Never modelled this in the software I use (Strand7), or any other software that is typically and often used for say post tensioned bridge analysis like LUSAS and MIDAS amongst others.
Now perhaps this can be of help, but I would still recommend to search the internet, because I am sure you will find some papers about this (I suppose abaqus is not used so much for designing of these structures, as mentioned LUSAS, Strand7, MIDAS, Sofistik and others are more common for this type of analysis).
1. Either with 3D elements (small model and few tendons) or beam elements (big model many tendons)
2. Edge to surface interaction can perhaps be used in Abaqus if you have relative movement between concrete and tendon modelled as beams (standard surface to surface general contact will handle both solid to solid interaction if tendons and concrete are modelled as solids, and solid to beam interaction if tendons are modelled as beams and concrete as solid). There seem to be some other interesting elements (tube inside tube) also that could be of interest. See: 36.3.2 Tube-to-tube contact element library in the help manual, and this paper for an example: Background to multi-scale modelling of unbonded Post-Tensioned concrete structures.
3. Apply a preload on the tendon, not sure exactly how though (in Strand7 one can apply a pretension on beams). This might give some ideas also:
on how to apply preload
4 Not sure about this, but perhaps see the paper mentioned above.
