I found this on the 'net' which seems to explain it. It is an alternative to grooving and some authorities consider a PFC coated wet runway to be equivalent to bare and dry.
8-3 . Porous friction surface course . Porous friction surface course
consists of an open graded bituminous concrete containing a large
proportion of one-sized coarse aggregate . The large void content
permits water to drain through the layer laterally out to the
shoulders . Porous friction courses are also described as "open graded
mix," "plant mix seal," and'"popcorn mix ." In addition to improving
skid resistance and preventing hydroplaning, porous friction courses
provide the following additional advantages :
- Improved visibility of pavement marking .
- Reduced tire splash and spray .
Some disadvantages include :
- Susceptibility to fuel spills .
- Susceptibility to clogging by mud, blow sand, and rubber .
Regards,
BH.