Pack the cavity with mineral wool or pillows, then chalk or puddy. Yu can also use the fireboard stuff too, in the recent past I cut all the holes, then cut a slit across them all so it was strips then pieced it back together between the pipes
Bob, some of the inspectors around here would want to see an approved section drawing sheet for the assembley listing of using concrete for penitrations. And allowable distances between them... I wonder if the concrete associations out there have such a drawing? RECTORSEAL makes a fire rated mortar...
