Concrete, as a heterogeneous materials, is powerful in compression however crumbles or cracks when the tensile stresses in it exceed 8 to 14 % of its compressive power. Due to this fact, while you use concrete in flexural members, one of many greatest drawbacks that you just come throughout is that all the part’s capability shouldn’t be totally utilized and solely the power of the half in rigidity 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 rigidity doesn’t crack too early and ultimately fail. The low tensile power of concrete is the largest disadvantage and it results in undeterred cracking within the member.
So as to prestress a concrete member, ideally an eccentric tensile drive is utilized to the prestressing metal strands. When the drive 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 drive prevents cracks from creating by eliminating or significantly lowering the tensile stresses on the crucial midspan and help sections.
As a consequence, the bending, shear, and torsional capability of the concrete member might be augmented. This permits the member to behave elastically and its full capability can successfully be utilized throughout all the member depth.
The extent to which a pre-stressing drive is utilized relies on plenty of 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 specifically pre-tensioning and post-tensioning.
- Pre-tensioning, because the identify signifies, refers to tensioning of the prestressing reinforcement earlier than the concrete is forged 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 forged into the formwork, completed and cured. As soon as the concrete will get cured, the prestress drive is launch by reducing the strands and the drive is transferred as a compressive drive into the member.
- However, 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 forged 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 drive is transferred to the concrete members solely by bearing on the ends of the member.
The metal reinforcement for prestressing might be within the type of both single wires, strands comprising a number of wires twisted to type a single ingredient, or excessive power metal bars. The most typical sorts of reinforcement utilized in the USA 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 attenuate the danger 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 fee of rest over time and are subsequently, well-suited for long-term pre-tensioning and post-tensioning purposes. To fabricate these strands, the high-quality metal is usually 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 power of such member has a direct bearing on the prestressing drive, which in flip relies upon upon the tendon space accessible.
What corrosion does is that it produces a powder known as rust and this reduces the accessible cross-section and in the end the second power of the prestressed member. This could result in a untimely failure of the structural system. Due to this fact, 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 ample concrete cowl is ensured. Nevertheless, in case of post-tensioned concrete members, the safety towards corrosion might be obtained by totally grouting the within of the duct the place strands are positioned and tensioned.
Pre-stressing Gear and Supplies
In case of pre-tensioning, the pre-stressing metal is tensioned towards unbiased anchorages earlier than pouring of the concrete. The anchorages are supported by massive bulkheads that help the very excessive localized forces on the tendon ends.
The pre-tensioning mattress is used to forged a number of such pre-tensioned members they usually 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 have an anchorage meeting that contains all of the parts required to switch the pre-stressing drive 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 standard reinforcement. It’s not like that you’ll omit all reinforcements in a member and substitute them with prestressed reinforcement. Nevertheless, pre-stressing is a complicated kind of reinforcement within the concrete that augments its properties and makes it use well 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 kind and size you wish to obtain as per your challenge 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 often a straight line at an eccentricity from the centroidal axis of the member.
The anchorage is often made from high-strength metal and it helps switch the pre-tensioning drive from the strands to the concrete. As soon as the strands are anchored, they’re tensioned or stretched to the required degree of drive.
As soon as the strands are tensioned, they’re jacked or locked within the stretched place. The concrete is then forged across the strands within the formwork until it will get hardened. It’s then cured for the required length until it positive factors the focused compressive power.
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 drive 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 often have a casting mattress the place plenty of members of the identical dimensions are forged and stretched on the identical time. These items are then bought as precast, pre-tensioned concrete members. Nevertheless, you might also customise the formwork and put together members with completely different geometry as required.
How is Submit-Tensioning of Strands Carried Out?
The post-tensioning of strands is often 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 can be to comprise the metal strands. The tendon profile in case of post-tensioned members is often not a straight line, it may be harped or draped, relying upon the variation of transverse loading on the member.
The ducts are often made up of plastic or steel and based mostly on the tendon profile, the ducts are put in in the identical profile. The concrete is then forged across the ducts to amass 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 way of the ducts.
The ends of the strands are anchored to specially-designed anchoring gadgets positioned on the ends of the ducts. The anchorage gadgets sometimes comprise wedges and bearing plates that switch the tensioning drive 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, subsequently, we would like the utmost eccentricity of the pre-stressing drive to be offered on the mid-span.
Nevertheless, 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 could 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.
Due to this fact, the profile of the tendon shouldn’t be a straight line and might 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.
However, in case of draped tendon profile, the variation of eccentricity from mid-span to the helps is parabolic and never linear. Due to this fact, the profile resembles a curve within the sagging course for beams.
Losses in Prestress
The preliminary pre-stressing drive 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 might be owing to plenty of causes, a few of which trigger instantaneous losses and different trigger time-dependent losses within the drive.
You will need to decide the losses in every stage from switch of pre-stressing drive to numerous different phases together with the appliance of service load and on the final stage. As an approximation, 25 to 30 % of the pre-stressing drive is misplaced because of numerous causes that we will elaborate as we go alongside within the dialogue.
The instantaneous losses happen throughout the development 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 drive that happen and should be accounted for in calculations in order that the member is ultimately designed as per the efficient pre-stressing drive and nit on the premise of preliminary drive to which it’s stretched on the time of tensioning.
Prestress Loss because 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 is because on this case the metal strands and concrete are bonded collectively and as soon as the strands are minimize, the prestress drive is transferred to the concrete. When the tendons bonded to the adjoining concrete concurrently shorten, they lose part of the pre-stressing drive.
Elastic shortening loss doesn’t happen in post-tensioned concrete members. It is because in post-tensioned members, the strands usually are not instantly bonded to the adjoining concrete; somewhat they’re current in sheathing or ducts contained in the hardened concrete. Nevertheless, that is solely true when all of the tendons are concurrently jacked.
If, however, the tendons are jacked sequentially and never all at a time, elastic shortening of concrete causes loss in pre-stressing drive 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 drive is transferred to the anchorage, such a loss in prestress takes place.
This loss may happen within the casting beds in case of pre-tensioned members when the drive is transferred to those beds.
The worth of slip because of anchorage loss is usually taken between 0.25 in. to 0.375 in. for two-piece wedges. The loss in prestress because 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 drive. 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 might be linearly interpolated.
Metal Leisure Losses
The tendons endure loss within the pre-stressing drive because of fixed elongation with time. The comfort of strands is a time-dependent loss and relies on the length of the sustained pre-stressing drive and in addition on the preliminary pre-stressing drive as a proportion of yield stress of the metal strand.
Prestress Loss because of Creep in Concrete
Creep in concrete takes place underneath sustained masses and it’s experimentally discovered that the creep pressure in concrete leads to some lack of pre-stressing drive. It’s a time-dependent loss and relies upon upon the stress degree at a specific loading stage.
Prestress Loss because of Shrinkage in Concrete
Similar to creep, concrete shrinkage with time is inevitable and in case of prestressed members, it leads to the lack of pre-stressing drive within the strands. Shrinkage in concrete relies on plenty of components together with combination proportions, mixture kind, cement kind, curing time, time between the top of curing and the appliance of pre-stressing drive, member dimension, and environmental circumstances.
Round 80 % of shrinkage in concrete takes place within the first yr when a construction is introduced into serviceability. Nevertheless, each creep and shrinkage losses contribute an awesome deal in cut back the prestress drive.
When all of the above losses are accounted for, the preliminary pre-stressing drive reduces to what we name efficient pre-stressing drive.
Parameters Affecting Concrete High quality
The 2 most necessary components in pre-tensioned concrete (or prestressed concrete, generally) are power and endurance. Lengthy-term detrimental results are anticipated to loosen up the prestressed strands and loosen up them. Due to this fact, strict high quality management measures should be adhered to at numerous phases of producing and set up.
For pre-tensioned concrete, excessive power concrete is a should requisite identical to excessive power metal strands are. That is to ensure the 2 supplies depict composite motion and present due compatibility towards the utilized loading.
If odd concrete (having some nominal worth of compressive power, 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 power of metal, say grade 280 or 420, is used, the strands will yield earlier than reaching to the required prestress worth.
It’s subsequently, necessary to make use of each high-strength metal and high-strength concrete in prestressed members. On this regard, the compressive power 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 power 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 Submit-Tensioned Concrete
Aside from financial causes, there isn’t a such concrete software the place prestressed concrete can’t be used. For causes of excessive power, sturdiness, and flexibility, using 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 principally the superstructure however can be used within the substructure in some instances. The reason being the flexibility of such concrete to face up to heavy masses and conveniently span lengthy distances.
- Prestressed concrete can be utilized in industrial, industrial and high-rise buildings due to its power and sturdiness.
- It can be utilized in transportation infrastructure reminiscent of highways, tunnels, and so forth. due to causes of power.
- Prestressed concrete pipes can be utilized in sewerage programs and excellently carry out their operate. When embedded underneath the bottom, these pipes provide probably the most resistance to heavy superimposed loading on the pure floor above them.
- Prestressed concrete can be used within the development of sports activities amenities reminiscent of stadiums, arenas, and so forth.
Benefits of Pre-tensioned and Submit-Tensioned Concrete
Prestressed concrete presents a plethora of benefits for every kind of development works and these embrace;
- The creation of everlasting prestresses earlier than the appliance of service masses eliminates or significantly reduces the online tensile stresses on the prime or backside.
- In case of pre-tensioned members, there’s a comparatively excessive managed restoration of cracking and deflection.
- Buildings which can be anticipated to expertise heavy vibrations from machines, and so forth. require resiliency and with the contribution coming from pre-stressing drive, they’ll simply be made inflexible.
- With prestressed concrete, you’re going to 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. Diminished depth means materials saving and extra accessible house. Nevertheless, this financial perspective will get blinded by the upper value of pre-stressing gear and supplies used on this concrete kind.
- 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 odd 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 Submit-tensioned Concrete
Though pre-tensioned and post-tensioned concrete have taken the development business to an entire new degree, their drawbacks additionally floor while you determine to go for them. On this regard, following are a few of the drawbacks that come flowing together with using such a concrete;
- The pre-tensioning and post-tensioning operations require specialised gear and an excessive amount 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 could incur added prices.
- The positioning and tensioning of metal tendons is a fancy process whether or not it’s carried in manufacturing unit, 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.
- Any such concrete presents restricted flexibility if the member is forged and the tendons are positioned and jacked. You can not alter the design specs as soon as the strands have been stretched to the required stress degree. Significantly speaking about pre-tensioned concrete members which can be forged in a manufacturing unit, there’s restricted flexibility accessible for a single development challenge. It is because should you plan to decide pre-tensioned members, you’ll want the identical dimension and member form to scale back the price of customizing one other formwork or mould within the manufacturing unit. Which means that all of the members may have the identical geometric specs for financial causes, abandoning lesser flexibility in customizing completely different designs for various members.
- The preliminary value of pre-tensioned and post-tensioned members is greater than their ordinarily bolstered counterparts. Nevertheless, if we weigh within the long-term advantages, this disadvantage typically subsides.
- These members require extra upkeep and inspection nearly on periodic foundation to make sure the tendons are in place and stay tensioned and that the concrete doesn’t present any indicators of harm or degradation. Which means that there’s extra value going into this area for prestressed concrete.
Pre-tensioned vs Submit-tensioned Concrete; Which One is Higher?
Each pre-tensioning and post-tensioning are wonderful methods for enhancing the efficiency of a structural concrete member. Nevertheless, the selection between the 2 relies upon upon a number of components such because the design necessities, development strategies to be adopted, the utilized loading and span, and so forth.
Basically, pre-tensioned members have a limitation on the span after which it’s nearly vital to modify to post-tensioned members. With pre-tensioned concrete, you’re going to get greater preliminary power and stiffness for the reason that strand are tensioned earlier than the concrete is forged. Nevertheless, the fabrication course of is carried out in a manufacturing unit and requires an excessive amount of precision.
In case of post-tensioned members, you possibly can attain a really lengthy span not like pre-tensioned members. As well as, additionally they provide better flexibility in design and development for the reason that strands are tensioned after the concrete has been forged.
Regularly 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 can be wound round a middle wire, identical to a typical seven-wire strand.
However, the phrase tendon is used to point an entire meeting of pre-stressing components. It contains anchorages and couplers, pre-stressing strands, ducts and grout caps.
What are the sorts of tendons in post-tensioned members?
In post-tensioning, there might be two sorts 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 shouldn’t be bonded to the concrete and is somewhat free to maneuver relative to it. On this case the drive 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 executed for a specific strand relies on 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 power. However, for post-tensioned concrete members, the strands are tensioned to a stress degree that corresponds to 60 to 70 % of their final power.
The way to decide the quantity of prestress loss that has taken place in a specific tendon at a specific time?
For this function, often 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.
Nevertheless, the loss can be computed theoretically at any specific 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 parts;
An anchor is a tool used for unbonded single strand tendons to accommodate the wedges and switch the pre-stressing drive from the strand to the concrete.
It’s a flat plate that transfers the tendon drive on to the concrete.
These are conduits used to accommodate the pre-stressing metal and might be grouted on the finish of the post-tensioning operation.
It is usually known 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 drive from the strand to the bearing plate.
Wedges are gadgets that maintain the strands and are all related 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 hooked up to the jacking meeting.
The place to acquire pre-tensioned concrete components?
If a pretensioned member is chosen from the producer’s listing and isn’t to be personalized, you possibly can procure it instantly from the manufacturing unit the place they’re manufactured. All firms focusing on pretensioned concrete have some bult-in shapes that they forged in bulk portions at their manufacturing unit and dispatch to the development web site.
You may instantly contact the product manufacturing firms and procure the required amount of pretensioned concrete members to your challenge.