## 2 Basic Threats to RC Structure Durability--Short Literature Review  
Reinforced concrete is a durable structural material and the steel reinforcement inside it is safe from corrosion. The above is true under the condition that the concrete and the reinforcing steel is well-protected against corrosion beforehand. In the case of relatively new concrete with correct composition and structure (high pH), the reinforcing steel surface is passivated and effectively protected against corrosion [2]. However, concrete surface pH decreases in time, which makes the concrete gradually lose its protective properties. Simultaneously, the threat of concrete contamination with aggressive chemical agents increases as they penetrate the concrete near the reinforcement. Concrete contamination is both aggressive ions (chloride, sulfate, nitrogen) from the environment, as well as carbon dioxide causing carbonation. If a structure is continuously exposed to aggressive chemical factors, this process becomes progressive. Corrosion processes can be significantly delayed by using various types of concrete surface protection against the penetration of aggressive factors [3; 4; 5] or the use of low absorption concrete [6]. Very simplified divisions group factors threatening RC structure durability into mechanical, atmospheric, biological, and chemical ones [7]. In practice, it is sometimes hard to distinguish between separate risk factors because their effects are often strongly interrelated (e.g., chemical aggression causes steel corrosion; then, swelling corrosion products burst concrete, which is a mechanical action). The selected examples presented below are aimed at depicting their influence on the possibility of a durable and effective structure repair. Purely mechanical damage often results from impact or other action which are incidental [1]. In such cases, the usefulness of the damaged element for repair must be determined by the size of the damage and the cost-effectiveness of the repair in relation to the costs of replacing a part of the structure. Another type of mechanical damage is concrete surface abrasion by a stream of liquid or gas--this does not influence structure suitability for an overhaul, either. A specific group is mechanical damage resulting from repeated repetitive fatigue loads. Reinforced concrete crane beams are a good example here. In this case, the occurrence of mechanical damage usually prevents effective repairs, because the structure of concrete and reinforcing steel is damaged. Most cases of atmospheric damage have the form of frost corrosion. Therefore, those do not preclude durable repairs, even though they do result in cover thinning, thus facilitating chemical aggression. Many biological factors also come down to mechanical action, for example when plant roots burst concrete. Sometimes the organisms living on the concrete surface are chemically aggressive as well. For example, lichens can cause chemical decomposition of concrete and pollute it with sulfur compounds. The activity of bacteria is equally detrimental as they can also pollute concrete with sulfur compounds [8; 9; 10]. Still, chemical agents form the most significant group of impact factors causing corrosion of concrete and reinforcement. Some of them are present in the atmosphere--an example is carbon dioxide which causes carbonation in concrete [7; 11]. Other typical corrosion factors are sulfur compounds, which form gypsum in concrete (through the reaction of sulfate ions with portlandite) and, in the subsequent phase, ettringite which can be, or not, expansive. Sulfur compounds are present, among other things, in the so-called industrial atmosphere and in a part of groundwater [7; 12]. The abovementioned biological corrosion is also a source of concrete contamination with sulfur compounds. Publication [13] contains an interesting overview covering the territory of Germany.