Evaluation of service life design models on concrete structures exposed to marine environment

Durability of reinforced concrete is primarily influenced by the penetration of aggressive substances into concrete, which are degrading concrete and reinforcement. For structures in marine environment chlorides are the most critical environmental load, which are causing serious corrosion damages. Data collected during the survey of the Krk Bridge, a large reinforced concrete arch bridge structure located on the Adriatic coast, is used as documented reference in this research. The structure has been exposed to the marine environment for over 25 years. Based on collected materials data and the exposure conditions, the service life of this structure is estimated using three currently available prediction models, two deterministic models, the North American Life-365 model and the Croatian CHLODIF model, and the DuraCrete probabilistic method. All these models are based on the chloride diffusion process, but with different detailing of the model parameters. The conclusion is an evaluation of the service life predictive ability of each of these three service life models.     

Simulation studies of methods to delay corrosion and increase service life for cracked concrete exposed to chlorides

The ingress of chlorides in reinforced concrete leads to the onset of steel reinforcement corrosion and eventually compromises a structure’s integrity. To extend its service life and improve safety, it is crucial to develop sound repair strategies for our nation’s infrastructure. In this paper, results are presented for numerical simulations to study the effectiveness of fillers for repair of cracks in concrete, so as to delay the onset of corrosion in reinforcing steel. Concretes without cracks and with either a 50 μm or 500 μm wide crack located directly above the steel reinforcement are simulated, with the addition of silica fume, a corrosion inhibitor, or epoxy-coated reinforcement being considered as additional scenarios. The effectiveness of the crack filler depends not only on its inherent diffusivity with respect to chloride ions, but also on its ability to penetrate and fill the damaged zone or interface between the open crack region and the bulk concrete. Additional simulations indicate that using continuum models instead of models that include details of the rebar placement can lead to underestimating the chloride concentration and overestimating the service life. Experiments are needed to study the ingress of chlorides in damaged (interfacial) regions adjacent to the crack or at the reinforcement surface, as the local transport properties of these regions can significantly influence service life predictions.     

Numerical modeling for predicting service life of reinforced concrete structures exposed to chloride environments

Degradation of reinforced concrete (RC) structures due to chloride penetration followed by reinforcement corrosion has been a serious problem in civil engineering for many years. In the present paper, a systematic and robust model for predicting service life of RC structures is developed which takes environmental humidity and temperature fluctuations, chloride binding, diffusion and convection, as well as the decay of structural performance into account. The interactions between the decay of structural performance, heat and moisture transfer are considered in a coupled thermal-hygro-mechanical model. The governing equations of heat, moisture and chloride transport into nonsaturated concrete are described particularly and solved numerically by finite element analysis in space and time domains. Comparisons of numerical results with analytical solutions and experimental observations are conducted to establish the validity of the proposed numerical model. Applications of the numerical model are demonstrated by predicting service life of a RC slab exposed to a chloride environment.     

General model for constitutive relationships of concrete and its composite structures

This paper presents a unified mathematical model that can be used to construct various constitutive relationships for concrete materials and its composite interfaces. The unified model has a single expression and a maximum of six variables/parameters, whereas in most cases, three to four parameters are sufficient. The model is simple, continuous, and easy to be integrated and differentiated. Its parameters can be assigned clear physical meanings, and used to control variations of different parts of the curve separately. Its greater versatility and flexibility enables possible applications in almost all kinds of constitutive relationships for concrete and its composite structures. For example, it is demonstrated to be suitable for stress-strain relationships of plain and confined concrete, bond-slip relationships of steel bar-to-concrete interfaces, externally-bonded fiber-reinforced polymer (FRP) interfaces, near surface mounted FRP interfaces, etc. Furthermore, a simple equation for the bond-slip relationship of steel reinforcement-to-concrete interface is derived using the unified model to replace the four-segment CEB-FIP code model.     

The possibility of evolving a theory for predicting the service life of reinforced concrete structures

The development in recent years with increasing air pollution, increasing need of energy saving and last not least the growing number of damage in old structures leads to a raising demand to calculate the durability of structures, too. The paper has been prepared by request of the RILEM Technical Committee 45-LTO to show that, in principle, it is possible to evolve a theory for predicting the service life of r.c. structures and to sketch a way how that could be achieved. A lot of necessary basic data can be found scattered in the literature. To collect these data will be the first step to take.     

Evaluation of surface sealers for concrete hydraulic structures exposed to low temperatures

Many hydraulic structures exhibit downstream cracks and surface degradation due to porous and non air-entrained concrete cast only a few decades ago, wetting–drying and freezing–thawing cycles. These degradations could be prevented by minimizing surface saturation and water infiltration by sealing the concrete surface. Many different types of sealers, such as silanes, siloxanes, boiled linseed oil, epoxies, urethanes and cement-based products, are presently available on the market. However, despite their ready availability, very few or no laboratory or field evaluations have ever been undertaken to evaluate their effectiveness, specifically for hydraulic structures exposed to extremely low temperatures. Consequently, 60 different surface sealers were selected for laboratory evaluation, aimed at minimizing water or moisture infiltration, and to extend the service life of hydraulic structures, besides reducing the harmful effects associated with concrete surface deterioration. The tests carried out on these sealers comprised reduction of absorption and evaporation capacity, durability under freezing–thawing and wetting–drying cycles, acid rain, UV radiation, effect of various parameters, such as concrete water:cement ratio (W/C), sealers application temperature, pH variation, presence of hydrostatic pressure, etc. Preliminary data showed that silanes and siloxanes exhibited satisfactory results, however, products from the same family yielded variable results. This paper summarizes a part of a large study on concrete sealers for hydraulic structures in cold climates.     

Influence of chloride threshold value in service life prediction of reinforced concrete structures

Extensive studies have been reported on the development of service-life prediction of reinforced concrete structures in the last two decades. Service life of a reinforced concrete structure is dependent on the corrosion process, specifically chloride threshold value. Chloride threshold value is a distinctive property and is dependent on several factors such as the supplementary cementitious material used, presence of surface cracks, water to binder ratio, type of steel reinforcement, exposure conditions, measurement methods, etc. Although chloride threshold value is an influential parameter in service-life prediction, a definitive chloride threshold value considering these factors is not reported in the existing literature and standards. Moreover, values adopted in many analytical tools for service life predictions based on the type of steel reinforcement alone are not appropriate and leads to inaccurate calculations. There is a gap in the available literature in understanding the selection of suitable chloride threshold values to be adopted for a specific system. Therefore, it is imperative to study the variation of chloride threshold value with exposure conditions as well as other influencing parameters, to achieve proper service life prediction. In the paper, several influencing parameters on chloride threshold value and its significance on prediction method are comprehensively reviewed. Moreover, suitable recommendations are highlighted for Indian and international standards at the later part of the study.     

Continual model of propagation of corrosion cracks for the evaluation of the service life of structures

On the basis of the mechanics of dispersed fracture, we develop a continual model of propagation of corrosion cracks for the evaluation of the service life of structural elements. The results of numerical simulation of the process of crack growth in specimens made of austenitic stainless steel of the 18-8 type performed on the basis of the experimental data available from the literature demonstrate the reliability of the proposed model. We obtain the solutions of the problem of service life of the rectilinear segments of heat-exchange pipes of steam generators of nuclear power plants and apply these solutions to establish a noticeable dependence of the service life on the size of the pipes, pressure in the primary coolant circuit, and chemical compositions of corrosive media.     

Doubling the service life of concrete structures. II: Performance of nanoscale viscosity modifiers in mortars

A new approach for increasing the service life of concrete structures is evaluated in a series of mortar specimens. The new approach consists of employing nanoscale viscosity modifiers to increase the viscosity of the concrete pore solution and concurrently and proportionally decrease the diffusion rates of deleterious ions such as chlorides and sulfates. In part I of this series, viscosities of bulk solutions of the admixtures in water and electrical conductivities of admixture solutions also containing potassium chloride were examined to verify the viability of this new technology. In the current paper, these studies are extended to quantifying the performance of one of these admixtures in mortars by measuring the penetration depth of chloride ions in cylindrical specimens exposed to a 1 mol/L chloride ion solution for up to 1 year. While significant reductions in the 1 year penetration depth are produced when the viscosity modifier is utilized via conventional addition to the mixing water, the best performance is achieved when a solution of the viscosity modifier is utilized to pre-wet fine lightweight aggregates that are then added to the mortar mixture. A scaling function appropriate for radial diffusion was used to estimate the relative effective diffusion coefficients. Compared to a reference mortar, the best mixture reduced the effective diffusion coefficient by a factor of 2.7, consistent with the overall objective of doubling concrete service life.     

Service life and life cycle cost modelling of cathodic protection systems for concrete structures

Cathodic protection (CP) of reinforcing steel has been applied successfully to concrete structures with corrosion damage for more than 25 years. Performance and maintenance data are reported from an inventory of CP systems in The Netherlands installed on about 100 structures between 1987 and 2010. The large majority provides corrosion protection for a long time. Degradation of components and overall systems seems to occur in limited numbers. Failure of components and total systems as a function of age is quantified. On the average, the time until minor repairs of parts is necessary is about 15 years. Global failure of the anode, which necessitates near complete replacement of the system, is rare. Based on the statistical analysis of field data, the cost of maintaining a CP system is predicted using a life cycle cost model.     

Methodology of calculating required chloride diffusion coefficient for intended service life as function of concrete cover in reinforced marine structures

For marine reinforced concrete (RC) structures, chloride initiated corrosion of reinforcement is generally accepted as the service life limiting degradation mechanism. A methodology is described for how the maximum required chloride diffusion coefficient (D) of a concrete for achieving an intended service life (t) can simply be calculated as a function of concrete cover thickness (x) over the reinforcement by D = constant·x²/t.The principle is based on the usual mathematical solution to Fick's 2nd law of diffusion. The constant is broken down into 3 factors; chloride concentration factor, aging factor and temperature factor, in which input parameters have to be selected. These factors are calculated for a range of selected input parameters illustrating their sensitivity. The largest uncertainty lays in proper selection of the aging factor of the diffusion coefficient. Some concrete recipes satisfying estimated chloride diffusion coefficients are proposed as part of the methodology.     

Predicting the performance of concrete structures exposed to chemically aggressive environment—Field validation

The behavior of two series of concrete slabs exposed to sulfate-bearing soils was investigated by a numerical model called STADIUM. In addition to the diffusion of ions and moisture, the model also accounts for the effects of dissolution/precipitation reactions on the transport mechanisms. The simulations yielded by the model were compared to the actual degradation of the slabs after 8 years of exposure. The microstructural alterations of concrete resulting from the penetration of magnesium, chloride and sulfate ions were studied by backscatter mode scanning electron microscope observations and energy-dispersive X-ray analyses. The comparison of both series of data indicates that the model can reliably predict the various features of the microstructural alterations of concrete.

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Résumé Le comportement de deux séries de dalles sur sol en béton exposées à des sols chimiquement agressifs a été étudié à l'aide d'un code de calcul numérique appelé STADIUM. Ce modèle permet de décrire le transport couplé de l'eau et des ions dans des matériaux poreux non-saturés en prenant en considération l'influence des réactions chimiques. Les résultats des simultations de la dégradation du béton après huit ans d'exposition à des ions chlore, sulfate et magnésium. Les observations ont été réalisées par microscopie électronique à balayage. Des analyses par dispersion des rayons X ont également été effectuées. Les données démontrent clairement que le modèle perment de prédire avec précision le comportement du béton soumis à différents types d'agression chimique.

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Editorial Note Laval University (Canada) is a RILEM Titular Member. Prof. J. Marchand was awarded the 2000 Robert L'Hermite Medal. He is Editor in Chief for Concrete Science and Engineering and Associate Editor for Materials and Structures. He participates in RILEM TC 186-ISA ‘Internal Sulfate attack’.     

Water repellent surface impregnation for extension of service life of reinforced concrete structures in marine environments: The role of cracks

The enhancement of long-term durability of marine structures is a matter of interest to many researchers. The study presented in this paper examines the effectiveness of a water reducer and chloride barrier surface impregnation of the concrete cover of reinforced concrete (RC) structures, exposed to a marine environment. Specific focuses is on how surface cracks created (1) before impregnation and (2) after impregnation, affect the effectiveness of the surface treatment. The experiments are conducted in an environment which is as close as possible to the real humid subtropical marine environment.A series of reinforced concrete (RC) prisms and concrete cylinders, each treated with various commercial surface impregnation agents, were exposed to cyclic sea water shower under an outdoor environment to accelerate the dry/wet cycles for 1 year. Six types of surface impregnation agents, including four types of silane-based water repellent agents and two types of sodium silicate-based pore blockers (water–glass) were applied. Three types of RC prisms were prepared to simulate the different cracking possibilities, which may occur in surface impregnated concrete structures, during their service life. No cracks were introduced in the first prism group, while cracks were introduced before and after surface impregnation, in the second and third groups, respectively. The time-dependent water absorption of all specimens was monitored during exposure to the dry/wet cycles. Finally the specimens were split open to measure the penetration depths of the surface impregnation agents and the chloride penetration profiles. The areas with corrosion evident in the steel reinforcement in the RC prisms were also measured.Sodium silicate-based pore blockers were found to be inefficient in preventing chloride penetration of concrete under simulated marine exposures. The long-term efficiency of water repellent agents used for surface impregnation was found to be highly dependent on the type of agent and whether impregnation was carried out before or after crack formation.     

Material model for the analysis of reinforced concrete surface structures

A material model for the analysis of reinforced concrete surface structures is developed. The constitutive model employs the smeared crack concept, i.e. only average stresses are considered at an integration point. While uncracked concrete is modelled with a plasticity approach, the paper focuses on the numerical treatment of cracked reinforced concrete in a state of plane stress. Special attention is given to the modelling of the tension stiffening effect, the re-orientation of the principal tensile strain direction and the compressive strength of cracked concrete. The material model has been implemented in the finite element program SEGNID and tested extensively. Here the comparisons of calculated and experimentally measured results on reinforced concrete surface structures-several panels, a plate subjected to torsion and a free-formed shell-are presented.List of symbols d bar diameter - D stiffness matrix - D _c stiffness matrix of concrete - D _s stiffness matrix of reinforcement - D rotational stiffness matrix - f stress - f _c stresses in concrete - f _c1 tensile stress in concrete - f _c2 compressive stress in concrete - f _c2,min concrete compressive strength - f _c cylinder crushing strength - f _s stresses in reinforcement - f _t concrete tensile strength - f _y yield strength of reinforcement - f ₁ applied principal tensile stress - f ₂ applied principal compressive stress - F tension force - F _y yield force - l length - M moment - Q dowel resistance - r _xy vector of unbalanced stresses - s applied load - s applied stress vector - T ₁, T ₂ transformation matrices - w crack width - x, y reference coordinate system - 1, 2 crack oriented coordinate system - curvature - _xy incremental strain in the reference coordinate system Extended version of the contribution Two-dimensional FE-analysis of reinforced concrete membrane elements to the International Conference on Computational Engineering Science, Atlanta, April 10–14, 1988, presented by the second named author     

Model for an engineering approach to the design of concrete structures in chemically aggressive environments

The paper presents a draft proposal for a procedure for designing concrete structures in chemically aggressive environments. Instead of empirical rules which are applied to produce durable concrete, a procedure has been proposed to analytically define the degree of environment agression. On the other hand, definitions have been given for concrete characteristics relevant for a durable concrete structure, as can be numerically set and checked during the construction and after the completion.     

Cracking of concrete structures due to imposed strains with regard to design of reinforcement

The principles for control of cracking in concrete structures due to imposed strain are investigated in this paper. The effects of uniform and non-uniform cooling and shrinkage are studied in medium-thick walls with regard to the amount of reinforcement necessary to distribute cracks. The influence of reinforcement on the behavior after the first crack is investigated, in particular its ability to reduce crack widths. The restraint force is significantly reduced for non-uniformly distributed imposed strain due to softening and cracking in the concrete. The amount of minimum reinforcement needed to distribute cracks is therefore significantly smaller for imposed strain in the form of drying shrinkage than for thermal imposed strain. Crack reinforcement is most effective for structures with large dimensions in the direction of external restraint.     

Automated design of a linear generator for wave energy Converters-a simplified model

An encouraging way to convert ocean wave energy into electricity through direct drive is by using a linear, synchronous, longitudinal-flux permanent-magnet machine (LFM), where the piston is driven by a buoy. In this paper, the speed of the piston is assumed to be constant or sinusoidal. The paper presents an automated method for optimizing the design variables of an LFM with a rectifier, using time-stepping finite-element analysis. The method yields feasible LFM designs tailored to a given ocean wave climate, assuming constant or sinusoidal piston speed. The method will help to avoid a large amount of field calculations to obtain an optimal LFM driven by a buoy. The systematic approach results in a transparent investigation, giving the engineer an easy way to determine the best design. The paper presents LFM designs for a calm site in the Baltic Sea as examples.     

Ageing management of safety-related concrete structures to provide improved bases for continuing the service of nuclear power plants

Resume On décrit les structures en béton se rapportant à la sécurité des installations nucléaires, et on détermine les facteurs de dégradation susceptibles de compromettre la conformité de ces structures aux critères de fonctionnement et de performance qui leur sont assignés. On donne ici, de fa?on résumée, les exigences d’inspection en service. On examine l’évaluation des performances des éléments en béton dans les installations nucléaires. Enfin, on trace les grandes lignes d’un programme pour gérer le vieillissement des structures en béton en rapport avec la sécurité des installations nucléaires, et améliorer leur capacité de service. On résume également les différents aspects du programme.      

人群简化模型与人行桥TMD参数设计研究

用两段连续弹性杆模拟静立人体,建立人群-人行桥-调谐质量阻尼器(TMD)振动系统。将静立人群简化建模为单个的广义人体,研究广义人体-人行桥-调谐质量阻尼器振动系统的动力特性及TMD参数设计。运用最小二乘原理确定广义人体的相关参数,通过与已有的实验数据对比,验证了将静立人群简化为广义人体模型的正确性。以均方根加速度作为人体舒适度的优化准则,分析了人行桥TMD的最优频率比和最优阻尼比。