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New Mexico State University researchers looking into eliminating cracks in bridges

New Mexico State University researchers are investigating ways to prevent cracks in concrete bridges.



Juan Lozoya, left, a graduate civil engineering student, and civil engineering professor Craig Newtson, measure concrete samples in an effort to learn more about the physical characteristics of different concrete mixtures. (NMSU photo by Darren Phillips)

oncrete cracks it creates a durability issue," said Craig Newtson, an assistant professor in the Department of Civil and Geological Engineering at NMSU. "You end up with water, salts and other chemicals that can get into the concrete. One primary concern is that salts cause corrosion of the reinforcing steel."



With the help of a $196,000 grant from the New Mexico Department of Transportation, NMSU researchers are trying to find cost-effective methods to reduce micro-cracking. They have been evaluating different misting practices at various stages of construction as well as different concrete mix designs.

Newtson said the project was initiated after cracking was observed in several bridge decks during the construction of the Big-I in Albuquerque in 2001. He said the bridge had 47 decks (a deck is the surface you drive on) and 25 were new. Out of the 25, 18 showed cracks.

"The fact that they got cracks at an early age, as soon as the burlap was stripped off, indicated that there was a durability issue," he said. "To remove any concerns about the Big-I they put a sealer on top of the bridge deck to mediate the effect of the cracks."

Newtson said it costs about $250,000 to seal one bridge.

Concrete cracks when it shrinks, Newtson said. When cement and water are mixed they create a chemical reaction that, as a side effect, produces volume changes. Additionally, concrete expands when it is hot and shrinks when it cools down.

"There are steel bars in concrete bridges. When the concrete shrinks the steel wants to hold it in its original location, so what is happening is that the concrete develops tensile stresses and once in awhile it cracks to alleviate that stress."

The first phase of the research involves the testing of concrete being misted at different ages.

"We are looking at misting the concrete at early ages, even during placement," Newtson said. "If we do that we don't lose water to evaporation, which will make the concrete shrink immediately."

Researchers are working with concrete slabs that measure 6 inches by 32 inches by 18 inches. The slabs are constructed inside a tent that has humidifiers at each corner.

"We wanted to make sure and develop a system that would allow us to measure the small shrinkage strains that occur at early ages," said graduate student Juan Lozoya of Santa Fe. "Everyone knows how to calculate the long-term effects, but the short-term effects may be equally important."

To measure the shrinkage strains LDVTs - linear differential variable transformers - were embedded into the concrete slab. LDVTs take readings when the concrete shrinks or swells and measures any displacement, Lozoya said.

In Phase II, researchers are evaluating concrete mixtures.

"We are playing with the water-cement ratio and are using different types of admixtures such as fly ash," Newtson said.
Researchers have found that fly ash - ash produced when coal is burned - helps with shrinkage and alkali-silica reaction, which is an expansive reaction that creates cracks. The reaction occurs when cement that contains alkali is mixed with a gravel source that contains silica.

"It breaks up the paste holding everything together," Newtson said.

Other techniques being evaluated include mixing tiny fibers into the concrete that provide reinforcement on a very small scale.

"We are trying to come up with a concrete that doesn't shrink or crack," Newtson said. "If we can alleviate the cracking problem, then we can alleviate the need to go out and seal bridges after construction, which is very costly."