Concrete solutions to aging, structurally deficient bridges

A Penn State release reports that in a cooperative effort with PennDOT, the researchers are evaluating 220 bridges in Pennsylvania, forecasting the extent of cracking over time, and working to optimize the type and timing of maintenance. By collecting and analyzing data on maintenance that’s been done over the last fifteen years, they can determine the costs and benefits of performing maintenance immediately, as opposed to waiting another few years. Most importantly, they are working to come up with an optimum maintenance schedule to help PennDOT determine which bridges must be repaired this year, next year, and further into the future and to get the most benefit out of the limited maintenance dollars available.

As well as concrete maintenance and durability issues, Rajabipour and his colleagues explore options for making concrete a greener and more environmentally friendly material.

Simply put, Rajabipour says, concrete is “a formable, manmade rock that is made of natural stone particles or aggregates, such as sand or gravel, that are glued together.”

The glue is a mixture of Portland cement and water, which react chemically and harden. Portland cement is relatively cheap and is produced using some of the world’s most abundant resources. At the same time, it is the most energy-intensive and environmentally negative element of concrete, Rajabipour explains. To make traditional Portland cement, limestone, sand and clay are ground up and heated to a high temperature (1,450 degrees Celsius or 2,600 degrees Fahrenheit), creating a cement powder that is highly reactive with water. This process uses a lot of energy and releases large amounts of carbon dioxide into the atmosphere. In fact, it is estimated that more than 5 percent of total human-generated carbon dioxide emissions is the result of cement production.

With funding from the National Science Foundation, Rajabipour and his colleagues and students are working to produce new cements that perform as well as Portland cement, while providing significant energy savings and environmental benefits. For example, they are studying recycled materials, such as ground glass bottles, and industrial byproducts, such as iron blast furnace slag and coal combustion residue (fly ash and gypsum), to produce new cements.

“When you’re using recycled and waste materials, you’re likely doing something that’s good for the environment,” Rajabipour says, “but at the same time you don’t want it to come at the expense of longevity and performance. Portland cement has been around for about 200 years. We know a lot about its properties and performance. But in the case of alternative cements, we have made a brand new material and we know little about its long-term quality. It doesn’t mean this material won’t be as good or better, it’s just that we don’t know yet. And we don’t have enough time or money to study these materials for another century or two to see if they’re good enough.”

To address this problem, the researchers are starting to use computer simulations to predict how materials will behave over a long period of time. They are performing what Rajabipour calls “virtual experiments” that simulate and accelerate the aging process for concrete. They use computer modeling to figure out the best recipe for combining different ingredients and processing or cooking materials to make an optimum cement and to determine how concretes made from these cements will perform over several decades of service life.

The release notes that over the years, Rajabipour has developed close relationships with members of the local and national construction industry. He collaborates with groups such as the Pennsylvania Aggregate and Concrete Association, a consortium of concrete producers in the state, and the Pennsylvania Coal Ash Research Group, a consortium of electric power producers.

“The electric power industry is very supportive of research on how we can use byproducts of burning coal,” Rajabipour says. “We continue to explore how we can use those products in a beneficial way, as opposed to putting them into landfills or impoundments. It’s a valuable working relationship — we’re the go-to place for the industry when they need basic or applied research to solve a practical problem.”