History of Pre-stressing
The art of pre-stressing concrete evolved over many decades and from many sources, but we can point to a few select instances in history that brought about this technology.
In the United States, engineer John Roebling established a factory in 1841 for making rope out of iron wire, which he initially sold to replace the hempen rope used for hoisting cars over the portage railway in central Pennsylvania. Later, Roebling used wire ropes as suspension cables for bridges, and he developed the technique for spinning the cables in place.
During the 19th century, low-cost production of iron and steel, when added to the invention of portland cement in 1824, led to the development of reinforced concrete. In 1867, Joseph Monier, a French gardener, patented a method of strengthening thin concrete flowerpots by embedding iron wire mesh into the concrete. Monier later applied his ideas to patents for buildings and bridges.
Swiss engineer Robert Maillart’s use of reinforced concrete, beginning in 1901, effected a revolution in structural art. Maillart, all of whose main bridges are located in Switzerland , was the first designer to break completely with the masonry tradition by putting concrete into forms technically appropriate to its properties – yet visually surprising. His radical use of reinforced concrete revolutionized masonry arch bridge design.
The idea of pre-stressing concrete was first applied by Eugene Freyssinet, a French structural and civil engineer, in 1928 as a method for overcoming concrete’s natural weakness in tension. Pre-stressed concrete can now be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete.
PRE-STRESSED CONCRETE
Pre stressed concrete, like reinforced concrete, is a composite material which uses to advantage the compressive strength of concrete, whilst circumventing its weakness in tension. Pre stressed concrete is made from structural concrete, usually of high strength, and high strength steel tendons which may or may not be grouped together. Prior to external loading the tendons are tensioned in one of two ways. With pretensioning the tendon are tensioned prior to the casting of the concrete and using post tensioning techniques the tendons are tensioned after the concrete has hardened. Some ordinary reinforcing steel is also often included both as subsidiary longitudinal reinforcement and as transverse stirrups to resist shear.
Pre-stressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Pre-stressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that offsets 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, inside poured concrete. The basic purpose of pre-stressing is to improve the performance of concrete members and this is achieved by inducing in the beam initial deformation and stresses which tend to counteract those produced by the service loads.
Since concrete is weak in tension in normal reinforced concrete construction cracks develop in the tension zone at working loads and therefore all concrete in tension is ignored in design.
Pre-stressing involves inducing compressive stresses in the zone, which will tend to become tensile under external loads. This compressive stress neutralizes the tensile stress so that no resultant tension exists, (or only very small values, within the tensile strength of the concrete). Cracking is therefore eliminated under working load and all of the concrete may be assumed effective in carrying load. Therefore lighter sections may be used to carry a given bending moment, and pre-stressed concrete may be used for longer span than reinforced concrete.
The pre-stressing force also reduces the magnitude of the principal tensile stress in the web so that thin-webbed I - sections may be used without the risk of diagonal tension failures and with further savings in self-weight.
The pre-stressing force has to be produced by a high tensile steel, and it is necessary to use high quality concrete to resist the higher compressive stresses that are developed. As the name itself suggests pre-stressing is the technique of stressing a structural member prior to loading to resist excessive tensile stresses.
The advantages of pre-stressed concrete as a construction material in multi storied frame can be listed as follows:
· Maximum utilization of provided section of the member.
· Provision of slender member for long span beams as compared to RCC.
· Use of high strength materials contribute to the durability of the structure.
· Pre-stresses concrete has considerable resilience and impact resistance.
· Proves to be economical only in long span beam-column frames compared to other materials.
· The intermediate distance between the columns can be in increased by using pre-stressed concrete as compared to reinforced cement concrete.
· Architectural design provisions and specifications can be achieved using pre-stressed concrete.
· Dead weight of concrete is reduced to a higher rate using pre-stressed concrete.
PRINCIPLE OF PRESTRESSING
The function of pre-stressing is to place the concrete structure under compression in those regions where load causes tensile stress. Tension caused by the load will first have to cancel the compression induced by the pre-stressing before it can crack the concrete.
Pre-stressing can be applied to concrete members in two ways, by pre-tensioning or post-tensioning. In pre-tensioned members the pre-stressing strands are tensioned against restraining bulkheads before the concrete is cast. After the concrete has been placed, allowed to harden and attain sufficient strength, the strands are released and their force is transferred to the concrete member. Pre-stressing by post-tensioning involves installing and stressing pre-stressing strand or bar tendons only after the concrete has been placed, hardened and attained a minimum compressive strength for that transfer.
METHODS AND SYSTEM OF PRE-STRESSING
There are two methods of pre-stressing concrete: -
1) Pre-cast Pre-tensioned
2) Pre-cast Post-tensioned
Both methods involve tensioning cables inside a concrete beam and then anchoring the stressed cables to the concrete.
Pre-cast Pre-tensioned: -
Pre-tensioning is a method of pre-stressing in which the steel tendons are tensioned before the casting of the member. In this method the tendons are tensioned using hydraulic jacks, which bear on strong abutments between which the moulds are placed. After the concrete attains full strength the tendons are released and the stress is transferred to the concrete by bond action.
Procedure of precast pre-tensioned concreting
Stage 1
Tendons and reinforcement are positioned in the beam mould.
Stage 2
Tendons are stressed to about 70% of their ultimate strength.
Tendons are stressed to about 70% of their ultimate strength.
Stage 3
Concrete is cast into the beam mould and allowed to cure to the required initial strength.
Concrete is cast into the beam mould and allowed to cure to the required initial strength.
Stage 4
When the concrete has cured the stressing force is released and the tendons anchor themselves in the concrete.
When the concrete has cured the stressing force is released and the tendons anchor themselves in the concrete.
Pre-cast Post-tensioned: -
Post-tensioning is a method of pre-stressing in which the steel tendons are tensioned after the casting of the member. In this method ducts or sheaths are placed in the required profile in the mould and the tendons are passed through the ducts. After the concrete had attained sufficient strength the tendons are tensioned using hydraulic jacks which bear on the member itself. The stress is transferred to the concrete by bearing action of tendons which are anchored using suitable anchorages. Finally the ducts are grouted and the anchor plates concealed by cement mortar.
Procedure of precast post-tensioned concreting
Stage 1
Cable ducts and reinforcement are positioned in the beam mould. The ducts are usually raised towards the neutral axis at the ends to reduce the eccentricity of the stressing force.
Cable ducts and reinforcement are positioned in the beam mould. The ducts are usually raised towards the neutral axis at the ends to reduce the eccentricity of the stressing force.
Stage 2
Concrete is cast into the beam mould and allowed to cure to the required initial strength.
Concrete is cast into the beam mould and allowed to cure to the required initial strength.
Stage 3
Tendons are threaded through the cable ducts and tensioned to about 70% of their ultimate strength.
Tendons are threaded through the cable ducts and tensioned to about 70% of their ultimate strength.
Stage 4
Wedges are inserted into the end anchorages and the tensioning force on the tendons is released. Grout is then pumped into the ducts to protect the tendons.
TYPES OF POST TENSIONED CONCRETING METHODS
1. BONDED POST-TENSIONED CONCRETE METHOD
2. UNBONDED POST-TENSIONED CONCRETE METHOD
BONDED POST-TENSIONED CONCRETE
Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). The concrete is cast around a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element. A set of tendons are fished through the duct and the concrete is poured. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete member itself. When the tendons have stretched sufficiently, according to the design specifications, they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. The duct is then grouted to protect the tendons from corrosion. This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobe clay) create problems for the typical perimeter foundation. All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure. Post-stressing is also used in the construction of various bridges, both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the bridge. The advantages of this system over unbonded post-tensioning are.
1. Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.
2. Tendons can be easily 'weaved' allowing a more efficient design approach.
3. Higher ultimate strength due to bond generated between the strand and concrete.
4. No long term issues with maintaining the integrity of the anchor/dead end.
UNBONDED POST-TENSIONED CONCRETE
Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process. The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab. The main disadvantage over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if damaged (such as during repair on the slab). The advantages of this system over bonded post-tensioning are:
1. The ability to individually adjust cables based on poor field conditions (For example: shifting a group of 4 cables around an opening by placing 2 to either side).
2. The procedure of post-stress grouting is eliminated.
3. The ability to de-stress the tendons before attempting repair work.
Picture number one shows rolls of post-tensioning (PT) cables with the holding end anchors displayed. The holding end anchors are fastened to rebar placed above and below the cable and buried in the concrete locking that end. Pictures numbered two, three and four shows a series of black pulling end anchors from the rear along the floor edge form. Rebar is placed above and below the cable both in front and behind the face of the pulling end anchor. The above and below placement of the rebar can be seen in picture number three and the placement of the rebar in front and behind can be seen in picture number four. The blue cable seen in picture number four is electrical conduit. Picture number five shows the plastic sheathing stripped from the ends of the post-tensioning cables before placement through the pulling end anchors. Picture number six shows the post-tensioning cables in place for concrete pouring. The plastic sheathing has been removed from the end of the cable and the cable has been pushed through the black pulling end anchor attached to the inside of the concrete floor side form. The greased cable can be seen protruding from the concrete floor side form. Pictures seven and eight show the post-tensioning cables protruding from the poured concrete floor. After the concrete floor has been poured and has set for about a week, the cable ends will be pulled with a hydraulic jack, shown in picture number nine, until it is stretched to achieve the specified tension.
The advantages of post-tensioning compared to pre-tensioning in the multi storied frame construction can be listed as follows
a) Tendons can be provided in any desired profile.
b) Stage pre-stressing can be adopted conveniently.
c) Costly factory equipments are not required.
d) Cast-in-situ construction procedure can be conveniently adopted.
e) It is possible to fabricate a beam with pre-cast and cast-in-situ elements, which are post-tensioned together to form a single structural unit.
f) Number of systems is available in this method.
Systems of pre-stressing are as given below
· Hoyer system – usually adopted for pre-tensioned members.
The system listed below are adopted for post-tensioning
· Freyssinet system
· Magnel Balton system
· Gifford Udall system
· PSC monowire system
· CLL standard system
· Lee -Macall system
ADVANTAGES OF PRECAST CONCRETE ELEMENTS IN BUILDING CONSTRUCTION
· Lower construction cost
· Thinner slabs, which are especially important in high-rise buildings where floor thickness savings can translate into additional floors for the same or lower cost
· Fewer joints since the distance that can be spanned by post-tensioned slabs exceeds that of reinforced construction with the same thickness
· Longer span lengths increase the usable unencumbered floorspace in buildings and parking structures
· Fewer joints lead to lower maintenance costs over the design life of the structure, since joints are the major locus of weakness in concrete buildings.
· One-stop shopping sources much of a building's shell in one efficient, precast contract.
· Fabrication of precast elements during permitting and/or site preparation saves time resulting in fast efficient construction regardless of weather conditions.
· Designing precast systems is easier.
· Precast components can be erected in winter conditions, maintaining tight schedules.
· With total precast systems, speedy erection allows the contractor to enclose the building quickly, giving interior trades faster access.
· Precast components are naturally fire protected, because they will not burn. Precast's inherent fire resistance eliminates the messy and time-consuming fireproofing required for a steel structure and subsequent repairs caused by other trades.
LOSS OF PRESTRESS
When the tensioning force is released and the tendons are anchored to the concrete a series of effects result in a loss of stress in the tendons. The effects are :
· Relaxation of the steel tendons
· Elastic deformation of the concrete
· Shrinkage and creep of the concrete
· Slip or movement of the tendons at the anchorages during anchoring
· Other causes in special circumstances, such as when steam curing is used with pre-tensioning.
Total losses in pre-stress can amount to about 30% of the initial tensioning stress.
Freyssinet system is the most widely adopted system in the construction of pre-stressed concrete structures. Pre-stressed Concrete is an architectural and structural material possessing great strength. The unique characteristics of pre-stressed concrete allow predetermined, engineering stresses to be placed in members to counteract stresses that occur when the unit is subjected to service loads. This is accomplished by combining the best properties of two quality materials: high strength concrete for compression and high tensile strength steel strands for tension.
REASONS FOR USING PRESTRESSED CONCRETE
Column-Free Long Spans
With fewer columns and more usable floor space, precast, prestressed concrete provides greater freedom for space utilization.
Conserves Energy
Pre-stressed concrete components can improve the thermal storage potential of a building. It effectively conserves energy required for heating and cooling.
Maintenance Free
Precast concrete does not require painting and is free from corrosion. Its durability extends building life.
Resists Fire
Durability and fire resistance mean low insurance premiums and greater personnel safety. Those who investigate life cycle costing will appreciate the precast concrete's excellent fire resistance characteristics.
Rapid Construction
Precast concrete construction gets the job done sooner. The manufacturing of prestressed members and site preparation can proceed simultaneously. Early occupancy provides obvious benefits to the client.
Versatility of Design
Precast concrete buildings are not only functional but beautiful as well. Numerous panel configuration design possibilities are available.
Sustainability
As with all concrete wall systems, precast offers high durability and strength as well as thermal mass, which contributes to increased energy efficiency. Precast systems use locally derived materials, and can incorporate recycled supplementary cementitious materials like fly ash and slag cement, one of the key reasons why they are often used in sustainable or “green” buildings.
As with all concrete wall systems, precast offers high durability and strength as well as thermal mass, which contributes to increased energy efficiency. Precast systems use locally derived materials, and can incorporate recycled supplementary cementitious materials like fly ash and slag cement, one of the key reasons why they are often used in sustainable or “green” buildings.
Variety, Flexibility, Utility
One of the biggest benefits of precast systems is in their design: tight controls mean more efficient mix designs, resulting in smaller structural members and longer spans. Construction waste is reduced because the exact amount of necessary components is delivered to the site; any spare components can be recycled, and their materials used again in another structure. Precast systems can adopt almost any aesthetic, incorporating a variety of colours and textures, or emulating natural stone. By crafting systems that not only look great, but also act as structural walls and support floor loads, designers can reduce material redundancy—and project costs.
Quality in Manufacturing
Because components are precast at manufacturing facilities, quality control measures ensure that every piece is made to specifications. The components can be cast and transported to the job site while designs are still being finalized, helping to speed construction schedules. Evolutions in self-consolidating concrete also promise to offer new options and challenges for designers using precast.
Applications
Prestressed concrete is the predominating material for floors in high-rise buildings and concrete chambers in nuclear reactors, as well as in columns and shear walls in the buildings intended for a high degree of earthquake and blast protection.
Unbonded post-tensioning tendons are commonly used in parking garages as barrier cable. Also, due to its ability to be stressed and then de-stressed, it can be used to temporarily repair a damaged building by holding up a damaged wall or floor until permanent repairs can be made.
GENERAL PRECAUTIONS IN PRESTRESSED CONCRETING
Working platforms
To provide a safe working environment, working platforms need adequate working space, appropriate edge protection, and safe access and egress.
They must also be designed and constructed to safely support all expected loads, including impact loads. Factors that will determine the selection of an appropriate working platform include:
· The type and number of items of stressing equipment that may be in use,
· The number of people required, or likely to be, on the work platform at any one time, and the likely material storage on the platform.
The platform should be large enough to enable the operators to remain clear of anchorages during stressing, and it should be at a height which eliminates or minimises risk of injury from over-reaching or awkward postures.
Formwork
In the case of cast in-situ concrete constructions, working platforms may be built integrally with the formwork.
In the case of cast in-situ concrete constructions, working platforms may be built integrally with the formwork.
Formwork and propping systems should be designed by a structural engineer experienced in formwork design.
Assembled formwork and propping systems should be checked by a competent person for compliance with the formwork design drawings and documented proof of such on-site checks should be readily available.
Provisions for stressing
Anchorages for stressing should be set out and tendon spacing marked on the ends. Since the wedging forces at anchorages are high, anti-burst provisions, such as special reinforcement, need to be installed and secured into position.
Pre-stressing ducts should be laid in accordance with the specified profile and adequately secured. Inadequate cover to duct tubes can result in concrete blow-outs during grouting operations.
Pre-stressing safety considerations
Stressing operations and associated preparations for stressing involve a variety of tasks which if, appropriate precautions are taken, could endanger the health and safety of workers carrying them out and/or those in the vicinity.
· The area where preparations and stressing are intended to take place must be fully barricaded with solid panels and signage prominently posted to keep unauthorised personnel clear of this area.
· All personnel involved in the tasks should wear the personal protective equipment. This will generally include safety goggles, gloves, sturdy protective footwear and safety helmets.
· All personnel involved in the tasks should have adequate training in identifying the hazards of stressing and their associated risks.
Uncoiling, Cutting and Placing Strands
· Coils are very heavy, typically weighing 3 to 4 tonnes, and therefore the structural adequacy of the area where coils are to be placed must be verified. Manual handling issues associated with handling the coils should be controlled.
· Ensure that the coils are restrained with uncut restraining straps when placing them onto the strand frame. Uncontrolled release of the coil can result in whip-back with sufficiently high force to cause serious injury.
· Strands should not be cut by heat-type cutting equipment such as oxy-acetylene or LP gas torches, as this may compromise their load-bearing capacity under tension.
· When assembling tendons, thoroughly inspect each individual wire or strand for obvious flaws.
Pushing Strands into Ducts
· All strands for each of the tendons should be pushed into place in accordance with the drawings, making sure all personnel are kept clear of the direct line of ducting to prevent injury from strands exiting from the other end of the duct.
· Once the specified number of strands is in place, ensure that a "dead end" is created for each strand by securing them at the end of the element.
· In strand set-ups where ends will protrude above the face of the concrete element and may create a hazard, they should be boxed or barricaded to prevent injury.
Concrete Pour
Concreting may be placed with either pumps or kibbles.
Concreting may be placed with either pumps or kibbles.
When concreting is being pumped, "chairs" or other means to support concrete pump lines above the reinforcement and tendons should be in place and well secured.
Where kibbles are used to place concrete, avoid dropping concrete in one place as tendons could be displaced. Concrete should always be allowed to flow in a controlled manner.
During concrete pouring:
· Ensure that the ducts and strands are not damaged during the pour. All damage should be promptly notified to the contractor's supervisor for repair. However, concrete around the anchorages needs adequate vibration to ensure a safe and sound seating for the anchorage.
· Concrete test cylinders should be taken at agreed intervals for storing and curing on site under conditions similar to those applying to the element being poured.
Stressing operationsBefore stressing operations.
Prior to commencing stressing operations, the post-tensioning supervisor should verify that:
· Concrete around the anchorages has been examined. The principal contractor should be notified if the concrete is of poor quality.
· All concrete test cylinders have achieved the specified strength.
· The grips in the jacks on the stressing equipment are clean and free from dirt or grit and in good condition.
· The stressing equipment, i.e. jacks and their gauges, has appropriate service records and up-to-date calibration certificates. All jacks should have a durable tag securely attached which clearly shows the following information:
o Final stressing pressure
o Diameter and grade of the strand for which the jack is being used
o Jack number
o Corresponding gauge number
o Date calibration expires
· The operator of the stressing equipment has documented evidence of appropriate training.
· A "NO GO" area of at least 2 metres radius is in place around the anchorages at the dead and live ends, with barricades behind the line of jacks and "Stressing in Progress. Keep Clear" signage prominently displayed at all appropriate locations
During stressing operations
· Tendons should be stressed in order from the furthest to the closest reachable to ensure that no person is standing in direct line of the jack or previously stressed strands.
· Ensure stress is applied gradually and evenly to tendons.
· Ensure that the specified initial and final stressing levels are not exceeded.
After stressing operations
· Gain the design engineer's approval prior to cutting off excess tendons.
· Seal anchorage recesses following approval and prior to grouting the ducts.
· Do not perform tasks requiring impact, such as hammering, drilling or coring in the vicinity until the grouting of the ducts has been completed.
Grouting
Build-up of excessive pressure during grouting can result in "blow-outs" of the concrete, which could injure personnel in the vicinity.
Build-up of excessive pressure during grouting can result in "blow-outs" of the concrete, which could injure personnel in the vicinity.
To prevent blowouts
· Ducts should be blown through to ensure there are no blockages.
· Avoid non-continuous grouting to ensure no blockages or voids are in the tubes.
· Monitor the gauge of the equipment throughout grouting to ensure that excessive pressure does not develop.
· Retain barricades used during stressing operations and also barricade at a lower level if formwork has already been removed.
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