I promise I will move on from San Francisco, but before I do, the 3rd thing on the list would surely be Alcatraz, and I was amazed/chuffed to see that engineering made its way into part of the tour!
"The Rock", a 12 acre island 1.5 miles from San Francisco is now a national park visited by droves of people every year. Its life has seen it as a military base, military prison, civilian federal prison and site of a major native indian demonstration before it became the tourist attraction it is today.
It is fascinating (and creepy) to walk around the jail, go into an isolation cell, and hear the stories of its past.
It's also interesting however that part of the exhibition discusses the jailhouse structure. This was constructed by military prisoners, and on completion in 1912 was apparently the largest reinforced concrete building in the world. It comprises 600 cells, a kitchen, dining hall, hospital, recreation yard and administration offices.
For anyone that knows us personally, it will not surprise you that yet again Ally and I have managed to be surrounded by scaffolding as we visit something! And this time, it's the cellhouse. Over 100 years of weathering the harsh climate of water and salt has deteriorated the structure, and there is now a major project underway to rehabilitate the reinforced concrete walls.
There is now a Concrete Preservation Institute programme partnered with the National Park Service and local universities to preserve and repair this historic landmark, and they're explaining it to the public too using information boards.
They explain the process of how water and salt has infiltrated the concrete and corroded the steel reinforcement bars. They even note that this is referred to as 'rebar'. The boards then go on to explain the solution of cleaning or replacing the damaged rebar and coating it with an epoxy resin. (Interestingly, along the same theme, I don't know if anyone saw, but recently some research into the use of epoxy resins for desalination systems has also been undertaken, https://www.google.com/amp/www.bbc.co.uk/news/amp/39482342#ampshare=http://www.bbc.co.uk/news/science-environment-39482342).
Along with rebar refurbishment, the repairs include applying an electric current and connecting rebar to a more easily corroded metal (in this case Zinc), and this sacrificial metal corrodes instead of the rebar.
Because of these walls, and the surrounding sea, this place was seen as a fortress. And yet, some things are apparently stronger than concrete and cold water...like spoons and determination. Immortalised by the 1979 Hollywood drama "Escape from Alcatraz" starring Clint Eastwood, in 1962 three prisoners used mess spoons to dig their way through their cell walls, into the ventilation shafts, and were never seen again. Alcatraz federal prison closed the following year. Surprisingly enough though, salt corrosion is included in design where appropriate, but spoon attack is yet to make the process!
