Structural Issues with Papercrete

Kelly Hart, who is your host at greenhomebuilding.com, has been involved with papercrete from the early days of its popularization. He included interviews with papercrete pioneers Mike McCain, Eric Patterson and Sean Sands in the video he produced: A Sampler of Alternative Homes: Approaching Sustainable Architecture. He also chronicled his own use of this amazing material in his video: Building with Bags: How We Made Our Experimental Earthbag/Papercrete House. Kelly has built and used both an electric barrel mixer, and a McCain-designed tow mixer. His house is plastered inside and out with papercrete and can be seen here. He can speak from his experience with this novel stuff, and is frank about both its pros and it cons.

Questions and Answers

Q: Is papercrete load bearing? I understand that it's untested for 2-3 story structures - but for a one story with a metal roof would the papercrete be load bearing? I'm thinking of building monolithic walls of the stuff.

A: I know of several buildings with walls of papercrete that support roof structures. These are all one story buildings. The amount of weight a papercrete wall could bear would depend on how thick the wall is and how dense the papercrete is. While papercrete may compress a bit under pressure, it is unlikely to actually fragment.

Q: Could you tell me if anyone knows the projected life span of papercete? I have seen one site saying their homes that incorporate it will last a hundred years, others say no one has a clue.

A: I think that your second opinion is probably most accurate. This material is simple too new for anyone to know. Also, there are many environmental factors that can affect the life of papercrete, as well as various formula, so it is hard to pin down. The more mineral material and the more cement in the mix, the more durable it is likely to be. The drier the climate and the less exposed to the weather, the longer it can be expected to last.

Q: I'm interested in what you think of a dome project using a 1 3/8" frame to support a 3-frequency, 4/9ths low profile dome, using papercrete sandwiched over screen/chicken wire both inside and outside as main shell. Do you think this concept would fail due to the poor tensile strength of papercrete? Or is tensile strength not a big issue when using the overall arch-truss theory behind dome building?

A: On your question about using papercrete sandwiched between chicken wire over a geodesic dome, I think it would hold up very well. It is amazing how strong such a structure can be when you add the mesh to the papercrete, especially on two sides as you propose. The lower mesh might have to be fairly fine to keep the wet papercrete from oozing through. Such a dome would have to be well sealed from moisture after it cures, or you would run the danger of having damp walls when it rains. Domes are inherently extremely strong.

Q: How close did you use papercrete by the chimney? I will use a big pipe and will do the a little smoke pipe of my oven through them in the top of my roof, so I have an isolation airspace. Which kind of papercrete is close to your chimney? Is it 1C.:3P.:4S.which you are using at the outside skin to stop burning?

A: For the chimney, I would be very careful, since the papercrete can burn, very slowly. I would suggest using really a lot of sand in it, or maybe even using no paper where it makes contact with the pipe. I used a kind of stove pipe that is designed to be right next to burnable things, so I didn't need to worry about this.

Q: I have spent some time thinking, there must be a way to recycle paper and to make it as strong as wood ; in fact I would like to find a way to make a strong kind of " paper mâché " to then make furniture. Indeed in some countries, Africa namely Mali, Niger or Burkina, wood is scarce and very expensive. Such a process would be very useful The idea would be to propose another way to create simple furniture, but solid and also, that could be either varnished or painted or decorated. In Africa, recycling paper is not so obvious, there is of course less paper than in our western developed world, you can easily guess it, and of course the "mixture" should include local product. That is of course the idea behind recycling and local craft. Do you think you could try some mixture using paper, sand plaster may be etc? One of the ways we could use it to make furniture would be to shape boards which could be used as ordinary wooden board and then painted or decorated. Otherwise it is to mold already shaped spare parts. I am sitting here in Paris, I live in a flat so you understand the size of the stove is hopeless and my bathroom is not big enough for such fancy handcraft.

A: This is an interesting idea you have to make furniture out of some kind of recycled paper material. I don't know anybody who has tried this, but it seems like it might be possible. The mix of materials that I am familiar with in plastering my house (paper, sand and cement) might not be durable enough to make furniture. Perhaps with some other ingredients, like gypsum, that sets up into a harder material that could then be coated with a very hard lacquer of some sort, then furniture could be made this way. You would probably need some heavy steel wire, bamboo, or cane to stiffen the shape even more. Another possibility would be to add other natural fiber material to the mix, such as hemp or whatever is available locally that would tend to make the furniture hold its shape better. I suggest that you experiment with various possibilities and see what might work. Much of this can be done in even a small kitchen, on a very small scale.

Q: My wife and I want to build 2 to 3 stories. Will papercrete work for this? It looks like all the buildings are only 1 story or so. Maybe with some sort of extra support it could work. Post and beam frame with walls filled in with papercrete. Please let me know your thoughts on building multi stories.

A: I would not be able to give you engineering data for building multiple stores with papercrete. I would say that the safest approach would be to consider the papercrete as infill in a post and beam or other structural system. If the papercrete were used as if it were rammed earth, with the thickness of the wall at least one tenth of its height, and perhaps tapering as it went up, it would likely be self-supporting, but I don't know of any experiments done on buildings higher than one story.

Q: Why can't one take up papercrete space in a papercrete block which will be used like a cement block. It would seem that the bottle would not be any heavier in the empty state than papercrete and yet be very strong once protected. Has anyone used intensive amounts of bottles in the papercrete, cob???

A: I know of several instances of glass bottles being incorporated into monolithic walls of both papercrete and cob. Doing it in blocks might weaken them when handling initially, so might not be so effective.

Comment about combining papercrete and steel framing: (Kelly) I have had a continuing concern with the basic concept of integrating steel framing material with papercrete to create a shell for a house. My concern is that because the steel will always be expanding and contracting with the fluctuations in temperature, this will create stress cracks in the papercrete where it joins the steel, no matter how hard you try to avoid this. The papercrete itself is extremely stable dimensionally, under all conditions (wet,dry, hot, cold), but the steel will force a separation. This will mean that moisture will find its way into the papercrete, even if you try to seal it from the outside. In fact, if you do seal it from the outside, this will inhibit that moisture from evaporating into the atmosphere. The presence of moisture in the papercrete, in a warm house is a perfect situation for mold to form, which can have negative health effects. If your climate is sufficiently dry, the best thing to do might be to not try to seal it at all. I hope I'm wrong about this...just thought I'd express my concern.

Response: This is not something I had considered, and I am glad you brought it up. I did some research, and found some figures: The coefficient of expansion for mild steel is somewhere around 12x10^-6 per degree C, or .000012/degree (depending on the exact alloy). My 33' dome has a circumference of approximately 103 feet at the largest place. That gives an expansion of 0.014832 inches per degree C around the "equator". The steel will be covered with a minimum of R12 on the outside of the steel and only R2 on the inside, so should be close to the inside temperature of the dome for all practical purposes. The intent is to keep the inside of the dome livable, so somewhere between 50 and 80 F, or 10 and 27 C. That 17 degrees C gives a total expansion of 0.252144 inches to the circumference at the equator of the dome. That would add .08 inches to the diameter IF I allowed such a wide temperature range inside.

While papercrete is stable dimensionally, it is compressible to some extent. For the sake of argument, let's say that the papercrete has 0 expansion from the temperature change. Even if that were the case, it would only have to distort 8/100 of an inch (in thickness) to absorb the expansion of the steel. I may be way off base here, and I'm sure someone will point it out if I am, but I don't think this is a problem.

Q: I saw in many constructions here in my country that people use concrete and steel bars to build the house ceiling with good results. Could I use papercrete to make the house ceiling?

A: Steel-reinforced concrete is much more rigid than papercrete would be if reinforced in the same way. Papercrete is a bit too spongy to expect it to hold its shape as a ceiling. I know somebody who made a papercrete vaulted roof this way, and he eventually had to add some supporting poles underneath it to help hold it up. Also, if the papercrete might get wet from the roof, this would not be good since it could get moldy.

Q: I am unsure about how to tie my papercrete walls into my foundation.

A: A papercrete wall can be tied to a concrete foundation with rebar, much the same way that other masonry materials would be.

Q: If possible I would like some advice on what to do and some of the building codes on papercrete (such as how far it can span and how much weight it can hold.)

A: To my knowledge, the are no building codes developed for papercrete...it has not been tested and adopted in this way. As far as spanning any distance, like is possible with reinforced concrete, I would not expect papercrete to perform well this way. It is more capable of supporting vertical loads, but even this has not been thoroughly tested.

Q: What do you about block sizes? What is the best?

A: In general, the smaller the block size, the quicker they dry out and the lighter they are to carry, but then the more you have to make to complete a project. Another consideration is how thick you want the wall to be, how insulating do you want it, etc...so the answer depends on all of these considerations.

Q: Here in my country, we use wavy griddle of fiber cement to make the roof of houses, due to hurricanes and summer storms. This kind of roof is heavy and this is my principal preoccupation. I have to make a wall with good thickness to support that weight.

A: I would think that a wall that is at least 30 cm thick could support your roof.

Q: I would like to build and addition to my current house. A master suite hobby room and utility room. I have also been thinking of making it two stories and add two additional bed rooms. Can papercrete be used in North Central Texas and can I do a two store building? I would use poured concrete footings and a poured concrete floor for the ground floor.

A: It is hard to say what your local authorities will allow because it varies so much from place to place. I have heard of papercrete being allowed because of its similarity to stabilized adobe, since it can have a fair amount of earthen material as part of the formula, and the paper portion would be considered a form of reinforcement, similar to the straw in adobe blocks. But you might need to convince the building authorities of this.

A two-story building with papercrete walls sounds like an ambitious project.  I would suggest engineering would need to done similar to reinforced concrete, or concrete block walls to assure that the whole edifice would hold up over time. You might want to get some engineering assistance.

Q: I need to make some ceiling insulation tiles to insulate the corrugated iron roof of my strawbale house. The tiles need to cover a span of 1500mm. They can be up to 75mm thick. Can you advise me on what sort of mix I should use and how thick the tiles need to be. I'm naturally after maximum insulation and minimum weight.

A: I am assuming that you are referring to the use of papercrete for this purpose. No matter what the mix is, I doubt that PC would be able to span that distance without some internal reinforcement, and even then it might tend to collapse in the middle. PC that thick would give you perhaps R-7 insulation, which is not great for the ceiling. I suggest that look for some other material or method of insulating that area.

Q: Can I use papercrete in constructing a footbridge?

A: I can think of no aspect of constructing a foot bridge where PC would be advantageous. It doesn't really have much structural strength.

Q: How about using the papercrete in the columns of a gazebo, tentatively with dimensions, 2.5 x 4 meters?

A: If the columns were thick enough and and perhaps reinforced with some wood or steel, this might work, but papercrete would not be my first choice of material for this.

How do we reinforce it with steel, the same with the conventional reinforced concrete?

Yes, you could form up the columns with rebar or mesh as a core and pour the papercrete into the form. This would probably be the strongest, since it would be monolithic. Or you could impale prefabricated PC blocks over rebar stakes...

Q: I have a couple questions about finished papercrete walls. Once up and finished, will the walls take a screw? I am wondering what to do about shelving. Could I a place a 2x2 in the walls as sort of a stud?

A: Yes you can put screws into PC fairly easily and they won't crack it. It would depend on how much weight the shelving might need to support how well this would work. I suspect that generally if you use long deck screws (3-4") you will be fine. Also, if you add some sand or mineral material to the PC so that it is more dense, it will hold the screws better. I don't think imbedding wood in the wall would work very well, and would actually weaken the wall in that area.

Q: My question is about papercrete. Is it as viable as adobe? Do I have to use post and beam or can I build like adobe?

A: You can use papercrete blocks in a similar manner to adobe blocks. They both need to be on good foundations, well above the ground. They both need a good roof with an eave to protect the walls. They both can be self supporting, without post and beam structure, but your building authorities may have other ideas about this. They both can use a slurry of the material used to make them as a mortar.

Q: I have gotten stuck trying to determine how papercrete blocks attach to the foundation.

A: For attaching papercrete blocks to a foundation, I would suggest carefully laying out the foundation with vertical rebar pins (sticking up maybe a foot) so that you can eventually impale the blocks on those pins to hold the wall in place. I would also suggest making sure that there is a moisture barrier between the concrete and the papercrete to keep moisture from wicking upward.

Q: Are there typically vertical support posts in the papercrete walls for the load bearing roof?

A: Papercrete is certainly capable of being load bearing, depending on the weight of the roof. It might be safer to confine the papercrete as infill between posts that support the roof. If you have to comply with codes, they would probably prefer this. But I should tell you that I am not an engineer, so if you want a definitive answer you should consult a professional.


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