This paper presents a critical comparison of the existing code provisions for the shear strength of concrete beams. The comparison is based on the computerized filtering-out of the inevitable statistical bias from the available multivariate database on shear strength, on an examination of the predicted size effects on shear strength and their underlying hypotheses and on the results of recent high-fidelity numerical simulations of shear failure. In addition to examining the existing models, the present comparison also provides several key considerations for testing the scientific soundness of any model of shear failure in concrete beams, which is necessary for future revisions to the design code provisions.

This paper presents a new theory for the shear capacity of reinforced concrete members without shear reinforcement. While recognizing that there are multiple failure mechanisms, the theory attributes the opening of a critical flexural shear crack as the lower bound of the shear capacity. It proposes that the shear displacement of an existing flexural crack can be used as the criterion for the unstable opening of the critical flexural shear crack. Based on the theory, the paper presents a simplified shear evaluation model. Compared with the current shear provisions in the design codes, the model is characterized by good accuracy and a solid physical background. It demonstrates a great flexibility for dealing with complex design conditions. As an example, the paper discusses the possibility of extending the theory to the shear resistance of higher-strength concrete. The suggested method provides a more logical and fluent transition from normal- to high-strength concrete and shows good agreement with experimental observations.

Most classic investigations on bonding properties in reinforced concrete have been performed on the basis of pull-out tests, where a reinforcement bar is pulled out from an uncracked concrete cylinder, prism or cube. In these tests, the bond is governed by the concrete strength and bar surface properties of the reinforcement (bond index, rib geometry) or by the splitting strength of the concrete (concrete cover). In the latter case, bond failure occurs due to uncontrolled cracking of the concrete specimen. In contrast to these fundamental tests, bond in many structural members is activated within already cracked concrete. This is particularly relevant for the reinforcement in beams and slabs (both for flexural and transverse reinforcement), as the reinforcing bars might be located at planes where flexural cracks develop. The opening of these cracks along the reinforcement is nevertheless not uncontrolled (as opposed to splitting failures), but it is governed by the bending deformations. The bond properties and strength of the reinforcement in actual members are therefore influenced by the opening of these cracks and are potentially different from those observed in classic pull-out tests.
The present paper aims to address this topic by presenting the results of an experimental investigation with 89 monotonic pull-out tests performed on cracked ties. The opening of the cracks was controlled while transverse bars - located in the plane of these cracks - were pulled out from the specimens. The tests were performed for crack openings ranging from 0.2 mm to 2.0 mm in order to cover conditions both at the serviceability and ultimate limit states. The results show a very significant influence of in-plane cracking on both strength and bond-slip stiffness, with decreasing mechanical performance for increasing crack openings. The performance of different actual anchorage types (straight, hooked, U-shaped and headed bars) - generally characterized through force-slip relationships - is finally analytically investigated and compared to the test results.

A concrete interface is a material discontinuity that requires special care with respect to structural design and assessment. Therefore, the definition of design expressions based on experimental testing data must ensure the necessary reliability depending on the type of structure and its use. The present work describes a new proposal for the design of concrete interfaces subjected to shear loading for different roughness profile types. The proposal is characterized by three linear branches (for monotonic loading) and an S-N curve (for cyclic loading) and is the result of a parametric analysis of existing experimental data (obtained by the authors and also from an extensive literature search) based on statistical and probabilistic methods. Design expressions were defined in order to minimize the dispersion and variability of the safety factor values for each experimental test considered and also to assure that those values are within a target range (defined according to reliability considerations). These improvements became clearer when the new proposal was compared with the most common design code recommendations.

The paper describes the design, development and experimental checking of a modified type of structural joint with limited length between concrete segments cast in-situ. The design concept is based on the developed length of an anchorage hook stiffened by transverse reinforcement bars and is particularly suited for the case of in-situ construction of staged box girder bridges, with the intention of possibly using lighter scaffolding.
The studies concentrated on the strength, stiffness and serviceability of the proposed joint are presented. The research work comprises the bending behaviour of reinforced concrete slabs with loop joints with regard to the diameter of the loop bar, loop joint width and ultimate and fatigue load. The results are compared to the behaviour of reinforced concrete slabs without joints. A total of 16 slabs were tested with static and fatigue loading tests. The present paper evaluates the flexural behaviour in static loading tests. The results of fatigue tests have also shown excellent performance.
In the static tests, crack widths and cracking pattern were observed at service load levels, and the ultimate behaviour was evaluated by means of tests to failure.
From the test results, the service performance of the loop joints was confirmed to be similar to slabs without joints. The static loading tests confirm the good performance and effectiveness of this loop joint type under static loading. Details of the loop joint design criteria are also proposed.

There are several methods available to decide appropriate design recommendations to prevent corrosion of reinforcing steel in prestressed concrete bridge girders. With respect to chloride-induced corrosion, in the present study two methods are considered. The first one is based on the target probability of corrosion initiation and the initial cost. The other method is based on the life-cycle cost that includes the initial cost, maintenance cost, and expected failure cost. This paper deals with the development of recommendations for durability design of structures in marine environments from the reliability point of view, taking into consideration the life-cycle cost of a structure. In order to address the problem, the chloride diffusion coefficient of a cracked area under service load is obtained considering opening and closing motion of cracks. Utilizing the diffusion coefficient of a cracked area, the development over time of the chloride concentration at the surface of reinforcement can be predicted. This information is used to quantify the probability of initiation of corrosion of prestressing steel as well as the distribution of life-cycle cost. Based on the findings, recommendations for durability design in various exposure environments are developed.

This article presents an experimental programme on reinforced concrete beams retrofitted with steel angles and pre-stressed stainless steel ribbons to increase their flexural strength and ductility. Two different configurations of the steel ribbon were designed, and two companion specimens for each type considered were subjected to a four-point bending test to facilitate a direct comparison in the analysis of the effectiveness of the retrofitting technique. The influence of longitudinal steel angles and transverse stainless steel ribbons is analysed, and the concrete confinement due to stainless steel ribbons examined. The strengthened beams show remarkable increments in flexural strength and ductility with respect to the as-built beam. Moreover, a simple cross-sectional model was adopted to calculate the flexural strength; then sophisticated numerical tools were used to reproduce both the experimental load-displacement curve and crack pattern for each specimen.

The primary objective of this new study of fibre-reinforced polymer (FRP)-reinforced concrete (RC) beams is to evaluate the mechanical performance of RC beams made of low strength concrete internally reinforced with FRP. The use of FRP rebars with low compressive strength concrete is desirable in order to avoid the accelerated corrosion processes that could occur with steel rebars. For this purpose, an experimental programme was designed to identify the failure modes and bending behaviour. The experimental results are compared with equations from ACI 440.1R-06, CSA S806-12 and other design codes as well as with other results from a review of the literature.
The comparisons indicate that the resistance moment is well predicted by codes for flexural failure. At ultimate loads, the deflection of the beams is further underestimated compared with the deflection of Reinforced Concrete beams with a higher compressive strength. An analytical simulation of 690 beams indicates that deflection is the major limiting criterion at the serviceability limit state.
Finally, the formulation problem of the optimal FRP design for reinforced concrete beams can be turned into a programming problem. The space of the feasible design solutions and the optimal solutions can be obtained using only a reduced number of design variables and an existing design method.

The dynamic response of a cracked beam subjected to moving loads has been studied extensively in the past decades. However, very little is known about the dynamic impact factors and crack propagation when vehicles move along the cracked beam. It can be reasonably postulated that a crack extension may occur when the vehicle loads cross the cracked bridge at a high speed. As a result, the dynamic response will be enlarged significantly due to the flexural rigidity reduction induced by cracks, which may result in a dangerous effect on structures. To address this problem, a three-dimensional vehicle-bridge model was developed to investigate the dynamic response of cracked bridges with crack breathing. Crack breathing is simulated at the crack surface using contact elements. The modified crack closure method is adopted to calculate the stress intensity factors. The results showed that the impact factors for the damaged bridge under a moving load could be notably larger than those for the intact bridge, and could exceed the value specified in the AASHTO bridge design code. Meanwhile, crack propagation may occur when the vehicles move along the cracked bridge at a high speed. So, it is very necessary to limit the velocity and transverse position of the vehicles to avoid further damage to the cracked bridge.

Self-compacting concrete (SCC) is defined as highly flowable and non-segregating concrete that does not require mechanical vibration during application. Load testing of bridge girders was investigated on full scale T-beams of pre-tensioned high strength self-compacting concrete (PHSSCC). The girders were monitored by fixing different types of sensors at different locations.
The results of ultimate moment are compared to evaluate if such girders can be designed using AASHTO Load and Resistance Factor Design (LRFD) Specifications; AASHTO Standard Specifications for Highway Bridges (STD) and the PCI Bridge Design Manual are intended for structures constructed using conventional (non-vibrated) concrete. It is concluded that good agreement in results exists for the two methods.
Investigations (theoretical and experimental) are needed on the issue of ductility in structural prestressed elements constructed in seismic zones or even, the issue of high strength in SCC. These have also been studied here, with the conclusion that suitable ductility for this type of structure, as would be needed in seismic regions, can be achieved.

The present paper summarizes the findings of an experimental programme to investigate a strengthening/repair technique through the introduction of a new RC layer to an existing slab. Six RC slabs, composed of twelve cantilever and six interior spans, were tested under monotonic transverse loading. The behaviour of statically determinate cantilever spans and indeterminate interior spans was examined by comparing the test results of these specimens to the results of reference slabs, in which the existing and additional layers were cast monolithically. The influences of recovering the permanent slab deformation before repair and the spacing of the shear connectors between the existing and additional layers were investigated. The tests indicated that recovering the permanent deformations of a slab before repair substantially reduces its rigidity while having little influence on the ultimate load. Furthermore, debonding of the reinforcement was observed to considerably decrease the load capacities and rigidities of the slabs.

Keine Kurzfassung verfügbar.

Keine Kurzfassung verfügbar.

At present, prescriptive regulations with regard to concrete cover and composition are applied to provide sufficient durability of reinforced concrete members under exposure conditions with different degrees of severity. In view of current knowledge on deterioration mechanisms and their modelling, it is planned to change from these deemed-to-satisfy specifications to a performance-based design approach in future standards. In such specifications, concrete durability design is based on the statistically characterized performance of concrete, determined in standardized tests with respect to defined classes of concretes with similar performance.
This paper presents the results of a study in which concrete mixes were tested and analysed with respect to their carbonation resistance. Compositions with similar performance are grouped into carbonation resistance classes. These classes are described statistically and requirements for performance testing are given. In addition, composition requirements are introduced in order to determine concrete performance depending on mix composition prescriptively. Finally, an example is given for the assessment of concrete performance with regard to carbonation.
This work was carried out at the request of JWG under CEN TC 250/SC2 and CEN TC 104/SC1 as an input and starting point for the ongoing committee work to implement the methodology from the fib Model Code for Concrete Structures 2010 in the next generation (2021) of European concrete standards.

The corrosion of reinforcement in concrete is the most common degradation phenomenon of reinforced concrete structures. Reinforced concrete elements subjected to corrosion generally crack due to the expansive nature of oxides. One very important task is estimating the corrosion level using a non-destructive method in order to establish both the actual safety of the structure and a priority intervention plan.
Many researchers have studied the relationship between the corrosion phenomenon and the corresponding crack openings and their evolution; several statistical analyses, based on test data from experimental campaigns under a wide range of test conditions, are available.
The present work attempts to contribute to finding a relationship between the crack opening and the amount of corrosion induced in the reinforcing bars. The result of the analysis is that only a reduced number of tests can be used to establish an empirical model based on a reliable set of test data. A simple relationship between crack opening and corrosion penetration is not recommended, due to the different parameters that are able to influence this correlation. Therefore, two fundamental parameters, the ratio of the concrete cover to the rebar diameter and the concrete strength, have also been considered. The considerations made regarding these parameter test results have been rearranged and the result is a formulation that shows reduced scatter.

An analytical model is proposed for bond stresses at the corroded steel-concrete interface in reinforced concrete. The concrete around the corroded bar is modelled as a thick-walled cylinder - consisting of an inner cylinder of an anisotropic material and an outer cylinder made of an isotropic material - subjected to internal pressure exerted by the growth of corrosion products on the concrete wall at the interface. A frictional model is used to combine the action of confining pressure due to radial pressure produced by principal bar ribs and the pressure resulting from expansion of corrosion products. The analysis results using the proposed model show good agreement with the experimental results of several researchers.

The methods of crack control for liquid-retaining RC tank walls are analysed taking into account external load (EN 1992-1-1) and imposed strain occurring at the construction stage (EN 1992-3), i.e. during the concrete-hardening period. The convergence ranges of the simplified method of crack control included in EN 1992-3 and the detailed calculation methods included in EN 1992-1-1 and EN 1992-3 are defined. Apart from the compatibility areas, overestimation of the acceptable reinforcing bar diameter &phgr;s*, illustrated in Fig. 7.103N in EN 1992-3, was proved. Coefficients k&phgr;1 and k&phgr;2 are defined, which enable the calculation of the acceptable reinforcing bar diameter &phgr;s* in order to obtain the values complying with the direct calculations. For practical purposes, graphs have been plotted to facilitate the definition of coefficients k&phgr;1 and k&phgr;2 without performing direct calculations. On the basis of the analyses performed and the relations proposed, it can be concluded that there is a possibility or a necessity to increase or decrease the acceptable reinforcing bar diameter &phgr;s* depending on the concrete mechanical properties and geometrical properties of an RC tank wall.

This paper presents a review of recent research and development work involving natural fibre-reinforced concrete (NFRC). The recent developments in NFRC reinforced with different types of natural fibre, such as sisal fibre, bagasse fibre, coir fibre, banana fibre, eucalyptus fibre, flax fibre, jute fibre and pinus radiate fibre, are covered. Natural fibres and their modification methods are introduced first and the development history of natural fibre-reinforced concrete and the relevant research into the mechanical behaviour of NFRC in both the short- and long-term are reviewed. The applications of NFRC are also summarized.

Previous research with fibre-reinforced slab elements has shown that the surface roughness of formwork and the presence of rebars affect fibre orientation and fibre volume distribution. This paper discusses the orientation and volume distribution of steel fibres in wall elements cast from a single point. Aparticular focus of the work was the effect of formwork tie ba rs on fibre orientation and distribution. Numerical simulations and X-ray computed tomography were appliedto quantify the fibre orientation and distribution, and the mechanical performance was determined using three-point bending tests on sawn beams. The Thorenfeldt model (applied in the Norwegian proposal for the new fibre-reinforced concrete guideline) was used to estimate the residual flexural tensile strength based on fibre orientation and distribution.
The simulation results show that the fibre orientation can be related to the flow pattern. The results indicate a large variation in fibre orientation, which was confirmedexperimentally. The fibre volume distribution was mostly uniform, except for an area with fewer fibres at the casting point. The large variation in fibre orientation was reflected in a large variation in residual flexural tensile strengths. Weak zones due to anisotropic fibre orientation, caused by formwork tie bars, were observed.

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.