Getting More with Less

Cement strength saves Westminster parking lot project

By Jeff Hawk

A parking lot paving job at a Westminster community college demonstrated that five times less cement produced a stronger product than the originally specified flyash.

Switching horses in midstream can sometimes be a good thing.

When estimators for subcontractor ARS Inc. bid the Front Range Community College parking lot reconstruction job in Westminster, they figured 8 percent Type C flyash would meet the specified 160 psi strength requirement.

The 30,000-sq-yd project called for pulverizing and stabilizing the parking lot's existing four-in. asphalt surface and clay/sandy clay subgrade 12 in. deep; then resurfacing with four in. of new asphalt.

Designers considered stabilizing the subgrade with lime but chose flyash because of a short construction deadline. Lime requires a mellowing and remixing process while cement or flyash can be processed the same day it's applied.

"We fast-tracked it because we had to get it done before the students came back to school," said Skip DeFalco, president, DeFalco Lee Construction, Longmont.

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Falling Short

Unfortunately, geotechnical testing of the site's soil provided by DeFalco showed that an 8 percent flyash mix came up far short of the five-day strength requirement.

Test results indicated it would take a 15 percent flyash mix design to meet the 160 psi requirement, nearly doubling the estimated amount of stabilizer, and severely cutting into ARS's profit margin.

"It went from being a good job to a major money loser," said ARS co-owner Kim Haarberg.

So the Littleton-based company decided to run a new mix analysis comparing cement to flyash. Geotechnical engineer and ACEC/C member-firm Kumar & Associates Inc. of Denver tested the two materials using a five-day, 100 F moist cure criteria for determining strength. Cement supplier Holcim Inc. provided the cement for the test.

Results showed that a much lower volume of cement produced a significantly stronger product, according to Jim Noll, a senior engineer for Kumar & Associates. The tests revealed that a 14 percent mix of Type C flyash yielded 214 psi while a 3 percent cement mix produced 295 psi. In other words, five times less cement produced a much stronger unconfined compressive strength.

"We ran a lot of flyash and didn't get the strength," said Noll. But the 3 percent cement mix produced a "really good product" that was also "pretty darn hard," he said. "It's a better product when you're dealing with stabilizing a pulverized layer."

Changing Over

DeFalco and Front Range officials allowed the change order to cement. The switch produced numerous benefits. ARS crews worked with much less stabilizer, which sped construction and reduced truck traffic on the college campus.

"Minimizing the impact to students was a big issue," said Noll.

Trucks hauled in 500 tons of cement for a project that would have required nearly 2,500 tons of flyash.

"And trucks are in scarce supply," added Haarberg. Recycling the existing asphalt with cement further reduced traffic and saved money.

"Why haul off good material?" Noll said.

The thinner application and higher density of cement compared to flyash required less water addition and also produced less dust during construction, Haarberg said. And cement is a "very consistent product," he added. "It's a product, not a by-product."

In August, torrential downpours threatened to put the project behind schedule. But the cement-treated subgrades held up under the pouring rains.

"We were able to get back on it immediately," said DeFalco.

The most significant benefits were the cost-savings and the stable platform that cement provided for DeFalco pavers.

"Not only did cement pull our butts out of the fire, but DeFalco said it's one of the best subgrades he's ever worked on," said Haarberg.

"I think we found something good," DeFalco said. The project represented the contractor's first cement stabilization project in its 45-year history.

DeFalco said he is currently pursuing other cement stabilization projects. "We're impressed with it. It's way better than what we've done before."

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