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Summer Flooding No Match for Cement-Bonded Base

Using cement proves 35 percent less expensive than asphalt alternative

By Jeff Hawk

When heavy rains flooded two road rehabilitation projects underway near Carrizo Springs, the outcome could have spelled disaster. Fortunately, both projects had built-in protection in the form of cement.

"A normal road project would have washed out but where we had cement, it didn't wash out," says Greg Howard, a Texas Dept. of Transportation area engi­neer in the Carrizo Springs office. Crews allowed the cement-treated sections to dry out a little and then went right back to work, says Howard, calling cement "pret­ty good insurance." "Uninsured" areas didn't fare as well.

Some untreated sections where crews placed new base "completely washed out," says Roger Benavides, assistant engineer for the office. "Cement really does hold it together."

The two identically designed projects - US 277 near Carrizo Springs and US 83 near Crystal City - represent the office's first effort at mixing cement with existing materials. The process, known as "full-depth recycling with cement," is rapidly becoming the choice for rebuilding dete­riorating asphalt roads in Texas. It involves pulverizing the existing asphalt surface and granular base and mixing the material with a low volume of cement.

Not only did the decision to use cement save the project from floating away, it also saved money. Before start­ing me project, TxDOT engineers explored using asphalt and asphalt-stabilized base to rebuild the roads.

"We considered going with just asphalt base but you needed so much, something like 20 inches of new base," says Bena­vides about the outcome of a roadway design program used by the office. Like­wise, the initial construction cost for an asphalt-stabilized road would have been 35 percent, or another $1.25 million, more than the cement option, according to Benavides.

Performing a complete reconstruction of the highways - a process that involves removing and replacing all existing materials with a new roadway - "jacks the price way up," says Howard. That process also requires building a detour to the side, "so it almost doubles the cost of the proj­ect," adds Howard.

Reconstruction also adds headaches, says Cliff Coleman, superintendent, San Antonio-based E.E. Hood & Sons. "You run into a lot more problems on a rebuild. These (recycling projects) are pretty much cut and dry," says Coleman. One reason is the simplicity of the recycling process. "It's a heck of a lot faster than remove/replace construction because you're using what's here. As long as you get enough water in the cement, I don't see how you could screw it up," says Coleman.

Rebuilding the roadway involves four or five more steps before reaching the top of the construction cycle again.

"My crews sleep at night and forget!" jokes Coleman, preferring recycling's repetitive process. "Once you get the crew trained, you can cut them loose," he says.

Working 2-mile sections at a time, E.E. Hood workers first mill off four inches of the existing surface and then spread about 34 pounds per square yard of cement over the exposed base. A pow­erful CMI 650 pulverizes and mixes the existing base with the cement to a depth of 8 inches.

After blading it to grade, the contrac­tor allows the new base to sit for at least 24 hours before micro-cracking it. Micro-cracking is a new practice that reduces the potential for shrinkage cracks in the newly bonded base by inducing a stress-relieving network of hairline "micro"-cracks.

The process involves running a 10- to 20-ton vibratory steel-wheeled roller over the new base at walking speed 24 to 72 hours after it's placed. One to four passes are rec­ommended to induce cracking. Crews top the new base with six inches of flex base, mix it with 1 percent of cement, final grade it, and then micro-crack it.

To prime the new base, E.E. Hood crews place a thin chip seal. Twenty-four hours later, workers top the roadway with another seal coat. At the end of the projects, E.E. Hood will finish the new highways with a 2-inch hot mix asphalt overlay.

Traffic management plans are also much simpler with recycling projects, says Coleman. Crews shut down alter­nating directions of traffic for about 15 minutes, using local off-duty law enforce­ment officers for traffic control. At night and on weekends, the roadway is turned back to the traveling public.

Even with the wait time, TxDOT's Benavides says the office has received fewer complaints than did a recently completed reconstruction project. That seven-mile rebuild project took almost 2 years to complete. The six-mile US 83 recycling project, in comparison, is tak­ing about 1.5 months per mile to com­plete, says Coleman.

With lower costs, fewer headaches, and quicker construction resulting in a stronger, more durable product, recy­cling with cement seems a tough option to beat. Says Coleman: "Out of all the jobs that I do, I like these the best."

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