huangyhg

formwork & concrete load
when placing a wall that is 7' tall with a pour rate of 7' per hour, how much extra load will be imparted on the formwork by the use of a plasticized mix?
is there something wrong with just using the entire weight of the wall and being done with it?
theroux,
this is covered in aci sp-4 "formwork for concrete".
there is a simple formula for this:
for rates of pour < 7 ft per hour:
maximum form pressure is p
p = 150 +9000*r/t
r = rate of pour
t = temperature of concrete in forms. (the example in the book uses t as ambient.)
from top to bottom, pressures are 150*h up to the max as calculated above, then below that is a uniform pressure.
at 63 degrees f, this formula is
p = 150 + 9000 * 7 / 63 = 150 + 63000/63 = 150+1000 = 1150 psf.
the pressure is obviously higher as the temperature goes down.
regards,
chichuck
one last point to avoid any confusion:
for a wall that is only 7 ft high, the pressure never reaches that max, it only goes up to
7 * 150 = 1050 psf.
so in this case, swivel63 has the correct pressure. a taller wall however, would be a different situation.
regards,
chichuck
hey, thanks. we have regular tests at work. and the correct answer is zero affect.
even took me a second, and i design formwork all day!
hydrostatic pressure is reasonable for this situation and appropriate for the desired answer, however, the equations presented are incorrect without a bit more info.
the equations presented in older versions of sp-4 are based on type i cement, with no admixtures or retarders, etc.
starting with aci 347-01 or 02 (i think), a series of chemistry coefficients were introduced to take appreciation of the affects of admixtures and pozzolans on concrete set times. so, in order to safely specify a pour rate, one must consider the added effect of the admixtures into the pour rate.
designing for full liquid head alleviates this problem, but many wall forms would be absurbly stout if designed for full liquid head.
please review the most current copy of aci 347 guide to formwork for concrete, or sp-4 formwork for concrete 7th addition for factors and additional info.
good luck and write your own question for next week! there is also a unit weight coefficient that can be added to address anything other than plain jane 150 pcf mud, so have fun with your question.
daniel toon

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