> 
>       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?
> 
>       Regards,
> 
>       Chris Chapman    

Chris,

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 
replaced.

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 
interesting experiment.

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.

Scott

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

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