Washington, D.C.'s overdue, overbudget Silver Line Metro route extension has hit another snag: Some 1,700 concrete panels laid on the $2.8 billion project are defective.

According to WRC-TV, the local NBC affiliate that broke the story yesterday, issues with the concrete include an unbalanced mix of cement and water, too little concrete covering steel inside the panels, and "insufficient safeguards for water to expand and contract," all of which could lead to cracking. Most of the concrete will have to be resealed, and at least 65 panels will have to be replaced entirely.

This added work supposedly will not affect the projected 2020 completion date. "We are confident at this point that the mitigations are solutions that will result in not just a safe system, but one that will last 100 years," the project's executive director, Charles Stark, told The Washington Post.

The construction of D.C.'s Silver Line, which is supposed to connect the western reaches of Maryland's Prince George County to Virginia's Dulles International Airport has suffered multiple delays during the project's 16-year history. The first phase of the Silver Line, which runs from Largo, Maryland to Reston, Virginia, was completed in 2014, six months behind schedule and $225 million over budget.

Phase 2, which will extend the Silver Line to Dulles International Airport and the suburban communities of Virginia's Loudoun County, has met a similar fate. Building began in 2014 with the goal a 2018 finish date. That got pushed back to 2020 about a year into construction. According to Marica McCallister, a spokeswoman for the project, stricter storm water management requirements were responsible for that delay, which added about $95 million in costs.

The bill for the second phase is being split between Loudon and Fairfax counties in Virginia and the Washington Metropolitan Airports Authority (WMAA), which is in charge of construction. The Federal Transit Administration loaned the counties and the WMAA a total of $1.9 billion to finance the project.

The WMAA blames the concrete mishap on the supplier, Universal Concrete Products, which is now the subject of a criminal probe.