Capteur chimique à fibres optiques pour la mesure des ions chlore dans le béton à un stade précoce

Francine Laferrière
EPFL, 2005
EPFL thesis

Abstract

In the national and worldwide context, countless reinforced concrete structures are in an advanced state of deterioration. A principal cause of such degradation is chloride induced corrosion of reinforcement bars. This phenomenon is accentuated in countries where de-icing salts are used for road safety, as well as in maritime zones. To date, no non-destructive method reliably quantifies chloride content during the corrosion initiation phase. Measurement of such a parameter is important for development of a better understanding of the complexity of corrosion phenomena and more practically, for better management of existing structures. The principal objective of this study is to propose a method for non-destructive measurement in order to monitor continuously and in real time free chloride content in concrete pores. In this context, a chemical sensor that employs optical fibres was developed and tested. The sensor functions using the fluorescence of an indicator dye that is sensitive to chlorides. Through fluorescence spectroscopy, variations in the concentration of free chlorides are related to intensity fluctuations of fluorescence. In this type of sensor, the role reserved for optical fibres is limited to data transport, thereby enabling remote measurement. The use of optical fibres also provides an advantage compared with existing electric non-destructive detection systems due to superior electromagnetic stability. Theoretical and experimental studies calibrated and validated the sensor for implementation within mortar and concrete. Free chloride concentrations between 0 and 350 mM can be detected, this represents 0.05% and 0.6% of cement weight. The system stability is approximately 0.02% [Cl-]/year. Measurement accuracy is generally lower than ± 8%. Two experiments reproduced climatic variations in a controlled environment. The first test simulated a hot maritime climate and the second test simulated a cold continental climate. These tests confirmed that it is possible to determine with precision free chloride content. A constant temperature and relative humidity test showed a good concordance between results from the measurement system and an existing chloride transport model. Although measurements indicate faster chloride ingress compared with model predictions, test results and model predictions coincide after a few days. In conclusion, fluorescence spectroscopy with optical fibres offers an innovative means for early and non-destructive detection of free chloride content in concrete. Experiments and model predictions demonstrated complementary strengths and weaknesses. As a result, there is much potential for synergy in order to improve the science of corrosion process understanding and to plan appropriately for preventive action in practical situations.

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Francine Laferrière
EPFL, 2005
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/53645?ln=en

About this result

Related concepts (18)

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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

Modèle d'évolution de l'état des ponts-routes en béton

Guido Roelfstra

The management of existing road infrastructures is a multidisciplinary activity that involves structural engineering, material science, management, economics and ecology. The objective is to achieve maximum availability of road links at minimum societal costs. Recently, tools (Bridge Management Systems, BMSs) have been developed to help decision makers to determine the optimal management strategies within available resources. The condition development model is a key element of the BMSs. Currently this model is using empirical approach making use of data collected during inspections and is formulated with Markov chains. The assessment units are structural elements and these are classified into condition classes according to their visual appearance. The condition assessment is therefore rather subjective since no measurements are involved in it. Consequently, the condition development based on visual inspections will reflect this subjectivity. In order to improve the objectivity of condition forecasts physical and chemical phenomena governing deterioration has to be incorporated in the condition development model. Observations made on a bridge sample representative of the Swiss highway bridges showed that chloride-induced corrosion is the main deterioration mechanism. For this reason this research was focused upon modeling the chloride-induced corrosion. In this research, the condition development model is established, based physical and chemical phenomena involved in chloride induced corrosion. This analytical model is able to yield quantitative results on condition development. Accuracy and availability of input data govern the level of accuracy of such an analytical model. The existing knowledge on chloride induced corrosion and inspection techniques available on site are examined in order to develop a simple and reliable condition development model. Firstly, non-destructive testing methods are used in addition to visual observations, to determine quantitative values (permeability and thickness of the concrete cover) of parameters governing the segment deterioration. The structural elements are divided into areas i.e. segments where the parameters governing chloride-induced corrosion are assumed to be constant. It is proposed to divide structural elements into segments based on three concrete cover permeability classes, concrete cover thickness and three types of surface exposures to chloride contaminated water. Secondly, a chloride transport model is developed to simulate the real exposure of reinforcement to chloride contaminated water, taking into consideration the transport mechanisms of water and chloride ions. Simulations are performed for the various segment parameters that have been identified in the segmental approach. Thirdly, a relation is established between the free chloride ion content and the corrosion initiation probability. Using the results of the chloride transport model, the development of the corrosion initiation is determined for road bridges segments. An analysis is made to predict the corrosion initiation time of existing bridges as a function of their segment parameters (exposure and concrete cover permeability and thickness), but also to establish recommendations for the construction of new bridges. Fourthly, the corrosion rate i.e. loss of steel cross-section is then determined according to the concrete cover permeability and the exposure to chloride contaminated water. Fifthly, the results of the presented mode1 are mapped to conditions States as defined in the BMSs. Three degradation matrices (Markov chains) are determined, corresponding to three condition developments: favorable, normal and unfavorable. The three condition developments can be estimated for given quantitative values of the concrete cover permeability, thickness and surface exposure to chloride-contaminated water. In this way, the segment parameters may be used in BMSs to forecasts future condition of segments, elements and the whole structure. Finally the structural behaviour of the concrete road-bridges until failure is studied. The determinant failure scenarios are identified for each bridge type and a case study is performed on the sample representative for the Swiss highway bridges. For this purpose the determinant segments for the structural safety are identified. Additionally the required concrete cover depth for a service life of 50 and 100 years is expressed as a function of the permeability and the exposure to chloride contaminated water. This research outlines the need to measure the concrete cover permeability and thickness with non-destructive test methods as well as to determine the exposure to chloride contaminated water. These values are essential to predict the condition development of concrete road bridges.

EPFL2001

Comportement structural des bétons de fibres ultra performants en traction dans des éléments composés

John Wuest

Ultra high performance concretes (UHPFRC) are characterized by a dense matrix and a high fibre content. These materials exhibit exceptional mechanical and durability properties making them an ideal material for rehabilitating existing structures. The primary interest in UHPFRC focuses on their uniaxial tensile performance. When they applied in a cast in place overlay configuration, they provide increased rigidity to the global element and localized protection to the steel reinforcement embedded in the concrete core during the service by minimizing surface cracking and inhibiting the diffusion of aggressive agents. At the ultimate state, and under certain configurations UHPFRC significantly improves the element's carrying capacity. The overall goal of this research is to study the UHPFRC tensile behaviour (hardening and softening behaviours) of characteristic test specimen and to apply this knowledge in analyzing the structural response of various composite elements. The objectives related to this study are: To determine the factors influencing the UHPFRC uniaxial tensile test specimen behaviour To illustrate the importance of UHPFRC tensile hardening and softening responses in UHPFRC structural plates and UHPFRC-reinforced concrete composite beams. To determine the rupture mechanism and overall contribution a UHPFRC overlay offers to a composite UHPFRC-reinforced concrete slab loaded to failure in flexural/punching shear. To study the parameters influencing the UHPFRC uniaxial behaviour in tensile composite structural elements. The behaviour of specimen with different configurations and compositions were studied in uniaxial tensile tests. The variables significantly influencing the fibre orientation were isolated by studying the fibre orientation at localized cuts in the various specimen. These controlling variables include: mixture viscosity, casting method, casting direction, fibre aspect-ratio and element geometry. Furthermore, a meso-level model was developed to predict the uniaxial tensile behaviour and was validated against the experimental test results. With the help of this model, the influence of the main fibre characteristics (Vf, Lf, df) on the resultant fibre orientation and matrix quality were examined. This model has determined that the extent of hardening does not increase linearly with the quantity of fibres but rather follows an asymptotic curve converging to a maximum possible hardening. A second model was developed to randomly orient and place fibres. This model has helped to explain the uniaxial tensile response variability of different mixtures ranging from conventional Fibre Reinforced Concrete (FRC) to UHPFRC. A finite element program has been employed to study the influence the UHPFRC softening behaviour has on the composite structural beam and UHPFRC plate responses. For a flexural plate (length 500mm, width 200mm, height 30mm), the bearing capacity was significantly increased by the very pronounced UHPFRC softening behaviour. Additionally, the UHPFRC layer noticeably increased the rigidity of the composite beam and slab. For example the UHPFRC layer increased the slab bearing capacity by 40% in comparison to a purely reinforced concrete slab by bridging the punching shear mechanism with a UHPFRC membrane effect. The structural responses of the two element types were then further analyzed using a finite element analysis program. An inverse analysis methodology was employed to obtain the UHPFRC tensile material laws in respective composite elements. These findings documented that a similar materials exhibited significantly different responses when applied in different applications. This analysis documents that the UHPFRC layer in a slab closely follows the uniaxial tensile test specimen response, while the UHPFRC layer in a composite beam exhibits a significantly reduced performance. The primary reasons for these variances are: the relative fibre distribution, the interface rugosity, the internal residual stresses and the extent of the maximum stress zone. Recommendations are made concerning the implementation of UHPFRC and the influence the fibre aspect ratio and volume (Vf, Lf, df) imposes on the resulting fibre orientation in different applications. Finally, a method for determining the UHPFRC characteristic curve which is required to design a composite UHPFRC-reinforced concrete element is presented.

EPFL2007