construction joints in poured walled

huangyhg

construction joints in poured walled
why is that most structural engineers specify in their notes that concrete poured walls shalled not be poured more than 40'-0" (or close to that) in one single pour.  is this a curing issue?  does this need to be true in control joints are placed every 20'-0"?  thanks
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here鈥檚 one reason: if the length of wall in a pour is very long, and the concrete isn鈥檛 poured fast enough, there may be portions of concrete at the bottom of the wall that cure before additional concrete is placed.  controlling the length of wall in a given pour can help prevent this situation.  controlling the rate of concrete placement is another alternative, although i don鈥檛 think that would be very practical option.
i鈥檓 a bridge engineer and in bridge applications we typically require a vertical construction joints every 25鈥?in an abutment or pier walls.
i gives us something with which to negotiate with the contractor.  
it has to do with attempting to limit the shrinkage potential. walls do not typically have much horizontal reinforcement and therefore have a high potential for shrinkage cracking. by limiting the length of a pour to 40 feet, you are minimizing the shrinkage stresses. if a longer pour is desired then either more steel should be used or more joints.
please note that control joints are often not 100% effective (in relieving shrinkage stesses) since we often run either all the steel thru or cut every other bar. even construction joints are not 100% effective especially when several days later the next section is poured (while the shrinkage is still happening).
a better method is to checkerboard the pour of long walls, let the walls cure and shrink and then fill in the gaps.  this is commonly done in large concrete lined flood control channels here, especially when constructing during the summer with low humidity and high temperatures.
placing wall concrete is a balancing act.  the contractor wants to place the concrete as fast as possible to reduce labor costs, but fast placement increases formwork loading. the engineer wants the concrete placed fast enough to avoid a cold joint. placed too fast, wall forms can bulge or breakout.  placed too slow and cold joints can develop when lower portions of the wall reach their initial set before the upper layers are placed.  
placing temperture and mix design have a major effect on concrete's initial set time and on shrinkage stresses.
mary hurd's book "formwork for concrete" has many good suggetions and details for concrete wall forming and placing, including a table of placement speed vs. formwork loading.
a 40' long section of 12" wall, 10' high contains 14.8 cy of concrete, which is basically two ready-mix truck loads.  if it is placed at the rate of 5'/hr, then it takes 2 hours.
placing stronger wall ties at the bottom section of the wall forms allows a faster pour rate.
equipment makes a big difference in placing time too. is the concrete placed by chute directly into the forms, with a pump or bucket or conveyor?
is your standard a construction joint every 40 feet and control every 20 feet? this is fine our standard has been (for many years!) a construction or control every 30 feet. the thought is that the concrete shrinkage is going to causse a crack at a control joint. you can add a caulking reglet to the detail to seal the crack if desired.