- Sep 13, 1999The 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.

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