MIT Researchers Discover Why Concrete Breaks Down
The old saying “step on a crack, break your mother’s back” may not apply to sidewalks for much longer now that MIT researchers have figured out why concrete breaks down. As a result of the discovery, structures like buildings, bridges, and yes, sidewalks, could last for hundreds of years longer than they currently do. A nuclear waste container built to last 100 years could, for example, last 16,000 years.
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According to MIT professor Franz-Josef Ulm, creep (the process that create cracks) is created when calcium-silicate-hydrates (CSH) rearrange at the nano scale. When mixed with water, CSH particles change in density from 64% to 74%. By adding silica fumes–a waste product from aluminum production–to concrete, overall density can increase to 87%. That’s a change that could eventually lead to longer-lasting, lighter structures.
If Ulm’s theories are put it into practice, the concrete industry and the planet could benefit immensely. 5 to 8% of all manmade CO2 comes from manmade concrete construction, so any reduction in the need to produce more of the stuff would slow global warming. And with twenty billion tons of concrete churned out annually, there’s plenty of room to cut down on production.
Interesting
How much does it cost for each ton?
Just because they say it is a way to use a waste product, it doesn’t mean it’s low cost.
All solutions need to be used worldwide and that means at a competitive cost.
You’re telling me that increasing the concrete density from 74% to 87% will increase the life span of a concrete structure from 100 years to 16,000 years? Any increse in lifespan is good but it appears the projections are a little generous.
This is news? Silica fume has been used in concrete for decades, beginning in the 1940’s. It is very beneficial in reducing concrete permeability and mitigating chloride attack; it doesn’t increase density significantly. Creep is not what causes cracks either. There really isn’t much accurate information anywhere in the article. 16,000 years? Dream on.
Do some research, silica fume has been used for years. The American Concrete Institute (ACI) started advocating its use in the mid 1980’s, there is an ASTM standard (ASTM C1240) concerned with this use of it. ACI has published ACI 234R-06. Guide to Silica Fume in Concrete. This is not news but standard good practice and it is not a new green miracle.
Please get facts checked before publishing a story. Use of silica fume in concrete is almost 25 year old practice and its benefits and disadvantages on concrete are well published.
This is not new, but nice to see confirmation of what is now available. Concrete industry has been using silica fume for 15+ years to make high-strength concrete, especially in factory precast concrete, to get 8000-12000 psi compressive strengths. High strength means less concrete needed. It does add cost, but extra strength balances it out. Better durability is known. Wonder if they are dosing are higher rates than we normally see.
How can increasing the density of concrete reduce lead to lighter structures? And creep does not itself cause concrete to crack. In fact, in many cases creep can reduce cracking by relieving stress.
Silica fume has been used in concrete production for years. FHWA’s High performance concrete program featured it to improve concrete durability, and virtually all high strength concrete uses the material. Silica fume is the by-product of producing silcon metal of ferrosilicon alloys in smelters. Costs in the neighborhood of $400 per ton compared to $100 a ton of ordiary portland cement. FHWA and the Silica Fume Association have published an excellent reference manual. I’m afraid the researcher is a little behind the times!
Articles like this don’t do anything for CleanTechnica’s credibility. Silica fume isn’t new, nor is knowledge of its performance.