The Ninian Central was finished in 1978. After 1978 the type of grout and method of application used to seal the prestressing tendon ducts in concrete
offshore platforms were improved. These improvements were made because it had been found the ducts, particularly horizontal ducts, were often not
fully filled. The cracks in the Ninian are vertical and of a considerable size. Given the harsh weather conditions it seems likely these cracks may
have reopened to some extent. I can't see how patching them could be expected to be 100% efficient.
This means most likely seawater has been in contact with a large number of horizontal prestressing tendons. Can any engineers suggest what effect that
is likely to have had on the integrity of the structure?
This is the wikipedia entry on prestressed concrete.
This is the relevant information from that entry.
"History of problems with bonded post-tensioned bridges
The popularity of this form of prestressing for bridge construction in Europe increased significantly around the 1950s and 60s. However, a history of
problems have been encountered that has cast doubt over the long-term durability of such structures.
Due to poor workmanship of quality control during construction, sometimes the ducts containing the prestressing tendons are not fully filled, leaving
voids in the grout where the steel is not protected from corrosion. The situation is exacerbated if water and chloride (from de-icing salts) from the
highway are able to penetrate into these voids.
Notable events are listed below:
The Ynys-y-Gwas bridge in West Glamorgan, Wales – a segmental post-tensioned structure, particularly vulnerable to defects in the post-tensioning
system – collapsed without warning in 1984.
The Melle bridge, constructed in Belgium during the 1950s, collapsed in 1992 due to failure of post-tensioned tie down members following tendon
Following discovery of tendon corrosion in several bridges in England, the Highways Agency issued a moratorium on the construction of new internal
grouted post-tensioned bridges and embarked on a 5-year programme of inspections on its existing post-tensioned bridge stock.
In 2000, a large number of people were injured when a section of a footbridge at the Charlotte Motor Speedway, USA, gave way and dropped to the
ground. In this case, corrosion was exacerbated by calcium chloride that had been used as a concrete admixture, rather than sodium chloride from
In 2011, the Hammersmith Flyover in London, England, was subject to an emergency closure after defects in the post-tensioning system were
Here is the wikipedia entry on offshore concrete structures.
I can't help noticing that structures were built in 73, 74, 75, 76, 77, 78, then a gap till 81, 82, 83, 84. Is this just coincidence or were lessons
edit on 6-3-2012 by Kester because: was to were