Princeton engineers create new oyster-inspired cement 17X more crack-resistant | Hacker News Reader

Princeton engineers create new oyster-inspired cement 17X more crack-resistant

engineering.princeton.edu

91 pointstoss131 comments

    ...creating alternating layers of tabulated cement paste and thin polymer can significantly increase crack resistance and the ability to deform without completely breaking (ductility)

A few years ago, my daughter and I did a science fair project[0] based on a question she asked while we were crossing bridge in a car and she noticed the rebar poking out out of an adjacent bridge which was under construction.

In our experiment, we tried various reinforcements to concrete and tested the weight loading capacity before cracking. Quite surprising, at #2, was basic acrylic yarn layered perpendicularly in an overlapping "weave". By far the cheapest, lightest, strongest reinforcement that we tried!

[0] https://youtu.be/9QlzRT69X-I

Mesh can even dramatically increase the load bearing capacity of sand: https://m.youtube.com/watch?v=0olpSN6_TCc

Very impressive! Even with an explicitly stated hypothesis. Peak asian parenting :)

> By far the cheapest, lightest, strongest reinforcement that we tried

In what sense is it strongest, strength-to-weight? Not sure the weight matters much compared to the weight of displaced concrete.

Also would be really curious to try nylon instead, could even beat the steel potentially

    > In what sense is it strongest, strength-to-weight

Weight, size, and cost. The galvanized wire was the clear winner (hairline crack at 300lbs), but also cost significantly more than the acrylic yarn. Would be interesting to revisit this experiment with different densities of the acrylic weave to see where the limits are. We used a very sparse weave, but I could see tripling the weave density raising the weight limit closer to 300 than 225.

Also probably need to prestress it for best results :)

If it's stretchy or wavy, the concrete may crack before it starts to produce tension

I wonder.

I hypothesize that one of the reasons the yarn performed so much better than the deer net is because it is more permeable and thus formed a better bond with the concrete.

In this scenario, I would imagine that it would be "locally tensioned" because the cement is effectively tightly bonded across any point of stress. Would be an interesting experiment!

Isn't the point of pre-tensioning is not the actual tension on the reinforcement, but the pre-loading of the concrete - so it needs to happen everywhere in the volume of the concrete, not just next to the reinforcement?

Interesting point; I see what you are saying like during the curing process, the tension in the yarn weave itself would pre-tension the dried slab. But I wonder if acrylic yarn would have a sufficient effect and to what extent.

If anyone has science fairs coming up, these seem like excellent experiments!

I was thinking about this. The yarn is fuzzy and so has a lot more surface area for bonding

It wouldn't work well: Coefficients of thermal expansion: Nylon: Typically ranges from 80-100 x 10^-6 per °C Steel rebar: Typically around 11-13 x 10^-6 per °C Comparison: The coefficient of thermal expansion for nylon is significantly higher than that of steel rebar - about 6-9 times greater. This means nylon expands and contracts much more with temperature changes compared to steel. That's not even taking into consideration the tensile strength of nylon and steel. But an interesting thought, for sure.

I would just call this "peak parenting". Half the world is Asian and doing experiments like this is rare.

Good things are always rare, the question is whether they are less rare in some cases compared to others

Did you try other yarns in the same weave? I'd be interested to see how wool, cotton, hemp, etc would compare.

How about some Kevlar?

Super cool!

I'm always looking for fun STEM things to do with my kids and will keep this in mind.

My father went to Princeton for a year for post graduate work when I was in kindergarten in the 1970's. We went to a Concrete Canoe race (like this one https://paw.princeton.edu/article/throwbackthursday-racing-c...), and for years afterward I remember him wearing a Concrete Canoe t-shirt.

It wasn't until I was much older that I realized that the impracticality of building a canoe out of concrete was the whole point of the endeavor. For years my childhood brain assumed that concrete was just one of the materials people commonly used to build boats. Want to build a canoe? Likely options are wood, animal hides, or concrete.

> For years my childhood brain assumed that concrete was just one of the materials people commonly used to build boats.

It was, depending upon availability of steel. So boat and ship were not uncommon, maybe less so with canoes.

https://en.wikipedia.org/wiki/Concrete_ship

https://www.iwm.org.uk/collections/item/object/30018296

There is a grounded concrete barge near where I grew up, now largely buried in sand dunes.

https://www.flickr.com/photos/36891793@N08/8033301173/

Our local one when I was in school was milk carton races. Those boats were quite odd.

The actual paper is posted in another thread:

https://news.ycombinator.com/item?id=40796770

Can we close this Princeton PR as a dupe and have the discussion over there instead?

I expect this one to perform much better due to the headline being more comprehensible

thx! I worked on making the headline comprehensible in 80chars, and made sure the link to the original paper was in the article if people wanted to easily find that level of detail.

Someone smarter than me can answer this: how would this be applied? If you normally pour cement, which presumably wouldn't work with this layered structure, how do you practically use it?

Precast concrete is often made at a factory then transported to the work site, a factory environment would enable all sorts of processes and this one doesn't look too difficult in principle - not to sell them short, it's great research and biomimicry is rad, but layering plates in polymer seems like it's definitely automatable.

It's a variation on the idea of reinforced concrete. It will take time to understand the interaction between cement and polymer. It took us time to understand that if moisture gets to the steel rods, the reinforced concrete beams crack.

>>if moisture gets to the steel rods, the reinforced concrete beams crack.

Exactly; and the crucial intermediate step in there is the moisture causing the steel to oxidize, the iron oxide increasing the volume of the rebar, and thus putting the concrete in tension from the inside, and since concrete really sucks in tension (but is great in compression), the concrete cracks.

I'm mystified that iron-based rebar is even still allowed in construction, when fiberglass-based rebar is readily available and proven to work better.

I asked this question here a while back and the answer I got was that at failure steel rebar stretches rather than breaks catastrophically and thats a very important safety factor when calculating dynamic loads, a fiberglass rebar structure would fail far more catastophically.

At, finally an answer that makes sense —Thx!

I'm sure adding a bit of kevlar in the rods would solve that problem, but modeling will still be difficult...

We have no data on such solutions and we keep building bigger, taller, more complex structures so there may be some resistance to new solutions, but I'm sure someone is working on them, as we reach the limits of the old ones.

And this is one of the big issues with RAAC (Reinforced Aerated Autoclaved Concrete, extensively used in the UK in the 60s and 70s. The concrete is aerated (think expanded polystyrene) so it is compressible. That means that you don't see any cracking on the surface as tye rebar corrodes. There is no obvious warning of imminent collapse. The panic in the UK last summer after RAAC ceilings in 2 UK schools collapsed (thankfully during the holidays) was because they had been recently inspected- one the week before.

The social flow on effects spread even wider.

Large numbers of low cost social housing used RAAC as a roofing solution that exists to this day. Councils that still manage low rent housing for people on assisted incomes are taking the financial hit for insurance and replacement.

Arguably worse is the situation of those people who scrimped and saved and bought their way out of povety and purchased the house they previously rented.

What was once an investment of their life savings into capital that would grow in value is now money lost in a house scheduled for demolition that has zero value with a defect no insurer will cover.

I wish my old neighborhood was more crack-resistant

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