# Risk-Based Selection of Inspection Method for External Post-Tensioning System of Bridges
## 2 Literature Review
### Deterioration of Post-Tensioning Elements
The second category shown in Figure 2 is the material-related factors. Hydrogen embrittlement could be the result of inappropriate steel production or improper cathodic protection. Due to the cold-drawing process, strands conforming to the ASTM A 416 are relatively resistant to hydrogen embrittlement; however, negligence in production or protection allows embrittlement to occur [24]. Cement and mix water contamination will make ion present in the grouted zone by additional chemicals [25]. Using dissimilar grouts during the construction or repair of the post-tensioning elements makes ions available and facilitates strand exposure [26]. The last factors under material are substandard duct material and grout segregation, which weaken the grouted area and cause strand exposure [23]. Substandard steel, which can be a significant issue for steel strands during the construction or rehabilitation process, is a potential manufacturing issue in the case of weak quality control in steel manufacturing plants. Although substandard steel does not relate to corrosion principles directly, it plays an important but indirect role and speeds up the corrosion of post-tensioning system. The lack of array for substandard steel in Figure 2 describes the indirect relation to corrosion principles. Substandard steel cannot be detected by well-known nondestructive inspection methods [23]. Corrosion cracking, fatigue corrosion, and fretting corrosion are three different corrosion-related factors that have been classified under the structural category. All factors under the structural category facilitate strand exposure and speed up the possible corrosion. The production of cracks in a corrosive environment is known as (stress) corrosion cracking. It can cause typically ductile metal alloys subjected to tensile stress to fail unexpectedly and suddenly, especially at high temperatures [27]. Fatigue corrosion is the mechanical degradation of a material under the joint action of corrosion and cyclic loading. All structures, specifically post-tensioned concrete structures, experience some form of alternating stress and are exposed to toxic environments during their service life [28]. Fretting corrosion refers to corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as generated, for example, by vibration [29]. The fourth class of factors that Figure 2 summarizes is construction. This category includes joint sealant barrier, delays in cap installation, bleeding, delays in grouting, and wrong grout injection method. While joint sealant barrier facilitates access of moisture content to post-tensioning elements, delays in cap installation, bleeding, and delays in grouting relate to two corrosion principles: moisture content and strand exposure. However, the wrong grout injection method provides strand exposure [19]. Different combinations of factors in the construction category will make the grout out of specifications which are named deficient grout [30; 31]. Deficient grout with voids plays an inevitable role in the corrosion of post-tensioning elements. In two ways, voids in ducts can compromise the strength of the tendon system. For starters, voids can cause tendon weakness due to a lack of stress redistribution inside the beam. Second, and most crucially, when strands are perched in cementitious material and exposed to air conditions through voids, corrosion can develop. Strands in grouted PTconcrete bridges will invariably be exposed to both environments because of the grouting materials and installation procedures [32]. Other factors can impact the corrosion rate, in addition to the varied exposure circumstances between strands perched in grout and those exposed to the void environment. The major defense for PT tendons is grout. Even yet, voids in grouted ducts are prevalent, especially when the workmanship and grouting material are of low quality.