Numerical simulation of the probability of corrosion initiation of RC elements made of reinforcing steel with improved corrosion performance

Steel reinforcing bars in concrete are protected from corrosion by a thin oxide film that is created on the steel surface in the highly alkaline environment of the concrete. Corrosion process begins once this oxide film is destroyed, for example, by chloride ions penetrating from the element surface into the concrete and to the steel rebar. Critical chloride concentration is highly variable owing to numerous factors such as the type of reinforcing steel used. The TransChlor (R) software is used to simulate the time history of chloride ion ingress and corrosion initiation into a typical reinforced concrete structural element with four different types of reinforcing steel, namely, carbon steel, ferritic stainless steel (1.4003) with rolling skin, duplex stainless steel (1.4462) and ferritic stainless steel (1.4003) with pickled surface. A climate model is used to simulate two different exposure conditions of the structural element, namely direct and mist exposure, representatives for winter conditions in mountainous and lowland areas. A probabilistic method is applied to consider uncertainty of material properties in the model. The results show that reinforcing steel bars with enhanced corrosion performance significantly improve the durability of reinforced concrete.

Etude expérimentale des couplages viscoélasticité-croissance des fissures dans les bétons de ciment

Emmanuel Denarié

Viscoelasticity and crack growth govern the long-term deformability of concrete and thus its service behaviour and its durability. For low load levels, viscoelasticity behaves quasilinearly and crack growth is inactive. On the other hand, for high load levels, cracks grow and interact with viscoelasticity. Numerous authors have long demonstrated the influence of microcracking on creep qualitatively. Moreover, rate effects on the fracture behaviour of concrete are clear for low and high loading rates. Nevertheless, the mechanisms involved in these effects are not yet clearly determined. Cohesive crack approaches extended to time dependent parameters, give some elements of explanation, although without providing a general tool capable of explaining all the phenomena. The dissipative nature of crack growth and viscoelasticity naturally encourages one to model their couplings by means of an energybased approach to fracture. This is the meaning of the continuum thermodynamics of dissipative multicracked granular bodies developed by Huet (1997). This theoretical formalism puts the emphasis on the effect of viscoelasticity on the driving (reactive) part of the propagation criteria (energy release rate). The aim of this experimental research was to investigate coupling effects between viscoelasticity and crack growth in concrete. With this aim in view, 4 different types of fracture test have been performed on the same material (concrete with a maximum aggregate size of 8 mm), with a constant specimen geometry (rectangular wedge splitting specimens 20 by 20 by 10 cm). The first type of test consisted of performing a series of successive relaxations, at various increasing load levels, before, on and after the peak force, following the envelope of failure. Special attention was devoted to the influence of the control parameter ("active": with respect to displacements measured on the specimen, or "passive": crosshead displacement) and loading history. The results showed a progressive deviation from the linear viscoelastic behaviour starting from a load level of approximately 50 % of the peak force, before the peak. This deviation was significantly higher with crosshead control ("passive"). It depended on the loading history. After the peak, in "active" displacement control, relative relaxations tended to be similar whatever the load level. The displacement parameters (other than control) measured on the specimen or crosshead during relaxation, showed an evolution dependent on the control parameter. This effect was significant at the beginning of the relaxation and tended to disappear later. Furthermore, acoustic emissions could be detected at the beginning and during some relaxations, for high load levels (nearby peak force and after). The second type of test consisted of one or more successive creep levels up to eventual failure under sustained force. In certain cases, during creep levels leading to fracture, alternating secondary and tertiary creep with multiple concavity changes correlated with measured acoustic emissions could be observed. The tertiary creep leading to fracture developed over several minutes. Unstable sudden crack propagation was only seen at the end of the tertiary creep levels, accompanied by a sudden and very fast increase of the number of acoustic emissions. Moreover, during tertiary creep, an increase of the crack length measured on the specimen's surface by means of a conductive crack graphite gauge could be observed, correlated with the evolution of the creep displacements. The third type of test was performed under constant displacement speed (displacement measured on the specimen in the axis of the splitting force) with a range of speeds from 5 x 10-4 to 5 x 10-1 mm/minute. The results show a clear influence of the displacement speed on the response in terms of force-displacement curve, in the vicinity of the peak. The maximum force decreases with the displacement speed. On the other hand, the displacement at peak force was not significantly influenced by the displacement speed. Surface crack length measurements by means of a conductive graphite gauge enabled the determination of the surface crack speed as a function of the imposed displacement. The obtained surface crack speed depended quasi linearly on the imposed displacement speed. The fourth type of test, complementary to the former ones, was especially for the study of the evolution of internal micromechanical parameters during crack growth. Two wedge splitting specimens were equipped internally, near to the potential crack path with optical strain gauges (Bragg gratings). The internal strain measurements, revealed two zones of very different behaviour, with similar tends for the two specimens. Optical strain gauges close to the fracture plane (less than 5 mm from gauge axis) measured high strains (around 1000 με) significantly greater than the ultimate tensile strain of normal concrete (0.1‰ or 100 με) and mostly irreversible. At the contrary, optical strain gauges farther from the fracture plane (over 10 to 15 mm) measured strains that were always smaller than 100 με , and mostly reversible. Finally, two types of computer simulation (homogeneous materials) were performed in order to complement the interpretation of experimental results. A linear viscoelastic calculation, for a constant crack pattern, was used to illustrate quantitatively the progressive deviation from linearity observed in the experimental successive relaxations. A non-linear calculation of the propagation of a discrete crack with softening behaviour, in a linear elastic material, accurately predicted some of the strains measured with optical strain gauges during crack propagation. The experimental results demonstrated many indications of crack growth activity during creep as well as relaxation levels. These measurements confirm the significant contribution of propagation and coalescence of microcracks to the non-linear viscoelastic response of concrete. Indications of microcrack growth during relaxation can be explained by unstable propagation phenomena triggered by the small size of the microcracks. These phenomena could be amplified by viscoelastic effects of the redistribution of internal stresses in the heterogeneous structure of concrete. All the work performed clearly shows the need to complement macroscopic measurements such as the reaction force of a specimen or external displacements, with the measurement of micromechanical parameters in order to distinguish the contributions of the different active phenomena and of their coupling.

EPFL2000

Effet du microclimat sur l'initiation de la corrosion des aciers d'armature dans les ouvrages en béton armé

David Conciatori

The deterioration of reinforced concrete structures and the associated rehabilitation work represent an economic cost of premier importance. In Switzerland, the structural maintenance expenditures amount to 40% to 65% of the initial construction cost (over a 100 year life-span). Among the maintenance and rehabilitation activities, those dedicated to combating corrosion constitute the largest portion of invested financial resources. For example, the curbs of each highway bridge need to be repaired every 20 to 30 years, resulting in extensive maintenance and user costs. The corrosion phenomenon is initiated by the presence of oxygen and water acting upon steel reinforcement with a compromised protective oxide film (a natural protective layer surrounding the steel). This protective layer is eliminated by carbonization of the concrete (the reaction between carbondioxide and concrete calcium-hydroxide paste), a sufficient chloride ion concentration around the steel reinforcement (introduced by the spreading of de-icing salts on the roads during winter), or a combination of the two phenomena. The water diffusion in the concrete cover facilitates the chloride ion movement, reducing the corrosion initiation time, and likewise, increasing deterioration rate. This research proposes a model, "TransChlor," to evaluate the duration of the corrosion initiation processes. TransChlor can therefore serve as a decision aid during the structural maintenance plan selection process and during the design of new structural elements to mitigate the steel reinforcement corrosion problem. The model can also complement and elaborate upon the existing codes. TransChlor is based on a structural analysis approach that groups the elements of a structure into distinct groups. Each group is defined by similar functional and structural qualities influencing the corrosion phenomenon (concrete cover permeability, degree of exposure to aggressive corrosive agents, local defects). The model in particular distinguishes the elements exposed to mist and liquid water. This model is based on in-situ measurements (structural monitoring) and inspections performed by trained inspectors. TransChlor employs a deterministic approach to simulate water diffusion, chloride ion diffusion, gas penetration, and thermal conduction. The penetration of liquid water into the concrete pours, called capillary suction, was experimentally studied in the laboratory at low temperatures to match in-situ conditions. These experiments formed the calibration base of this model. TransChlor also incorporates a probabilistic model to consider the variable and indeterminate aspects of the corrosion problem, such as the permeability space, the concrete cover thickness, and the reinforcing steel physical-chemical resistance to chloride ion attack. The deterministic model is refined by the probabilistic approach and together TransChlor offers a more exact evaluation of the corrosion deterioration risks and can facilitate the objective dialogue and discussion between owners and maintenance specialists. For locations exposed to liquid water and brine, the standards should impose additional protective provisions such as periodic hydrophobic impregnations, implementation of stainless steel reinforcing bars, etc. For other locations (exposed to salt spay and carbonated concrete not exposed to chloride ions), the codes should require complementary construction and maintenance provisions.

EPFL2006

Structural Behavior of Tension Members in Ultra High Performance Concrete

Jörg Jungwirth, Aurelio Muttoni

The paper presents a study on the tensile behavior of UHPC conducted at the structural concrete laboratory at the Swiss Federal Institute of Technology in Lausanne (EPFL). Material tests and tests on structural members with reinforcement bars have been performed. A schematic representation of the material behavior allows a general description of the behavior of structural members in tension. The mechanical behavior for tension members and a simple model are presented.