# Degradation Prediction Model for Friction of Road Pavements with Natural Aggregates and Steel Slags
## 2 Materials and Methods  
The intention of the experimentation was to verify the deterioration of the skid resistance of a road pavement in the province of Trieste, Italy, whose wearing course was made by a mixture of aggregates that included steel slags and limestones. The in-situ skid resistance in previous tests had shown that the surface wear of aggregates, due to the traffic polishing action, is not linear. The skid resistance, as measured by the skid tester, does not proportionally decrease with an increase in traffic levels. Therefore, the decision was made to carry out specific experiments, using an accelerated polishing machine, in an attempt to confirm these observations in the laboratory and, possibly, to find the correlation between in-situ measurements and traffic levels. In the first part of this study, the following types of aggregates were investigated (Figure 1): slag, basalt, diabase, limestone 1 (silica- limestone), limestone 2, PSV control stone and friction tester reference stone (Criggion stone), in addition to the slag and limestone mixture at 50%, used on the wearing coarse on the investigated sites. Table 1 summarizes the main chemical composition of materials (slag and limestone) used to mix the design of wearing coarse. The laboratory study included several steps:  
* Determination of the polished stone value (PSV) according to the standard UNI EN 1097-8 [15];
* Monitoring the same samples through an extended period of polishing;
* Determination of the skid resistance of the specimens, not only at the end of the polishing process, but also at fixed time intervals, to compare the polishing behavior in the various samples. * Careful investigation of the first phase of the 'polished stone value' (PSV) test or initial conditioning. To determination the PSV, according to UNI EN 1097-8, an accelerate polishing machine was used (Figure 2). The samples for the test (Figure 3) were therefore prepared with all the materials being analyzed and with the reference stone, according to the standard. The specimens have been polished through the procedure of the UNI EN 1097-8. The first three hours of testing correspond to the conditioning phase, with the main task of cleaning the surface of the specimen from any resin residues used for packaging. The next three hours of testing concern the polishing phase, as required by the standard, and, finally, the test is extended for a further three hours, to better understand long-term behavior, not required by the standard. Skid resistance was evaluated through the measurement of the British portable number (BPN) during all phases with the British pendulum skid tester (Figure 4). During the conditioning phase (the first 3 h) the values of BPN were monitored at 10, 20, 30, 60, 120 and 180 min. During standard polishing phase (3 h after the conditioning phase) the BPN was monitored at 15, 30, 45, 60, 120 and 180 min, and the skid resistance of the specimens was measured and compared with the results from specimens made with a 'control stone' to calculate the PSV. Polishing time was further 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. Figure 4: British pendulum skid tester. Figure 3: Specim example. As outlined in the introduction, the purpose of this work was to investigate the behavior of a particular kind of asphalt concrete, influenced by traffic polishing action. In order to test the behavior under field conditions, the aggregates of this asphalt concrete were subjected to a laboratory analysis, using a polishing device, and, subsequently, results were compared with on-site measurements. All the selected sites presented suitable features to preserve homogeneity among field measurements. All the sites were part of the road network of the Province of Trieste; they were located in the road tangent and belong to two-lane rural roads.