Prestressed concrete is a type of concrete that is strengthened by pre-tensioning or post-tensioning steel wires or bars inside it. This technique allows for the construction of stronger, more durable structures that can withstand greater loads and stresses than traditional reinforced concrete.
The advantages of prestressed concrete include improved structural efficiency, reduced material usage, and increased durability. However, there are also some disadvantages, such as higher construction costs, specialized expertise required for design and installation, and a risk of corrosion over time.
Table of Contents
What Is Prestressed Concrete?
Prestressed concrete is a system into which internal stresses are intentionally induced without any form of external loads to improve its performance. The internal stresses induced in the concrete structure are used to counteract the stresses originating from the external load application.
Concept Of Concrete Prestressing:
As mentioned earlier, prestressing is the application of an initial force on the concrete structure, so that the structure is capable to counteract or withstand the stresses coming due to the service loads. The idea can be clearly understood by the example of a barrel.
A barrel used in the old era to transports liquids and food grain is tightly bound by metal bands as shown in the figure given below. These metal bands are fitted very tightly so that they make a hoop compression around the barrel. When this barrel is filled with liquid, it exerts hoop tension on the surface of the barrel.
The hoop compression that is created by the metal bands assists to counteract the hoop tension created by the fluid within. This is called a prestressing arrangement.
Likewise, effective internal stresses are evoked into the concrete by means of tensioned steel bars before the concrete structure is susceptible to any service loads. This stress counteracts external forces.
The Requirement Of Prestressing Concrete:
The requirement of prestressing in concrete can be justified by the following issue:
- As we know, Concrete is weak in tension and strong in compression. This weak point of concrete that results in early flexural cracks primarily in flexural members like beams and slabs. To stop this, the concrete is evoked with compressive stress intentionally (prestressing) and this stress counteracts with the tensile stress the structure is subjected to during service condition. Thus, the chances of flexural cracks are decreased.
- The pre-compression that is induced as a part of prestressing assist to increase the bending capacity, the shear capacity, and the torsional capacity of the flexural members.
- A compressive prestressing load can be applied concentrically or eccentrically in the longitudinal direction of the member. This stop cracks at critical midspan and supports at service load.
- A prestressed concrete section acts elastically.
- The full capacity of the concrete in compression can be utilized over the entire depth under full loading in the case of prestressed concrete.
Methods Of Prestressing:
The prestressing can be performed by two methods as described:
- Pre-tensioning
- Post-tensioning
1. Pre-tensioning:
In the pre-tensioning technique, the stress is induced by initially tensioning the steel tendons. These are wires or strands that are tensioned between the end moorings. After this tensioning procedure, the concrete casting is performed.
After casted concrete has hardened sufficiently, the end anchorages arranged are released. This releasing transfers the prestress load to the concrete. The bond between the concrete and the steel tendons assists this transfer of stress.
As given in the below image, the tendons that are protruding at the ends are cut and a finished look is achieved. In order to generate prestress force in the pre-tensioning method, a large number of tendons and wires are used. This system hence demands a large area of surface contact to make the bond and stress transfer achievable.
2. Post-tensioning:
The process in post-tensioning is depicted in the figure below. Here, the steel is prestressed only after the beam is cast, cured, and achieves the strength to take the prestress. Within the overlay, the concrete is cast. For the passage of steel cables, ducts are formed in the concrete structure.
After casted concrete hardens completely, the tendons are tensioned. One end of the tendon is anchored and the other end is tension. In some specific cases, the tensioning can be performed from either side and anchored subsequently.
After the prestressing process, there is space between the tendons and the duct. This leads to:
- Bonded Construction
- Unbounded Construction
1. Bonded Construction:
In bonded construction, the gap between the duct and the tendon is filled with cement grout. The grouting process assists the steel to resist corrosion to a large extent. The final strength is enhanced as this method increases the resistance to live loads acting. The grout mixture is cement and water compounded with or without admixture. No sand is utilized in this grout.
2. Unbounded Construction:
If no grout is utilized to fill the gap between the duct and tendon, it is called unbonded construction. In this process, the steel is galvanized to protect it from corrosion. Waterproofing material is exploited for galvanizing.
Protection Of Tendon Bars In Pre-Stressed Concrete
Pre-stressed concrete structures require special attention to ensure the durability of the tendons and protection against corrosion. Tendons in pre-stressed concrete structures are steel cables or bars that are pre-tensioned or post-tensioned to provide compressive stresses to the concrete.
Corrosion is a primary factor that can affect the durability of tendons in pre-stressed concrete structures. Corrosion occurs when steel in the tendons reacts with oxygen and moisture in the environment, leading to the formation of iron oxide.
To protect tendons from corrosion, designers and builders can use corrosion-resistant materials such as stainless steel or galvanized steel, which have a higher resistance to corrosion than plain carbon steel.
Adequate concrete cover to the tendons is an effective measure to protect them from exposure to the environment and reduce the risk of corrosion. However, a too thick cover can increase the risk of cracking due to shrinkage and thermal expansion.
Regular maintenance and inspections of pre-stressed concrete structures are essential for the durability of the tendons. Regular inspections can detect any signs of corrosion or damage to the tendons, and appropriate repairs can be made to prevent further deterioration.
Advantages Of Prestressed Concrete:
The main advantages of Prestressed Concrete are:
- The prestressing of concrete by using high tensile steel better the efficiency of the materials.
- The prestressing system works for a span of more than 35 meters.
- Prestressing increase shear strength and fatigue resistance of concrete
- Dense concrete is used by prestressing systems hence it improves durability.
- Prestress concrete is best for the construction of sleek and slender structures.
- Prestress concrete helps to cut down the dead load of the concrete structure.
- Prestressed concrete remains uncracked even at service load conditions which proves the structural efficiency of the concrete member.
- Composite construction by exploitation the prestressed concrete unit and cast-in-unit derives the economical structure.
Disadvantages Of Prestressed Concrete:
- Material cost is very high.
- The prestressing system is an extra cost in the concrete casting process.
- Formwork is more difficult than for RC (flanged sections, thin webs) – hence, precast not as ductile as RC.