4 ms·

I did a quick check, and it looks like rebar concrete has a lifespan of about 50 years, and I had to check the lifespan of post-tensioned concrete because I sus

by grp000 5y ago

I did a quick check, and it looks like rebar concrete has a lifespan of about 50 years, and I had to check the lifespan of post-tensioned concrete because I suspected it has a shorter lifespan. Most sources are saying 15-20 years for post-tensioned. Any structural engineers able to report in? If that is the case, it's a bit surprising, given how expensive buildings are, although technically I believe post-tensioning can be replaced, but I also imagine that replacement cost would also be high. Edit: Thanks, civeng, this is a good warning to be aware of validation bias when googling. Here is a paper that cites comparisons of post-tensioned concrete bridges. From the paper: "As an example, two optimal post-tensioned concrete box-girder road bridges located in an eastern coastal region of Spain were assessed... In addition, maintenance was optimized to ensure that the bridge complied with all the performance requirements during its life-span of 150 years." https://www.researchgate.net/publication/320444421_Life-Cycle_Assessment_A_Comparison_between_Two_Optimal_Post-Tensioned_Concrete_Box-Girder_Road_Bridges https://www.researchgate.net/publication/320444421_Life-Cycl...

civeng 5y ago
Post tension should not decrease the lifespan, if anything in some applications it could increase the lifespan since there can be smaller or less cracks, which is easier to maintain. Maintenance is the main requirement to get 100+ years in a normal environment.
tingletech 5y ago
If that is true; how could I refinance my 30 year loan on a 20 year old post tension pedistal slab condo? And why isn’t replacing the slab in our reserves budget?
- tingletech 5y ago
  Also, the first rebar structure, in golden gate park, is still extant https://99percentinvisible.org/episode/episode-81-rebar-and-the-alvord-lake-bridge/ https://99percentinvisible.org/episode/episode-81-rebar-and-...
- escape_goat 5y ago
  Let me confer my profound hope that providence will triumph over irony.
- redis_mlc 5y ago
  What is being learned in Surfside is this: - according to two recent independent structural model calculations based on the original plans, the building that collapsed was deliberately designed to meet only 100% to 115% of the original static loads to save money, but not the dynamic loads of rolling vehicles or interior upgrades like granite counter tops and marble wall displays. Also, there were a few 12" columns, no column caps, and not enough shear wall areas or elevator core tie-in to resist twisting from wind loads or prevent domino column failures. - any number of events could have started the collapse because there was no safety margin, including water intrusion and previous hurricane wind loads, but it was probably vehicles rolling around the lobby level near the pool. To answer your question, it would cost more than the building was worth to replace the columns and slabs. The 40-year review for that building really meant a teardown had they redone the structural model. The owners were already looking at a $100,000 to $200,000 bill each for cosmetic repairs. Because of absentee owners (lessors and airbnb), that level of investment for a rebuild doesn't fly. It's believed that the former building inspector gave a positive building review speech so that cronies could continue selling units, as negative news would have to be reported to potential buyers, killing any sales. Note that builders who looked at the post-collapse photos even one day afterwards all wrote, "Nothing looks right in those fotos." What bothered them the most were the columns that punched through with no debris stuck to the top (pejoratively called "those toothpicks"), and yards of rebar bundles that also looked too clean.
  - comex 5y ago
    FYI, you're shadowbanned.
    - Cthulhu_ 5y ago
      Why can people see shadowbanned people anyway? Doesn't that defeat the whole purpose? As is pointing out that they are.
      - nathancahill 5y ago
        It's a setting in your profile: showdead
dirtyid 5y ago
Gossip from a prominent building technologist is 40-80 years for mass produced concrete condos. 80+ years is expected lifespan, if all goes right. 40 is serious structural issues from corner cutting. This is in Canada with bad weather cycles. Usually civil infra has better oversight. It was... a lot less than I expected. But long enough to flip condos for a few more cycles until crazy contingency fees + insurance, which I guess is enough.
jaclaz 5y ago
>Most sources are saying 15-20 years for post-tensioned. Which sources? Not a structural engineer (but I have been site manager for quite a few post-tensioned bridges building sites), you may be confusing pre-tensioned with post-tensioned. In pre-tensioned there is no difference with "normal" rebar, the steel is in direct contact with the concrete. In post tensioned you cast the concrete embedding pipes where you later insert the cables, then you tension the cables and inject in the pipes (and around the cables) some sort of corrosion preventing mixture, this used to be a mix of cement + additives (very, very fine, no sand nor other aggregates), but depending on the project it can nowadays be also some mix of grease/wax (in this latter case cables can be de-tensioned, removed, replaced and re-tensioned). So, unlike the pre-tensioned structures and common rebars the cables in post-tensioned structures the cables are better protected and less likely to suffer corrosion deriving from infiltration of rain or water, so that part of the structure is - in theory - less prone to aging. Still, the cables only represent a part of the structure, the rest is "kept together" by "common" re-bar steel, and these are - generally speaking - nearer to the surface of the structure, so I would expect properly built post-tensioned structure to have the same life-span of normal reinforced concrete. IMHO 50 years is a very low threshold for the lifespan of any reinforced concrete structure, including pre-tensioned structures, while post-tensioned should have - at least for the cables - a longer lifespan. Anecdotally, the first bridges I contributed to build are now around 40 years old and show no particular signs of decay.
throwaway0a5e 5y ago
>I did a quick check, and it looks like rebar concrete has a lifespan of about 50 years, The variability based on type of rebar and the environment it is installed in is too high to make statements like that. Galvanized rebar in concrete in non-marine environment will last hundreds of years. The concrete will probably crumble around it before the rebar corrodes if there's any appreciable moisture and freezing temps. Epoxied rebar will last less, bare rebar less still and improperly installed epoxy rebar will last less than that. There are tons of 100yo buildings in fairly moist temperate climates with (poorly, by modern standards) reinforced concrete foundation that used bare rebar and are doing just fine.

by grp000 5y ago

civeng 5y ago

Post tension should not decrease the lifespan, if anything in some applications it could increase the lifespan since there can be smaller or less cracks, which is easier to maintain. Maintenance is the main requirement to get 100+ years in a normal environment.

tingletech 5y ago

If that is true; how could I refinance my 30 year loan on a 20 year old post tension pedistal slab condo? And why isn’t replacing the slab in our reserves budget?

tingletech 5y ago
Also, the first rebar structure, in golden gate park, is still extant https://99percentinvisible.org/episode/episode-81-rebar-and-the-alvord-lake-bridge/ https://99percentinvisible.org/episode/episode-81-rebar-and-...
escape_goat 5y ago
Let me confer my profound hope that providence will triumph over irony.
redis_mlc 5y ago
What is being learned in Surfside is this: - according to two recent independent structural model calculations based on the original plans, the building that collapsed was deliberately designed to meet only 100% to 115% of the original static loads to save money, but not the dynamic loads of rolling vehicles or interior upgrades like granite counter tops and marble wall displays. Also, there were a few 12" columns, no column caps, and not enough shear wall areas or elevator core tie-in to resist twisting from wind loads or prevent domino column failures. - any number of events could have started the collapse because there was no safety margin, including water intrusion and previous hurricane wind loads, but it was probably vehicles rolling around the lobby level near the pool. To answer your question, it would cost more than the building was worth to replace the columns and slabs. The 40-year review for that building really meant a teardown had they redone the structural model. The owners were already looking at a $100,000 to $200,000 bill each for cosmetic repairs. Because of absentee owners (lessors and airbnb), that level of investment for a rebuild doesn't fly. It's believed that the former building inspector gave a positive building review speech so that cronies could continue selling units, as negative news would have to be reported to potential buyers, killing any sales. Note that builders who looked at the post-collapse photos even one day afterwards all wrote, "Nothing looks right in those fotos." What bothered them the most were the columns that punched through with no debris stuck to the top (pejoratively called "those toothpicks"), and yards of rebar bundles that also looked too clean.
- comex 5y ago
  FYI, you're shadowbanned.
  - Cthulhu_ 5y ago
    Why can people see shadowbanned people anyway? Doesn't that defeat the whole purpose? As is pointing out that they are.
    - nathancahill 5y ago
      It's a setting in your profile: showdead

dirtyid 5y ago

Gossip from a prominent building technologist is 40-80 years for mass produced concrete condos. 80+ years is expected lifespan, if all goes right. 40 is serious structural issues from corner cutting. This is in Canada with bad weather cycles. Usually civil infra has better oversight. It was... a lot less than I expected. But long enough to flip condos for a few more cycles until crazy contingency fees + insurance, which I guess is enough.

jaclaz 5y ago

>Most sources are saying 15-20 years for post-tensioned. Which sources? Not a structural engineer (but I have been site manager for quite a few post-tensioned bridges building sites), you may be confusing pre-tensioned with post-tensioned. In pre-tensioned there is no difference with "normal" rebar, the steel is in direct contact with the concrete. In post tensioned you cast the concrete embedding pipes where you later insert the cables, then you tension the cables and inject in the pipes (and around the cables) some sort of corrosion preventing mixture, this used to be a mix of cement + additives (very, very fine, no sand nor other aggregates), but depending on the project it can nowadays be also some mix of grease/wax (in this latter case cables can be de-tensioned, removed, replaced and re-tensioned). So, unlike the pre-tensioned structures and common rebars the cables in post-tensioned structures the cables are better protected and less likely to suffer corrosion deriving from infiltration of rain or water, so that part of the structure is - in theory - less prone to aging. Still, the cables only represent a part of the structure, the rest is "kept together" by "common" re-bar steel, and these are - generally speaking - nearer to the surface of the structure, so I would expect properly built post-tensioned structure to have the same life-span of normal reinforced concrete. IMHO 50 years is a very low threshold for the lifespan of any reinforced concrete structure, including pre-tensioned structures, while post-tensioned should have - at least for the cables - a longer lifespan. Anecdotally, the first bridges I contributed to build are now around 40 years old and show no particular signs of decay.

throwaway0a5e 5y ago

>I did a quick check, and it looks like rebar concrete has a lifespan of about 50 years, The variability based on type of rebar and the environment it is installed in is too high to make statements like that. Galvanized rebar in concrete in non-marine environment will last hundreds of years. The concrete will probably crumble around it before the rebar corrodes if there's any appreciable moisture and freezing temps. Epoxied rebar will last less, bare rebar less still and improperly installed epoxy rebar will last less than that. There are tons of 100yo buildings in fairly moist temperate climates with (poorly, by modern standards) reinforced concrete foundation that used bare rebar and are doing just fine.