## 4 Discussion
### Consequences and Measures to Ensure the Compliance of Wind Turbine Foundations  
The casting process used to build the foundations, and especially their top inclined surface, did not appear to have led to non-conformities in terms of the compressive strength, since the results from the tests on the cores indicate that the concrete fulfils the requirements for the strength class C35/45, as specified in the design. It should be noted that standards allow the field-measured in situ strength to be 15% lower than the one measured on standard test specimens during the casting process to achieve the same compressive strength class [25]. This deviation forms part of the partial safety factor for concrete used in design [23]. The likelihood of the occurrence of certain deviations during construction is considered in the design of concrete structures. The possibility of worsened bond conditions is taken into account in EN 1992-1-1 [23] by a reduction factor, \(\eta_{1}\). This factor reduces the bond strength by 30% in cases where poor bond conditions may occur, e.g., in the case of bars placed in the top 300 mm during concreting of elements with a height of more than 600 mm. For wind turbine foundations, this applies to the top layers of bars, which may be affected by plastic settlement cracks and not optimally enveloped by the concrete. Carbonation developed quickly and locally in the cracks since the concrete around the cracks comes into contact with carbon dioxide more easily and rapidly. Differences can occur depending on the crack width but also the moisture condition in the crack. Carbonation has been observed on a single test to penetrate the concrete along a crack and reach a depth corresponding to the cover layer, which is not necessarily critical for corrosion [17]. It is important to ensure that the crack widths are limited according to the requirements of the standard. The decision was made between the different actors of the construction project to seal cracks that were wider than 0.3 mm. This crack width can be compared to the limiting calculated crack width according to the design standards. However, it should be noted that the design crack limitations are intended for load-induced cracks while in the studied project, the observed cracks are production related. The applied environmental exposure class (XC4/XF3) led to the requirement of a maximum water-to-cement ratio of 0.55 [26]. As the design service life of the foundations is 25 years, the limiting design crack width was chosen to be equal to 0.4 mm, which corresponds to the requirement for a design service life of 50 years, according to the Swedish provisions on the application of the European construction standards [27]. Indeed, the shorter design service life covered by the standards is 20 years, for which no design crack width requirements are specified. The purpose of these design requirements is to determine the reinforcement required to withstand the theoretical design load effects while limiting crack widths in the concrete. Furthermore, the design crack width limitations are specified at the reinforcement level while the observed ones are measured at the concrete surface. Besides, concrete has self-healing properties to some extent in the presence of water for cracks that are under a certain width of less than 0.2-0.3 mm [28]. Although the minimum concrete cover was specified on the drawings as 50 mm by the designer, a lower concrete cover would have been sufficient to satisfy the requirements of the design standards. Indeed, the specified value was obtained by rounding up the nominal concrete cover according to the design standards (i.e., 42 mm), which also include an allowance of 10 mm for deviations. It is the minimum concrete cover requirement regarding the bond of the reinforcement that governed this value (equal to the maximum bar diameter of 32 mm), with the requirement regarding durability being lower (equal to 30 mm for the exposure class XC4/XF3 and design life of 50 years, also including the 10 mm allowance for deviations).