Flexing its muscles

Test sites prove concrete durability, resilience

By Angelle Bergeron

Concrete is flexing its pavement muscles.

Key test projects in the South Central region demonstrate concrete’s durability and flexibility. The price of petroleum products continues to rise, and concrete innovations hustle to keep pace, making the familiar construction material increasingly desirable.

“We certainly think concrete is a lot more competitive with the way asphalt products have gone up in cost,” says Al Crawley, Mississippi director of the American Concrete Pavement Association’s Southeast Chapter.

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That may be one contributing factor to the fact that the Mississippi Department of Transportation seems to have a lot of whitetopping projects (concrete overlayed on asphalt) coming down the pipe, Crawley says. Or it may simply be that MDOT has recognized the superiority of concrete as an overlay material, he adds.

“I think these whitetoppings are more tailored for intersections that have been heavy maintenance,” Crawley says. “There have been a lot of problems with asphalt overlay for stopping and turning movement. Most whitetopping we see here are areas that have periodic rutting.”

MDOT has done whitetopping since 1996, with mixed results, leading to improved specifications, Crawley says. Now, whitetopping contracts seem to be popping up everywhere.

Last year, APAC-Mississippi Inc. of Jackson, Miss., along with concrete paver American Field Service Corp. of Madison, Miss., performed a 6-in concrete overlay on asphalt at an interchange of Interstate 59 in Meridian, Miss.

Warren Paving of Hattiesburg, Miss., again with American Field Service as the paver, are scheduled to begin another whitetopping this month in Forrest County near the University of Southern Mississippi. The asphalt pavement will be milled to a depth of 6 in and the whitetopping will bring the grade back to its existing elevation. The intersection has a history of rutting and shoving in the asphalt and several asphalt overlays have been placed during the last 20 years.

Bids were also taken April 22 on another whitetopping project in Scott County.

“It is unusual that they are doing a lot of whitetopping projects,” Crawley says. “MDOT has only done about nine in the past 12 years and now it has three within the span of a year.”

For heavy-traffic intersections, MDOT is looking for the “increased performance based on longevity” that concrete provides, says Adam Brown, the agency’s concrete field engineer. But there is a lot that plays into how a particular pavement performs with respect to underlying materials, and each district has the freedom to determine which pavement is best suited to a particular application, Brown says.

For example, whereas in the past MDOT has specified the use of microfibers to reduce plastic shrinkage cracks and increase tensile strength, some contractors are opting for macrofibers. “MDOT does not determine the mix design,” Brown says. The contractor develops a design that must meet certain performance criteria and is subject to MDOT’s approval.

In Louisiana, the alternate bid process is definitely making concrete more attractive, says John Eggers, area construction engineer for the Louisiana Department of Transportation and Development. The process allows the department to perform a 30-year lifecycle cost analysis, comparing lifespan and rehabilitation cycles of both asphalt and concrete pavement for any given project, Eggers adds.

“If those two future costs are within 20% or 25% of one another, we will offer that project as an alternate bid where the contractor can choose what he prefers,” he says. Changing times and costs make either option more desirable, Eggers adds. “Right now, with the cost of oil, I would think it is probably making concrete more attractive,” he says.

The La. DOTD is proceeding with its first test project using the International Roughness Index to gauge surface profiles on concrete. Eggers says the IRI requirements have been used on asphalt for some time, but a $43.5 million extension of General Ambassador Caffery Parkway in Lafayette by Gilchrist Construction Co. LLC of Alexandria, La. will be the state’s first for concrete.

“In the past we used a PI or profile index,” Eggers says. “IRI is a lot faster, gives you a better seat-of-your-pants feel about it, so the contractors gets quicker feedback on paving performance.” Using lasers, the IRI can test can be run in a golf cart or van doing 45 mph, as opposed to the former, handheld instrumentation, Eggers adds.

DOTD has also reduced the minimum cement requirement for pavement specifications to 50 lbs per cu yd, which reduces carbon dioxide emissions. The department has improved the gradation on aggregates, making is possible to use 50% slag substitution for cement. That also reduces CO2, is more economical and makes contractors less dependent on foreign markets because slag is primarily domestic, Eggers says. 

Smith’s Ready Mix Inc. of Nashville, Ark., spent a few months last year coming up with a high performance concrete mix that would provide the Arkansas Highway Department with the durability for what was rated as a 100-year bridge. Smith’s experimented with 15 different concrete mixtures before providing contractor Southern Pavers Inc. of Pine Bluff, Ark., with the right material for the $6.8 million widening and demonstration project on an overpass at Interstate 30 and Arkansas Hwy. 245 in Texarkana.

“They were trying to demonstrate a special concrete mix that would prevent salts from going into the concrete and to minimize cracking, which generally occurs due to the amount of cement,” says Hix Smith, company president. “They wanted to limit the amount of cement by specifying the maximum strength.” 

Only 5.5 sacks of cement were allowed, compared to the normal 6.5, Smith says. The maximum strength was 6,000 psi in 56 days; the minimum was 3,500 psi in 28 days.

“That’s a real tight window for variance,” he says.

In addition to strength, Smith’s was required to test the resistance of the concrete to an electrical charge. Although the highway department has not yet designated the mix for future projects, Smith says he is already using lessons learned from the test to improve mixes for other customers.

In Alabama, roller-compacted concrete is expanding its influence, says Robert Taylor, Alabama director for the southeast chapter of the American Concrete Pavement Association. What has been the favored solution for heavy manufacturing parking lots is moving into ports and roadways, thanks to good promotion and A.G. Peltz Group, LLC of Birmingham, the leader in RCC projects nationwide, Taylor says. Since 1999, A.G. Peltz has installed more than 4 million yds of RCC and related products. “With RCC you get roughly the initial price of asphalt with the life cycle of traditional concrete,” Taylor says.

This year, A.G. Peltz will be installing roughly 80 acres of RCC at the Port of Mobile. The application will be 16 in, double the thickness previously used at car manufacturers throughout the state.