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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