It turns out that replacing the conventional aggregates of concrete--sand, gravel, crushed stone, even volcanic ash--with glass is not really a stretch of the imagination.

The Romans might have no trouble building their roads, buildings and dams with glass-aggregated concrete. But MODERN concrete is still problematic for two reasons:

  1. It's not as tightly packed as ancestral concrete, meaning it's more porous
  2. It's fitted with steel reinforcement bars for better durability, higher relative strength and higher tolerance of tensile stress.

For these two reasons, modern metropolitan areas are estimated to survive only an average of 100 years in a Life After People.

In this alternate scenario, glass has been the aggregate of both Roman and Modern concrete. Hoover Dam is now a vast wall of glass-aggregated concrete. New York City is a concrete jungle of cement, water and glass. Route 66 is now one of many roads made from glassphalt concrete. Special precautions are made to make sure that Hoover Dam is curing at a slow pace, making it stronger.

Now...is it practical to reinforce concrete with a glass aggregate? If yes, then in a New York and Chicago climate, how long would they stand in a Life After People?

  • $\begingroup$ What was their motivation to use energy-expensive glass instead of whatever rock they had laying around? $\endgroup$ – JDługosz Aug 19 '16 at 3:38
  • $\begingroup$ seas.columbia.edu/earth/wtert/sofos/meyer_egosi_paper.pdf $\endgroup$ – JohnWDailey Aug 19 '16 at 5:11
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    $\begingroup$ @JohnWDailey, the paper you linked lays it out as an economic and recycling case, top of page 5 specifically mentions bonding issues between cement and glass aggregate, making glass concrete weaker. $\endgroup$ – Josh King Aug 19 '16 at 12:52
  • $\begingroup$ sand is the glass - why you have to replace it $\endgroup$ – MolbOrg Aug 19 '16 at 13:29
  • $\begingroup$ @MolbOrg Because sand is sand and glass is glass. $\endgroup$ – JohnWDailey Aug 19 '16 at 14:16

Your question seems to lack some basic understanding of what concrete is. People often use the terms cement and concrete synonymously, but these two are not the same thing. Cement is a binding agent (aka glue) made mostly of calcium carbonate. Concrete is made of a mixture of a binding agent (usually cement), mixed with a filler/aggregate (normally sand and small rocks or gravel), and also water to activate the binder.

Changing the aggregate is not going to greatly alter the lifespan of the concrete. Concrete normally fails by cracking or wearing due to weakness in the cement binder (which is weaker than the rock aggregate), often when under tensile loads pulling it apart (which is why steel is added to concrete to reinforce it). Glass concrete aggregate would not be much stronger than normal concrete. Silica glass is actually very similar to other mineral rock sources commonly used as aggregate in concrete.

Roman concrete lasted longer because the volcanic ash added to ancient cement is created a binding agent that performs differently than modern Portland cement.

I personally would be interested seeing the reference source that says that modern cities would only last a hundred years unattended. Buildings might possibly fail structurally and fall down, but other than skyscrapers any concrete is going be around for a long time; and even a failed tall building is going to leave a substantial rubble pile. Hoover dam is was completed in 1936 and is in no danger of disappearing, and it isn't as though they have replaced substantial sections of concrete or modified it structurally. Some estimates say that it could last for another 10,000+ years.

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  • $\begingroup$ Yeah -- modern concrete can last quite a while as well -- fly ash pozzolans are the key to this (they aren't far off from the volcanic ash pozzolans used during Roman times). We also have innovations such as superplasticizers which allow us to ship and place high grade concretes (water-to-concrete ratios below 0.5) in bulk (or even up high!) without driving everyone nuts. $\endgroup$ – Shalvenay Aug 19 '16 at 4:59
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    $\begingroup$ «question seam to lack some basics of what concrete is» this was reviewed in previous questions, where the nomenclature was initially quite mixed up. Here, he’s talking about the aggregate as the rocks, which is right. There were also some links powted to serious info about Roman concrete, and as I recall some discussion on why modern building would fail (according to that book and a few special cases). $\endgroup$ – JDługosz Aug 19 '16 at 5:57
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    $\begingroup$ Maybe he got nomenclature bit better this time, but still lacks basic understanding of what actually cracks, what's responsible for what etc. "Reinforce concrete with glass aggregate" is misconception - glass is no reinforcement whatsoever, if used as aggregate. $\endgroup$ – Mołot Aug 19 '16 at 16:03
  • $\begingroup$ @Mołot I'm not talking about glass as reinforcement. I'm talking if it's possible to fit concrete with a glass aggregate with steel rebar. What am I not getting here? $\endgroup$ – JohnWDailey Aug 19 '16 at 21:05
  • $\begingroup$ @JohnWDailey: Although there is no reason why you can't embed steel rebar inside glasscrete, one thing you may have neglected is that the cement/binder may have difficulty actually griping the aggregate. Glass normally has both smooth surfaces (preventing mechanical bonding) and is non-reactive to chemicals (preventing a chemical bond). Unless you use something superior to cement for bonding chunks of glass together, your glasscrete may be very weak indeed. On the other hand, remelting glass chunks so that it bonds to itself works really well, but that results in glass blocks. $\endgroup$ – Mark Ripley Aug 21 '16 at 8:29

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