laying of roof

you need not to worry for corse aggregate you saw at the slab top surface, only thing is that proper mixing may not have done. you keep the shuttering of slab for min 14 days do proper curing and then apply any cement based epoxy coating to top of slab, this will provide the covering to slab area also stops leakage.

secondly laying of 3- more slabs is not related to this problem

while further concreting keep good attention to compaction of concrte use 40mm needle vibrator . this will do.