ANNAPOLIS – Professionals familiar with the concrete used in the Chesapeake Bay bridge project disagree whether, as the contractor has claimed, the material can be applied in colder weather, a technique now under scrutiny after the material cracked.
Cianbro Corp. used a mix of Portland cement and silica fume, a byproduct of producing silicon metal or iron-silicon alloys, according to the Silica Fume Association Web site.
The concrete was discovered to have cracked early in 2004, according to Jack Cahalan, Maryland Department of Transportation spokesman. Work on the William Preston Lane Jr. Memorial Bridge began in January 2002.
All of the resurfacing done on the westbound span of the bridge will have to be redone, a job that could add $7 million to the $60 million project, Cahalan said.
The Maryland Transportation Authority is investigating why the new concrete cracked. Its findings could determine who must pay for the excess cost.
Microsilica concrete was used because it allowed contractors to work with the material in colder weather, Cahalan said.
“What they (contractors) were trying to do was utilize a product which would allow them to expand the amount of time they would be able to operate during colder months and minimize the impact to motorists,” said Cahalan.
However, engineers have contradicting opinions regarding its use in colder temperatures.
The mixture produces a higher heat of hydration, which allows the material to gain strength faster than regular cement in colder weather, according to a Portland Cement Association engineer. When water is mixed with cement, the temperature of the mixture increases and that reaction is called the “heat of hydration,” according to the association.
Typically, it takes a regular cement mixture 28 days to reach its full strength, but at colder temperatures the early rate of strength gain is slower, PCA engineer Terry Collins said. Microsilica provides a faster early strength gain in all temperatures, Collins said.
However, a consultant for the Silica Fume Association said he is not familiar with any benefits to using microsilica concrete in the cold.
Microsilica concrete has fine particles that reduce the ability of water to pass through the concrete, said Terence Holland, a consulting engineer in concrete materials.
“If you reduce the permeability, it means it takes longer for deicing (material) to get down to the reinforcing steel,” Holland said. “For this type of application, that’s the driving force.”
Silica fume is generally added in proportions of 5 percent to 10 percent by weight of the Portland cement that is in regular concrete, Holland said.
Microsilica concrete is used in marine and corrosive environments because of its ability to withstand corrosion, said John Depman, president of Central Atlantic Contractors in Abingdon, Md. But, he said he is not familiar with using the mixture in cold weather.
Central Atlantic applied the finishing to some of the overlays on the bridge in the summer of 2003 and it deals with microsilica concrete “all the time,” Depman said.
There are some problems associated with the mixture. Compared to regular concrete, microsilica does not bleed, Collins said. In a regular mixture, moisture escapes to the top, preventing rapid drying. Special precautions have to be taken in the finishing process to prevent the microsilica concrete surface from “crusting,” Collins said.
A failure to address this situation could cause the cement to shrink before it hardens, resulting in cracking. But, that is only typical in the first two to six hours of the mixture’s life, Collins said, not months later as is the case with the Bay Bridge.
Another possible cause of the cracking, Holland said, could be the movement of the bridge while carrying traffic.
“Doing an overlay on a flexible bridge is one of the hardest concrete operations you can do, whether you have silica or not, because you’re putting new concrete on top of old (concrete) on a structure that is very flexible,” Holland said.
Shrinkage cracking, where the mixture gives up moisture and loses mass, may also be the cause, Collins said.
On a bridge, the concrete is poured over steel reinforcement, so when the concrete tries to shrink, it is restrained by the steel and cracks, according to Collins.
There could be other reasons for the cracking on the Bay Bridge, but without firsthand knowledge Collins declined to speculate.
“When you take a look at the volume of use, the percentage of problems isn’t any more common than problems with regular Portland cement concrete mixtures,” Collins said. “There are relatively few problems, but there is no way to use it that is absolutely problem-free.”
Fixing the cracked concrete may result in more delays on the bridge, but the original summer 2006 completion date is still expected to be met, Cahalan said.
Pioneer Contracting Company Inc., which manufactured the cement, refused to comment Thursday, saying it is forwarding everything to its lawyers then declined to provide the attorneys’ names.
Cianbro Corp. said it did all the necessary work for the job.
“We did the work as per the plans and specs of our contract,” said Michael Hart, vice president and general manager of Cianbro’s Baltimore facility.