US prosecutors have launched a probe into a firm accused of using sub-standard concrete in one of the biggest public works project in US history, Boston’s $14.6 billion "Big Dig," which has been plagued by leaks and delays.  Two months ago, state police raided three offices of Aggregate Industries in Massachusetts and found fraudulent paperwork used to disguise shipments of poor quality concrete, Massachusetts Attorney General Tom Reilly said on Thursday. 
 
"We found a scheme that reissued new paperwork for concrete that had been rejected elsewhere," Reilly told a news conference. "We consider this to be a violation of a contract and we also are looking at potential violations of criminal law." 
 
A whistle-blower tipped off prosecutors and Reilly’s office is working closely with the US Attorney’s office, which is also investigating the project, the Attorney General said.  This is not the first time problems have been found at the Big Dig. Leaks were found in the highway tunnels this year. That matter is being investigated separately, Reilly said. 
 
While condemning the use of rejected concrete shipments, Reilly also assured thousands of motorists who drive through the tunnel daily that the concrete is strong and durable.  "It is comforting to realize in terms of safety, in terms of strength, and in terms of durability there may be no impact," Reilly said. But he added, "While that is comforting it does not excuse this behavior. The taxpayers paid millions of dollars and they did not get what they contracted for." 
 
The Big Dig was started more than 14 years ago and involves building a 12.6km underground roadway through downtown Boston. Aggregate Industries, is now a unit of Swiss company Holcim Ltd, and reportedly the biggest supplier of concrete used in the project.