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CHANTILLY, Va. (AP) — Concrete panels installed in the $2.6 billion project extending the D.C. region’s Metrorail Silver Line to Dulles International Airport are not as durable as they should be.

Charles Stark, director of the Silver Line project, said the concrete is supposed to last 100 years but was not mixed properly by a subcontractor. To fix the problem, the concrete will have to be periodically coated with a special substance to enhance its durability.

Stark said the additional costs will be borne by the contractor and will not delay the anticipated 2020 opening of the Silver Line’s second phase.

Stark said another 60 panels are being replaced because the reinforcing steel mesh is positioned in a way that it could too easily rust.

The issue was first reported by WRC-TV.