1.1 History and Development of PC Structures

The creation of reinforced concrete (RC) occurred in contemporary times. It is widely recognized that French gardener Joseph Monier devised and patented it in 1849 and 1867 respectively, featuring reinforced concrete flower pots and highway guardrails with reinforced concrete beams and columns [7]. The world's first reinforced concrete edifice was built in 1872, located in New York, USA, signaling the start of a new era in human architecture. Reinforced concrete constructions became popular in the engineering profession after 1900. Prestressed concrete (PC), a new type of reinforced concrete construction, was introduced in 1928 and was widely used in engineering practice after World War II.

Concrete is made up of aggregates (stone as coarse aggregate and sand as fine aggregate) and cement (typically Portland cement). When water is added into the cement, it hydrates and forms a small opaque lattice structure that wraps and seals the aggregate into a solid structure. Concrete constructions typically have high compressive strength (about 28 MPa), but poor tensile strength (such as beam bending). Any significant tensile stress will break the microscopic hard lattice of concrete, resulting in cracking and separation. However, since most structural components require tensile stress, unreinforced concrete is rarely employed in engineering without reinforcement.

1.1.1 Reinforced Concrete Structures

Compared to concrete, the tensile strength of steel is generally above 200 MPa. People usually add steel and other tensile reinforcing materials in concrete to work together, which makes up a reinforced concrete structure. The tensile force is borne by the steel, the concrete bears the compressive stress part. According to the force of the structure, the reasonable configuration of the reinforcing steel can form a higher load-bearing capacity and a greater stiffness of the structure.

For example, a simply supported beam under bending is shown in Fig. 1.1. When the load P is applied, the beam cross-section is subjected to compression at the top and tension at the bottom. At this point, the reinforcement configured at the bottom of the beam bears the tension, while the concrete shown in the upper shaded area bears the compression.

Fig. 1.1
A diagram represents a rectangular beam with 2 horizontal strips and a load P applied from the top. On the right, it presents a tall block divided into two halves, titled 1 and 2.

Reinforced concrete force characteristics

Full size image

1.1.2 Prestressed Concrete Structures

Although reinforced concrete structures make reasonable use of the force performance characteristics of both steel and concrete, forming a structure with better integrity and durability, reinforced concrete needs to face two main problems in actual use.

  1. (a)

    Concrete structures generally work with cracks. When a reinforced concrete structure is loaded, it will inevitably deform, resulting in cracking. The existence of cracks not only reduces the stiffness of the structure but also renders it impossible to apply the reinforced concrete structure in situations where cracking is not allowed.

  2. (b)

    Concrete structures cannot make full use of high-strength materials. As the load increases, it is not economic to increase the cross-sectional size of reinforced concrete structures or the amount of reinforcement to control the cracks and deformation of the structures. Additionally, the self-weight of concrete structures has also been increased, especially for bridge structures, with the increase in span, the proportion of the role of self-weight also increases, which will obviously limit the application of reinforced concrete structures in bridge engineering.

The emergence of prestressed concrete structures is a good solution to these problems. Prestressed concrete structures use steel cables to provide pressure at the ends of the concrete structure to establish a state of stress within it, the magnitude and distribution of which are used to resist or eliminate tensile stresses generated by the application of loads. This type of concrete structures, in which prestressing steel is configured and then established by tensioning or other methods, is called prestressed concrete structures [17].

1.1.3 Main Methods of Prestressing

Pretensioning method: The pretensioning method is the method of tensioning the reinforcement first and then pouring the concrete of the structure afterward. As shown in Fig. 1.2a, after the concrete reaches the required strength (generally not less than 75% of the design strength), the temporary anchorage is released and the tension is slowly relaxed, allowing the retraction of prestressing reinforcement. The retraction force of the reinforcement is transferred to the concrete through the bond between the prestressing reinforcement and concrete, so that the concrete can obtain precompression stress. This kind of concrete structure in which the prestressing tendons are tensioned on the pedestal and the concrete is poured and the prestressing force is transferred through the bond is called pretensioned concrete structures.

Fig. 1.2
2 diagrams. A. Pre tensioning method comprises pre-stressed steel with tensioning pedestals, a concrete beam with temporary anchorage, and relaxation of tensile strength. B. Post-tensioning methods comprise tensioning of prestressing steel followed by pressurized water slurry injection.

Types of prestressed concrete structures

Full size image

Post-tensioning method: The post-tensioning method is a method of casting concrete elements first, and then tensioning and anchoring prestressing steels after the concrete has hardened. As shown in the Fig. 1.2b, concrete structures are poured first and holes are reserved in it. After the concrete strength reaches the required strength, the prestressing steels are threaded into the reserved holes, the jacks are supported at both the ends of the concrete structures, and the prestressing steels are tensioned and anchored to the concrete structures so that the concrete obtains and maintains its precompressive stress. Finally, cement slurry is injected into the reserved orifice to protect the prestressing steel from rusting and to make the prestressing steel bonded to the concrete as a whole. This kind of structure is called post-tensioned concrete structures after the concrete is hardened by tensioning the prestressing tendons and anchoring them to establish prestressing.

1.1.4 Characteristics of PC Structures

Compared with reinforced concrete structures, PC structures have the following main advantages.

  1. (1)

    It can improve the crack resistance and stiffness of concrete structures. After applying prestressing to the structures, the cracks may not appear or may be greatly delayed under the service load, thus effectively improving the serviceability of the structures, increasing the stiffness of the structures, and improving the durability of the structures.

  2. (2)

    It can save materials and reduce the self-weight of structures. Since prestressed concrete can reasonably use high-strength materials, it can reduce the cross-sectional size of the structures and reduce the dead load of the structure. This is a significant advantage for large span bridges where dead load is the main effect.

  3. (3)

    The vertical shear and principal tensile stresses in concrete beams can be reduced. The curved reinforcement of prestressed concrete beams can reduce the vertical shear force near the support in the beam; due to the presence of precompressive stresses in the concrete cross-section, the main tensile stresses in the concrete under load will be reduced accordingly.

  4. (4)

    Prestressing can be used as a means of connecting structures, promoting the development of new systems and construction methods for bridge structures.

  5. (5)

    The fatigue resistance of the structure can be improved, which is beneficial for bridge structures subjected to dynamic loads.

PC structures have the following main disadvantages.

  1. (1)

    The upper arch degree of structures induced by prestressing is not easy to control. The prestressing effect may make a large upper arch degree, resulting in unevenness of the bridge deck.

  2. (2)

    The construction cost of prestressed concrete structures is large for the projects with a small spans and small number of structures.

1.2 Practical Application of PC Structures

1.2.1 Application of Prestressing Technology in Bridges

The use of prestressing technology in engineering can greatly reduce the amount of concrete and steel, effectively reduce the self-weight of reinforced concrete components, and thus improve the crack resistance of concrete. The application of prestressing technology not only improves the quality of the bridge project but also improves the economic benefits of the project. It can further improve the aesthetic effect of the bridge project and prolong the service life of the bridge.

In bridge construction, the concrete structure is very important, which will directly carry most of the carrying load of the bridge. Therefore, applying prestressing technology to the concrete structure is to strengthen the main structure of the bridge, thereby improving the overall quality and stability. The implementation of prestressing technology in the concrete structures of bridge construction can solve the common problems, such as easy cracking and deformation of concrete to a certain extent. So that the stability and service life of the concrete structure can be greatly improved, thereby improving the overall quality of the bridge.

The selection of steel strands is directly related to the quality and performance of bridge construction. There are four main types of prestressing steel strands used in bridge engineering in China, namely prestressed steel bars, ordinary prestressing steel strands, low-relaxation steel strands, and prestressing steel strands with straightening and tempering properties. The low-relaxation steel strands are widely used in bridge engineering due to their characteristics of durability and low cost. Scientific and reasonable selection of steel strands can greatly reduce the use of steel on the basis of ensuring the quality of the project, thereby improving the economic effect of the project [2].

The choice of anchorage for prestressing technology is based on both friction anchorage and mechanical anchorage. Friction anchorage is the formation of prestressing steel anchor rotating effect and fixing the steel tightly. The advantage of this technology is easy to wear the cable, the defect is not enough convenient connection, and the loss is large. Mechanical anchorage is the use of mechanical processing to form the anchor working conditions suitable for the use of prestressing steel end and the construction method of anchorage [29].

In bridge construction, the construction quality of flexural components is a key factor affecting the construction quality and service life of the entire bridge [14]. Therefore, improving the construction quality of the flexural members can effectively improve the overall quality and service life of the bridge. To ensure the bridge’s stability and safety during actual operation, it is essential to construct the components in accordance with the construction specification.

In bridge construction projects, the positive and negative bending moment area of multi-span continuous beams is a very important but easily neglected part. Prestressing technology is applied in the construction of this structure to improve the stability and resistance of positive and negative bending moment structures. The pressure capacity of the bridge will also have a significant improvement effect on the overall quality of the bridge. Usually, multi-span continuous bridges have the characteristics of large span and high strength. The quality of the bridge is very strict, and the large deformation and cracking cannot occur. Therefore, prestressing technology can be used in the construction of key positive and negative bending moments to improve the stability of critical instability, and then combined with prestressed concrete cast-in-place construction, to greatly improve the quality level of multi-span continuous bridges.

The prefabricated board is a common template in engineering construction, and the quality of the prefabricated boards affects the quality of the bridge construction [16]. Prestressed technology is applied in preplate production projects, and people choose high-strength and relaxed steel strands as prestretched bands to increase seismic performance and stability of the prefabricated plates.

With the increase in traffic, the bearing capacity of some bridges designed and constructed in the early stage has been difficult to meet the current traffic requirements. Therefore, many bridge constructions are buried with many dangerous factors, and bridges need to be reinforced in time. The prestressing technique reinforcement method is a fairly mature and widely practical method. In recent years, with the continuous progress of bridge construction technology, prestressing technology has also been developed rapidly, which is able to carry out reinforcement construction in key areas such as bridge main structure, bridge deck layer, bridge structure, etc. By reinforcing different areas of the bridge, it can play a great role in repairing and improving the overall quality of the bridge and extending the service life of the bridge.

1.2.2 Examples of Prestressing in Bridges

Modern prestressed bridge structures have been widely used in the field of highway and railway bridge construction due to their good performance and superior spanning capacity. In the USA from the 1950s to the 1990s, compared with steel bridges, reinforced concrete bridges, and other types of bridges, the application of prestressed concrete bridges was increasing, and traditional steel bridges in the range of large and medium span are being gradually replaced by modern prestressed concrete bridges [15].

Prestressed concrete is ideally suited for long-span bridge construction. The Parrotts Ferry Bridge in California has a main span of 195 m. The Pasco-Kennewick cable-stayed bridge in Washington is 299 m. The central span of the cable-stayed Vasco de Gama Bridge in Lisbon, Portugal, is 420 m. The Sunshine Sky Bridge, a typical cable-stayed bridge of 365 m main span, constructed at Tampa Bay, Florida, USA. The Chaco-Corrientes Bridge constructed in Argentina, South America, is the longest precast prestressed concrete cable-stayed box girder in the world at the time. California Guide Ways is the typical use of prestressed concrete simple-span box girders for the Bay area rapid transit system in San Francisco, California. The Lubha Bridge, once the longest single-span 172 m prestressed concrete box-girder-type continuous bridge in India, built across a 30 m deep gorge of the Lubha river in Assam. Zuari Bridge at Goa, is 807 m long, comprising prestressed concrete cantilever box girders with 4 main spans of 122 m, two end spans of 69.5 m, and a via duct with 5 spans of 36 m. Ganga Bridge at Patna, the once longest prestressed concrete bridge in the world, has a total length of 5575 m, consisting of continuous spans of 121.65 m long prestressed concrete girders of variable depth. Cable-stayed prestressed concrete bridge across the Brahmaputra at Jogighopa, Assam, with a span of 286 m between the two towers and two side spans of 114 m, comprises a single-cell prestressed concrete box girders [22].

Bridges play an important role in the normal operation of road traffic as the structures built to cross road obstacles. Bridge engineering has been unprecedentedly developed with the rapid development of social economy, showing a thriving scene. There are more than 912,000 highway bridges and 10,000 km of high-speed railway bridges in China in 2020. China began to develop prestressed concrete technology around the 1950s and was the first to apply prestressed reinforced concrete to the accessory of sleepers in railway tracks. With the acceleration of the construction of China's transportation industry, the prestressed concrete technology is gradually popularized and used nationwide. Especially in bridge engineering, it has grown the fastest. In the late 1970s, all kinds of bridges built in China basically used prestressed concrete structures. In the twenty-first century, the main materials of modern prestressed concrete are high-quality steel and high-strength concrete. The high-efficiency prestressed concrete is formed through very advanced production technology through modern design concepts and methods.

The Nanjing Yangtze River Bridge was completed in 1968. It is the first prestressed concrete bridge in China that integrates design and construction. The main bridge has ten holes and a total length of 1577 m. The successful completion of the Nanjing Yangtze River Bridge demonstrates that China's bridge construction has risen to a new level in both scale and technology. The Chongqing Yangtze River Bridge, built in 1980, is a prestressed concrete box girder structure with a main span of 174 m. The prestressed concrete T-beam bridge, built in Feiyunjiang, Zhejiang, in 1988, has a maximum span of 62 m. In the same year, the Luoxi Bridge, the main traffic road connecting Guangzhou urban area to Panyu, was built in Guangdong, with a total length of 1916 m. Its main span of 180 m also created a precedent for the construction of long-span PC continuous rigid frame bridges in China. Over the next ten years, more than 100 PC beam bridges with spans greater than 120 m were built nationwide. At the same time, cable-stayed bridges were gradually printed on the list of bridges in China. In 1975, the first cable-stayed bridge with a main span of 76 m was built in Yunyang County, Chongqing. The cable-stayed bridge on the Yellow River Highway in Jinan, opened to traffic in 1982, mainly adopts prestressed box girder in structure, with a main span of 220 m.

Under the condition of comprehensive strength, the development of modern prestressing technology on bridges is faster. For example, cable-stayed bridges such as Shanghai Nanpu Bridge and Yangpu Bridge, and suspension bridges such as Hong Kong Tsing Ma Bridge and Jiangyin Yangtze River Bridge from the perspective of design, construction, materials, equipment, corrosion protection, etc. continuously explore the characteristics and accumulate experience to provide foundation for the updated technology.

1.3 Corrosion of Strand in Prestressed Concrete

Prestressed concrete has been diffusely used in bridge construction because of its high strength, good compactness, small cracks, and superior spanning ability [4]. However, the durability degradation of these bridges has been gradually found during the serviceability period, caused by the bad construction quality, environmental erosion and material deterioration [27], as shown in Fig. 1.3. Strand corrosion can cause concrete cracking, degrade bond performance at the strand–concrete interface, lead to prestress loss, and deteriorate the capacity of bridges [3]. Therefore, it is important to study the durability and remaining service life of corroded prestressed concrete bridges for ensuring their standard operation and safety utilization.

Fig. 1.3
Three photographs titled A, B, and C of the rust accumulated on the collectively twisted strands used in P-C bridges.

Corrosion of prestressing strand in bridges

Full size image

Strand corrosion is one of the major reasons for the performance deterioration of PC bridges. In worldwide, accidents of prestressed concrete (PC) bridges caused by corrosion have been widely reported. A footbridge collapsed suddenly in Hampshire, England, in 1967. In 1980, a large number of corroded prestressing tendons were found on the Angell Road Bridge in north London. The Ynys-Y-Gwaa Bridge, UK, collapsed in 1985 due to the corrosion of post-tensioning tendons at the segment joints after only 32 years of service [12]. The Welsh Bridge in the USA suddenly failed due to strand corrosion at the joint positions in 1985. Additionally, Italy’s Saint Stefano Bridge failed due to pitting corrosion of the prestressing steel near the box girder joints in 1999, after 40 years of service. The collapse of Lake View Drive Bridge in the USA in 2005 was caused by strand corrosion [11].

It is found that more than 200 durability problems of bridges caused by prestressing strand corrosion had been reported in worldwide during 1951–1979, which caused huge economic losses. A survey in 1982 indicated that ten safety accidents of bridges caused by strand corrosion had been reported in USA during 1978–1982 [23]. In China, numerous of PC bridges have been built with the economic development. Under the combined effects of traffic growth and environment worsening, the durability of PC bridges has been gradually emerged during the serviceability. Reports from the railway department in 1994 indicated that more than 6000 defective concrete bridges were under the operation in China, accounting for 18.8% of the total number. Structural deterioration caused by corrosion has cause huge economic losses, which needs to be paid high attention.

Strand corrosion has been found to be one of the primarily common problems in PC bridges [5, 13]. Corrosion can weaken the strand section area, the mechanical strength and bond properties of strand. The structure would be deteriorated due to these factors. The failure of prestressed concrete bridges would exhibit brittle characteristic without warning due to the high stress state of the strand, which leads to a huge economic loss. Existing investigations are mainly centered on the corrosion of reinforced concrete structures, and the related corrosion mechanism of ordinary steel has been studied extensively [8, 9, 28]. Few works on strand corrosion have been reported.

The corrosion morphology of strand is more complicated than that of ordinary steel owing to the combined effects of electrochemical corrosion, stress corrosion, and crevice corrosion [10]. First, the corrosion process of strand under the high stress state is faster than that of reinforcement steel. Second, the multiple steel wires are usually used as the prestressing strand in bridges, and the gap between the steel wires will provide a channel for the longitudinal migration of erosion medium, which promotes the corrosion propagation. How does strand corrosion evolve? What is the effect of high stress on corrosion-induced cracking? How does the bond between strand and concrete degrade? How to evaluate the effective prestress of strand after corrosion? How do these factors affect the bearing capacity of corroded PC beams? These problems need to be resolved.

The corrosion mechanism between the pretensioned and post-tensioned concrete bridges may be different because of their construction techniques [10]. The grouting defect will exist in the post-tensioned concrete bridges owing to bleeding and construction problems [20]. This defect not only weakens the ability of strand and concrete to work together but also weakens the strand protection, causing corrosion induced by erosion medium [21]. Without bellow protection, strands in pretensioned concrete bridges are easily to be corroded. Strand corrosion causes concrete cracking and bond degradation. Moreover, prestressing strand in pretensioned concrete beams transmits the prestressing force to concrete through interfacial bond stress, and the effective bond is peculiarly important compared with other concrete structures [6]. Corrosion-induced bond degradation not only reduces the ability of strand to work together with concrete but also affects the stress transfer, which can be easy to cause the anchorage failure of beams [1]. As mentioned above, the mechanical properties of corroded pretensioned and post-tensioned concrete structures are different, which needs to be discussed in several ways.

1.3.1 Mechanisms of Electrochemical Corrosion

The electrochemical corrosion generally occurs in strand. The passive film of strand in concrete is easily destroyed by the environmental media, such as carbon dioxide and chloride ions. When corrosive media including \({\text{CO}}_{2}\) and \({\text{Cl}}^{ - }\), the reduction of the alkalinity in the concrete will partially or completely destroy the passivation state of the strand surface. A potential difference will occur at different parts of the strand surface, forming anodes and cathodes, which will lead to strand corrosion [18].

Liquid water in concrete usually exists in the form of \({\text{Ca}}\left( {{\text{OH}}} \right)_{2}\) solution, which will make the concrete in a highly alkaline state. The strand in a highly alkaline state is easily oxidized by oxygen to form a dense passive film on the surface. The main component of the passive film is \(n{\text{Fe}}_{2} {\text{O}}_{3} \cdot m{\text{H}}_{2} {\text{O}}\), which can resist erosion by external harmful substances. With the continuous reaction of \({\text{CO}}_{2}\) in the air with \({\text{Ca}}\left( {{\text{OH}}} \right)_{2}\) solution to form \({\text{CaCO}}_{3}\), the concrete becomes less alkaline and the passive film is constantly damaged. When the passive film breaks down, the strand section, the passive film, and the pore water form a closed-circuit electrolytic cell. The closed-circuit will cause electrochemical corrosion of the strand. Electrochemical corrosion can be divided into three processes:

  1. (1)

    Oxidation reaction occurs at the anode region. The strand loses electrons becoming ferric ions where the passive film breaks down. The reaction can be schematically written as

    $$ {\text{Fe}} - 2e^{ - } = {\text{Fe}}^{2 + } . $$
    (1.1)
  2. (2)

    Reduction reaction occurs at the cathode region. The strand gains electrons to \({\text{OH}}^{ - }\) where oxygen and pore water penetrate. The reaction can be schematically written as

    $$ {\text{O}}_{2} + 2{\text{H}}_{2} {\text{O}} + 4e^{ - } = 4{\text{OH}}^{ - } . $$
    (1.2)
  3. (3)

    Corrosion products are created as a result of corrosion. The \({\text{OH}}^{ - }\) generated by the reduction reaction moves to the place where the passive film of the strand is damaged. Then the \({\text{OH}}^{ - }\) forms \({\text{Fe}}({\text{OH}})_{2}\) with the \({\text{Fe}}^{2 + }\) at the passive film break down. In an oxygen-rich environment, \({\text{Fe(OH)}}_{2}\) will be oxidized to a red corrosion product \(({\text{Fe}}_{2} {\text{O}}_{3} )\). In an oxygen-deficient environment, a part of \({\text{Fe}}({\text{OH}})_{2}\) will be oxidized to black corrosion products \(\left( {{\text{Fe}}_{3} {\text{O}}_{4} } \right)\) The reaction can be schematically written as

    $$ {\text{Fe}}^{2 + } + 2{\text{OH}}^{ - } = {\text{Fe(OH)}}_{2} , $$
    (1.3)
    $$ 4{\text{Fe(OH)}}_{2} + {\text{O}}_{2} = 2{\text{Fe}}_{2} {\text{O}}_{3} + 4{\text{H}}_{2} {\text{O}}, $$
    (1.4)
    $$ 6{\text{Fe(OH)}}_{2} + {\text{O}}_{2} = 2{\text{Fe}}_{3} {\text{O}}_{4} + 6{\text{H}}_{2} {\text{O}}. $$
    (1.5)

When the PC structures are exposed to a chloride ion rich environment, the chloride ions will continuously spread to the surface of the strand. The strand passive film will be damaged as the critical chloride ion concentration is reached. The chloride ion will act as a catalyst with \({\text{Fe}}^{2 + }\) to produce the corrosion products (\({\text{Fe}}_{2} {\text{O}}_{3} , {\text{Fe}}_{3} {\text{O}}_{4}\)), as shown in Fig. 1.4. The reaction can be schematically written as

$$ {\text{Fe}} - 2e^{ - } = {\text{Fe}}^{2 + } , $$
(1.6)
$$ {\text{Fe}}^{2 + } + 2{\text{Cl}}^{ - } + 4{\text{H}}_{2} {\text{O}} = {\text{FeCl}}_{2} \cdot 4{\text{H}}_{2} {\text{O}}, $$
(1.7)
$$ {\text{FeCl}}_{2} \cdot 4{\text{H}}_{2} {\text{O}} = {\text{Fe}}({\text{OH}})_{2} + 2{\text{Cl}}^{ - } + 2{\text{H}}^{ + } + 2{\text{H}}_{2} {\text{O,}} $$
(1.8)
$$ {\text{Fe}}({\text{OH}})_{2} \to {\text{Further oxidized to corrosion products}} \,\,{\text{Fe}}_{2} {\text{O}}_{3} ,{\text{Fe}}_{3} {\text{O}}_{4} . $$
(1.9)
Fig. 1.4
A schematic of a strand of passive film in the middle of the concrete beams. It displays the oxidation reaction at the anode region and the reduction reaction at the cathode region.

Schematic diagram of electrochemical corrosion mechanism

Full size image

1.3.2 Mechanisms of Stress Corrosion

The existence of high stresses in the strand will change the corrosion mechanism of the strand. Under the coupled action of high stress and corrosive environment, strand will suffer from traditional electrochemical corrosion and stress corrosion [25], as shown in Fig. 1.5. In the coupling of high stress and corrosive environment, micro-cracks occur on the strand surface. The stress corrosion mechanism includes two types of anodic dissolution stress corrosion and hydrogen-induced cracking stress corrosion [26]. Micro-cracks lead to brittle fracture of the strand at well below the tensile strength in a form of corrosion, which is called as the stress corrosion.

Fig. 1.5
A diagram of a crack at the anode region in the passive film of the strand along with the transfer of an electron from the anode region to the cathode region.

Schematic diagram of stress corrosion mechanism

Full size image

High stresses in the strand will cause local cracking of the passive film to expose fresh fracture surfaces. The fractured surface acts as the anode. The rest of the unbroken passive film acts as the cathode. The pore water acts as the electrolyte solution to form a closed-circuit electrolytic cell. Cracks extend internally as the strand fracture surface at the anode dissolves. Due to the existence of high stress, the cracks at the fracture surface are constantly cracking which makes it hard to form a new passive film. Under the continuous action of the electrochemical reaction, the strand at the anode fracture surface dissolves and the cracks continue to expand to the inner depth. The brittle fracture strength of stranded wire is much lower than the tensile strength. This corrosion behavior is known as anodic dissolution stress corrosion [24]. The hydrogen and chloride ions absorbed by the strand during the manufacturing process undergo an electrochemical reaction to produce hydrogen. Hydrogen accumulates at the fracture surface of the strand caused by high stresses. When the hydrogen concentration reaches the critical hydrogen concentration, it will cause the strand to become brittle and crack at the fracture face. Constant cracking of the strand leads to hydrogen brittle fracture of the strand. This corrosion behavior is known as hydrogen-induced cracking stress corrosion [19].

1.3.3 Influence Factors of Strand Corrosion

Corrosion of strand in concrete is influenced by many factors, e.g., the medium (gas, liquid, solid), temperature, humidity, freezing, etc. The surrounding environment is the external factor that affects the corrosion of strand. The position of the strand, the diameter of the strand, the type of concrete, permeability, cracking, alkalinity, the use of additives, the thickness of the protective layer, the strength level, and quality of the concrete are the internal factors affecting the corrosion of the strand. The factors affecting the corrosion of strand in general are as follows.

  1. 1.

    PH value of concrete

For strand in concrete, when PH is greater than 11.5, they are completely passivated and corrosion will not occur. When the PH gradually decreases from 11.5 to 9, the strand passive film is gradually damaged and the corrosion rate gradually increases. When the PH is between 9 and 10, the strand is completely depassivated and the corrosion rate is not affected by the PH. When the PH is less than 4, the corrosion rate of the strand increases sharply.

  1. 2.

    \({\text{Cl}}^{ - }\) concentration in concrete

The sources of \({\text{Cl}}^{ - }\) in concrete are both internal mixing and external penetration. The internal \({\text{Cl}}^{ - }\) is mainly derived from antifreeze such as \({\text{CaCl}}_{2}\), which is added during the concrete mixing process. Most of the \({\text{Cl}}^{ - }\) is adsorbed by the cement slurry and is present in the form of bound \({\text{Cl}}^{ - }\), which have little effect on strand corrosion. Concrete in seawater environment and highway concrete with deicing salt on pavement are the sources of external penetration type \({\text{Cl}}^{ - }\). The \({\text{Cl}}^{ - }\) in the external environment gradually accumulates at the concrete–strand interface through the concrete protective layer, so that the concentration of \({\text{Cl}}^{ - }\) in the strand surface solution gradually increases. When \({\text{Cl}}^{ - }\) reaches the critical concentration, the strand begins corrosion.

  1. 3.

    Environmental conditions

Environmental conditions such as temperature, humidity and drying alternation, seawater splash, and sea salt penetration are the external factors that cause the corrosion of strand, which have obvious effects on the corrosion of strand in concrete structures. When the self-protection ability of concrete does not meet the requirements or the concrete cover is cracking and other defects, the influence of external environmental factors will be more prominent. The actual investigation results show that the service life of concrete structure in dry and non-corrosive media is 2–3 times longer than that in wet and corrosive media.

  1. 4.

    Thickness of protective layer, integrity, and density of concrete

Concrete protective layer prevents infiltration of corrosive medium, oxygen, and water into the structure. The thicker the protective layer, the smaller the oxygen concentration gradient, and the slower the corrosion rate. However, the excessive thickness of concrete cover will not only reduce the ultimate bending resistance of concrete members but also change the angle of oblique section of punching failure and slightly reduce the ultimate punching resistance of concrete members. The intact degree of concrete cover has obvious influence on strand corrosion, especially on prestressed concrete structures in wet environment or corrosive medium. The density of concrete affects the permeability of concrete and the strand in the concrete with high permeability are more prone to corrosion.

  1. 5.

    Cement varieties and admixtures

Mineral admixtures, such as fly ash, can reduce the alkalinity of concrete, thereby affecting the corrosion of strand. Adding high-quality fly ash and other admixtures can reduce the alkalinity of concrete. At the same time, it can improve the density of concrete and change the internal pore structure of concrete. This can prevent the infiltration of external corrosive medium and oxygen and water, which is undoubtedly beneficial to prevent the corrosion of the strand. In recent years, research work has also shown that the addition of fly ash can enhance the corrosion resistance of concrete.

  1. 6.

    Carbonization degree of concrete

Carbon dioxide in the atmosphere diffuses into concrete and reacts with calcium hydroxide produced during hardening. Chemical reaction reduces the original strong alkaline of cement. When the PH drops to around 8.5, carbonation of the concrete occurs, which gives the strand the possibility of depassivation. The degree of concrete carbonation has a significant effect on the corrosion of the strand.

1.4 Contents of This Book

This book introduces the research results on the performance deterioration of existing prestressed concrete (PC) structures, clarifies the mechanical behavior of corroded prestressing strands, corrosion-induced cracking, bond degradation, prestress loss, and structural performance deterioration of PC structures and proposes the corresponding prediction models, which has an important guidance for the durability and maintenance design of PC structures.

This chapter provides the introduction, history and development of PC structures, practical application of PC structures, and corrosion of strand in prestressed concrete.

Chapter 2 is organized as follows. First, corroded prestressing strands are obtained by the artificial climate conditions. Then, the number and shape of corrosion pits are measured to investigate its probability distribution. Furthermore, static tensile tests are carried out to study the mechanical property of corroded prestressing strands.

Chapter 3 conducts an experimental study on corrosion-induced cracking in PC structures at first. Next, it proposes theoretical models for predicting the corrosion-induced cracking. Following this, it establishes the numerical model to simulate the concrete cracking induced by helical strand corrosion.

Chapter 4 studies the bond behavior between prestressing strands and concrete. First, the effect of corrosion on residual bond stress of the strand is clarified based on the pull-out test. Next, the bond behaviors of corroded strand in pretensioned concrete beams are investigated by the bending test.

Chapter 5 aims to develop an analytical model for predicting the bond strength of strand considering rotation effect at first. Then, the effect of corrosion-induced concrete cracking on the ultimate bond strength of the corroded strand is discussed. Following this, a simplified model is proposed to predict the bond stress–slip relationship between corroded strand and concrete.

Chapter 6 proposes an analytical model to evaluate the prestress loss in corroded pretensioned concrete structures, incorporating the coupling effects of concrete cracking and bond degradation. A model, combining the coupling effect of the Hoyer effect and the cracking caused by corrosion, is also proposed to predict the transfer length of pretensioned concrete beams.

Chapter 7 designs an experimental study with five specimens to explore the secondary anchorage, secondary transfer length, and residual prestress in locally corroded post-tensioned concrete beams after strand fracture. A numerical model is established to reproduce the process of strand fracture and the secondary anchorage of fractured strand.

Chapter 8 designs an experimental study with twenty post-tensioned concrete beams to study the influence of grouting defects, strand corrosion in insufficient grouting, and strand corrosion in full grouting on the flexural performance of post-tensioned concrete beams.

Chapter 9 proposes an analytical model to predict the flexural behavior of locally ungrouted PC beams at first. Then, a model is proposed to predict the bearing capacity of corroded PC beams considering bond degradation.