# Degradation Prediction Model for Friction of Road Pavements with Natural Aggregates and Steel Slags
## 3 Results and Discussion  
The values reported in Table 4 are the rounded averages of the specimens for each aggregate and mixture. The conditioning phase in the polishing test showed similar trends with all types of aggregates. An initial increment of the values of BPN was followed by a subsequent slight decrement. The results of the conditioning phase in the polishing test are shown in Figure 9. The BPN reported are the averages of the values of four specimens for each aggregate using the normal PSV scale for the small slider, without any correction. The trend of the values is in accordance with what normally occurs in these tests, with an initial increase, due to the removal of the residues of the resin used for the realization of the samples, followed by a decay of the values due to the initial wearing. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Aggregates**} & \multirow{2}{*}{**PSV**} & \multicolumn{6}{c}{**BPN at Given Times (min.), 180 min. is after Conditioning**} \\ \cline{3-10}  & & **180 (min.)** & **195** & **210** & **225** & **240** & **300** & **360** & **540** \\ \hline Slag & 54 & 63 & 57 & 55 & 54 & 52 & 51 & 51 & 50 \\ Silica limestone & 37 & 53 & 49 & 43 & 42 & 40 & 37 & 36 & 34 \\ Limestone & 34 & 51 & 42 & 39 & 38 & 37 & 35 & 32 & 30 \\ Basalt & 53 & 65 & 61 & 57 & 55 & 53 & 51 & 49 & 45 \\ Diabase & 57 & 64 & 58 & 55 & 54 & 54 & 54 & 52 & 49 \\ Criggion stone & 60 & 67 & 63 & 59 & 58 & 57 & 56 & 53 & 53 \\ Mixture & 47 & 59 & 54 & 51 & 49 & 46 & 44 & 45 & 43 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Results of the laboratory test. Figure 8: Plan of in-situ measures; “S” and “C” are shoulder and lane width. At the beginning of the polishing phase, the highest BPN values were 67, 65, 63 and 63, respectively, for Criggion stone, basalt, diabase and slag. Limestone samples had the lowest values--53. BNP values of the control stone after the polishing activity, ranging from 47 to 50, were used to calculate the PSV. According to the UNI EN 13036-4, the PSV was determined using the following equation:  
\[\text{PSV}=\text{S}+52.5-\text{D}, \tag{1}\]  
where S is the mean value of skid resistance for the aggregate test specimens and D is the mean value of skid resistance for the PSV control stone specimens. The following remarks are derived from the laboratory results shown in Table 4.  
The polishing tests showed that, overall, skid resistance rapidly decreased with increased traffic conditions during initial test stages; however, as the test time was extended, the polishing slowed down or did not show observable changes. With all the aggregates, most polishing effects occurred within the first fifteen minutes of treatment and later continued at a progressively decreasing rate. The slag lost 17.5% of the BPN value in the first hour of polishing, while samples with limestone showed a reduction of BPN values of the order of 26-31%. The slag showed the smallest loss of BPN value throughout the entire polishing process, about 21%, against 40-45% of limestone, 33 of the basalt and 23% diabase. The decreasing trend in BPN values during a three-hour polishing period is confirmed by previous observations in the literature, namely, that the behavior of the mixture is directly proportional to the quantity of each material. The BPN values of the samples, consisting of a 50% mixture between the two aggregates, are roughly the average of the measurements obtained for the two materials, analyzed separately. Percentage reductions of 22% were observed for the samples made with slag and limestone. Polishing time was extended to 3 h in order to understand and better describe the polishing mechanism. Moreover, the other aggregates were subjected to the same test conditions. The analysis of BPN data versus polishing time (t) allowed us to empirically derive a regression model of the following form:  
\[\text{BPN}(\text{t})=\left(\frac{\text{A}}{\text{t}}\right)^{4}+\text{B-PSV}, \tag{2}\]  
Figure 9: Conditioning phase in the polished stone value (PSV) tests.