PSN-L Email List Message
Subject: Re: concrete tests
From: beezaur beezaur@..........
Date: Wed, 12 Mar 2003 21:45:50 -0800
> Is the curing rate and the final properties significantly
> temperature dependant?
> Are the properties of the top surface any different from the
> moulded surfaces?
> Can you inhibit surface micro cracking by coating the inside of
> the mould with carbon or other fibres? Alternatively, can you post coat
> with carbon or glass to any advantage?
> Chris Chapman
Yes, curing is very sensitive to temperature. Type 1 portland cement is
the kind you usually find at the hardware store. It likes to cure in
~40 deg F. Projects cast in hot or cold temperatures require different
types of portland cement aor strength suffers considerably. The final
crystalline structure of the hardened cement is much stronger if it
cures at its design temperature.
The most important factor is the ratio of water to cement by mass in the
mix. A water/cement ratio of 0.7 only gives half the strength of a mix
with a 0.4 ratio. This is more or less independent of proportions of
sand and gravel, which are essentially fillers.
If the concrete is kept moist during cure, the hydration process
proceeds at an exponentially decaying rate. Removal of the moisture
stops the process, but some further curing can take place if moisture is
Smoother surfaces contribute to higher strength, but this is usually
most true where the material is under tension. A very rough surface can
locally magnify the stress by a factor of 2 to 3 or more.
As an example, a 12x12 in concrete beam 3 ft long, supported at its
ends, will be under 7 psi tension at the middle of the bottom face from
its own weight. A very rough surface might make this 21 psi locally.
Double this stress (at least) to account for decelleration during
placement. Tensile strength is conservatively 5% of compressive
strength, so a garden-variety, overly watered mix will have a tensile
strength of 0.05 * 1000 psi = 50 psi. Things now look pretty bad for
the garden concrete. Roughness is a factor, but not a large one. The
stuff I hope to make may have a tensile strength of 300 psi. There are
also much more efficient sections - this example weighs 450 lbs.
Explosion and shock resistant structures often have metal "whiskers"
mixed in, and even nylon fibers in the mix will greatly reduce cracking.
External application to the bottom, either before or after casting,
would help by taking the tension load. Bridges work exactly this way,
except that the reinforcement is embedded in the bottom of the beam. A
coat of penetrating, flexible epoxy may be enough.
I worry about noise with reinforcement. Apparently gravel and air
pockets cause noise. There is a possibility that a surface coating
could generate a "surface tension" like that in water. I would expect
fructuations in such tension to cause noise by whatever mechanism rock
and air inclusions produce noise. Or maybe not. It would be an
One could mechanically separate the reinforcement from the concrete and
apply the force evenly at the ends of the beam. Prestressed concrete
beams have cables (tendons) which are often run through greased conduit
and fixed to thick end plates under high tension. Tension rods could
easily be centered in longitudinal holes or run outside the beam.
A day without math is like a day without sunshine.
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