A wide body Captain reported that on taxi out from the gate at EDDF (FRA) ground observers spotted asphalt and other debris adhering to the main gear tires from a repair area on the ramp.

Narrative

After pushback and engine start; taxiing the aircraft to Runway 25R; EDDF Tower notified us that we were to hold short of Taxiway D-West for an aircraft inspection. A Safety Inspector from the Airport Authority noticed a build up of debris on the outer [main] gear tires. The debris was from a recent repair of the Taxiway Golf. We taxied the aircraft to Taxiway Alfa holding area short of Delta. Maintenance and Airport Safety Authorities inspected the tires and notified us via aircraft interphone that the tires needed to be removed before flight. The cabin was opened for the Flight Crew to inspect the tires. I did inspect the tires and also inspected the area where the debris was clinging to the tires. The temperature at the airport was above 25 degrees Centigrade. The tarmac was even hotter with jet engines running and also high stresses from the heavy aircraft in the vicinity of the area. The aircraft was stopped during the pushback over the area in question and the outer tires actually sunk into the asphalt. The outer tires became [covered] in tar and gravel [that] clung to the outer edges of the tires in very large clumps; over five inches in some areas. The forward tire was scraped clean of debris; the rear tire was removed and replaced. After an hour delay we departed without further incident. It was a heads-up and a very alert ground safety staff that made a very good call. Our takeoff speed at rotation was going to be 164 knots. The debris was certainly going to be expelled upon takeoff and may have caused damage to the aircraft; but it would have been even worse for any other aircraft taking off behind us for foreign object damage on the taxiways and runways. The airport authorities closed Taxiway Golf for engineers to survey and repair the area.