What are those markings on runway 25L at Frankfurt Airport?
UPDATE:
July 2016 imagery shows the runway as closed, top image is c. 2015.
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1$\begingroup$ Zooming in further makes it look like vehicle tracks, like someone was going back and forth over the runway at 15 meter intervals. Looking further down the runway, it looks as though it is made up of 15 meter wide concrete plates (in lack of a better name). Perhaps the tracks are a sign of some mainteinence of the mortar between these plates? $\endgroup$– 60levelchangeCommented Nov 20, 2016 at 19:43
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3$\begingroup$ I don't know if it's related, but there is a second threshold on runway 25L: 26L. The (deleted?) markings are just before this threshold. See AIP. 26L is equipped with High Approach Landing System: "For operation on THR 26L, special markings and lighting are installed which deviate from the ‘Guidelines for the Markings and Lighting at APTs’, as well as ICAO. For detailed depiction refer to page 10-9H.". $\endgroup$– minsCommented Nov 20, 2016 at 21:42
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1$\begingroup$ @mins Very interesting, never heard of that. From what I've been able to find, the concept was known at HALS/DTOP, and is no longer in use $\endgroup$– 60levelchangeCommented Nov 21, 2016 at 19:15
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$\begingroup$ @mind Indeed. What you found is not from the AIP, but rather an old Jeppesen chart dated 2008, and it even says "Notice: Printed from an expired revision" :) By looking in the current AIP, I can confirm that the markings no longer exist $\endgroup$– 60levelchangeCommented Nov 22, 2016 at 5:22
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Zooming in further makes it look like vehicle tracks, like someone was going back and forth over the runway at 15-meter intervals. Looking further down the runway, it looks as though it is made up of 15-meter wide concrete plates (in lack of a better name). Perhaps the tracks are a sign of some maintenance of the mortar between these plates?
It's possible to have been WIP (work in progress) for the removal of the unique experimental approach system Frankfurt Airport tested. In which they had two separate touchdown zones, with the one farthest down labelled 26L, thereby allowing two glide-slopes—one higher than the other—to reduce the wake turbulence separation for parallel runway operations. That system is no longer in use (obsolete AIP link).
If not, then the runway closure in the second frame would indeed confirm some sort of intermediate maintenance until the runway could be closed for a proper overhaul.
HALS / DTOP was the experimental special landing procedure at Frankfurt Airport (tested from 1999 to 2004) with the aim of increasing the landing capacity on the existing runways.
HALS is no longer in use and all approach procedures are suspended, as this operational method failed to prove itself in practice. At the end of 2008 or beginning of 2009, all the navigation, corresponding markings, and firing systems to runway 26L were removed.
The acronyms HALS / DTOP stand for High Approach Landing System and Dual Threshold Operation.
— de.wikipedia via Google Translate [edited]
Special thanks to J. Hougaard and mins, they basically answered the question.