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The safety of existing bridges may be assessed using data from monitoring. The fatigue damage induced during observation period is extrapolated to the total service duration of a structure, and the reserve capacity is estimated. However, due to the randomness of road traffic, there exists a risk of underestimation of damage using a short-term monitoring campaign. This paper analyses the influence of monitoring duration on the obtained results using datasets collected during two long-term (>3 years) dynamic monitoring campaigns. Two road bridges with different nature of traffic are considered: (a) a heavily loaded two-lane highway viaduct and (b) a road viaduct with a bi-directional traffic. The deck slabs of both bridges are monitored using strain gauges installed on reinforcement bars. The daily, seasonal, year-to-year and COVID-19 lockdown-induced variations of measured action effects and calculated equivalent fatigue damage are discussed. The resampling method is used for simulation of possible short-term monitoring campaigns during the period considered, with different order of heavy vehicles arriving. The Extreme Value Theory serves to answer what is the required monitoring duration for reliable results. Possible ways of considering the monitoring duration in the fatigue damage calculation are discussed, and the methodology for damage extrapolation proposed.

This paper discusses the influence of fiber orientation on elastic limit tensile stress of ultra-high performance fiber reinforced cementitious composites (UHPFRC). An original model relating elastic limit tensile stress to fiber orientation is proposed, and three determination methods of the elastic limit are compared. Five specimens are tested under uniaxial tension and characterized with digital image correlation, acoustic emission and displacement transducers. Before testing, the fiber distribution (local dosage and orientation) of each specimen is determined using a magnetic probe. After testing, three methods (offset with 2 different threshold values, and deformation modulus drop) are applied to determine the elastic limit tensile stress. The test results show that the proposed model estimates well the elastic limit tensile stress on the basis of fiber orientation. The modulus-drop method yields the most representative value for the elastic limit considering the underlying physical phenomena. It may be taken as definition of elastic limit tensile stress of UHPFRC.

Two full-scale PC box girders with typical damage were derived from an in-situ bridge that was in service about 20 years, to understand the strengthening effects of prestressed CFRP plates for the entire bottom slab, casting of a RC layer in composite action with the entire top slab and steel plates anchored on the webs of the shear span. The flexural tests were conducted on the two full-scale PC box girders with and without strengthening measures, and comparative analysis was conducted regarding flexural capacity, failure mechanism, rigidity, ductility and overall structural behavior. The test results showed that the strengthening measures improved the girder rigidity by 40 % before reaching cracking state and increased the yielding and ultimate capacity by 39 % and 28% respectively. The theoretical analysis was conducted to discuss failure modes for PC box girder strengthened by prestressed CFRP plates. Theoretical equations are derived to calculate the flexural capacity of strengthened PC box girder according to typical failure modes. Good agreement with tested results was obtained. The strengthening design of the prestressing CFRP plate was verified by application in a PC box girder bridge.

The forms of U-shaped UHPFRC beams have not been investigated for the highway footbridge. Compared with the traditional section forms, the U-shaped UHPFRC beams can reduce the material consumption under the condition of providing the same bearing capacity. Furthermore, prestressed U-shaped UHPFRC beams are rarely reported in the existing research. This paper explores the flexural behavior of prestressed ultrahigh-performance fiber-reinforced concrete (UHPFRC) beam bridge having unique design and the material properties of prestressed reinforcement combined with UHPFRC. Based on the unique shape of the U beam, the flexural performance test of the full-scale model of the prestressed UHPFRC U beam is conducted. Then, the finite element model considering material nonlinearity and structural ductility is established using Midas FEA software. Finally, the failure mode, failure process, cracking moment, ultimate moment, and strain of the full-scale model are studied. The calculation formulas of the flexural capacity of UHPFRC U beam considering ductile failure are derived. The comparative analysis results show that the prestressed UHPFRC U beam has an excellent flexural performance. The bending failure of a U-shaped beam belongs to the group of ductile failures, which is characterized by the main crack along the central rib and the loading center, which is accompanied by multiple microcracks. The failure process can be divided into four stages: linear deformation, microcracks development, main cracks development, and bearing capacity decline. The incorporation of steel fiber and the interaction between UHPFRC and reinforcement can effectively reduce the development of cracks. The U-beam bending moment is 50–55% of the ultimate bending moment. In the UHPFRC bridge design, the deformation can be used as a control index, and material advantages of the UHPFRC can be used to a certain extent. The strain-hardening characteristics of the UHPFRC are obvious in the loading process. The finite element analysis results show that the maximum strain value appears at the central rib, followed by the transverse strain value of the bottom plate, while the minimum strain is the longitudinal strain value of the bottom plate. The deformation of the rib plate is the largest, and the strain of the other measuring points changes slowly. The farther away from the center the measurement point is, the slower its strain changes. Therefore, the load is mainly caused by the central rib and the loading center plate. With the increase in the deformation, the load on both sides continuously moves to the central rib along the plate surface. This study can provide a useful reference for theoretical analysis and design of prestressed U-UHPFRC bridges.

Ultra High-Performance Fiber-Reinforced Cementitious Composite (UHPFRC) is increasingly popular for new structural designs thanks to its high resistance both in tension and compression. When UHPFRC is complemented with steel reinforcing bars or prestressing tendons, the structure remains waterproof and crack-free under service conditions, significantly improving the durability compared to conventional reinforced-concrete designs. The Aiguillon Bridge is one of the first railway bridges entirely made of UHPFRC. Built in 2021 in Switzerland, this bridge has a single span of 6.6 m and a width of 5.4 m. The bridge is designed for a narrow track and 2 walkways. Two prefabricated elements in UHPFRC have been assembled on-site by a longitudinal cast-in-place UHPFRC keying joint. This paper presents the full-scale laboratory experiment to validate the keying joint suitability for railway bridges. This experimental investigation involves a static test on a transverse beam composed of two precast elements and the keying joint. Results show that the structure presents a post-peak ductile behavior as well as an elastic behavior under service loads. Digital-Image-Correlation measurements provide details on the cracking patterns of the interface between the keying joint and the precast elements. As the maximum resistance obtained during the experiment is in agreement with the analytical-model predictions, the concept of the keying joint is validated.

Ultra high performance fiber reinforced cementitious composites (UHPFRCs) have demonstrated their outstanding efficiency as structural materials. Determination of the bending resistance of members combining UHPFRC with steel reinforcement bars (R-UHPFRC) using material properties obtained by material testing gives satisfactory results. However, the synergetic interaction between rebar and UHPFRC in the tensile action due to bending, especially under loading–unloading of the structure is not yet well understood. This paper compared methods of analytical inverse analyses, as well as finite-element modeling and nondestructive testing (NDT), for determining UHPFRC material properties based on plates and R-UHPFRC members’ test results. It was demonstrated that UHPFRC in the tensile zone of R-UHPFRC members can enter into compression during unloading, influencing the structural response under service loading.

Ultra-High-Performance Fibre-Reinforced Cementitious Composite (UHPFRC) has been developed to design lightweight structures and enhance existing designs. As the environmental footprint of the construction industry must be significantly reduced, the potential to lower environmental impacts of structures using UHPFRC needs to be explored. While the greenhouse gas emissions of a volume of UHPFRC are higher than that of the same volume of concrete, UHPFRC enables the reduction in the amount of material required in structural designs and improves the durability of structures. The environmental impacts of structural designs must thus be compared on the cradle-to-grave use cycle of the design at a project scale. In this study, a methodology is proposed to evaluate the ecological burdens of several bridge designs involving various structural elements in UHPFRC. The method proposes an analysis over three time horizons: first, the construction phase, then including the scheduled maintenance, and finally, adding the elimination. A case study of a short-span bridge in Switzerland is used to assess three alternatives of bridge designs: a conventional reinforced-concrete structure, a composite timber–UHPFRC bridge, and a full-UHPFRC solution. The results show that timber–UHPFRC structures can significantly reduce the environmental impacts of bridge designs, showing promising results in terms of sustainable development. The use of the methodology supports bridge owners in assessing the environmental impacts of structural designs.

The State Representation Methodology (SRM) combined with the Frequency Slice Wavelet Transform (FSWT), which is a new time–frequency analysis tool, is proposed for assessing the condition of bridges on the basis of bridge monitoring data. First, this paper presents an overall description of the SRM method via FSWT analysis. It then shows, through numerical simulations, some novel characteristics and advantages of FSWT analysis in contrast to the conventional wavelet approach and the feature extraction accuracy of SRM analysis for the detection of bridge damage on the basis of monitoring data. The principal results obtained through this study can be summarized as follows: (1) details of a newly proposed SRM and its application to bridge condition assessment based on bridge monitoring data are introduced. The proposed SRM combined with the FSWT is validated as a novel time–frequency analysis tool for assessing bridge condition on the basis of bridge monitoring data. (2) New properties of FSWT analysis are demonstrated, and advantages in contrast to the traditional wavelet method are highlighted. Feature extraction in SRM analysis is precise for damage detection in a bridge system on the basis of monitoring data and using numerical simulations.

The fatigue behavior of a reinforced UHPFRC (Ultra High Performance Fiber Reinforced Cementitious composite) T-shaped beam under four-point bending is investigated. The beam was subjected to a fatigue loading range equal to 49% of the static resistance and failed after 0.88 million cycles. It was instrumented with extensometers, strain gauges and distributed fiber optic sensors for strain monitoring. The fatigue process consists of three stages: with rapid, stable and again rapid growth of strains during 10%, 80% and 10% of total number of fatigue cycles, respectively. Except of the first 10%, this process takes place locally; therefore, it cannot be followed with the deflection measurement. During the stable stage, growth of strain occurs at minimum loading level in the fatigue cycle, indicating a fatigue damage process under tensile-compressive response of UHPFRC. Advanced fatigue crack propagation in the reinforcement bar determines the location of rupture of the beam. When the remaining cross-section of the rebar does not suffice to carry the tensile load, stress is transmitted to the encompassing UHPFRC causing its fast deterioration. Complete rupture of the rebar occurs only at the end of the test, when the beam collapses.

The behavior of reinforced-concrete structural elements under moving loads involves many complex parameters related to material properties, vehicle loading, environmental changes and structural response. It is difficult to generate the real conditions of loading when performing laboratory tests or numerical simulations. Due to these difficulties, many uncertainties and non-conclusive results prevail. This paper presents a fatigue-monitoring concept to evaluate the cracking activity of a reinforced-concrete slab of an existing road viaduct under moving loads. Both steel rebars and concrete of the deck slab were instrumented and are being monitored continuously. The monitoring system comprises strain gauges to measure the strain in the rebars and acoustic emission (AE) system to evaluate the AE activity of the concrete under traffic loading. Parametric study of AE features and b-value analysis were performed to quantify six weeks of acoustic emission data. Results show that the AE activity is due to the opening and closing of microcracks. The monitoring system indicates successfully on-site the activity of microcracks in concrete when subjected to stress reversals due to passing vehicles.

Riveted railway bridges are commonly considered as being “old”, having fatigue damage and are therefore replaced. While this practice should be by now obsolete by using novel engineering methods like structural monitoring, prolongation of the service duration of railway bridges often requires an enhancement of the railway track system, in particular when there is an open deck with the sleeper-rail-system directly fixed on the steel structure. Three examples are described where the novel Ultra-High Performance Fiber Reinforced Cementitious composite material (UHPFRC) has been used to design lightweight trough and deck slab elements for new ballasted and fixed railway tracks respectively. Acting as a composite UHPFRC – riveted steel system, the structural resistance is significantly increased leading to important reduction of fatigue stresses. Subsequently, safety verifications at Fatigue and Ultimate Limit States are satisfied in view of future higher traffic loading.

The article “An analytical inverse analysis to determine equi-biaxial tensile properties of strain-hardening UHPFRC from ringon-ring test”, written by Xiujiang Shen, Eugen Bruhwiler, Emmanuel Denarie and Wanghu Peng was originally published electronically on the publisher’s.

The equi-biaxial tensile properties of strain-hardening UHPFRC are determined and investigated based on an original analytical inverse analysis of results from ring-on-ring tests. First, the analytical inverse analysis method is developed based on the elastic slab bending and yield line theories. Using this method, a new objective criterion for the determination of the elastic limit stress of strain-hardening UHPFRC is provided, and a point-by-point inverse analysis is used to obtain the strain value at the end of hardening. This method reduces uncertainties regarding assumptions and avoids any iterative procedures. The inverse analysis results are put into perspective with experimental evidence, particularly based on DIC measurements. Moreover, the uniaxial tensile properties are also derived from the inverse analysis of 4PBT results and compared with the equi-biaxial tensile properties from the proposed inverse analysis. The inverse analysis results show a 18% lower elastic limit stress, and almost equivalent tensile strength of UHPFRC subjected to equi-biaxial stresses, compared with the corresponding values from uniaxial stress. Moreover, a relatively small equi-biaxial strain at the end of hardening is highlighted.

Steel reinforcing bars in concrete are protected from corrosion by a thin oxide film that is created on the steel surface in the highly alkaline environment of the concrete. Corrosion process begins once this oxide film is destroyed, for example, by chloride ions penetrating from the element surface into the concrete and to the steel rebar. Critical chloride concentration is highly variable owing to numerous factors such as the type of reinforcing steel used. The TransChlor (R) software is used to simulate the time history of chloride ion ingress and corrosion initiation into a typical reinforced concrete structural element with four different types of reinforcing steel, namely, carbon steel, ferritic stainless steel (1.4003) with rolling skin, duplex stainless steel (1.4462) and ferritic stainless steel (1.4003) with pickled surface. A climate model is used to simulate two different exposure conditions of the structural element, namely direct and mist exposure, representatives for winter conditions in mountainous and lowland areas. A probabilistic method is applied to consider uncertainty of material properties in the model. The results show that reinforcing steel bars with enhanced corrosion performance significantly improve the durability of reinforced concrete.

The material UHPFRC is not yet used in the field of bridge construction regularly. The reasons are missing codes and allurements for innovation in the structural engineering. Nevertheless, UHPFRC has many advantages, which are significant compared to the use of concrete in this case: the replacement of a railway bridge. This paper documents two tests of reinforced UHPFRC beams. This tests has been done in the context of the validation of the dimensioning: one bending test and one shear forcer test. Furthermore, the reasons for the choice, the specimen geometry as well as the test results are presented. Finally, the investigated load bearing capacities will be compared with analytical models.

Bridges of high cultural value and aesthetic quality deserve respectful treatment, and consequently, construction interventions must balance these assets with the severe requirements of utilisation. This is particularly relevant to structural engineers and bridge owners involved in rehabilitation or modification interventions. This paper presents examples of how interventions are performed with adequate respect to cultural value. It is argued that the preservation of cultural value may go hand-in-hand with socio-economic, environmental and technical requirements following the principles of sustainable development. These requirements are met through the application of advanced structural engineering methods, including monitoring of structural behaviour and using the UHPFRC Technology. Extending the service duration means adding value to bridges as well as appreciating the art of structural engineering and the identity of structural engineers.

Ultrahigh-performance concrete (UHPC) offers significant potential to address a variety of needs in bridge design, construction, and performance enhancement. Bridge owners have shown willingness to embrace novel solutions that could address specific challenges related to the cost, speed of construction, durability, and service life of their projects. There are hundreds of bridges worldwide that, largely in the past decade, have utilized UHPC. These applications range from minor field-cast closures to precast segments for long-span bridges to kilometer-long bridge deck overlays on a signature structure. The objective of this paper is to promote the application of this class of cementitious material in bridge engineering by presenting the progress that has been made in different regions of the world in the past two decades. Today, UHPC is being widely used in Malaysia to design and construct many bridges of different types and spans as they build out their roadway network. In South Korea, the unique characteristics of UHPC are being utilized to advance the state-of-the-art in long-span bridges. The French were early adopters and pioneers in building a strong foundation for using UHPC in a variety of bridge applications. In Switzerland, UHPC is employed to address major bridge rehabilitation needs. The United States bridge sector has embraced UHPC for a variety of field-cast connections. Current research and development efforts are promoting the use of UHPC in major rehabilitation projects and construction of primary bridge components. The adoption of UHPC solutions into the bridge sector is progressing rapidly because of the unique opportunities provided by the strength and durability of the material. It is expected that additional innovations and refinements of solutions will occur as knowledge of the material proliferates. (C) 2020 American Society of Civil Engineers.

This paper investigates experimentally the equi-biaxial flexural fatigue behavior of thin circular slab specimens made of Ultra High Performance Fibre Reinforced Cementitious Composite (UHPFRC), using ring-on-ring test method. The circular slab-like specimens represent an external tensile reinforcement layer for reinforced concrete (RC) slabs or a new thin slab element for the application, e.g. in bridge construction. Four series of flexural fatigue tests under constant amplitude fatigue cycles up to Very High Cycle Fatigue (20 million cycles) are conducted with varying stress levels S ranging from 0.47 to 0.76, targeting at the endurance limit of UHPFRC material and fatigue damage propagation under equi-biaxial stress condition. By means of digital image correlation (DIC) system, the tensile surface of each slab is monitored continually during testing, and the fatigue damage evolution is studied in terms of central deflection propagation. Based on the fatigue test results, a fatigue endurance limit is determined to be S=0.50, and four stages of fatigue damage evolution are characterized for the UHPFRC slabs with fatigue failure.

This work investigates the fatigue resistance of reinforced Ultra High Performance Fiber Reinforced Cementitious composite (R-UHPFRC) beams with special consideration of steel rebar – UHPFRC interaction under tensile stresses due to bending. Experimental testing of full-scale beams reveals that the fatigue resistance depends on both minimum and maximum loadings in the cycle. The increase of stress range in the rebar during the fatigue process is discussed and quantified. Finally, recommendations for fatigue verification of R-UHPFRC beams are given.

Existing riverbank riprap could face the risk of failure if the flood regime changes in future. Additionally, changed sediment transport in rivers, as a possible result of climate change, impacts the failure risk of flood protection measures. Evaluation of this potential failure is the primary issue of riprap stability and safety assessment. The consequences of the bank failure are probably uncontrolled erosion and flooding with disastrous consequences in residential areas or damage to infrastructures. Thus, a probabilistic analysis of riprap failure considering different mechanisms due to the flood and sediment transport uncertainties is required to assess embankment stability. In this article, the concept of a probabilistic assessment model based on Monte Carlo simulation method, moment analysis methods, and Rosenblueth point estimation method are presented to define the failure risk of riprap as the river bank protection. The probability of failure in different modes, namely direct block erosion, toe scouring and overtopping, has been defined by taking into account the river bed level variation based on bedload transport described with a probabilistic function of the peak discharge. The result of three models comparison revealed a good agreement (the average deviation of less than 2%) in estimation of riprap failure probability. This model is a strategical tool to search the critical river reaches and helps to evaluate the risk maps. So that, the model could cover the engineering aspect of environmental stability in the rivers with riprap as the bank protections.

Adding a thin layer of Ultra-High Performance Fiber Reinforced cement-based Composite (UHPFRC), with or without steel rebars, over a Reinforced Concrete (RC) slab is an efficient reinforcement method for existing structures. The thin layer of UHPFRC serves as a tensile reinforcement for the RC slab, creating a composite element. A recent experimental campaign showed that the layer of UHPFRC significantly increases the rigidity and the punching shear resistance of a RC slab submitted to a point force. An analytical composite model is developed herein to predict the global bending behavior of the composite slab and the punching shear resistance. A multilinear moment–curvature relation for composite sections is proposed to calculate the global force-rotation behavior of a slab which can then be used in combination with a composite failure criterion to predict the punching shear resistance. The contribution of the concrete section to the punching shear resistance is obtained with existing models from the literature. The UHPFRC layer resists to punching shear by out-of-plane bending over a limited length equal to its height. This mechanism induces tensile stresses perpendicularly to the interface with the concrete. The contribution of the UHPFRC layer to the punching shear resistance thus depends on the tensile strength of concrete. The results of this analytical composite model are in good agreement with the experimental result. A method is also proposed to consider pre-existing deformation of the RC section for a post-installed UHPFRC layer.

A novel monitoring scheme oriented towards the structural examination of reinforced concrete slabs subject to railway traffic action is presented. Monitoring is based on the measurement of the mean axial strain profile variation in the cross-section of the slab. The strain profile is reconstructed under the assumption of plane sections by measuring the variation of two distances between two pairs of points materialized at two different levels under the slab on rigid attachments fixed on the slab. The distance variations are measured by means of strain gages measuring the deformation of two very thin strips connecting the two attachments. The implementation of the above scheme on a short-span railway bridge for a period of 7 months and the processing of the acquired signals are presented in detail. Some interesting monitoring records are provided over which the efficiency of the proposed scheme is discussed.

The application on existing Reinforced Concrete (RC) slabs of cast on site Ultra High Performance Fiber Reinforced cement based Composite (UHPFRC) layers is a very efficient reinforcement technique currently spreading. For the design of these tensile reinforcement \{UHPFRC\} layers with thicknesses between 25 and 50 mm a representative in plane tensile response of the \{UHPFRC\} has to be determined considering the effects of fiber orientation. To do so theoretical analysis tools are derived to (1) analyze the relation between fiber orientations in perpendicular directions and (2) determine the average effect of fiber orientation on fiber efficiency at pull out. A comprehensive material testing campaign of a strain hardening \{UHPFRC\} is then carried out on specimens with various thicknesses and casting processes and a meso mechanical model is calibrated on the results. The results of this campaign as well as results taken from the literature are used to validate the theoretical developments. Finally the most likely fiber orientation in a layer of \{UHPFRC\} for the casting method considered herein is estimated based on the results of the theoretical developments and the material characterization and a representative tensile response is estimated using the calibrated meso mechanical model.

Timber-Concrete Composite (TCC) structures allow taking synergistic advantage of the properties of both materials to optimize the overall performances in terms of lightness, slenderness, acoustic insulation, vibrational behaviour and environmental footprint. In the last years, ductile shear connectors have been developed to allow the structural ductility of TCC structures. Considering the limitations of current design methods, this work aims at developing a closed-form solution for accurately predicting the nonlinear structural response of a TCC structure directly from the materials' property and the shear law of ductile connectors. In particular, we have assumed a generalized shear law based on 3 parameters which allow considering shear law from a pure elasto-plastic to pure brittle behaviour. After a short introduction, Section 2 briefly presents the basics of the well-estabilished elastic theory for a 2-layer composite beam with a linear shear law in terms of horizontal shear vs. slip (Vh-s) law; Section 2.3 extends the semi-analytical method proposed by Bažant and Vitek for composite structures with a generalized shear law; Sections 3 extends the previous method by developing a new closed-form analytical solution for predicting the TCC structural response for a generalized elasto-plastic Vh-s shear law described by an initial linear response up to Vmax followed by a plastic plateau at a constant load, which can range from 0 to Vmax; Section 4 compares the model results with the ones of existing methods and FEM analysis. Furthermore, a parametric analysis is carried out to investigate the model sensitivity to the connector parameters; Finally, Section 5 presents a simple point-by-point design procedure of which prediction accuracy of the ultimate moment, deflection and slip was statistically assessed for a large range of possible TCC structures against FEM analysis. Eventually, the effect of concrete cracking is also considered and a correction factor is proposed for engineering purpose.

The influence of thermal curing on the evolution of the material properties and the UHPFRC behaviour was investigated. Tests results showed a beneficial effect of a high temperature curing on the early age material properties due to the thermo-activation effect on the hydration process. However, an inverse effect was observed at long-term. In our study, activation energy of UHPFRC was evaluated from experimental data by means of empirical models. The traditional maturity-function based on Arrhenius law, generally used to describe thermally activated physical or chemical processes, was used to predict the evolution of the UHPFRC autogenous shrinkage and to validate the applicability of this concept for such cement-based materials. Results showed that the concept based on Arrhenius law could describe correctly temperature effects on UHPFRC for temperature lower than 30A degrees C.

The results of tests on two continuous composite beams combining a reinforced concrete (RC) beam with a layer of reinforced ultra-high performance fiber reinforced concrete (R-UHPFRC) are presented. The R-UHPFRC element acts both as a tensile membrane and a flexural element. The tests show the element’s contribution to the member capacity by allowing the redistribution of the internal forces. The continuous beams are placed on two intermediate supports; the shear span-depth ratios and stirrup content are chosen to provoke two successive formations of local flexure-shear collapse mechanisms, forming a plastic hinge at each support. With the formation of the first support hinge, the stresses redistribute. As the applied actuator displacement increases, the member continues to resist the increasing force up to the formation of a second support hinge that causes the member to collapse. The member deflection and resistance at collapse were respectively 4.5 and 1.3 times greater than the corresponding values at the formation of the first hinge. The response demonstrates the redundancy in RC beams with additional R-UHPFRC reinforcement, which can be used for designing structures against progressive collapse.

Ultra high performance fibre reinforced concrete (UHPFRC) early age viscoelastic behaviour under tension was investigated. The tests results showed a high creep potential due to the high volume paste (88%). This result is of major importance because the viscoelastic properties contribute to mitigating the high early age stresses generated under restrained shrinkage. This beneficial effect was reflected by the increased linear-relationship between tensile creep and shrinkage. As expected, UHPFRC tensile creep behaviour was also sensitive to the loading level. Above 35% of the tensile strength at the loading age, the material exhibited viscoplastic behaviour. A Maxwell chain model was applied to predict the early age UHPFRC tensile creep and confirms the induced non-linear response.

This paper presents experimental results on creep and relaxation of \{UHPFRC\} at early age (3–8 days) under low to high tensile stresses (1.2 2.7 5.4 and 8.1 MPa—estimated as 13 30 60 and 90 of the tensile strength respectively) up to the strain hardening domain. Various loading programs (single and incremental loading steps) were used to reveal non linear viscoelasticity. For a very low load level of 1.2 MPa an unexpected important decrease of the creep response was observed. For the 2.7 \{MPa\} load level the viscoelastic responses obtained for creep and relaxation were in the same range. Non ageing linear viscoelastic models were used to analyze and discuss the obtained results. Non linear viscoelasticity was observed in several test series and possible underlying mechanisms were discussed. The hypothesis of shrinkage being the same for free and loaded specimen remains an open question for the analysis of test results.

Ultra-High Performance Fibre Reinforced Composites (UHPFRC) is a cementitious material showing relatively high tensile strength and significant tensile strain-hardening behaviour (given a certain volume of fibres). Adding a layer of UHPFRC or UHPFRC combined with steel rebars (R-UHPFRC) to structural members is an efficient method for strengthening of reinforced concrete structures. This paper presents empirical fatigue damage models for UHPFRC and R-UHPFRC. The tensile fatigue behaviour of UHPFRC is analysed based on elementary damage mechanics theory. Damage grows at a constant rate until fatigue fracture, which is considered to be due to the capacity of UHPFRC to redistribute local deformation increases. Difference in damage evolution between fatigue fracture tests and run-out fatigue tests is highlighted, and it is understood that when significant damage is caused in UHPFRC in the early stage of the fatigue life, UHPFRC fractures due to tensile fatigue. An average curve of damage evolution of fatigue fracture tests is proposed as a bi-linear damage evolution model of UHPFRC. The damage evolution model is used to determine the remaining fatigue life of UHPFRC by correlating the damage-fatigue strain relationship for UHPFRC. Considering that stress transfer from UHPFRC to steel rebars is characteristic of the R-UHPFRC tensile fatigue behaviour and is caused by fatigue damaging of the UHPFRC part, evolution of the modulus of deformation, i.e. the ratio of stress to strain of the UHPFRC part of the R-UHPFRC specimens is investigated. Among all the R-UHPFRC specimens similar behaviour is observed in the fatigue damaging curves of the deformation modulus of the UHPFRC part. An empirical relationship between the modulus of deformation of the UHPFRC part in the R-UHPFRC element and the number of fatigue cycles is proposed to characterise the R-UHPFRC tensile fatigue behaviour. (C) 2015 Elsevier Ltd. All rights reserved.

To date, there have been few reports of fatigue problems in medium-span reinforced or prestressed concrete road bridges, primarily due to the high ratio of the dead load to live load in such structures. Where fatigue problems have arisen, they have generally been due to poor detailing issues, sometimes combined with durability problems. One such example is the construction joints of certain post-tensioned concrete road bridges built in Europe during the 1960s and 1970s, which have been found to be fatigue vulnerable. This paper outlines a new approach to the fatigue safety verification of such details incorporating a direct monitoring approach. The study examines the remaining life of construction joint details in a real case study in Switzerland where detailed modelling combined with traditional fatigue load models were found to be insufficient in explaining the real structural behaviour. A long-term monitoring study was performed over three years to accurately determine the real action effects in the reinforcement elements due to thermal and traffic effects and this information is subsequently injected into fatigue damage calculation models. The seasonality of fatigue damage is assessed and the influence of the monitoring duration is examined. (C) 2015 Elsevier Ltd. All rights reserved.

Investigations on the fatigue strength of steel reinforcement bars (rebars) mainly involves fatigue tests with hot rolled (HR) and cold worked (CW) steels. However, in the last few decades, HR and CW rebars were replaced by quenched and self-tempered (QST) rebars with hardened surface layer. There still remains a lack of research on fatigue strength of QST rebars especially in the very high cycle domain i.e., number of stress cycles surpassing 5 million. This study is part of a further detailed investigation on the fatigue behaviour of HR, CW and QST rebars in the very high cycle domain. It aims to investigate the fatigue performance of QST rebars axially tested at number of stress cycles in the range of 106–108. A preliminary study of the gripping method is followed by fatigue test results including non-destructive inspection of the rebar surface and fractographic analyses. The rebar surface is examined with liquid penetrant to reveal fatigue crack location and size in specific frequency interval monitored during the tests. Fractured surface analyses are performed by scanning electron microscopy to detect the location from where fatigue cracks initiate. Cross sectional area reduction resulting from fatigue crack propagation is also determined. Fractographic investigations are compared with the fractured surfaces of HR, CW and QST rebars from the literature.

An effective method to strengthen existing Reinforced Concrete (RC) structures is to add a thin layer of Ultra-High Performance Fiber Reinforced cement-based Composite (UHPFRC), with or without steel rebars, over the concrete slab to create a composite element. It was demonstrated by previous test series that this method increases rigidity, bending and shear strength of one-way RC members. This paper presents the results of punching tests on 6 composite slabs without transverse reinforcement. The parameters of the tests included the thickness of the UHPFRC layer and the amount of reinforcement in it. All slabs failed in punching mode with a drop in resistance after maximum resistance was measured. For a layer of 50 mm of UHPFRC, the normalised resistance was at least 1.69 times greater than the normalised resistance of the RC reference slab. The layer of UHPFRC increased the rigidity of the slab and provided added shear resistance to the cracked RC section by out-of-plane bending. By doing so, it allowed more deformation to take place in the RC section before punching shear failure. This results in rotations and deflections at maximum resistance similar to what was observed for the reference RC slab.

Current design procedures for structural concrete elements are based on member level (or local) safety checks, in the sense that the safety condition is evaluated at each cross-section individually. In the examination of existing structures, the exploitation of the redundancy of the structural system and its redistribution capacity may prove necessary to fulfil the safety requirements, demanding for a system level (or global) safety evaluation. This can be achieved using nonlinear finite element analysis (NLFEA) procedures and requires a safety format different than that based on partial safety factors. In this work, a simple safety format tailored for NLFEA of existing structures is described. It is shown how to define a global resistance safety factor based on a simple semi-probabilistic approach in line with the recommendations of the new Swiss standards for existing structures [1,2] that can capture the sensitiveness of the structural system resistance, R, to the random variation of the input variables. Besides enabling the use of updated information regarding the material properties, the proposed procedure allows performing reliability differentiation based on risk analysis, being therefore suitable for the safety examination of existing bridges. The definition of the semi-probabilistic global resistance safety factor is based on the assumption of a log-normal probability density function for the resistance R and on an estimate of its coefficient of variation, v(R). The existing proposals for estimating v(R) are reviewed and compared. For statically determined structures with a single dominant failure mode (axial compression, bending or shear) the examination values of R computed with global resistance safety factors are shown to compare well with those obtained with partial safety factors. The reliability of the examination values of R is also evaluated through a comparison with the Monte Carlo simulation procedure and it is shown that the global resistance safety factor method is sufficiently accurate. Finally, a case-study is presented illustrating the application of the proposed procedure in the structural safety examination of an existing prestressed concrete bridge. (C) 2014 Elsevier Ltd. All rights reserved.

Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) combined with steel rebars, subsequently called R-UHPFRC, is a promising building material implying a novel technology for the improvement of concrete structures. Steel rebars enhance effectively the resistance of UHPFRC while reducing variability in the tensile behaviour of monolithic UHPFRC due to variation in fibre distribution and orientation. When a thin layer of R-UHPFRC is overlaid on top of a concrete bridge deck slab, it is subjected to repeating wheel loads and fatigue limit state needs to be considered. This paper presents the results of tensile fatigue tests on R-UHPFRC elements for the determination of its fatigue behaviour. Experimental results show a fatigue endurance limit at 10 million cycles at a solicitation level of S = 0.54 for S being the ratio between the maximum fatigue force and the ultimate strength. Over the fatigue life of the specimens, stress was transferred from UHPFRC to steel rebars. Fatigue resistance of R-UHPFRC shows that it has a significant potential for fatigue strengthening of reinforced concrete structural elements like bridge deck slabs.

An original concept is presented for the durable rehabilitation and strengthening of concrete structures. The main idea is to use ultra-high performance fibre reinforced concrete (UHPFRC) complemen...