# Performance Evaluation of Silane in Concrete Bridge Decks Using Transmission X-ray Microscopy
## 2 Materials and Methods
### Diffusion Test  
NCHRP Report 244 [22] is one of the widely used documents for following and testing laboratory silane quality. This study tried to use a direct observation method to evaluate the effectiveness of the silane. The diffusion test method explained in this study is an alternative to the test method represented in NCHRP Report 244. From each of the 14 samples, four cores of 19.05 mm (3/4 in) diameter were extracted: two cores from the finished surface coated with silane and two cores from the side without silane. An overview is shown in Figure 3a. These cores were also inspected to be sure that they did not contain a large aggregate near their surface as this would interfere with the diffusion measurement. If this occurred, then another core was taken. Next, the cores were oven-dried at 50 \({}^{\circ}\)C until less than 0.03% weight change in 24 h was observed. A constant mass was reached in 5 days. After this, the average degree of saturation (DoS) of all the cores was recorded equal to 2.05% \(\pm\) 0.48%. Next, the sides of the cores except the surface were coated with a hydrophobic wax to force the outside fluid to penetrate the unwaxed surface. An overview of this is shown in Figure 3b,c. After waxing, a PVC hexagonal nut was attached to the bottom of the samples. This nut was matched with the hexagonal stage of the X-ray machine to load the samples in a consistent manner. The diffusion test in this study used a potassium iodide (KI) tracer on top of the unwaxed surface of the samples. X-rays were used to take images before and after applying KI on top of the samples to study the one-directional mass transport into the samples for 28 days [3; 5; 47]. In this test, the gravity and moisture movement in the samples were in the same direction. Since TXM can capture materials with high electron density, potassium iodide (KI) was chosen to be used as the penetrating tracer. The iodide in the KI strongly absorbs X-ray waves due to its high electron density. Furthermore, iodide and chloride ions are similar in size (iodide's radius is 206 pm versus chloride's radius of 167 pm) which can provide valuable insights into the chloride diffusion mechanism of cement-based materials [3]. Moreover, another study proved that the concentration profile and diffusion coefficients obtained from the intrusion of iodide can represent the intrusion of chloride into a cement-based system [3]. The concentration of the KI solution was chosen equal to 0.6 mol/L to get enough contrast between the sample and the solution in the radiograph images. This matches previous procedures used to study the diffusion coefficient of cementitious systems [3]. Graphical representations of the absorbed X-rays are known as radiographs. Radiographs taken from the original samples before starting the experiment are known as reference radiographs, and these are compared with radiographs taken from the same samples at times after starting the experiment by applying KI solution to the samples. The concept behind this technique is that, when a solution with a high electron density penetrates a sample, the penetrated depths in the radiographs look darker in comparison to the reference radiograph before KI was placed on the surface. A profile of the concentration of iodine versus the depth of the sample can be produced by using calibration curves, as described in future sections. From the concentration profile the apparent diffusion  
Figure 3: (**a**) Extraction of 25 mm diameter cores from each cylinder; (**b**) application of wax; (**c**) a real sample after applying a hydrophobic wax around the sample. coefficient, surface concentration, and depth of penetration can be calculated. The apparent diffusion coefficient combines the fluid transport mechanisms of diffusion, absorption, convection, and chemical binding in one term. A Skyscan 1172 \(\upmu\)CT scanner with the settings summarized in Table 1 was used to conduct the experiments and capture radiographs at the desired intervals. More details on the TXM method are provided in Appendix B.