I am working on some in N at the moment. They are a foot tall so in G you must be getting on a bit more

? IMO The key issue is to make sure that although the thing is made up of tubes (in my case, cardboard) it does n't end up looking like a exactly that, a collection of tubes stuck together.
Some concrete silos/elevators are made from a continuous pouring process and leave characteristic lines as along the length of the structure as the formwork rises. Also the cylinders have a curved "fill" between them. If this is omitted the resulting structure does look very convincing to me.
Here are some shots of mine so far, I used postal tubes with card overlay pre-printed with some concrete textured card I drew up in photoshop from original photos. To me it's the only way of capturing the complex patterns of man and nature in my lifetime. I just could n't get a realistic effect trying to hand paint the concrete.
Granted not much use outside, so the overlay would have to be 20thou plasticard or similar, to do away with "these are tubes stuck together effect". In my case there are 27 silos(only 9 on this segment, its cut away at the back to conceal a track up against a wall), but
the theory is good for 2 or 20. As always get yourselves some real pictures to work from, it's suprising how a subject that one thinks as realistic turns out to be total poo when you do that.
Laters