Sir George Cayley,
Not quite ad idem with you on the grooving and grip issue.
Surface texture (which some call macrotexture) for normal asphalt varies but an average asphalt sand patch macrotexture is around 0.65 mm. Grooving adds to the texture of the surface, although how much it adds does indeed depend on width, depth and number of grooves per metre as you suggested.
The groove style varies by airport/authority, as shown in the following table, and the resultant
net effect on the texture of the grooved asphalt can be seen.
I think that it adds more than a small amount to the texture on airports which use the larger grooves.
Grooving has its limits – it won't totally cope with standing water due to ruts and birdbaths in the runway (common in worn-out runways). Grooving won't totally cope with deep standing water due to heavy rainstorms and it won't cope with standing water due to all the grooves and texture being filled up with rubber.
However grooving does make a difference to the grip on a wet runway if as the water gets deeper on the runway. The work of Benedetto shows how when the macrotexture increases, the decrease of skid resistance during heavy rainstorms is greatly reduced. His conclusion is important because it underlines the ICAO requirement for both friction and texture depth:
The numerical analysis on the case study of Milan Airport shows some interesting outcomes:
(a) the macrotexture plays a fundamental role especially during the heavy rainstorms: in fact, if the skid resistance in good weather conditions depends substantially on the microtexture, in poor weather conditions the skid resistance depends on the macrotexture, because of only the greater pavement irregularities can penetrate the water film and can assure of the tire-pavement contact;
His Figure illustrates how skid resistance (shown on the Y-axis) reduces as speed (shown on the X-axis) increases. The depth of water is shown as n. For a dry surfacing with no water on (water depth or n = 0mm), there is minimal change. But for a surfacing with water on (the Figure shows depths from 0.1mm to 1.5mm), the skid resistance drops off rapidly with increasing speed. The deeper the water, the more pronounced the drop in skid resistance. This effect continues beyond the depth of 1.5mm. What grooving does is work to offset the effect of water on skid resistance. Even if the depth of water is more than the groove depth, Benedetto found that the grooving still acted to reduce the loss of wet friction at speed.
If the runways at CDG are not grooved, then the French must be providing macrotexture in other ways. The French do pay attention to macrotexture for runways. They get their macrotexture from the choice of surfacing, its grading, and rugosity. While their standard BBA and BBME asphalts have a macrotexture around the 0.6-0.8 mm mark, their BBM, BBTM 0/10, ECF asphalt mixes have macrotexture over 1mm.
And in their airport asphalt guide:
STBA 2003: Guide d’application des norms. Enrobés hydrocarbonés et enduits superficiels pour chaussées aéronautiques. Direction Générale de l'Aviation civile. Bonneuil-sur-Marne, France.
the French say
Le chapitre 7 de l’ITAC indique qu’une macrotexture exprimée en hauteur au sable (NF P 98 216 1) inférieure à 0,5 mm risque de conduire à une glissance excessive; lorsque la valeur dépasse 1 mm, l’adhérence est satisfaisante, pour peu que la microtexture soit conforme
which I poorly translate as being
Chapter 7 of the Instruction Technique sur les Aérodromes Civils (ITAC) indicates that a macrotexture (sand patch method) less than 0.5 mm is likely to lead to excessive slipperiness; when the value exceeds 1 mm, skid resistance is satisfactory if the microtexture also conforms.
Their airport asphalt guide goes on with a lot of detail about macrotexture. I don’t know enough about the exact asphalt wearing course mix on any of the runways at CDG. However the French are more advanced than most with their asphalt surfacings (IMHO including Australia, USA and the UK), and I think that they could well be providing their macrotexture through innovative asphalt engineering rather than grooving.