Post Tension Wedge

Post tension wedge is a key component of the tendon anchor system in the construction and mining industry. There have two tensioning methods in civil engineering. One is pre-tensioning, the other is post-tensioning. Post-tensioning is a reinforcement way in which tendon systems is tensioned after construction cement solidified and hardened. As post-tensioning has occupied a dominant position, so the industry people used to call it post-tensioning wedge.

According to the definition of Post Tensioning Institute, it is pieces of tapered high-strength heat-treated steel with serrations (teeth) that penetrate the prestressing steel strand during the transfer of prestressing force.

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Wedge Classification

PTI demonstrated post tension wedges

The post-tension anchor wedge is classified against steel strand size or the specific application it will be used. Because 1×7 steel wire strand has size 0.5″, 0.6″, 0.7″, and 0.85″, consequently the wedge profile will be varied slightly.  Besides, some anchorage systems use two-parts wedges and some use three-parts wedges. Thereby, the choice is up to the field application purpose. The two pieces wedge is the component of tendon system, while the three pieces are used in the prestressing tool that makes tensioning on tendon system. The left picture shows multi-size wedges that are used in the construction and mining industry.

Anchor Wedge Material Specification

The material of post-tension wedge is steel. Although free machining steel has ever been the prime material at the earlier development stage of the civil engineering industry, alloy steel 20CrMnTi (China steel grade) has the dominant position now because of its higher tensile strength advantage (minimum 1080Mpa).

In addition, the surface hardness of anchor wedge is critical to the function of tendon system. Normally it is between 58~63HRC(or 80.1~82.8HRA) after heat treatment. However, surface carburizing and nitriding is necessary to achieve this tight and extremely high hardness specification. Heat treatment practice shows that the alloy steel 20CrMnTi is the perfect material that is suitable for quenching without crack or other defect happening.

Post Tension Wedge Manufacturing Process

As mentioned, old tradition makes anchor wedge from stock bar through CNC machining. But the trend has changed now. The industry has set the standard that makes wedge cone from cold forging process. The 20CrMnTi material has a very nice cold formability after spheroidizing annealing. Most importantly, the wedge cone has become much tougher and strength enhanced after cold forging.

Read on: Metal forging process that hammers your strength and toughness to learn how cold forging workmanship produces wedge cone.

As long as wedge cone gets CNC turning finished, the anchor block wedge will go through thread tapping, splitting, heat treatment, and sorting inspection consequently.

As soon as good strand wedges are sorted out, manufacturer will dip them into anti-rust oil to prevent rusting in transition and stocking. Excess oil will be spun out.

Furthermore, packing method is also very important to prevent them from rusting or damage during transportation or stocking. The industry-standard method is to put post-tension wedges in a plastic bucket with a sealing lid, then in wooden box. To make it tight waterproof, a large plastic bag is also put inside of the wooden box normally.

The below figure shows the packing material/method of the anchorage wedge.

Strand Wedge packing plastic bucket post-tension wedge pacing wooden box

Quality and Inspection

Critical quality parameters of post-tension wedge are surface hardness, case-hardens depth, and teeth sharpness. To make the finished anchor block wedge meet the these requirements, heat-treatment factory must provide test certification and report for each furnace heat batch.  Most importantly, quality inspectors or vision equipment will make 100% sorting. This is to make sure that wedges with flat or damaged teeth are sorted out, and there has no foreign particle inside of wedge teeth.

How does Post Tension Wedge Work In Field

In construction practice, operators will put wedges in tapered hole of the anchor block or anchor head when hydraulic stressing jack prestress steel strand and reach the nominated tensile strength. Once hydraulic stressing jack releases force, the serration on wedges will hold up strand to prevent it back to its original status. As a result, the tensioned cable transfers stressing force on concrete members.

Post-tension wedge working with anchor block

Field Problem and Trouble Shooting

Occasionally when wedges fail to hold up steel strand, the most common cause is cement slurry in the tapered cavity of the anchor or wedge plate. If a separation between pocket former and anchor occurs, cement slurry can flow into the anchor cavity and form up a ring around the strand at the back of the anchor. As such, this will stop the wedges from penetrating into the anchor. Consequently, it will prevent wedge from sitting at a proper distance in the tapered cavity and result in strand slippage. If there has a slippage happening, then the tensioning job needs reworking.
Actually, there exist multi-factors that may lead the tensioning work to fail, and the most are visible. The first one is rust in teeth serration, and the second is flat or damaged teeth as the figure indicated. The dull teeth are not able to bit into and hold up strands, and the rusted wedge lost its surface harness layer.
Finally, serration hardness out of specification will be a serious issue that can happen in very few chances. As customers will reject the whole heat batch wedges if heat treating not pass, so the supplier will put much focus on the critical features.

In conclusion, a post-tension wedge is an alloy steel anchorage component, plays the role of holding up a prestressed steel strand, and transfer force onto the concrete member.

Read more on the Post-tensioning tendon system in the construction and mining industry to get a thoughtful viewpoint on what it is and how it works.