Concrete cracking is linked to alkali
Projects in state not holding up
Some concrete projects across Arkansas are cracking because of high-alkali cement that was used from 1997 through 2002, according to researchers at the University of Arkansas at Fayetteville.
The pattern of cracks can resemble an alligator’s hide or lines on a road map. Sometimes gel oozes from the cracks.
Not only did cement have a higher alkali content in the 1990s, concrete producers used more cement at that time thinking it would make their concrete stronger, said Micah Hale, a UA professor of civil engineering.
But the extra alkali reacted with chalcedony and chert, two minerals in Arkansas River sand, which is used for most concrete construction in the state.
State regulations that began to be implemented in 2003 require a lower alkali content in cement.
But some projects built
about 15 years ago are crumbling from the alkali-silica reaction.
The process creates a gel that absorbs water, causing the concrete to expand and crack. The classic symptom is a pattern called map cracking, alligator cracking or crocodile cracking.
Alkali-silica reaction has damaged interstates, airport runways and bridge foundations in Arkansas, requiring that they be repaired or replaced earlier than expected.
Contractors can’t be held responsible for the damage because they were following regulations that were in place when the projects were constructed, said Danny Straessle, a spokesman for the Arkansas Highway and Transportation Department, and Kelly Johnson, director of the Northwest Arkansas Regional Airport in Highfill.
Alkali-silica reaction ruined a runway at the Highfill airport. It cost $30 million to replace the runway, said Barbara Busiek, director of construction and grant administration for the airport.
The runway was constructed in 1997 and 1998. Normal life expectancy for an airport runway is 20-30 years, said Busiek. Because of the alkali-silica reaction, the runway had to be replaced after 16 years.
In Little Rock, the state Highway Department is planning to replace a 2.14-mile section of Interstate 30 between 65th street and the spot where Interstates 30, 440 and 530 converge. The six-lane interstate is damaged by an alkali-silica reaction, primarily in the middle and outside lanes in both directions, said Tony Evans, construction engineer for the Highway Department’s District 6 in central Arkansas.
The I-30 work is scheduled for bid letting in the coming December. The Highway Department doesn’t have an estimate yet on the cost.
Randy Ort, a spokesman for the Highway Department, said that section of I-30 services an average of more than 90,000 vehicles per day, and tractor-trailer rigs are limited to the right two lanes, where most of the pavement damage has occurred.
“The need to patch potholes in that area has become a much more frequent occurrence over the past 12 to 18 months,” said Ort.
The most visible epidemic of alkali-silica reaction in Arkansas could very well be the Interstate 49 median barrier wall near Winslow in southern Washington County. Some sections of the 3-foot-tall wall just north of the Bobby Hopper Tunnel are crumbling.
The Highway Department plans to spend $1.6 million to replace a 1.9-mile section of the wall, said Straessle.
That amount will also cover the cost of treating another 2.4-mile section of the wall with silane, a compound of hydrogen and silicon, to slow or stop the effects of the alkali-silica reaction.
The repair work should begin in April and be completed in four months, said Straessle. The silane treatments will continue for about seven months. Koss Construction Co. of Topeka, Kan., was awarded the bid to do the repair work.
Silane will also be used to treat the adjoining roadway pavement, which shows evidence of alkali-silica reaction, said Richard Deschenes Jr. of Rogers, a graduate student in the College of Engineering at UA in Fayetteville.
Deschenes and others at the university have been researching the effects of silane on alkali-silica reaction over the past few years. Treating the surface with silane reduces deterioration, he said.
“However, the silane is only effective if applied before the expansion and cracking becomes severe,” Deschenes said via email. “The untreated concrete has continued to deteriorate rapidly, and the cracking has allowed more moisture to enter the concrete, which increases the ASR [alkali-silica reaction] related expansion. Expansion
has also occurred in the winter, when ASR is dormant, indicating that moisture within the concrete is leading to freezing and thawing related distress and exacerbating the deterioration.”
Hale said the Highway Department has given UA more than $500,000 in grants to research the alkali-silica reaction problem.
With the Highway Department’s support, the UA researchers have learned that they can mitigate alkali-silica reaction by using silane when the reaction is first spotted, said Hale.
“We’ve also learned that we can prevent it using the right amount of fly ash in our concrete, which is a locally produced byproduct from electric generating plants such as the one in Redfield,” said Hale.
While silane has slowed or stopped the expansion of the alkali-silica reaction in the I-49 median wall, Hale said, it’s too early to tell if it will work on the road pavement.
But he doesn’t expect the I-49 pavement to expand further because the gel in the pavement has consumed all of the alkalis in the cement, and the silane treatments reduce the relative humidity in the pavement, which will stop the reaction. Alkali-silica reaction occurs when the relative humidity is 80 percent or greater inside the concrete.
Hale said there are concrete structures in Arkansas that are 50 years old and show no evidence of alkali-silica reaction.
But in the 1990s, concrete producers began to use more cement in their products thinking it would result in stronger concrete, said Hale.
The total alkali content in the I-49 barrier wall ranged from 0.40 percent to about 1.1 percent when it was constructed, said Hale. Today, the alkali content of the cement from that same plant is around 0.60 percent to 0.65 percent, he said. The Highway Department limits the alkali’s use in state projects to 0.60 percent or less.
Alkali-silica reaction isn’t just an Arkansas problem. According to the Federal Highway Administration, it’s a problem worldwide and was first documented 75 years ago.
“Alkali-silica reactivity was first identified as a cause of concrete deterioration in 1940,” according to the federal agency. “Since that time, much has been learned about the cause and prevention of ASR. However, ASR still continues to be a problem. Distresses caused by ASR have been noted in concrete structures that are over 100 years old and in structures that are only a few years old. The problem of ASR is not limited to certain types of structures, but has occurred in large structures, bridges, and pavements.”
Older Arkansas sidewalks often show evidence of the crocodile cracks. But the large and severe cases in Arkansas so far have fallen within that narrow time-frame of the late 1990s and early 2000s.
When asked why many concrete structures from ancient Rome still stand, Hale said, “We see only their best structures. There are many more Roman structures that have fallen down. If the world is around in 2,000 years, I think you’ll see some of structures that were built in our day and age still standing too.”