What is External post tensioning of concrete?
Post-tensioning of concrete is an advanced method used to enhance the load-bearing capacity and structural efficiency of concrete elements. This technique involves the application of tension to prestressing steel cables, known as tendons, after the concrete has cured.
The tendons, positioned within ducts or channels inside or adjacent to the concrete, are tensioned and then anchored to the concrete, compressing it and improving its strength and performance characteristics. Post-tensioning is particularly effective in bridges, parking structures, and buildings, offering a dynamic solution to increase flexural and shear strength, counteract tensile stresses, and manage structural deficiencies due to under-design, construction errors, or increased loading demands.
What is External post tensioning of concrete used for?
External post-tensioning of concrete is a specialized technique used to enhance the load-bearing capacity of structures where the original design lacks sufficient flexural strength, or where increased demands have been placed on the structure due to construction faults or additional loading. Ideal for structures in good condition without deterioration, this method efficiently corrects under-designed elements, supporting increased dead or live loads with minimal disruption.
Adhering to EN 1504-9:2008 and EN 1504-4:2004 standards, external post-tensioning ensures structural strengthening and durability, making it a cost-effective solution for extending the lifespan of various concrete infrastructures, including bridges and buildings, by addressing specific structural deficiencies.
Attention must be paid when deterioration processes such as creep are present to ensure the forces resulting from this is accounted for in the design of post tensioning systems.
How does External post tensioning of concrete work?
Post-tensioning of concrete fundamentally transforms the structure's loading profile and mechanical behavior, enhancing its capacity to bear loads more efficiently and effectively. This technique applies a compressive stress to the concrete, which is inherently strong in compression but weak in tension. By inducing a state of compression throughout the concrete element before any service loads are applied, post-tensioning improves the material's ability to resist tensile stresses that would otherwise lead to cracking and failure.
The technical essence of post-tensioning lies in its active engagement with the concrete's structural dynamics. The tension applied to the tendons exerts a compressive force on the concrete, effectively preloading the structure. This preload counterbalances potential tensile forces encountered during the structure's use, such as those due to live loads, thermal expansion, and shrinkage. As a result, the overall structural system is less likely to experience tension-induced cracking and has a higher flexural strength.
Moreover, by altering the stress distribution within the concrete element, post-tensioning allows for more slender and efficient structural designs. The concrete sections can be made thinner and spans can be longer, reducing the quantity of concrete and reinforcement required. This not only leads to material savings but also facilitates architectural flexibility and innovation.
Additionally, post-tensioning can rectify structural deficiencies and adapt existing structures to new load requirements, significantly extending their service life and usability. By actively changing the loading profile of the structure, post-tensioning ensures that new and existing concrete elements are optimized for performance, durability, and sustainability.
How do I repair concrete structures using strengthening by is External post tensioning?
Based on standards like BS EN 1504 for structural strengthening and BS EN 13391:2004 for mechanical tests on post-tensioning systems, a step-by-step procedure for carrying out post-tensioning on a concrete structure can be outlined as follows:
Conduct a thorough assessment of the concrete structure to identify the need for post-tensioning. This includes evaluating the current structural capacity, identifying areas of under-design, construction faults, or increased loading.
Plan the post-tensioning process in accordance with BS EN 1504-9:2008, focusing on structural strengthening to address the identified needs.
Design the post-tensioning system, considering the specific requirements of the structure. This includes determining the type and number of tendons, their layout, and the anchorage and deviator positions.
Prepare the concrete surface where the post-tensioning system will be applied. This involves cleaning the surface and ensuring it is in good condition, with no ongoing deterioration processes like corrosion or creep.
Install the end anchorages and deviators according to the designed system layout. Ensure that these components are securely fixed and properly aligned.
Lay out the unbounded prestressing cables through the deviators and anchor them at the end anchorages. Care should be taken to ensure that the tendons are correctly positioned and tensioned.
Apply tension to the cables using hydraulic jacks. The tensioning should be done in stages, monitoring the force applied to each tendon to achieve the desired level of prestress.
For systems requiring grouting, fill the ducts housing the tendons with grout to protect the tendons from corrosion. The grout must meet the specifications outlined in relevant standards, ensuring durability.
Seal any openings and apply protective coatings as required to protect the post-tensioning system and the concrete from environmental exposure.
Conduct thorough inspections and tests to ensure the post-tensioning system meets the required standards, including mechanical tests as specified in BS EN 13391:2004.
Document the entire process, including materials used, design calculations, tensioning records, and inspection results, for future reference and maintenance.
Establish a schedule for regular inspections and maintenance of the post-tensioned structure to ensure its long-term performance and durability.
It's important that all procedures comply with the relevant British and European standards, ensuring the safety, durability, and performance of the post-tensioned concrete structure. Collaboration with experienced professionals and adherence to detailed specifications from the planning phase through to execution and maintenance are crucial for the success of post-tensioning projects.
What equipment and expertise are required for repairing concrete structures using External post tensioning?
Equipment required for post tensioning includes:
High-strength steel tendons and coil dispenser.
- Hydraulic Jacking equipment for applying required level of tension to the pre-stressing tendons including equipment such as pumps gauges and hoses.
- Anchorage systems to securely hold the tendons in place after tensioning.
- Ducting to house the tendons within the concrete, protecting them and allowing for post-tensioning adjustments.
- Grouting equipment for filling the ducts with grout post-tensioning to protect the tendons from corrosion.
Significant expertise is required to design and install external post systems successfully. The need to understand the loading that the structure must sustain and design tensioning systems that can efficiently achieve this requires a high level of structural design knowledge. Failure to design external post tensioning systems correctly can further undermine the structural integrity of concrete structures and redistribute loading to areas that were not designed to endure them and therefore do not have the requisite reinforcement bars present.
- Surveying equipment to allow accurate placement and monitoring of the tendons and anchorages.
What are the advantages of repairing concrete structures using External post tensioning?
External post-tensioning can significantly increase the load-bearing capacity of existing structures, making it ideal for buildings and bridges requiring strengthening due to increased usage demands or design deficiencies.
Both ultimate limit state (ULS) and serviceability limit state (SLS) can be improved.
This method allows for the strengthening of structures where traditional reinforcement methods are not feasible. It can be applied without significantly altering the structure's original appearance or function.
External post-tensioning can often be executed with minimal disruption to the structure’s use. It's particularly beneficial for bridges and commercial buildings where prolonged downtime is not viable.
Compared to demolishing and rebuilding, external post-tensioning can be a more cost-effective solution for extending the life of concrete structures.
By applying compression to the concrete, external post-tensioning can help close cracks and improve the structure's resistance to environmental degradation, thereby extending its service life.
The method offers a solution for various defects, including insufficient flexural, shear strength, or deflection issues, without significant structural alterations.
Proven technology that is easy to inspect and monitor.
In case of premature failure individual tendons can be replaced and re-tensioned.
What are the disadvantages of repairing concrete structures using External post tensioning?
The process requires meticulous planning and design by experienced engineers. Incorrect application can lead to inadequate strengthening or even additional stress points.
Post-tensioning systems, especially external ones, may require more maintenance than traditional concrete structures to ensure the tendons remain effective over time.
External tendons and anchorages can affect the visual appearance of a structure, which may not be desirable in certain architectural contexts.
While cost-effective in the long term, the initial investment for external post-tensioning can be higher than some traditional repair methods due to the specialized equipment and materials required.
What are the limitations of repairing concrete structures using External post tensioning?
External post-tensioning, though beneficial for structural reinforcement, faces limitations.
Structural suitability is a primary concern; not all buildings or bridges can effectively utilize this method due to their design, condition, or material composition. Logistical challenges also arise, particularly with installation that requires exterior access, which may not be feasible in densely built areas or where aesthetics are crucial. Furthermore, environmental exposure risks such as corrosion necessitate ongoing maintenance to ensure the system's integrity.
The successful application of external post-tensioning requires specialized knowledge and experience, limiting its use to projects with access to skilled professionals.
Ancillary information
Service disruption: Yes
Preliminary works: Yes
Posterior works: Yes
Time consumption: Medium
Cost: High
References and further information