Post-Tensioning in Structures
Installation of bearing plates and ducts
The bearing plates are anchored to the formwork structure with boxes, in order to guarantee the correct alignment of active frameworks during start-up, leaving the minimum clearance dimensions needed to insert the hydraulic jack after the formwork removal operations. Ducts and steel fixings are installed at the same time.
Said operations comply with the specifications described on the plans, supporting these elements on the passive framework every 1 or 2 metres, depending on the complexity of the layout and the unit used.
Joint coupler are used to fix ducts. Said joints will be sealed with tape to prevent grout from being poured inside during the concrete pouring operations.
Special parts (vents) are installed on the highest points, which include a plastic tube that allows the discharge of air to the exterior during the injection operations, to make sure that there are no air-locks inside the duct.
This operation is carried out before the concrete pouring operations, in order to prevent the risk of potential dents or cracks on the duct during such operations, which would prevent the execution of the threading operations.
After assembling the coil on the strand, the reel will be aligned with the alignment unit.
A bullet-shaped metallic piece will be installed on the edge of each wire to avoid any deflection effects and from damaging the duct.
One of our experts will thread the strands used in the project along the length, as required for the subsequent tensioning operations.
Tensioning of the tendon
After the concrete pouring and formwork removal operations, one of of our experts will install the anchor heads and wedges.
The tensioning process starts when the adequate concrete resistance value has been reached, with the corresponding compression test confirmation. Said situation will be reported by the Site Management Area to one of our experts.
The tensioning process will be carried out in compliance with the instructions of the tensioning program that has been designed and drafted at our offices, including the following:
- Partial tensioning stages
- Tensioning order in each stage
- Tensioning order in each stage
During the tensioning operations, the expert will record the different load steps and elongation that has taken place during the operation on the corresponding report.
The measurement equipment will be adequately calibrated. Likewise, elongations will be measured with a minimum precision of 2% of the total length.
Cutting edges and sealing boxes
After the completion of the tensioning operations and once the elongations have been checked, the corresponding supervisor will give the instructions to cut the tensioning edges with a radial saw, 3 cm from the edge of the wedges. The boxes will then be sealed with a non-shrink grout.
The tubes used for the injection will be placed before sealing the boxes, connecting them to the openings on the bearing plates.
The conduits are washed with water before the injection process, which will eliminate all water-soluble oils during the thread operations and it will also discharge the remaining water injected with compressed air. In case no oils have been used, the cleaning operations can be carried out with injected air.
The injected grout has the following composition: 100 Kg of cement, TYPE I, 40 to 43 l of water and 450 g of additives.
In addition, the cleaning operations can be used to detect obstructions and communications between ducts or between ducts and recesses.
The grout injection process requires the use of a 3 to 5 m3 per hour capacity grout injector, with a double mixer, designed so as to prevent any interruptions during the operations.
Post-Tensioning in bridges built in formworks
Nowadays, most Civil Engineering works have plenty of structures, such as overpasses and underpasses, which are usually executed on formwork.
The use of Post-Tensioning techniques is crucial for the construction of this type of structures which are built on site with the following main advantages:
- Reduced costs
- Larger spans
- Smaller deck depths
- Elimination of joints
- Greater durability
These structures can be executed in one or several phases, depending on their length.
We offer many different solutions that cover all post-tensioning requirements for this type of structures:
- Anchors, from 1 to 37, 0.6
- Continuity anchors
- Active and Passive Anchors
- Vertical and horizontal tendons used to join abutments
- Adjustable anchors
- Cement, wax or grease grout injections
Post-Tensioning in bridges in self-launching formworks
Nowadays, Civil Works viaducts are built with larger spans and at greater heights.
Therefore, the construction techniques have been improved with the use of Self-launching Formwork Structures, Launching Carriers or Launching Techniques.
The coordination of the post-tensioning works (alignment, tensioning and grouting) with the rest of the activities is crucial in this type of constructive method, since any delay or failure in the execution would lead to important delays in the weekly schedule.
In order to comply with or even improve the weekly progress, we offer detailed procedures for an optimal and faster execution.
Post-Tensioning of precast segment viaducts
We are one of the most experienced company in the precast segment bridges execution.
This type of viaduct has a deck built in segments of approximately 2.4 m, which are then set with the launching carrier.
The sections start being placed on the pile section and advance in a T-shaped projection until they meet the corresponding T-shaped projection of the next pile.
Logically, segments are joined with post-tensioning operations. Sometimes, the second phase Pretensioning operations are available, in order to facilitate the performance of the launching operations, which will be responsible for absorbing the service loads.
The execution of tensioning operations for this type of viaducts is highly complex, as a result of the following:
- Post-tensioning is regarded as the critical path to launch the next section or segment. This is how 10 Segments have been launched per launcher and per day.
- The space is limited for the execution of tensioning operations on viaducts with a variable depth, which can be less than 1.20 ml. Therefore, we have developed special tools to attain a high performance in highly reduced.
- The hazards involved in working close to a free fall area.
Post-tensioning in balanced cantilever viaduct
This method is based on carriage formwork travelers which advance symmetrically from the pier in segments between 3-10 meters in length.
This method is used in viaducts with long spans where the use of other methods is impossible.
We have built among others the Pujayo and Montabliz Viaducts in the Cantabria-Meseta Highway.
The Montabliz Viaduct has not only one of the tallest piers in Spain but is also the longest span.
The cantilever segment have a length of 10 ml and it has post-tension tendon of 31 strand and 162 meters length.
We have carried out the Despe-te que Suas Viaduct in the Azores Viascut with a main free spam of 185 m.
We have executed these projects with all the necessary means to achieve more than a segment per week and per pier.
Post-Tensioning in special structures
The use of Post-tensioning operations is vital in the construction of special structures, such as:
- Water tanks
- Waste water digesters
- Liquefied Gas Tanks
- Nuclear Power Plants
- Telecommunications Towers
These structures require a strict quality control process and a perfect execution.
Post-tensioning operations in building works
The use of Post-Tensioning Operations in Building Works offers the following advantages:
- Costs savings
- Greater open spaces
- Greater distance between pillars
- Smaller slab depths
- Less number of joints
- Control of deflections
- Smaller foundation structures
- Greater durability
We have carried out the post-tensioning operations of many slabs in different types of special projects and buildings:
- Shopping Centres
- Conference Centres
- Train Stations
We have used different solutions in these projects, such as its non-adherent single-strand systems, its 31 0.6 tendons or 4 0.6 units with plain duct.