# Durability and Safety Performance of Pavements with Added Photocatalysts
## 2 Materials and Methods
### Methods
#### 2.2.2 Freeze-Thaw Resistance  
In order to evaluate the freeze-thaw resistance, a modified version of the test described in CEN/TR 15177:2006 was applied. In this method, specimens are exposed to freeze-thaw cycles with demineralised water as the freezing medium. The modifications introduced relative to the standard procedure primarily affect specimen size, casting and conditioning. The tests were conducted on approximately 15 \(\times\) 8 cm cores removed from the outdoor exposed slab materials. All but the test surfaces of the specimens were sheathed in rubber secured to the material with silicone. The top edge of the rubber sheath protruded 20 \(\pm\) 1 mm over the test surface. The sheathed samples were pre-saturated for 3 days. The test surface of the paving tiles was covered with a mean 3 mm deep layer of liquid protected from evaporation with a thick polyethylene film. In the asphalt samples, as the liquid water seeped into the specimen rather than remaining on the surface, a soaked sponge positioned on the surface of the cores was used as the freezing medium. The specimens were insulated with white cork on all but the test surfaces (Figure 1a).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Substrate** & **Commercial Photocatalytic Material** & **Initial RNOx (\%)** & **Label** \\ \hline \multirow{4}{*}{Asphalt (A)} & - & - & Ref-A \\  & Slurry 1 & 26.91 & S1-A \\  & Slurry 2 & 7.22 & S2-A \\  & Emulsion 1 & 2.78 & E1-A \\  & Emulsion 2 & 6.36 & E2-A \\  & Emulsion 3 & 13.63 & E3-A \\ \hline \multirow{4}{*}{Paving tile (T)} & - & - & Ref-T \\  & Emulsion 1 & 10.84 & E1-T \\  & Emulsion 2 & 18.97 & E2-T \\  & Emulsion 3 & 29.71 & E3-T \\ \hline Untreated paving tile 1 & - & - & Ref-T1 \\ \cline{1-1}  & Incorporated in tile & 4.6 & T1 \\ \hline Untreated paving tile 2 & - & - & Ref-T2 \\ \cline{1-1}  & Paying tile 2 & Incorporated in tile & 22.93 & T2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Material description and labels.  
After 3 days of preconditioning prior to freezing, water was removed from the specimen surfaces and the first ultrasonic reading, defined as the reference value, was taken. Since some of the samples were so narrow that the ultrasonic sensors protruded beyond the sides of the specimen, readings were taken with the sensors spaced at 4 cm on the active surface. The specimens were subsequently exposed to successive freeze-thaw cycles. During thawing, air temperatures of up to 35 \({}^{\circ}\)C were reached, for such values are readily reached in the air in Madrid in summer. The temperature of the freezing medium at the centre of the test surface was continuously recorded on one specimen in each freezing chamber. The data for three cycles are graphed in Figure 1b. The excess freezing medium was poured off and the scaled material collected after 7, 14, 28, 42 and 56 cycles. The ultrasonic pulse transmission times (UPTT) were then measured, the scaled material was weighed, and new freezing medium was poured onto the test surface, or the sponge was replaced, before returning the samples to the freezing chamber. The results were assessed against RILEM TC 176-IDC [52], calculating relative UPTT, \(\gamma\), as defined in Equation (1):  
\[\gamma=\left(\frac{t_{n}}{t_{0}}\right)\times 100 \tag{1}\]  
where \(t_{n}\) is the transmission time measured after \(i\) freeze-thaw cycles (\(\upmu\)s) and \(t_{0}\) the initial transmission time (\(\upmu\)s).  
The RILEM classification of concrete by relative UPTT is listed in Table 2. As explained in the aforementioned publication, the damage levels given in the table should not be mistaken for acceptance criteria.