Monostrand anchor is a key component of post-tensioning tendon systems in the construction and mining industry. The formal name is called anchor by PTI definition, but industry people call it anchor block normally.
According to the definition of Post-Tensioning Institue, it is a device that houses the post-tension wedges and transfers prestressing force to the concrete. It is a single tendon anchor that an unbonded tendon system adopts normally in the ground or floor slab application.
According to the definition from PTI, a device that houses post tension wedge is called anchor. As such, wedge plate that houses wedges in multistrand bonded tendon system has same anchorage function as anchor. To avoid confusion, anchor that is used in unbonded tendon system is called as monostrand anchor. The following pictures shows their profile difference.
Monostrand Anchor Material Specification
The design of monostrand anchor is identical in civil engineering industry. Traditionally, iron casting material is sufficient to meet the requirements of design and construction engineering. As such, casting material is much popular choice for cost-saving purposes.
Read article sand casting: the manufacturing craftmanship that shape world to learn inside of casting process and quality control.
However, sand casting can not avoid casting defect's impact on parts' performance. In cases that construction project has strict demand on anchor's material toughness and strength, manufacturer may select carbon steel material, and make anchor through forging workmanship.
Construction project application gives the way of material selection for monostrand anchor. Since unbonded single strand tendon system is applicable in building ground or floor slab, the slabs are structure member, not a load-bearing member. As such, the design material of monostrand anchor is ductile iron, and it is made from sand casting process traditionally without any change until now.
There has no doubt that this choice has full consideration on construction economic, but all is under the condition that the quality of construction meets the design intent.
However, that does not mean the material and manufacturing process has no chance to change. Under the special condition that a minor porosity in casting may lead construction project failure, carbon steel forging blank is the better choice. As the forging process optimize metal grain flow, this will enhance monostrand anchor's tensile strength and toughness. Consequently, this will improve its performance in the anchoring tendon system. The right picture shows metal grain flow simulation in CAM software for hot forging blank of anchor.
Quality and Inspection
Quality is the most concern of iron castings. Failure of tendon system in field comes from casting defects, such as porosity, or sand inclusion. To make sure that anchor quality is guaranteed, foundry supplier has to take actions, such as X-ray or other sorting method, to detect internal nonconformities if necessary for high grade construciton project. In additon, shipment certification has to be provided for each shipment batch. This is to show customers that material chemical or mechanical characteristics meets customer demand or related national standards.
Monostrand Anchor Application
Monostrand tendon system is the dominant unbonded tendon system in construction of groud or buliding floor slabs.
Over the half-century, unbonded post-tensioning has become the dominant construction choice for home and commercial concrete building over the world because of its excellent performance, economical and versatile application.
Moreover, monostrand anchor tendon system utilizes a compact anchorage device, recess pocket former, and light-weight stressing jacks, this makes it much simple and convenient to make stressing operation on site.
In conclusion, monostrand anchor tendon system is a reletively simple and versatile tendon system to build up post tensioning thin slabs and narrow beams. As a result, its proven performance and economical advantage give its a steady growth in civil construction industry.
Read more: Post-tensioning system-technology changes the world of civil construction to learn more about this unbonded tendon system.