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12 Deterioration Conditions Introduction When PT concrete bridges are properly designed and constructed, they may prove to be the most durable bridge type, also requiring the least amount of maintenance (Clark 2010). However, when bridge post-tensioning systems are not designed, detailed, or constructed properly there are several deterioration conditions that may affect the strength and lon- gevity of the bridge. Environmental conditions also play a criti- cal role in the deterioration of bridge post-tensioning systems. If inspection and repair of these systems are not done properly, early failure and public endangerment can occur (Trejo et al. 2009). Deterioration conditions such as corrosion, breakage, section loss, voids, water infiltration, and compromised grout can pose a serious threat to the safety of in-service bridges. For example, significant corrosion rates can cause early failure of stay cable and external post-tensioning systems if necessary repairs are not carried out (Trejo et al. 2009). This chapter reviews the various types of deterioration con- ditions that are typically observed in a post-tensioning or a stay cable system. The various deterioration conditions considered in this chapter are corrosion, section loss, tendon or strand breakage, various compromised grout, voids, and water infil- tration. The causes of these deterioration conditions and the significance of the deterioration conditions on the structural performance and structural safety of post-tensioning and stay cable systems are also discussed. Finally, the NDE methods that can be used to identify these particular deterioration conditions are also discussed. Corrosion Corrosion is an electrochemical process that can negatively affect the strength capacity or service life of any metal element. The most critical and prevalent condition causing cable deteri- oration is corrosion of the steel tendons in both stay cable and external post-tensioning systems (Grant 1991). Corrosion is also a condition that exists across many other fields, including buildings, automobiles, and storage tanks, to name a few. The estimated annual cost of atmospheric corrosion has been esti- mated as 3.1% of the GNP of the United States, costing about $276 billion in 1998 (Griffin 2006). It is well documented that moisture and oxygen are required for the corrosion process to occur, but corrosion is a more com- plex process requiring the transfer of an electric charge in an aqueous solution. The complete electrochemical reaction of cor- rosion can be described in four individual steps. First, the iron is oxidized, which is an anodic process that transfers electrons, cre- ating positively charged iron ions that are hydrolyzed and pro- duce acidity. Next, oxygen is reduced by the transferred electrons, which produces alkalinity. As the negatively charged electrons are transferred from the anodic region produced by iron oxi- dation to the cathodic region produced by oxygen reduction, a nominal positively charged electrical current is created and flows in the opposite direction of the electrons. Finally, the cir- cuit is completed as ions have the ability to be transferred in a pore solution, typically water (Bertolini et al. 2013). Although moisture and oxygen are the only necessary entities for the cor- rosion process to begin, there are several other factors that can significantly affect the rate or type of corrosive activity of metal, including temperature, wind, airborne contaminants, alloy con- tent, biological organisms, and others (Griffin 2006). However, a corrosion test on several strand specimens concluded that most severe corrosion occurs at or near the grout-air-strand (GAS) interface (Trejo et al. 2009). Depending on these factors and many more, there are numerous different types of corrosion, including uniform corrosion, atmospheric corrosion, chloride induced corrosion, carbon dioxide concentration, pitting cor- rosion, corrosion cracking, fretting fatigue, galvanic corrosion, and hydrogen-induced stress corrosion. Types and Causes of Corrosion There are multiple mechanisms of corrosion that can be present. Corrosion mechanisms include uniform or atmo- C h a p t e r 3
13 spheric corrosion, pitting corrosion, crevice corrosion, stress corrosion cracking, hydrogen cracking, corrosion fatigue, and electrolytic corrosion (Elliott and Heymsfield 2003). In atmo- spheric corrosion, the presence of other chemicals in the tendonâs surrounding area can cause corrosion to occur and often serve to accelerate the rate of corrosion. Substances in the atmosphere, such as carbon dioxide, chlorine, and sulfur compounds, can lead to atmospheric corrosion of the strands. This type of corrosion is highly dependent on the degree of these chemicals present in the environment and the length of the exposure time to the steel in the post-tensioning and stay cable systems. For example, chlorides from deicing salts can contaminate the tendons at expansion joints and other tendon anchorage areas, causing severe damage. Pitting corrosion is a form of localized corrosion. When the passivating layer of the steel is deteriorated by means such as high chloride concentrations or acidic solutions, crevices are created. These small areas become anodic, while the sur- rounding environment becomes cathodic, leading to extremely localized corrosion and often galvanic corrosion. Galvanic cor- rosion occurs due to the close presence of different metals, which contain differing electrode potentials. This difference can result in galvanic coupling and therefore accelerate the rate of corrosion. Creep and cable relaxation may lead to differential elonga- tion of the three components (strand, grout and duct) that form the stay cables (Watson and Stafford 1988). This differ- ential elongation of the cables causes the steel strand, grout, and plastic duct to move independently and can form a gap between the interior surface of the duct and the grout layer, creating cracking (Watson and Stafford 1988). Chlorides and moisture seep into the duct and reach the steel tendon to facilitate corrosion. Corrosion can occur even if the duct is completely filled with grout. The rate of the tendon corrosion largely depends on the amount of chloride contamination and the surrounding grout quality. Although, regardless of chloride levels, corrosion can occur at the location of voids and low grout pH levels (Venugopalan 2008). Significance of Corrosion The exposure of PT and stay cable tendons to environments ideal for corrosion is a critical concern in maintaining bridge structures. Although the tendons are designed to be encased in grout and duct, undesired conditions such as voids and moisture exposure are often present to facilitate corrosion. In post-tensioning and stay cable systems, corrosion of the tendons results in loss of metallic area which directly cor- responds to their loss of tensile strength. This strength loss negatively impacts the load-carrying capacity of the bridge, making corrosion a significant condition to take note of dur- ing bridge inspection. Corrosion can cause many types of premature failure of strands in the PT or cable-stayed bridge elements, endanger- ing the structure as a whole. The first type of failure that can occur is a brittle failure due to corrosion pitting, in which local- ized corrosion essentially notches the strand, reducing its abil- ity to hold the required load. Second, a common phenomenon called hydrogen-induced stress corrosion cracking occurs when a crack forms and propagates under the influence of a corro- sive environment and tensile stresses. This method of failure can occur without visible corrosion product, making them particularly difficult to detect (Nürnberger et al. 2003). Corro- sion also severely affects the fatigue life of prestressing strands. Fatigue tests on parallel wire cables have shown that the yield and ultimate strengths of the strands are somewhat unaffected by uniform corrosion, but their ductility greatly decreased and the variability in mechanical properties increased significantly (Li et al. 2011). Similarly, the fatigue life of corroded strands was significantly less than noncorroded wires, as corrosion sped up fatigue crack propagation and ultimate fatigue fracture of the strands (Li et al. 2011). Although the negative effects of corrosion on the strength and serviceability of prestressing steel are known and numerous corrosion protection systems have been developed, corrosion continues to be a major problem in PT tendons and stay cable MTE design and construction. Watson and Stafford (1988) claimed that no cable protection method is foolproof and that many cable-stayed bridges are in danger of collapse unless the corrosion problems can be stopped. The authors traveled the world and surveyed over half of the cable-stayed bridges built at the time, concluding that premature corrosive degradation was found in cables everywhere (Watson and Stafford 1988). Because of the prevalence and extremely harmful nature of cor- rosion on stay cable MTEs worldwide, it is essential for bridge inspections to include corrosion detection protocols. NDE Methods for Corrosion Based on this investigation, the NDE methods that are capable of detecting corrosion in external tendons include MFL, MMFL-permanent magnet, MMFL-solenoid, and EIS. The accuracy of the methods in detecting corrosion ranges from moderate to high. None of the NDE technologies that were investigated as part of the current study is capable of detecting corrosion in internal tendons. However two NDE methods, USE and IE, could identify voids within the protective grout ducts of inter- nal tendons. Detecting voids may be useful in pinpointing areas of potential corrosion issues and is discussed further in the following sections on compromised grout, voids, and water infiltration. The flowcharts in Appendix A provide information about the capability of each NDE method in detecting corrosion
14 in internal/external tendons and metal/nonmetal ducts. In addition, it also provides the specific accuracy levels for the NDE methods that successfully detect corrosion defects. Section Loss Section loss occurs due to excessive corrosion or breakage of tendons in post-tensioning and stay cable systems, resulting in a loss of strength or fatigue resistance. Corro- sion induced section loss is highly dependent on the envi- ronment surrounding the steel. In the presence of oxygen and moisture, often with other substances, the strands can corrode to the extent of section loss in the tendon. Breakage can occur due to corrosion, fatigue loading, or over-loading of the strands and can also lead to notable section loss of the tendon. Other possible causes of sec- tion loss are fatigue, inter-strand wear, or intra-strand wear (Weischedel 2004). Types and Causes of Section Loss Section loss can occur due to breakage or corrosion of strands and/or tendons. Significance of Section Loss Section loss occurs as a result of excessive corrosion or breakage in tendons of PT and cable-stayed bridge struc- tures. Once the damage reaches the level of significant steel section loss in the tendons, the tensile strength of the load- bearing systems can be significantly decreased. In the case of extreme section loss, repair or rehabilitation methods are often necessary to correct the undesired condition of the steel and maintain the safety of the bridge. Therefore, tendon section loss is a vital concern during bridge inspections of post-tensioning and stay cable systems. In addition, the ten- sile strength of post-tensioning strands and stay cable MTEs are closely related to the cross-sectional area of the prestress- ing steel, meaning that any section loss can reduce the total load-carrying capacity. NDE Methods for Detecting Section Loss Based on this investigation, the NDE methods that are capable of detecting section loss in internal tendons include MFL, MMFM-permanent magnet, and MMFM-solenoid. All methods have moderate accuracy in detecting section loss. None of the NDE technologies that were investigated as part of the current study is capable of detecting section loss in internal tendons. However two NDE methods, USE and IE, could identify voids within the protective grout ducts of internal tendons. Detecting voids may be useful in pinpoint- ing areas of potential corrosion issues, and such areas can be monitored for any further signs of corrosion. The flowcharts in Appendix A provide information about the capability of each NDE method in detecting section loss in internal/external tendons and metal/nonmetal ducts. In addition, it also provides the specific accuracy levels for the NDE methods that successfully detect section loss defects. Breakage Breakage of steel strands or wires can occur as a result of numerous activities and can also significantly reduce the capacity of the tension element. Broken wires or nicking have been known to develop in multistrand ropes because of the torque-balancing assembly of the wires. This is because the different layers of wires touch at an angle, meaning when these ropes are stressed and bent at an anchorage or saddle region, they are subjected to a combination of radial load- ing, bending stresses, and relative motion between the wires, which can wear the rope (Weischedel 2004). This method of breakage can be difficult or impossible to see by the naked eye because this breakage typically happens between the inner steel wire layers of the strand. Wires can also break as a result of fatigue or shear loading on the strand (Hamilton III 1995). In addition, a strand or wire can be mechanically damaged due to lack of proper quality control measures during the manufacturing process, or in the field prior to placement by dragging it on the ground, dropping equipment, and driving machinery over the strands. Types and Causes of Breakage Strand or wire breakage can occur as a result of corrosion, fatigue loading, or over-loading in the post-tensioning and stay cable systems of bridge structures. Corrosion induced breakage is highly dependent on the environment surround- ing the steel strands. In the presence of oxygen and moisture, often with other substances, the wires or strands can corrode, resulting in breakage. Breakage due to fatigue loading can occur as fatigue of the individual wires or by fretting fatigue. Cyclic loading on bridges can initiate and propagate fretting fatigue of the PT steel tendons (Wollman et al. 1988). This fretting fatigue is experienced as wire breakage and gradually can escalate in severity. The majority of this type of tendon fracture occurs as a result of the steel tendon severely rubbing on the inte- rior duct surface at locations of tendon curvature. This ten- don curvature causes lateral pressure on the steel. Fretting fatigue breakage locations have been found to be prevalent at locations of steel slipping along the duct at flexural cracks and where surface micro cracks in the steel are present. Duct material and the stress range of the steel also play a role in the
15 occurrence of fretting fatigue and later breakage (Wollman et al. 1988). Significance of Breakage Wire and strand breakage in the tendons of post-tensioning and stay cable systems is significant, in that it can be a pre- cursor to section loss of the entire tendon. Breakage of prestressing steel is important to detect because such a local- ized breakage can reduce capacity and possibly lead to a brittle failure of the strand (Nürnberger et al. 2003). Locating breakage during bridge inspections can enable preemptive repair or rehabilitation to prevent noteworthy loss in tensile strength. NDE Methods for Breakage Based on this investigation, the NDE methods that are capable of detecting breakage in external tendons include MFL, MMFM-permanent magnet, and MMFM-solenoid. The accuracy of the methods in detecting breakage ranges from low to high. None of the NDE technologies that were investigated as part of the current study is capable of detecting corrosion in internal tendons. However two NDE methods, USE and IE, could identify voids within the protective grout ducts of inter- nal tendons. Detecting voids may be useful in pinpointing areas of potential corrosion issues and is discussed further in the following sections on compromised grout, voids, and water infiltration. The flowcharts in Appendix A provide information about the capability of each NDE method in detecting breakage in internal/external tendons and metal/nonmetal ducts. In addi- tion, it also provides the specific accuracy levels for the NDE methods that successfully detect breakage defects. Compromised Grout Grouting serves as a corrosion resistant layer protect- ing the steel tendons (VSL 2002). However, if good quality grouting is not achieved during construction, grout cannot aid in increasing the durability of the tendons. Voids can occur during the grouting process of post-tensioning and stay cable systems due to improper grout mixing and place- ment procedures. Improper grouting procedures may include not injecting grout at the low point of the tendon profile, injecting grout in the wrong direction, and using improper grouting pressure. Voids in post-tensioning and stay cable systems at locations within the ducts can be det- rimental to the bridge structure, as it prevents proper bond- ing of the materials. Voids can also facilitate an environment prone to corrosion by the presence of oxygen and potentially other gaseous substances. Water presence due to grout bleeding nearly always occurs during grouting of the ducts unless special precautions or grout mixtures are used (Goodwin 2002). Bleeding occurs as the water separates from the cement due to the materialsâ different densities. Bleeding happens as a function of the changing duct elevation, grout mixture and mixing efficiency, temperature, and interstices in the strands (Goodwin 2002). The gaps in the strands serve as wicks for the bleed water to reach the highest points in the duct. Bleed water can evaporate to form air voids in the duct or even freeze in the appropri- ate climate, possibly causing damage to the ducts. This water infiltration in stay cable and external post-tensioning systems is highly undesirable. There are several other compromised grout defects that can occur within grouted tendons other than voids and water infiltration, including grout segregation and unhydrated grout. Both of these conditions have the ability to compro- mise the corrosion protection that the grout provides to the steel strands, making them susceptible to defects. Segregation can exist in many states, such as a wet plastic, a white powdery residue, or a dry low density grout (Merrill 2014). There are numerous causes of segregation, including insufficient mix- ing of grout, poor or expired materials, and improper admix- tures being added. Unhydrated grout can be caused by either not adding enough water or water at too high of a tempera- ture so that evaporation can reduce the water content of the grout. This is why PTI recommends not to use black water containers in regions with warmer climates, including Texas (Merrill 2014). Types and Causes of Compromised Grout Compromised grout such as segregated grout, white paste, soft grout, unhydrated grout, and gassed grout can occur dur- ing the grouting process of post-tensioning and stay cable systems. Segregated grout, white paste, and soft grout are the result of excess water in the grout. Unhydrated grout results from insufficient water in the grout mix, and gassed grout can be the result of gas being introduced into the grout mix dur- ing grout placement. These types of compromised grout are all detrimental to the bridge structure. Without the presence of good grout between the steel tendons and the duct walls, proper bonding does not occur and the grout does not create the protective environment necessary to prevent corrosion of strands. Significance of Compromised Grout Compromised grout can occur in large amounts at par- ticular regions in the ducts during grout placement and can be detrimental to the bridge structure. Without the presence of good grout between the steel tendons and the duct walls,
16 proper bonding does not occur and other detrimental condi- tions such as water infiltration can follow. This can eventually lead to the corrosion of strands. NDE Methods for Detecting Compromised Grout Based on this investigation, the NDE methods that are capable of detecting compromised grout include USE, IE, sounding, IRT, GPR, LFUT, and ECT. While most of the methods have low accuracy in detecting compromised grout, sounding and IE have moderate accuracy. The flowcharts in Appendix A provide information about the capability of each NDE method in detecting compromised grout in internal/ external tendons and metal/nonmetal ducts. In addition, it also provides the specific accuracy levels for the NDE methods that successfully detect compromised grout. Voids Injection of grout is a common practice to provide corro- sion protection to the PT tendons and stay cable MTEs. How- ever, there are several issues that can cause problems with this system. Grout is not only intended to form a barrier from intruding moisture and chlorides due to its low permeability, but the alkalinity of the cementitious grout provides excel- lent corrosion protection (Salas et al. 2004). This protection system becomes ineffective if a voided region occurs in the tendon grout, which can leave the steel prestressing strand bare and unprotected from the environment. Types and Causes of Voids There are several possible causes for voids within a tendon, including improper grouting procedures or poor materials. Voids can be caused by human error, including incomplete grouting, inadequate grouting pressure, leaks within the tendon, and insufficient grout discharge at vents (Pielstick 2014). Similarly, grouting materials are just as important, as bad grouting mixtures can cause bleed water pockets, which can evaporate, leaving voided regions within the duct (Salas et al. 2004). Significance of Voids Voids can be a major problem in all post-tensioning and stay cable systems with grouted tendons, but can be espe- cially problematic near coastal regions where moisture and chlorides can enter the ducts and create a corrosive environ- ment within the duct very quickly. Although grouting is also used to bond and transfer load from internal steel strands in post-tensioning systems, this is not the case in external post-tensioning and stay cables systems, where the only purpose of the grout is corrosion protection (Pielstick 2014). Since grout in stay cables serves no load-carrying purpose, the existence of a void does not necessarily mean there is a structural problem with the cable. Voided regions serve more as indicators that corrosive activity is likely to occur in that region, as engineering knowledge and experi- ence has proven that corrosion occurs significantly more often in voided regions than in nonvoided regions (Salas et al. 2004). Voids can also facilitate an environment prone to corro- sion by the presence of oxygen and potentially other gaseous substances. The threat of corrosion due to voided regions is one that needs to be taken into account during bridge inspec- tions, as voids can be excellent indicators of corrosive activity. The specification for grouting of PT structures by PTI (2012), requires that in no less than 24 hours after a grouting opera- tion all outlets and grout caps shall be inspected and filled with fresh grout if they are not full. Similarly, PTI recom- mends inspection of exit ports of vertical vent tubes at the trumpet end, vertical stand pipe, and any other inspection ports. If grouting uncertainties arise after the grout has set, the construction engineer shall arrange for nondestructive testing of the system (PTI 2012). NDE Methods for Voids Based on this investigation, the NDE methods that are capable of detecting voids include IE, USE, sounding, IRT, GPR, LFUT, and ECT. While the accuracy of most methods in detecting voids ranges from moderate to high, USE has low accuracy. The flowcharts in Appendix A provide information about the capability of each NDE method in detecting voids in internal/external tendons and metal/nonmetal ducts. In addition, it also provides the specific accuracy levels for the NDE methods that successfully detect voids. Water Infiltration Water infiltration of a grouted tendon has similar con- sequences to grouting voids, in that, it can allow corrosive environment to be created around the steel strand. Water infiltration after construction is usually focused near the anchorage regions of PT and stay cable systems due to access points in the ducts. Types and Causes of Water Infiltration One possible cause for water infiltration within a tendon includes the existence of a void allowing water to accumulate in the voided area through either an opening in the duct or an accumulation of humidity. Another cause, which is more
17 commonly an issue in vertically angled tendons, such as stay cable MTEs, is bleed water. Bleed water is typically caused by poor mixing, poor materials, or excessive water in the grout, but is essentially the formation of a water pocket that forms at the highest point of the grout as the particles settle downwards and force the water upwards. Because of the vertical orientation of the tendons, the required pumping head cannot be obtained to pump the entire tendon full of grout in one operation but rather it is done segmentally from bottom to top. This means that not only could bleed water accumulate at the top of the ten- don but also at the interfaces between grouting operations, which are referred to as intermediate lenses, as shown in Figure 3-1 (Pielstick 2014). Another possible cause of water infiltration is using water to flush the duct. After the ducts are placed, it is common practice to flush the ducts in order to clean them prior to inserting the steel strands. Prior practices flushed the tendons with water, but this led to many problems, including water infiltration and over-hydrated grout. Currently, PTI requires ducts to be flushed with air as opposed to water to help avoid these problems (Pielstick 2014). A water infiltrated region within the duct has the potential to be more harmful than a voided region, as moisture is one of the constituents of the corrosion process. The presence of water can accelerate the corrosion process in comparison to a dry void leading to a reduction in strength or fatigue life of the stay cable MTE and post-tensioning systems. Proper grouting procedures and use of bleed-resistant grouts can be used to minimize the chance of water infiltration, but inspec- tion is still necessary to detect the condition. Significance of Water Infiltration Water infiltration in the ducts of post-tensioning and stay cable systems can cause problems in its interaction with the grout and steel. Moisture is needed for corrosion to initi- ate and propagate and therefore water infiltration can be an early warning sign for the onset of corrosion. Depending on the state of the grout at the time of water infiltration, com- promised grout can be formed from the excess water in the duct. The grout could form into soft grout, white paste, or segregated grout. NDE Methods for Water Infiltration Based on this investigation, the NDE methods that are capable of detecting water infiltration include IE, USE, sound- ing, IRT, GPR, LFUT, and ECT. Sounding and IRT have high accuracy in detecting water infiltration; the accuracy of the other methods ranges from low to moderate. The flowcharts in Appendix A provide information about the capability of each NDE method in detecting water infiltration in internal/ external tendons and metal/nonmetal ducts. In addition, it also provides the specific accuracy levels for the NDE meth- ods that successfully detect water infiltration. Tendon Deterioration in the Anchorage System The anchorage region is one of the most critical parts of the post-tensioning and stay cable systems, and any deterio- ration in these regions could have significant impact on the performance of the structure. Types and Causes of Tendon Deterioration in the Anchorage System Strand breakage, corrosion, section loss, water infiltration, voids, and compromised grout occur in a large degree in the anchorage region of post-tensioning and stay cable systems. These anchorage regions are especially vulnerable to the atmosphere and must be subject to thorough bridge inspec- tions to ensure they are properly connected and sealed from the elements. Significance of Tendon Deterioration in the Anchorage System The secure anchorage of the post-tensioning and stay cable tendons is critical to carrying the load of the bridge and should Figure 3-1. Illustration of bleed water and intermediate lense.
18 be thoroughly examined during bridge inspections. Sealed ducts in the anchorage zone are vital to protect the steel tendons. NDE Methods for Tendon Deterioration in the Anchorage System USE is capable of detecting tendon deterioration in the anchorage region, however the accuracy of this method is low. For investigating tendon deterioration in the end caps of the anchorage regions, sounding and IRT may be used, and their accuracy ranges from moderate to high. Closing Remarks Six deterioration conditions that are most commonly observed in bridge post-tensioning and stay cable systems are discussed in detail in this chapter. The conditions discussed include corrosion, section loss, breakage, compromised grout, void, and water infiltration. The types and causes of these defects, their significance in terms of its effects on the bridge structures, and NDE methods that are capable of detecting these defects are also discussed. Finally, the effects of these defects in the anchorage regions are discussed.
