PSI - Issue 84

Enrico Mittiga et al. / Procedia Structural Integrity 84 (2026) 867–873

871

of this static scheme minimizes the size of the precast elements and greatly simplifies assembly operations, allowing installation even in the presence of traffic. Attention was paid to limiting the thickness of the precast elements to 10 cm, with local increases only at connection points and in areas where it was necessary to reshape the profile. This choice required careful design of the reinforcement and rigorous verification of stresses and deformations during the most demanding phases. Once the static scheme and construction approach had been defined, the main design challenges faced were: • Limit the size and weight of precast elements to facilitate handling and installation. • Verify the stresses and deformations induced by temporary loads related to the pouring of concrete fill. • Define repeatable, accurate and safe installation procedures. The precast elements were developed in close collaboration with a specialist manufacturer to balance structural requirements with production constraints. Each element has a longitudinal length of approximately 8.26 m and a width of 2.40 m, dimensions that represent an optimal compromise between transport and handling constraints and installation speed. The triangular section steel truss beams with reduced height are integrated into the element and are designed to play a fundamental structural role during the transitional phases. Unlike the solutions commonly adopted in traditional prefabrication, longitudinal reinforcement bars with significantly larger diameters, ranging from 20 to 26 mm, made of B450C (UNI EN 1992-1-1 – EC2) steel and checked at the factory, were used. The welding of the reinforcement bars was also carried out and verified at the factory, ensuring high quality and repeatability. The use of large-diameter longitudinal reinforcements makes it possible to limit: During the transitional phases and the pouring of concrete, the truss beams act as primary structural elements, withstanding the stresses induced by the pressure of the fresh concrete without the need for temporary shoring. The concrete used for the precast elements was designed with particular attention to durability, considering the aggressive environment typical of road tunnels. A strength class of C35/45 (UNI EN 1992-1-1 – EC2) was adopted, with type IV cement (CEM IV) (UNI EN 197-1), exposure class XF4 (UNI 11104) and a low water/cement ratio, to ensure adequate resistance to frost and freeze-thaw cycles. 3.4. Connections and Behaviour during Assembly Phases The overall behaviour of the system largely depends on the effectiveness of the connections between the precast elements and between the elements and the existing structures. At the base of the elements, the connection to the side walls is achieved using expanding-type anchoring systems combined with M30 bolts fitted with adjustment nuts. This solution compensates for any irregularities in the substrate and ensures the correct geometric alignment of the cladding during installation. At the top, the connection between elements is made using steel inserts embedded in the prefabricated concrete, connected by M30 bolts. Each element is equipped with two bolts: one connecting it to the previous element and one to the next panel, using dedicated sleeves. During assembly, temporary structural steel profiles are used to stabilize the geometry of the arch; these elements are removed only after the concrete filling has been completed, once the system has reached a structurally stable configuration. 3.3. Characteristics of Precast Elements • Elastic deformations of the element. • Cracking of the precast concrete. • Demand for base anchorage systems.