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Cover Story - June 2004

A new kind of mix
Concrete design innovations seek to reduce job costs

By Sam Barnes

A reduction in job costs - often achieved through reductions in placement times or materials - is the ultimate goal of most new innovations in the concrete industry.

In the building market, two of the latest attempts toward this financial objective include self-consolidating concrete and pervious mix designs.

Craig Duos, executive director of the Concrete and Aggregates Association of Louisiana, said self-consolidating mix is gaining a lot of attention because it offers a much faster concrete placement alternative.

"Self consolidating concrete is a mix that flows more easily around the reinforcing steel and through the form," Duos said.

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Mike Seal, a sales representative with Lafarge Corp. of New Orleans, said the greatest benefits of the self-consolidating mix are its flowability and uniform distribution of aggregates.

The mix has fairly small pea gravel as its aggregate and has the properties of a fluid during placement, Seal added. It can achieve an outward flow of up to 550 to 650 mm.

His company's self-consolidating mix, called Agilia, has architectural and vertical applications.

The architectural form of the product is typically used for heavily reinforced or highly congested applications, seismic zone construction, walls, columns, spans, precast and applications with an architectural finish requirement.

Agilia's vertical application is used for spans, precast, walls, columns, heavily reinforced or highly congested applications, and applications with a high quality surface requirement.

"The product is most useful for surfaces that require an excellent finish, heavily reinforced structures or members, and structures or members with complex forming," Lafarge sales representative Steve Salzer said. It also provides the architectural and engineering communities with a product that allows for enhanced design creativity, such as for exposed walls and columns in large-scale commercial or residential projects.

Duos said pervious concrete is also gaining in popularity because it allows water to seep through the mix. This makes it attractive to owners and contractors who wish to control site runoff.

"It was originally developed for underdrain systems and for areas that cannot permit any additional runoff into the system," Duos said. "It allows moisture to percolate through the concrete into the soil below."

Reducing the "fines" in the mix and using more intermediate or larger stones as aggregate help create the porous concrete.

"Now it's starting to see the light of day as a system for use in parking areas and places that have to comply with stormwater retention regulations," Duos said. "In some cases they'll say you can put a building in a certain location but you can't increase stormwater runoff."

Highway applications. A section of Interstate 10 in east Baton Rouge will soon provide the testing grounds for a new method of determining flexural strength in pavement concrete.

James Construction Group is building the $40 million I-10/Picardy Avenue Interchange and should begin paving this fall.

John Eggers, an engineer with the Louisiana Transportation Research Center, said Department of Transportation and Development designs often specify flexural strength even though there has never been an accurate way to measure it.

"In the past, quality control testing has been based upon compressive, not flexural, strength," Eggers added. "But if the concrete pavement is designed using a flexural strength it needs to be measured that way."

At the I-10 site, a flexural test will measure the concrete's tensile strength.

"Samples of the concrete mix will be used to create a rectangular test beam and we'll use a flexural testing machine with a three-point loading system to determine its strength," Eggers said. The testing will occur throughout the project for quality assurance.

He added that a flexural strength of 750 psi is specified on the I-10 project, while most jobs have a typical strength of 600 psi.

"A higher flexural strength will allow us to decrease the thickness of the pavement, which will in turn save us money," Eggers added. Varying the gradation of components in the mix by adding more crushed stone or cement can help achieve the higher flexural strength.

"It's ultimately up to the contractor and the concrete supplier to determine how they're going to achieve it," Eggers said.

Duos said increasing the flexural strength will allow DOTD to decrease the amount of concrete necessary for a project and therefore make the material less costly and more attractive for road construction.

Mark Morvant, LTRC's pavement & geotechnical research administrator, said his office is researching the use of cement as soil stabilization for road subgrades.

Traditional lime treatment has been ineffective in soils with a high concentration of silt.

"We've basically switched from lime to cement under certain conditions," Morvant said. "Lime requires more cohesive soils to have a reaction for stabilization. So with silty soils, there's no long-term performance."

Most of the silty soils have been found in west and northeast portions of the state. Most recently, a four-laning project on LA 15 near Sicily Island specified a cement-stabilized subgrade.

Morvant said tests this summer at DOTD's Accelerated Load Facility in Port Allen will enable state engineers to determine if other cost-saving changes can be gained by using cement.

"Until now, we haven't given it any value in our design process," he added. "With the ALF section, we're trying to find out if we can reduce the depth of the pavement layers when using the cement as stabilization."

F. G. Sullivan Jr. Contractor LLC of Baton Rouge will build the test section at the ALF facility this summer in conjunction with an existing overlay project on LA 1.


Useful Source:

For more information and updates about LTRC research, go to: http://www.ltrc.lsu.edu/research.html

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