MICROBIAL INDUCED REINFORCED CONCRETE DETERIORATION AND INNOVATIVE PROTECTION TECHNIQUES

Reinforced concrete deterioration due to steel reinforcing bars corrosion is recognized as a serious problem, affecting the durability of various types of civil structures. Many efforts have been dedicated to studying the steel corrosion process induced by chloride penetration, being by far the most frequent cause for reduced durability of reinforced concrete structures. Much less attention has been focused on the microbial degradation of reinforced concrete structures occurring as a consequence of the presence of bacteria involved in the sulfur cycle (Sulfate Reducing Bacteria and Sulphur Oxidizing Bacteria) whose metabolic products react with the cementitious matrix yielding to fast deterioration processes. According to literature, in such field, most of the attentions were focused on concrete deterioration, while if and how bacteria metabolites eventually affect the behaviour of the steel reinforcing bars appears to be still an open issue. The research project aims at filling up the knowledge gap above mentioned, that is: assessing how risky and aggressive a bacterial environment is, especially focusing on corrosion of steel reinforcing bars. On this purpose, abiotic solutions, simulating bacteria metabolic products were used for all tests. While a second part of the work was devoted to the development of a "smart" inhibitive system able to protect steel rebars from the corrosion induced by biogenic acidity that slowly neutralize cement alkalinity. The prolonged protective effect was meant to be achieved by encapsulating the active substances into pH sensitive microbeads, that would release the inhibitor only when reached by the acidification front, thus preventing a premature leaching of the inhibitor. As a common practice in concrete science, investigations were started from the easier system of the steel rebars directly immersed into simulating solutions. Thus, a stable passive layer was growth on the steel surfaces by means of a three days immersion into a saturated Ca(OH)2 solution. Afterwards, samples were moved into sulfides containing alkaline solutions or diluted sulfuric acid, aiming to simulate the metabolic products of SRB and SOB respectively. The acidic conditioning, into diluted sulfuric acid solutions, easily dissolved the passive layer and caused a fast onset of generalized corrosion, whose extension was found to be dependent on the acid concentration as indicated by both gravimetric and electrochemical tests. Conversely, sulfides were proven to induced localized corrosion, their interactions with the steel surface resulted to be significantly affected by the pH of the environment. Actually, for a prevailing of hydroxyl ions over the sulfides ones the steel surface remained protected by the oxide layer, while at lower pH steel-sulfides interaction were promoted, yielding to a porous, conductive, and thus non protective layer of iron sulfides. Once clarified the corrosion mechanisms for steel directly immersed in simulating solutions, reinforced mortar samples were cast, cured, carbonated and then conditioned into two different model media: a sulfides containing solution and diluted sulfuric acid solution. The combination of several electrochemical techniques such as OCP, LPR and EIS pointed out the active behavior of the embedded steel rebars. Visual inspection performed at the end of a 500 days conditioning period confirmed that the acidic conditioning yielded to more severe damages. On the basis of such findings, a "smart" corrosion inhibitor was developed by combining together calcium phosphate and methylene blue dye, being both active compounds in terms of protection of carbon steel from sulfuric acid corrosion. The resulting product was an organic/inorganic hybrid where the organic molecules were entrapped into a porous inorganic matrix, granting their release as a function of a pH drop leading to the dissolution of the latter. Anodic and cathodic potentiodynamic polarizations performed in sulfuric acid solution and in presence of the hybrid inhibitor confirmed that both the anodic and the cathodic processes were hindered as a consequence of the combined effect of methylene blue dye and phosphate ions. The effectiveness of the slow release mechanism was evaluated by means of LPR and EIS monitoring by comparing the responses of the hybrid, its two components singularly used and the free corroding system as a control case. The effect of the organic inhibitor was that of significantly increasing the polarization resistance. Such effect was rapidly lost in the case of methylene blue dye alone, while its slow release, together with the synergic effect of phosphate ions contributed to the prolongation of the protective effect. Finally the interactions of the hybrid and its components with the cementitious matrix were investigated by means of isothermal calorimetry and standard compressive strength tests. The response of both the hydration rate and the strength evolution were found to be independent from the chemical admixtures. Once excluded the onset of negative side effects, reinforced mortar samples containing different amount of the unloaded HAP and the hybrid MBD-HAP were cast and exposed to a sulfuric acid environment for about four months. However such a conditioning period proved not to be long enough to induce the corrosion of the steel reinforcing bars.