Glow-in-the-dark concrete

Sure it's novel, but it can have a serious side too

With a recent innovation in concrete, "green" takes on a whole new meaning. This concrete is green because it is solar-powered. It's also green because that's literally the colour of the glow-in-the-dark light it gives off.

Daniel Clark, president of Maryland-based Glow Inc., has been in the business of phosphorescent pigments since April 2000. He "fell" into it after his computer company sponsored a haunted house during Halloween.

"We bought some glow-in-the-dark material and were quite disappointed with the results," he says during a phone interview.

"One of my employees decided to find the best phosphorescent material on the earth, which he located at a little company in China."

Glow Inc. started selling the material on the side and eventually shut down the computer company. "We found out a lot more people had the same need that we did," he says. "It took right off and we've been doubling in size yearly ever since."

The company sells glow-in-the-dark pigment, which can be added to the concrete before it is poured. The fishing industry is the company's largest market. (Apparently fish can't resist a glow-in-the-dark lure.) But the concrete application has started to generate some interest, particularly in Canada.

Glow-in-the-dark concrete may sound like a novelty, but it does have a serious side. It can be used to light driveways, walks, and curbs, thereby improving safety at night.

How does phosphorescent pigment work? In simple terms, light shines on the material, excites it on a molecular level, and the particles start moving. After getting charged up during the day, the light is released at night, emitting a green glow.

The major difference between fluorescent and phosphorescent light is that fluorescent light takes ultra-violet light and converts it to visible light instantly, explains Clark; whereas phosphorescent light does it very slowly.

Typical applications include painting warehouse floors, emergency exit lighting, and for hobby use, such as star and galaxy wall and ceiling murals. Until a few years ago, the pigments weren't suitable for outdoor use.

That's where it gets interesting.

A client contacted Clark about making a concrete pool that glowed in the dark, so he began investigating how it could be executed so the "glow" material didn't dissolve when exposed to the elements.

"There was a process [that coated the particles and sealed them off] offered by a company in China," he explains. "That's what we call coated phosphorescent."

The coated pigment is composed of strontium aluminate sealed in a plasticizer. It can be used for outdoor use and will only degrade by 5 per cent over 10 years.

Glow Inc.'s website describes two different application methods for glow-in-the-dark-concrete. One method involves mixing the "coated" glow pigment with concrete. This method delivers smooth results but can be pricey, especially for a big job. The second, more economical method involves pouring the concrete and then shaking on the pigment over top of the concrete. A professional can get the results of the first method for just a fraction of the cost.

But is the green glow aesthetically appealing?

"Some people feel it is a downright turnoff," Clark concedes.

The problem is that green glows the brightest. According to the rating scale mcd/m2 (millicandelas per square metre), green's luminosity is 33,000 mcd; blue, the next brightest, measures about 9,600 mcd. Other colours come in around 2,000 to 3,000 mcd.

Still, there is a growing market for those who don't mind the green glow. Glow Inc. receives the most orders for pigment to be used in concrete from Canada.

"The pigments improve yearly," explains Clark. "Up until this year, they weren't instantly satisfying. This year's brightness is becoming more utilitarian. Now you can really create outdoor things."

He offers a few tips to lovers of the glowing concrete: when direct sunlight hits the concrete, it begins charging; when light is no longer direct, it goes into discharge mode. So, for instance, at night a sidewalk on the east side of a building will glow much brighter than a sidewalk on the west side.

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