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The civil engineering-oriented channel Practical Engineering has a pretty good video on Roman concrete vs. modern. https://www.youtube.com/watch?v=qL0BB2PRY7k h

The civil engineering-oriented channel Practical Engineering has a pretty good video on Roman concrete vs. modern. https://www.youtube.com/watch?v=qL0BB2PRY7k https://www.youtube.com/watch?v=qL0BB2PRY7k The primary tldw; is the fact modern concrete uses steel rebar for tensile strength, whereas the Romans relied on architectural techniques to minimize tensile forces on their structures. Steel rebar is the primary route through which concrete deteriorates.

leoc 5y ago
There's also a YT channel about concrete run by a concrete-engineering academic, Tyler Ley: https://www.youtube.com/channel/UCrvfiHNDS_QI-FgKQSmTITQ https://www.youtube.com/channel/UCrvfiHNDS_QI-FgKQSmTITQ .
- anitil 5y ago
  He is so compelling!
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.
nojokes 5y ago
It is also possible to use carbon fiber reinforced polymer rebar.
- robbedpeter 5y ago
  And now graphene nanoflakes in the concrete, https://www.specifyconcrete.org/blog/graphene-reinforced-concrete-the-possibilities https://www.specifyconcrete.org/blog/graphene-reinforced-con... If you can get an economical, efficient route from co2 to graphene, using graphene flakes in composite materials might be an excellent method of carbon sequestration.
Eric_WVGG 5y ago
Architectural design isn’t everything, it appears that their concrete “recipe” was in fact different: “University of Utah geologist Marie Jackson… and her colleagues have found that seawater filtering through the concrete leads to the growth of interlocking minerals that lend the concrete added cohesion.” http://unews.utah.edu/roman-concrete/ http://unews.utah.edu/roman-concrete/ includes some tasty microscopic photography
mrtnmcc 5y ago
We couldn't use seawater for concrete today, again because of the rebar.
- drran 5y ago
  We should replace iron rebar with corrosion resistant fiberglass rebar.
  - bluGill 5y ago
    Steel and concrete have similar thermo coefficient of expansion factors, while fiberglass does not. Thus fiberglass rebar works in temperature stable environments, but if the temperature changes a lot it is prone to failure. The above is a generalization. See a proper engineer if you are actually building something to get the correct details for your application.
picsao 5y ago
Eh rebar can be almost eternal if protected by solar and a https://en.m.wikipedia.org/wiki/Galvanic_anode https://en.m.wikipedia.org/wiki/Galvanic_anode in the ground?

leoc 5y ago

There's also a YT channel about concrete run by a concrete-engineering academic, Tyler Ley: https://www.youtube.com/channel/UCrvfiHNDS_QI-FgKQSmTITQ https://www.youtube.com/channel/UCrvfiHNDS_QI-FgKQSmTITQ .

anitil 5y ago
He is so compelling!

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.

nojokes 5y ago

It is also possible to use carbon fiber reinforced polymer rebar.

robbedpeter 5y ago
And now graphene nanoflakes in the concrete, https://www.specifyconcrete.org/blog/graphene-reinforced-concrete-the-possibilities https://www.specifyconcrete.org/blog/graphene-reinforced-con... If you can get an economical, efficient route from co2 to graphene, using graphene flakes in composite materials might be an excellent method of carbon sequestration.

Eric_WVGG 5y ago

Architectural design isn’t everything, it appears that their concrete “recipe” was in fact different: “University of Utah geologist Marie Jackson… and her colleagues have found that seawater filtering through the concrete leads to the growth of interlocking minerals that lend the concrete added cohesion.” http://unews.utah.edu/roman-concrete/ http://unews.utah.edu/roman-concrete/ includes some tasty microscopic photography

mrtnmcc 5y ago

We couldn't use seawater for concrete today, again because of the rebar.

drran 5y ago
We should replace iron rebar with corrosion resistant fiberglass rebar.
- bluGill 5y ago
  Steel and concrete have similar thermo coefficient of expansion factors, while fiberglass does not. Thus fiberglass rebar works in temperature stable environments, but if the temperature changes a lot it is prone to failure. The above is a generalization. See a proper engineer if you are actually building something to get the correct details for your application.

picsao 5y ago

Eh rebar can be almost eternal if protected by solar and a https://en.m.wikipedia.org/wiki/Galvanic_anode https://en.m.wikipedia.org/wiki/Galvanic_anode in the ground?