Updated: Feb 28
BASIC PRINCIPLES OF PRESTRESSING
A beam resting on supports at each end trends to bend under its own weight and under applied loading. This causes compression along the top part of the beam and tension along the bottom part. In other words, there is a tendency for the bottom of the beam to stretch.
Concrete is strong in compression, but weak in tension, and for this reason, a plain concrete beam has little strength. The tensile weakness of concrete is overcome by casting steel bars into the sections where tension is likely to occur. When a load is applied on beam, cracks still occur in the concrete, but the tension is carried by the steel reinforcement.
The principle of prestressing is to compress the beam before it is loaded in such a way that stresses are induced in the section which is opposite in action to those arising under loading. Thus the bottom of the beam is compressed by the prestressing so that tension arising when it is loaded will be entirely neutralized.
Furthermore, the compression in the concrete is also of great importance in resisting shear. If one imagines a prestressed beam as a row of blocks pressed together, it is easy to see that if they are pressed together sufficiently tightly they will not fall out when a load is applied. This condition aided by the device of sweeping cables upwards at the end of the beam will usually eliminate the need for steel reinforcement to resist shear stresses.
ADVANTAGES OF PRESTRESSED CONCRETE
The rapid increase in the use of prestressed concrete is due to the fact that it is technically and economically superior to other methods of construction.
Since the size of prestressed members is less than that of the conventional reinforced concrete members, the dead load of the structure is often reduced sufficiently, hence savings of materials in structural members.
The use of prestressed concrete as a structural medium for bridge construction has been gaining popularity. With the increase in transportation, the requirement for bridges became acute and the shortage of general building materials gave an impetus to use of prestressed concrete in all civil engineering activities.
The development of prestressed concrete technology over the last two decades can be attributed to the research, development, improvement and advancement of materials, technology and construction techniques. The existing materials used in construction were better utilized and their properties enhanced, new materials developed to suit the special needs of prestressed concrete. The development in technology also seen in the concept of form-work and analytical techniques.
APPLICATIONS OF PRESTRESSED CONCRETE
Prestressed Concrete Piles: Prestressed concrete piles have been used extensively in the construction of buildings and marine structures. Due to its high strength for handling and a high degree of durability in seawater and other adverse environments, the use of prestressed concrete piles became very popular in the construction of marine structures. The prestressed concrete piles have many advantages in comparison with conventional piles - a few of them are:
High load-carrying capacity.
Crack-free under handling and driving.
Ability to take up-lift (tension).
Can bear hard-driving and can penetrate hard strata.
Durability in an adverse environment.
High column strength.
In view of the above advantages, the prestressed concrete pile is an ideal choice for deep foundations with heavy loading on weak soil. At present, prestressed concrete piles are being used as sheet piles, fender piles and soldier piles. It also used for carrying vertical loads with different soil strengths and found to be durable in varied environments ranging from sub-arctic to the desert.
Rock /soil Anchors: Prestressing techniques are now used for strengthening an existing structure by anchoring it to the rock or soil. In places where rocks are not available immediately below the ground level, rock-anchors are used to anchor the pile to the rocks that are situated at very large depth. The use of prestressed anchors avoids the driving of the pile all the way to the rock which is available at very large depth. The pile is driven only to a certain depth, depending on the soil condition and prestressed cable is sent through the pile to the rock. The cable is then stressed and grouted.
The spectacular contribution of prestressed concrete can be seen in the construction of superstructures of bridges. It has been extensively used in both rail and road bridges.
The technique of prestressing lends itself beautifully to the construction of different types of bridges.
a. Simply Supported Bridges: They are adopted for medium and short spans. The cross-sections of these beams maybe I, T, two T's or Box shape. The girders can be pre or post-tensioned. These beams may be precast or cast-in-situ and are usually supported by neoprene or other types of bearings at either end.
b. Cantilever Bridges: This method is usually adopted for longer span bridges. In this method, there will be cantilevers extending from each of the piers. There will be a suspended span of the shorter length to connect the cantilevers. The cantilevers are usually extended by anchoring precast segments of short length. Each segment is anchored to the balancing extension on the other side of the pier.
c. Cable-Stayed Bridges: Extremely long spans constructed by using this method of construction. In this type of construction, the deck or slab is held by a number of prestressed cables anchored to the anchor tower. Using this method spans up to 300 m can be constructed.
Other types of bridges like bridges with Bow String Truss, Stressed Ribbon Deck and Arch Bridges are included.
3. MARINE STRUCTURES
Prestressed concrete has gained acceptance in the field of marine structures due to its durability, strength and economy. Its application to foundations has already been discussed in the earlier section. Prestressed concrete is now being applied increasingly in the super-structures of the marine projects. A few types of marine structures where prestressed concrete has been adopted are:
In these structures, the prestressed concrete elements may be in the foundation, such as bearing pile, sheet pile, etc. or in the super-structure, such as the deck, beam slab, etc.
It is a well-known fact that marine construction has many problems of its own in its construction procedures. The difficulty at the site especially for the movement of materials and workers, makes quality control a difficult exercise.
Further, highly skilled labour is required in such projects. Such labour is either not available or available at a very high cost. All such factors show that precasting is an ideal choice for marine structure. In precasting, the efficiency and economy can be increased by means of prestressing, especially by pre-tensioning. The pre-tensioned materials can then be made to act monolithically by post-tensioning them subsequently.
4. WATER CARRYING STRUCTURE
a. Aqueducts: Prestressed concrete is found to be the ideal choice for construction of aqueducts due to its water tightness and crack-free surface. Prestressed concrete, due to its high strength, enables the construction of long-span aqueducts with high water carrying capacity.
b. Water Tanks: Circular water tanks are also constructed by using prestressed concrete. They withstand higher circumferential stress than R.C.C. The wall thickness of the prestressed concrete tanks is much less than that of R.C.C because of its high strength. With these advantages, the use of prestressed concrete for the construction of overhead water tank and reservoirs is gaining popularity.
5. INDUSTRIAL STRUCTURES
Application of prestressed concrete in the field of construction of industrial structures is getting momentum. The tie members of the trusses are usually prestressed. The advantages of using prestressed concrete are:
longer spans of trusses can be constructed.
The aesthetic look of the structure is enhanced.
6. PRETENSIONED PRODUCTS
In the field of pretension, much progress has made to great advantage. The extensive manufacture of prestressed electric transmission poles is just one of the many application of pretension. The recent and important addition in the list is railway sleeper. A number of plants manufacturing these sleepers springing up in every corner of the world. Precast pre tensioned members are also used extensively for prefabricated houses.
This application has potential and offers excellent scope for development and diversification.
7. NUCLEAR STRUCTURE
In this atomic age, the concept of prestressed concrete lives up to its reputation as the technology that can offer solutions even to the most difficult and intricate problems faced by the civil engineering industry. The designers of the reactors have realized the advantages of prestressed concrete and are now designing their pressure-vessels and container-vessels of the reactors, recommending the use of prestressed concrete.
This application should prove the versatility and superiority of the concept of prestressed concrete over conventional methods.