Showing papers in "Structural Concrete in 2013"

Fibre‐reinforced concrete in fib Model Code 2010: principles, models and test validation

Abstract: In the fib Model Code for Concrete Structures 2010, fibre-reinforced concrete (FRC) is recognized as a new material for structures. This introduction will favour forthcoming structural applications because the need of adopting new design concepts and the lack of international building codes have significantly limited its use up to now. In the code, considerable effort has been devoted to introducing a material classification to standardize performance-based production and stimulate an open market for every kind of fibre, favouring the rise of a new technological player: the composite producer. Starting from standard classification, the simple constitutive models introduced allow the designer to identify effective constitutive laws for design, trying to take into account the major contribution in terms of performance and providing good orientation for structural uses. Basic new concepts such as structural characteristic length and new factors related to fibre distribution and structural redistribution benefits are taken into account. A few examples of structural design starting from the constitutive laws identified are briefly shown. FRC can be regarded as a special concrete characterized by a certain toughness after cracking. For this reason, the most important constitutive law introduced is the stress-crack opening response in uniaxial tension. A wide discussion of the constitutive models introduced to describe this behaviour, which controls all the main contributions of fibres for a prevailing mode I crack propagation, is proposed. The validity of the models is discussed with reference to ordinary cross-sections as well as thin-walled elements by adopting plane section or finite element models.

Reliability‐based non‐linear analysis according to fib Model Code 2010

Abstract: The fib Model Code 2010 for Concrete Structures introduces numerical simulation as a new tool for designing reinforced concrete structures. The model of resistance based on non-linear analysis requires adequate model validation and a global safety format for verifying designs. The numerical simulations combined with random sampling offer the chance of an advanced safety assessment. Approximate methods of global safety assessment are discussed and compared in a case study. An example of a bridge design supported by non-linear analysis is shown.

Background to the fib Model Code 2010 shear provisions – part I: beams and slabs

Abstract: This paper examines the evidence for the one-way shear model developed for the fib Model Code for Concrete Structures 2010 and provides examples of its application. For the design and analysis for shear, for members with and without shear reinforcement, the fib Model Code 2010 procedures have been developed from physical-mechanical models that are based on observed behaviour at the meso-scale level; they represent a significant advance over previous standardized empirical methods. In addition, an approach referred to as “level of approximation” (LoA) is incorporated where advanced models are simplified in a consistent and conservative way such that the designer can select the effort needed to justify their design. To illustrate the practical use of the models and the LoA approach, two examples are presented. The first is a deck slab of a cut-and-cover tunnel where design and possible refinements are discussed for a given configuration. The second is a prestressed concrete bridge girder, which is considered for the cases of design and for the analysis of an existing structure.

Design recommendations for interface shear transfer in fib Model Code 2010

Abstract: Interface shear transfer between differently aged concretes is a topic that crops up frequently and in different situations in structural design. In the fib Model Code for Concrete Structures 2010 [1] the fundamental basics of concrete-to-concrete load transfer are given in section 6.3 and the corresponding design rules in 7.3.3.6. The different potential mechanisms contributing to the shear resistance along the interface, i.e. adhesive bond, aggregate interlock, friction and dowel action, are thus combined and their relationship taken into account by interaction factors. This article summarizes the most important results from past and ongoing studies and presents the background to the theory forming the design basis of fib Model Code 2010, the "extended shear friction theory" (ESF).

Cracking of RC members revisited: influence of cover, ϕ/ρs, ef and stirrup spacing – an experimental and theoretical study

Abstract: This article describes an experimental programme aimed at studying the effect of cover, ratio between diameter and effective reinforcement ratio (ϕ/ρs, ef) and the influence of stirrup spacing on the cracking behaviour of reinforced concrete elements. The experimental programme was conceived in order to contribute to the debate – fuelled by the publication in recent years of Eurocode 2 EN1992-1-1 and the revision of the Model Code under way when the tests were carried out (and now published as a finalized document) – regarding the influence of these parameters on cracking. Important theoretical aspects are discussed, including where the crack width is estimated by current code formulations and what relevance this may have on the correlation between crack opening and durability of RC structures, especially with regard to structures with large covers. The effect of stirrup spacing, a variable absent from current codes, is also discussed.

Background to fib Model Code 2010 shear provisions - part II: punching shear

Abstract: This paper outlines the theoretical background of the punching shear provisions implemented in the fib Model Code 2010 and presents a practical example of its applica tion. It is the aim to explain the mechanical model that forms the basis of the punching design equations, to justify the relevance of the provisions and to show their suitability for the design and assessment of structures.

Design for SLS according to fib Model Code 2010

Abstract: This paper provides an overview of serviceability specifications given by the fib Model Code for Concrete Structures 2010 (fib MC2010 [1]) First, the reasons behind crack control and deflection control are discussed, then specific design rules are provided Simple rules as well as detailed models are also presented Numerical examples are provided in order to assist in the application of the design recommendations for crack control and deflection control (reinforced and prestressed concrete elements) Simple rules mean indirect control of cracking or deflections without calculations Indirect crack control may include limitation of stresses and selection of maximum bar diameter or maximum bar spacing Indirect deflection control normally means limiting the span-to-depth ratio Detailed models are based on physical and mathematical approaches to cracking and deflections The design crack width is expressed as the maximum bond transfer length multiplied by the mean strain between cracks Deflection analysis can be provided by integrating curvatures or by using a simplified or refined method Vibrations and numerical modelling of cracking are also briefly discussed

Global safety format for non‐linear analysis of reinforced concrete structures

Abstract: Semi-probabilistic safety formats for the non-linear analysis of reinforced concrete structures are of practical interest for structural designers. The safety format proposed in EN 1992-2 enables a safety assessment through a non-linear structural analysis and the application of a global safety factor, which is defined as the ratio between the representative and design values of the material resistances. A more realistic estimate of the global safety factor can be obtained from the distribution of the structural response. This paper proposes a safety format based on the mean values of the material resistances and a global resistance factor. Its practical application in the structural design of concrete beams and columns is also presented.

Residual compressive and flexural strength of a recycled aggregate concrete following elevated temperatures

Abstract: A recycled aggregate concrete (RAC) with different replacement percentages of recycled coarse aggregates (RCAs) (i.e. 0, 30, 50, 70, 100%) was investigated experimentally at elevated temperatures. The residual compressive strength as well as the residual flexural strength of the RAC following elevated temperatures was studied and evaluated. A relationship between the residual compressive flexural strength of RAC and the elevated temperature was proposed. Furthermore, the relationship between the residual flexural strength and the compressive strength of the RAC was compared and analysed. It was found that both the residual compressive strength and the residual flexural strength of the RAC decrease with a rise in temperature, and the effect of the RCAs replacement percentage on the residual flexural strength and the residual compressive strength of RAC was found to be obvious.

Concrete: treatment of types and properties in fib Model Code 2010

Abstract: Section 5.1 “Concrete” of the fib Model Code for Concrete Structures 2010 contains basic definitions and well-established constitutive relations for structural concrete. However, it also presents various new approaches and updated models compared with the earlier CEB-FIP Model Code 1990. This is particularly true for the strength, stress and strain characteristics of structural concrete, for creep and shrinkage and for sophisticated durability-related processes. The validity of the models has been extended to several types of concrete such as high strength concrete, self-compacting concrete and lightweight aggregate concrete. The durability-related models are either suitable for facilitating a full probabilistic service life design or for applying simpler code-type approaches. This article provides a concise and selective overview of some of those models. Background information is summarized and there is a focus on improvements achieved by the updated models. In addition, some simple design aids are given to allow pre-design, for example

Design for service life: implementation of fib Model Code 2010 rules in the operational code ISO 16204

Abstract: CEB/FIP Model Code 1990 (MC-1990) did represent the technology and focus some 20 years ago. However, it soon became evident that the document had some notable lacunas. In 1995 the general assemblies of the two organizations endorsed CEB/FIP bulletin No. 228, extensions to MC 1990 for high-strength concrete, and in 2000 a similar extension to MC 1990 for lightweight aggregate concrete as bulletin No. 4. The fib approved bulletin No. 34 Model Code for Service Life Design (MC SLD) in 2006. All these three additions have since matured and are now incorporated in the new fib Model Code for Concrete Structures 2010 (MC-2010). The main purpose of an fib Model Code is to act as a model for operational standards. The obvious counterpart for a body such as fib operating worldwide is ISO. The initiative taken by MC SLD has therefore further matured in ISO TC-71/SC-3/WG-4 and it was accepted as ISO 16204 “Durability – Service Life Design of Concrete Structures” during the summer of 2012. According to the obligations given in the WTO Agreement on Technical Barriers to Trade, it is hoped that these principles will be further implemented in national and regional standards. This article describes the need for a transparent methodology when dealing with service life design, and the process – originating from a group of enthusiasts one decade ago – through fib and finally reaching international consensus in ISO.

Development of a mix design methodology for high-performance geopolymer mortars

Abstract: This paper presents the development of a mix design method for geopolymer mortar mixes with a principal focus on compressive strength. The method is based on the hypothesis that the overall strength of geopolymer mortar depends on the strength of the geopolymer binder materials, the packing density of the aggregates in the mortar and the amount of geopolymer binder. First, the strength of the binder was evaluated experimentally. Fly ash obtained from two different sources and ground granulated blast-furnace slag were used as the main ingredients in the production of the geopolymer binder. Correlation between the packing density of the aggregates in the mortar and their influence on the binder demand and compressive strength were investigated. Finally, a conceptual framework for a geopolymer mortar mix design approach is proposed. In addition to describing the mortar mix design methodology in a rational way, it is hypothesized that the approach can also be extended to the development of a geopolymer concrete mix.

Design for punching of prestressed concrete slabs

Abstract: Prestressing in flat slabs helps in controlling deformations and cracking under service loads and allows reducing the required slab thickness, leading thus to more slender structures and being therefore an economic solution for long spans. However, as a consequence of the limited thickness of these members, punching is typically governing at ultimate limit state. Investigations on the topic of punching shear strength have shown that the presence of prestress in flat slabs has a number of potential beneficial effects, namely the vertical component (force) carried by inclined tendons, the in-plane compression stresses and the bending moments developed near the supported region. The approach provided by codes of practice for punching design in presence of prestressing may however differ significantly. Some neglect the influence of the introduced bending moments due to prestressing and the sections at which deviation forces of the tendons are considered may be located at different distances from the edge of the supported region. In this paper, the influence of prestressing on the punching shear strength of members without shear reinforcement is investigated by using the fundamentals of the Critical Shear Crack Theory. On that basis, and accounting also for 65 tests available in the scientific literature, the suitability and accuracy of a number of design codes, such as Model Code 2010, Eurocode 2 and ACI 318-11, is investigated and compared.

Crack healing of cementitious materials using shrinkable polymer tendons

Abstract: This paper presents an experimental study on autogenous healing in a recently developed shrinkable polymer cementitious material system. Small-scale hollow prismatic mortar beams with unbonded shrinkable polymer tendons were loaded until a crack of predefined width had formed. The specimens were then subjected to a range of combined heating/curing regimes to activate the shrinkable polymer and to promote autogenous healing of the cementitious material prior to reloading to failure. Upon reloading, specimens loaded with the shrinkable polymer tendons remaining after heating and curing achieved between 120 and 195% of their initial peak loads, whereas specimens tested with the shrinkable polymer tendons removed after the heating and curing stages recovered on average 80% of their initial peak loads. The overall conclusion was that, provided a supply of water is present and sufficient heat is applied for polymer activation, the material system is effective at not only closing cracks and applying a prestress, but also at enhancing the self-healing of cracks in cementitious materials.

Shear resistance evaluation of prestressed concrete bridge beams: fib Model Code 2010 guidelines for level IV approximations

Abstract: The design shear resistance of an existing structure can be evaluated with analytical design procedures and numerical procedures provided by non-linear finite element analyses. The new fib Model Code 2010 proposes different calculation methods that fall into four different levels of approximation. As the level of approximation rises, so the complexity and the accuracy of the calculated shear resistance increases. Non-linear finite element analyses belong to the highest level of approximation, but although they are more and more becoming a customary tool in the daily design process, building codes do not provide guidance on how to perform these analyses. This paper describes non-linear finite element analyses performed on prestressed beams, which underwent shear failure during experimental loading, in order to assess and criticize the finite element approaches. The aim of this work is to propose guidelines for numerical simulations in order to reduce model and user factors. The results obtained from the non-linear finite element analyses have been compared with the analytical results using different levels of approximation. The design shear resistance obtained with the highest level of approximation, level IV, derived from non-linear finite element analyses, turned out to be higher than the design shear resistance obtained with analytical procedures (levels I/II/III).

fib Model Code for Concrete Structures 2010: mastering challenges and encountering new ones

Abstract: The Model Code for Concrete Structures 2010 is a recommendation for the design of structural concrete, written with the intention of giving guidance for future codes. As such, the results of the newest research and development work are used to generate recommendations for structural concrete at the level of the latest state of the art. While carrying out this exercise, areas are inevitably found where information is insufficient, thus inviting further study. This paper begins with a brief introduction to the new expertise and ideas implemented in fib Model Code 2010, followed by a treatment of areas where knowledge appeared to be insufficient or even lacking and where further research might be useful.

Compressive, tensile and bending basic creep behaviours related to the same concrete

Abstract: This paper concerns the relationship between compressive, tensile and flexural creep behaviours related to the same concrete. Experimental tests and a numerical simulation are performed in the scope of this work. The main conclusion of this study is that due to the strong scale effect related to the tensile and bending creep behaviours of a “standard” concrete mix design, it is not possible to simulate numerically (with classical Kelvin-Voigt chains) the bending creep behaviour of that type of concrete knowing the compressive and tensile creep behaviours obtained using the specimen geometries normally used in laboratories.

Concrete shells - towards efficient structures: construction of an ellipsoidal concrete shell in Switzerland

Abstract: Concrete shells have been widely used in the past as economical and suitable solutions for a number of structures such as roofs, silos, cooling towers or offshore platforms. Taking advantage of their single or double curvature, bending moments and shear forces are limited and the structure develops mostly membrane (in-plane) forces, allowing to span large distances with limited thicknesses (typically of only some centimetres). In the last decades, the advances on numerical modelling, formwork erection and concrete technology open a new set of possibilities for use of concrete shells. In this paper, the design and construction of a shell with the form of an ellipsoid (93×52×22 m) and with variable thickness between 100 and 120 millimetres is described. The shell was built using sprayed concrete and also ordinary concrete in some regions. A number of tailored solutions were also adopted such as post-tensioning, addition of fibres and shear studs to ensure satisfactory performance both at serviceability and ultimate limit states.

Performance- and displacement-based seismic design and assessment of concrete structures in fib Model Code 2010

Abstract: The scope of fib Model Code for Concrete Structures 2010 includes the fully fledged performance- and displacement-based seismic design of new structures and assessment of existing ones. This part of fib Model Code 2010 covers buildings, bridges or similar concrete structures and aims to provide well-defined performance levels for specific seismic hazard levels. Detailing of members for ductility is not based on opaque prescriptions, as in current codes, but on transparent, explicit verification of inelastic deformation demands against capacity limits. The reference analysis method is non-linear dynamic, but under certain conditions inelastic deformation demands may be estimated from linear analysis and the 5%-damped elastic response spectrum; in that case force demands on force-controlled, brittle failure modes are estimated from the plastic mechanism through equilibrium. In order to predict the seismic deformation demands with some confidence, the analysis should use realistic values for the member secant stiffness up to the yield point. The paper explains the background to the expressions given for this property in fib Model Code 2010 as well as of those for the deformation limits used when verifying seismic deformation demands. The modifications to the shear resistance approach of fib Model Code 2010, which takes cyclic loading into account, are also explained and justified.

Reliability in the performance-based concept of fib Model Code 2010

Abstract: The design philosophy of the new fib Model Code for Concrete Structures 2010 represents the state of the art with regard to performance-based approach to the design and assessment of concrete structures. Given the random nature of quantities determining structural behaviour, the assessment of structural performance cannot be well established by deterministic methods, instead requires a probabilistic approach. The performance-based approach is introduced in Part I of fib Model Code 2010 by applying the concept of performance requirements and reliability management during service life. Correct understanding of the reliability concept of fib Model Code 2010 is a basic prerequisite for applying its design philosophy in an appropriate manner. Therefore, the main objective of this paper is to explain some decidedly non-trivial issues related to safety and reliability management aspects. In this context, this paper indicates how this general philosophy in fib Model Code 2010 is further developed into a set of operational rules for the design and assessment of concrete structures. Copyright © 2013 Ernst & Sohn Verlag fur Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

Restoring force model for steel reinforced concrete columns with high steel ratio

Abstract: Key points on the load-displacement skeleton curves of steel reinforced concrete (SRC) composite cantilever columns are established based on the experimental results in the literature. Influencing parameters such as the ratio of encased steel, the transverse reinforcement characteristic value, the size of the cross-section, etc. are considered in the study. A degrading trilinear restoring force model applicable to SRC columns with an encased steel ratio of 10–20% is suggested. The main results show that the influencing parameters have different effects on the values of key points. The proposed restoring force model, which considers various influencing parameters, can predict the test results more accurately and can be applied to the non-linear dynamic analysis of structures with such SRC column types.

3D numerical modelling of concrete structural element reinforced with ribbed flat steel rebars

Abstract: Construction company MATIERE has developed a new type of reinforcement based on ribbed flat steel in recent years The objective of the present work is to obtain information about the bending behaviour of RC structural elements reinforced with these ribbed flat steel bars and also about the cracking process they induce (number of cracks and crack opening), especially at the serviceability limit state The structural element chosen for this work is a small slab-beam (330 × 15 × 80 cm) subjected to three-point central bending Concurrent with this experimental study, a 3D finite element model was developed at IFSTTAR to consolidate the experimental results It appears that the numerical modelling strategy chosen in this work is relevant for analysing both the bending behaviour of an RC structural element and its cracking process

Fibre-reinforced polymer reinforcement enters fib Model Code 2010

Abstract: Most applications of fibre-reinforced polymers (FRP) deal with externally bonded reinforcement as a means to repair and strengthen reinforced concrete (RC) structures or to retrofit RC structures in seismic regions. As internal reinforcement, FRP rebars or (more rarely) prestressing elements, are used in special projects, combining material strength and durability characteristics. Over the last years several national and international design guidelines have become available, specifically for the design and application of FRP strengthened or reinforced concrete structures. These efforts demonstrate clearly the interest in FRP as a novel reinforcing material for concrete construction. Hence, the time was there to introduce FRP reinforcement also in the new Model Code 2010 (MC2010). Main contributions to MC2010 relate to chapters 5.5 “Non-metallic reinforcement” and 6.2 “Bond of non-metallic reinforcement”. The material presented in these two chapters is further elaborated in this contribution.

Finite element study of bond strength between concrete and reinforcement under uneven confinement condition

Abstract: It is well known that many factors affect the bond behaviour between concrete and steel reinforcement. Most of them characterize the degree of confinement, which is necessary for an appropriate determination of the bond strength and bond-slip relationship according to most extended models. Nevertheless, there are situations where the confinement is not known in advance or it can hardly be defined. That is, for example, the case of reinforced concrete under transverse pressure or tension. Under such conditions the confinement varies both longitudinally and circumferentially. This paper presents a 3D finite element approach to study the bond capacity of cases where the bond strength of the specimen is the result of the different contribution provided along the steel-concrete interface and cannot be described uniquely. A surface-to-surface contact model with frictional behaviour and debonding is used to study the influence of the local stress state at the interface. The model is able to reproduce the different failure modes (splitting or pull-out) obtained in experiments and provide a rational explanation of the influence of transverse tension.

Two-layer concrete bridge beams as composite elements

Abstract: Two-layer high-performance RC beams can be used in the traffic (longitudinal) direction of long-span bridges. The beam section consists of two parts: a bottom part made of normal-strength concrete (NSC), which has a U-type section, and an upper one made of steel fibre-reinforced high-strength concrete (HSC), which has a rectangular section. Using two-layer beams is logical because as the beam span becomes longer and the service load increases, so a higher concrete strength is required in the beam's compression zone to withstand rather large bending moments. As concrete in the tension zone of the section contributes little to the beam's loadbearing capacity, this zone is made of NSC. This is an important economical advantage of two-layer beams. Steel fibres are only used in the compression zone made of HSC. The addition of steel fibres results in a higher ductility for the HSC layer, allowing proper design of two-layer bridge beams for dynamic loads. The design parameters for such beams include the HSC class, where the HSC layer depth is known and is equal to the depth of the rectangular section (above the U-section). The bottom NSC part (U-section) of a long-span beam is usually prestressed. Taking into account this circumstance, the total beam section depth should be checked for the serviceability limit state.

Bilateral loading experiment on and analysis of concrete piers using mortar-jointed ultra-high-strength fibre-reinforced concrete precast formwork

Abstract: The seismic performance of a reinforced concrete pier structure depends greatly on the plastic hinge region at the base of the pier. To improve the plastic hinge region of the structure, we propose using ultra-high-strength fibre-reinforced concrete (UFC) formwork for the concrete cover at the base of the pier. This study describes the structural characteristics of a pier with a base cast using mortar-jointed UFC formwork. To investigate the structural characteristics, a bilateral circular cyclic loading experiment and a simulation analysis using a fibre model were conducted. The results show that the flexural strength remained stable under a large rotation angle. Even at the rotation angle at the ultimate limit state, crushing failure of the mortar did not occur and the ultimate limit state did not result from mortar damage, because the tensile strength of mortar-filled joints is close to zero, and thus excellent flexural crack distribution performance can be achieved.

Sound engineering through conceptual design according to the fib Model Code 2010

Abstract: Conceptual design is the approach that creates an idea in order to find a solution to a new proposal for a structure or solve a detail in a specific structure. It is a personal approach that is learned over time and with experience. It is not normally dealt with at university, but is vitally important for producing sound structures. The fib Model Code for Concrete Structures 2010 introduced this concept in the first section of chapter 7 “Design”. The content of that section explains the general approach to developing conceptual design. This paper will show different examples of conceptual design following the general guidelines stated in the Model Code.