Post tensioning system

Types of post tensioning system

A post tensioning system is a technique used in the construction of reinforced concrete elements. It is a method of applying tension to high-strength steel strands or tendons within concrete elements to improve their structural integrity and performance. Here are the key components and an overview of how a post-tensioning system works:

• Tendons:

  Tendons are the key to post-tensioning. They consist of high-strength steel strands or cables. The tendons are typically grouped in bundles of three to five strands and are enclosed in a plastic sheath. The tendons are placed in the concrete elements before the concrete is poured. Their positioning depends on the design requirements. For example, tendons may be located closer to the bottom of beams and slabs where tensile stresses are highest. Alternatively, they can be positioned toward the top in some situations to counteract uplift forces.

• Anchors:

  Anchors are devices used to secure the tensioned tendons at the ends of post-tensioned concrete elements. They transfer the high tensile forces generated by tensioning the tendons into the concrete. There are two main types of anchors: swaged anchors and wedge anchors. Swaged anchors consist of a series of steel components that are progressively reduced in diameter through a swaging process. This process increases the anchor's strength and allows it to carry higher tension forces. Wedge anchors, on the other hand, use mechanical wedges to grip the steel tendon. They provide a reliable and adjustable means of anchoring the tendons under tension.

• Sheathings:

  Sheathings are tubular casings that encase the post-tensioning tendons. They serve multiple purposes. First, they prevent the tendons from coming into direct contact with the concrete, reducing friction. Second, they allow for the smooth movement of tendons during tensioning. Third, sheathings provide protection to the tendons from corrosion and other environmental factors. Additionally, they create a hollow space within the concrete element for future injection of grout.

• Couplers:

  Couplers are devices used to connect individual tendons or strands in a post-tensioning system. They are essential for creating continuous tendons that span long distances or connect multiple anchors. Couplers consist of a smooth steel tube that encloses the strands and is filled with high-strength grout. The use of couplers can enhance the efficiency and reliability of post-tensioning systems, especially in large-scale construction projects.

Features and Functions of Post Tensioning System

There are several features and functions of a post-tensioning system, which include:

• Tendons

  The tendons are the main feature of a post-tensioning system. They are responsible for transferring loads to the foundation. The tendons run through the ducts in the concrete slab. They are made of high-strength steel cables or strands. The tendons are encased in plastic sheathing to prevent corrosion. The sheathing also provides a way to allow the tendons to move during tensioning.

• Ducts

  The ducts are tubes that create a pathway for the tendons. They are usually made of plastic. The ducts are installed in the concrete before it is poured. They help to ensure the tendons are positioned correctly.

• Anchors

  The anchors are critical components of the post-tensioning system. They secure the tendons after they have been tensioned. The anchors come in two main types: Wedge anchors and Nut and Plate anchors. The Wedge anchors provide a quick and efficient locking mechanism. They consist of steel wedges that grip the tendons when tensioned. The nut and plate anchors offer a more robust locking mechanism. They use a steel nut and a flat plate to create a permanent grip on the tendons.

• Junction boxes

  The junction boxes are metal boxes that house the anchors. They protect the anchors from damage during construction. The boxes help to prevent corrosion of the anchors.

• Hydraulic pump

  The hydraulic pump is a crucial component of the post-tensioning system. It generates high-pressure hydraulic fluid. The pump is connected to a hydraulic cylinder. When the pump is activated, it pushes hydraulic fluid into the cylinder. This creates high tension in the tendons, which are then tightened.

Scenarios of post tensioning system

Pre-tensioning and post-tensioning are two types of tensioning systems in construction. They are used in various scenarios, including:

• Pre-tensioning

  This involves stretching the steel cables before pouring concrete. The cables are pulled tight and anchored to beams or supports. Once the concrete hardens, the tension cables are released. The concrete is now under compression, making it stronger. Pre-tensioning is mainly used in:

  Long span beams and bridges. The tensioned concrete beams can support greater distances without sagging.

  Precast concrete products like railway sleepers, parking garage beams, and hollow core slabs for flooring. These products are made off-site and installed quickly.

  Parking garages and bridges. The pre-tensioned concrete structures are durable and require less maintenance.

• Post-tensioning

  In this method, ducts are placed in the concrete slab or beam before it sets. Once the concrete cures, high-strength steel cables are threaded through the ducts. They are then pulled tight and anchored to compress the concrete. Post-tensioning is mainly used in:

  Large concrete structures like stadiums, parking garages, and mat foundations for skyscrapers. It allows for larger structures with fewer columns.

  Concrete floors in buildings and bridges. Adjusting the amount of tension helps control deflection and make level floors.

  Bridges and parking garages. The tension can be adjusted after construction to optimize strength and stability.

  Slopes and hilly areas. Tensioned anchors stabilize and secure to the bedrock or stable soil layers.

  Large storage tanks and silos. The tensioned cables evenly distribute pressure and prevent bulging or collapsing.

How to choose post tensioning system

• Load Capacity and Design Requirements

  Consider the anticipated load capacity and design specifications for the project. Different systems offer varied load-bearing capabilities. For higher loads, opting for systems like high-strength strands is beneficial.

• Slab Thickness and Span Length

  The thickness of the slabs and the length of the spans significantly influence the choice of a post-tensioning system. Tendons and ducts work well for thicker slabs and longer spans. For thinner slabs and shorter spans, unbonded tendons may suffice.

• Environmental Conditions

  The environmental conditions at the construction site play a crucial role in determining the most suitable post-tensioning system. If the project is located in an area with high humidity or frequent rain, consider using corrosion-resistant strands or ducts to protect the tendons from moisture and prevent corrosion. In coastal regions, where there is exposure to saltwater, opt for galvanized or epoxy-coated tendons for added protection against corrosion.

• Construction Schedule

  The construction schedule can influence the choice of post-tensioning systems. If speed is a priority, consider using bonded post-tensioning systems, as they allow for quicker concrete curing and early strength development. Unbonded tendons may require more time for concrete placement and curing.

• Cost Considerations

  While it is essential to ensure that the chosen post-tensioning system meets the project's structural requirements, cost considerations are also crucial. Compare the initial costs of different systems, such as bonded and unbonded post-tensioning. Bonded systems may have lower material costs, while unbonded systems could save construction time. Consider long-term costs, including maintenance and durability. Unbonded systems may require more maintenance over time, while bonded systems offer long-term durability benefits.

• Construction Expertise

  The expertise of the construction team can influence the choice of post-tensioning systems. If the team has more experience with specific systems, such as bonded or unbonded post-tensioning, leverage that expertise to ensure a successful project execution. Experienced teams can handle more complex systems, such as hybrid post-tensioning, while less experienced teams may prefer simpler solutions.

post tensioning system_2 Q&A

Q1: What is the purpose of a post-tensioning system?

A1: Post-tensioning systems strengthen concrete slabs and beams, allowing them to span longer distances without support. This is achieved by tensioning high-strength steel strands or bars within the concrete after it has cured, compressing the concrete and reducing deflection and cracking.

Q2: When should post-tension cables be used?

A2: Post-tension cables are typically used when a structure requires enhanced load-carrying capacity, durability, and resistance to deflection and cracking. This includes parking structures, bridges, high-rise buildings, water tanks, and slabs-on-grade in residential construction.

Q3: How common is post-tensioning in construction?

A3: Post-tensioning is a common practice in modern construction, especially for large-scale projects requiring efficient and economical structural solutions. It is widely used in commercial, industrial, and infrastructure projects.

Q4: What are the disadvantages of post-tensioning?

A4: While post-tensioning offers several advantages, such as improved structural performance and reduced deflection, it also has some disadvantages. These include the need for specialized knowledge and skills for installation, potential corrosion of tendons if not properly sealed, and the requirement for precise construction techniques to avoid problems such as tendon misalignment.