concrete placement in sub freezing temperatures

huangyhg

concrete placement in sub freezing temperatures
good day - any experience with concrete placement at temperatures in the 20f to 40f range without use of heat?  some manufacturers admixtures are advertised to protect the concrete down to 20f. ever the skeptics, we are seeking input from unbiased, experienced sources.  situation is lightweight concrete on metal deck, 16 story building.  currently using heaters, tarps, and moisture control with apparent, albeit costly, success.  aci 306 is our bible.  specifically the question is, have you had success using an accelerating admixture and placing concrete without use of heat at temperatures approaching 20f?  if so, did you implement any other procedures to help ensure success?  and finally, was there any discernible loss of strength evident in the test cylinder breaks?  thanks in advance for any feedback.
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wiff,
it has been my experience that placing concrete in cold weather the key is to let it develop its initial set (500 to 1,000 psi) then it can cool until it freezes with out (too) much damage to the matrix.
several solutions:
add an excelerator, to the normal strength concrete.
increase the strength by 1,000 psi.
then cover the slab, top and bottom, to conserve the heat the concrete is generating while curing.
field cured cylinders (cured on site under the blankets on the edge of the slab) will prove if you have been effective.
as an asside, the slower concrete sets the stronger it becomes.  after the initial set, the cold concrete will develope a higher 'ultimate' strength than  concrete that is hot.  that is one reason that cooling pipes were set in the mass concrete placed to form hoover dam.
if you are pouring concrete on a metal deck in tempertures below freezing, then i think the method you are currently using is the way to go, expensive, but the best solution.  
thin sections on metal deck don't generate a lot of heat, at least they don't retain enough when poured on a frozen metal deck.  warm the deck before the pour, heat the poured slab for 24-48 hours, then let it cool down slowly and move to the next slab.  
that's my two cents.
rjeffery & jheidt - thanks for the input.  in way of feedback, we are staying with the heat and increasing the strength by at least 500 psi.  concrete is being covered with visqueen for 2-3 days, weather dependent.  the much bigger challenge  turns out to be how to soak the lightweight aggregate in sub freezing temperatures when the plant "doesn't have the capability".  for this we've gone to aci 304 which permits some water addition on site to account for absorbed moisture.  ultimately, slump at pump discharge, cylinder breaks, and follow on testing will be the deciding criteria.  again, thanks. - wiff
rjeffry was right on the money and a better solution.  insulated tarps on the top and insulation against the underside of the metal decking.  i have poured slabs like this and have had to occasionally let some cool air in.  it will stay warm a minimum of 3 days if insulated properly.