No! Flexural strength of 50MPa!!! And rammed earth only has a compressive strength of 4.3MPa! Concrete has a flexural strength of around 30MPa, and I don't know math so I don't know what the reinforced concrete is like, but this is still stronger than even cooked bricks.
It looks like because of the composition of the martian soil they figured this would never work without a bonding agent and that it would require heating, and then somebody tried it anyway, and it worked. But here's the neat takeaway: The method they used to form it, along with particle size, and whacking it versus slowly loading it, significantly strengthened the resulting bricks. Nobody expected these things to happen, it seems.
From article:
"Loading rates were either quasi-static [..] or impact [..]. The lateral boundary condition of compaction loading was either rigid (confined by a steel wall), free, or flexible (confined by an elastomeric wall)"
"Compacted Mars-1a solids were cut into beams and subjected to three-point bending tests"
"It is remarkable that the lateral boundary condition of compaction loading significantly influences the strength and the shape of the compacted solid, specifically, the thickness-to-diameter ratio. Compared with the boundary condition, the effect of loading rate is secondary."
"With the rigid boundary condition, the compacted Mars-1a samples are structurally integral but the flexural strength is relatively low, comparable with ordinary clay bricks. When the lateral boundary is free or flexible, R is nearly 3 times higher than that of rigid boundary condition.
Between the free and flexible boundary conditions, the flexible boundary condition reached high R at much lower Pmax and the resultant solid thickness was only slightly smaller than that of the rigid boundary condition, much larger than that of the free boundary condition. The strength-wise efficacy of forming inside a flexible boundary is ~150–200% greater than the free boundary for R ~30 MPa, taking into consideration the flexible boundary’s larger sample sizes."
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Concrete alone has a shitty tensile strength, compared to its compressive strength. If you add steel cables to the concrete, the hybrid material's tensile strength increases.
Similarly, Cod is what you get when you add natural fibers and other material to rammed earth. Its tensile strength is also significantly higher due to the addition of the fibers.
So then my next question is: could they bring along some cheap fibers (like mylar?) to strengthen these Mars-1a bricks even more? Maybe the way the materials are forming now, they don't need added tensile strength, or the fibers could interrupt the formation somehow. But it's worth trying...
Impressive! Thanks for the paper link. The impact of flexible/free boundary condition is very interesting. It points to a key role for shear during compaction. Maybe grains rolling and locking?
For fibers, I wonder if asbestos would be available. Maybe too hazardous.
Edit: From the paper:
> With the flexible boundary condition, the particle motion is localized. Clearly, during compaction it is critical to allow rotation and transverse motion of Mars-1a particles, so as to maximize the effective contact area and to promote bond formation. Excessive particle motion, however, may introduce defects, which explains why the free boundary compensates with higher pressure to achieve similar R as the flexible boundary.
This is very cool. Martian regolith seems ideal for rammed earth construction.
I wonder how uniform Martian soil is. We have explored only a tiny fraction of the surface with actual robots. On Earth there is a huge variation between different regions. I would expect it to be similar on Mars.
Martian regolith is believed to be fairly uniform across the planet, and this is backed by analysis at several widely-scattered locations. The dust is definitely well-mixed, but coarser parts are mostly of local origin and can vary, but because of the lack of biological elements are much less varied than on Earth.
This is making me wonder if it might be a viable/useful construction technique on Earth, depending on how hard it is to make the Martian soil simulant.
https://en.wikipedia.org/wiki/Adobe (mudbrick with straw as structural reinforcement) it already is, it was also similarly used for reinforcing statue casts (with hairs iirc)
>Historically, additives such as lime or animal blood were used to stabilize it, while modern construction adds lime, cement, or asphalt emulsions.
Curious to know what filler material is practical on Mars. I suspect animal blood is out of the question, unless of course, we consider the human settlers. I can see some potential for an amusing modern-day Modest Proposal with the problem of colonizing Mars.
> Researchers also investigated the bricks’ strengths and found that even without rebar, they are stronger than steel-reinforced concrete.
That's under compression, I'm sure.