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Comment by schiffern | Hacker News Reader

So the question becomes, how do existing concrete manufacturers determine whether locally sourced materials ("random rocks") are suitable?

In traditional dry stone construction, strength of each rock is determined by sound and by sight. It seems crazy, but nowadays this should be feasible with a neural net. If you want to get fancy (or achieve high reliability) add ultrasound in the grippers or other sensor modalities.

Or... you target a market other than building foundations? :) I suspect different groups could pursue both paths.

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A Deep Learning Based Method for the Non-Destructive Measuring of Rock Strength through Hammering Sound Appl. Sci. 2019, 9(17), 3484; https://doi.org/10.3390/app9173484

Acoustic inverse problems are solved routinely in seismology and exploration geophysics. Seeing trough a rock the cracks and internal structure using acoustics seems solvable.

> how do existing concrete manufacturers determine whether locally sourced materials ("random rocks") are suitable?

They test individual samples from each pour on a calibrated press. For large buildings and critical pours, they'll keep & test multiple samples that undergo accelerated aging to make sure they're up to spec for the anticipated lifetime of the building (or until the concrete fully stabilizes over a few months / years).

So could catagorization of local stone based on acoustic and visual indicators, combined with random sampling and strength testing provide the assurance needed that a wall will have the required structural properties?

The point is that the concrete manufacturers pulverize and homogenize the aggregate. This makes random sampling acceptable.

If you haven't pulverized and mixed the inputs, a random sample is merely a random sample. It isn't meaningful unless it comes from a high-entropy source.

So are you saying we couldn’t estimate the structural properties of autonomous built dry walls from local stone? Or that the error bars would be unacceptably large? Or just that this approach would only work with certain stone in certain locations?

I think the problem is more that you cannot at all guarantee a specific performance of the structure ahead of time, because you can't know what the properties of the local rocks will be before you are on site and have measured them all.

Geotechnical engineering is all about taking samples and using wide enough safety margins. And in the vast majority of cases, the end result is a building that doesn't fall down.

Sure, you might have been really unlucky that all your test samples of soil and rock were really strong granite, but the exact spot you put the main structural pile, unknown to you, was all mud and silt... But the chances are so vanishingly unlikely we accept it.

The interesting thing here is you can feasibly measure them all.

Not ahead of time, but as you build.

Sure, but then it's somewhat up to luck precisely what properties you get. This variation might me manageable if you design enough redundancy into the design, but by necessity you will need more margins than if you used a more controlled material.

Doesn't that apply to wood too? Yet we use wood to build houses almost everywhere.

People rarely build entirely using found wood on site, at the very least each tree to be used gets selected before you bother felling it. But the overwhelmingly most common use of wood is in the form of processed product: using quality controlled lumber of standardised size.

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