Concrete, as a heterogeneous material, is strong in compression but crumbles or cracks when the tensile stresses in it exceed 8 to 14 percent of its compressive strength. Therefore, when you use concrete in flexural members, one of the biggest drawbacks that you come across is that the entire section’s capacity is not fully utilized and only the strength of the part in tension governs how long the member will remain intact.
How about augmenting the performance of a concrete member by utilizing its full capacity across its depth? One method of doing so is going for pretensioned and post-tensioned (prestressed) concrete.
Table of Contents
What is meant by Pre-stressing?
Prestressing is a technique that artificially induces compressive stresses in a concrete member by stressing the prestressing steel in it so that when the member actually gets loaded, the part of it in tension does not crack too early and eventually fail. The low tensile strength of concrete is the biggest drawback and it leads to undeterred cracking in the member.
In order to prestress a concrete member, preferably an eccentric tensile force is applied to the prestressing steel strands. When the force is released, it imparts compressive stresses into the member, especially in the part that is expected to develop tensile stresses upon service loading. This prestressing force prevents cracks from developing by eliminating or considerably reducing the tensile stresses at the critical midspan and support sections.
As a consequence, the bending, shear, and torsional capacity of the concrete member can be augmented. This enables the member to behave elastically and its full capacity can effectively be utilized across the entire member depth.
The extent to which a pre-stressing force is applied depends on a number of factors including the type of structural system, the span length of the member and the required degree of slenderness of the concrete member.
Methods of Pre-stressing
Pre-stressing can essentially be achieved by two methods namely pre-tensioning and post-tensioning.
- Pre-tensioning, as the name indicates, refers to tensioning of the prestressing reinforcement before the concrete is cast into the formwork. In other words, in this method, the steel strands or any form of prestressing reinforcement is placed inside the formwork, preferably at an eccentricity to the centroidal axis of the member and stretched to the required stress level and anchored. The concrete is then cast into the formwork, finished and cured. Once the concrete gets cured, the prestress force is release by cutting the strands and the force is transferred as a compressive force into the member.
- On the other hand, in case of post-tensioning, the steel strands or prestressing reinforcement is placed inside small ducts. The ducts or sheathings are placed at suitable places inside the member before casting. The concrete is cast into the surrounding formwork and cured. Once the concrete gets hardened, the tendons are stretched to the required level and the prestressing force is transferred to the concrete members only by bearing at the ends of the member.
The steel reinforcement for prestressing can be in the form of either single wires, strands comprising several wires twisted to form a single element, or high strength steel bars. The most common types of reinforcement used in the United States are as follows;
- Uncoated stress-relieved or low-relaxation wires
- Uncoated stress-relieved strands and low-relaxation strands
- Uncoated high-strength steel bars
The term stress-relieved means that they are designed to minimize the risk of developing internal stresses and they are prepared by heating high-strength steel strands to a very high temperature, around 1000 degrees Celsius followed by rapid cooling.
By low-relaxation it is meant that these strands will exhibit a lower rate of relaxation over time and are therefore, well-suited for long-term pre-tensioning and post-tensioning applications. To manufacture these strands, the high-quality steel is typically coated with a layer of grease or wax that helps reduce friction. Their manufacturing process involves heat treatment and chemical composition control.
In case of prestressed concrete members, protecting the prestressed reinforcement from the attack of corrosion is more critical because the strength of such member has a direct bearing on the prestressing force, which in turn depends upon the tendon area available.
What corrosion does is that it produces a powder called rust and this reduces the available cross-section and ultimately the moment strength of the prestressed member. This can lead to a premature failure of the structural system. Therefore, protecting the prestressed reinforcement plays a pivotal role in structural integrity and durability.
In case of pretensioned members, the concrete that surrounds the prestressing reinforcement suffices to protect it from corrosion provided adequate concrete cover is ensured. However, in case of post-tensioned concrete members, the protection against corrosion can be obtained by fully grouting the inside of the duct where strands are placed and tensioned.
Pre-stressing Equipment and Materials
In case of pre-tensioning, the pre-stressing steel is tensioned against independent anchorages before pouring of the concrete. The anchorages are supported by large bulkheads that support the very high localized forces at the tendon ends.
The pre-tensioning bed is used to cast multiple such pre-tensioned members and they can have different tendon profiles. The tendon profile refers to the shape or manner in which a pre-stressing tendon is placed inside a concrete member.
In case of post-tensioning, you will need an anchorage assembly that comprises all the components required to transfer the pre-stressing force from the strand to the concrete member.
It is to be kept in mind that the pre-stressing reinforcement in case of a prestressed concrete member is placed in addition to the conventional reinforcement. It is not like that you will omit all reinforcements in a member and replace them with prestressed reinforcement. However, pre-stressing is an advanced type of reinforcement in the concrete that augments its properties and makes it use 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 whatever strand type and length you want to achieve as per your project specifications. The strands are then fixed to the anchorage. The term anchorage means a support that houses the strands and enables them to be stretched to the required stress level. The strand profile for pre-tensioned member is usually a straight line at an eccentricity from the centroidal axis of the member.
The anchorage is usually made of high-strength steel and it helps transfer the pre-tensioning force from the strands to the concrete. Once the strands are anchored, they are tensioned or stretched to the required level of force.
Once the strands are tensioned, they are jacked or locked in the stretched position. The concrete is then cast around the strands in the formwork till it gets hardened. It is then cured for the required duration till it gains the targeted compressive strength.
Once the concrete gets cured for the required duration, the strands are released by cutting them from both the ends. Consequently, the pre-stressing force is transferred to the concrete member. The ends of the member are then sealed to prevent the corrosion of strands.
For pre-tensioned members, we usually have a casting bed where a number of members of the same dimensions are cast and stretched at the same time. These units are then sold as precast, pre-tensioned concrete members. However, you may also customize the formwork and prepare members with different geometry as required.
How is Post-Tensioning of Strands Carried Out?
The post-tensioning of strands is usually carried out on the site where a post-tensioned member is to be installed. The first step is to install the ducts that are to contain the steel strands. The tendon profile in case of post-tensioned members is usually not a straight line, it can be harped or draped, depending upon the variation of transverse loading on the member.
The ducts are usually made up of plastic or metal and based on the tendon profile, the ducts are installed in the same profile. The concrete is then cast around the ducts to acquire the desired shape. Once the concrete hardens, it is cured and after curing it for the required duration, the strands are passed through the ducts.
The ends of the strands are anchored to specially-designed anchoring devices placed at the ends of the ducts. The anchorage devices typically comprise wedges and bearing plates that transfer the tensioning force from the strands to the member ends by bearing.
The strands are stretched to the required stress level using hydraulic jacks. Following this, the ducts are grouted to bond them with the concrete member and also to prevent the corrosion of strands.
In case of prestressed concrete members, the profile of the tendons can vary along the member length. In other words, the eccentricity of the strand with respect to the centroidal axis of the member can vary.
The reason for such a variation in profile is because for simply-supported end conditions, the maximum bending moment or stresses are expected to be developed at the mid span, therefore, we want the maximum eccentricity of the pre-stressing force to be provided at the mid-span.
However, away from this critical section (mid-span), there is essentially no need to maintain the same eccentricity throughout because this can lead to very high compressive stresses at the member ends. Hence, as we move towards the member supports, this eccentricity is reduced.
Therefore, the profile of the tendon is not a straight line and can be either draped profile or harped profile.
In case of harped profile, the mid-span has the maximum tendon eccentricity and away from it, the variation is liner towards the support sections. In other words, the tendons have a V-shaped profile along the length of the member.
On the other hand, in case of draped tendon profile, the variation of eccentricity from mid-span to the supports is parabolic and not linear. Therefore, the profile resembles a curve in the sagging direction for beams.
Losses in Prestress
The initial pre-stressing force to which the steel tendons or strands are subjected to experiences progressive loss in prestress over a period of around five years. The partial loss of prestress can be owing to a number of reasons, some of which cause instantaneous losses and other cause time-dependent losses in the force.
It is important to determine the losses in each stage from transfer of pre-stressing force to various other stages including the application of service load and at the ultimate stage. As an approximation, 25 to 30 percent of the pre-stressing force is lost due to various reasons that we shall elaborate as we go along in the discussion.
The instantaneous losses take place during the construction process whereas the time-dependent losses take place once the member is subjected to service loads and can continue up to five years after which the effective prestress in the strand or tendons becomes essentially constant.
Following the losses in pre-stressing force that take place and must be accounted for in calculations so that the member is eventually designed as per the effective pre-stressing force and nit on the basis of initial force to which it is stretched at the time of tensioning.
Prestress Loss due to Elastic Shortening of Concrete
Elastic shortening in concrete in the longitudinal direction causes prestress losses in the steel strands in case of pre-tensioned members only. This is because in this case the steel strands and concrete are bonded together and once the strands are cut, the prestress force is transferred to the concrete. When the tendons bonded to the adjacent concrete simultaneously shorten, they lose a part of the pre-stressing force.
Elastic shortening loss does not take place in post-tensioned concrete members. This is because in post-tensioned members, the strands are not directly bonded to the adjacent concrete; rather they are present in sheathing or ducts inside the hardened concrete. However, this is only true when all the tendons are simultaneously jacked.
If, on the other hand, the tendons are jacked sequentially and not all at a time, elastic shortening of concrete causes loss in pre-stressing force in the tendons.
Anchorage Seating Loss
Anchorage seating loss only occurs in case of post-tensioned concrete members. Owing to seating of the wedges in the anchors when the jacking force is transferred to the anchorage, this type of loss in prestress takes place.
This loss can also take place in the casting beds in case of pre-tensioned members when the force is transferred to these beds.
The value of slip due to anchorage loss is generally taken between 0.25 in. to 0.375 in. for two-piece wedges. The loss in prestress due to anchorage seating becomes significant in case of short-span members.
In case of post-tensioned members, friction between the tendons and the surrounding concrete ducts reduces the pre-stressing force. The magnitude of frictional loss depends upon the tendon alignment and also on the local deviations in the alignment.
The maximum frictional losses take place at the far end or extreme of a member if the member is jacked at one end. The variation of frictional losses is linear from the end the member is jacked to the other extreme and in between these extremes, the magnitude of the loss can be linearly interpolated.
Steel Relaxation Losses
The tendons suffer loss in the pre-stressing force due to constant elongation with time. The relaxation of strands is a time-dependent loss and depends on the duration of the sustained pre-stressing force and also on the initial pre-stressing force as a percentage of yield stress of the steel strand.
Prestress Loss due to Creep in Concrete
Creep in concrete takes place under sustained loads and it is experimentally found that the creep strain in concrete results in some loss of pre-stressing force. It is a time-dependent loss and depends upon the stress level at a particular loading stage.
Prestress Loss due to Shrinkage in Concrete
Just like creep, concrete shrinkage with time is inevitable and in case of prestressed members, it results in the loss of pre-stressing force in the strands. Shrinkage in concrete depends on a number of factors including mixture proportions, aggregate type, cement type, curing time, time between the end of curing and the application of pre-stressing force, member size, and environmental conditions.
Around 80 percent of shrinkage in concrete takes place in the first year when a structure is brought into serviceability. However, both creep and shrinkage losses contribute a great deal in reduce the prestress force.
When all the above losses are accounted for, the initial pre-stressing force reduces to what we call effective pre-stressing force.
Parameters Affecting Concrete Quality
The two most important factors in pre-tensioned concrete (or prestressed concrete, in general) are strength and endurance. Long-term detrimental effects are expected to relax the prestressed strands and relax them. Therefore, strict quality control measures must be adhered to at various stages of manufacturing and installation.
For pre-tensioned concrete, high strength concrete is a must requisite just like high strength steel strands are. This is to make sure the two materials depict composite action and show due compatibility against the applied loading.
If ordinary concrete (having some nominal value of compressive strength, say 3000 psi) is coupled with high-strength steel strands, the transfer of very high compressive stresses once the strands are release will crush the concrete at the ends. Conversely, if nominal strength of steel, say grade 280 or 420, is used, the strands will yield before reaching to the required prestress value.
It is therefore, important to use both high-strength steel and high-strength concrete in prestressed members. In this regard, the compressive strength of concrete should preferably be more than 6000 psi at 28 days.
In addition, owing to high creep and shrinkage losses in concrete, the effective pre-stressing can only be achieved by using very high-strengths steels having a tensile strength of greater than or equal to 270,000 psi (1862 MPa). These will help counterbalance the prestress losses in the surrounding concrete.
Applications of Pre-tensioned and Post-Tensioned Concrete
Apart from economic reasons, there is no such concrete application where prestressed concrete cannot be used. For reasons of high strength, durability, and versatility, the use of both pre-tensioned and post-tensioned concrete is increasing. Following are some of its very common applications;
- Pre-tensioned and post-tensioned concrete members are used in bridges for mostly the superstructure but can also be used in the substructure in some cases. The reason is the ability of such concrete to withstand heavy loads and conveniently span long distances.
- Prestressed concrete can also be used in commercial, industrial and high-rise buildings because of its strength and durability.
- It can also be used in transportation infrastructure such as highways, tunnels, etc. because of reasons of strength.
- Prestressed concrete pipes can be used in sewerage systems and excellently perform their function. When embedded under the ground, these pipes offer the most resistance to heavy superimposed loading on the natural ground above them.
- Prestressed concrete can also be used in the construction of sports facilities such as stadiums, arenas, etc.
Advantages of Pre-tensioned and Post-Tensioned Concrete
Prestressed concrete offers a plethora of advantages for all kinds of construction works and these include;
- The creation of permanent prestresses before the application of service loads eliminates or considerably reduces the net tensile stresses at the top or bottom.
- In case of pre-tensioned members, there is a relatively high controlled recovery of cracking and deflection.
- Structures that are anticipated to experience heavy vibrations from machines, etc. require resiliency and with the contribution coming from pre-stressing force, they can easily be made rigid.
- With prestressed concrete, you will get a shallower depth section in comparison to a conventional concrete member for the same design specifications (e.g., span and loading conditions). To quantify the depth difference, a prestressed member has a depth of around 65 to 80 percent of the depth of an equivalent reinforced concrete member. Reduced depth means material saving and more available space. However, this economic perspective gets blinded by the higher cost of pre-stressing equipment and materials used in this concrete type.
- Pre-tensioned concrete members result in long-term savings in the form of reduced maintenance costs, a longer working life, and lighter foundations because of relatively less weight of superstructure to be borne by them.
- Pre-tensioning and post-tensioning operations substantially reduce the cracking in concrete and enable the member to support greater loads and span longer distances.
- Pre-tensioned and posttensioned concrete members are more ductile than ordinary reinforced concrete members. Be it flexure or compression, the performance and mechanical properties of the former are way better than the latter.
Drawbacks of Pre-tensioned and Post-tensioned Concrete
Although pre-tensioned and post-tensioned concrete have taken the construction industry to a whole new level, their drawbacks also surface when you decide to opt for them. In this regard, following are some of the drawbacks that come flowing along with the use of this type of concrete;
- The pre-tensioning and post-tensioning operations require specialized equipment and a great deal of quality control is to be required to ensure their high performance. In addition, these operations also demand highly skilled labor and this can incur added costs.
- The positioning and tensioning of steel tendons is a complex task whether it is carried in factory, as in case of pre-tensioned members, or on the construction site as in case of post-tensioned members. The complexity adds on to the construction costs.
- This type of concrete offers limited flexibility if the member is cast and the tendons are placed and jacked. You cannot alter the design specifications once the strands have been stretched to the required stress level. Particularly talking about pre-tensioned concrete members that are cast in a factory, there is limited flexibility available for a single construction project. This is because if you plan to opt pre-tensioned members, you will prefer the same size and member shape to reduce the cost of customizing another formwork or mold in the factory. This means that all the members will have the same geometric specifications for economic reasons, leaving behind lesser flexibility in customizing different designs for different members.
- The initial cost of pre-tensioned and post-tensioned members is more than their ordinarily reinforced counterparts. However, if we weigh in the long-term benefits, this drawback often subsides.
- These members require more maintenance and inspection almost on periodic basis to ensure the tendons are in place and remain tensioned and that the concrete does not show any signs of damage or degradation. This means that there is more cost going into this domain for prestressed concrete.
Pre-tensioned vs Post-tensioned Concrete; Which One is Better?
Both pre-tensioning and post-tensioning are amazing techniques for enhancing the performance of a structural concrete member. However, the choice between the two depends upon several factors such as the design requirements, construction methods to be followed, the applied loading and span, etc.
In general, pre-tensioned members have a limitation on the span after which it is almost necessary to switch to post-tensioned members. With pre-tensioned concrete, you will get higher initial strength and stiffness since the strand are tensioned before the concrete is cast. However, the fabrication process is carried out in a factory and requires a great deal of precision.
In case of post-tensioned members, you can attain a very long span unlike pre-tensioned members. In addition, they also offer greater flexibility in design and construction since the strands are tensioned after the concrete has been cast.
Frequently Asked Questions (FAQs)
What is the difference between a strand and a tendon?
In the terminology of pre-stressing equipment, the term strand is used to indicate high-strength steel wires that are wound around a center wire, just like a typical seven-wire strand.
On the other hand, the word tendon is used to indicate a complete assembly of pre-stressing elements. It comprises anchorages and couplers, pre-stressing strands, ducts and grout caps.
What are the types of tendons in post-tensioned members?
In post-tensioning, there can be two types of tendons. You can either have a bonded tendon or an unbonded tendon.
Bonded tendon means that you fill the duct with grout at the end of the post-tensioning operation so that it gets bonded to the concrete. In this case, the tendon is prevented from moving relative to the concrete.
Unbonded tendon is one in which the pre-stressing steel is not bonded to the concrete and is rather free to move relative to it. In this case the force is transferred to the concrete only by anchorages or deviators.
To what stress level should the strands be tensioned both in case of pre-tensioning and post-tensioning?
The amount of tensioning that is to be done for a particular strand depends on various factors such as the design requirements, materials used, etc. and also on whether you are carrying out pre-tensioning or post-tensioning.
As a normal practice, in case of pre-tensioned members, the strands are stretched to a stress level that corresponds to 70 to 80 percent of their ultimate strength. On the other hand, for post-tensioned concrete members, the strands are tensioned to a stress level that corresponds to 60 to 70 percent of their ultimate strength.
How to determine the amount of prestress loss that has taken place in a particular tendon at a particular time?
For this purpose, usually strain gauges are mounted on the concrete members and corresponding to the strain, the stress levels are determined. These can then be compared to the initial prestress that was applied to the strand.
However, the loss can also be computed theoretically at any particular stage of the concrete member using data from the design specifications and formulae available in the codes of practice.
What is an anchorage assembly in case of post-tensioned members?
In case of post-tensioned members, the anchorage assembly comprises the following components;
An anchor is a device used for unbonded single strand tendons to house the wedges and transfer the pre-stressing force from the strand to the concrete.
It is a flat plate that transfers the tendon force directly to the concrete.
These are conduits used to accommodate the pre-stressing steel and can be grouted at the end of the post-tensioning operation.
It is also called a trumpet and is a connection piece between the bearing plate and ducts.
It is a device that houses the wedges and transfers the prestressing force from the strand to the bearing plate.
Wedges are devices that hold the strands and are all connected to the wedge plate. Normally, a two-piece wedge is used.
This equipment is used for stressing the tendons and it comprises a hydraulic jack, a hydraulic pump and calibrated strain gauges attached to the jacking assembly.
Where to procure pre-tensioned concrete elements?
If a pretensioned member is selected from the manufacturer’s list and is not to be customized, you can procure it directly from the factory where they are manufactured. All companies that specialize in pretensioned concrete have some bult-in shapes that they cast in bulk quantities at their factory and dispatch to the construction site.
You can directly contact the product manufacturing companies and procure the required quantity of pretensioned concrete members for your project.