A chemical reaction in ancient Roman concrete makes it stronger over time (2017)
science.howstuffworks.com
science.howstuffworks.com
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
Last year: https://news.ycombinator.com/item?id=20482050
2017: https://news.ycombinator.com/item?id=14690329
Is is definitely interesting to read about some unexpected properties that ancient concrete with volcanic ash as one of its ingredients has. For modern cement there is a different set of choices for binding materials, were you can choose the ingredients that are locally good available.
Also there are definitely mistakes that get made in modern construction. - Maybe the structure has enough rebar to handle the forces that act on the structure, but not enough to prevent the small cracks that hurt durability (although that is something that should be checked). - Maybe the detailing of the rebar is not correct. Lapping lengths are too short. Often I see that the workers who put in the rebar come up with creative solutions but understanding why it now doesn't work well is outside their expertise. - Maybe the concrete layer that protects the rebar (covering) from rusting is to thin or to porous. The amount of covering required depends on the exposure classification. And the concrete mixture influrences how porous the concrete becomes over the years.
For modern engineering you specify the exposure classes that a structure falls in. Carbonatation (https://en.wikipedia.org/wiki/Carbonatation), a reaction where calcium in the concrete with CO2 in the air, which changes the acidity in the concrete. Chlorids, from sources such as seawater or de-icing salts, also impact the acidity of the concrete. Chemical acids, basically everything biological such as milk, beer, manure, and other acids dissolve the calcium. And frost can cause parts of the concrete to flak off. All those effects can get worse if the concrete gets porous. But all are well understood problems that inform the necessary mixture.
From the abstract of `Bacteria-Based Self-Healing Cementitious Composite for Application in Low-Temperature Marine Environments` [3]: "The composite displayed an excellent crack-healing capacity, reducing the permeability of cracks 0.4 mm wide by 95%, and cracks 0.6 mm wide by 93% following 56 days of submersion in artificial seawater at 8 ◦C. Healing of the cracks was attributed to autogenous precipitation, autonomous bead swelling, magnesium-based mineral precipitation, and bacteria-induced calcium-based mineral precipitation in and on the surface of the bacteria-based beads. "
[1] https://www.tudelft.nl/citg/over-faculteit/afdelingen/materi...
[2] https://www.basiliskconcrete.com/hoe-werkt-het/?lang=en
[3] doi:10.3390/biomimetics2030013
I would love to hear why a chemical analysis doesn't solve the riddle here. Scientists were able to isolate a strain of the coronavirus and publish the entire genetic sequence in a matter of days, but we can't analyze a sample of concrete?
> The [balanced chemical mass] of C-S-H in cement paste is variable and the state of chemically and physically bound water in its structure is not transparent, which is why "-" is used between C, S, and H
The article actually misses how modern concrete (portland) get stronger over the first few years as the crystalline structures grow. So, I can well understand how it's a bit tricky to find out what the original roman mix was, just using observations 2000 years later: because a lot of things have happened from the original cake mix to what we see now.
AFAIK wood ash is predominantly mineral oxides, particularly of calcium. The carbon in the wood is the fuel that burns up and goes out the chimney as carbon dioxide. When wood burns it first boils and burns off various volatile compounds, leaving behind black charcoal coals, made predominately of carbon. This charcoal also burns, leaving behind wood ash.
Finding Mona Lisa in the Game of Life
https://news.ycombinator.com/item?id=22552006
From the article: "This looks kinda cool, but what if we want to find a Life state that eventually, after following the rules of Life for a few rounds, reaches a state that looks like Mona Lisa? This requires working backwards instead of forwards from the target picture, which is a much more difficult problem. (...)
We call Life state A the "parent" of state B if A turns into B by following the rules of Life. The reason that it's difficult to find the parent of a state is that the rules of Life are non-reversible. There's no direct way to go from a Life state to its parent, and in fact, it's possible for a state to have multiple parents or even no parents. "
With regards to the featured article, I'd say multiple factors are at play - Roman concrete has amazing properties and structures that used it combined with good quality-control (i.e. correct material ratios in the concrete) and good engineering survived.
The human elements, quality-control and design, are less interesting than a magical substance.
The top comment there links to two other discussions.
Overall, considering we build far more things per day than the Romans did, I reckon there's a good chance more of our buildings are around in 2000 years than the Romans have left now
With "modern" cement (Portland) it is relatively easy to get high strength at 4 weeks/28 days, while (good ol') pozzolanic cement the reaction/hardening is much slower but over time (and with adequate humidity) it can reach and beat the "better" portland cement.
Almost nothing we build today will be there in 2000 years, but not because of the concrete itself, but because of the reinforcing steel we use (and because of the different use of concrete in much slimmer structures).
Anecdata: in the '80's/'90's I was working in a large tunnel project and we used in tunnels pozzolanic cement (not reinforced) for the lower part of the lining, and it was not easy to get the "right" (according to norms) 28 days strength (250 Kg/cm^2 at the time), but, once the tunnel was finished (roughly 3-5 years laters) and we did further testing of the structures, we found that it reached strength of the order of magnitude of 500-600 Kg/cm^2 whilst the corresponding Portland based concrete, targeted at the same 250 Kg/cm^2, reached "only" 300-350 Kg/cm^2.
EDIT: Looks like they are: http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=displa...
Concrete is only a good material to resist compression forces - generally speaking - it was used by Romans only in structures that were exclusively (or almost exclusively) subject to compression (arches).
The revolution that reinforced concrete made was about combining two materials (steel very apt to resist tensile forces and concrete very apt to resist compression) with very similar other characteristics (thermal expansion) and compatible between them, if you want it is one of the first examples of composite material, to obtain something that could be used in structures subject to tensile and compression (besides shear) forces.
Our recently (last 100 years or so) reinforced concrete structures are very lean and elastic, which implies that they move and crack.
All the research on new concrete is about making mixtures where cracks are reduced to the minimum, as before or later through these (micro) cracks air and water penetrate, oxidizing (or rusting) the steel.
As well in the years the norms about the cover (i.e. the minimal distance from steel to the outside) has been increased (it depends on countries and types of structure but 30 years ago 2 or 2.5 cm were common, nowadays 4 or 5 cm are common).
Who knows how much knowledge has been lost, but my guess is that the Romans developed their processes much more deliberately than haphazardly. You don't have to understand the 21st century chemistry of why something works in order to build a systematic and even scientific methodology. It was part and parcel of a mortar makers job to understand how to make use of regional materials and handle local environmental conditions.
2. Only over time
3. And actually we still do not know.
Slightly disappointing.
Here’s a good textbook on concrete: Concrete: Microstructure, Properties, and Materials Textbook by Paulo J. M. Monteiro and Povindar Kumar Mehta
The average person is too ignorant to know or care anyway.
We prefer to build quickly (more efficient use of land multiplied by time) and prefer our buildings not fall over in earthquakes.
Also much of residential construction is wood frame (edit: US), which also has pretty bad longevity unless extremely overbuilt.
Why is this?
My feeling when I went to the US was that many houses felt incredibly "flimsy" as if you could just punch your fist through the wall.
one of the interesting tidbits about passivhaus construction is that it tries to minimize even more the wood content of a building (in favor of insulation), so studs can even go to 24" apart.
Also, wood houses can be built to much higher quality than what people are building new to sell on the open market. Square footage has a much higher profit margin than thicker and more insulating walls or better wood or more careful and accurate framing. Even putting in something highly visibly like way better windows and doors isn't going to get you jack shit of a return if you are building to sell rather than ever paying for heating and cooling costs or what not.
I mostly agree with the other points you make but not this.
Modern wood-frame and plywood construction with vapor barriers, Air-conditioning and impermeable facades are very sensitive to moisture. Once water enters, it has nowhere to go and this has to be addressed quickly and thoroughly or else there's a serious and relatively quick structural deterioration, mold issues and a downward-spiral of problems.
Mcmansion owners who want to keep their house in excellent condition better be on top of this stuff, or else they or the suckahs they sell to are going to be paying a hefty price. Avoiding this requires decidedly more than half-assed maintenance.
Like mold. If it appears, you got a moisture source that needs to be discovered and dealt with. Crawlspaces need to be encapsulated and have negative pressure. It's amazing how simple stuff like water leaking from a gutter can find it's way into a house with even the smallest gap at a window interface or if the drainage is bad where the house meets the ground.
No it isn't - if you ask the people actually building American wooden houses how long they think they'll last - they'll tell you almost what I did - just 60 years! Check national building standards 7543:1992. They increased it under pressure a few years ago, but that's what they were designing against until recently!
Of course some houses will survive longer (and 170 is a pretty modest age to be boasting about) but even the people building them don't have confidence that they will.
That's great, but that wooden house you live in is vastly better in terms of quality of wood than what is common today in USA construction. If OSB had existed 170 years ago, I assure you would have long ago turned to dust by today. You probably have magnificent wood beams and single-piece shipboards spanning all studs and joists.
It all comes down to material-quality, maintenance, design, expectations and weather. Sadly, all of those factors take a back seat to cost and speed where modern housing developers are concerned. Modern building techniques make it easy to fool unsavvy consumers into an illusion of durability and quality.