safetypee,
And I share your general observation that concrete runways have reduced friction characteristics in comparison to asphalt. I was going to say it earlier, but I couldn’t find any data to support it so I crossed it out of my draft post.
Thanks for the link to NASA’s (Yager’s) Factors influencing aircraft ground handling performance. That is clearly one of the foundation documents of wet runway knowledge.
In NASA/Yager’s document, on page 7 and again in Figure 5, he seems to indicate that in his tests, the ratio of wet/dry stopping distance is worse for ungrooved asphalt compared to ungrooved concrete, as “a result of lower surface macrotexture”. I think that this result is specific to the particular runways tested in that programme, and that the real difference in wet/dry stopping distance between concrete and asphalt comes back to the macrotexture provided.
Across the world, concrete is popular at busy airports (meaning lots of rubber deposits), military airports (meaning little maintenance until the next war), and in countries/continents where maintenance is less prevalent (meaning little runway cleaning is done). That may be why the observation can be made that concrete runways have reduced friction characteristics in comparison to asphalt.