Beating the Heat

Concrete Gets Even Cooler Thanks to Liquid Nitrogen

By Rob Patterson

The centuries-old Roman invention of concrete, one of the world's most common, durable, versatile and easily usable construction materials, proved the notion that it is hard, if not impossible, to build a better mousetrap.

Yet for all its countless merits, concrete does come with a glitch - it's difficult to keep it cool enough in warm climates so that it won't develop structural problems as it hydrates and, as a result, heats up.

In Texas, where the summers are anything but cool, the collective efforts of contractors and TxDOT are helping to make this reliable material even better. They're doing it with the help of liquid nitrogen, a relatively new and increasingly common cement-mix coolant to replace ice and chilled water in the warmer months. In addition to a substantial cost savings on some jobs, liquid nitrogen's use is creating benefits in scheduling including less need for nighttime pours. And it may lengthen the distance ready-mix can be transported.

Liquid nitrogen was a key factor in mass pours for the SH 45/U.S. 183 interchange in Austin.

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"I don't think we could have done it without it," said Mike Brown, project manager for Austin Bridge & Road of Dallas. "It was probably the only way to maintain the temperature differential. It also helped us avoid losing loads while they were waiting to be placed.

"If the concrete heated up too much, we could send it back and have them inject more nitrogen to cool it back down."

"It's a good process for us," said Joe Dan Johnson, who most recently served as the QA/QC manager for Transit Mix of Austin. "Everyone's been pretty happy about it." Other Texas concrete suppliers using the technique include Dallas-based TXI and Cemex in El Paso.

TxDOT took its cue on liquid nitrogen from Arizona, one of the many DOT's using the technique on highway jobs across the nation. Another such project is the new San Francisco-Oakland Bay Bridge. "I've had calls from Dubai in the United Arab Emirates about it," Johnson said.

TxDOT is sponsoring a $237,000 research project at the University of Texas at Austin to investigate the technique and to ensure that it doesn't negatively affect the concrete's structural properties. The study could determine any possible material benefits. While the results are still not in, TxDOT is advocating the use of liquid nitrogen in mass pours on Austin-area jobs such as SH 45, SH 130 and Loop One, just a few examples of the growing trend in the Lone Star State.

"We found that in the summer we were adding so much ice to the concrete that there wasn't any room for water and we still weren't maintaining our temperature requirements," said Jeff Tolson, project supervisor for TxDOT's turnpike division. "Liquid nitrogen was easier, faster and cheaper, and it was half the price of ice."

On all pours of 5 ft. or greater, TxDOT requires that the concrete temperature not exceed 160º Fahrenheit during hydration, which means that it needs to be cooled to at least 75ºF when it is poured. If the temperature rises above 160ºF, it can result in thermal cracking due to delayed ettringnite formation. If moisture seeps into the ettringnite, it expands and can weaken and crack the concrete.

"There's a good chance that if you're starting out at about 100ºF with concrete, you won't get to 75ºF with ice." Johnson said. "There's only so much water in the mix that you can replace with ice, and it takes a lot of time to load. You need 160 lbs. of ice per yard. So with a 10-yd. load, you might need 1,600 lbs."

Chilled water can sometimes be used, and icing the rock pile helps reduce the temperature. "You have to jump through hoops to get it to 75ºF," Johnson said.

Liquid nitrogen, on the other hand, takes only a blast of a minute or two of the gas - which has a temperature of 320ºF below zero - into the full drum of a ready-mix truck, to cool the slump enough to reach a nearby job site at 75ºF.

The process requires a liquid nitrogen vessel, vaporizer and a simple bridge assembly that the truck pulls in underneath. Atop the assembly is a retractable wand - made of copper or treated steel to withstand the cold temperature - that is inserted into the drum and sprays the slurry to chill it to the needed temperature.

Currently, the set-up requires a large-volume concrete job to be cost effective. One danger is that the liquid nitrogen, if not correctly aimed, can crack the mixing drum. Similarly, it can also freeze the concrete.

"One of the drawbacks of cooling concrete is that it slows the hydration rate and your pour is going to take a little longer," said Ralph Browne, area engineer for TxDOT's North Tarrant district office.

"But contractors are aware [of potential problems] and can adjust for that slower placement rate," Browne added.

Liquid nitrogen's success as a coolant prompted speculation on how the process works and what its full effects are, good or bad. "We started asking some technical questions on how it affects concrete and what does it do its strength and structure," Browne said. "We developed a research problem statement and submitted it to our research committees."

UT's Construction Materials Research Group was selected to do a study in the lab and field. "We're doing a lot of concrete studies and chemical analyses to better understand the effects of liquid nitrogen on the system," said Maria Juenger, associate professor in UT's civil engineering department and research supervisor of the study.

"The initial prognosis is that all looks good," Juenger said. "It may be affecting some things in the chemistry of the system, but it's not clear yet whether those will make a difference in terms of the long-term properties. My guess is that it's going to be fine."

Further refinements of the technique include injecting liquid nitrogen during mixing at plants and perhaps smaller and more portable systems that can cool mixes just prior to pouring.

"If we can make this a viable method economically and scientifically, it can only give us good things," Juenger said.