Q.: Is there an allowable time limit for successive placements of concrete during a monolithic pour? In other words, once one truckload has been discharged, how long can I wait before placing the next truckload?

A.: We don't find any reference to a set time limit in the American Concrete Institute (ACI) building code or specifications for structural concrete. That's probably because time isn't the only factor to consider. 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-87) gives the following advice:

"When the placement consists of several layers, concrete delivery should be scheduled so that each layer is placed while the preceding one is still plastic to avoid cold joints. If the underlying layer has stiffened just beyond the point where it can be penetrated by the vibrator, bond can still be obtained by thoroughly and systematically vibrating the new concrete into contact with the previously placed concrete; however, an unavoidable joint line will show on the surface when the form is removed."