One layer of concrete.  What?  You mean a solid block of concrete right?  The target structure consisted of a reinforced concrete block measuring 7m (23ft) square and having a thickness of 3.66m (12ft). Air bearings were installed in the platform beneath the target to ensure that the target was able to float on a thin layer of air. The thin layer of air created by inflating the air bearings enabled the concrete block to move freely during impact. A force of approximately 0.8t (1.8kips) or 0.2% of the target weight was able to move the reinforced concrete block. The concrete block weighed about 470t 42 (1000kips) which was approximately 25 times the weight of the impacting F4 Phantom aircraft.  So we have a massive concrete block that can move freely on an air cushion. This is nothing like a building.