## 3 Chemical Tests
### Permissible Values of Tested Parameters  
The recommendations contained in the European standards and the model standard, _fib_-Model Code 2010, concerning concrete durability in relation to corrosion factors are described in detail in [21] and other publications. In general, the design of an overhaul or modernization of an RC structure has to be based on the recommendations contained in the standards (in the scope of the permissible content of harmful ions) and utilize generally accepted, examination-based pH values.  
The conducted chemical examinations focused on three basic parameters: The content of chloride ions (CI\({}^{-}\)) and of sulfate ions (SO\({}_{4}\)\({}^{2-}\)), as well as the concrete reaction connected to the carbonation process.  
Under PN-EN 206+A1. Concrete--Specification, performance, production and conformity [36], the maximal content of chloride ions (CI\({}^{-}\)) in concrete in relation to the cement mass (including type II additives) is: 0.1% or 0.2% in structures with prestressing steel reinforcement; 0.2% or 0.4% in structures with ordinary steel reinforcement; 1.0% in structures without reinforcement. Such strict requirements (as well as their differentiation depending on the presence and type of reinforcement)  
Figure 1: The colors used to determine the pH value of concrete and its carbonation level.  
stem from the fact that chloride ions do not only cause concrete corrosion, but also--or actually first and foremost--corrosion of steel reinforcement and other elements [7; 14]. Determination of chloride content in relation to cement is convenient because it allows obtaining reproducible results and presents the proportions between the aggressive action of chlorides and the inhibiting effect of cement [37]. At the same time, the literature contains a number of author's concepts concerning critical chloride content as well as chloride content examination methods [38; 39; 40]; unfortunately, some of them lead to certain interpretation contradictions, thus hindering assessment of results. It should be highlighted that under PN-EN 197-1:2012 standard [41] chloride content in cement as the construction material must not exceed 0.1%.  
The maximal content of sulfate ions (SO\({}_{4}\)\({}^{2-}\)) is 0.5% of the concrete mass. Given the cement content of 300 kg/m\({}^{3}\) of concrete mixture, this means approx. 3% in relation to binder mass. This value is relatively high, but one must be aware that a certain amount of sulfur compounds is already contained in cement and their excessive amount that penetrates the material comes from the surroundings of the structure.  
The concrete reaction is not regulated by provisions of standards, but numerous tests conducted allow to state certain universal recommendations. In the first phase of material existence, the reaction is strongly basic--a pH of more than 12. The reaction of carbon dioxide with calcium hydroxide creates calcium carbonate, which initially has a good influence on the structure because it seals the structure surface [11]. At the next stage, calcium carbonate is transformed into soluble bicarbonate ions which are then washed out from the structure. This process lowers concrete reaction and deteriorates concrete structure. It is a progressive phenomenon, both in terms of concrete reaction lowering in a given layer and the depth at which it takes place. If concrete pH is lowered to approx. 11.8, the passivation layer on the reinforcement is damaged for the first time, which initiates corrosion processes. Further pH decrease results in the complete destruction of the passivation layer and a risk of serious corrosive damage to the reinforcement. Long-lasting carbonation may lower concrete pH even to 8.3 [42], causing the anti-corrosive properties of the cover to disappear completely.