Dave:

The phenomenon you are seeing is commonly referred to as "honeycombing". Honeycombing can be recognized by exposed coarse
aggregate on the surface without any mortar covering or surrounding the aggregate particles. The honeycombing may extend deep into the concrete. It can be caused by a poorly graded concrete mix, by too large of a coarse aggregate, using unclean form boards, or by insufficient vibration at the time of placement. Honeycombing will result in further deterioration of the concrete due to freeze-thaw because moisture can easily work its way into the honeycombed areas. Severe honeycombing should be repaired to prevent further deterioration of the concrete surface. Most engineers recommend repair with an epoxy-resin patching material or "grout".

Aaron