For the assessment of concrete structures in the new fib Model Code 2020 (fib MC 2020), three categories are distinguished: (1) the residual capacity of existing structures without damage, (2) the residual capacity of structures suffering deterioration, and (3) the residual capacity of structures with noncompliant details. In the accompanying paper by Walraven and Dieteren in this volume, the backgrounds for this subdivision have been explained, and indications for the assessment of existing concrete structures in those categories have been given. In the actual paper, examples are given how to perform an assessment of concrete structures for the categories mentioned above. These examples have contributed to the formulations of the recommendations proposed for fib MC 2020.

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Existing structures often no longer meet the demands formulated in contemporary design codes with respect to structural safety and serviceability. This occurs, for instance, if the loads on existing structures, like traffic loads on bridges, are larger than assumed in the original design. A second potential reason is that structures are subject to deterioration, like alkali–silica reaction within the concrete or corrosion of the reinforcement due to chloride attack or carbonation. A third possible reason is that in recent codes, additional criteria have been introduced based on new theories and/or negative experiences with older structures. The new fib Model Code for Concrete Structures 2020 will be valid both for the design of new structures and the assessment of existing structures. This paper shows how the design and assessment of concrete structures are integrated into this new code concept.

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A large number of bridges in the Netherlands have transversely post tensioned deck slabs cast in-situ between flanges of precast girders and were found to be critical in shear when evaluated by Eurocode 2. To investigate the bearing (punching shear) capacity of such bridges, a 1:2 scale bridge model was constructed in the laboratory and static tests were performed by varying the transverse prestressing level (TPL). A 3D solid, 1:2 scale model of the real bridge, similar to the experimental model, was developed in the finite element software DIANA and several nonlinear analyses were carried out. It was observed that the experimental and numerical ultimate load carrying capacity was much higher than predicted by the governing codes due to lack of consideration of compressive membrane action (CMA). In order to incorporate CMA in the Model Code 2010 (fib 2012) punching shear provisions for prestressed slabs, numerical and theoretical approaches were combined. As a result, sufficient factor of safety was observed when the real bridge design capacity was compared with the design wheel load of Eurocode 1. It was concluded that the existing bridges still had sufficient residual bearing capacity with no problems of serviceability and structural safety.

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An investigation is carried out into the applicability of self-compacting high-performance fiber concrete (HPFC) in foundations. A concrete mixture has been designed with a concrete cube strength of about 110 MPa. The concrete contains 60 kg/m³ steel fibers. The properties of the HPFC developed are very suitable for structural applications, especially because the post-cracking tensile strength, provided by the fibers, is higher than the axial tensile strength of the concrete so that hardening in tension occurs after crack formation, often characterized by multiple cracking. This not only results in a high bearing capacity but as well in substantial durability. As a potential application foundation elements are considered. Experiments have been carried out to determine the pre- and post-cracking strength properties, the shear resistance of short beams with loads near to the supports, the anchorage length of reinforcing bars, and the shear capacity of pile caps. The results of the tests are used for verification of the applicability of the general design rules for fiber concrete, as found in the fib Model Code 2010, to the HPFC developed. The HPFC developed is characterized by high strength and ductility, is durable and self-compacting. The research program showed that the design of structures with the HPFC considered can be based on existing design rules with some extensions.

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An investigation was executed into the applicability of self-compacting high performance fiber concrete in foundations. The applied concrete has a concrete cube strength of about 110 MPa and contains 60 kg/m³ hooked-end steel fibers (L_F = 30 mm, D_F = 0.38 mm). This publication consists of two parts: (1) Experimental assessment and verification of design rules and (2) assessment of fiber orientation and distribution. In the first part, experiments are described which were carried out to determine the pre- and post-cracking strength properties, the shear resistance of short beams, the anchorage length of rebars, and the shear capacity of foundation slabs supported on piles. The test results were used for a verification and extension of design rules for fiber reinforced concrete (FRC) found in the fib Model Code 2010. The application of the FRC developed can lead to substantial savings in concrete and reinforcing steel. In the present second part, cross-sections of two slabs and a beam of this testing series have been analyzed with regard to fiber orientation and distribution. An image analysis, executed on 111 concrete areas, indicates that a preferred fiber orientation could not be identified throughout the assessed elements. The data supports the conclusion of a good fiber distribution as well; a strong correlation was obtained for the data set of full images between measured and theoretical relation of fiber orientation and fiber density.@en

Service life of concrete infrastructure is severely compromised because of chloride-induced corrosion and measuring the chloride content is crucial to determine the remaining service life. DuraCrete provides a chloride ingress model based on Fick’s 2nd law. Although the diffusion coefficient is modelled as a time-dependent variable, the DuraCrete solution averages it to a constant value. This simplification leads to inaccurate estimation of the chloride content. A new analytical solution that addresses the underlying mathematical discrepancy has been proposed. However, the time-dependent diffusion coefficient is still based on an empirical factor. In this study, a real-time durability monitoring system has been developed using remotely operated resistivity sensors. Such a system is able to monitor the time dependent diffusion coefficient without the need to incorporate empirical factors. Additionally, a numerical technique to find an approximation of the proposed improved analytical solution is presented using real-time resistivity measurements from laboratory and real structures. The results show that the discrete sensor data measurements over time provide a good approximation of the proposed analytical solution. The system developed in this study is used as a data-driven input parameter to supplement the existing chloride models.@en

A high performance fibre concrete with a cylinder strength of about 80 MPa was developed and tested both under laboratory and site conditions. The post-cracking properties were determined in a series of three-point notched beam tests according to EN 14651. Subsequently, double punch tests on 150 mm cubes were carried out to verify whether such tests could be used for control of the properties of the concrete cast at site. It was shown that the same relations can be obtained by using simple conversion factors. Additionally, it was studied if it is possible to test round panels according to ASTM C1550-02 to verify the properties of the fibre concrete. Also here, it turned out that it is possible to derive simple conversion factors to relate the results of the round panel tests to those of the notched beam tests. The findings provide useful information for a practical quality assessment with regard to the execution of fibre reinforced concrete structures.@en

The Netherlands has a large number of thin, transversely prestressed concrete bridge decks, cast in-situ between flanges of prestressed concrete girders dating back to the 1960s and 1970s. These bridges are critical in shear when analyzed using EN 1992-1-1:2005; however, in reality, they show no significant signs of distress, possibly because of residual bearing (punching shear) capacity arising from compressive membrane action. Since these bridges are old, it is an astute approach to check whether they can be used for a few more decades, provided they are safe and reliable against modern traffic loads. The results could then be applied to a wider range of structures, especially in developing countries facing economic constraints. A prototype bridge was selected and experimental, numerical and theoretical approaches were used to investigate its bearing capacity. Respective coefficients of variation of 11% and 9% were obtained when the experimental and the finite element analysis punching loads were compared with the theoretical results. This led to the conclusion that the existing transversely prestressed concrete bridge decks still have sufficient bearing capacity and considerable cost savings can be made if compressive membrane action is considered in the analysis.@en

An analytical model was developed to predict the tensile behaviour of a corroded steel bar. The model was established based on cross sectional analysis and was validated using experimental data for slotted steel bars and electrochemically corroded steel bars. The model was further used to predict the tensile behaviour of a corroded steel bar. The corrosion mode of the steel bar was supposed to be pitting corrosion and the distribution of the corroded section was supposed to follow a lognormal distribution. A power law between the parameters of the lognormal distribution and the average corrosion rate of the steel bar was used to predict the statistical distribution of the cross section area of a corroded steel bar. Based on these assumptions, the yield strength, ultimate strength and ductility of a corroded steel bar were predicted with different corrosion rates. The predicted behaviours are compared to collected experimental results from various sources. It is found that the numerical results of yield strength and ultimate strength agree well with the collected experimental results. The model slightly underestimates the ductility of the corroded steel bars. The result of the model would be helpful for the prediction of the tensile behaviour of reinforced concrete member subjected to chloride induced corrosion.

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Discrete fibres have been added to concrete to improve its performance. Besides the post-cracking strength also other performance aspects can be enhanced. Fibre Reinforced SCC (FRSCC) and less flowable fibre concretes form the general group of fibre concrete. FRSCC distinguishes itself from conventional vibrated concrete in three main dimensions: strength, fibre content and flowability, the last two aspects being a prerequisite for FRSCC.
This chapter starts with a definition of FRSCC, which is a broad group of different types of fibre-containing mixtures. After consideration of their influence on the characteristics in the fresh state by adequate mix design and appropriate execution FRSCC can be applied to produce outstanding concrete structures. The discussion includes a broader view on execution, potential applications, material properties, structural behaviour and durability performance.@en

Most of the Dutch bridges were built around middle of the last century and it is vital for designers to find out if these bridges can still be considered safe for the traffic of modern times. The capacity in shear is especially critical as it was not considered in design recommendations before 1976. Therefore, experiments on a 1:2 scale model of a transversely prestressed concrete bridge deck cast between concrete girders were carried out to investigate the bearing (punching shear) capacity. The scale was selected based on the space available in the laboratory and the expected failure loads that would have to be applied. Also, a three-dimensional, solid, nonlinear finite element model was developed in the Finite Element Analysis software package TNO DIANA to study the structural behavior of deck slabs and is the focus of this paper. The results of the experimental and numerical analyses leads to the conclusion that existing bridges still have significant residual strength due to the presence of transverse prestressing and the membrane forces, and nonlinear finite element models can predict the load carrying capacity quite accurately.@en

Most Dutch bridges were constructed around the middle of the twentieth century and considering the fact that traffic has increased exponentially since, it is important to find out if these bridges are still safe for use. Experiments on a 1:2 scale were carried out in the laboratory of the Delft University of Technology to investigate the bearing capacity of transversely prestressed concrete bridge decks subjected to concentrated wheel loads. All the tests showed failure in punching shear at loads much higher than expected from the current codes. This paper presents the results of the experimental parametric study including the effect of the transverse prestressing level (TPL), location of the load, number of loads, size of the loading area (wheel print/loading plate), and influence of previous damage to the deck slab panel, on the bearing capacity.@en

Traditionally industrial foundation elements are manufactured using reinforced concrete. For these elements a lot of reinforcement is necessary as these foundation elements are usually loaded by the combination of large shear forces and bending moments. Moreover, splitting and spalling reinforcement is crucial to transfer the internal stresses as very large concentrated loads are present. Due to the combination of the different reinforcement requirements the production process of these foundation elements is complex and requires a lot of time and labor. Simplification of the production process for foundation works will result in a more effective execution and de-risking of the construction schedule. Therefore, the idea was born to use self-compacting high performance fibre reinforced concrete (SCHPFRC) reinforced with traditional steel only for bending. The steel fibres will take over the functions of all other types of traditional reinforcement, i.e. shear reinforcement, transverse reinforcement, skin reinforcement, minimum reinforcement and additional reinforcement required to control crack widths. To test the capabilities and the behavior under large concentrated loads of this SCHPFRC, two extensive test series are performed at the Magnel laboratory for Concrete Research at Ghent University. The first test series consisted of the development of a mix design for the SCHSFRC, 12 CMOD tests on SCHPFRC prisms, 6 full scale loading tests on SCHPFRC beams subjected to a concentrated load to test the shear resistance and 6 full scale loading tests on SCHPFRC slabs. In the second test series additional tests are performed to confirm the findings from the first test series, to investigate the influence of the steel fibers on the anchorage length of the steel rebars and to investigate the punching resistance and the three-dimensional load-transfer of the foundation elements in more detail. Based on the outcome of the experiments the applicability of SCHPFRC is evaluated and practical design guidelines are derived.@en

Traditionally industrial foundation elements are manufactured using reinforced concrete. For these elements a lot of reinforcement is necessary as these foundation elements are usually loaded by the combination of large shear forces and bending moments. Moreover, splitting and spalling reinforcement is crucial to transfer the internal stresses as very large concentrated loads are present. Due to the combination of the different reinforcement requirements the production process of these foundation elements is complex and requires a lot of time and labor. Simplification of the production process for foundation works will result in a more effective execution and de-risking of the construction schedule. Therefore, the idea was born to use self-compacting high performance fibre reinforced concrete (SCHPFRC) reinforced with traditional steel only for bending. The steel fibres will take over the functions of all other types of traditional reinforcement, i.e. shear reinforcement, transverse reinforcement, skin reinforcement, minimum reinforcement and additional reinforcement required to control crack widths. To test the capabilities and the behavior under large concentrated loads of this SCHPFRC, two extensive test series are performed at the Magnel laboratory for Concrete Research at Ghent University. The first test series consisted of the development of a mix design for the SCHSFRC, 12 CMOD tests on SCHPFRC prisms, 6 full scale loading tests on SCHPFRC beams subjected to a concentrated load to test the shear resistance and 6 full scale loading tests on SCHPFRC slabs. In the second test series additional tests are performed to confirm the findings from the first test series, to investigate the influence of the steel fibers on the anchorage length of the steel rebars and to investigate the punching resistance and the three-dimensional load-transfer of the foundation elements in more detail. Based on the outcome of the experiments the applicability of SCHPFRC is evaluated and practical design guidelines are derived.@en

Discussions have been underway in fib (Fédération Internationale du Béton) about advancing the fib Model Code for concrete structures. These include the fib international workshop in The Hague (June 2015), the fib MC2020 Core Group meeting in Madrid (December 2015), and a series of follow-up worldwide consultations on the fib ambition regarding the new developments in structural codes, including the special session on Model Code in the fib Symposium in Maastricht (June 2017). This paper discusses the main aspects of the development of fib MC2020, which is envisaged as a single-merged general code fully integrating the provisions for the design of new concrete structures with matters relating to existing concrete structure. It needs to deal effectively with both the design of structures and all the activities associated with the through-life management of existing concrete structures, including matters such as their in-service assessment and interventions upon them. To that end, MC2020 will take sustainability as a fundamental requirement, based upon a holistic treatment of societal needs and impacts, life-cycle cost and environmental impacts. This paper discusses the main aspects of the development of fib MC2020. As part of this, the envisaged contribution of fib T10.1: Model Code 2020 is reviewed. However, recognizing the overall ambition of the fib MC2020 project, it is clear that all fib commissions, along with other bodies able to make contributions on relevant topics, will need to work together to assemble the breadth of knowledge and expertise which will be required for the fib MC2020 project.

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An experimental program was carried out to investigate the shear capacity of High-Performance Fiber-Reinforced Concrete (HPFRC) I-beams. The main parameters were assigned as the fiber content and presence of shear reinforcement. To study the effect of these main parameters on the shear capacity, testing of six I-beams and other control specimens was conducted. It can be observed from the results of the experimental study that the presence of fibers and shear reinforcement significantly improves the ultimate capacity and structural behavior of HPFRC members. Finally, the experimental results are discussed, and the shear capacity of HPFRC can be estimated by extending the code provisions stated in AFGC-Sétra 2013.

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This paper presents a new concept to evaluate the shear capacity of reinforced concrete beams without shear reinforcement having flexural shear failure. Based on experimental observations, it is proposed that the opening of the critical inclined crack can be considered as the lower bound for the shear capacity of a structural member. It is also proposed that the unstable opening of the critical inclined crack is triggered when the shear displacement in an existing flexural crack reaches a critical value Δ_cr. Thus the critical shear displacement is used as a failure criterion, and based on that a new shear evaluation method is proposed. The method shows agreeable accuracy when compared with test results in literature.

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The paper investigates the effect of various geometrical and material parameters on the bearing (punching shear) capacity of transversely prestressed concrete deck slabs by numerical methods. Experiments on a 1:2 scale model of such a bridge were carried out in the laboratory and a 3D nonlinear finite element (FE) model was developed in the finite element analysis software package TNO DIANA (2012) to study the structural behavior in punching shear. A comparison of the experimental and numerical ultimate loads show that the non-linear FE models can predict the load carrying capacity quite accurately with a standard deviation of 0.1 and the coefficient of variation of only 10%. The effect of varying the transverse prestressing level, the presence and size of the ducts, size of the loading plate and the concrete class is also described as part of the parametric study. It was observed that sufficient saving in cost could be made if calibrated numerical models are employed to investigate existing structures rather than doing expensive experimental studies.@en

Building codes are a necessity. The first codes were simple, easy to apply for designers and offered residual capacity for a low price. Meanwhile codes are significantly more complicated. This is partly due to the development of more scientific background knowledge and partly to the upcoming significance of regulations in society. National codes disappear and are replaced by continental codes like the Eurocodes, the ACI Code and the Asian Code, with some space for national exceptions, to be negotiated. On the one hand a nostalgic desire develops to go back to more simple and transparent codes, but on the other hand there is a need for more advanced models, in order to deal with the existing infrastructure in an economic way. There is as well a remarkable development of high performance materials, improved calculation and measuring techniques, which ask for modernized codes. Finally new targets are formulated, like life cycle design and sustainable solutions. The development of user-friendly, future-oriented and innovation-stimulating codes is a large challenge.

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