PSN-L Email List Message
Subject: Concrete testing done
From: beezaur beezaur@..........
Date: Mon, 05 May 2003 16:16:17 -0700
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.
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
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.
A day without math is like a day without sunshine.
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