Current infrastructure management frameworks typically involve replacing bridges at the end of their intended service duration or when significant structural deficiencies arise, resulting in high costs and environmental impacts. Novel structural-strengthening methods using ultra-high-performance fiber-reinforced cementitious composite (UHPFRC) have allowed the preservation of hundreds of bridges in several countries. Their service duration has been extended, and their performance has been improved to match that of a new structure. Examining the Swiss federal network (3903 bridges), it is found that interventions using the UHPFRC method are feasible on more than 99.7% of structures, demonstrating that the structural intervention can be technically applied to most bridges in this network. On the given case study, systematically applying the UHPFRC method would lead to savings of up to 7.7 MtCO2eq, and 18.5 billion CHF over the next 80 years compared to current engineering practice. This study highlights the significant potential of systematically implementing the UHPFRC method for sustainable and cost-effective infrastructure management.

The tensile fatigue behavior of Ultra-High-Performance Fiber-Reinforced-Cementitious Composites (UHPFRC) is critical for the long-term performance of UHPFRC structures, yet its response under variable amplitude loading remains unexplored. This paper experimentally investigates the tensile fatigue behavior of UHPFRC under two-level block loading, representing a simplified form of realistic structural loading histories. Six specimens were tested after being statically preloaded to a tensile strain of 1.5 parts per thousand, representative of typical maximum strain in structural applications. Prior to fatigue testing, the local fiber volume and orientation of each specimen were quantified. Fatigue loading consisted of stress cycles both below and above the theoretical constant amplitude fatigue limits (CAFL), while specimen responses were monitored using digital image correlation and displacement transducers. It was found that local fiber orientation governs both fatigue deformation and fracture location. The evolution of global deformation followed three stages: an initial rapid increase, a prolonged phase of gradual growth, and an accelerated and unstable increase leading to fracture. In the first and middle stages, deformation growth and specimen stiffness degradation were contributed primarily by the higher stress cycles. During the middle stage, partial recovery of deformation and stiffness occurred during the lower stress cycles, a behavior identified for the first time in UHPFRC and analogous to damage-retardation mechanisms reported in steel, which likely contributed to enhanced fatigue resistance. In the final stage, stress cycles both lower and higher than the CAFL contributed to strain accumulation, showing that stress cycles below the CAFL can also be damaging.

Fatigue‐induced damage in reinforced concrete bridge slabs subjected to traffic loading evolves gradually, often with limited visible signs until critical damage occurs. Monitoring this process is essential for timely maintenance and improved assessment of structural safety. This paper investigates how concrete microcracking evolves spatially and temporally under traffic loading and temperature variations and how microcracking can be characterized using acoustic emission (AE) monitoring. An 18‐month AE monitoring campaign was conducted on a reinforced concrete bridge slab, combining Ib‐value analysis, absolute energy quantification, and three‐dimensional localization of AE events. Two novel fatigue damage indices, one for progressive damage and another for stationary damage, are proposed to quantitatively differentiate microcracking mechanisms and assess fatigue progression. The results show that early‐fatigue damage of the concrete in the reinforced concrete slab is dominated by stationary microcracking, with occasional progressive microcracking events. Spatial analysis revealed concentrated microcracking in regions experiencing significant shear and bending stresses. Additionally, microcracking displayed seasonal cyclic patterns linked to temperature‐induced stress variations. The developed fatigue damage indices and integrated monitoring approach provide practical methods for early‐stage fatigue assessment, supporting proactive management of reinforced concrete structures under in situ operational conditions.

The work focuses on the strength and ductility performance of UHPFRC-RC composite structures, promoting UHPFRC as a protective/reinforcing layer for new construction. Design standard SIA 2052:2016 boosted UHPFRC applications in Switzerland. Data analysis shows that the national technical community regards UHPFRC mainly as a material to enhance the resistance and durability of existing structures. While the increase in strength provided by UHPFRC is evident, assessing deformation capacity for new or repaired structures is critical. Moreover, the ductility of structural elements is pivotal in various approaches, from seismic design to traditional plasticity theory methods for moment redistribution, from warning before failure by large deflections to resistance against imposed deformation. To fully exploit UHPFRC as a valid alternative to traditional materials for new construction, research must focus on this critical aspect at an experimental and numerical level to provide engineers with sound structural design tools for safety checks and assessment. The paper showcases M-N interaction domains and moment-curvature curves for different structural and sectional configurations and critically compares numerical and experimental results. Additionally, it demonstrates how traditional numerical methods for the non-linear analysis of RC structures fit the assessment of (R)UHPFRC-RC composite structures. UHPFRC jacketing of traditional RC structures is a promising solution to retain adequate strength and deformation capacity while preventing aggressive deterioration mechanisms. Finally, the work lays the foundation for further reflections on the actual deformation capacity of structural members repaired with UHPFRC, addressing some common criticisms and ensuring the widespread application of a sustainable and durable material.

UHPFRC stands for ultra-high-performance fibre-reinforced-cementitious composite material, that is complemented by reinforcing steel bars to form a R-UHPFRC strengthening layer. UHPFRC "upcycling" has the goal to enhance the resistance and durability of deficient structural elements in reinforced concrete. It helps to avoid the still widespread and material- and cost-intensive practice of "demolition-replacement" of deficient reinforced concrete bridges. As UHPFRC "upcycling" often offers significant cost and environmental benefits, it should become the default option when dealing with deficient reinforced concrete bridges. This report is intended to serve for the structural engineering community to profit from the 20-year long application experience in Switzerland to broadly apply this new technology.

Renewal of reinforced concrete bridges by means of UHPFRC – Experiences from 400+ applications Ultra-High-Performance Fiber-Reinforced Cementitious composite UHPFRC is a novel building material that is characterized by high mechanical strength and durability. UHPFRC is used in Switzerland for 20 years to rehabilitate and strengthen existing reinforced concrete bridges. This article first appraises more than 400 UHPFRC applications. The findings from numerous applications when casting UHPFRC on bridge deck slabs are prepared in such a way that they can be adopted ready for use by the interested reader. Finally, a recently completed example of a 65-year-old reinforced concrete bridge strengthened with UHPFRC shows how the basic concepts and the great potential of the UHPFRC technology can be transferred into practice, while preserving resources and building culture values.

Passive ultrasonic stress wave, or acoustic Emission (AE), monitoring is a highly effective technique for continuously assessing the structural health of materials, aiding in the prevention of potential failures. AE refers to elastic waves generated during fracture processes, which are detected and recorded by ultrasonic transducers. Quantitative geophysics-based methods enable processing of recorded waveforms to monitor and characterize the spatio-temporal growth of fractures in brittle materials such as concrete and composites. Due to the complexity of the recorded elastic signals and the non-homogeneous nature of the medium, data processing is often performed manually. The high processing costs associated with large datasets, often exceeding terabytes, have limited the practical application of this approach in real-world scenarios. Therefore, an automated methodology is required to reduce costs while maintaining high precision, enabling its integration into Structural Health Monitoring (SHM) and Non-Destructive Evaluation (NDE) frameworks. This paper presents the application of a novel automated and high-precision AE monitoring algorithm and software, SIMORGH-SHM, designed for applications ranging from materials testing to seismicity. The software is compatible with various standard data formats and is capable of processing both trigger-based and continuous data streams. After introducing the software package, initial results from AE monitoring of a 4.2-meter-long Ultra-High-Performance Fiber-Reinforced Cementitious Composite (UHPFRC) T-beam are discussed. The beam was equipped with 24 novel embedded ultrasonic transducers and tested in EPFL’s Structures Laboratory under cyclic loading to failure. Source localizations were performed and damage mechanisms were estimated using Moment Tensor Inversion (MTI) techniques.

The rehabilitation of existing bridges should always be the first investigated solution because it usually reduces construction costs, environmental impacts, and traffic disruption. Nonetheless, bridges are too often replaced due to the lack of effective structural strengthening schemes. The latter reason is particularly true for short-span, simply supported structures that cannot plastically redistribute additional loads. Ultrahigh-performance fiber-reinforced cementitious composites (UHPFRCs) and their technology offer new solutions for enhancing the performance of existing reinforced concrete bridges. The UHPFRC technology allows for the rehabilitation and strengthening of the deck and girder of bridges due to its high mechanical properties and high durability. Conventional UHPFRC interventions are realized by casting an additional layer on the deck, but this solution enables only limited improvement in the case of simply supported structures. This manuscript presents the innovative strengthening concept for short-span concrete bridges with UHPFRCs, consisting of clamping supports to modify the boundary conditions and, thus, the static system. This intervention was recently realized on a prestressed concrete bridge with a single span of 35 m built in 1958 in Switzerland. The bending structural capacity increased by 47%, allowing for the widening of the deck from 5.3 to 7.9 m. Load tests were performed before and after the intervention, and data collected validated the strengthening scheme. Life-cycle cost and environmental analysis showed significant savings (42% and 55%, respectively) compared to the previously proposed deconstruction-reconstruction solution. This case study demonstrates the potential of this strengthening strategy for managing short- to medium-span bridges, respecting sustainable and cost-effective infrastructure management.

Hydraulic structures made of reinforced concrete show damage as a result of hydroabrasion and alkali-silica reaction. This damage can be repaired efficiently using the novel ultra-high-performance, fiber reinforced cementitious composite material UHPFRC (also called UHPC). UHPFRC is applied in relatively thin layers to exposed concrete surfaces with the aim of permanently repairing the concrete damage typical of hydraulic structures and protecting the reinforced concrete. This means that hydraulic structures can be strengthened for a long, continued service life. In Switzerland, the UHPFRC technology has been used for 20 years primarily for the maintenance of bridges and buildings. First applications for the rehabilitation and protection of hydraulic structures were already carried out. In this article, the hydro-mechanical abrasion behavior of UHPFRC is first described by means of laboratory tests and compared with concrete. Two hydraulic structures, a river weir and the turbine house of a hydroelectric power plant, are then described with concrete surfaces that were rehabilitated and protected with a UHPFRC layer ten resp. two years ago. First findings are available as to how the UHPFRC protective layer has proven itself in the case of the river weir.

To explore the governing mechanism underlying the tensile fatigue behavior of Ultra-high Performance Fiber Reinforced Cementitious Composites (UHPFRC), this study tested eight specimens using four advanced nondestructive measurement techniques. First, magnetoscopy is conducted on each specimen to determine the local fiber orientation and volume. Afterward, seven specimens are statically preloaded to the tensile strain of 1.5 parts per thousand, identified as the typical maximum strain of UHPFRC in structural applications; while one specimen to the strain of 0.19 parts per thousand, within the tensile elastic domain. During testing, the specimen response is monitored using digital image correlation and acoustic emission, in addition to displacement transducers. All specimens show similar evolution of fatigue deformation, characterized by three development stages. It is found that the local fiber orientation governs the fatigue deformation behavior. Fatigue deformation concentrates in low fiber orientation zones and fatigue fracture always occurs at the zone with lowest fiber orientation coefficients. The acoustic emission measurement, represented by cumulative energy curve and Ib-values, can appropriately characterize specimen damage degree and distinguish cracking patterns.

This article focuses on the characterization of the early-age properties of Ultra-High-Performance Fiber-Reinforced Cementitious Composite (UHPFRC), which is becoming popular for designing lightweight and durable structures. Due to the large proportion of cement in the mix, the hardening of UHPFRC is significantly faster than conventional concrete. Therefore, the development of UHPFRC properties, such as the elastic modulus, is difficult to monitor as it happens while elements are within the formwork. For this reason, the hydration process of UHPFRC elements is not fully understood yet. A combined passive (or acoustic emission) and active ultrasonic stress wave monitoring approach has the potential to characterize structures made of cementitious materials over their entire service duration. Using a network of embedded ultrasonic transducers, monitoring can start only a few instants after casting. A UHPFRC beam with a T-shaped cross-section and a length of 4.2 m was constructed and instrumented with 24 transducers as well as 15 thermocouples. Monitoring results lead to the characterization of the development of the early-age UHPFRC properties on the structural-element scale while the specimen is within the formwork. The continuous monitoring approach enabled accurate estimations of the spatial and temporal evolution of the modulus of elasticity. Thanks to this novel combination of monitoring techniques, the early age properties of UHPFRC, which were measured at the material scale, are confirmed at the structural scale for the first time.

This paper investigates the high cycle tensile fatigue behavior of steel rebar reinforced - UHPFRC elements, at a fatigue load ratio, i.e., R-ratio of 0.3, representative for structural applications. Prior to testing, magnetoscopy is conducted on each specimen to determine the local fiber orientation and volume inside UHPFRC. During testing, global specimen deformation is recorded by displacement transducers; specimen surface is monitored by digital image correlation; and strain along rebars inside the specimen is measured by fiber-optic sensors. Based on the test results, an S-N diagram with a high regression coefficient is obtained. Hereby, the normalized fatigue force S is defined as the ratio between the maximum fatigue force and the estimated specimen ultimate tensile resistance. The fatigue endurance limit is identified as being about S = 0.40. It is found that fatigue deformation of the specimen mainly occurs in the zones with low fiber orientation coefficient mu 0,y of UHPFRC (mu 0,y decreases when average angle between fiber axis and principle tensile direction changes from 0 degrees to 90 degrees), where the strain along steel rebars also have their higher value and increase rates during fatigue testing. The lowest UHPFRC fiber orientation determines the locus of crack localization and of fatigue fracture of steel rebars, thus final fracture of the elements.

This paper investigates the fracture mechanism of ultra-high-performance fiber-reinforced cementitious composites (UHPFRC) using acoustic emission (AE), digital image correlation (DIC), and magnetoscopy testing. Four specimens undergo uniaxial tensile loading, preceded by magnetoscopy testing to determine local fiber volume and orientation. DIC captures matrix discontinuities, crack initiation, and propagation. Acoustic emission monitors fracture mechanisms at different loading phases. During the elastic phase, matrix discontinuities and fiber debonding are observed to occur. A higher density of matrix discontinuities during this phase enhances hardening behavior and tensile performance. The softening phase of UHPFRC is found to be characterized by three stages based on AE parameters: emergence and competition of multiple fictitious cracks, propagation of a dominant fictitious crack, and real crack formation. The rate of dominant fictitious crack propagation can be determined by analyzing the evolution in AE parameters with stress decrease. Uniform fiber distribution limits the initiation and propagation of fictitious cracks.

This paper outlines concepts using ultrahigh-performance fiber- reinforced composite (UHPFRC) to address structural deficiencies in modern fixed railway track systems of concrete, focusing on enhancing structural safety, durability, and fatigue resistance. It introduces UHPFRC as a key material known for its high compacity and ability to remain crack-free in the serviceability limit state. In the context of modern railway infrastructure, where robust, durable, and crack-free construction is necessary for safe train operation, UHPFRC has the potential to provide costeffective solutions for critical, heavily stressed areas of slab-like ballastless railway track systems and to improve the load-bearing structures of railway bridges with fixed railway tracks. Its use is expected to significantly reduce maintenance costs and extend the service duration of fixed railway tracks to align with the service duration of load-bearing structures of more than 100 years.

This paper quantitatively studies the influence of fiber orientation on the high cycle tensile fatigue resistance of ultra-high performance fiber reinforced cementitious composites (UHPFRC) within and beyond elastic domain. Eight tensile fatigue tests and re-analysis of sixteen fatigue tests from a previous study are conducted, which have specimens statically preloaded to the strain from 0.19 %o to 0.5 %o and from 1.1 %o to 2 %o, considered to be in the elastic and post-elastic domains in tension, respectively. For the eight fatigue tests, the local fiber orientation and dosage of each specimen are determined before testing using a magnetic probe. During testing, specimen behavior is characterized by displacement transducers, digital image correlation and acoustic emission. Based on the above, the fatigue endurance limit at 10 million cycles is determined to be aboutSUte,10% = 0.9 of the elastic limit tensile stress for UHPFRC within elastic domain. For UHPFRC preloaded beyond the elastic domain, the fatigue resistance decreases with increasing pre-applied strain and decreasing average fiber orientation (increasing average angle between fibers and principle tensile direction). With pre-applied strain from 1.1 %o to 2.0 %o, the endurance limit may be taken asSUte,10% = 0.60 to 0.65 for typical UHPFRC structures. Two original fatigue resistance models are proposed, which describe the relationship between average fiber orientation, number of cycles and maximum fatigue stress for UHPFRC within and beyond the elastic domain.

In this study, the structural behavior of a short threaded anchor with a 20 mm diameter and an embedment length of 50 mm (2.5Ø) in UHPFRC is investigated using non-linear Finite Element models. UHPFRC is assumed to exhibit tensile strain-hardening behavior, with tensile strengths of 7 MPa and 11 MPa, respectively. The modelled anchor was subjected to a continuously increasing uniaxial pull-out force. The results indicate that the fracture mechanism of threaded anchors in UHPFRC is primarily characterized by the formation of a tensile membrane within the UHPFRC, which acts as the main resisting element against the pull-out force. Additionally, the influence of UHPFRC's tensile properties on the pull-out behavior and ultimate resistance of the threaded anchors was determined.

Combined passive [or acoustic emission (AE)] and active ultrasonic stress (US) wave monitoring has been shown to provide a more holistic picture of ongoing fracture processes, damage progression, as well as slowly occurring aging and degradation mechanisms in concrete structures. Traditionally, different data analysis techniques have been used to analyze the data generated from these two monitoring approaches. For AE data analysis, for instance, signal amplitudes, hit rates, source localization, and b-value analysis have been used to detect and locate cracking. On the other hand, amplitude tracking, magnitude squared coherence (MSC), and coda wave interferometry (CWI) are examples that have been employed for US data analysis. In this presentation, we explore these data analysis techniques and show where their respective applications and limitations might be. After providing an overview of the monitoring approach and the different data analysis techniques, results and observations from select laboratory experiments, as well as an in-service structure are discussed. Finally, suggestions for further work are proposed.