Since the introduction of post-tensioning technology in the construction market about 60 years ago, its evolution has been continuous in terms of performance and durability. In this context, it must be acknowledged that the efforts over so many years have heralded major improvements in the materials, products, specifications and working procedures.
Local standards and regulations were being adapted, at the same time as the technological advances were being made in post-tensioning systems.There is absolutely no question about the economic and technical efficiency of post-tensioning technology – and this is the reason that the time, cost and energy which have been poured into developing the technology further are totally justified. A further advantage of technological development has been the ability to deal with issues surrounding durability concerns in particular.
In the early projects carried out using PT technology, durability was not a concern – in the sense that it was considered an intrinsic feature of the technology itself – just as with reinforced concrete. Practical experience in the last few decades has demonstrated this belief to have been illfounded and has forced a more holistic consideration of durability – as a problem relating not only to the PT system in isolation, but also to the concrete structure as a whole.
With the introduction of the multilayer protection strategy, protection against corrosion is provided by waterproofing, dense impermeable concrete, sealed ducts and good quality grouts.The erstwhile design principles have been refocused to improve durability – and items such as waterproofing, drainage, concrete cover and concrete quality are taking a higher profile. In the case of PT, this has been provided with a first protective barrier against corrosion – namely, grout.The second protective barrier is provided by the duct. Commonly used metal corrugated or smooth ducts do not necessary constitute permanent corrosion protection. So, to mitigate the risks, the sealing of the ducts – and the entire tendon – has been improved, and the use of plastic ducts has been introduced.
When looking at corrosion protection for bonded tendons as a whole, grout is a key element. This is the last protection layer in the fight to prevent corrosion and to improve durability. For unbonded systems, soft filling materials such as grease and wax or plastic sheathing fulfil this function. Grout has two main tasks in a bonded tendon: - To protect the prestressing steel from corrosion.
- To provide the necessary bond between the strands and the structure member, in accordance with the design.
Cementitious grout provides excellent protection for prestressing steel, thanks to the highly alkaline environment which passivates it against corrosion. All the analysis demonstrates that the vast majority of PT structures have behaved satisfactorily – demonstrating excellent durability of the post-tensioning tendons with, at the very least, good design principles and best practice installation techniques. However, enough deficiencies were found to prompt a review of the quality of the grout specified for use – but most of all, of the grouting practice itself.
Throughout the years, the BBR experts have been working to produce a truly remarkable grout in terms of quality – using prime products in the market, leading equipment and grouting technology. This has made it possible for our
grout and grouting process to be fully compliant with all of the demanding requirements contained within the new standards – even long before they came into force.The BBR Network was so conscious of the importance of this aspect, that most grout tests – which nowadays are compulsory – had, for some years, already been conducted on most projects as a BBR internal requirement.
Whilst no single protection layer will guarantee durability, a good quality grout and good grouting practice are of prime impor tance for the robustness of any posttensioning installation. Grout can provide excellent protection for post-tensioned structures – even more than any other protection layer – and this vital tool is in our hands!
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