A: At its most basic level, post-tensioning (PT) is a fiendishly clever way of reinforcing concrete while you are building – occasionally even allowing the construction of something which might otherwise have been impossible!

The use of post-tensioning allows thinner concrete sections, longer spans between supports, stiffer walls to resist lateral loads and stiffer foundations to resist the effects of shrinking and swelling soils. Concrete has what engineers call ‘compressive’ strength – meaning that it happily bears its own weight within a structure. As soon as you introduce the ‘live’ loads of everyday usage, such as vehicles in a car park or on a bridge, the concrete tends to deflect or sag which leads to cracking, thus weakening the structure.

Concrete lacks ‘tensile’ strength. Alone, it does not always offer the flexibility needed. That’s why steel reinforcing bars – ‘rebar’ – are often embedded in the concrete to limit the width of cracks. However, rebar provides only passive reinforcement – that is, it does not bear any load or force until the concrete has already cracked.

This is where post-tensioning comes in. PT systems provide active reinforcement. The function of post-tensioning is to place the concrete structure under compression in those regions where load causes tensile stress. Post-tensioning applies a compressive stress on the material, which offsets the tensile stress the concrete might face under loading. PT is applied by the use of post-tensioning ‘tendons’ – a complete assembly including the very high strength prestressing steel strands or bar, the sheathing or protective ducting, plus any grout or corrosion-inhibiting coating surrounding the steel strands or bar and the anchorages needed at both ends.

A typical steel strand used for post-tensioning has a diameter of 15.7mm and a tensile strength around four times higher than an average non-prestressed piece of rebar. Sheathing or ducting houses the prestressing steel This allows it to move as necessary when the tensioning force is applied after the concrete cures. The steel stretches as it is tensioned and it is locked into place using an anchoring component, thus maintaining the force in the strand for the life of the structure.

A: The one-size-fits-all concept is no longer a viable one – optimization is needed to achieve the most environmentally-friendly structure. Opting to use post-tensioning means that a wider product range is available.

At BBR, we have now grouped our tendons so that you only use what you actually need. We offer more than 25 different sizes of multi-strand anchorages – ranging from 1 to 73 strands. All the size groups can be handled and stressed with just one piece of gear – the kit has been rationalized. In this way, we are helping to ensure that only the amount of anchoring material which is really needed goes into the structure, whilst minimizing additional transportation impacts for construction gear.

Post-tensioning tendons come in a number of varieties and cover a wide range of applications. There are four main types:

• Internal bonded tendons – where one or more strands are inserted into a metal or plastic duct that is embedded in the concrete. By filling the duct with special grout, the tendon is ‘bonded’ with the surrounding concrete. Internal bonded tendons are installed before concrete is poured and become locked in concrete. This approach is often used for bridges and heavily loaded beams in buildings – flat internal systems are also an excellent choice for thin slabs.
• Internal unbonded tendons – where the prestressing steel is not actually bonded to the concrete that surrounds it, except at the anchorages. They are used in slabs and slabs-on-ground for buildings and parking structures but also more and more in infrastructure projects.
• External unbonded tendons – these are installed on the outer surface of concrete structures. This type of post-tensioning allows access for maintenance and replacement, this is therefore the solution of choice for bridge enhancements and refurbishments.
• Ground anchors – these are used to stabilize sides of excavations, hillsides and tunnel walls. They are also used for resisting uplift for towers or seismic strengthening. CE-marked PT products are constantly under surveillance to ensure they continue to fulfill all requirements of their European Technical Approval.

A: The short answer is ‘easily’ – that is, by a Specialist Post-tensioning Specialist whose staff have completed the relevant training and have the backing of a major international organization.

Post-tensioned systems should only be designed and installed by PT Specialist Companies, such as those within the BBR Network. Designing the shape of the PT layout and specifying the right system is a vital stage in the process and requires sound engineering consideration in order to maximize the benefits for all the stakeholders in a project.

Work on applying the post-tensioning system can begin after any formwork or bottom reinforcement has been installed. Typically, for a bonded system, ducting will be laid out first, then strands will be installed with the help of a strand-pushing machine and end anchorages will be placed. But also off-site prefabrication of post-tensioning tendons can offer great benefits for certain construction projects.

Concrete placement follows next. When the concrete has attained its minimum strength, the strands are anchored in a common anchorage device and stressed with a large, multi-strand jack or mono-strand stressing tool. When the jack reaches the correct load, it releases the tendon. The tendon then retracts slightly, activating the anchoring system and creating a tight lock on the tendon. The anchor maintains the applied force in the tendon and transfers it to the surrounding concrete.

The duct is then filled with a cementitious grout that provides corrosion protection to the strand and bonds the tendon to the concrete surrounding the duct.