Oh, I am not asking what software you use to create the geometry, but what element type you use for the rebars, solid or beam? This is just out of curiosity as I'd like to know how do you bond the rebar and the concrete together because for beam elements, it would be a bit difficult.
For the mesh, I would usually do a metrics check to make sure my mesh quality is good. Again may not be directly related to your problem but it is a practice I would do. You can find more information at: https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v201/en/wb_msh/msh_metrics.html?q=mesh%20metrics . It can help you get an idea of whether you have some geometric defects (such as small faces or small edges) that caused an extremely fine mesh in some regions.
If your geometry do have some small faces or edges, you may use the "repair" tool in Spaceclaim to fix it before importing to Mechanical: https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v201/en/spaceclaim/Discovery/user_manual/repair_overview.html?q=spaceclaim%20geometry%20repair