$123$:$$;"What are the key principles behind the design and construction of reinforced concrete structures in civil engineering? How do engineers determine the optimal reinforcement layout and calculate the required concrete strength to ensure structural integrity and durability? Furthermore, what factors influence the selection of appropriate construction techniques for different types of reinforced concrete elements, and how do these techniques contribute to the overall efficiency and safety of the project? Delve into the fundamental theories and practical considerations governing reinforced concrete in civil engineering."