# Application of Mastic Asphalt Waterproofing Layer in High-Speed Railway Track in Cold Regions
## 4 Field Validation  
### Investigation of Surface Condition  
Two field investigations were carried out since the completion of the test section, in November of 2015 and August of 2017, respectively. Field investigations aimed to evaluate the serviceability of MAWL. Thermal cracking and construction joint failure due to weak bonding are the main distresses resulting in poor waterproofing performance of the waterproofing layer, therefore, cracking and construction joint failure of the test section were mainly investigated in comparison with the conventional concrete waterproofing layer. Due to security reasons, the field investigations were carried out at midnight, during which no trains were scheduled on the high-speed railway line. All the visible cracks of MAWL within the 200 m test section were measured and recorded, including its width and length. The number of construction joint failure spots was also recorded. At the same time, a field investigation was also conducted on an adjacent 200-m normal section.  
The investigation results show that lateral thermal cracking is the major distresses of both mastic asphalt and concrete waterproofing layers. In the meantime, cracks are more frequently observed on the shoulder than the middle area between track slabs. The majority of the cracks in mastic asphalt layer are short cracks developed from the outside edge of the shoulder, and they usually occur at the position above the construction joints of the concrete side slope (Figure 10). The expansion of construction joints, which is caused by the contraction of the concrete side slope, results in cracking of the waterproofing layer, while the cracks in the concrete waterproofing layer are mostly long cracks that cut through the whole cross section.  
Figure 10: Typical cracking of (**a**) mastic asphalt waterproofing layer; and (**b**) concrete waterproofing layer.  
Figure 9: The layout of water content sensor in test section and normal section (cross Section 1–3)The features of cracks measured in two field investigations are shown in Figure 11. Details, including length, width, and number of cracks within the mastic asphalt waterproofing layer, are listed. Cracks in the 200 m of normal concrete waterproofing layer are shown in the figure as well. In the field investigation of November 2015, the majority of cracks in the test section were shorter than 60 cm, and their width varied from 0.5 mm to 2.0 mm, while, for the normal section, the total number of recorded cracks within 200 m was much higher than that of the test section, and half of the cracks cut through the whole cross section of waterproofing layer (250 cm long). The results of the field investigation in August 2017 indicate that the distribution of cracks in the test section is roughly the same as that of 2015, some of the cracks even turn out to be narrower and shorter, benefitting from the self-healing property of asphalt mixture. However, the number of cracks in the 200-m normal section increased sharply from November 2015 to August 2017, and all the cracks recorded in August 2017 cut through the cross section of waterproofing layer. Moreover, after two years of operation, most part of the construction joint between MAWL and concrete base maintained in good condition (Figure 12a), only several local cracks resulted by debonding were found, as shown in Figure 12b.