Post-Tensioning system for repair and rehabilitation

2431
Post-Tensioning

Post-tensioning can be explained as the method of reinforcing concrete or other materials with high-strength steel strands or bars, typically referred to as tendons. Their applications include commercial buildings, residential buildings, parking structures, slabs-on-ground, bridges, sports stadiums, rock and soil anchors, and water-tanks. Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Afterward, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete.

Elements of the post-tensioning system for repair and rehabilitation

The basic element of a post-tensioning system is called a tendon. A post-tensioning tendon is made up of one or more pieces of prestressing steel, coated with a protective coating, and housed inside of a duct or sheathing. A tendon will have anchors on each end to transmit the forces into the structure. Long tendons may also have intermediate anchors along their length.

Anchorages are another critical element. Anchor Head is an important part of the post-tensioning system. It works in tandem with the Wedge that holds the individual tensile element. The prestressing load is then transferred to the Bearing Plate. For some small tendon sizes, this prestressing force can be transferred directly to the structure without the need for Bearing Plate. In the case of some mono anchorage system, the Wedge will hold an individual tensile element directly to the Bearing Plate.

Internal bonded tendons – where one or more strands are inserted into a metal or plastic duct that is embedded in the concrete. By filling the duct with special grout, the tendon is ‘bonded’ with the surrounding concrete. Internal bonded tendons are installed before concrete is poured and become locked in concrete. This approach is often used for bridges and heavily loaded beams in buildings – flat internal systems are also an excellent choice for thin slabs.

Internal unbonded tendons – where the prestressing steel is not bonded to the concrete that surrounds it, except at the anchorages. They are used in slabs and slabs-on-ground for buildings and parking structures but also more and more in infrastructure projects.

External unbonded tendons – these are installed on the outer surface of concrete structures. This type of post-tensioning allows access for maintenance and replacement, this is therefore the solution of choice for bridge enhancements and refurbishments.

External unbonded tendons
External unbonded tendons

Another important element is the hydraulic tension jack. Tension is applied to prestressing steel by using a hydraulic stressing jack. The jack bears against one of the anchors that are embedded in the concrete and pulls the steel to a predetermined force. As the tensioning is occurring, the steel is being elongated, and the concrete or masonry element is being compressed.

In unbonded construction, the plastic sheathing acts as a bond breaker between the concrete and the prestressing strands. It also protects against damage by mechanical handling and serves as a barrier that prevents moisture and chemicals from reaching the strand.

Bearing Plate is a very important element in a post-tensioning system. It serves the purpose of transferring prestressing force derived from the tendon to the concrete. And it is precisely in this, that explains what we want to achieve in prestressed concrete structures, and thus we take special interest in the Bearing Plate.

Advantages of the post-tensioning system for repair and rehabilitation

There are post-tensioning applications in almost all facets of construction. In building construction, post-tensioning allows longer clear spans, thinner slabs, fewer beams and more slender, dramatic elements. Thinner slabs mean less concrete is required. Also, it means a lower overall building height for the same floor-to-floor height. Post Tensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors. This reduces the foundation load and can be a major advantage in seismic areas.

Lower building height can also translate to considerable savings in mechanical systems and façade costs. Another advantage of post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other. Structurally, this is much more efficient than having a beam that just goes from one column to the next.

Post-tensioning is the system of choice for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations. Post-tensioned parking garages can be either stand-alone structures or one or more floors in an office or residential building. In areas where there are expansive clays or soils with low bearing capacity, post-tensioned slabs-on-ground and mat foundations reduce problems with cracking and differential settlement.

Shallow foundations

Post-tensioning allows bridges to be built to very demanding geometry requirements, including complex curves, variable superelevation and significant grade changes. Post-tensioning also allows extremely long-span bridges to be constructed without the use of temporary intermediate supports.

In stadiums, post-tensioning allows long clear spans and very creative architecture. Post-tensioned rock and soil anchors are used in tunneling and slope stabilization and as tie-backs for excavations. Post-tensioning can also be used to produce virtually crack-free concrete for water-tanks.

Installation of a post-tension system for repair and rehabilitation

Expert advice and supervision are important for post-tensioning works. The sequence of work also plays a key role.

  • Designing the shape of the PT layout and specifying the right system is a vital stage in the process
  • Work on applying the post-tensioning system can begin after any formwork or bottom reinforcement has been installed.
  • Typically, for a bonded system, ducting will be laid out first, then strands will be installed with the help of a strand-pushing machine and end anchorages will be placed
  • Cables are arranged as indicated by the engineer and chaired to run through the center of the slab.
  • Commercial foundations will have much more steel. Tendons can be easily routed around obstructions.
  • The concrete placement follows next. When the concrete has attained its minimum strength, the strands are anchored in a common anchorage device.
  • The duct is then filled with a cementitious grout that provides corrosion protection to the strand and bonds the tendon to the concrete surrounding the duct.
  • PT tendon placement and stressing is usually done by companies with certified workers who specialize in this work.

Different products of post-tension for repair and rehabilitation

There are several products of post-tension for repair and rehabilitation. We will discuss the most important ones.

PT Anchorages

Anchorages are used to fix the ends of the tendons in post-tensioning systems. Various designs are available, depending on the system supplier, but basically, they are steel blocks through which single or multiple strands pass and are anchored by wedges. The strands may be tensioned individually or as a group.

The anchorages are cast into the concrete and transfer the entire load from the strands into the concrete. This causes high local bursting forces. To prevent the concrete splitting additional reinforcement is required close to the anchorage. Where high tensile strength bars used for prestressing, the bars are threaded (either just at the ends or over their whole length) and anchored after tensioning using nuts that bear against steel plates set in the concrete. Again additional reinforcement is needed to prevent bursting of the concrete.

PT Anchorages
PT Anchorages

Strands

Strands are made from 7 individual cold-drawn wires, 6 helically wound outer wires and one center wire (king-wire). The mechanical properties of the strand as well as corrosion protection properties are most important to DSI. For a maximum in corrosion protection, we offer electrically isolated systems using polyethylene (PE) or polypropylene (PP) ducts.

Strands
Strands

PT Duct & Grouting Systems

The purpose of grouting is to provide permanent protection to the post-tensioned steel against corrosion and to develop a bond between the Prestressing cables and the surrounding structural concrete. Grouting shall be carried out as early as possible, but generally not later than two weeks of stressing. Whenever this stipulation cannot be completed with for unavoidable reasons adequate temporary protection of the cables against corrosion by methods or products, which will not impair the ultimate adherence of the injected grout shall be ensured till grouting. Different equipment includes- Grout Mixer Mechanical type, Grout Pump, Grout Screen, Connection and air vents, Generator etc.

PT Duct
PT Duct

Monostrand PT systems

Monostrand PT systems can be used as an economical method in non-corrosive environments. Some applications include indoor residential or office areas. To provide a cost-effective system, the anchorage devices are not encased in an extruded jacket and plastic sleeves/grease caps are not included. However, our regular system still includes steel tendons with the special corrosion-inhibitor inside our distinctive green plastic sheathing.

Monostrand PT systems
Monostrand PT systems

Multi-Strand system

 Multistrand system comprises three to fifty-five strands (either 12.7 or 15.2mm diameter), round duct and anchorages. Prestressing force is applied to the tendons after the casting and curing of the surrounding concrete. All strands are stressed simultaneously using a hydraulic jack and are fastened at the live end by wedge grips. The free space inside the duct is then pressure-filled with cement grout.

Multi-Strand system
Multi-Strand system

Vacuum-assisted grouting for bonded PT repair

Before grouting, a vacuum chest is used to pump the air in the prestressed channel to make the vacuum in the channel more than 80%. Then the squeegee is used at the other end of the channel. A positive pressure greater than 0.7 MPa presses the cement slurry into the prestressed tunnel. Because there are only a few air holes in the channel, it is difficult to form bubbles; at the same time, due to the positive and negative pressure difference between the channel and the squeegee, the plumpness and density of the hole paddle are greatly improved. Vacuum-assisted grouting for bonded PT repair.

Vacuum-assisted grouting
Vacuum-assisted grouting

Conclusion

It is important to take the opinion of expert guidance for post-tensioning. It is a very important and complicated process. There are various services offered by different companies such as External PT for Bridge girders & Slabs, Floor System and Suitable Arrangement for Slab and many more others. So choose the com[pny that fits your requirement.

Source – Tmgglobals, reaerchgate, bbrnetwork, dsiamerica,.dywidag-systems, posttensioning.co.nz