Toward a probabilistic design of reinforced concrete durability: application to a marine environment

This work aims to propose a complete design methodology for concrete durability problems based on a probabilistic method including an original chloride diffusion modelling compatible with the new performance-based approaches. This is illustrated by the study of a concrete immersed in sea water. The physical and chemical processes that lead to the corrosion of the concrete reinforcement bars are presented first. A chloride penetration model based on Fick’s second law is proposed. Next, the durability modelling parameters chosen among the durability indicators (Baroghel-Bouny, Concrete design for structures with predefined service life—durability control with respect to reinforcement corrosion and alkali–silica reaction, 2004; Alexander et al., Mater Struct 41:921–936, 2008) are assessed. They depend on the concrete formulation and the chemical composition of the cement. These indicators are characterized by their statistical distributions, which are realistically specified from a wide literature review. The whole probabilistic modelling is included in a Bayesian network so that it can be easily updated to include new experimental data. The evaluation of the time dependant corrosion risk is estimated for two types of cement: CEM I and CEM I with silica fume. The result shows the effect on the Lind–Hasofer reliability index of the type of cement, the concrete quality and the design options. The quality is integrated through the mean value and the standard deviation of the modelling parameters. The method could be used either directly for cover design or for semi-probabilistic design code calibration.     

混凝土结构耐久性设计方法与寿命预测研究进展

为量化钢筋锈蚀与余震对钢筋混凝土结构抗震性能的影响,选取两栋按我国现行规范设计的中国东南沿海地区钢筋混凝土框架结构为研究对象,考虑未锈蚀和由低到高三种不同锈蚀率(5%、10%和15%)的四种工况,采用真实主余震序列作为输入,选取Park-Ang损伤指数作为结构损伤指标,开展了主余震序列作用下的未锈蚀与锈蚀钢筋混凝土框架结构的地震损伤评估与易损性分析。计算结果表明：锈蚀率的提高加剧了主余震序列作用下的结构累积损伤,其增长率最大可超过50%。由钢筋锈蚀单一因素引起的结构损伤在主余震累积损伤中的占比最高可超过30%。此外,钢筋锈蚀因素会导致结构的主余震易损性曲线发生显著提升。当锈蚀率较大时,钢筋锈蚀对结构易损性的影响与余震对结构易损性的影响相接近。钢筋锈蚀和余震两个因素的耦合作用会使结构的易损性水平发生更为显著的提升。因此,十分有必要在既有钢筋混凝土结构抗主余震性能评估中考虑钢筋锈蚀因素的影响。     

Durability design of concrete structures—Committee report 130-CSL

The idea of the durability design of concrete structures is developed as statistical theory which is based on the probability of failure taking into account the time as a parameter of the performance function and of the loading function. The generalised loading includen environmental effects into the structure like mechanic loading and weathering (sun, temperature changes, frost, moisture, pollutants, oxygen etc.). As generalised capacities like loading capacity, tightness, deflections, cracking, smoothness of the surface and visual fitness are treated. The statistical theory is transformed into practical deterministic durability design applying partial safety factor method. Important addition into the ordinary partial safety factors is the lifetime safety factor which is introduced into a design for first time here. The static and dynamic performance parameters and the durability performance parameters are combined together in the final phase of design. Performance and service life calculation models is introduced into design method, and presented for six decradation processes. The article is based on the report of RILEM TC 130—CSL [31], where the author was serving as chairman.

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Résumé La notion de durabilité des constructions en béton est développée ici comme une théorie statistique basée sur la probabilité de rupture prenant en compte la donnée temporelle comme un paramètre de la fonction de performance et de la fonction de chargement. Le chargement généralisé comprend les effets environnementaux dans la construction tel que le chargement mécanique et les effets des intempéries (exposition au soliel, écarts de températures, gel, humidité, polluants, oxygène, etc.). Lorsque les capacités globales telles que les capacités de chargement, l'étanchéité, la flexion, la fissuration, la douceur de la surface et l'aspect visuel ont été traitées, la théorie statistique est appliquée dans une démarche de conception pratique et déterministique de la durabilité, mettant en œuvre un facteur partiel de sécurité introduit ici pour la première fois dans la conception. Les paramètres de performances statiques et dynamiques et les paramètres de performances de durabilité sont associés dans la phase finale de conception. Les modèles de calcul de performance et de durée de vie sont incorporé aux méthodes de conception, et présentés pour une série de six processus de dégradation. L'article est basé sur le rapport de la commission technique RILEM TC 130_CSL: ‘Calculation methods for service life design of concrete structures’, qui était présidée par l'auteur du présent rapport.

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Prof. A. Sarja is a RILEM Senior Member, and works at the VTT, a RILEM Titular Member, He is also the chaiman of RILEM TC 172-EDM/CIB TG 22: ‘Environmental design methods in materials and structural engineering’. He is active in the RILEM Technical Advisory Committee (TAC) which he chaired from 1996 to 1998.     

A design concept with the use of RUHTCC beam to improve crack control and durability of concrete structures

Aiming at design issues of strictly anti-cracking structures or aseismic design in crucial locations of structures when using ultra high toughness cementitious composite (UHTCC), investigations on flexural behavior of reinforced ultra high toughness cementitious composite (RUHTCC) members are carried out due to excellent crack dispersion and strain energy absorption abilities of UHTCC. According to elastic theory, a calculation model of strain-hardening composites flexural members including theoretical calculation of moment, deflection and curvature, as well as critical reinforcement ratio is proposed in detail. Then experiment on RUHTCC beams without web reinforcement is performed to verify theoretical equations. For RUHTCC beams, there is a good agreement between test results and theoretical calculation. The safe calculated ductility indices can be used to predict ductility of structures. Compared with reinforced concrete beams, UHTCC delays yielding of reinforcements and improves load bearing capacity and ductility of structures, then steel reinforcement is saved; low reinforcement ratio is propitious to exert advantages of UHTCC. Under service load conditions, crack width in RUHTCC beams is limited to 0.05 mm and can be considered without negative influence on durability. Durability of structures will be significantly improved by using UHTCC instead of concrete.     

Durability of reinforced concrete wharves in Norwegian harbours

Summary A comprehensive field inspection has recently been carried out on 219 reinforced concrete wharves constructed in Norwegian harbours during the past half century. The structures represent more than 190 000 m² of reinforced concrete decks on more than 5 000 slender, reinforced concrete pillars poured under water. The overall good condition observed below low water proves that concrete can be successfully placed under water even for reinforced concrete members with relatively narrow cross-sections. Within the tidal zone the observations indicate that the deterioration here seems to develop very slowly. Above high water the deck beams showed to be particularly vulnerable to steel corrosion, while on the whole, neither deck slabs nor seawalls had their strengths seriously impaired. In the discussion of the field behavior it is emphasized that deck beams generally should be made very shallow or preferably be avoided in all reinforced concrete structures exposed to severe marine environments.

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Résumé On a procédé récemment à une vérification totalein situ de 219 quais en béton armé construits au cours des 50 dernières années en Norvège. Ces constructions représentent plus de 190 000 m² de planchers en béton armé sur plus de 5 000 piles minces en béton armé coulées sous l'eau. L'état satisfaisant observé sur l'ensemble à un faible niveau d'eau témoigne qu'on obtient des résultats satisfaisants en immergeant le béton, même pour des éléments en béton armé de sections assez étroites. A l'intérieur de la zone de marée, on note que la détérioration se développe très lentement. Au-dessus d'un niveau d'eau élevé, les poutres supérieures se montrent particulièrement vulnérables à la corrosion de l'acier alors que, dans l'ensemble, la résistance des dalles du plancher comme celle des parois exposées à la mer n'a pas été sérieusement compromise. A propos du comportementin situ, on souligne que les poutres de tablier devraient avoir une section moins haute ou de préférence évitées dans toutes les constructions en béton armé exposées à des conditions marines rigoureuses.

     

Concepts and requirements of durability design for concrete structures: an extensive review of CCES01

Acquiring enough durability for concrete structures in their design phase is a crucial step against unexpected deterioration during their service life. This article gives an extensive and in-depth review of the first Chinese national guide, CCES01-2004, destined exclusively for durability design of concrete structures. Based on the state-of-art knowledge on durability and in place experiences, this guide formulates its durability requirements from several fundamental concepts: environmental action, durability limit state and structure service life. For each environmental class and expected service life, the guide details its requirements on raw material constituents, concrete curing conditions, material performance as well as structure details, in particular the concrete cover to reinforcement steel. The relevant quantitative requirements are compared with those from other codes such as EuroCode2, EN206, ACI318 and CSA. As a special case, the durability of post-tensioned prestressed concrete structures is treated separately and a multi-layer, environment-based strategy is proposed for the prestress tendon and anchorage protection. At the end, the remaining crucial questions are brought forth for durability design of concrete structures.     

Reliability assessment and design load factors for reinforced concrete containment structures

The current ASME code for reinforced concrete containment structures is not based on probability concepts. This paper develops reliability assessment and probability-based design load factors for reinforced concrete containment structures. The purpose of constructing reinforced concrete containment structures is to protect against radioactive release, and so the use of a serviceability limit state related to cracking that can cause the emission of radioactive materials is suggested as a critical limit state for reinforced concrete containment structures. Load factors for the design of reinforced concrete containment structures are proposed. The proposed load factors are also examined in terms of a set of code performance objectives and consistency in limit state probability.     

Durability issues of FRP rebars in reinforced concrete members

The use of fibre reinforced polymers (FRPs) as rebars in reinforced concrete (RC) elements is a viable means to prevent corrosion effects that reduce the service life of members employing steel reinforcement. However, durability of FRP rebars is not straightforward as it is related to material properties as well as bar–concrete interaction. A state of the art of durability of FRP rebars is presented herein in order to highlight issues related to the material properties and interaction mechanisms which influence the service life of RC elements. The design approach implemented in international codes is discussed and the reduction factors taking into account the durability performances are summarized.     

Durability model for AR-glass fibres in textile reinforced concrete

Textile reinforced concrete (TRC) is an innovative material for thin-walled, structural elements with a high load-bearing capacity. For a safe design of TRC load bearing structures comprehensive investigations were carried out to predict the time-dependent loss in strength of the AR-glass reinforcement embedded in fine grained concrete as a consequence of weathering. The present work describes two different approaches to calculate the amount of strength loss of the reinforcement as a function of material, humidity and temperature. Both models, however, do not take into account outdoor weathering of TRC components which leads to a strength loss that is too high to be realistic. One of these approaches is combined with the component temperature and humidity constantly measured during outdoor weathering to derive a basis for a durability model which can take into account arbitrarily complex weathering. Consequently, this work presents the basis for a quantitative prognosis of the loss in strength of AR-glass reinforcement in TRC adapted to the respective material and weathering conditions.     

The effect of steel fibres on the earthquake-resistant design of reinforced concrete structures

The results of an experimental investigation are presented, studying the effect of fibres on the behaviour of reinforced-concrete (RC) structures designed in accordance with Eurocode 8. Twelve two-span continuous RC columns, eight with and four without steel fibres, were tested to failure, under constant axial force and monotonic or cyclic lateral displacement. Specimens without fibres suffered in some cases premature brittle failure, reflecting the incompatibility between post-peak concrete behaviour and the theoretical model underlying RC design. It was shown that it is possible to correct for this incompatibility through the use of steel fibres, resulting in a behaviour that satisfied current performance requirements for strength and ductility.     

A site study of durability indexes for concrete in marine conditions

The paper describes an investigation into the validity of durability index tests when used in a site situation and to evaluate the effectiveness of site curing methods. Concretes manufactured and cured under site conditions in a warm, humid coastal environment were investigated using recently developed durability index tests.Three blended binders (GGBS, FA and CSF) were used to cast a series of wall and slab elements. The elements were cured using practical site methods currently employed in the industry. Cores were extracted at early (28-day) and later (120-day) ages and used to determine the durability index properties.The results indicated that it is possible to manufacture, place and cure site concrete to achieve acceptable durability properties. Full wet curing proved to be the most effective method, as expected. While variation in potential durability properties existed at early age (28 days) between site and wet cured samples, at a later age (120 days) the variations had reduced such that, in practical and general terms, the different site curing methods were virtually indistinguishable. It was clear that environmental curing continues after 28 days provided climatic conditions are conducive for this to occur. Thus, on-going environmental curing largely governs the potential durability. To develop a concrete durability performance specification, it is imperative that a system is developed to quantify the effects of the environment on potential durability.     

Mix design and mechanical behaviour of some steel-fibre-reinforced concretes used in reinforced concrete structures

This article examines the mechanical behaviour of metal-fibre-reinforced concretes (MFRCs) on two different scales: that of the material and that of the structure. As regards the intrinsic mechanical behaviour of the material, defined as the behaviour observed before the stage of crack localization, it is shown that with MFRCs having the workability of a concrete (a method for optimizing the compositions of MFRCs is proposed) for use in reinforced concrete structures, the compressive and tensile strengths are in all cases less than those of a conventional concrete. On the scale of the structure, on the other hand, the fibres transmit forces across a macrocrack just like the reinforcements of a reinforced concrete. It is therefore on the scale of the structure, and not on that of the material, that metal fibres add ductility. Lorsque l’on considère la littérature portant sur le comportement mécanique des bétons de fibres métal-liques (BFM), on constate que les informations obtenues sont très nombreuses et souvent contradictoires. Ainsi que cela concerne les résistances mécaniques, les problèmes d’adhérence entre les armatures de béton armé et les BFM, ou encore les problèmes de ductilité, les résultats présentés par les divers chercheurs qui travaillent dans ce domaine, sont souvent très différents. Il y a, à notre avis, deux raisons à ce constat:

锈蚀对钢筋混凝土结构耐久性的影响研究

钢筋混凝土结构在工程建设中得到广泛的应用,然而结构中的钢筋锈蚀是影响钢筋混凝土结构耐久性的主要因素。文章阐述了混凝土结构的耐久性理论,锈蚀影响混凝土耐久性的因素以及预防锈蚀的措施等。     

Probabilistic design and management of environmentally sustainable repair and rehabilitation of reinforced concrete structures

This paper presents a probabilistic sustainability design framework for the design of concrete repairs and rehabilitations intended to achieve targeted improvements in quantitative sustainability indicators. The framework consists of service life prediction models combining deterioration mechanisms with limit states and life cycle assessment models for measuring the impact of a repair or rehabilitation. Both types of models (service life or LCA) are formulated stochastically so that the time to repair and the accumulated sustainability impact are described by probability density functions. This leads to a probabilistic calculation of cumulative impacts throughout the structure’s service life, from initial repair to functional obsolescence (end of life). The methods discussed are in accordance with sustainability design requirements within the 2010 fib Model Code. A case study is presented which computes the probability that reinforced concrete repair strategies using thicker concrete cover will meet future greenhouse gas emission reduction targets proposed by the UN Intergovernmental Panel on Climate Change.     

钢筋混凝土结构的温度场

火灾会导致钢筋混凝土性能退化,该文基于元胞自动机(Cellular Automata,CA)与纤维模型提出了一种用于火灾下钢筋混凝土梁非线性分析的方法。建立CA模型模拟了火灾中梁截面温度扩散演化过程,得到不同升温时间下截面各处的温度;引入损伤模型得到钢筋损伤因子-升温时间变化关系;将元胞自动机模型与纤维模型结合,对火灾下钢筋混凝土连续梁进行时变非线性分析。结果表明：CA模型能够有效模拟钢筋混凝土梁截面的温度扩散过程;引入的损伤模型可有效将钢筋损伤进行量化;编制的MATLAB计算程序可有效用于高温损伤后钢筋混凝土梁的非线性分析;CA模型与纤维模型的有效结合可分析钢筋混凝土梁的变形与承载力时变演化过程,并对其耐火寿命进行评估。该方法可为火灾下钢筋混凝土结构的耐火寿命预测与评估提供理论工具。