This paper focuses on the reliability of the design approach proposed in the fib Model Code for Concrete Structures 2010 for estimating the ultimate capacity of fibre-reinforced concrete (FRC) elevated slabs on the basis of different tests for material characterization. The fracture properties of the material are determined through three-point bending tests on notched beams and through double edge wedge splitting (DEWS) tests carried out on cylinders cored in the full-size test structure. As a case study, an FRC elevated flat slab 0.2 m thick is considered which consists of nine bays (panels) measuring 6 × 6 m (overall size 18.3 × 18.3 m) and is supported by 16 circular concrete columns. The ultimate bearing capacity of the slab determined experimentally is compared with the design value predicted by means of a procedure based on limit analysis following fib Model Code 2010. The results show that the method proposed in fib Model Code 2010 using the characteristic values and the classification is reliable. Even if the tests are affected by a significant standard deviation and the two experimental campaigns with three-point bending tests give a significant difference between class “5c” and class “3e”, the structural test results in a loadbearing capacity that is always larger than the predicted one, which considers a safety coefficient for the material &ggr;F = 1.5.

Punching shear is usually the governing failure criterion when selecting the depth of reinforced concrete footings. Despite the fact that large experimental programmes aimed at the punching strength of slender flat slabs have been performed in the past, only a few experimental campaigns on full-scale compact reinforced concrete footings can be found in the literature. This paper presents the results of an experimental programme including eight reinforced concrete footings with a nominal thickness of 550 mm. These experiments investigated the influence of column size, member slenderness and the presence of compression and shear reinforcement. The tests were performed using an innovative test setup to ensure a uniform soil pressure. The experimental results show that slenderness influences the punching shear strength as well as the effectiveness of the shear reinforcement. The experiments also show that an important interaction occurs between bending and shear for high levels of shear force near the column (the typical case of compact footings or members with large amounts of shear reinforcement). Different continuous measurements recorded during the experimental tests allow a complete description of the kinematics and strains at failure. On that basis, experimental evidence is obtained showing that crushing of the concrete struts near the column is the phenomenon that triggers the punching failure of compact footings.

A significant amount of research on self-consolidating concrete (SCC) technology has been devoted to evaluating the suitability of the material for its use in structural applications. However, more research is required to confirm the adequacy of SCC structural members for resisting gravity and seismic loads. This study focuses on the experimental investigation of the seismic performance of interior reinforced concrete beam-column connections with SCC. Four beam-column connection specimens, three with SCC and one with normally vibrated concrete (NC), were designed for this experimental study. Factors such as concrete type (SCC or NC) and axial load ratio (0, 7.5 and 15 % of column section capacity) were assumed to be the variables in designing the specimens. Beam-column connections made with NC and SCC were studied and compared under reversed cyclic loading. The performance of SCC specimens is comparable with NC specimens in terms of strength, displacement and ductility, but SCC specimens show lower energy dissipation capacity.

Damage processes in fatigue loaded concrete structures depend on the number and amplitude of the load cycles applied. Damage evolution is linked to a reduction in concrete stiffness, and it is thought that this reduction causes stress redistributions at component level which have a favourable impact on the service life of a structure. Until now, the stiffness reduction and stress redistribution have never been successfully measured in laboratory tests or in situ. It is only known that the real service life is longer than the calculated one and that indicators of stiffness reduction, such as component deflection, increase with the number of load cycles applied.
Ultrasonic measurement techniques are considered to be well suited to detecting degradation processes caused by cyclic loading. It is expected that the stiffness reduction in fatigue loaded concrete structures can be recorded reliably with ultrasonic pulse velocity measurements. In the light of this, fatigue tests were performed on small-scale concrete specimens. The aims of the tests were to understand the correlation between the observed stiffness degradation of the specimens and the results of ultrasonic pulse velocity measurements and to estimate the potential for using ultrasonic pulse velocity measurements in continuous structural health monitoring.

The fatigue behaviour of concrete is gaining new relevance against the backdrop of continuous developments in concrete construction. Modern types of concrete are achieving ever higher strengths; hence, concrete structures are becoming increasingly attractive for new fields of application such as onshore and offshore wind turbines. The fatigue of concrete has a special relevance for these cyclically loaded structures and knowledge of the number of cycles to failure is no longer sufficient for their design. There are further questions concerning strain and stiffness development and the combination of fatigue loading and maritime environmental conditions which have been investigated with new testing methods at Leibniz Universität Hannover within the scope of the “ProBeton” research project. The first results of this project, which is supported by the Federal Ministry for Economic Affairs and Energy, are presented here.

Today, the development of innovative shear connectors for steel-concrete composites is accompanied by a large number of experimental investigations, which are obligatory when proposing suitable design formula and carving out their limitations of use. Using the example of the so-called pin connector, the present paper illustrates to what extent validated finite element models of novel shear connectors can be used to replace expensive and time-consuming shear tests and how these finite element models can support the deduction of design concepts. The pin connector considered was developed for connecting steel sections to very slender high-strength concrete slabs in which conventional shear connectors such as headed studs cannot be used due to the limited embedment depth. In order to clarify the shear behaviour and load-carrying mechanisms of these novel connectors, non-linear finite element models were set up using the commercial FE software Abaqus. Subsequently, the FE models were used to perform systematic parametric studies. This paper describes the numerical results and also explains the stepwise development of an entire engineering model for determining the longitudinal shear capacity of small-scale pin connectors, including all the necessary limitations of use. The proposed modelling strategy and the methodology for the deduction of design rules can be transferred and assigned to other types of shear connectors.

The prediction of the seismic behaviour of reinforced concrete elements using numerical models has become a field of growing interest in recent years due to the importance of the effects induced by seismic loads applied to reinforced concrete structures. The simulation of the hysteretic behaviour of the plastic hinges generated in the structure when the seismic load acts requires the use of models that are able to describe the sectional behaviour of structural members. Thus, the main objective of the present paper is the adjustment of several empirical expressions that reduce the computational time needed to simulate the yield and ultimate behaviour of a given reinforced concrete rectangular section under either monotonic or cyclic loading. The expressions are calibrated with a selection of tests, taken from a published database of more than 1000 tests, according to the criterion that the selected specimens comply with the seismic construction requirements of the main international building codes (EC-2, EC-8 and ACI-318). Owing to their robustness and the acceptable computation time for low-dimensional problems, genetic algorithms are used for this calibration. The equations proposed can be employed by structural engineers for the design and analysis of actual structural elements used in ordinary reinforced concrete buildings located in seismic areas, and provide more accurate results than other expressions.

The load-carrying capacity and deformability of concrete columns can be substantially enhanced by confining with post-tensioned steel straps. As interest in high-strength concrete (HSC) grows among structural engineers and researchers due to its superior performance, this confining technique is being extended to HSC columns with the hope that it can eliminate the undesired properties of HSC, especially its brittleness. However, experimental studies involving confined HSC columns subjected to eccentric loads are comparatively limited. It can be seen from past research that most studies of external confinement were conducted on small-scale normal-strength concrete (NSC) cylinders subjected to concentric loads. Since most columns are subjected to eccentric loads in reality, the scarcity of test data has prevented the potential of this confining technique from being fully exploited. In this paper, this confining technique is called the steel-strapping tensioning technique (SSTT) for brevity. Nine HSC columns were tested under eccentric loads. The specimens were grouped into three groups with each group having an unconfined HSC column as a control specimen, a two-layer SSTT-confined HSC column and a four-layer confined HSC column. The experimental results show that the flexural capacities of HSC columns can be enhanced with SSTT. The deformability of confined HSC columns is significantly improved with such confinement.

A new modified mix proportioning method for producing normal-strength concrete using recycled concrete aggregate, called the equivalent coarse aggregate mass (ECAM) method, is proposed in this paper. The basic concepts of the proposed method with calculations for mix design are presented by designing 14 mixes and testing 99 concrete samples (57 cubes and 42 cylinders). Experimental work was carried out in two phases. In the first phase, an experimental programme was conducted to verify the proposed mix design method by studying a single parameter - uniaxial compressive strength. Five different mixes with initial 0, 25, 50, 75 and 100 % replacement by mass were designed, cast and tested in this phase. It was concluded from the first phase that the proposed method can be adopted for designing the recycled concrete up to a nominal replacement ratio of 50 %. Accordingly, the second phase of experimental study was carried out to design three different grades of concrete strength using the proposed method to investigate the mechanical properties of the recycled concrete. Seven different mechanical properties - compressive strength, splitting tensile strength, modulus of elasticity, Schmidt hammer test, ultrasonic pulse velocity test, fresh density and hardened density - were investigated and are presented and discussed here.

11th fib PhD International Symposium in Tokyo
CONSEC 2016
fib Bulletin 79
New COM7
Brazil NMG hosts fib event
Short notes
Obituaries
Congresses and symposia

Keine Kurzfassung verfügbar.

Keine Kurzfassung verfügbar.

The paper reports on a benchmark of European deemed-to-satisfy rules for exposure class XC (carbonation exposed structural members). The benchmark of the descriptive rules was carried out following the probabilistic design approach for carbonation-induced corrosion developed in [1] and adopted in fib bulletin 34: Model Code for Service Life Design (2006) [2] and fib Model Code for Concrete Structures 2010 [3], respectively. To perform a representative study, three groups of European countries were selected, representing different parts of Europe, south (Spain, Portugal), middle (Netherlands, Great Britain and Germany) and northern Europe (Denmark, Norway). Reliability ranges for carbonation-induced depassivation of rebar were calculated for “favourable” and “unfavourable” design situations in exposure classes XC2, XC3 and XC4 [4]. In each design situation the deemed-to-satisfy rules of selected countries were followed. The probabilistic calculations were mainly based on short-term carbonation data. However, some calculations were also based on long-term observation. The latter was implemented for independent validation purposes. The calculated reliability ranges are very broad and in some “unfavourable” situations, the deemed-to satisfy requirements do not guarantee the required limit state (LS) arget reliabilities for the particular exposure. In “favourable” situations less stringent demands would have been sufficient.

This paper presents the results of the corrosion fatigue behaviour of profiled reinforcing steel bars. Cyclically loaded rebar was simultaneously exposed to different corrosive environments - moderate to severe corrosive environments simulating XC or XD/XS exposure. Corrosion was configured naturally without any external polarization. Rebar was exposed to the corrosive solutions either directly or, when embedded in concrete, indirectly. In this latter case, corrosive agents penetrated towards the steel surface through an open crack. Low frequency was applied to enable extended corrosion periods. The potential drop method was utilized to detect and quantify the crack initiation and crack growth of the rebar. Using this method it was possible to determine the ratios between the number of cycles to crack initiation and the cycles to failure. Based on this method, the Nini/NF ratios were almost always between 0.8 and 0.9 - values that are similar to ratios determined for rebar tested in air (reference). This indicates that the fatigue life of rebar in carbonated concrete or concrete containing chloride is strongly dependent on crack initiation and less on crack growth. The S-N curves derived from the corrosion fatigue tests deviate significantly from the curve that was measured during the reference fatigue tests (tests in air). The S-N curves of rebar tested under corrosion fatigue load were linear, with a slope that was much steeper than the slope of the reference rebar tested in air.

Structural health monitoring (SHM) can be defined as the process of developing and implementing structural damage detection strategies. Ideally, this detection should be carried out in real-time before damage reaches a critical state and impairs structural performance and safety. Hence, it must be based on sensorial systems permanently installed on the target structures and on fully automatic detection methodologies.
The ability to detect damage in real-time is vital for controlling the safety of old structures or for post-retrofitting/post-accident situations, where it might even be mandatory for ensuring a safe service. Under these constraints, SHM systems and strategies must be capable of conducting baseline-free damage identification, i.e. they must not rely on comparing newly acquired data with baseline references in which structures must be assumed as undamaged.
The present paper describes an original strategy for baseline-free damage detection based on the application of artificial neural networks and clustering methods in a moving windows process. The proposed strategy was tested on and validated with numerical and experimental data obtained from a concrete cable-stayed bridge and proved effective for the automatic detection of small stiffness reductions in single stay cables as well as the detachment of neoprene pads in anchoring devices, requiring only a small number of inexpensive sensors.

The main objective of this study is to investigate the effect of pre- and post-jacketing corrosion and loading damage on concrete-jacketed reinforced concrete (RC) columns under uniaxial loading and to develop a methodology for predicting the corresponding compressive strength. The pre- and post-damage involved preloading up to 50 % of the peak load of the core column, an electrochemical process to accelerate the migration of chlorides from an external electrolyte into the test columns and a wetting-drying cycle process with a controlled current to speed up the corrosion of the steel reinforcing bars in the test columns. Uniaxial loading tests were performed to determine the structural performance of the concrete-jacketed columns with and without corrosion damage. The failure mode and load-displacement and load-strain responses of the test columns were recorded, and the related mechanisms are discussed. A model capable of evaluating the compressive strength of unjacketed or jacketed RC columns with and without corrosion damage was then developed. The analytical approach considered the effect of reinforcement corrosion on the effective loadbearing area of the concrete and the confinement effect of the stirrups. The analytical results agree well with the experimental results, indicating the reliability and effectiveness of the models developed.

The analysis of the mechanical behaviour of a reinforced concrete tie subjected to a monotonic loading in the stabilized cracking stage is performed here by way of a theoretical general model that considers the effect of the so-called Goto cracks (secondary cracks). It is shown, in particular, that the average bond stress along the transmission length depends not only on the concrete strength as assumed by the fib Model Code for Concrete Structures 2010, but also on reinforcement ratio and bar diameter. In this regard, tabulated theoretical values of the average bond stress are proposed as a function of the aforementioned parameters. Moreover, the secondary cracks reduce the effect of tension stiffening on the relative mean strain. On the basis of the main results obtained with the general model, some improvements are suggested for the calculation methods proposed by fib Model Code 2010 and Eurocode 2 concerning the average value of the bond stress and taking into account the influence of the secondary cracks on the mean deformation. An improved calculation method is therefore performed. Finally, the theoretical results of crack spacing and crack width obtained with the general and improved methods are compared with experimental data obtained from extensive research on RC ties.

When it comes to the efficient design of reinforced concrete beams and frames, moment redistribution is used to: reduce the absolute maximum magnitude of the moment in the critical region, equalize the critical moments on either side of interior columns and fully utilize the capacity of the non-critical regions of a member. Although important, historically, moment redistribution has proved to be difficult to quantify due to the complexity of quantifying hinge rotations. Although numerous empirical expressions exist for plastic hinge lengths, i.e. the length over which the ultimate curvature can be integrated in order to give hinge rotations, a comparison with a global dataset yields poor results. Using a recently developed mechanics-based moment-rotation approach, it is possible to quantify the moment-rotation characteristics of reinforced concrete hinges. In the tension region, the approach applies partial interaction theory directly to simulate the mechanisms associated with slip of the reinforcement relative to the surrounding concrete as cracks widen, whereas in the compression region, partial interaction shear-friction theory is used to describe the formation and failure of concrete softening wedges. It is shown how the moment-rotation approach explicitly allows for the size dependency. Furthermore, mechanics-based solutions for moment redistribution are then derived and it is shown how these can be simplified at the ultimate limit state for use in the design office.

Structures made of a material with a very high standard deviation, such as fibre-reinforced concrete, exhibit an exceptionally safe prediction of the maximum bearing capacity when this is derived from characteristic values identified by means of small specimens. This is emphasized when the structures are characterized by high redundancy. In this regard, two reference tests representing two extreme situations are considered: a) simply supported unnotched full-scale beams characterized by a statically determinate loading scheme and b) full-scale slabs on the ground characterized by a statically indeterminate loading scheme. The Italian standard and, more recently, the fib Model Code for Concrete Structures 2010 have introduced a coefficient (structural redistribution factor) that is able to take into account the reduced variability of mechanical bearing capacity when associated with a large volume involved in the failure process and/or when the structure is able to redistribute stresses significantly, thus favouring the average rather than the minimum strength. A numerical procedure taking into account the expected heterogeneity of the mechanical characteristics in the structure is introduced for the first time to evaluate the redistribution factor.

An experimental programme was proposed and carried out to assess the effectiveness of the mineral-based composite (MBC) technique for the flexural strengthening of negative moment regions in continuous reinforced concrete slabs. In addition to the testing of the two reference specimens, the experimental programme included the testing of nine continuous RC slab specimens with different strengthening techniques, namely, using ordinary steel bars and MBC material. This experimental programme was conducted to study the failure modes, the load-deflection behaviour and the failure loads.
Furthermore, we present and describe a comparative study between the two strengthening techniques, namely, steel reinforcing bars with MBC or steel bars with epoxy mortar. Based on the experimental results presented, both strengthening techniques for continuous slabs are evidently efficient. In this study the measured results for the average crack spacing were compared with the limits stipulated in CEB-FIP code 1990 and the failure load calculations were extended with an analytical approach based on the ultimate theory for the failure load calculation. In conclusion, the results obtained from the analytical model are in agreement with the experimental results.

Fibre-reinforced polymer (FRP) composites have been employed in the last few decades largely for the strengthening and seismic retrofitting of existing reinforced concrete (RC) structures. Several studies are available in the literature and different analytical models have been proposed for evaluating the FRP contribution in strengthened RC elements. This paper analyses the accuracy of analytical models widely used for evaluating the flexural and shear contributions provided by the FRP. Some of those models are included in design guidelines. In particular, the analytical models for evaluating the FRP strain at intermediate crack-induced debonding failure are analysed. The accuracy of each formulation is assessed comparing the analytical provisions with the experimental results collected from two databases, one for bending and one for shear. The results obtained show that most of the analytical flexural models achieve a good level of accuracy and only a few models provide inadequate results. A new formulation proposed for evaluating the FRP shear contribution is shown to be generally conservative, which comes at the expense of accuracy.

The shear resistance mechanisms of a reinforced concrete (RC) member with shear reinforcement can be divided into the contributions of the concrete and the shear reinforcement. The shear resistance mechanisms of concrete can be further divided into the shear resistance of the intact concrete in the compression zone, the aggregate interlock in the cracked tension zone and the dowel action of the longitudinal tension reinforcement. The shear demand curves and potential shear capacity curves for both tension and compression zones have been derived in this study, with the assumption that the shear failures of RC members are dominated by the flexural-shear strength. The shear capacity model was also proposed on this basis. In the proposed model, the crack width and the local stress increase in reinforcement were calculated based on the bond behaviour between the reinforcement and its surrounding concrete. Further, the crack concentration factor was introduced to consider the formation and propagation of the critical shear crack that developed from the flexural cracks. The results of a total of 1, 018 shear tests were collected and compared with the analysis results provided by the proposed model. It was demonstrated that the proposed model provides a good estimate of the shear strengths of RC beams.

The fib Model Code for Concrete Structures 2010 introduces a new design concept for punching shear based on critical shear crack theory. This paper presents and provides the background to the design provisions for punching shear according to fib Model Code 2010, Eurocode 2 and the corresponding German National Annex to Eurocode 2. The different punching shear design provisions are critically reviewed by means of parameter studies and a comparison of the calculated resistances and test results. The safety levels of the code provisions are verified and the influence of the different punching parameters on the calculated resistances is examined in detail.

Strut-and-tie modelling constitutes a powerful tool for the design of complex structural reinforced concrete elements. It has been proved numerically that strut-and-tie (ST) models obtained using structural optimization methods produce designs that are more efficient. However, to the best of the authors' knowledge, no experimental evidence of such results has been published. This paper presents experimental results for nine test specimens; five of them were designed using optimal models derived from a full homogenization structural optimization algorithm, and the remaining four using conventional ST models for comparison purposes. Although all specimens carry loads higher than the factored design load, specimens based on ST models derived using full homogenization with reinforcement parallel to the ties exhibit better structural performance regarding crack growth control, more ductile modes of failure and a greater increase in load capacity.

This paper concerns the shear capacity of keyed joints that are transversely reinforced with overlapping U-bar loops. It is known from experimental studies that the discontinuity of the transverse reinforcement affects the capacity and the failure mode. However, to the best knowledge of the authors, previous theoretical works and current design equations in standards do not account for this important effect. This paper introduces a detailed model based on finite element limit analysis to assess the effect of the discontinuous reinforcement. The model is based on the lower bound theorem and uses the modified Mohr-Coulomb yield criterion, which is formulated for second-order cone programming. The model provides a statically admissible stress field as well as the failure mode. Twenty-four different test specimens were modelled and the calculations compared with the experimental results. The results of the model show satisfactory agreement with the experimental observations. The model produces estimates of the shear capacity that are significantly better than those of the Eurocode 2 design equations.