Recent years were marked with fast development of post-tensioning system that assures protection of all steel parts of the system against moisture for the whole useful life of the structure. Corrosion was always a problem for any steel structures, and steel strands and anchors of post-tensioning system is not an exception.
Bad experience is valuable experience
Different American specialists mentioned that several significant events drew attention to the problems of corrosion of strands in post-tensioning concrete. It happens because water penetrates in anchors because of bad concrete pouring and sealing of pockets. We can make conclusion thereof that it is possible to eliminate the corrosion problem by making necessary technology modifications.
In addition to that modification of technology can smooth over some negative prejudices relative to the corrosion of strands. Several years ago Post-Tensioning Institute (PTI) started to elaborate this question and examine the ways to apply established technologies in order to avoid the moisture penetration.
Installation and standards of post-tensioning system Installation of post-tensioning system consists of placement of sheathed strands and high-strength steel anchors in the formwork. Anchorage is intended to transfer post-tensioning forces to the concrete. Anchorage includes anchor, bearing plate and wedges. After concrete has hardened, end of post-tensioning strand is pulled to several centimeters using hydraulic jack. Once the jack stopped, wedges set into the anchor cavity, thus keeping strand in its position. Then we cut strand tail till the anchor and inject the pocket with nonshrink grout. We use grout to ensure anti-corrosion protection, but if the injection has not been properly made, water can penetrate in the system. Anchorages are generally more sensitive to the corrosion; that is why most standards and specifications require the use of encapsulated anchorage system for projects situated in aggressive environment. (Encapsulated tendon is entirely covered with protective waterproof sheath, from one end to another, including anchors, strand protective coating, grease, sleeves and caps installed on the strand tails at both its ends.)
Changing impression on the USA market
According to the questioning of representatives of different post-tensioning industries in the USA, made in 2014, encapsulated system is used with different intensity, but all of them are sure that during coming years this system will be used in 100% of cases.
Today the application of encapsulated strands and anchors in construction of commercial buildings is broadly spread in the region. This system is more often used for projects situated in aggressive environment with high humidity. In dry areas though it is used not that often. In open structures, such as for instance parking, encapsulated system is installed in 90% of cases.
But as we can see, encapsulated system became more popular not only in the projects situated in aggressive environment, but also for projects where structure has quite good protection against external influence. We can explain it very simple: «Quality of works is one of the most important factors affected tendon bearing capacity. More importantly, sometimes strands can suffer from water because of some unexpected circumstances during construction works ».
Certainly, system with additional protection can’t cost less than the one without anti-corrosion protection. But we also need to take into account several additional factors here:
2. Relative share of post-tensioning system cost in the price for entire building frame.
3. Relative share of building frame in the price for whole building.
For this reason, taking all these factors into account, it’s understandable that even if the cost of encapsulated anchorage system is a little higher, it is nothing if we compare with the total cost of the building! And advantages of this system are obviously more important:
1. Due to encapsulated system we create the positive impression about post-tensioning system in minds of our Clients.
2. Installation of post-tensioning system provides long-term protection and, with that, guarantees the correspondence of structural member designed load capacity.
3. Concrete durability is increased.
4. Application range of post-tensioning system is getting wider, and, as consequence, it provides the possibility for development of new engineering decisions and architectural forms.
Finally, the increasing efficiency of unbonded tendons make a positive effect on the whole post-tensioning industry.
Despite the fact that encapsulated system is just started to gain its popularity, it improved already the image of post-tensioning system and increased its application in building structural systems. Encapsulated system will turn post-tensioning into the most sustainable construction solution.
According to estimates of American specialists, construction norms should be amended accordingly within five years.
And here we have to admit that if in our country all new materials and technologies proved their effectiveness, were introduced into norms within five years, Russian construction market, probably, would be absolutely different.