# Risk-Based Selection of Inspection Method for External Post-Tensioning System of Bridges
## 2 Literature Review  
### Deterioration of Post-Tensioning Elements  
Typically, post-tensioning (PT) tendons are comprised of high-strength steel strands encased in a duct (cover pipe) and grouted to fill the internal space of the duct for corrosion protection purposes. For long-span bridge constructions, post-tensioning can be a cost-effective and time-saving solution [11]. Post-tensioning is commonly used in the building of new bridges, as well as the restoration and strengthening of existing bridges [12]. Post-tensioning systems are categorized as internal or external depending on where the tendons are located. An internal tendon is described as one that is placed inside the concrete, whilst an external tendon is defined as one that is placed outside the concrete. In general,  
Figure 1: General procedure for risk-based selection of inspection method. segmental PT bridges may have either or both of these tendon systems. External post-tensioning systems are less complicated to monitor, repair, and maintain than internal post-tensioning systems because the tendons are not encased in hardened concrete [13]. External tendons, on the other hand, might be more sensitive to corrosion than internal tendons, even when exposed to identical conditions, due to the lack of protection from the concrete cover and the potential existence of undesirable air-voids [14]. Although post-tensioning systems offer several benefits for designers and builders, they have also highlighted concerns about their corrosion [15]. Unlike traditional reinforced concrete systems, where corrosion distress is visible as staining, cracking, or spalling of the concrete cover, corroding post-tensioning systems seldom exhibit similar surface distress signs [16]. Because the tendons are embedded in ducts away from the structure's exterior surface or inside the ducts, these distress indications are often not visible [17]. Accordingly, structural performance of the post-tensioning systems is more sensitive to corrosion than conventionally reinforced systems. Furthermore, tendon replacement can be very costly [18]. Post-tensioning systems are complicated, and environmental, construction, material, and structural factors can be involved in their deterioration. The literature suggest that the presence of ions (corrosive ions), moisture content, and strand exposure are the three main causes of corrosion, all of which are needed to initiate and sustain the corrosion [19]. Ions that result in partial or complete loss of the passive layer of steel lead to active corrosion. Chloride and carbonation-induced ions are those most often mentioned in the literature [20]. Figure 2 shows the classification of factors that can be involved in the corrosion of post-tensioning elements and the relationship with the essential corrosion causes. Figure 2: Classification of factors involved in corrosion of external post-tensioning system. The first class of factors is environmental, as indicated by Figure 2. In some climate conditions, deicing is inevitable for the operation of highways during the winter; however, side effects influence the long-term use of road and bridge elements. Salt consumption for deicing purposes is extremely high. For instance, the salt consumption in Denmark, which has only 43,000 km\({}^{2}\) roads, is approximately 300,000 tons per year [21]. Salt supplies chloride as a corrosive ion, which is one of the corrosion causes, and increases the deterioration rate of a post-tensioning system [22]. While seawater and river water supply both ion and moisture content, rainfall and weather humidity will replenish moisture content. Another important factor is proper storage of materials before application. For example, grout mix may be delivered in bags but should be stored in a weatherproof building. Storage in the open may be allowed, providing that the materials are on a raised, dry platform with adequate weatherproof covering. Neglecting these critical practices will result in grout storage issues [23].