## 2 Method  
The construction of foundations at an onshore windfarm in the north of Sweden was used as case study in this work. The wind farm covers a large area of land with a range of different geotechnical conditions that required different types of foundations. This study focuses on the about two dozen reinforced concrete gravity-based foundations that represent part of the foundations built in the project. The foundations were designed for a design service life of 25 years. The design was conducted in accordance with the European Eurocode design standards taking into account the environmental exposure class XC4/XF3. The allowable crack width was chosen as 0.4 mm, which corresponds to the standard requirement for a design service life of 50 years. A freeze-thaw resistant concrete, of strength class C35/45, based on CEM 1 42.5 N SR3 MH/LA Portland cement was used, with a water-to-cement ratio of 0.55. The construction of the foundations was spread over a period of one year in the first half of the 2010s. The gravity-based foundations were designed as circular foundations with radial and circular top reinforcement and a square net of bottom reinforcement (see Figure 2a,b). The shear reinforcement used consists of so-called T-headed bars, which have a hook at one end (top) and a welded plate at the other end (bottom), as shown in Figure 2c. The foundations have a diameter and height of approximately 21 m and 2.3 m, respectively, which required a volume of concrete of about 550 m\({}^{3}\) (see Figure 2c). They were cast on blinding concrete over a compacted filling of more than 500 mm of crushed rock. The top of the foundations is sloping and was cast without a top form. The different stages of the construction of the foundations are illustrated in Figure 3.  
Figure 2: The studied reinforced concrete gravity-based foundation studied: (**a**,**b**) plan views of a quadrant of the foundation showing (**a**) the top reinforcement layout and (**b**) the bottom reinforcement layout; and (**c**) the radial section of the foundation. Following the construction of the foundations, visual inspection was carried out. The first observations of cracks were made at the inspection round at the end of the construction. The cracks observed were located on the surface of the foundations and observed on foundations that were not backfilled. Based on these observations, further inspections were carried out by the general contractor and consulting engineers during the following years. Excavations were conducted to uncover all foundations for inspection and repair purposes. The actions that were undertaken included visual inspection of the foundations, the measurement and mapping of surface cracks, the measurement of the concrete cover, the extraction of samples from the foundations, and the repair of the identified defects. Crack widths were visually measured using crack comparator cards (i.e., graduated transparent plastic cards with lines of various widths), as shown in Figure 4a. The concrete cover was assessed by a sonic surface measurement tool to map the distance between the surface and the top layer of steel reinforcement (see Figure 4b). The concrete cover was measured on average at 170 points spread over the foundation surface (up to 560 points on critical foundations). The measured crack widths together with the position and layout of the cracks, and the concrete cover measurements were reported on drawings of the foundations. Cores were drilled out from a number of foundations, as illustrated in Figure 4c, in order to measure the concrete's in-situ compressive strength and density at different depths. The carbonation, the homogeneity of the concrete and the distribution of the aggregates were also assessed, and the bond between the concrete and the reinforcement bars was visually checked.