Ever-Versatile

By Debra Wood

Concrete continues to evolve as vendors find new ways to refine a reliable product that is gaining recognition for its green properties.

CTLGroup measures solar reflectance of a variety of materials, which are available in decorative finishes for walls and floors and help commercial buildings take advantage of thermal storage effect.

"Concrete is a product that can save energy," says David Shepherd, director of sustainable development for the Portland Cement Association in Chicago. He cites the material's thermo-performance attributes and heavy mass that has the ability to absorb heat during the day. ""And it slowly radiates to the interior in the nighttime," Shepard says.

Martha VanGeem, a principal engineer with CTLGroup in Chicago, says that concrete acts as a thermal-storage medium and saves the most energy when the outdoor temperature is above 55 degrees and below 75 degrees.

In an office setting, for example, if interior walls are concrete and not covered with drywall, the concrete will absorb heat generated by bodies as well as large electronics such as copy machines, so the air-conditioning does not need to run as often. Later, as the room cools after workers leave, the concrete will give the heat back.

VanGeem says concrete is now available in decorative finishes for walls and floors, which helps the building take advantage of the thermal effect.

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"The key to a zero-energy building is concrete plus insulation," VanGeem adds.

Insulating concrete forms prevent air infiltration and enhance the energy savings, Shephard says. Concrete is poured into a 6-in. void in the 4-ft-long by 2-ft-tall polystyrene forms, held together with steel ties. The forms remain in place to insulate the building, explains Joe Bailey, president of PolySteel of Austin, a manufacturer of the forms.

In addition, concrete is a relatively light-colored product, which helps reduce heating and cooling costs.

"Instead of energy hitting the surface and being converted to long-wave radiation, much more of it bounces back into the atmosphere, and it can provide a cooler surface and cooler air," Shepherd says. "If you can reduce the amount of hot air created by dark surfaces in an urban environment, you can reduce your heating and cooling."

CTLGroup recently completed a study evaluating 45 concrete samples, including some with fly ash and slag cement. It found that the cement's solar reflectance had more effect on the concrete's ability to reflect sunlight than any of the other material.

An environmental drawback of concrete is that a chemical reaction occurs while making cement. Concrete production emits carbon dioxide. Cement manufacturers, however, have begun taking steps to control emissions.

Plans for the Central Texas CEMEX Balcones cement plant expansion in New Braunfels call for installation of emission-control technology, an energy-efficient vertical roller mill and a fuel-efficient kiln. The new equipment will allow the company to double production at the plant, without increasing emissions.

TxDOT innovates The Texas Department of Transportation also is thinking green, using combined or optimized gradations for roads, bridges, piles and drill shafts. Because less cement is required, these mixes offer a more environmentally friendly approach.

TxDOT crews are using combined or optimized gradations for roads, bridges, piles and drill shafts. Because less cement is required, the mixes offer a way to cut energy consumption. Photo courtesy TxDOT.

"It goes back to our goal of reducing energy consumption," says Ralph Browne, North Tarrant Area engineer in the Fort Worth district. "We have one of the highest energy-consuming products with cement. We've cut the energy needed to produce concrete almost by half."

Dennis Warren, executive director of the Texas Concrete Pavement Association, says the process is not new nationally, but it is to Texas.

"It maximizes the aggregate and minimizes the cement for a denser, more durable final product," Warren says.

"Because of aggregate manipulation, we are able to decrease the cement content by 100 lbs per yd," says Browne, adding that the mix includes three sacks of cement and 1.5 sacks of fly ash rather than 5.5 sacks of cement in a normal mix.

Browne adds that its specs allow for fly ash, which fills voids in the concrete and decreases the amount of cement paste needed. He says the refined mix improves the sheer resistance and dimensional stability and reduces shrinkage.

Lisa Lukefahr, manager of the rigid pavement and concrete construction branch of TxDOT, says it should be cheaper, since the price of aggregates is less than cement. But that's not always the case, with pricing often dependent on the contractor's comfort level with the product.

"We give contractors the option," Lukefahr says. "They can go with a traditional gap-graded concrete mix or they can use an optimized mix. We are big believers that if we give contractors options, they will find the most economical."

TxDOT also is using thick unbonded overlays, placing concrete atop existing asphalt pavement that's in good shape but needs repaving. It eliminates the need to rework the base or haul in new base material.

"You can incorporate the existing pavement into the overall structure, so there is a cost savings," Lukefahr says. "There could be clearance issues with bridges, utilities and drainage, but if it's a large project, materials and mobilization savings outweigh the height adjustments."

Prefabrication advances Speed Fab-Crete, a design-build general contractor in Fort Worth, produces TxDOT-approved prefabricated, modular bridges for Contech Bridge Solutions of West Chester, Ohio. The sections can range in length from 12 to 48 ft, and widths can vary from 4 to 8 ft.

TxDOT says specs allow for fly ash, which fills voids in the concrete. The amount of cement paste needed is decreased and the result improves sheer resistance and dimensional stability and reduces shrinkage.

Speed Fab-Crete produces the precisely designed sections more quickly and economically than a cast-in-place structure and loads them two units at a time onto a flatbed trailer. Once at the jobsite, Contech workers off-load and erect them.

Speed Fed-Crete recently poured seven arch-bridge sections, spanning 22 ft, for a $1.7 million city of Mesquite project. Its bridges also have been used in resorts and golf courses to traverse small creeks.

"The main advantage is these are made offsite in a manufacturing environment, and that helps accelerate the construction process," says Russ Bloxom, vice president of marketing for Speed Fab-Crete.

Tilt-up thin-shell panels C.F. Jordan of El Paso, working at Fort Bliss as a subcontractor to Hensel Phelps Construction of Austin, is placing approximately 750,000 sq ft of thin-shell concrete panels instead of 6- to 10-in.-thick tilt-up panels. Metal studs strengthen the 2-in.-thick concrete veneer.

"You eliminate the process of frame-out by carpenters," says John Goodrich,

vice president of infrastructure and concrete specialties for Jordan. "The panel is ready for whatever inside application is going to be used, such as drywall or metal."

Goodrich says thin-shell costs about the same as a typical concrete tilt panel. Although such panels are available as precast, Jordan prefers to pour onsite to eliminate the need for trucking and to better control the schedule. Goodrich says the high-strength concrete sets up rapidly, with the company is able to erect the panels after three days, rather than the standard seven to 10 days.

"For speed, it's tremendous, and the panel is not as heavy," Goodrich says. "You can erect it with a smaller crane and do quite a few of the panels in a day."