## 1 Introduction
Cracks in concrete structures are often classified in two main categories depending on whether they occur before or after hardening of the concrete [14; 15]. Under each of these categories, various types of cracks are associated with different underlying causes, as described in Figure 1. Cracks may be structural (i.e., load-induced) or non-structural. Generally, structural cracks are included in the design as they mobilise the tensile properties of the reinforcing steel, which is adequately placed in the structure to make up for the low tensile strength of the concrete. Non-structural cracks encompass many different types and causes that are related to the nature of the concrete material, its constituents, and its interaction with the environment. A thorough examination is often required to categorize different types of cracks. For instance, plastic settlement cracks and plastic shrinkage cracks, both appearing in the first hours after casting before the concrete hardens, are particularly hard to distinguish. The former are characterized by a crack pattern that follows the pattern of the surface reinforcement, which constitutes the main aspect distinguishing them from the latter [16]. Cracks that form in the early stages may increase in width and size over time, sometimes over several years, due to a combination of different causes. It is crucial to control the cracking of concrete structures since uncontrolled cracking leads, in many cases, to poor function and durability of the structures. The most common concern is that cracks will render the concrete more permeable and facilitate the ingress of moisture, gas, chloride ions, and other chemicals within the concrete, which will in turn increase the risk of corrosion of the reinforcement bars during the service life of the structure. The high pH of the concrete pore solution normally protects the reinforcing steel by forming a naturally passivated surface around it. However, the ingress of chlorides in the concrete may break this passive layer and lead to the initiation of corrosion in the presence of oxygen and water. The passive layer may also be destroyed due to carbonation, when carbon dioxide and moisture penetrate the concrete and lead to a series of reactions with the hydrated cement and the pore solution, which results in a pH drop of the concrete [17; 18]. Corrosion induced by chlorides is mostly a concern for wind turbines that are located offshore or in coastal regions onshore [19]. Corrosion of the reinforcement bars leads to further degradation of the structure due to the expansive nature of the corrosion products, causing concrete spalling and loss of bond [15]. Reinforcement corrosion is one of the main mechanisms of deterioration of concrete structures, and it is considered by the concrete community to be directly related to the environmental exposure condition of the structure, to the number of cracks, and the concrete quality and cover thickness to the reinforcement [15; 20]. Cracks can also impair the water or gas tightness and negatively affect the aesthetics of a structure, but these requirements are normally not applicable to wind turbine foundations, which are commonly almost entirely buried and are not subject to specific tightness requirements. Today, cracks are controlled by reinforcement using either traditional steel bars or fibres. The purpose of crack control is to keep the crack widths below a defined limit. An extensive description of crack control in concrete and the theory of cracking is given in [21; 22]. The width of the cracks--independently of the causes--are commonly limited to between 0.3 and 0.4 mm for reinforced members and 0.2 mm for prestressed members [23]. The aim of this study was to assess defects that emerged in the construction of foundations for wind turbines and analyse their relation to production aspects and design requirements. For this purpose, available field data from the construction and inspection of foundations at an onshore wind farm in Sweden were compiled and critically analysed.