Post Tensioning

This simple video captures some process in our production of post tensioning accessories, namely Anchor Head and Wedge.

Anchor Head (also commonly known as Wedge Plate) is an important part of the post tensioning system. It works in tandem with the Wedge that holds the individual tensile element. The prestressing load is then transfer to the Bearing Plate. For some small tendon sizes, this prestressing force can be transferred directly to the structure without the need of Bearing Plate.

In the case of some mono anchorage system, the Wedge will hold an individual tensile element directly to the Bearing Plate.

Anchor Head and Wedge is subjected to Static Load and Fatigue (Reliability) Test. Hence its design and production quality control is of paramount importance.

Static Load Test requires the Anchor Head and Wedge to hold to a specific percentage (>95%) of the tensile element ultimate strength with a minimum elongation (>2%), with without premature failure of anchorage components, without undue deformations in the components, and without disproportionate relative movements between tensile elements and anchorage components.

As for Fatigue Test, the Anchor Head and Wedge have to go through 2 million cycles of fatigue loads (dynamic strength, 80MPa) without exceeding a specified percentage loss of tensile element cross section (<5%).

We hope this video provides some insight of our production process and gives our operators a better understanding of our commitment to quality control.

Bearing Plate is a very important element in a post tensioning system.  It serve the purpose of transfering prestressing force derived from the tendon (tensile element, eg. PC strand) to the concrete. And it is precisely in this, that explains what we want to achieve in prestressed concrete structures, and thus we take a special interest on the Bearing Plate.

In our earlier blog post we shared on the testing we conducted on ‘Load Transfer and Static Load Test of Anchors in accordance to European Technical Approval (ETAG 013)’, we posted some photographs of Bearing Plate casted into concrete blocks for Load Transfer Test. Well, this is to test the ultimate load bearing capability of the Bearing Plate (by means of a Stressing Jack) under a real-life situation. As ETAG 013 specification clearly states under Load Transfer to the Structure; “Post Tensioning systems shall be able to transfer a specified percentage of the tensile element ultimate strength from the anchorage into the concrete structure, of a defined strength class of concrete, without undue cracking of the structure, and at deformations which stabilise within a given time frame”.

However, we are not going in-depth on this test procedure and result. Rather we are taking an interest on the manufacturing of the Bearing Plate. Bearing Plate is generally manufactured with ductile iron (eg, ASTM A536, Grade 80-55-06) or grey iron (eg, HT200, GB/T9439-2010), casted in a steel foundry. There are numerous ways in the production of Bearing Plate. From an automatic production line (eg, DISA) to a brick & mottar method of using moulds, very much like how we produce precast concrete products.

We take a particular interest in a method we are familiar with, and that is, the usage of polyfoam to bring our Bearing Plate to life!

The polyfoam is produced to the exact same shape and size as the actual Bearing Plate. And then a layer of clay is coated over the polyfoam to form an ‘temporary’ mould over it. Molten iron is then poured into this temporary mould through a small opening, and the polyfoam inside will then be burnt away. With the disappearance of the polyfoam, the molten iron will then filled up the temporary mould. Once the molten iron cools down and harden, the removal of the exterior clay will then give us the Bearing Plate we wanted.

We posted some photographs of our Bearing Plate production process and we hope this will provide some insights of our daily production activities.

Duct is an important element for bonded post tensioning work. It is essential as it creates a void for the tendon or tensile element. By and large, steel corrugated duct provides the most economical solution to bonded post tensioning work.

Steel corrugated duct are manufactured from a long galvanized strip of steel, normally in the range of 0.25 – 0.40mm in thickness and can be 30 – 80mm in width. It depends on the type of duct forming machine used. These strips are then pressed into a corrugated form before they are fed into a turning wheel to make the edges interlock, and forming into a pipe. And in post tensioning terms, we call this pipe a ‘duct’. And duct can come in 2 types of shapes, round and flat. A steel corrugated flat duct is pressed into shape from a steel corrugated round duct and it is generally used for slab anchorage (not limited).

Duct plays a critical role in providing a corrosion protection environment to the tensile element or tendon (eg. PC Strand). And steel corrugated duct offers excellent bonding behaviour to tendon grout and also the concrete around it. And it is precisely because the duct is a protection to the tendon, its quality is paramount.

Due to the way the steel corrugated duct is produced, bad quality duct can cause huge problems like ‘bleeding’ during the construction process. Most of the time, it is the bad interlocking of the edges between adjacent strips that causes the duct to collapse under the weight of the concrete. This results the concrete to leak into the duct (term ‘bleeding’) and bond with the tendon even before stressing commence.

Operators are well aware of the repercussion bleeding can cause. During stressing, it causes the tendon elongation to fall under the required length even though the stressing jack and pump had worked to its required load. To sum it up, we call this prestressing lost (due to frictional effect). This is a situation everybody wants to avoid.

Duct is also available in plastic form, generally made of polyethylene / polypropylene material. Plastic duct has lesser bleeding issue and it is more commonly found in corrosive environment like waste water treatment tanks, acid tanks, silos etc. However steel corrugated duct is still the preferred choice for providing a homogenous environment to prestressed concrete structure, primarily due to its excellent bonding characteristic. And in some countries, plastic duct are barred because of fire safety regulation. This is because the burning of plastic during a fire hazard can emit poisonous gases and it is harmful to humans. 

TMG visited a few project sites and with the blessing of the main contractors, we managed to film the process of steel corrugated duct production. The source of the duct forming machine remains unknown.

PC strand is widely used in prefabricated or precasted concrete elements like hollow core slabs, girders, beams, railway sleepers, foundation piles, poles etc., and also in post tensioning constructions such as bridges, buildings, dams, tanks, silos etc. And also as tendons in bridge cable or stay cable. Its application is far and wide, both for pre-tensioning and post tensioning, bonded and unbonded applications.

TMG is one of the world largest PC strand manufacturer, our Low Relaxation 7-wire PC strand are manufacture with high quality wire rod in a variety of steel grades. In 2014 alone, we produced over 330,000 ton of PC strand and shipped to more than 35 countries around the world.

TMG is capable of producing PC strand with these standards ASTM A416, BS 5896, JIS G3536, AS/NZS 4672.1, EN 10138 etc., with a variety of sizes. Ranging from 9.30mm to 28.60mm in diameter. And all PC strand are delivered in precision wound coils with excellent pay-off properties. Standard coil weight is around 3 tonnes. Special coil weight is also available on request. And anti-rust proofing is also a common practice for coils that requires longer transhipment duration.

Below is a video showing our manufacturing facilities.