Subpar material could add to woes on tunnel project

The use of 5,000 truckloads of diluted, old concrete in the Big Dig could cause its tunnels and roadways to crack and crumble, several independent engineers said yesterday, even as state and federal officials sought to reassure drivers that the massive highway project is safe.

Concrete has to be mixed and poured precisely and quickly for the material to endure, according to civil engineers.

But prosecutors say that Aggregate Industries NE Inc., the main supplier of concrete to the Big Dig, violated the project's specifications.

State and federal officials said the faulty concrete does not have to be replaced immediately, although it could pose long-term maintenance problems.

Aggregate is accused of waiting well beyond the required maximum 90 minutes to pour the material and in other instances of mixing old, hardening batches with fresh concrete. Prosecutors also say that workers diluted concrete with extra water to make it appear fresh. Engineers said that such dilution could weaken the concrete and cause it to disintegrate eventually.

''The worst-case scenario is it collapses," said Paul J. Tikasky, a civil engineer at Penn State University and a board member of the American Concrete Institute. ''It depends how bad it is, but it could happen, sooner than later."

Federal and state officials, however, tried to reassure the public that the roads are safe.

''There's no evidence to suggest that the structural integrity of the tunnel has been compromised," US Attorney Michael J. Sullivan told reporters at a press conference yesterday.

He cited the assessment of engineers at the US Department of Transportation, the Massachusetts Highway Department, and independent analysts hired by the Big Dig. ''People should not feel unsafe using the tunnel," Sullivan said.

Indictments handed down yesterday against six employees of Aggregate, charging them with conspiracy to commit fraud and other offenses, did not indicate to what extent the firm's concrete was allegedly watered down.

Aggregate was the main supplier of concrete for the Big Dig, responsible for 60 percent of material used on the project, Sullivan said.

Civil engineers were shocked yesterday that the prominent firm had been accused of flouting basic industry regulations.

''Shame on them," said Michael W. Grutzeck, a Penn State chemist who studies concrete. ''My gut feeling is that was not a good thing, and there could be long-term issues. If they cut corners, I would want them to redo it."

Civil engineers said they were most concerned that Aggregate had compromised the strength of the concrete used to build the Big Dig's tunnels. They also raised concerns about the safety of highway ramps and roads, which could degrade more quickly than those made with properly mixed concrete. The resulting rough roads would make braking more difficult and slow traffic.

Federal officials said they did not know how much damage if any the faulty concrete had caused and could not estimate how much repairs might eventually cost.

''Strength is what you're really concerned about," said William P. Henry, past president of the American Society of Civil Engineers, who stressed he had not specifically analyzed Aggregate's concrete.

He and other engineers raised concerns that the addition of old concrete to batches of new concrete would result in a lumpy mixture that would not solidify as strongly.

''If you have the lumps that are too big, you might not have the uniform mixture of sand, cement, and gravel that you need for tight concrete," Henry said. ''It could form cracks. It could have internal bonding issues."

Made properly and poured quickly, concrete can last 50, 75, even 100 years, depending on weather and road conditions, according to Tikasky. Made poorly, with too much water and chunks of hardening concrete, the mix might last only 5 to 15 years before deteriorating, he said.

''There should be signs: the rebar [metal reinforcements] would become exposed, and usually there are some sign of stress before they go," Tikasky said. ''That's the one good thing about concrete structures: They bend a lot before they break."

Aggregate is accused of pouring the substandard concrete into the walls in both directions on the Interstate 93 and Interstate 90 tunnels, as well as on I-93 ramps and in a seawall at Fort Point Channel. Sullivan said the substandard concrete -- totaling 50,000 cubic yards, or 1.2 percent of all the concrete used on the Big Dig -- might need to be repaired sooner then expected, but he could not say when that might be.

Attorney General Thomas F. Reilly said use of the substandard concrete could result in ''long-term maintenance issues," but did not give specific examples.

He said he had met with engineers who had conducted inspections of the tunnels and declared them safe.

Sullivan said some of the panels in the tunnel that developed leaks had been constructed with Aggregate's faulty concrete but he said he didn't know whether it played a role in the leaks.

Some independent engineers suggested yesterday that the state would need to take samples from the tunnel walls made with the poor quality concrete and test them to make sure they will not leak.

Henry said he hoped the indictment would send a message to construction contractors that they need to respect even the most basic rules of the trade, because the consequences can be dire for them as well as the public.

''You obviously don't like it because the owner should get what they paid for," Henry said ''and the owner in this case would be the citizens of Massachusetts."

Michael Levenson can be reached at mlevenson@globe.com.