The Italian Guidelines for special inspections of post-tensioned concrete bridges: review and case study

Post-tensioned (PT) concrete bridges are structural systems characterized by intrinsic fragility. The assessment of their health condition is very difficult due to their nature, which prevents the detection of degradation in PT elements through conventional investigation methods and/or visual inspections. For this reason, the Italian Guidelines for the maintenance of existing bridges envisage special inspections to assess the condition of the post-tensioning system. According to the dedicated Guideline, the special inspection procedure consists of four steps. First there is Step O, which identifies the number and location of PT samples to be investigated based on the historical•critical review of available documentation and the visible defects in the post-tensioned members. Then, in Step 1 a preliminary analysis of the identified PT elements is performed, identifying the presence and location of active defects and grout voids, as well as the probability of corrosion of the reinforcement through non-destructive tests. In Step 2, the analysis of defects becomes more accurate, determining the probability of corrosion and estimating the level of stress in the concrete and in post-tensioned cables through locally destructive tests. Step 3 is applied if the outcome of Step 2 is uncertain and involves the experimental analysis of the overall structural response of the bridge. Eventually, in case the presence of damaged PT elements is confirmed, a load assessment of the structure is performed. The objective of this study is to evaluate the effectiveness of the investigation method proposed by the Italian Guidelines, highlighting its strengths and potential weaknesses. The procedure is applied to the Dora Riparia River Bridge, located in Avigliana (Turin) on provincial road SP197. The bridge consists of seven spans, 21.4 m long, and is characterized by a simply supported static layout; each span has four post-tensioned I-shape beams and four post-tensioned diaphragms.