Concrete Answers

An assistant professor of civil engineering at SIUC has paved the way for coal to not only power our buildings, but to help hold them up. 

Coal-burning power plants in Illinois alone generate more than 3 million tons of combustion waste every year, mostly lightweight "fly" ash and heavier "bottom" ash that settles on the floor of the boilers. The bulk of this ash goes straight to landfills, at disposal costs of $30 or so a ton.

Sanjeev Kumar came to SIUC in 1998 with extensive industrial experience in foundation design and construction. Once here, he learned about SIUC research to use coal combustion byproducts in construction materials, including concrete for parking lots and roads. He began to think about other concrete products that might use a substantially greater amount of these wastes, making a bigger dent in the amount going to landfills. 

His answer? Deep foundations. 

Much of America's infrastructure rests on deep foundations: concrete pillars that support bridges and multi-story buildings, transferring a structure’s load to greater depths. "Some of these pillars are huge—they use a lot of concrete," says Kumar. 

The pillars—skinny ones are known as piers, thicker ones as piles—usually range from 10 to 60 feet long and from one to 10 feet in diameter. Depending on the building site, they either are poured in place (these are called drilled shafts), or are pre-cast and then driven into the ground. 

Based on his engineering experience in the St. Louis and southern Illinois area, Kumar estimates that some 1 million cubic feet of concrete is used annually in this region of the country alone to construct deep foundations. Potentially, a lot of ash might be recycled, diverting it from our swelling landfills.

Fly ash has been used for years to partially replace cement in concrete, but bottom ash, which eats up much more landfill space, has not commonly been used in construction. Kumar thought that this ash—a light-colored, gritty, sand-like substance—might substitute nicely for the fine aggregate (sand) in concrete. But would concrete made with bottom ash be as strong and durable as the conventional concrete mixes used in deep foundations? 

To find out, he teamed up with Nader Ghafoori, a former SIUC professor of civil engineering now at Tennessee Technological University, and Vijay Puri, another civil engineering professor at SIUC, to explore this idea. Ghafoori already had tested certain types of coal combustion waste in pavement (see "Green Highways," Perspectives, Fall 1995). Puri would contribute expertise in field testing and data analysis.

With a $128,828 grant from the Illinois Clean Coal Institute (ICCI), and with ash donated by City Water, Light & Power of Springfield, Ill., they and their students began lab-testing nine mixes of concrete that substituted various percentages of bottom ash for sand. They needed to test strength under heavy loads. They needed to test stiffness—how much the concrete deforms under those loads. And they needed to test durability. 

They compressed the concrete samples, tried to split them, tried to bend them, measuring deformation under all of these tests. To simulate severe weather conditions, they saturated chunks of concrete with water, put them in the freezer for a day, removed them to thaw for a day—then repeated that cycle 50 times, measuring exactly how much material was lost under these conditions. They also measured how much material was lost when samples were soaked in a sulfate solution. (Some soils have a high sulfur content, which can degrade concrete over time.) Lab studies convinced Kumar that no significant amount of pollutants, such as heavy metals, should leach from the new concrete mixes. 

The team chose two of the experimental mixes to test in the field against conventional concrete. One mix replaced 50 percent of the sand with bottom ash; the other replaced all of the sand with ash. 

At the Illinois Coal Development Park in Carterville, Ill., the team constructed drilled shafts by drilling 25-foot-deep holes about one foot in diameter and filling them with the mixes. Then they tested the shafts for the types of loads that foundations are designed to withstand: compression, uplift, and lateral forces. They even used a sophisticated hydraulic jack at the bottom of some of the shafts to apply compression and uplift forces simultaneously—a specialized process called the Osterberg cell test.

Enduring the heat and humidity of a southern Illinois summer, several graduate and undergraduate students labored alongside Kumar and industry consultants, sometimes starting work at dawn and not quitting until 2 a.m. As Kumar explains, "It can take a day just to set up a test and several hours to perform it. Once the test starts, you have to finish it; you can't just stop."

The results were worth the trouble. Both of the new mixes performed similarly to conventional concrete. Used in actual building foundations, they should be just as strong and durable, says Kumar.

"The advantage of bottom ash is that, unlike sand, it has some cementitious properties," he notes. "In our research we have seen the strength actually to be slightly higher, long-term, than with conventional concrete."

With that experience, Kumar, Ghafoori, and Puri won a follow-up ICCI grant of  $175,746 to try the same with pre-cast concrete piles. They kept the 50-percent bottom-ash formula and the 100-percent bottom-ash formula. For both mixes, however, they increased the proportion of cement to make the concrete stronger. Why? When pre-cast piles are driven 10, 20, 30 or more feet into the ground, they sustain tremendous stresses. "The concrete strength you need for pre-cast piles is much higher than what you need for drilled shafts," Kumar says.

After these new concrete mixes were tested in the lab, Egyptian Concrete (Salem, Ill.) helped the SIUC team use them to construct piles for field testing. The piles, along with some made from regular concrete, were then trucked to Carterville, where a pile driver mounted on a crane was used to drive them into place. 

Besides load testing, the piles underwent stress testing with special equipment designed by GRL and Associates Inc. Kumar explains, "When you hit the pile, a stress wave travels down through the pile and reflects back up from the bottom. How fast it moves tells you a lot about the properties of the material, including the pile capacity."

The tests the team performed are "not done every day on every job," he says. "They are very expensive to do, and high skill is needed to do them. Many engineers don't get the opportunity to see this type of testing in their entire career. It’s great hands-on training for our students."

Choreographing the flow of contractors, consultants, students, and visiting observers on site was a challenge for Kumar. Besides the firms mentioned above, Illini Ready Mix, Illini Drilled Foundation Co., Holcomb Foundation Engineering Co., Loadtest Inc., and E. T. Simonds Construction Co. worked to prepare concrete mixes, drill shafts, pour concrete, pile-drive the pre-cast piles, set up and monitor testing equipment, and so forth. The Illinois Department of Transportation did soil drilling at the site. And Subsurface Constructors Inc. and Geotechnology Inc. of St. Louis provided technical assistance with the testing.

"Making sure things worked OK on the site, with so many players involved, was difficult," Kumar says. "But these projects have succeeded only because of industry involvement, and these companies have helped us with enormous in-kind contributions—more than $200,000 worth."

He adds, "I couldn't have done it without the help of my students either. They are the ones who really worked hard."

In the most recent phase of the research, the Illinois Department of Commerce and Community Affairs kicked in $151,828 for a demonstration project at the site of a hospital complex under construction in Marion, Ill. 

"We constructed two shafts at their facility, one with our concrete mix and one with theirs, and tested both," says Kumar. "We also made samples which we tested in the lab. We successfully demonstrated that the mixes would perform similarly, and now we're looking to move to commercialization."

Use of the new concrete mixes in foundations could begin soon. 

"Whenever you develop a new product [for construction], you have to develop a whole suite of data—lab data, field data—to convince the engineering community that the material will behave a certain way," says Kumar. He now has the evidence they need, he says, and he plans to continue testing the new concrete mixes.

But the motivating factors for industry to adopt them will be financial and environmental. Each cubic yard of concrete made with bottom ash instead of sand would save society about $28, Kumar estimates. Construction companies would save 10 to 15 percent on making the concrete. And utility companies would see significant savings by giving the ash away rather than paying landfill charges.

It all adds up to a big plus for both industries, Kumar says—and one they could implement right away. 

"We have enough [data] right now that the public should be interested," he says.

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