In the previous post I pointed out that the Freeman data used in the Colebrook article only had a maximum relative roughness of 0.006. I have examined the Colebrook article and the Moody article and I don't see data with higher values.
Commercial pipes, including concrete
Moody reports on the roughness of concrete pipes based on data obtained by Scobey. The absolute roughness ranged from 0.001 to 0.01 ft. The smallest standard size of concrete pipe is 1 ft., so the largest relative roughness would be 0.01. Thus, for most commercial pipes, with concrete pipes being the roughest, we can expect relative roughness to be 0.01 or less. The Colebrook-White (CW) formula for the friction factor should work fine for commercial pipes.
For relative roughness > 0.01
If you have a very unusual pipe with a higher relative roughness, I believe the CW formula will predict a higher friction factor than actual in the transition region. I base this belief on the observation by others that the friction factor curves for all pipes converge to the smooth pipe value near Reynold's number of 3500. The friction factor at Reynold's number of 4000 is between 0.04 and 0.05. As shown in Fig. 1 of Colebrook & White, the friction factor for a sand roughened pipe needs to increase to the fully turbulent value of 0.06 (Nikuradse data). However, the CW formula would start out with a higher friction factor and decline to 0.06.
In most cases, use of a higher friction factor than actual will be a conservative estimate: actual pressure drop for a given pipe size and flow rate will be lower than predicted for the transition region. Knowing this possible limitation for the CW formula might ward off loss of faith in the formula for normal usage if you should experience this rare occurrence of a pipe with a relative roughness above 0.01
Colebrook, C.F., J. Inst. Civil Eng. (London) (Feb. 1939)
Colebrook, C.F., and C.M. White, Proceedings of the Royal Society London: A 161 (1937)
Moody, L.F., Trans. A.S.M.E. (1944): 671–84.
Nikuradse, J., NACA Tech. Mem., 1292 (1937)
Scobey, F.C., Bulletin 852, U.S. Dept. of Agriculture (1920)