Concrete, as a heterogeneous materials, is robust in compression however crumbles or cracks when the tensile stresses in it exceed 8 to 14 % of its compressive energy. Subsequently, whenever you use concrete in flexural members, one of many greatest drawbacks that you simply come throughout is that your complete part’s capability just isn’t totally utilized and solely the energy of the half in stress governs how lengthy the member will stay intact.
How about augmenting the efficiency of a concrete member by using its full capability throughout its depth? One technique of doing so goes for pretensioned and post-tensioned (prestressed) concrete.
What is supposed by Pre-stressing?
Prestressing is a way that artificially induces compressive stresses in a concrete member by stressing the prestressing metal in it in order that when the member truly will get loaded, the a part of it in stress doesn’t crack too early and ultimately fail. The low tensile energy of concrete is the largest disadvantage and it results in undeterred cracking within the member.
To be able to prestress a concrete member, ideally an eccentric tensile power is utilized to the prestressing metal strands. When the power is launched, it imparts compressive stresses into the member, particularly within the half that’s anticipated to develop tensile stresses upon service loading. This prestressing power prevents cracks from creating by eliminating or significantly decreasing the tensile stresses on the crucial midspan and help sections.
As a consequence, the bending, shear, and torsional capability of the concrete member will be augmented. This allows the member to behave elastically and its full capability can successfully be utilized throughout your complete member depth.
The extent to which a pre-stressing power is utilized depends upon numerous components together with the kind of structural system, the span size of the member and the required diploma of slenderness of the concrete member.
Strategies of Pre-stressing
Pre-stressing can primarily be achieved by two strategies particularly pre-tensioning and post-tensioning.
- Pre-tensioning, because the identify signifies, refers to tensioning of the prestressing reinforcement earlier than the concrete is solid into the formwork. In different phrases, on this technique, the metal strands or any type of prestressing reinforcement is positioned contained in the formwork, ideally at an eccentricity to the centroidal axis of the member and stretched to the required stress degree and anchored. The concrete is then solid into the formwork, completed and cured. As soon as the concrete will get cured, the prestress power is launch by reducing the strands and the power is transferred as a compressive power into the member.
- Alternatively, in case of post-tensioning, the metal strands or prestressing reinforcement is positioned inside small ducts. The ducts or sheathings are positioned at appropriate locations contained in the member earlier than casting. The concrete is solid into the encircling formwork and cured. As soon as the concrete will get hardened, the tendons are stretched to the required degree and the prestressing power is transferred to the concrete members solely by bearing on the ends of the member.
The metal reinforcement for prestressing will be within the type of both single wires, strands comprising a number of wires twisted to type a single ingredient, or excessive energy metal bars. The commonest varieties of reinforcement utilized in the US are as follows;
- Uncoated stress-relieved or low-relaxation wires
- Uncoated stress-relieved strands and low-relaxation strands
- Uncoated high-strength metal bars
The time period stress-relieved signifies that they’re designed to reduce the chance of creating inside stresses and they’re ready by heating high-strength metal strands to a really excessive temperature, round 1000 levels Celsius adopted by fast cooling.
By low-relaxation it’s meant that these strands will exhibit a decrease charge of leisure over time and are due to this fact, well-suited for long-term pre-tensioning and post-tensioning purposes. To fabricate these strands, the high-quality metal is often coated with a layer of grease or wax that helps cut back friction. Their manufacturing course of entails warmth therapy and chemical composition management.
In case of prestressed concrete members, defending the prestressed reinforcement from the assault of corrosion is extra crucial as a result of the energy of such member has a direct bearing on the prestressing power, which in flip relies upon upon the tendon space accessible.
What corrosion does is that it produces a powder referred to as rust and this reduces the accessible cross-section and finally the second energy of the prestressed member. This could result in a untimely failure of the structural system. Subsequently, defending the prestressed reinforcement performs a pivotal position in structural integrity and sturdiness.
In case of pretensioned members, the concrete that surrounds the prestressing reinforcement suffices to guard it from corrosion offered sufficient concrete cowl is ensured. Nonetheless, in case of post-tensioned concrete members, the safety towards corrosion will be obtained by totally grouting the within of the duct the place strands are positioned and tensioned.
Pre-stressing Tools and Supplies
In case of pre-tensioning, the pre-stressing metal is tensioned towards impartial anchorages earlier than pouring of the concrete. The anchorages are supported by giant bulkheads that help the very excessive localized forces on the tendon ends.
The pre-tensioning mattress is used to solid a number of such pre-tensioned members and so they can have completely different tendon profiles. The tendon profile refers back to the form or method wherein a pre-stressing tendon is positioned inside a concrete member.
In case of post-tensioning, you will want an anchorage meeting that contains all of the elements required to switch the pre-stressing power from the strand to the concrete member.
It’s to be saved in thoughts that the pre-stressing reinforcement in case of a prestressed concrete member is positioned along with the traditional reinforcement. It’s not like that you’ll omit all reinforcements in a member and change them with prestressed reinforcement. Nonetheless, pre-stressing is a complicated sort of reinforcement within the concrete that augments its properties and makes it use definitely worth the effort required.
How is Pre-Tensioning of Strands Carried Out?
The pre-tensioning of strands is carried out by first fabricating the strand, be it a seven-wire strand or no matter strand sort and size you need to obtain as per your mission specs. The strands are then mounted to the anchorage. The time period anchorage means a help that homes the strands and allows them to be stretched to the required stress degree. The strand profile for pre-tensioned member is normally a straight line at an eccentricity from the centroidal axis of the member.
The anchorage is normally manufactured from high-strength metal and it helps switch the pre-tensioning power from the strands to the concrete. As soon as the strands are anchored, they’re tensioned or stretched to the required degree of power.
As soon as the strands are tensioned, they’re jacked or locked within the stretched place. The concrete is then solid across the strands within the formwork until it will get hardened. It’s then cured for the required length until it beneficial properties the focused compressive energy.
As soon as the concrete will get cured for the required length, the strands are launched by reducing them from each the ends. Consequently, the pre-stressing power is transferred to the concrete member. The ends of the member are then sealed to stop the corrosion of strands.
For pre-tensioned members, we normally have a casting mattress the place numerous members of the identical dimensions are solid and stretched on the similar time. These models are then offered as precast, pre-tensioned concrete members. Nonetheless, you may additionally customise the formwork and put together members with completely different geometry as required.
How is Publish-Tensioning of Strands Carried Out?
The post-tensioning of strands is normally carried out on the location the place a post-tensioned member is to be put in. Step one is to put in the ducts which might be to include the metal strands. The tendon profile in case of post-tensioned members is normally not a straight line, it may be harped or draped, relying upon the variation of transverse loading on the member.
The ducts are normally made up of plastic or metallic and primarily based on the tendon profile, the ducts are put in in the identical profile. The concrete is then solid across the ducts to accumulate the specified form. As soon as the concrete hardens, it’s cured and after curing it for the required length, the strands are handed by the ducts.
The ends of the strands are anchored to specially-designed anchoring gadgets positioned on the ends of the ducts. The anchorage gadgets usually comprise wedges and bearing plates that switch the tensioning power from the strands to the member ends by bearing.
The strands are stretched to the required stress degree utilizing hydraulic jacks. Following this, the ducts are grouted to bond them with the concrete member and in addition to stop the corrosion of strands.
In case of prestressed concrete members, the profile of the tendons can fluctuate alongside the member size. In different phrases, the eccentricity of the strand with respect to the centroidal axis of the member can fluctuate.
The rationale for such a variation in profile is as a result of for simply-supported finish circumstances, the utmost bending second or stresses are anticipated to be developed on the mid span, due to this fact, we would like the utmost eccentricity of the pre-stressing power to be offered on the mid-span.
Nonetheless, away from this crucial part (mid-span), there’s primarily no want to keep up the identical eccentricity all through as a result of this will result in very excessive compressive stresses on the member ends. Therefore, as we transfer in the direction of the member helps, this eccentricity is diminished.
Subsequently, the profile of the tendon just isn’t a straight line and will be both draped profile or harped profile.
In case of harped profile, the mid-span has the utmost tendon eccentricity and away from it, the variation is liner in the direction of the help sections. In different phrases, the tendons have a V-shaped profile alongside the size of the member.
Alternatively, in case of draped tendon profile, the variation of eccentricity from mid-span to the helps is parabolic and never linear. Subsequently, the profile resembles a curve within the sagging course for beams.
Losses in Prestress
The preliminary pre-stressing power to which the metal tendons or strands are subjected to experiences progressive loss in prestress over a interval of round 5 years. The partial lack of prestress will be owing to numerous causes, a few of which trigger instantaneous losses and different trigger time-dependent losses within the power.
You will need to decide the losses in every stage from switch of pre-stressing power to varied different levels together with the applying of service load and on the final stage. As an approximation, 25 to 30 % of the pre-stressing power is misplaced on account of numerous causes that we will elaborate as we go alongside within the dialogue.
The instantaneous losses happen in the course of the building course of whereas the time-dependent losses happen as soon as the member is subjected to service masses and might proceed as much as 5 years after which the efficient prestress within the strand or tendons turns into primarily fixed.
Following the losses in pre-stressing power that happen and have to be accounted for in calculations in order that the member is ultimately designed as per the efficient pre-stressing power and nit on the premise of preliminary power to which it’s stretched on the time of tensioning.
Prestress Loss on account of Elastic Shortening of Concrete
Elastic shortening in concrete within the longitudinal course causes prestress losses within the metal strands in case of pre-tensioned members solely. It’s because on this case the metal strands and concrete are bonded collectively and as soon as the strands are reduce, the prestress power is transferred to the concrete. When the tendons bonded to the adjoining concrete concurrently shorten, they lose part of the pre-stressing power.
Elastic shortening loss doesn’t happen in post-tensioned concrete members. It’s because in post-tensioned members, the strands are usually not immediately bonded to the adjoining concrete; fairly they’re current in sheathing or ducts contained in the hardened concrete. Nonetheless, that is solely true when all of the tendons are concurrently jacked.
If, then again, the tendons are jacked sequentially and never all at a time, elastic shortening of concrete causes loss in pre-stressing power within the tendons.
Anchorage Seating Loss
Anchorage seating loss solely happens in case of post-tensioned concrete members. Owing to seating of the wedges within the anchors when the jacking power is transferred to the anchorage, the sort of loss in prestress takes place.
This loss can even happen within the casting beds in case of pre-tensioned members when the power is transferred to those beds.
The worth of slip on account of anchorage loss is mostly taken between 0.25 in. to 0.375 in. for two-piece wedges. The loss in prestress on account of anchorage seating turns into vital in case of short-span members.
In case of post-tensioned members, friction between the tendons and the encircling concrete ducts reduces the pre-stressing power. The magnitude of frictional loss relies upon upon the tendon alignment and in addition on the native deviations within the alignment.
The utmost frictional losses happen on the far finish or excessive of a member if the member is jacked at one finish. The variation of frictional losses is linear from the top the member is jacked to the opposite excessive and in between these extremes, the magnitude of the loss will be linearly interpolated.
Metal Rest Losses
The tendons endure loss within the pre-stressing power on account of fixed elongation with time. The comfort of strands is a time-dependent loss and depends upon the length of the sustained pre-stressing power and in addition on the preliminary pre-stressing power as a share of yield stress of the metal strand.
Prestress Loss on account of Creep in Concrete
Creep in concrete takes place beneath sustained masses and it’s experimentally discovered that the creep pressure in concrete leads to some lack of pre-stressing power. It’s a time-dependent loss and relies upon upon the stress degree at a specific loading stage.
Prestress Loss on account of Shrinkage in Concrete
Identical to creep, concrete shrinkage with time is inevitable and in case of prestressed members, it leads to the lack of pre-stressing power within the strands. Shrinkage in concrete depends upon numerous components together with combination proportions, mixture sort, cement sort, curing time, time between the top of curing and the applying of pre-stressing power, member dimension, and environmental circumstances.
Round 80 % of shrinkage in concrete takes place within the first yr when a construction is introduced into serviceability. Nonetheless, each creep and shrinkage losses contribute a terrific deal in cut back the prestress power.
When all of the above losses are accounted for, the preliminary pre-stressing power reduces to what we name efficient pre-stressing power.
Parameters Affecting Concrete High quality
The 2 most vital components in pre-tensioned concrete (or prestressed concrete, generally) are energy and endurance. Lengthy-term detrimental results are anticipated to calm down the prestressed strands and calm down them. Subsequently, strict high quality management measures have to be adhered to at numerous levels of producing and set up.
For pre-tensioned concrete, excessive energy concrete is a should requisite similar to excessive energy metal strands are. That is to ensure the 2 supplies depict composite motion and present due compatibility towards the utilized loading.
If unusual concrete (having some nominal worth of compressive energy, say 3000 psi) is coupled with high-strength metal strands, the switch of very excessive compressive stresses as soon as the strands are launch will crush the concrete on the ends. Conversely, if nominal energy of metal, say grade 280 or 420, is used, the strands will yield earlier than reaching to the required prestress worth.
It’s due to this fact, vital to make use of each high-strength metal and high-strength concrete in prestressed members. On this regard, the compressive energy of concrete ought to ideally be greater than 6000 psi at 28 days.
As well as, owing to excessive creep and shrinkage losses in concrete, the efficient pre-stressing can solely be achieved through the use of very high-strengths steels having a tensile energy of better than or equal to 270,000 psi (1862 MPa). These will assist counterbalance the prestress losses within the surrounding concrete.
Purposes of Pre-tensioned and Publish-Tensioned Concrete
Aside from financial causes, there is no such thing as a such concrete utility the place prestressed concrete can’t be used. For causes of excessive energy, sturdiness, and flexibility, the usage of each pre-tensioned and post-tensioned concrete is rising. Following are a few of its quite common purposes;
- Pre-tensioned and post-tensioned concrete members are utilized in bridges for largely the superstructure however can be used within the substructure in some instances. The reason being the power of such concrete to face up to heavy masses and conveniently span lengthy distances.
- Prestressed concrete can be utilized in business, industrial and high-rise buildings due to its energy and sturdiness.
- It can be utilized in transportation infrastructure comparable to highways, tunnels, and so forth. due to causes of energy.
- Prestressed concrete pipes can be utilized in sewerage techniques and excellently carry out their operate. When embedded beneath the bottom, these pipes provide essentially the most resistance to heavy superimposed loading on the pure floor above them.
- Prestressed concrete can be used within the building of sports activities amenities comparable to stadiums, arenas, and so forth.
Benefits of Pre-tensioned and Publish-Tensioned Concrete
Prestressed concrete provides a plethora of benefits for every kind of building works and these embody;
- The creation of everlasting prestresses earlier than the applying of service masses eliminates or significantly reduces the online tensile stresses on the high or backside.
- In case of pre-tensioned members, there’s a comparatively excessive managed restoration of cracking and deflection.
- Constructions which might be anticipated to expertise heavy vibrations from machines, and so forth. require resiliency and with the contribution coming from pre-stressing power, they will simply be made inflexible.
- With prestressed concrete, you’ll get a shallower depth part compared to a standard concrete member for a similar design specs (e.g., span and loading circumstances). To quantify the depth distinction, a prestressed member has a depth of round 65 to 80 % of the depth of an equal bolstered concrete member. Decreased depth means materials saving and extra accessible house. Nonetheless, this financial perspective will get blinded by the upper price of pre-stressing gear and supplies used on this concrete sort.
- Pre-tensioned concrete members end in long-term financial savings within the type of diminished upkeep prices, an extended working life, and lighter foundations due to comparatively much less weight of superstructure to be borne by them.
- Pre-tensioning and post-tensioning operations considerably cut back the cracking in concrete and allow the member to help better masses and span longer distances.
- Pre-tensioned and posttensioned concrete members are extra ductile than unusual bolstered concrete members. Be it flexure or compression, the efficiency and mechanical properties of the previous are method higher than the latter.
Drawbacks of Pre-tensioned and Publish-tensioned Concrete
Though pre-tensioned and post-tensioned concrete have taken the development trade to a complete new degree, their drawbacks additionally floor whenever you determine to go for them. On this regard, following are a few of the drawbacks that come flowing together with the usage of the sort of concrete;
- The pre-tensioning and post-tensioning operations require specialised gear and quite a lot of high quality management is to be required to make sure their excessive efficiency. As well as, these operations additionally demand extremely expert labor and this will incur added prices.
- The positioning and tensioning of metal tendons is a fancy process whether or not it’s carried in manufacturing facility, as in case of pre-tensioned members, or on the development web site as in case of post-tensioned members. The complexity provides on to the development prices.
- The sort of concrete provides restricted flexibility if the member is solid and the tendons are positioned and jacked. You can’t alter the design specs as soon as the strands have been stretched to the required stress degree. Notably speaking about pre-tensioned concrete members which might be solid in a manufacturing facility, there’s restricted flexibility accessible for a single building mission. It’s because for those who plan to decide pre-tensioned members, you’ll desire the identical dimension and member form to cut back the price of customizing one other formwork or mould within the manufacturing facility. Because of this all of the members may have the identical geometric specs for financial causes, forsaking lesser flexibility in customizing completely different designs for various members.
- The preliminary price of pre-tensioned and post-tensioned members is greater than their ordinarily bolstered counterparts. Nonetheless, if we weigh within the long-term advantages, this disadvantage typically subsides.
- These members require extra upkeep and inspection virtually on periodic foundation to make sure the tendons are in place and stay tensioned and that the concrete doesn’t present any indicators of injury or degradation. Because of this there’s extra price going into this area for prestressed concrete.
Pre-tensioned vs Publish-tensioned Concrete; Which One is Higher?
Each pre-tensioning and post-tensioning are superb strategies for enhancing the efficiency of a structural concrete member. Nonetheless, the selection between the 2 relies upon upon a number of components such because the design necessities, building strategies to be adopted, the utilized loading and span, and so forth.
On the whole, pre-tensioned members have a limitation on the span after which it’s virtually needed to modify to post-tensioned members. With pre-tensioned concrete, you’ll get greater preliminary energy and stiffness for the reason that strand are tensioned earlier than the concrete is solid. Nonetheless, the fabrication course of is carried out in a manufacturing facility and requires quite a lot of precision.
In case of post-tensioned members, you possibly can attain a really lengthy span not like pre-tensioned members. As well as, in addition they provide better flexibility in design and building for the reason that strands are tensioned after the concrete has been solid.
Ceaselessly Requested Questions (FAQs)
What’s the distinction between a strand and a tendon?
Within the terminology of pre-stressing gear, the time period strand is used to point high-strength metal wires which might be wound round a middle wire, similar to a typical seven-wire strand.
Alternatively, the phrase tendon is used to point a whole meeting of pre-stressing parts. It contains anchorages and couplers, pre-stressing strands, ducts and grout caps.
What are the varieties of tendons in post-tensioned members?
In post-tensioning, there will be two varieties of tendons. You may both have a bonded tendon or an unbonded tendon.
Bonded tendon signifies that you fill the duct with grout on the finish of the post-tensioning operation in order that it will get bonded to the concrete. On this case, the tendon is prevented from shifting relative to the concrete.
Unbonded tendon is one wherein the pre-stressing metal just isn’t bonded to the concrete and is fairly free to maneuver relative to it. On this case the power is transferred to the concrete solely by anchorages or deviators.
To what stress degree ought to the strands be tensioned each in case of pre-tensioning and post-tensioning?
The quantity of tensioning that’s to be achieved for a specific strand depends upon numerous components such because the design necessities, supplies used, and so forth. and in addition on whether or not you’re finishing up pre-tensioning or post-tensioning.
As a traditional follow, in case of pre-tensioned members, the strands are stretched to a stress degree that corresponds to 70 to 80 % of their final energy. Alternatively, for post-tensioned concrete members, the strands are tensioned to a stress degree that corresponds to 60 to 70 % of their final energy.
Tips on how to decide the quantity of prestress loss that has taken place in a specific tendon at a specific time?
For this goal, normally pressure gauges are mounted on the concrete members and equivalent to the pressure, the stress ranges are decided. These can then be in comparison with the preliminary prestress that was utilized to the strand.
Nonetheless, the loss can be computed theoretically at any explicit stage of the concrete member utilizing information from the design specs and formulae accessible within the codes of follow.
What’s an anchorage meeting in case of post-tensioned members?
In case of post-tensioned members, the anchorage meeting contains the next elements;
An anchor is a tool used for unbonded single strand tendons to accommodate the wedges and switch the pre-stressing power from the strand to the concrete.
It’s a flat plate that transfers the tendon power on to the concrete.
These are conduits used to accommodate the pre-stressing metal and will be grouted on the finish of the post-tensioning operation.
It’s also referred to as a trumpet and is a connection piece between the bearing plate and ducts.
It’s a machine that homes the wedges and transfers the prestressing power from the strand to the bearing plate.
Wedges are gadgets that maintain the strands and are all linked to the wedge plate. Usually, a two-piece wedge is used.
This gear is used for stressing the tendons and it contains a hydraulic jack, a hydraulic pump and calibrated pressure gauges connected to the jacking meeting.
The place to acquire pre-tensioned concrete parts?
If a pretensioned member is chosen from the producer’s checklist and isn’t to be custom-made, you possibly can procure it immediately from the manufacturing facility the place they’re manufactured. All corporations focusing on pretensioned concrete have some bult-in shapes that they solid in bulk portions at their manufacturing facility and dispatch to the development web site.
You may immediately contact the product manufacturing corporations and procure the required amount of pretensioned concrete members in your mission.