Construction workers with Capital Rail Constructors lift a 75-ton girder into place on the Dulles extension of the Silver Line of the Metro on Tuesday, April 21, 2015 in Dulles, Virginia. (Photo by Pete Marovich For The Washington Post)

Officials at the Metropolitan Washington Airports Authority said that crews have resumed placing some of the concrete girders that will support tracks at Dulles International Airport, but questions  remain about what caused cracking in the largest of the concrete supports and whether the ones that have already been installed will have to be torn out.

In July, project officials told members of the MWAA board of directors that crews had stopped placing the support beams after cracks were found in six of the eight girders that had been put into place. The concrete beams, which resemble a capital “I,” help support the aerial tracks that will carry Silver Line trains through airport property.

Charles Stark, executive director of the Silver Line rail project, said the cracks were first discovered in the larger 96-inch girders in April, but officials were not concerned because cracks can appear in these types of precast concrete structures. But after the size of the cracks grew, project officials suspended work later that month until they could determine what was causing the problem.

New problems with the Silver Line’s second phase.

Stark said that the cracks were in the top flange of the concrete structures and were of less concern because they are not part of the main body structure. Still any cracking can affect the life span of a beam, he noted.

The girders were manufactured by Coastal Precast Systems, which is based in Chesapeake, Va. Of the 100 that were cast, as many as 60 had cracks, said project spokeswoman Marcia McAllister.

Chris Paolino, an MWAA spokesman, said that officials have been able to resolve problems with the 72-inch and 84-inch girders and so resumed work placing those structures. But questions still remain about what is causing the larger 96-inch girders to crack. MWAA and Capital Rail Constructors, the contractor building the second phase of the rail line have each hired their own consultants to investigate the cause of the problems.

It is not clear whether the problems with the larger girders will further delay the $5.8 billion project that already is 13-months behind schedule.  Stark said in July that if the problems were not resolved by Labor Day, additional delays were possible. Also not known is whether contractors will have to remove and replace the 96-inch girders that already have been set.

Silver Line may not carry passenger until 2020.

The first phase of the 23.1 mile Silver Line opened last July with five stops– four in Tysons Corner and one in Reston at Wiehle Avenue. The project’s second phase will have six stops including one at Dulles Airport and will, for the first time extend Metro service into Loudoun County.

MWAA is overseeing construction of the Silver Line, which ultimately be operated by Metro.