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New Technique Addresses Shrinkage
Cracking -
TxDOT applies recycling on State Highway 47
 Inducing
cracks on soil-cement pavements may go against traditional wisdom,
but the highway and street industry is beginning to explore the
benefits of "micro-cracking" - a new technique of creating
a network of hairline cracks to reduce stress on cement-treated
materials and reduce typical macro-cracking.
In
1995, Austrian researchers found that running a 10- to 20-ton vibratory
roller over a cement stabilized layer 24 to 72 hours after compaction
induces a series of "micro-cracks" that translate the
normal shrinkage cracks and mitigate reflective cracking through
the asphalt surface. The continuing hydration of the cement restores
the design strength in a short time.
 The
micro-cracking concept is being applied on projects near Bryan,
Texas, where TxDOT is repairing five patches that run along an 8-mile
stretch of four-lane State Highway 47. Hunter Industries, San Marcos,
is performing the work, which entails tearing up and pulverizing
12 inches of the existing asphalt and base, adding 3-percent cement
(by weight of dry material) and then mixing it to a depth of 14.5
inches. After compaction and final shaping, crews wait 24 hours
before micro-cracking the new base.
"I
didn't think we would be able to crack it, but we're using a 12-ton
steel-wheeled vibratory roller and it's cracking after four passes,"
says supervisor Gary Tackert of San Marcos-based Hunter Industries
Ltd., the project's contractor. Tackert says recycling with cement
expedites rehabilitating roads. "If you're not laying cement,
you've got to lay base and each course would take up to seven days,"
says Tackert. "With lime (stabilization), you have to come
back and do the final mixing three or four days later." But
with cement, he adds, "after a five-day turnaround, I'm opening
the road to traffic."
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