How would you

I'm not entirely sure what you are talking about. Do you mean the seams of concrete that come out between the boards on forms? If so I would, I use pieces of styrene cut to the sizes between seams. I would think you could then use a solvent type glue and press the pieces together until the softened styrene pushes out like the seams of concrete. In ho scale the seams would be very small. Remember that 1/16th of an inch is very close to 6 inches, so the seams would protrude no more than 1/32 or even 1/64th of an inch.
 
When they make a retaining wall from concrete they have to pour it in steps. I think they call that "lifts". It depends on the hight and thickness of the wall what the code will allow in one pour. Each time they do, there is a visable difference between one pour and the one above it. I would simulate that by just making the concrete color a bit off shade from the concrete below it and do it in a straight line.

I'm guessing that this is what you are talking about.:rolleyes:

Don
 
Yes that is what I am refering to the concrete lines that can be seen on the pours. I am starting to model the Moffat Tunnel's East Portal in N and was just wondering if there was a easy way to model this or if it was overkill.