Depending on the strength and elasticity of the carbon fabric you are using, try solving the moment capacity like you would steel reinforcement in a composite concrete or masonry section. Basic assumptions: the concrete and carbon fabric are perfectly composite, therefore strain from bending is linear, elastic (before yield of steel)... use your equilibrium equations - compression in concrete (and possibly top reinforcing steel) is equal to tension in fabric (and possibly lower reinforcing steel). Your maximum capacity is going to depend on which material "fails" first.
The carbon fabric is a very clean, straight-forward remedy to increase capacity of structures already in place, but it's pricy.