Hi all,

I don't know if you remember, but I was asking about making the base of 
a Lehman type seismometer out of concrete earlier.  This particular mix 
is useful because it is seismically quiet, being relatively homogenous 
for lack of gravel.  I made up two batches of concrete and tested it 
today at my engineering school.  Below is a summary of the results.

Concrete type: "Low noise" mix of cement powder and sand only. 
Ingredients by mass are 1:1:0.4-0.5 cement:sand:water.  Cement was 
generic "type I/II" from the local hardware store; sand was recycled 
crushed concrete (not the best, but easily available).  The mix was 
prepared in a buket with a trowel.  It had the consistency of cookie 
dough - about at the limit of being mixable by hand.

Two mixes were tested, 0.4 water/cement (w/c) by mass, and 0.5 w/c. 
Three cyliners were cast of each mix: 3" dia, 6" tall.  They were tested 
to failure in axial compression at 5 weeks age (not quite moist cure) on 
a 300,000 lb testing machine.  I'll probably go to Hell for it, but I 
did not cap the ends with testing compound.  The strength of plain-ended 
cylinders is perhaps 20% lower than the real copmressive strength, and 
is highly variable.

results:
0.5 w/c: 5400 psi test, maybe 6500 psi corrected for end conditions
0.4 w/c: 6250 psi test, 7500 psi corrected

For comparison, concrete from a truck should be 3000-4000 psi.  "Back 
yard, wet-mixed" concrete is usually less than 1000 psi.  This is very 
strong stuff.

Other engineering properties are:
estimated tensile strength: 600 psi
estimated elastic modulus: 5e6 psi (1/6 as stiff as steel)
specific weight: 8.4e-2 lbs/in^3 or 145 lb/ft^3

Translation: You can cast whatever you want out of this mix, so long as 
you vibrate it adequately to remove air pockets according to the 
intended purpose.  The section I am designing for my Lehman base will 
weigh ~ 68 lbs at 36" length and will at least as stiff as any available 
steel section (like I-beams or structural tubing) of the same weight. 
If this proves to be adequately noise-free, it will be a very cheap 
(cement and sand cost <$20) way to build very stiff structural members.

Scott

-- 
A day without math is like a day without sunshine.

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