## 4 Results and Evaluation  
The detailed visual inspection carried out in conjunction with the research allowed for detecting and ratifying the existence of serious damages in the structure, which were concomitant with other execution defects. In this sense, the most relevant damage detected in the structure means a serious problem in post-stressed concrete slabs due to their implications on structural safety, are the following:  
* Concrete degradation process due to aggregate-alkali reaction, chloride contamination because of the use of fluxing salts and lack of waterproofing of the deck;
* Systematic breakage of the transverse reinforcement due to a corrosion-fatigue process;
* Damage to the prestressing strands with a stress corrosion phenomenon and, consequently, the breakage of some wires. The above damages mean a significant reduction of the safety conditions of the structure. However, although the structure under study had other minor defects, the previous ones have been highlighted because of the relation between them and the extraordinary and unexpected phenomenon of brittle fracture in transverse steel bars, which is described below. Thus, the deterioration process of the structure began with a concrete sensitive to the aggregate-alkali reaction, that is, concrete with potentially reactive aggregates (reactive silica) that, in the presence of water and the alkalis from the cement paste, gave rise to the formation of an expansive gel that produces internal stresses in the concrete mass. In this  
Figure 11: Lack of connection between venting pipe and prestressing sheath (Colmenar Bridge). Figure 10: On the left, corroded cords and broken wires; on the right, unusual grout consistency with foam formation. sense, the lack of a waterproof treatment played a determining role in the activation of the aggregate-alkali reaction, since for this to occur it is necessary to provide water (usually from an external source); therefore, it is possible to slow down, or even stop this reaction by minimizing the entry of water into the structure, meaning with appropriate waterproofing. The internal stresses that occur in the concrete as a result of the aggregate-alkali reaction, caused a high number of cracks, that in a massive element or idealized model would form a homogeneous and multidirectional cracking network, but in resistant elements such as bridge decks, the orientation of these cracks is conditioned by the geometry of the element and the orientation of the stresses. In the case of the structure presented in this article, cracks showed a parallel direction to the compression isostatics lines. The longitudinal cracks observed with large crack openings (over normative limits) together with the lack of an adequate waterproof treatment cause the entry of chlorides from the de-icing salts used during winter into the structure, easing the corrosion of the passive bars and post-tensioning strands. The bars of the transverse reinforcement showed full-section ruptures with a brittle appearance. In this sense, although the breakage of steel elements by a stress corrosion process is certainly a common phenomenon, in the case of the Colmenar Bridge, these breaks are completely different from the usual forms of breakage due to chloride contamination, which appears as local corrosion that progresses inwards the bar in an inverted cone form. In the structure covered by the paper, the bars showed a macroscopic appearance of brittle fracture that pointed to the presence of mechanical actions concomitant with chloride attack. Although the breaks detected in the bars are still being analyzed using tests in specialized laboratories, the most likely hypothesis is that it is a corrosion-fatigue phenomenon, in which the steel bars breakage was caused by the simultaneous action of cyclic stress and the presence of an aggressive medium that caused localized corrosion of the steel.