A customer asked our field rep whether there is an allowable time limit for successive concrete placements during a monolithic pour? I think the customer was asking this in reference to cold joints.
The American Concrete Institute (ACI) literature on building codes or structural concrete specifications provide no reference to a set time limit That's probably because time isn't the only factor to consider in determining initial set. In cool, damp weather you can wait longer to place the next batch than you can during hot, dry weather. As long as the previously placed concrete is still plastic (an internal vibrator will penetrate it), you can safely place the next layer of concrete without producing a cold joint. ACI's Guide for Consolidation of Concrete, ACI 309R-96, gives the following advice:
"To avoid cold joints, placing should be resumed before the surface hardens. For unusually long delays during concreting, the concrete should be kept live by periodically re-vibrating it. Concrete should be vibrated at approximately 15-minute intervals or less depending upon job conditions. However, concrete should not be overvibrated to the point of causing segregation. Furthermore, should the concrete approach time of initial setting, vibration should be discontinued and the concrete should be allowed to harden. A cold joint will result and suitable surface preparation measures should be applied."