Greetings Armine:
Unfortunately, I can't tell by your description whether this bark loss is natural, environmental, or related to biological factors. I wouldn't worry too much if the limbs are small in diameter and few in number--it is common for tree branches to die, lose bark, fall to the ground, etc. Factors that cause this may include environmental factors such as wind, shading, mechanical damage, drought; biological factors such as insect, pathogens, or wildlife damage, etc.
I would not be worried based on your description of an 80-ft tall tree with a couple dead, small diameter branches.