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.

This paper presents the results of 28-month-long monitoring of a slab portion of the Chillon viaducts in Switzerland using strain gauges and thermocouples. This post-tensioned reinforced concrete structure was strengthened with a layer of reinforced ultrahigh-performance fibre-reinforced cementitious composite (UHPFRC). The strain gauges are used to measure stresses in the bottom layer of reinforcement bars in the slab, while thermocouples explain the behaviour of the structure due to temperature variation. The response under both traffic and thermal actions is discussed. It is demonstrated that the stress variation due to the thermal action can be as large as the response due to traffic action even in such a massive structure. Furthermore, recommendations for analysing both traffic and thermal-induced stresses are given. The commonly used simplified method for calculation of fatigue stress is shown to be highly conservative, leading to overestimation of structural action effects by a factor of four.

Following the significant improvement in their properties during the last decade, Distributed Fiber Optics sensing (DFOs) techniques are nowadays implemented for industrial use in the context of Structural Health Monitoring (SHM). While these techniques have formed an undeniable asset for the health monitoring of concrete structures, their performance should be validated for novel structural materials including Ultra High Performance Fiber Reinforced Cementitious composites (UHPFRC). In this study, a full scale UHPFRC beam was instrumented with DFOs, Digital Image Correlation (DIC) and extensometers. The performances of these three measurement techniques in terms of strain measurement as well as crack detection and localization are compared. A method for the measurement of opening and closing of localized fictitious cracks in UHPFRC using the Optical Backscattering Reflectometry (OBR) technique is verified. Moreover, the use of correct combination of DFO sensors allows precise detection of microcracks as well as monitoring of fictitious cracks’ opening. The recommendations regarding use of various SHM methods for UHPFRC structures are given.

Under service conditions, R-UHPFRC (Reinforced Ultra High Performance Fiber Reinforced Cementitious composite) beams exhibit residual deflection after loading–unloading. This is due to the tensile strain hardening behavior of UHPFRC. The precise calculation of deflection is thus relevant and was not addressed previously. This paper proposes a material model for UHPFRC under loading–unloading and a numerical layered model for the calculation of stress and strain distribution in the cross section. Then, a curvature-based analytical model is presented for calculation of deflection of a beam. This method is finally compared and validated against experimental results as obtained from four-point bending of full-scale R-UHPFRC beams. This research reveals the need for a specific material model for UHPFRC subjected to loading–unloading for the precise calculation of the structural response of elements and members under repetitive loading, such as service or fatigue loading.

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.

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.