[theoh]

You have to build formwork to construct with in-situ concrete, so there's still carpentry (or equivalent) involved.

It's a good point that existing architectural concrete 3d printing creates "mass concrete" not reinforced concrete, but mass concrete can be structural.

[jethro_tell]

What sort of structure would you use a concrete without a tensile support like rebar or steel of some kind? How would that stand up in an earthquake?

[burfog]

Rebar makes earthquakes worse, as seen in California's freeway collapse. The rebar depends on the concrete to prevent corrosion, but that is only good for roughly 50 years. Rust causes expansion, which cases cracking. The outer concrete falls away, leaving rebar to get crushed under the load. The obvious fixes, like stainless steel rebar, have thermal expansion coefficients that don't match concrete, so instead you get cracking even without corrosion.

Lots of Roman stuff is still standing in areas that get earthquakes. The solution is to use a conservative design, with arches and thick walls. Domes are good. We can improve on this with 3D printing, using a structure like mammalian bone: solid near the surface, and spongy in the middle.

[wjnqx]

Would you be able to provide links to modern structures built with the methods you're describing or research about them? Does this conservative design resemble something like Gaudi's Sagrada Familia, or is that the wrong way to think about it?

[theoh]

Here's one modern example: https://pagethink.com/v/project-detail/Wiss-Janney-Elstner-A...

It really means massiveness and stability, in order to have an acceptable margin of safety.

One aspect of the theory is the notion of a line of thrust: https://en.wikipedia.org/wiki/Line_of_thrust

Arch dams might be the only type of "contemporary" looking structure that is habitually made in this way out of unreinforced concrete.

Although Gaudi was interested in structural optimization (using catenary models), he is an outlier in terms of design. He didn't comprehensively consider seismic aspects, though apparently he didn't do too badly: https://blog.sagradafamilia.org/en/divulgation/seismic-activ...

[tomcam]

These links were eye-openers. Thanks

[wjnqx]

Thanks! Those were some interesting reads.

[theoh]

"Primitive" structures and other structures engineered to avoid tension in the concrete. Concrete doesn't have zero tensile strength. Apparently it's a few MPa, so a fifth or a tenth of the compressive strength. http://www.engineeringtoolbox.com/amp/concrete-properties-d_...

The large dome of the (Roman) Pantheon is unreinforced concrete. Most compressive structures built before the modern period (arches, domes, buttresses) were not reinforced with tension elements, just plain masonry.

[crwalker]

The concrete in the Pantheon is exposed to max ~20 psi tensile stress, and the concrete can handle about ~200 psi tensile stress.

http://www.romanconcrete.com/docs/chapt01/chapt01.htm