In article , Jim Hawkins
writes
The treads are about 700mm X 25mm
The steps are outside our back door.
Adhesive must be fairly thick (not runny) and
must not go off too quickly.

I'd suggest some sort of waterproof 'sticks like ****' product applied
in the traditional squiggle fashion but with a fine bead front, back and
sides (as a bed) to keep the water out. My fave at the mo is Plumbers'
Gold which is an adhesive and sealant that will bond to damp masonry if
reqd. The slight flexibility should help too:

http://www.toolstation.com/shop/p61679

Do mask off the surrounds as it will stick to anything and will likely
squeeze out when you bed the step.
--
fred
it's a ba-na-na . . . .