Re: The Boston Arch (Aquarium parking garage)
Jeez people. I didn't realize that everyone in this thread has a PhD in concrete durability.
Nitpicking about deicing salts aside, my question still stands. A concrete structure exposed to 40years worth of freeze thaw cycles, salt intrusion and cyclic loading will not last forever. Does anyone have a ballpark on the design life of concrete parking garages? Eventually they will HAVE to come down and be replaced by something else. If that time is within the near future (~20 years IMO), why settle when we can wait for something more appropriately sized?
I don't have a PhD but definitely do have some relevant experience in this subject. Some of the comments regarding this issue have been pretty silly.
thinkbiggerboston was perfectly correct. Exposed reinforced concrete particularly in a chlorine rich environment (e.g. road salt and near the ocean) are major issues in the structural lifecycle of reinforced concrete structures.
Here's a
Globe article on exactly this issue - impact of environment on reinforced concrete structures. Read this if you want a good summary of the issue. For a story written by a journalist and not an engineer it's not bad but not entirely correct. It unfortunately tries to tie the issue to climate change which further confuses the issue.
"[There are] two processes that slowly eat away at concrete: carbonation, in which carbon dioxide diffuses into concrete, and chlorination, in which chloride ions, dissolved in water, are absorbed into concrete (especially a problem near the ocean, where chloride-rich salt spray coats buildings). Eventually the carbon dioxide or chlorides reach the rebar inside, causing it to corrode and expand. The process is largely invisible until a building’s façade cracks or chunks of concrete fall from a highway overpass."
This process is exacerbated in structures like bridges and garages that are directly exposed to road salts. And also that have significant live loads from moving cars and therefore have cyclic loading and deflections. The potential for corrosion is mitigated by using
epoxy-coated rebars which seek to keep the chlorine away from the steel and providing deeper concrete '
cover' of the reinforcement (i.e. the perpendicular distance from the face of the concrete to the edge of the reinforcement.
There is not pat answer to how long such buildings will last. As usual it depends on the original detailing, the concrete mix design and the quality of the original construction and probably mostly on the level of maintenance. There are also several remediation measures that can be undertaken. The article discusses a lot of these issues. Structures built in the 60's like the garage will be particularly susceptible to these issues. The article refers to a code-based 75 year expected design life which might then be shortened by 20 years or more because of climate change. The point is that despite climate change the 75 years is already proving optimistic (subject to varying capex for maintenance).
Frankly I'm not sure I find the technical study referred to in the article about the impact of climate change on these structures all that persuasive. But it is another thing to consider. All of this said, with respect to the garage it can be assumed that Chiofarro knows these issues fairly well as he will have had a technical appraisal of the condition and potential problems of the structure performed at the time of the purchase and likely more recently in order to perform maintenance.
Note that these issues are very similar as for the now condemned parking structure beneath UMass Boston and precisely for these reasons. Refer to
this study by Simpson Gumphertz & Heger who are very much the local experts in this field. This building was built at the same time (early '70s) as the Harbor Towers Garage and is located in a similar environment (ocean, parking) and was decommissioned almost 10 years ago.
The conclusion of SGH's report is:
"Exposure to salt-laden water over many years has caused severe and widespread corrosion damage at the upper and lower levels; this is not unusual for structures of this vintage and exposure. The predominant corrosion damage is delaminated and/or spalled concrete over corroded reinforcement in the top of the slabs.
Unlike the upper and lower levels, the plaza level structure was protected from the elements for many years by waterproofing. Consequently, the amount of deterioration of the plaza level structure is far less than that of the upper and lower levels."
Just as in the case of the UMass building a plan for long-term repair can be undertaken. However, as in that case, it is likely to be cost prohibitive and indicate a preference to take it down and build something new.
