Post-Tensioning in the Precast Industry
Post-tensioning is not commonplace in most precast production. Its usually found in segmented bridge construction, cast-in-place structures and specialized construction. Why is it not used more? One factor is the cost – post-tensioning requires at least 3 components to work:
- Tendons / Strand
And in some cases a 4th component – grout – where bonded post-tensioning is used. All of this adds up to more material and labor cost compared to traditional pre-stressing where all that’s required is the strand in most cases.
Despite the added cost and complexity, there are cases where post-tensioning can be extremely beneficial compared to prestressing. To understand that, first:What is post-tensioning?
Post tensioning is applying tensile forces to strands or other special reinforcement within a concrete element after casting and anchoring these forces within the element such that it produces a compressive force. It is different from pre-stressing in that the compressive force is transferred to the member at a later stage in the life cycle.How is this done? There are actually 2 methods:
1. Unbonded Post Tensioning
Where bond between the concrete and the tensioning reinforcement is prevented, either by installing it in a duct or sleeve, or by using coated strand (with a plastic or rubber sheath). In the case of ducts or sleeves, the strand may be installed after casting, but coated strand will be installed before. Anchorages are usually installed at each end of the element and are similar to modified chucks. Anchorages facilitate tensioning and transfer of the forces to the element, while sealing the end of the duct or sleeve. Some sleeves are filled with grease to reduce friction during pulling and as a corrosion barrier on the strand. Ducts can be straight or draped in a variety of patterns as long as it allows the strand to pass freely without pinch points. The stressing force is applied to the element at the anchorages primarily and not along the length of the member.
2. Bonded Post-Tensioning
Where bond between the concrete and the tensioning reinforcement is promoted by installing it in ducts or sleeves which are then grouted (typically with a grout pump) on site. Special corrugated sleeves or ducts are used to transfer the force between strand, grout and concrete. The tension is usually applied first to an anchorage and then to to the grout once it is set, similar to pre-stressing. The force is therefore applied along the length of the reinforcement, not just at the ends.
So where can post-tensioning be useful?1. In large tanks or other retaining structures that require lateral/transverse compressive forces to seal joints and to counteract fluid pressure.
In this example, vertical panels are erected with transverse ducts which align between panels. The cables/strands are pulled through the ducts to the anchorages. The force acts on the whole structure to keep it in a circumferentially compressive state.2. In pre-stressed elements to create span continuity at supports or between members
Commonly used in bridge construction either for continuing spans or for segmented elements. For example ducts will align between adjacent elements, primarily along the axis of the element. When the tensile reinforcement is installed, it ties the elements to the support or together, to create continuity.3. In floor systems to counteract deflections
Thin cast in place or precast floors can be erected with ducts aligning in transverse and longitudinal directions, shored and then post-tensioned to achieve continuous one or two way floor plates which can be very economical for lightly loaded buildings such as residences, hotels and offices in multi-storey applications where floor to floor height is a premium.
For example precast ‘light walls’ are a type of primary structural shear wall element that are commonly found in parkades. They are moment-resisting frames characterized by large openings which make them susceptible to failure during handling. They also require a lot of non-prestressed reinforcement to resist lateral loading which makes it difficult to produce on a traditional prestressed bed. As a result, post tensioning can be deployed soon after casting to impart compressive forces to the member
Prestressed elements may be produced with straight or harped profiles which can be limiting in terms of counteracting special ultimate loading scenarios. By adding a post-tensioning duct with a freely varying eccentricity to the member’s longitudinal axis, a highly optimized application of compressive forces can be achieved.
- In molds
- On tilt tables
- On floor pallets
- In carrousel plants
Let us know what you think. Are you a precaster who has used post-tensioning in a unique way? If so we would love to hear from you on your experiences.http://precast.org/wp-content/uploads/2018/03/CP3-Post-tensioning-vs.-Prestressing-Miroslav-Vejvoda_FINAL.pdf