科学美国人:New Concrete Recipes Could Cut Cracks

60秒科学节目(SSS)是科学美国人网站的一套广播栏目,英文名称:Scientific American - 60 Second Science,节目内容以科学报道为主,节目仅一分钟的时间,主要对当今的科学技术新发展作以简明、通俗的介绍,对于科学的发展如何影响人们的生活环境、健康状况及科学技术,提供了大量简明易懂的阐释。

There's a stretch of highway in Pennsylvania, along US-422. "And like every probably 20 feet you see a big pothole or cracking at the joint. Like everywhere. It was so bad." Yaghoob Farnam is a construction materials engineer at Drexel University in Philly. And this road is pretty much his worst nightmare. "Yeah and just imagine I was driving like 60 miles per hour, I could see, I could feel it, I was driving, I was so mad, like, 'what is going on with this?'" 

The culprit, he says, may be calcium chloride road salt, used to de-ice highways in the winter. Because calcium chloride reacts with a compound in concrete called calcium hydroxide to form something called calcium oxychloride. "It's a huge molecule that causes a lot of pressure inside concrete. And starts degradation of concrete." 

The solution? Novel blends of concrete that use cheap leftover materials from the coal and steel industries: fly ash, silica fume and slag. In his latest work, Farnam and his team created plugs of these experimental concretes, and submerged them in salty solutions—along with plugs of conventional concrete. Then they eavesdropped on any cracking with high-sensitivity acoustic sensors. And they tracked heat flow through the material, to monitor chemical reactions.

The results: concrete slugs made with ingredients like fly ash and slag held up remarkably well after more than a month. Whereas normal concrete was cracked to pieces in just a week. Their recipes are in the journal Cement and Concrete Composites. [Yaghoob Farnam et al, Evaluating the use of supplementary cementitious materials to mitigate damage in cementitious materials exposed to calcium chloride deicing salt]

Farnam says some states have actually started using this sort of concrete—because it's already known to make the material more durable against other factors, like corrosion of internal steel reinforcement. As for those cracks on US-422, and elsewhere? Farnam has another project in the works—to apply a bacterial slurry, which forms limestone when it interacts with salt, plugging up the gaps. But he says that work is still a ways…down the road.

—Christopher Intagliata

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