.
I'll guess you'll have to keep looking for an answer to your problem.


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RE: Why does hypertufa crack?

@Club53... I have a background in engineering and materials, unfortunately its in polymers and not concrete. However the principles of analysis of applied forces/stresses, mechanics of design and physical properties still remain the same and are wholly applicable to cementitious type mixtures. What I needed to do was research the specifics of cement and various types of concretes and then apply that knowledge to hypertufa. This I've been doing and have found that you are entirely correct in your advice. Strength gain from submerging is going to be marginally greater that simply being stuffed in a sealed plastic bag. A 7-day cure is mandatory but 2 weeks is even better and 3 weeks will add a bit more and a 28-day cure a little bit more, etc. etc. etc.

A major take-away from my research is: ALL cementitious materials WILL crack at some point, this is an accepted fact within the industry and is considered as inevitable... as you say the stuff is inherently "cracky". The physics of hydration demand this. Cement typically has a tensile strength that is only 5-10% of its compressive strength and since the hydration process will continue for 30 years or more, this means it continually shrinks over time yet has little tensile strength to resist the ever increasing stresses. This constant shrinking guarantees it will crack... and when little cracks get together they become bigger cracks and is the ultimate failure mode for even perfectly made and cured cementitious constructions.

The addition of 'light' aggregates (perlite, vermiculite, polystyrene) greatly reduce ALL the mechanical properties, both compressive, tensile, flexural, shear and impact. The addition of organic matter only worsens this by creating large channels of space after the peat, coir, sawdust, etc rots away... this can happen within mere months in the case of peat. This means the mode of destruction for all properly made hypertufa is going to be the same... they will self-destruct by crack propagation... as you say 'joining the cracking brothers union', that is inevitable. So the only question is what steps to take to delay the inevitable for as long as possible. Your advice is well thought out.

Everything that will increase compressive strength will likewise increase tensile strength (and flexure as a by-product) too as this is still going to be found as a 5-10% ratio of compressive strength. The submerged curing for increased length of time is a good suggestion as is to tightly compact the lip since this puts it under compression which is similar in principle to the concept of why pre-stressing concrete works to allow greater tensile loads to be absorbed. To your excellent suggestions I will add and say that a lower water to cement ratio will also increase strength... lowering the volume of organic matter incorporated as a percentage will also increase strength. The addition of anything that has inherent tensile strength (fibers do this especially well because of their aspect ratios... meaning their shapes are long and skinny like a rope being used in a tug-o-war) and a strong bond to the cement will increase tensile strength by preventing the small micro-cracks from growing (joining the union)... there are polymer additives that will enhance this fiber bonding thereby increasing this transfer of tensile stresses to the fibers to resist.

I too am beginning to sell my quite unique looking products to the public and so cannot afford to have them begin showing cracks a few months or in a year or two even... doesn't matter if its only the aesthetic 'character' type cracks that don't really affect it structurally. I believe I've effected a solution in my situation using modern technology and a re-design that keeps the lightness of standard hypertufa while adding tremendously to the tensile strength and impact resistance of my wares and yet only increases cost per item by mere pennies... however only the test of time will tell me for sure how well they actually do hold up.

Bottom line is ALL cementitious composites WILL crack and the only strategy is to try to delay the onset of the inevitable crack propagation failure.


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RE: Why does hypertufa crack?

What do you add to your mixture to give it more strength?

Adding more concrete, after a point, defeats one of the purposes of hypertufa -- by adding weight back in.

Yes, if the hypertufa mixture is too wet, it does make the product weaker. I tell my students "you can pay me now or you can pay me later". The mixture may be easier to deal with today, but you will lose it on the other end with a shorter life to the pot. Same thing about curing. I do three weeks minimum, usually 28 days. I suppose if a trough were to be planted and put outside, rain, dew and plant watering will help in keeping up with the cure if someone is in a hurry.


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RE: Why does hypertufa crack?

  • Posted by slowjane USDA 10 - Sunset 21 (My Page) on
    Sun, May 11, 14 at 12:23

hypertufa newbie here!

just reading through this thread among others and curious what your final recipe was monomer? (are you still here?) if you solved it, sure would be curious to know!


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RE: Why does hypertufa crack?

  • Posted by slowjane USDA 10 - Sunset 21 (My Page) on
    Sun, May 11, 14 at 12:24

and billie_ann what is your recipe? if you've posted it elsewhere, my apologies. you seem to have a ton of experience and was wondering what works for you.

thanks all!


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