Concrete is poured around tensioned tendons. This method produces a good bond between the tendon and the concrete, which protects the tendon from rust, and allows direct transfer of stress. The concrete or set concrete adheres to the bars, and when the tension is released, it is transferred to the concrete in the form of compression through friction. However, strong external anchor points are required between which the tendon is stretched and the tendons are generally in a straight line. Therefore, most elements prestressed in this way are prefabricated in the workshop and must be transported to the construction site, which limits their size. Prestressed elements can be balcony elements, lintels, floor slabs, foundation beams or piles.

It is the descriptive phrase for the application of compression after the pouring and subsequent in situ drying process of the concrete. A plastic, steel or aluminum sheath is placed inside the concrete mold to follow the most convenient route inside the piece, following the strip where, otherwise, traction would be registered in the structural element. Once the concrete has set, the tendons are passed through the conduits. These tendons are then stressed using hydraulic jacks that react against the concrete piece itself. When the tendons have been stretched sufficiently, according to the design specifications (see Hooke's law), they are trapped in position by wedges or other anchoring systems and maintain tension after the hydraulic jacks are removed, thus transferring the pressure to the concrete. The conduit is filled with grease or cement slurry to protect the tendons from corrosion. This method is commonly used to create structural elements of civil works or buildings subjected to significant tensile stresses. For example, post-tensioning is used in the construction of concrete bridges, being practically essential in construction systems using cantilevers, push and prefabricated segments, etc.

The idea of ​​prestressing, from a general approach, is by no means an "invention" typical of structural engineering and there are numerous examples of human activity in which it is possible to find prestressed solutions, in the sense of "previous application of forces that lead to a desired state of stress or deformation." An example can be seen in the way barrels or barrels intended to house some type of liquid are manufactured (as is the case with wine barrels). These "structures" are made up of tongue-and-groove wooden staves that are girded with metal rings that are heated before embracing the barrel so that, when they cool, they compress the staves together. If it were not done this way, the pressure of the liquid contained in the barrel would open the joints and render it useless. Another example is that of a bicycle wheel, which is a structure formed by an external ring linked to another internal ring through spokes, which are thin metal elements. In order for this structure to support the weight of the cyclist without deforming, the spokes are screwed into housings arranged for this purpose to put them in traction.

Although various attempts have been made over time to reduce the cracking of concrete under tension, according to Freyssinet, prestressing a structural element consists of creating in it, by means of some specific procedure, before or during the application of external loads, forces of such magnitude that, when combined with the results of said external forces, they nullify the tension forces or reduce them, keeping them under the admissible stresses that the material can resist.

It is true that 40 years earlier there had been other inventors, the most important contributions to its solution are usually attributed to the French engineer Eugène Freyssinet, who turned the idea of ​​prestressing concrete elements into a practical reality. He is credited with being the first to realize the importance of the phenomena of delayed shrinkage deformations and, above all, creep, and their importance for the effectiveness of long-term prestressing, thanks to his tenacity and knowledge in the field of structural analysis.

Let's look at some of the most important milestones in relation to prestressed concrete:.

1886:

This year the above principle was applied to concrete when P. H. Jackson, an engineer from San Francisco, California, obtained patents for tying steel rods into artificial stones and into concrete arches that served as floor slabs.

1888:

About this year, C. E. W. Dohering, of Germany, secured a patent for metal-reinforced concrete that had a tensile stress applied before the slab was loaded.

1908:

C. R. Steiner of the United States suggested the possibility of readjusting the reinforcing bars after some shrinkage and creep of the concrete had occurred, in order to recover some of the losses.

1925:

R. E. Dill of Nebraska tested high-strength steel bars coated to prevent adhesion to concrete.

After placing the concrete, the rods were tensioned and anchored to the concrete with nuts at each end.

1928:

The modern development of prestressed concrete begins in the person of Eugène Freyssinet, from France, who began using high-strength steel wires for prestressing. Such wires had a breaking strength as high as 18,000 kg/cm², and a yield strength of more than 12,600 kg/cm².

1939:

Freyssinet produced conical wedges for the end anchors and designed double-action jacks, which tensioned the wires and then pressed the male cones into the female cones to anchor them to the anchor plates. This method consists of stretching the wires between two pillars located several tens of meters apart, placing shutters between the units, placing the concrete and cutting the wires after the concrete reaches a specific design strength.

1945:

The shortage of steel in Europe during World War II gave impetus to the development of prestressed concrete, since much less steel was needed for this type of construction compared to conventional reinforced concrete.

While France and Belgium led the development of prestressed concrete, England, Germany, Switzerland, Holland, Russia and Italy quickly followed. About 80% of all bridges built in Germany are made of prestressed concrete.

In 1945 Pacadar prefabricated the first prestressed beam in Spain.

1949:

Work began in the United States with linear prestressing when the construction of the famous Philadelphia Walnut Lane Bridge was carried out. The Bureau of Public Roads has investigated and shown that during the years 1957-1960, 2,052 prestressed concrete bridges were authorized for construction, totaling a length of 68 mi, with a total cost of 290 million dollars.

1951:

The first prestressed bridge is built in Mexico. The city of Monterrey being the godmother of such an event, by carrying out the construction of the "Zaragoza" bridge, which has 5 sections of 34 m each and whose purpose is to provide circulation across the Santa Catarina River "Santa Catarina (Nuevo León)").

1952:

There is a meeting in Cambridge, at which an international society is created under the name of Fédération Internationale de la Précontrainte (FIP). The main objective of this group of visionary engineers was to spread the message and enlighten the world about the relatively unknown concept of prestressed concrete construction, which would be done by encouraging the integration of national groups in all countries that had a particular interest in the subject and by facilitating an international forum for the exchange of information.

1958:

The Tuxpan bridge (Mexico - Tuxpan highway) is built with a total length of 425 m. Main structure of three spans of 92 m of prestressed concrete, built with the double cantilever procedure (first bridge of this type in Latin America).

1962:

The Coatzacoalcos bridge is built with a total length of 996 m. Sections of prestressed beams of 32 m and a section of drawable metal reinforcement of 66 m of span, supported on reinforced concrete piles.

Prestressed concrete is the predominant material in girder bridges, in bridges built in situ with long spans between piers, or built by special methods such as cantilevering, pushing, etc. It is also widely used in floors of skyscrapers, in nuclear reactor chambers, as well as in the pillars and resistant cores of buildings prepared to resist a high degree of earthquake" and protection against explosions").[4]

An advantage of prestressed concrete is the lower construction cost thanks to the use of lighter elements, such as thin slabs - especially important in tall buildings where the weight savings of the floor can translate into additional floors for the same and less cost.

Increasing lengths increases usable space in buildings; reducing the number of joints, which leads to a reduction in maintenance costs during the design life of a building, since these joints are the main scenario of weakness in concrete buildings.

The first prestressed concrete bridge in North America is the Walnut Lane Memorial Bridge&action=edit&redlink=1 "Walnut Lane Memorial Bridge (Philadelphia, Pennsylvania) (not yet drafted)") in Philadelphia, Pennsylvania. It was completed and opened to traffic in 1951.[5]​.

The study of the behavior of prestressed concrete involves its analysis over time since the mechanical properties of the materials degrade and are not constant throughout the entire useful life of the system.

The two critical stages of the system take place: (a) during the moment of testing -immediate effects-, and (b) at the end of the useful life -deferred effects-. Therefore, in general, the analysis must consider the following losses.

(a) Immediate losses:.

(b) Deferred losses:.

Depending on the extent to which the preliminary tension states counteract the requesting tension states, there are the following three possible degrees of prestressing: total, partial or limited.

Total prestressed.

It occurs when, in the most unfavorable fiber of the critical cross section, the preliminary stress states completely counteract the requesting stress states. Thus, a zero resulting tension state is obtained on it.

In this way, two important characteristics are achieved regarding the entire transverse profile of the section: (1) it is all in compression and, therefore, (2) there is no cracking in the concrete.

In the early days of the use of prestressing this is the degree of prestressing that became popular. Although its consideration is evident, however, it soon became known that it suffered from two important disadvantages: (1) completely canceling the final stress states is onerous because it demands a significant amount of tensioning force with the consequent over-requirement of tesa reinforcement and jack power, and (b) the (although scarce but existing) tensile strength of the concrete is wasted (of the order of 10% of its compressive strength).

Attempts to optimize the technique led to the consideration of the next degree of prestressing: partial prestressing.

Partial prestressing.

With this degree of prestressing, the preliminary tension states only partially counteract the requesting tension states so that, in the most unfavorable fiber of the critical cross section, final tensile tension states are accepted but of small magnitude and lower than the tensile capacity of this fiber.

In this way, the following two characteristics are obtained on the profile of the critical cross section: (1) although the majority of the section is in compression, there is a small portion of it that is slightly tensed to a magnitude lower than its tensile capacity and, therefore, (2) the section is not cracked either.

With this guarantee, two improvements were achieved at the same time: (1) the amount of tesa reinforcement necessary was reduced - as well as the consequent jacking power required to install it -, and (2) the cost of prestressing was reduced.

But this led to something else. Since prestressed concrete technologists were in the quest to optimize its application, and since they had achieved this with the introduction of the partial degree of prestressing, it was logical that they questioned whether it ended there or if there was the possibility of pushing the limit even further. They soon found that the answer to this question was yes, which gave way to the emergence of the next degree of prestressing: limited prestressing.

Limited prestressing.

When setting service limit states (ELS), modern structural concrete standards include three components to verify: (1) deformation -arrows-, (2) cracking, and (3) vibration.

On the one hand, evidently, in a given structural system, only one of these three components is critical and dominates its behavior; On the other hand, these standards impose admissible values ​​for these three parameters: maximum admissible deflection, maximum admissible crack opening and minimum vibration frequency.

The limited degree of prestressing consists of adopting for the structural system such a degree of prestressing such that traction is allowed on the transverse profile of the section, but in such a way that either its maximum deflection or its maximum crack opening or its vibration period is limited so that the admissible limit values ​​set by standard are not exceeded.

Evidently, with this approach the amount of stiff reinforcement required as well as the necessary jacking capacity is further reduced (compared to the partial degree of prestressing), obtaining a substantial optimization of prestressing.

The prestressing stress can be transmitted to the concrete:.

Normally, when applying this technique, high-strength concrete and steel are used, given the magnitude of the induced stresses.

As indicated, prestressing can be achieved in two ways: prestressing (with pre-tensioned reinforcement) and post-tensioning (with post-tensioning reinforcement). In this way, the general word prestressed is used to simultaneously refer to both prestressed concrete and post-tensioned concrete, where what changes is the moment in which the tensioning of the cables occurs. Thus, in general, we speak of prestressed structures, when we do not want to refer to the moment in which the tensioning of the cables occurs and, we will say prestressed structures or post-tensioned structures when this moment is important.

Concrete is poured around tensioned tendons. This method produces a good bond between the tendon and the concrete, which protects the tendon from rust, and allows direct transfer of stress. The concrete or set concrete adheres to the bars, and when the tension is released, it is transferred to the concrete in the form of compression through friction. However, strong external anchor points are required between which the tendon is stretched and the tendons are generally in a straight line. Therefore, most elements prestressed in this way are prefabricated in the workshop and must be transported to the construction site, which limits their size. Prestressed elements can be balcony elements, lintels, floor slabs, foundation beams or piles.

It is the descriptive phrase for the application of compression after the pouring and subsequent in situ drying process of the concrete. A plastic, steel or aluminum sheath is placed inside the concrete mold to follow the most convenient route inside the piece, following the strip where, otherwise, traction would be registered in the structural element. Once the concrete has set, the tendons are passed through the conduits. These tendons are then stressed using hydraulic jacks that react against the concrete piece itself. When the tendons have been stretched sufficiently, according to the design specifications (see Hooke's law), they are trapped in position by wedges or other anchoring systems and maintain tension after the hydraulic jacks are removed, thus transferring the pressure to the concrete. The conduit is filled with grease or cement slurry to protect the tendons from corrosion. This method is commonly used to create structural elements of civil works or buildings subjected to significant tensile stresses. For example, post-tensioning is used in the construction of concrete bridges, being practically essential in construction systems using cantilevers, push and prefabricated segments, etc.

The idea of ​​prestressing, from a general approach, is by no means an "invention" typical of structural engineering and there are numerous examples of human activity in which it is possible to find prestressed solutions, in the sense of "previous application of forces that lead to a desired state of stress or deformation." An example can be seen in the way barrels or barrels intended to house some type of liquid are manufactured (as is the case with wine barrels). These "structures" are made up of tongue-and-groove wooden staves that are girded with metal rings that are heated before embracing the barrel so that, when they cool, they compress the staves together. If it were not done this way, the pressure of the liquid contained in the barrel would open the joints and render it useless. Another example is that of a bicycle wheel, which is a structure formed by an external ring linked to another internal ring through spokes, which are thin metal elements. In order for this structure to support the weight of the cyclist without deforming, the spokes are screwed into housings arranged for this purpose to put them in traction.

Although various attempts have been made over time to reduce the cracking of concrete under tension, according to Freyssinet, prestressing a structural element consists of creating in it, by means of some specific procedure, before or during the application of external loads, forces of such magnitude that, when combined with the results of said external forces, they nullify the tension forces or reduce them, keeping them under the admissible stresses that the material can resist.

It is true that 40 years earlier there had been other inventors, the most important contributions to its solution are usually attributed to the French engineer Eugène Freyssinet, who turned the idea of ​​prestressing concrete elements into a practical reality. He is credited with being the first to realize the importance of the phenomena of delayed shrinkage deformations and, above all, creep, and their importance for the effectiveness of long-term prestressing, thanks to his tenacity and knowledge in the field of structural analysis.

Let's look at some of the most important milestones in relation to prestressed concrete:.

1886:

This year the above principle was applied to concrete when P. H. Jackson, an engineer from San Francisco, California, obtained patents for tying steel rods into artificial stones and into concrete arches that served as floor slabs.

1888:

About this year, C. E. W. Dohering, of Germany, secured a patent for metal-reinforced concrete that had a tensile stress applied before the slab was loaded.

1908:

C. R. Steiner of the United States suggested the possibility of readjusting the reinforcing bars after some shrinkage and creep of the concrete had occurred, in order to recover some of the losses.

1925:

R. E. Dill of Nebraska tested high-strength steel bars coated to prevent adhesion to concrete.

After placing the concrete, the rods were tensioned and anchored to the concrete with nuts at each end.

1928:

The modern development of prestressed concrete begins in the person of Eugène Freyssinet, from France, who began using high-strength steel wires for prestressing. Such wires had a breaking strength as high as 18,000 kg/cm², and a yield strength of more than 12,600 kg/cm².

1939:

Freyssinet produced conical wedges for the end anchors and designed double-action jacks, which tensioned the wires and then pressed the male cones into the female cones to anchor them to the anchor plates. This method consists of stretching the wires between two pillars located several tens of meters apart, placing shutters between the units, placing the concrete and cutting the wires after the concrete reaches a specific design strength.

1945:

The shortage of steel in Europe during World War II gave impetus to the development of prestressed concrete, since much less steel was needed for this type of construction compared to conventional reinforced concrete.

While France and Belgium led the development of prestressed concrete, England, Germany, Switzerland, Holland, Russia and Italy quickly followed. About 80% of all bridges built in Germany are made of prestressed concrete.

In 1945 Pacadar prefabricated the first prestressed beam in Spain.

1949:

Work began in the United States with linear prestressing when the construction of the famous Philadelphia Walnut Lane Bridge was carried out. The Bureau of Public Roads has investigated and shown that during the years 1957-1960, 2,052 prestressed concrete bridges were authorized for construction, totaling a length of 68 mi, with a total cost of 290 million dollars.

1951:

The first prestressed bridge is built in Mexico. The city of Monterrey being the godmother of such an event, by carrying out the construction of the "Zaragoza" bridge, which has 5 sections of 34 m each and whose purpose is to provide circulation across the Santa Catarina River "Santa Catarina (Nuevo León)").

1952:

There is a meeting in Cambridge, at which an international society is created under the name of Fédération Internationale de la Précontrainte (FIP). The main objective of this group of visionary engineers was to spread the message and enlighten the world about the relatively unknown concept of prestressed concrete construction, which would be done by encouraging the integration of national groups in all countries that had a particular interest in the subject and by facilitating an international forum for the exchange of information.

1958:

The Tuxpan bridge (Mexico - Tuxpan highway) is built with a total length of 425 m. Main structure of three spans of 92 m of prestressed concrete, built with the double cantilever procedure (first bridge of this type in Latin America).

1962:

The Coatzacoalcos bridge is built with a total length of 996 m. Sections of prestressed beams of 32 m and a section of drawable metal reinforcement of 66 m of span, supported on reinforced concrete piles.

Prestressed concrete is the predominant material in girder bridges, in bridges built in situ with long spans between piers, or built by special methods such as cantilevering, pushing, etc. It is also widely used in floors of skyscrapers, in nuclear reactor chambers, as well as in the pillars and resistant cores of buildings prepared to resist a high degree of earthquake" and protection against explosions").[4]

An advantage of prestressed concrete is the lower construction cost thanks to the use of lighter elements, such as thin slabs - especially important in tall buildings where the weight savings of the floor can translate into additional floors for the same and less cost.

Increasing lengths increases usable space in buildings; reducing the number of joints, which leads to a reduction in maintenance costs during the design life of a building, since these joints are the main scenario of weakness in concrete buildings.

The first prestressed concrete bridge in North America is the Walnut Lane Memorial Bridge&action=edit&redlink=1 "Walnut Lane Memorial Bridge (Philadelphia, Pennsylvania) (not yet drafted)") in Philadelphia, Pennsylvania. It was completed and opened to traffic in 1951.[5]​.

The study of the behavior of prestressed concrete involves its analysis over time since the mechanical properties of the materials degrade and are not constant throughout the entire useful life of the system.

The two critical stages of the system take place: (a) during the moment of testing -immediate effects-, and (b) at the end of the useful life -deferred effects-. Therefore, in general, the analysis must consider the following losses.

(a) Immediate losses:.

(b) Deferred losses:.

Depending on the extent to which the preliminary tension states counteract the requesting tension states, there are the following three possible degrees of prestressing: total, partial or limited.

Total prestressed.

It occurs when, in the most unfavorable fiber of the critical cross section, the preliminary stress states completely counteract the requesting stress states. Thus, a zero resulting tension state is obtained on it.

In this way, two important characteristics are achieved regarding the entire transverse profile of the section: (1) it is all in compression and, therefore, (2) there is no cracking in the concrete.

In the early days of the use of prestressing this is the degree of prestressing that became popular. Although its consideration is evident, however, it soon became known that it suffered from two important disadvantages: (1) completely canceling the final stress states is onerous because it demands a significant amount of tensioning force with the consequent over-requirement of tesa reinforcement and jack power, and (b) the (although scarce but existing) tensile strength of the concrete is wasted (of the order of 10% of its compressive strength).

Attempts to optimize the technique led to the consideration of the next degree of prestressing: partial prestressing.

Partial prestressing.

With this degree of prestressing, the preliminary tension states only partially counteract the requesting tension states so that, in the most unfavorable fiber of the critical cross section, final tensile tension states are accepted but of small magnitude and lower than the tensile capacity of this fiber.

In this way, the following two characteristics are obtained on the profile of the critical cross section: (1) although the majority of the section is in compression, there is a small portion of it that is slightly tensed to a magnitude lower than its tensile capacity and, therefore, (2) the section is not cracked either.

With this guarantee, two improvements were achieved at the same time: (1) the amount of tesa reinforcement necessary was reduced - as well as the consequent jacking power required to install it -, and (2) the cost of prestressing was reduced.

But this led to something else. Since prestressed concrete technologists were in the quest to optimize its application, and since they had achieved this with the introduction of the partial degree of prestressing, it was logical that they questioned whether it ended there or if there was the possibility of pushing the limit even further. They soon found that the answer to this question was yes, which gave way to the emergence of the next degree of prestressing: limited prestressing.

Limited prestressing.

When setting service limit states (ELS), modern structural concrete standards include three components to verify: (1) deformation -arrows-, (2) cracking, and (3) vibration.

On the one hand, evidently, in a given structural system, only one of these three components is critical and dominates its behavior; On the other hand, these standards impose admissible values ​​for these three parameters: maximum admissible deflection, maximum admissible crack opening and minimum vibration frequency.

The limited degree of prestressing consists of adopting for the structural system such a degree of prestressing such that traction is allowed on the transverse profile of the section, but in such a way that either its maximum deflection or its maximum crack opening or its vibration period is limited so that the admissible limit values ​​set by standard are not exceeded.

Evidently, with this approach the amount of stiff reinforcement required as well as the necessary jacking capacity is further reduced (compared to the partial degree of prestressing), obtaining a substantial optimization of prestressing.