Six men who worked for the Big Dig's largest concrete supplier were arrested Thursday on federal charges alleging they falsified records to hide the poor quality of concrete delivered to the massive highway project.

The six, all current or former employees of Aggregate Industries, face a variety of charges including making false statements, mail fraud and conspiracy to commit highway fraud, said FBI spokeswoman Gail Marcinkiewicz.

"My understanding is what they did was mix, commingled leftover concrete with new concrete," she said. The actions allegedly took place from 1999 to 2003.

Lawyers for Aggregate Industries have defended the quality of its concrete and said it never delivered any that did not meet strength specifications called for in its state contract.

Nancy Sterling, a spokeswoman for Aggregate, said the company had no immediate comment Thursday. R. Robert Popeo, an attorney for Aggregate, did not immediately return a call.

The allegations about concrete strength stemmed from a whistleblower suit filed in May 2005 in Suffolk Superior Court.

The Big Dig, formally called the Central Artery and Third Harbor Tunnel project, buried Interstate 93 in tunnels beneath downtown and connected the Massachusetts Turnpike to Logan Airport with a third tunnel beneath Boston Harbor.

The project was plagued by long delays and soaring costs that ballooned from $2.6 billion to $14.6 billion.

Earlier this year, after more than a decade of traffic detours, the last major section of the project opened. The heavy construction had started in 1991.

Aggregate provided 60 percent of the concrete used to build the Big Dig, according to the highway project's Web site. The company was acquired this year by Holcim Ltd., a Swiss company with U.S. headquarters in Maryland.

The men were expected to appear in federal court later Thursday.

Marcinkiewicz identified those arrested as: Robert Propseri, 63, general manager; Mark Blaise, 36, dispatch manager; John Fahar, 42, dispatch manager; Jerard McNally, 53, quality control manager; Gregory Stevenson, 53, district operations manager; and Keith Thomas, 50, dispatch manager.

Fahar was arrested early Thursday in Connecticut and Stevenson was arrested in Pennsylvania, Marcinkiewicz said. She did not know which of the men were current employees and did not give their hometowns.