Anchorage block is a force transfer component of the post-tensioning tendon system in construction and mining industry. The formal name is bearing plate by PTI definition.
According to the definition of Post-tensioning Institue, anchorage bearing plate(irregular) is any hardware that transfers tendon force directly into the concrete and does not meet the analytical design requirements of PTI " Acceptance Standards for Post Tensioning Systems". Typically it includes spiral confinement reinforcement, works with post-tension anchor head as one set anchorage system.
Anchorage Block Classification
Anchorage block is a load-bearing and transferring structure component in unbonded multi-strand post-tension system. As such, it provides sitting for anchor head and transfers prestressed force from steel strand to concrete member directly. Since multi-strand anchorage tendon system is diversified in practice, therefore the bearing plate profile is also varied against the field application.
In general, it can be classified as flat or round type as below pictures showed. However, anchorage size or profile is different case by case. That is to say that it depends on what kind of construction to build and how many strands to use.
Material and Manufacturing Process
According to anchorage block type and its application field, design material selection and manufacturing process is varied. Hot rolled steel is used for round type anchorage system, while grey iron casting workmanship makes bearing plate of flat type anchorage system.
The casting process has the advantage that makes structure complex part an economic way but has an inherent disadvantage that produces casting defects, such as porosity, sand inclusion, shrinkage, etc. As a result, anchorage block casting will explode under prestress tension force if these defects affect casting strength. In case there has an anchorage casting explosion, the tendon system will lose its tensioning force to some degree even it is an integrated bonded tensioning system.
Read more: sand casting-the oldest craftsmanship that shapes the world to learn anchorage block manufacturing casting process.
In addition, packing shall be considered to prevent rusting or damage during transportation or stocking. Industry practice makes use of solid wood or plywood to make wooden packing boxes. Furthermore, a large plastic bag shall be put inside of the wooden box, bearing plates is dipped with rust inhibitors.
Anchorage Bearing Plate Quality and Inspection
Anchorage bearing plates quality is the most concern of post-tensioning anchorage assembly. In case it is failed as an explosion that results from porosity or sand inclusion defect, the prestress tensioning force will lose. For a bonded multi-strand tendon system, the tensioning fore will lose completely, but still, it affects the concrete member strength or toughness.
As such, the foundry has to take action to sort out anchorage castings that has internal porosity, shrinkage hole or sand inclusions.
In addition, shipment certification has to provide for each shipment batch. This is to demonstrate to customers that material chemical and mechanical property meets customer requirements or related national standards.
How does Bearing Plate Work
Anchorage bearing plates work together with wedge plate, loads, and transfers tensioning force directly on concrete member that is tensioned through bonded post-tensioning system.
Bonded post-tensioning systems consist of tendons with multiple strands or bars. The strands or bars are placed in corrugated galvanized steel, high-density polyethylene (HDPE), or polypropylene(PP) ducts. Depending on the construction site situation or tensioning system used, strands may be installed before concrete is placed or ducts may be installed without strands put in advance.
The right figure gives a layout of how the anchorage bearing plate works in this unbonded post-tension anchorage system.
Once concrete hardened and strand stressed, ducts will then be filled with grout. The grout provides an alkaline environment and protects the prestressing strands from corrosion. Also, it bonds the strand to the surrounding concrete to make the whole anchorage system integrating with concrete member as one unit.
In conclusion, anchorage block is a functional post tensioning structure component, specific application may affect its design structure and profile.