THE EVALUATION OF THE EVOLUTION OF ROAD SURFACE FRICTION: A CASE STUDY IN NAPLES' DISTRICT

Road surface friction, such as all pavement surface characteristics, undergoes to a degradation through time. The prediction of friction progression with time represents a key topic in pavement maintenance since it is important for road managers to know where and when the maintenance intervention is needed in order to restore a satisfactory friction level and, as a results, to guarantee an adequate road safety. For these reasons, it is necessary to quantify how road friction decreases with the increase of time and in which manner it is affected by different factors involved in the degradation phenomenon, among which heavy commercial vehicle traffic flow and spectrum play a critical role. To this purpose, following a brief review of skid resistance degradation models available in literature, a friction deterioration model has been derived from site measurements carried out in a dual carriageway road within the Naples area. Traffic flow and spectrum monitoring, on one hand, and friction (through British Pendulum) and macrotexture (through Sand Patch method) measurements, on the other, have been performed for almost two years on a conventional asphalt concrete wearing course and on an experimental high performance modified asphalt concrete mix. By making use of the differential surface wear occurring in the transversal direction due to the variability of lateral position of vehicular traffic, it has been possible to derive a friction deterioration model for both wearing courses within a reasonably short period of time according to two equivalence damage criteria. Preliminary numerical analysis seems to confirm that a strong relationship exists between friction values and cumulated traffic expressed in terms of equivalent light vehicles for both the bituminous mixes if an equivalence damage criterion based on total tyre footprint area of vehicle is adopted. Furthermore, friction level provided by the highperformance mix appears to be always larger than that provided by the conventional asphalt concrete wearing course.