From Name
John

Concrete concrete cure is like the linear no threshold
theory; theoretically it just continues to cure indefinitely. 28 days is a time
at which the required compressive strength is attained (typically expressed in
psi).  On inspection required
placements 28 day samples are analyzed and  3 day and/or 7 day tests are done to
predict the 28-day compressive strength and speed up replacement due to failure.
 Concrete is put into a number of
cylindrical samplers at the time of placement and cured in a lab and in the
field.  These samples are broken in
a hydraulic device that records the psi at failure.  28 days is also the minimum application
time guideline used by paint, adhesive, caulk, and thin set
manufacturers.
 
Concrete today is quite a technical process compared to
the old days.  There are numerous
variations in batch designs (recipes). 
Admixtures, entrainers and aggregate options are varied  to best match weather conditions
(placement & use), critical service strength (compressive/tensile),
exposures (water, chemical, etc.), climates and geologies.  Reinforcing  materials and procedures have also become
more sophisticated along with insulation strategies.   Structural and material design and
properties are available in national and local design codes like ACI (American
Concrete Institute). 
 
You can call the batch plant that the concrete
contractor uses and talk to their engineers.  They will know the concrete properties
and placement parameters for your job better than anyone.
 
John Mallon
 
 
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