# Degradation Prediction Model for Friction of Road Pavements with Natural Aggregates and Steel Slags
## 1 Introduction  
The by-product of steel production waste, named slag, whose chemical properties are similar to effusive rocks, is transformed into coarse aggregates and they are used for bituminous surfacing as a valuable replacement for natural aggregates, which are becoming increasingly rare. Moreover, the coarseness made by the wearing this material maintains good values of micro-texture and, therefore, friction resistance, even after prolonged exposure to the action of the traffic loads. Several experimental studies [1] have suggested that the number of wet-pavement skid road accidents increases as skid resistance decreases and skid resistance depends on the road pavement surface characteristics: macrotexture and microtexture. Macrotexture refers to the irregularities in the road surface associated with voids between aggregate particles, which relate to the size, shape, and distribution of coarse aggregates used in pavement, as well as the construction method of the surface layer. The magnitude of macrotexture can increase or decrease during pavement service life [2; 3; 4]. Microtexture refers to roughness in the surfaces of the aggregate and is a function of aggregate mineralogy. The size of microtexture also varies over time during the pavement service life, depending on the initial roughness of the aggregate and the ability of the aggregate to maintain roughness against the polishing action of traffic and weather [5] whereas microtexture always has a decreasing trend over time. Past studies have been dedicated to the improvement of methods aimed at predicting deterioration before pavements are constructed, in order to achieve appropriate design life and to enable advanced planning of maintenance interventions [6]. The PSV is the most widespread laboratory test, used worldwide to assess the skid resistance performance of surface aggregates; this test has, however, some limitations that affect its ability to predict the long-term performance of road surfaces. In particular, the standard PSV test uses a set polishing time of six hours, hence there is a possibility that the aggregates, in this case greywacke and melter slag, have not reached the equilibrium skid resistance by then [7]. The first research examining the effect of traffic on pavement skid resistance was carried out by TRRL report LR504 [8]. The analysis of traffic volumes and skid resistance measurements, made in the UK road network, allowed us to identify correlations between the side-force coefficient SFC, and the independent variables in traffic flow in commercial vehicles per day, or the traffic flow in total vehicles per day, and the polished stone value of some aggregates, calcined bauxite, gristtone and granite. [PERSON] and [PERSON] [9] developed a deterioration model of microtexture with cumulative polishing over time using a PSV machine for five aggregates: carbonate rock, gneiss, basalt, argillite and limestone. The laboratory-measured microtexture decay rate, as a function of the polishing hours, tends to an asymptotic value. This asymptote is referred to as'minimum polish value' (PV) associated with a specific aggregate. The deterioration of microtexture is determined by the laboratory BPN after \"t\" h of polishing. However, in the model there is no correlation with traffic volume. In the TRL report 322, [PERSON] and [PERSON] [10] subdivided the measure sites according to the investigatory level and correlated their skid resistance (mean summer SCRIM coefficient--MSSC), with PSV and the traffic level (commercial vehicles per day, CVD). Only three types of aggregates were analyzed: basalt, gritstone and porphyry. [PERSON] and [PERSON] [11] correlated the on-site SN (skid number) and the BPN10 measured in the laboratory, after 10 h of polishing, with a PSV machine for two aggregates: steel slag and dolomite. In this note there is no reference to traffic. [PERSON], [PERSON] and [PERSON] [12] identified a decay curve of indicators of friction as a function of cumulative vehicle passages for a mixture of steel slags and limestone.