Bacteria are your friend

From Rocket Science, an article about a "self-healing"concrete that uses limestone-secreting bacteria activated by water that seeps into tiny cracks.


E Hines said...

I have a couple of questions.

1) the micro-cracks would seem to lend a measure of flexibility to the concrete. Will significantly reducing/eliminating these cracks make the concrete stronger, or increase its brittleness?

2) In what regimes will these bacteria be able to function?
2a) here in Plano, the city's answer to two inches of snow on the streets is to apply sunlight. This seems relatively benign for the bacteria. In Germany, the national answer to two inches of snow was to apply 2 inches of salt. This seems somewhat toxic.
2b) for how long can the bacteria last without water? There are significant parts of the Southwest that don't get any for long periods of time.

3) How deeply does the water penetrate into the roadway's 12 inches of concrete? Will we get a tightly sealed skin with the bulk of the interior hollowing out?

On the other hand, if we just moved to hover cars, the whole problem would become moot.

Eric Hines

Grim said...

Q3 is a good one. When I was a kid, the dentists gave us fluoride treatments and fluoride vitamins. The result was a very hard enamel on the teeth, which has almost entirely prevented me from developing cavities. However, they're subject to that very concern -- even a tiny cavity in the enamel can lead to massive hollowing out underneath before you become aware of the damage.

It's not been a problem for me, but it's something I've been warned about by dentists since.

Texan99 said...

I will defer to any concrete experts who may be in the Hall, but my understanding is that concrete is terrific in compression and awful in tension. We put rebar in it to give it some strength in tension, and that is the source of any flexibility it may boast.

The article implied that the bacteria would hibernate next to some kind of sugary food source until water penetrated, at which pint they'd wake up and find something to eat.

E Hines said...

...the bacteria would hibernate next to some kind of sugary food source until water penetrated, at which pint they'd wake up and find something to eat.

But this raises further questions:

4) what keeps the water in the mixing and setting stages from activating the bacteria--if it's the "coating" used to protect the spores during mixing, what "deactivates" the coating so that later water additions can do their trick?

5) what's the shelf life of the calcium lactate?

5a) what keeps it from "rotting," which is both a bacterial process (from different species, but there's nothing that would keep these bacteria from entering) and a simple chemical break down process?

5b) do the bacteria consume all of their food supply in the first go, or do they encyst/enspore/?? for a subsequent go? For how many such cycles will the food last, if it's not rotting?

6) what's the mass of calcium (lactate) that's added for conversion to significant masses of limestone? In the end, this seems the long rebar in this slab.

Eric Hines

douglas said...

Well, Tex has the basics introduced, and I’m not a materials or chemistry or biology expert, but I do know a little about concrete, and in passing, a little chemistry and biology, so I’ll wade in and give the questions a go:

1) Concrete has virtually no flexibility (yes, there is deflection, but very, very small amounts), and is brittle by nature. The micro cracks themselves don’t really affect overall strength beyond normal safety factors, but minimizing them reduces further crack development over time that can cause issues in strength and performance.

2) Couldn’t say, though it would be pointless if they couldn’t be set in the concrete with viability.

2a) I’m not a biologist, but I’m sure there are bacteria in salt mines too. It likely depends on the specific bacteria specie. (see Halophiles)

2b) 2b-1:Spores can survive dormant for seemingly indefinite periods of time. Apparently, they’ve been found to survive in glacial ice over 500,000 years. 2b-2: It rains at least once a year in most areas of the Southwest. It’s not the Atacama desert.

3) What exactly would be hollowing out the interior? The point is that where water intrudes (and is the vehicle for damage to the concrete), the bacteria will be activated and stem the damage early.

4) This is, I’m sure, the real bugaboo. You have to add in the harsh chemistry of the setting concrete (though they said they’ll survive the alkalinity), along with the raised temps the chemical process produces. We’ll see if they figure it out.

5) As a mineral, it’s probably got a long shelf life, unless affected by other chemicals and water.

5a) I think either process is unlikely as if water enters, which could dissolve the mineral, the bacteria are activated, and other bacteria probably aren’t going to prosper as they wouldn’t find the right menu or environmental chemistry.

5b) This seems to me a good question, though there is a fair amount of air space in the concrete matrix, and in some mixes it’s increased with an air-entraining admixture, so perhaps this is accounted for.

6) This I really don’t have a clue on.

I think it’s a fascinating idea, and perhaps may be useful in someday repairing the Frank Lloyd Wright textile block Ennis and Freeman Houses here in Los Angeles that are falling apart at a molecular level because he insisted on using decomposed granite sand from the site to ‘tie the building to it’s site’, but the chemistry was all wrong. I’d written them off as irreparable and their only hope being reconstruction, but I could be wrong with this development.