In structural Engineering, broad areas areas are material technology, Analysis of structure, design of structure, earthquake engineering, durability and performance of structure, repairs and retrofittting of structures and so on ......
Since you are interested in Concrete technology, analysis and design of structure, you may take topic related to analysis of reinforced or fibre reinforced concrete member.. In my opinion, you can take a topic related to performance of thin walled fibre reinforced concrete beam subjected to flexural action. The beam may be similar to I section..