I am not sure, if you need the neutral axis for cracked or uncracked sections.

For uncracked sections you can use the methods for composite and prestressed sections (n-values, consideration of creep in prestressed members).

For cracked sections the neutral axis is dependent from the applied loads. Assume a neutral axis, apply an axial load only (no bending moment), calculate the inner forces NR and MR for the assumed neutral axis. The shift of the real neutral axis is given by MR/NR. 

This article may helps you. 

https://newtonexcelbach.files.wordpress.com/2008/03/depthna.pdf