The M25 in Surrey between Reigate and Chertsey is concrete, I'm not sure about the other sections as I don't often go beyond Heathrow or the A21 junction. I think that section isn't the oldest or most recent part of the M25 (early 1980s?), so for some reason they went back to tarmac for the remaining sections. It is really noisy though to say the least!
1971- 72, I worked for Errut Products, here, near Chesterfield. They were a "London" firm, who had relocated here.
Harry, the foreman was from Chertsey. And, dearly wanted to get back there.
They were developing, the "Plastic Groover". Part of the roadlaying train, which spans the carriageway.
This machine, tamped the almost set, concrete surface, with 1/4" square grooves, for water run off.
They had already discovered that equi spaced grooves were too tyre noisy.
(I remember driving a section of the M1, around J15 (?), which was frightening! And, I knew what caused the unexpected racket!).
The tamping plate covered around 4 feet per pass. The random spaced grooves, on the M18, produce a relatively pleasant, repeating rhythm......
