In message o.uk, Dave
Liquorice writes
On Mon, 11 May 2015 11:11:59 +0100, gareth wrote:

As these things seem to rot at ground level and just below but not
above, then perhaps the remedy is to seal with bath sealant where the
post enters the concrete,

Don't use concrete it just traps water in the timber. It's not needed
anyway provided there is enough post in the ground and the soil well
firmed in from the base of the hole up when the post is installed.

... and then smear further sealant up the post for a further couple
of inches above ground level to prevent extended contact with water?

It's not so much the water as the availabilty of water that is the
problem. Posts rot at or just above the ground level as that is where
the conditions are "just right" for the wood rot fungi/bacteria WHY
to thrive. In the ground is generally too wet, up in the air too dry.
Capilary action from the wet timber in the ground to the dry timber
in the air maintains a damp gradient and the just right conditions
along that gradient.

All I can see sealing doing is moving the position of the just right
conditions. Well treated, good quality, timber preferably pressure
treated with CCA (if that hasn't been banned...) or submerge the
posts in proper creosote for 24/48 hours or at the very least stand
the ends in creosote to at least a foot above where the ground level
will be again fro 24/48 hours. A good couple of liberaly brushed on
coats of creosote for the in air portion.

It might be worth considering drilling a hole (1/4" to 1/2") to the
depth of the centre of the post - about 3" above the ground, and at an
angle of 45 degrees. Every so often, top it up with your favourite
preservative (several fills, and allow the preservative to soak in
well), then fit a water-tight bung.
--
Ian