underpinning with concrete and drypack

huangyhg

underpinning with concrete and drypack
i am specifying underpinning with concrete, with the top 2 inches to be drypacked.  should i provide a time interval between the concrete placement and the drypacking work.
i had in mind 4 days from concrete placement to drypacking work (drypacking with non shrink grout) and then additional 3 days for drypack to cure before going to the next set of pins, thus for a total of 7 days.
does concrete shrink only between the time of placement and initial set, or is this a gradual process continuing.