1. Provide a brief history of prestressed concrete.
The concept of prestressed concrete appeared in 1888 when P.H. Jackson was granted the first patent in the United States for prestressed concrete design. Jackson’s idea was perfect, but the technology of high strength steel that exhibited low relaxation characteristics was not yet available. It was not until Eugene Freyssinet defined the need for these materials that prestressed concrete could be used as a structural building material. Unfortunately, although Freyssinet, a brilliant structural designer and bridge builder, lacked the teaching qualities necessary to communicate his ideas to other engineers. It would take Gustave Magnel to write the first book of design in prestressed concrete, communicating this idea to designers worldwide. Magnel designed and built the legendary Walnut Lane Bridge in Philadelphia, which revolutionized prestressed concrete in America. Simultaneously, Urlich Finsterwalder, the German bridge builder and designer, was revolutionizing the construction means and methods for prestressed concrete bridges.
Prestressed concrete is a method for overcoming concrete's natural weakness intension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebars, inside poured concrete. Prestressing can be accomplished in three ways: pretensioned concrete, and bonded or unbonded post-tensioned concrete.
2. Describe the principle of prestressed concrete. Your description should relate to material characteristic and stress distribution both for an eccentric and noneccentric prestressing.
Principle of prestressed concrete are prestressing is a method for overcoming the concrete's natural weakness in tension. In a prestressed concrete beam, steel tendons (generally of very high tensile strength alloy steel) are stretched to introduce a pre-compressive force into
the member. The prestressing force offsets the tensile stress and eliminates the tensile strain allowing the beam to resist further higher loading or to span longer distance. Material characteristic: 1. The plasticity of concrete enables it to be moulded readily into different shapes, whilst its relatively high fire resistance enables it to protect the steel reinforcement embedded in it. 2. Upon hardening, concrete bonds firmly to steel reinforcement so that, when loads are applied, the two act as though they are one. The tensile forces in any area are carried by the reinforcement. 3. The compressive strength of the concrete is used to advantage by applying an external compressive force to it which either keeps it permanently in compression even when loads are applied to it during its service life (fully-prestressed) or limits the value of any tensile stress which arises under load (partial prestressing). 4. Concrete provides an alkaline environment to steel embedded in it. This protects the steel from rusting and, because concrete is relatively inert to chemicals other than acids, it continues to do so for long periods of time in all but very hostile environments.
Figure 1 : Internal and External Stresses in Member without Eccentricity
Figure 2 : Internal and External Stresses in Member with Eccentricity
3. What are the two kind of prestressing? Explain.
The normal method for applying prestress force to a concrete member is through the use of steel tendons. There are two basic methods of arriving at the final prestressed member ; Pretensioning, and Post-tensioning.
i. Pretensioning
Pretensioning can be defined as a method of prestressing concrete in which the tendons are tensioned before the concrete is placed. This operation, which may be performed in a casting yard, is basically a five-step process:
i.
The tendons are placed in a prescribed pattern on the casting bed between two anchorages. The tendons are then tensioned to a value not to exceed 94% of the specified yield strength, but not greater than the lesser of 80% of the specified tensile strength of the tendons and the maximum value recommended by the manufacturer of the prestressing tendons or anchorages.
ii.
If the concrete forms are not already in place, they may then be assembled around the tendons.
iii.
The concrete is then placed in the forms and allowed to cure. Proper quality control must be exercised, and curing may accelerated with use of steam or other methods. The concrete will bond to the tendons.
iv.
When the concrete attains a prescribed strength, normally within 24 hours or less, the tendons are cut at their anchorages. Since the tendons are now bonded to the concrete, as they are cut from their anchorages the high prestress force must be transferred to the concrete. As the high tensile force of the tendon creates a compressive force on the concrete section, the concrete will tend to shorten slightly. The stresses that exits once the tendons have been cut are often called the stresses at transfer. Since there is no external load at this stage, the stresses at transfer include only those due to prestressing forces and those due to the weight of the member.
v.
The prestressed member is then removed from the forms and moved to a storage area so that casting bed can be prepared for further use.
Pretensioning members are usually manufactured at a casting yard or plant that is some what removed from the job site where the members will eventually be used. In this case, they are usually delivered to the job site ready to be set in place. Sometimes, a casting yard may be built on the job site to decrease transportation costs.
Figure 3 : Pretensioned Member
ii.
Post-tensioning
May be defined as a method of prestressing concrete in which the tendons are tensioned after the concrete has cured. The operation is commonly a six-step process:
i.
Concrete forms are assembled with flexible tubes placed in the forms and held at specified locations.
ii.
Concrete is then placed in the forms and allowed to cure to a prescribed strength.
iii.
Tendons are placed in the tubes. In some systems, a complete tendon assembly is placed in the forms prior to the placing of concrete.
iv.
The tendons are tensioned by jacking against an anchorage device or end plate that, in some cases, has been previously embedded in the end of the member. The anchorage device will incorporate some method for gripping the tendon and holding the load.
v.
If the tendons are to be bonded, the space in the tubes around the tendons may be grouted using a pumped grout. Some members use unbounded tendons.
vi.
The end anchorages may be covered with a protective coating.
Although post-tensioning is sometimes performed in a plant away from the project, it is most often done at the job site, particularly for units too large to be shipped assembled or for unusual application (Figure 4).
Figure 4 : Post-tensioned Member
4. What are then important benefits of prestressed concrete.
i. Smaller Section Sizes Since PSC uses the whole concrete section, the second moment of area is bigger and so the section is stiffer:
ii. Smaller Deflections The larger second moment of area greatly reduces deflections for a given section size.
iii. Increased Spans The smaller section size reduces self weight. Hence a given section can span further with prestressed concrete than it can with ordinary reinforced concrete.
iv. Durability Since the entire section remains in compression, no cracking of the concrete can occur and hence there is little penetration of the cover. This greatly improves the long-term durability of structures, especially bridges and also means that concrete tanks can be made as watertight as steel tanks, with far greater durability.
5. List the reasons for losses in prestressing force transfer.
Losses in Pre Tensioning During the process of anchoring, the stressed tendon tends to slip before the full grip is established, thus losing some of its imposed strain or in other words, induced stress. This is known as loss due to anchorage draw -in. From the time the tendons are anchored until transfer of prestressing force to the concrete, the tendons are held between prestressing force to the concrete, the tendons are held between the two abutments at a constant length. The stretched tendon during this time interval will lose some of its induced stress due to the phenomenon known as relaxation of steel. As soon as the tendons are cut, the stretched tendons tend to go back to their original state, but are prevented from doing so by the interfacial bond developed between the concrete and the tendons. The concrete will therefore be subjected to a compressive force, which results in an instantaneous shortening of the member. Since the tendons are bonded to the concrete, they will lose an equal amount of deformation, meaning a reduction of induced stress. This is known as loss due to elastic deformation. Subsequent to the transfer of prestress, concrete keeps on shrinking due to the loss of free water and continues shortening under sustained stress, thus resulting in a loss of tension in the embedded tendon. These are known as loss due to shrinkage and loss due to creep respectively. Also, loss due to relaxation of steel continues.
Losses in Post-Tensioning Friction loss Creep of the concrete The tendons are contained inside ducts, and the hydraulic jack is held directly against the member. During stressing operation, the tendons tend to get straightened and slide against
the duct, thus resulting in the development of a frictional resistance. As a result, the stress in the tendon at a distance away from the jacking end will be smaller than that indicated by the pressure gauge mounted on the jack. This is known as loss due to friction. With regard to elastic shortening, there will be no loss of the prestress gauge records the applied stress after the shortening has taken place. However, if they are tensioned one after another in sequence, all tendons, except the last one, will lose stress due to elastic shortening of concrete caused by forces in the subsequent tendons. Once the stressed tendons are anchored, the time-dependent losses caused by shrinkage and creep of concrete and relaxation of steel begin.
