Hello asfeiza,
1. the discrete bars only have axial stiffness, i.e., no bending nor shear resistance (nor shear stresses nor strains). If you thing the bending or shear response of the reinforcement is important, please explain (there are "thicks" to consider the effects, depending on the situation being modelled; replacing the bars with volume elements usually lead to extreme amount of elements...).
2. from mentioning "line" as possible interface I assume you work with a 2-dimensional model. It is not clear if you have already read section "2.1.8 How to model carbon or glass fibre wrap (CFRP, GFRP) strengthening in ATENA?" of the ATENA Troubleshooting Manual? The easiest way is to model the FRP as a reinforcement bar near the concrete surface, eventually with "bond for reinforcement". As the FRP is typically very thin, modeling using normal 2D elements is usually NOT suitable - therefore, the question of having interface or fixed contact between it and the concrete sounds surprizing?
