I'd consider it as resting on it but not as "in contact", that's more for explosives tied to the surface with tape or such. So just normal DV. So you'd take the 16P/-2 against the 10+16 soak dice of Plascrete, then apply the resulting damage. In other words, we're reducing 16P with 24 dice, and if we bring it down to 9 or less it won't take out the floor. That's roughly 3/4 chance. But I consider it strange that the example suggests an explosion bounces against the wall it's next to. I'd consider it even stranger if it bounches off the floor.
By the way: A frag grenade wouldn't pierce the plascrete even if it exploded on it, because it wouldn't pierce the Armor value.