Ok, after extensive research (5 minutes really), I have reached a solution.
Make sure all your runway textures begin with the same few letters.....i.e. runw.
Then in your config.cpp, in the CfgSurfaces section, add the following:
class RunwayRes
{
   access=2;
   files="runw*";
   rough=0.001000;
   dust=0.006000;
   soundEnviron="road";
};
Remember 'runw' is only an example, change it for whatever you decide upon.
P.S. I have Resistance and I strongly suspect 'RunwayRes' relates to that, so, if this doesn't work for those who don't have Resistance, try changing 'RunwayRes' for 'Runway'
Planck