What is the material I have highlighted here ?.
That needs to be removed as it is trapping water and keeping the base of the post damp.
It might be that the deterioration is limited to the outer surface of the timber and the central cores of the post are still sound.
If that is the case and the posts are just standing on the concrete ( not sunk below the surface ) thenn rescue may be possible.
The important thing is to seal the end grain of the post and arrange for water to flow away from the post.
I would
1) remove that surround.
2) lift the post and slide a sheet of 4 mm thick lead pad under the post.
Cut the lead so that it extends for about an inch all round the post.
3) lower the post onto the lead
4) fold the sides of the lead up to make a cup around the post.
Fill the cup with a good timber preservative such as Sadolin Classic
Allow the preseravtive to soak into the timber for several days. topping up when necessary
Paint the post with several coats of Sadolin
5) trim the lead to be the same size as the base of the post
This is based on experience of building a post and beam timber framed house where the posts rested on lead pads.
Built in 1980 the posts show no sign of significant deterioration after more than 35 years .