Anchor block is a force transfer component of the post-tensioning tendon system in construction and mining industry. The formal term is called wedge plate in PTI definition. However, industry people also call it to wedge block or anchor head according to its different profile and varied application.
According to the definition of Post-Tensioning Institue, it is a device that houses the post-tension wedges in the tapered cavity and transfers the prestressing force to a bearing plate for multistrand tendons.
Anchor Block Classification
In general, post-tension system has bonded and unbonded two types. While unbonded tendon system use monostrand anchor to make a single strand tendon system, the bonded tendon system use anchor block to build multi-strand anchoring system.
According to the definition of PTI, anchor block is used in concrete multistrand unbonded post-tensioning tendon system. Since multi-strand unbonded tendon system is diversified, therefore its profile is varied against the field application. However, anchor may be classified as round or flat two types in general as the below picture indicated.
Wedge Plate Material Specification
According to field application requirements, wedge block profile will be different. As such, the design of anchor material varies from casting iron to middle carbon steel. In addition, the wedge plate size or strand quantity to hold up is different case by case. That is to say that it depends on what kind of construction to build. For example, bridge construction usually takes round profile anchor to build lateral cable tensioning system or hollow girder or choose flat one to construct bridge member or beam.
The material defines manufacturing workmanship. Wedge block that has iron or casting steel material is produced through casting process traditionally. However, casting process has an inherent disadvantage that has casting defects, such as porosity, sand inclusion, shrinkage, etc. Hence, these casting defect has serious impact on anchor performance and may result in disaster in a severe situation.
Read more: sand casting-the oldest craftmanship that shapes the world to learn how cast iron anchor manufacturing process.
Thereby, it emerges a trend to make construction anchor block from hot rolled steel bar through CNC machining process. As such, the manufacturing cost may increase, but it is worthable to bear comparing with quality stability and durability from the improved mechanical performance.
Furthermore, packing shall be considered to prevent them from rusting or damage during transportation or stocking. As anchor is heavy steel or iron casting, industry practice makes use of solid wood or steel belt enforce Polywood for wooden box. In addition, a large plastic bag shall be put inside of wooden box to make it waterproof.
Quality and Inspection
Quality is the most priority consideration in anchor block concrete construction. In fact, construction site problem comes mainly from casting material defects, such as internal porosity or sand inclusion. To make quality-assured, the foundry has to do X-ray or other method sorting if required in high-grade casting material. In addition, shipment certification has to be provided for each shipment batch. This is to demonstrate material chemical and mechanical characteristics meet related national standards. The right picture shows an internal porosity defect in anchor block casting that may possibly result in disaster if failed.
How does Wedge Plate Work in Tendon System
In construction practice, anchor block works together with anchorage block (bearing plate) as an integrated unit. The operator will put post-tension wedge in the tapered hole of anchor block after hydraulic jack prestressing force on steel cable reach nominated tensioning force. Consequently, the anchor will hold up wedges in the tapered hole once the hydraulic jack releases tensioning force, and transfer tensioning force from the strand onto construction member. The right picture shows how an anchor works in a tendon system.
In sum, anchor block is a functional structure component of bonded tendon system. The specific application may affect not only the selection of its material, but also manufacturing process and its profile.