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PRESTRESSED CONCRETE - WHAT IS IT? WHEN TO USE?

 

A little history

Although it is true that there is a bibliography that mentions that prestressed concrete/concrete was patented by “an engineer” from San Francisco - the USA in 1886, it was not until half a century after that this construction material was accepted by the construction sector. construction.

The steel shortage in Europe after World War II, coupled with "technological advances" in concrete and high-strength steel, made prestressed concrete the building material of choice during postwar European reconstruction.

However, the first prestressed concrete structure in North America, the "Walnut Lane Memorial Bridge" in Philadelphia, Pennsylvania was completed until 1951.


What is prestressed concrete (concrete)?

Prestressed concrete refers  to concrete containing steel subjected to strong prior and permanent traction.

The basic idea of  prestressing is to put the concrete under compression before loading it, in all those areas where the loads produce tractions.

To generate this internal compression force, prior to pouring the concrete, steel bars, wires or cables are placed, which are tensioned before pouring the concrete, making the mixture solidify on the already tensioned reinforcements.

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In this way, until these compressions are canceled, no tensions appear in the concrete. In conventional reinforced concrete, the high tensile strength of steel combines with the high compressive strength of concrete to form a structural material that is strong in both compression and tension. The principle behind prestressed concrete is that the compressive stresses induced by high-strength steel tendons in a concrete member before the loads are applied will balance the tensile stresses imposed on the member during service.


These metal bands are fitted so tightly that it creates rim compression around the barrel. When this barrel fills with liquid, it puts stress on the rim.


So how does prestressed concrete work?

The barrel example is the base theory that prestressing experts and creators were guided by, but now we will explain it by idealizing beams and the stresses that occur in prestressing.

In real life, high tensile steel cables are inserted into the beam section, stretched and anchored, then released. Now the steel tendon wants to return to its original length and the tensile stresses are transformed into compressive stress in the concrete. Now after loading, there are two types of forces in the beam:

1.      1.  Internal prestressing force

2.     2.   External force (The dead load that is going to be applied to the structural element and counteracts the tension produced by the steel cables).

Next, I will show in images how a "normal" beam and a prestressed one behaves at external loads:

Conventional beam exposed to external loads


Prestressed concrete materials

As per AASHTO, seven-strand high-strength steel wire, high-strength steel wire, or grade and type alloys (as specified by the designer) must be used in prestressed concrete.

In addition, it is required to use much stronger concrete in prestressing than in conventional. In general, a minimum concrete strength of 5000 Psi should be used.

Why?

If the concrete is not strong enough, it can crack or fail when the tendons contract.

  1.      Excellent traction, which improves the efficiency of the materials.
  2.      It works for sections greater than 35 meters.
  3.      Improves resistance to cutting and resistance to fatigue.
  4.      Provides dense concrete, hence durability.
  5.     The best choice for slim and elegant structures.
  6.      Helps reduce dead load.
  7.      Some experts claim that it remains seamless.


Why is prestressed concrete necessary?

Concrete is weak in tension and strong in compression. This is a weak point in concrete that produces early bending cracks mainly in bending members such as beams and slabs. To avoid this, the concrete is induced with compressive stress (prestressing), and this stress counteracts the tensile stress to which the structure is subjected during service conditions. Therefore, the chances of flex cracks are reduced.

The precompression that is induced as part of the prestress helps to improve the bending capacity, shear capacity, and torsional capacity of the bending members.

A compressive prestressing force can be applied concentrically or eccentrically in the longitudinal direction of the member. This stems from cracks in the critical media space and supports the service load.

A section of prestressed concrete behaves elastically.

The full capacity of concrete in compression can be used throughout the depth under full load for prestressed concrete.

Prestressing Methods:

  • 1.       Prestressed
  • 2.       Post-tensioned

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Advantages of Prestressed Concrete

  1. Greater resistance: it provides an internal compressive stress that counteracts the tensile stress produced by loads of the structural elements.
  2. Durability in construction: elimination of fissures and cracks in all stages of loading and minimizing the possibility of steel corrosion
  3. Saving of materials.
  4. Weight reduction.
  5. Increased lighting.
  6. Use of spaces.
  7. More efficient and slender architectural elements.
  8. Greater quality control, cost reduction, and high production.
  9. Use in works and large-scale projects.
  10. Faster construction.

Disadvantages of Prestressed Concrete:

  1. Higher costs in materials
  2. Prestressing is an additional cost.
  3. The formwork is more complex.

Also read:

Uses of prestressed concrete

The uses of prestressed concrete that we can highlight for its lightness, strength, and durability are in the construction of bridges. This is because the placement of beams manufactured using this technique achieves greater structural strength, especially in bridges with a high vehicular flow.

It is also used in skyscraper floors due to its structural characteristics, it has a greater resistance to external traction with a lower weight.

Some large parking lots and warehouses also use prestressed concrete, since these precast elements allow a greater separation between columns, favoring the roofing of large areas. It is possible to make better use of the space, generating more lighting in a natural way with a reduction in electricity consumption.

Application mode

When making this building material, concrete is poured around tensioned rods or cables, creating a bond between the rod and the concrete.

This joint prevents oxidation of the bar and allows the direct transfer of stress. Therefore, a correct application of this material supposes the prefabrication in the workshop in order to allow the correct exchange of tension.

Once this concrete is precast, each piece is moved to the construction site (it may be the construction of a bridge, for example) where it is installed and other construction elements are added to it.

Why choose prestressed concrete?

The use of prestressed concrete pieces will depend a lot on the type of structure that you want to build. Thus, if what you want is to build a solid and firm structure that saves money and time, then prestressed concrete is the best option.

Due to the tension forces that are present in this material, it is possible to provide rigidity to the structure while preventing the formation of cracks and fissures that can affect the durability of the construction.

If what you are looking for is to have a firm, durable and avant-garde structure, then it is best to choose to use prestressed concrete pieces.

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