Funny thing is, when I last visited Stonehenge 4 years ago I never even noticed the reinforced concrete. I did this time although to be fair it probably 'blends in' as best it can. I don't suppose there are too many options if you want to ensure the stone stays standing?
Sarsen, a type of sandstone laid down in warm shallow seas, varies consideiahly in its composition and hardness, The slabs into which it naturally fractures often have one rough side and one that is much smoother.
Holes made by the roots of aquatic plants and burrowing marine creatures create weaknesses that weather can modify and enlarge, the stones sometimes taking on fantastic shapes and contours. The base of Stone 60 had eroded into what was effectively a tiny cave, big enough for four people at a squeeze.
In 1959 Stone 60 was straightened and, at the same time, in order to deal with what was seen as a weakness in its structure, the 'cave' in its base was filled in.