Implementación de metodologías de protección frente a la corrosión para extender la vida útil de estructuras de hormigón armado

Abstract

INTRODUCTION AND OBJECTIVES The service life of a reinforced concrete structure is the time from the mentioned structure starts to being used until it reaches a limit state previously defined [1] that, in chloride-contaminated environments, used to coincide with the depassivation of rebars as consequence of the corrosion processes. The time to depassivation is known as corrosion initiation period and it mainly depends on the difficulty of chlorides to move through concrete and the corrosion resistance of reinforcement. In consequence, the forms to eradicate or delay the corrosion act in/on concrete and rebars. Among the first forms, the employment of cements with supplementary cementitious materials able to bind chlorides, like slag, must be mentioned. Among the second forms, the protection of rebars with some type of coating like galvanized coating [2] or the employment of materials with higher corrosion resistance, like stainless steel, in the fabrication of rebars [3] must be highlighted. However, although it is well known the higher chloride contents that galvanized steel rebars and stainless steel rebars are able to tolerate with respect to conventional rebars [4] and the higher delay in the chloride transport when slag cement are employed, there is a lack of knowledge regarding the quantification that supposes the employment of these materials in the extension of the service life of a reinforced concrete structure. In the case of galvanized steel rebars, there also exists a debate about whether they attain enough bond with concrete [5], due to the high disparity of results in the literature. All of the above, the main objective of the doctoral thesis has been to quantify the extension of service life of reinforced concrete structures when galvanized and stainless steel rebars are employed. To this aim, partial objectives were stablished, depending on the corrosion protection method analyzed: • To define the durability limit state (DLS) that will mark the service life of the reinforced concrete structure • To determine the chloride content that promotes the durability limit state previously defined • To determine the response against the entrance of chlorides into concrete when ordinary Portland cement (OPC) and a ternary binder composed of OPC, blast furnace slag and limestone filler is employed. • With the preceding results and other results coming from literature, to predict the time to be reached the durability limit state for the reinforced concrete configuration analyzed • To determine the life-cycle cost of the different alternatives of reinforced concrete In addition to that, a second objective of the doctoral thesis has been to know the bond behavior of galvanized rebars with concrete and to identify the reasons of the existent controversy. METHODOLOGY To get the objective of clarifying the effect of galvanizing on the bond ability of rebars, it was firstly checked if the surface geometry of rebars accomplished with the requirements to their embedment in concrete. Additionally, the bond-slip curves obtained from pull-out tests and parameters like the stiffness of steel-to-concrete bond and the critical and ultimate bond stress were analyzed. The tests were carried out on concrete specimens fabricated with OPC or ternary binder composed of OPC (64%), blast furnace slag (30%) y limestone filler (6%), more resistant to the chloride entrance and named as SL and embedding carbon or galvanized rebars. The tests were done in specimens exposed to different curing times with the aim to know if this parameter could be determining in the comparison of the bond of carbon and galvanized rebars with concrete. To reach the objective of quantifying the service life extension of reinforced concrete structures as consequence of the employment of galvanized or stainless steel rebars, the first step is to predict the service life of these structures and the structures embedding conventional rebars. The reinforced concrete configurations for which the service life was predicted were the result of combining galvanized or stainless steel rebars with the OPC and SL concrete before mentioned. The galvanized rebars had a coating thickness, in average, of 229 ±37 µm. In relation to the stainless steel rebars, 3 types of duplex stainless steel with similar Cr content (20-23%) but different content in Ni, Mo and Mn, were employed, in particular the grade 1.4462 (Ni/Mo/Mn 4.7/3.4/1.7 %), 1.4362 (Ni/Mo/Mn 4.2/0.1/1.6 %) and 1.4482 (Ni/Mo/Mn 1.8/0.2/4.1 %). For the determination of the end of the service life, the method of the durability limit state (DLS) was employed. The first DLS was associated to the corrosion initiation. In the case of galvanized rebars, a second DLS was proposed, the total destruction of the galvanized coating in a localized area. The thesis includes tests oriented to determine the parameters of concrete and rebars that influence the time to be reached the DLS considered and that, known, allow to determine the service life. Such parameters are the chloride diffusion coefficient and the chloride content that initiates the corrosion on both types of reinforcement, although in the case of galvanized rebars the chloride content that leads to the total destruction of the coating in a localized area was also a parameter studied. To promote the natural transport of chlorides through concretes, unidirectional diffusion tests were carried out and to favor the accelerated transport, migration tests until reaching the steady state were done. The tests were carried out on OPC and SL concrete specimens that were cured during different periods. In the case of the natural penetration, the specimens were exposed to the chloride penetration for different periods. For the detection of the two DLS defined for the galvanized rebars, two types of tests were done: 1) For the first DLS, the interaction of chlorides with the galvanized coating without the presence of the passive layer was considered by adding these ions during the mix (in contents of 0.6, 1.5 and 3 % by weight of binder) and 2) For the second DLS, the interaction of the chlorides penetrating by diffusion into concrete with the galvanized rebars already passivated was considered. The detection of the corrosion onset of the galvanized coating was done from the corrosion potential and the corrosion current density and, in the case of natural penetration, the content of chlorides cumulated at the rebar level was determined. When stainless steel rebars were employed, the DLS was associated to the corrosion onset promoted on the passivated reinforcement by the chlorides penetrating by diffusion into concrete. From the results obtained, predictions of service life were done following the Model Code for service life design prepared by the Fédération International du Béton (FIB) considering three of the exposure classes prescribed in EN 206:2013. Finally, a life cycle cost analysis was done for each alternative of reinforced concrete with the aim to value the economic suitability of the implementation, in reinforced concrete structures, of the corrosion protection methods analyzed in the present thesis. To this aim, a specific volume of reinforced concrete structure from the combination of OPC or SL concrete with carbon, galvanized and stainless steel reinforcement and permanently immersed in sea water was considered. The life cycle cost was calculated, for a period of 100 years, as the sum of the construction cost and the repair cost every time that DLS was reached, and it was presented relativized to the construction cost of a reinforced concrete structure with OPC concrete and conventional rebars. CONCLUSIONS The results of the thesis have revealed, for the materials object of study, that: - The study of the surface geometry of carbon and galvanized rebars, inexistent in the literature until now, has revealed that the galvanizing slightly modifies the surface geometry of the rebars but they accomplish the requirements for a reinforcement to be embed into concrete. - The pull-out tests have allowed to understand the disparity of the results found in the literature and to know the parameters that determine whether the bond of galvanized rebars with concrete is higher or lower with respect that attained by conventional rebars. The results revealed that the bond of galvanized rebars with OPC concrete is lower than that attained by conventional rebars when the curing lasts for 7 days but higher when the curing is extended to 28 days and that is always lower when the concrete with supplementary cementitious materials is employed. In addition to that, it has been demonstrated, and reflected in literature for the first time, that the analysis of the of the bond in the ultimate limit state produces the same relative result between the two types of reinforcement than in the serviceability limit state but higher differences between the bond stresses in absolute value. - It has been confirmed that the employment of the cements with supplementary cementitious materials reduces the porosity of the concrete with respect to the concrete fabricated with OPC and that diminishes the rate of chloride penetration to the half value. It has been also demonstrated that the extension of the curing from 28 to 90 days is not an effective measurement to delay the chloride entrance into concrete, even in the concrete with supplementary cementitious materials. - The corrosion tests on the galvanized rebars allow to understand the disparity of results found in the literature as regard to the corrosion resistance of this type of reinforcement. The DLS has been revealed as one of the factors that contribute to those differences, because while chloride contents between 0.6 and 1.5% by weight of binder initiate the corrosion of the galvanized coating, values around one order of magnitude are needed to discover the steel base at a certain area of the galvanized rebars. - The joint consideration of the chloride transport tests and the corrosion tests of galvanized rebars has demonstrated that, although the corrosion kinetics and the attack is more pronounced when the cement with supplementary cementitious materials is employed, even for less amount of chlorides, the delay produced in the penetration of these ions when this blended cement is employed leads into longer service life. - The study of the service life of concrete structures with galvanized rebars has allowed to discover the importance of the adoption of the DLS as the corrosion initiation of the coating or the base steel, since two very different service lives are obtained depending on the DLS chosen. The life-cycle cost of a concrete structure with galvanized rebars is at least half than that of a structure embedding conventional steel rebars. - The study of corrosion of the stainless steels has revealed that the grade 1.4462 tolerates higher chloride contents than the grade 1.4362 and 1.4482 before the corrosion initiation, revealing the importance of Ni and Mo in the protection against corrosion initiation of the duplex stainless steels. Furthermore, this study, not found in the literature, has allowed to know the influence of the cement with supplementary cementitious materials in the chloride contents before corrosion onset that are able to tolerate the duplex stainless steels analyzed when are embedded in concrete. Other new result has been the demonstration that the passivation corrosion current density of the duplex stainless steels depends on the type of cement and mainly on the content of the alloying elements, being higher for the grades with higher content in Ni and Mo. - The study of the service life of the concrete structures with stainless steel rebars has demonstrated that these structures satisfy service lives higher than 100 years even when the minimum concrete cover thickness required for bond is given to these structures. Furthermore, the life-cycle cost analysis has demonstrated that the higher initial investment of incorporating this corrosion protection method can be justified, since the life-cycle cost is around 25 times lower than that of the structures embedding conventional steel rebars.