Six years ago we installed some oak sleepers in soldier format for a client. We import the oak 400 sleepers at a time and use it for many varied external applications. We concreted the sleepers into the ground and told the client there was no great need for a treatment and that they would weather naturally and last a long time.
The client decided to treat the sleepers in a black treatment(variety not known at this time), and now is complaining that some have rotted.
ALL the other applications we used the same sleepers for are absolutely fine.

Can anyone shed any light on why this rot may have occurred in a relatively short period. Could it be something in the soil which surrounds their feet?
Could it be moisture that has been trapped in by a non breathable treatment?
Any suggestions would be welcomed as we have used green oak for years without a problem

Thanks in advance