## 3 Infrastructure Inspection and Monitoring Carried Out
### Pathologies Detected in the Visual Inspection Initial Phase  
The evaluation of the conservation status of a structure and the early detection of damage requires the realization of programmed and systematized inspections, which make it possible to know the structure's performance level at a specific time. Therefore, it is essential to define a conservation strategy that, in addition to other aspects, establishes the inspection levels and frequency with which they are carried out. With this purpose, different inspection levels can be defined according to the personnel that carry them out, the material resources used and the frequency with which they are executed. The aim of this article is not to define the management of the structure's conservation. However, is important to highlight the importance of carrying out scheduled and systematized inspections, considering that they are a fundamental aspect of the early detection of pathologies and the starting point for the subsequent phases of study and evaluation. In the case of the structure under study, the first alarm about the conservation status was detected within these programmed inspection works. During the main inspection  
Figure 1: Structure location in the road junction Nudo de Colmenar. Figure 2: Cross-section of Nudo de Colmenar Structure. (visual inspection by expert structural technicians, without special means of access and carried out every 4 years) excessive deflections in some spans and abundant cracking in the deck were noticed. In this way, the detection of the above damages within the programmed inspection works constitutes the first alarm about the state of conservation and gives rise to the subsequent evaluation phase. In this evaluation phase, a detailed visual inspection was carried out by engineers specializing in structural pathology, and a campaign of auscultation and testing was performed. The goal of these tasks was to determine the real scope of the pathologies detected and to assess their implications in the structural safety rate. Thanks to the previously cited scheduled and systematized inspections, the identified damages revealed the existence of a serious pathological process were, fundamentally, the presence of a concrete degradation process by an aggregate-alkali reaction. It manifested itself in a series of particular damages, including the cut-off in the transverse reinforcing bars of the deck upper face and injection defects of the post-tensioning sheaths. The process of concrete degradation by an aggregate-alkali reaction was manifested in the following observable damage:  
Abundant longitudinal cracking on the bottom surface of the section, oriented parallel to the main prestressing compressions, can be seen in Figure 3.  
Vertical marked deflections, especially in three spans: span # 5 (longest span over the M-40) and spans # 8 and # 11. The deformation of these spans pointed to a loss of concrete stiffness because of aggregate-alkali degradation. This deformation can be observed in Figure 4.  
Longitudinal cracks in the deck's upper face coincide with the direction of the prestressing. These cracks, detailed in Figure 5, were observed after milling the pavement. Figure 4: Marked deflections in spans # 5 and # 8 of Nudo de Colmenar structure. Figure 3: Longitudinal cracking on the bottom surface of the deck. Moreover, in addition to the cracks observed on the deck's upper face, cores were extracted in the alignment of the cracks with internal fracturing of the concrete mass. In the visual examination of the cores extracted in the alignment of the fissures, the presence of a white halo around some aggregates was observed, as can be seen in Figure 6. It was compatible with the formation of silica gel of expansive nature, as a result of the aggregate-alkali reaction.