PSI - Issue 84

Alessandro Vari et al. / Procedia Structural Integrity 84 (2026) 975–982

977

3. Specific Challenges and Limitations of External Prestressing Techniques 3.1. Systemic Criticalities and Asset Vulnerability An analysis of Reinforced Concrete (R.C.) and Prestressed Reinforced Concrete (P.R.C.) decks reveals that performance deficits stem from the convergence of two antagonistic macro-factors: • Physico-chemical degradation: Carbonation fronts in concrete and corrosive phenomena affecting reinforcement (both ordinary and prestressing steel) lead to a progressive reduction in the effective resistant cross-section and a loss of the steel-concrete bond. • Evolution of Load Demand: Traffic flow intensity and axle loads have consistently increased relative to original design standards, as evidenced by the gap between the actions mandated by Circular LL.PP. 1962 and the current NTC 2018. This evolution induces unforeseen static stress states and subjects structures to high-frequency fatigue cycles, accelerating decay through cumulative damage. Consequently, the global safety factor (Capacity/Demand ratio) is drastically reduced. Retrofitting via external prestressing emerges as the most effective strategy in this scenario, aiming to substantially modify structural behavior rather than merely restoring local capacity. 3.2. The Primacy of Serviceability Limit States (SLS) in Bridge Durability External post-tensioning offers different and convergent benefits: it restores lost compression in P.R.C. structures due to relaxation and creep, and mitigates cracking in R.C. structures, effectively transforming the element into a partially prestressed system.

(a) (b) Fig. 2. (a) Installation complexity and high interference of anchorage assemblies for post-tensioned cables; (b) Typical deviator fixed to the intrados for post-tensioning with cables or bars. A paradigm shift from civil building construction is essential here: for bridges and viaducts, the critical condition for design and verification lies not strictly in Ultimate Limit States (ULS), but in Serviceability Limit States (SLS). Unlike static structures, bridges endure moving loads that generate continuous stress variations and fatigue loading trains. While ULS verification remains an essential condition for safety, SLS control determines the asset's longevity. The primary objective of reinforcement must be the expansion of the linear-elastic operating regime. Ensuring a state of coaction that prevents or limits crack opening (decompression state) interrupts the degradation cycle, as intact concrete protects the armature from oxidation and preserves internal component fatigue resistance. Prestressing thus functions not merely as mechanical reinforcement, but as a potent tool for active durability.