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34[hreg] Papercrete/Fidobe Strength Tests

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  • Kim & Garth Travis
    Sep 13, 1999
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      The following is from Earth Quarterly issue No. 5. In this article they
      refer to fidobe as paper adobe. I have inserted the appropriate fomulae
      in square brackets.
      Kim

      Papercrete Strength Tests

      In May we had the good fortune to meet Kenneth Leitch, a graduate
      sstudent in Civil
      Engineering at New Mexico State University, who agreed to do some
      compression and tensile
      strength tests for us. We made samples of 5 different formulations, and
      met with Kenneth on
      August 19 at NMSU's Civil Engineering materials testing lab, where we
      tested them with a couple
      of Tinius Olsen multitesters, which are essentially huge hydraulic
      presses with calibrated
      dials that tell you how much pressure is being applied.
      For the compression tests, we had made papercrete cylinders, 6" in
      diameter, and 12" tall.
      The cylinders of pure paper pulp were only 10" tall, because of excess
      shrinkage.
      Here are the formulas we used:
      #1. Pure paper pulp (newsprint).
      #2. 1/2 bag of cement in a 200gallon batch; no sand. This is the "roof
      panel mix" described on
      page 16 of this issue. [160 gallons water; 60 lbs. paper; 47 lbs
      cement.]
      #3. 1/2 bag of cement + sand This is the "formula for large tow mixer"
      on page 16 of this issue.
      [160 gallons water; 60 lbs. paper; 47 lbs cement; 66lbs sand.]
      #4. Identical to #2, but with a full bag of cement rather than 1/2 bag.
      #5. Paper adobe, using the formula on pages 16-17.[160 gallons water; 60
      lbs paper; 240 lbs dirt.]
      We made three cylinders of each formula. One of the paper adobe
      cylinders broke when wet,
      so we only tested two cylinders of this formula.
      When testing these cylinders, we noticed how elastic papercrete
      and paper adobe are. Under
      a compressive load, they behave more like wood than like concrete. Wood,
      when subjected to
      modorate compressive loads, will compress down without breaking.
      Concrete, on the other hand,
      will retain its original shape as pressure is applied, until it
      eventually breaks.
      Papercrete/paper adobe behaved like an accordion--we could compress
      the cylinders from their
      original 12" down to about 9' when we got to the 85 psi range, but they
      would regain about half
      of this when the pressure was released.
      We decided to measure how much pressure was required to compress
      each sample by 2". Here
      are our averages:
      #1 59 psi
      #2 68
      #3 74
      #4 86
      #5 143
      As one would expect, the higher the non-elastic content (cement,
      sand, or dirt), the more
      pressure is required to deform the sample.
      We then took one cylinder of each formula to a larger multitester
      to see if we could
      destroy them. This unit was set up for concrete testing and had a
      limited range of motion, and
      couldn't smash samples 1-3 small enough to cause them to fail. (For
      example, we applied 12,000
      pounds of force (424 psi) to the pure paper sample, and compressed it
      from its original 10"
      down to 3 1/4". When the pressure was released, it rebounded to 4 3/4".)

      Samples 4-5, more brittle with a higher non-paper content, did
      fail. #4 failed at 248 psi,
      and #5 failed at 212 psi. (After the pressure was released, samples 4
      and 5 were both 8 1/2"
      tall 3 1/2" less than their original height.)
      The formula for #4 is similar to that used by Mike McCain; the
      "260 psi" figure we've
      been quoting in EQ is based on a compression test he had done in
      Alamosa, CO (EQ #1, p. 13).
      Considering the range of experimental error (a single test done on two
      differem samples),
      the 248 psi figure we obtained at NMSU is equivalent to the 260 psi
      we've been quoting all along.
      But-and this is an important issue--long before papercrete and
      paper adobe lose structural
      integrity, they will compress down as more pressure is applied. So
      rather than asking, "At what
      pressure will papercrete/paper adobe lose structural integrity," a more
      immediate question would
      be, "What level of compressional shrinkage is acceptable?" If you put X
      amount of weight on top
      of a wall and it squeezes down by-Y inches, is this acceptable?
      The papercrete pioneers would answer: Yes of course it is
      acceptable. In the real world,
      the weight of the wall itself is insufficient to compress the bottom
      layers at all; even adding
      the heaviest possible roof (vigas, heavy planks, etc.) will not compress
      the wall much, if at
      all; any compressional shrinkage can be easily compensated for, and the
      structural integrity of
      the wall will not be compromised.
      From the point of view of structural engineers and building codes
      people, who are used
      to dealing with inelastic wall systems, there might be some issues here.
      I think that straw bale
      walls, which surely compress a little when heavy loads are applied to
      the top, could provide a
      precedent. It might well be that more conservative building codes will
      insist that papercrete
      be used only as infill with post-and-beam walls. However, I remain
      convinced (and I think that
      most seatof-the-pants papercrete experimenters would agree) that
      load-bearing papercrete and
      paper adobe walls are perfectly safe, particularly if they are
      reinforced with rebar.
      For the tensile strength tests, we made little beams, 30" long
      with a 3x3" cross section.
      We supported these beams on their ends and applied a load in the middle.
      As expected, papercrete
      had a low tensile strength, much like unreinforced concrete. Samples 1
      and 2 could support
      approximately 20pounds; the other three samples could support
      approximately 40pounds. Kenneth
      told us that a piece of wood this size could support 1000pounds. So
      clearly, tensile strength
      is not papercrete's strong point.
      The conclusion to be drawn from all this is that papercrete/paper
      adobe are unique
      materials, with much more compressive strength than tensile strength. I
      wish we had tested to
      see how much pressure was necessary to cause the samples to deform even
      slightly, since I know
      that people will be asking this question. My sense is that any kind of
      stable (non-earthquake)
      real-world loads will cause, at most, only a slight deformation, not
      enough to be concerned
      about. But I think that further testing is called for, because I know
      that the elasticity of
      papercrete/paper adobe will be of concern to people who are used to
      working with perfectly
      rigid materials.
      page 21