Index
What is Prestressed Concrete?
Pre-stressed concrete is a type of concrete in which pre-tensioned tendons or steel strands are used to counteract the compression caused by load on the concrete.
To understand this better, let us study the diagram below.
In figure 1, A unstressed concrete beam is shown. Any load on the beam will exert a downward pressure on the beam causing the beam to compress as shown in figure 2.
To counteract this compression a tension needs to be created that would push the structure upwards, thus creating a stable structure. For this purpose steel strands or tendons are stressed (pulled using a hydraulic jack) as shown in figure 3.
Since the tendons are stretched with force, as shown in figure 4, they tend to compress inwards. This creates an upward deflection known as hobbing shown in figure 5.
When you combine all these forces, the downward force of the load on the beam and the upward force of the prestressed concrete result in a nullified deflection making the surface flat as depicted in figure 6.
Prestressing can cut down on the amount of concrete used in construction, as well as the amount of materials used and transported, while also improving durability and life span.
Concrete has a high resistance to compressive forces, but it has a significantly lesser resistance to tension. Reinforcement can be added to provide tensile capacity. Such reinforcement could be tensioned during production or construction to apply a preparatory compression to those areas of the concrete surface that will be under tension when it is placed and in service. This is referred to as prestressing.
Prestressed concrete is divided into two types: pre-tensioned and post-tensioned.
What is Pre-Tensioning or Prestressing?
Pre-tensioning is achieved by stretching wires or strands, known as tendons, between two anchorages to a predefined amount prior to pouring concrete. The bond along the lengths of the tendon transfers the prestressing force to the concrete. Pre-tensioning is often done on permanent beds at precasting factories producing pre-tensioned concrete elements for the construction industry.
What is Post Tensioning or Post-stressing?
Post-tensioning is a concrete strengthening procedure. Before the concrete is poured, post-tensioning tendons, that are prestressing steel rods inside plastic ducts or casings, are put in the forms. The wires are then pulled tight, or stretched, and fastened to the concrete's outer edges once the concrete has achieved strength and before the service load is applied.
Applications and Advantages of pre-tensioning & post-tensioning
Applications of Pre-tensioning Concrete
Prestressed concrete is a highly adaptable construction material because it is an optimal blend of high-strength steel and modern concrete. The biggest reason prestressed & post stressed concrete has received attention is their use in development of metro structures. Most metro structures are pre stressed in a casting yard. This has also increased a demand for engineers with the knowledge of stressing in India. Almost all bridges, viaducts, and structures within highways are also increasing using stressed concrete in construction.
Few other ways that stressing is applied in construction are–
To counter in-service loadings, prestressing concrete permits "load-balancing" stresses to be introduced into the structure.
Prestressing is often used in short-span bridges with spans of 10 to 40 metres (30 to 130 feet) in the form of concrete pre-tensioned girders or slabs.
Since the mid-1930s, the prestressing method has been used in concrete dams to reduce uplift and improve overall stability. Dam rehabilitation projects usually include the installation of prestressing.
Prestressed concrete has been shown to be a suitable construction material when it comes to high-pressure nuclear structures such as containment buildings and nuclear reactor tanks.
Prestressed concrete ground surfaces and pavements that are heavily laden are prone to cracking and consequent traffic-driven degradation. As a result, prestressed concrete is commonly used in such constructions because its pre-compression allows the concrete to withstand the crack-inducing tensile stresses caused by in-service loading.
The Advantages of Pre-tensioning
A broader span length allows for more clear floor area and parking.
Thinner slabs are useful for high-rise buildings since they can produce more slabs for the same cost as traditional thicker slabs.
Durability for a long period of time.
It only necessitates a reduced number of building materials.
It can withstand larger pressures than standard Reinforced structures and is crack-free.
Applications of Post-tensioning Concrete
Post-tensioning has applications in practically every aspect of construction. Post-tensioning allows for longer unobstructed spans, narrower slabs, fewer beams, and more sleek, dramatic features in building construction. When compared to a normal concrete structure with a similar number of floors, post-tensioning can result in a significant weight reduction.
Developing crack-free tennis courts is a very good application for post-tensioning slabs. The trophy goes to post-tensioned concrete courts for its durability, efficiency, and long-term worth.
External post-tensioning is a relatively new application of PT for strengthening the existing structures, particularly as a seismic improvement.
Both cast-in-place concrete and precast segmental construction have been used by bridge designers. Longer spans and tighter fissures are possible with PT.
To reduce crack breadth and leakage, concrete storage tanks are frequently post-tensioned.
Post-tensioning masonry walls are commonly done with a strong steel bar attached to the foundations and stressed with a screw at the top of the wall.
Advantages of Post-tensioning
It minimises or eliminates shrinkage cracking, enabling the use of no or fewer joints.
When cracks do appear, they are carefully held together.
It allows for thinner slabs as well as other structural elements.
It enables us to construct slabs on extensive or spongy soils.
It allows us to create greater spans in elevated elements such as floors and beams.
Reinforcement or Reinforced concrete
The term "reinforced concrete" refers to a mixture of regular cement concrete and reinforcements (steel bar). This combination is designed to take advantage of both concrete's compressive and steel's tensile strengths at the same time.
The components of reinforced concrete work together to withstand a wide range of loads. Compression forces are resisted by concrete, while tension forces are resisted by steel reinforcement.
Reinforced concrete is becoming increasingly popular as a cost-effective building material. It is commonly employed in a variety of structures all around the world.
Difference between pre-stressed concrete and reinforced concrete
Prestressed Concrete | Reinforced Concrete |
The fatigue threshold is really high. (The number of stress cycles applied to a member below the permitted limit without generating deformation is referred to as fatigue.) | The fatigue threshold is comparatively low. |
The cost of materials is high. | The cost of materials is low. |
Tendons with high tensile strength are employed. Tendons are made of high-tensile steel consisting of a specific wire or a strand of many wires. | The materials used are steel and deformed bars. |
To anchor the tendons, a unique approach is required, which necessitates the use of skilled labour. | Because no unique technique is required, skilled labour is not a significant necessity. |
In terms of stress handling, prestressed concrete is effective on all sides. | For RCC, the concrete of the stress area is ineffective and vice versa. |
Steel corrosion is less because concrete fractures are minimised. | When it comes to RCC, steel corrosion is a concern. |
Applications: Rail sleepers, bridges, dams, and other structures | Applications: Used widely in building constructions. |
Advantages & Disadvantages of Reinforcement
Advantages
In comparison to other building materials, reinforced concrete has high compressive strength.
Reinforced concrete may also endure a lot of tensile stress because of the reinforcing.
The reinforced concrete composite technology outlasts all other types of construction.
Like a fluid material, reinforced concrete can be affordably moulded into an almost unlimited range of shapes at first.
Reinforced concrete has a very low maintenance cost.
Disadvantages
Reinforced concrete's tensile strength is about a tenth of its tensile strength.
Blending, casting, and curing are the three primary phases in employing reinforced concrete. The final strength is impacted by everything.
The forms that are used to form RC are much more expensive.
The RCC columns section is bigger than that of the steel section for multi-story buildings because RCC has a lower compressive strength.
Shrinkage causes cracks to emerge causing the material to lose its strength.
Concrete is a rigid material, thus is susceptible to breaking, cracking and damage. To increase concrete's strength and tensile properties it undergoes different types of treatment. The most common is concrete reinforcement in which steel bars are embedded within concrete during casting. Stressing however is a comparatively advanced technique in which tendons or high tensile steel strands are stressed using hydraulic jack and placed inside the concrete slab/bar/girder etc. When these stressed strands are placed before casting the concrete, the process is called pre-stressing and when these strands are inserted and stressed after the casting process, it is called post stressing.
While stressing is a more advanced and reliable method of strengthening concrete, it also comes at a higher cost. This is why reinforced concrete is much more abundantly used in construction. Since stressing requires machinery, it is a very technique-oriented and specialised job in the construction industry. At Cemtech Infra Solution Pvt. Ltd. we employ specialised engineers and technicians for this process.
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