A federal investigation of alleged bid rigging in the asphalt paving industry has spread to Pennsylvania and is being aided by a Baltimore paving executive who pleaded guilty to conspiracy yesterday in U.S. District Court in Alexandria.

The businessman's attorney told a federal judge that his client, Barton S. Mitchell, has been cooperating with authorities investigating alleged corruption in highway paving contracts in several states, including Pennsylvania, under a May 28 plea agreement with the Justice Department.

The 17-state federal investigation, which a spokesman said yesterday is the most widespread antitrust probe in department history, already has resulted in numerous indictments and convictions in Virginia and a federal grand jury investigation in Maryland. The spokesman confirmed yesterday that grand juries in Philadelphia and Harrisburg, Pa., also are weighing evidence of alleged bid rigging.

Mitchell was indicted last month in Alexandria, along with two companies he heads, E. Stewart Mitchell Inc. of Baltimore, and Superior Paving Corp. of Fairfax County. Under the plea agreement, Mitchell and Superior both pleaded guilty to conspiracy in exchange for dismissal of separate mail fraud counts. A conspiracy charge against E. Stewart Mitchell Inc. was dropped yesterday.

District Judge Richard L. Williams sentenced Mitchell to 150 days in prison and fined Superior $250,000. Williams said he would accept a government request that Mitchell be confined at Allenwood, Pa. Mitchell will appear as a prosecution witness in the scheduled July 6 trial in Alexandria of two other Northern Virginia paving firms and their officers indicted with Mitchell last month, Justice attorney Hays Gorey Jr. said.

Gorey told Williams yesterday that Mitchell took part in illegal meetings in 1977 and 1978 with fellow pavers to arrange who would win Northern Virginia contracts let by the Virginia Department of Highways and Transportation. The secret sessions occurred at motels in Gaithersburg and Tysons Corner and at a hotel in Richmond, Gorey said.

In a separate development yesterday, Judge Williams agreed to modify the 60-day sentence imposed April 29 on Northern Virginia paving executive Peter W. Herring. Williams said he would allow Herring, who has been at Allenwood since early May, to complete his sentence at a halfway house in Norfolk.

In another development, a U.S. District Court jury in Richmond acquitted a South Carolina paving firm and one of its vice presidents Thursday on charges they conspired to fix prices on a 1979 construction contract at the Roanoke airport.