Early 21st-century droughts in Europe have been broadly regarded as exceptionally severe, substantially affecting a wide range of socio-economic sectors. These extreme events were linked mainly to ...increases in temperature and record-breaking heatwaves that have been influencing Europe since 2000, in combination with a lack of precipitation during the summer months. Drought propagated through all respective compartments of the hydrological cycle, involving low runoff and prolonged soil moisture deficits. What if these recent droughts are not as extreme as previously thought? Using reconstructed droughts over the last 250 years, we show that although the 2003 and 2015 droughts may be regarded as the most extreme droughts driven by precipitation deficits during the vegetation period, their spatial extent and severity at a long-term European scale are less uncommon. This conclusion is evident in our concurrent investigation of three major drought types - meteorological (precipitation), agricultural (soil moisture) and hydrological (grid-scale runoff) droughts. Additionally, unprecedented drying trends for soil moisture and corresponding increases in the frequency of agricultural droughts are also observed, reflecting the recurring periods of high temperatures. Since intense and extended meteorological droughts may reemerge in the future, our study highlights concerns regarding the impacts of such extreme events when combined with persistent decrease in European soil moisture.
The aim of this paper is to describe mix design of Ultra High Performance Fiber Reinforced Concrete (UHPFRC) and its response to deformable and non-deformable projectile impact. UHPFRC represents a ...class of cementitious composite in which stress–strain response in tension undergoes strain hardening behaviour accompanied by multiple cracking, leading to a high strain prior to failure. The compressive strength of the resulting UHPFRC mixtures exceeded 130 MPa and direct tensile strength was in the range of 10 MPa. Several UHPFRC mixtures with different content of fibers were subjected to deformable projectile impact. It was found that specimens containing 2% of fibers by volume have optimal resistance against deformable projectile impact. Slabs containing 2% of fibers were further subjected to a non-deformable projectile impact. In addition, response of slabs made of traditional fiber reinforced concrete (FRC) is discussed. The magnitude of the damage was assessed based on the penetration depth, crater diameter and loss of mass.
•The mixture formulation and material properties of the UHPFRC are presented.•Composites containing more than 2% of fibres by volume exhibit tensile hardening.•Slabs made of this material are loaded by deformable and non-deformable projectiles.•Fibre content beyond 2% by volume has no effect on reducing the crater diameter.•Addition of fibres reduces penetration depth compared to reference concrete.
Drought is one of the main threats to food security and ecosystem productivity. During the past decades, Europe has experienced a series of droughts that caused substantial socioeconomic losses and ...environmental impacts. A key question is whether there are some similar characteristics in these droughts, especially when compared to the droughts that occurred further in the past. Answering this question is impossible with traditional single-index approaches and also short-term and often spatially inconsistent records. Here, using a multidimensional machine learning-based clustering algorithm and the hydrologic reconstruction of European drought, we determine the dominant drought types and investigate the changes in drought typology. We report a substantial increase in shorter warm-season droughts that are concurrent with an increase in potential evapotranspiration. If shifts reported here persist, then we will need new adaptive water management policies and, in the long run, we may observe considerable alterations in vegetation regimes and ecosystem functioning.
Versuche an Stahlbetonbalken ohne Bügel dienen häufig als vereinfachte Grundlage zur darauf aufbauenden numerischen Simulation von Stahlbetonplatten. In diesem Beitrag wird eine experimentelle Studie ...des Verhaltens von Stahlbetonbalken unter Stoßbelastung vorgestellt. Die Versuche wurden im 11 m hohen Fallturm des Otto‐Mohr‐Laboratoriums der TU Dresden mit zylindrischen Impaktoren von 100 mm Durchmesser durchgeführt, welche mit Druckluft auf Geschwindigkeiten zwischen 2,7 m/s und 25,8 m/s beschleunigt wurden. Die verwendeten Impaktoren wogen 14,22 kg und 21,66 kg. Die Impaktor‐ und Auflagerkräfte, die Balkenverschiebungen und ‐beschleunigungen wurden an ausgewählten Stellen erfasst. Die Sensordaten wurden durch digitale Bildkorrelation (DIC) ergänzt. Die Balken wiesen je nach Aufprallgeschwindigkeit und ‐masse unterschiedliche Versagensarten auf. Scherrisse bildeten sich ab Aufprallgeschwindigkeiten von etwa 11 m/s, was beim leichteren Impaktor einer kinetischen Impaktorenergie von 860 J entspricht. Unter Aufprallgeschwindigkeiten von etwa 22 m/s wurde die Bildung eines Durchstanzkegels in der Nähe des Belastungspunkts beobachtet. Diese Ergebnisse liefern wichtige Erkenntnisse zur Bewertung von Stahlbetonbalken ohne Querkraftbewehrung unter Impaktbelastung. Weiterhin dienen die Ergebnisse der Verbesserung von numerischen Modellen, sodass diese präziser und verlässlicher komplexe Großstrukturen abbilden können.
Translation
Experimental study of reinforced concrete beams without stirrups under impact: performance evaluation and structural response
Experiments on reinforced concrete beams without stirrups are often utilized as a simplified basis to calibrate numerical models of reinforced concrete slabs. This paper details an experimental study on the behavior of reinforced concrete beams without stirrups subjected to impact loading. For this study, the 11‐meter‐high drop tower at the Otto Mohr Laboratory of TU Dresden was used to accelerate a cylindrical impactor with a diameter of 100 mm. Compressed air accelerated the impactor to velocities between 2.7 m/s and 25.8 m/s. The impactor masses were 14.22 kg and 21.66 kg, respectively. Data on impactor and support forces, displacements, and accelerations were collected. Digital image correlation supplemented the measurements. The study revealed that the behavior of the beams exhibited different failure modes depending on the impactor velocity and mass. The shear cracks started to form at impactor velocities of approximately 11 m/s, corresponding to the lighter impactor's kinetic energy of 860 J. Furthermore, the formation of a punching cone near the loading point at impactor velocities of around 22 m/s was observed. These findings provide valuable insights for designing and thoroughly assessing the performance of shear‐deficient reinforced concrete beams under impact loading. The results serve as a foundation for enhancing the precision and reliability of numerical models, facilitating further investigations into complex structural systems.
Compound extreme events, such as flash drought, have received wide attention in recent decades due to their far-reaching effects on the ecosystem. Thus, a new concept of flash drought has begun to ...spread globally in the scientific community, and it is continuously being developed. This study offers for the first time an overview of the global trends in flash drought research from 2000 to 2021. The analysis was based on the Scopus database in order to investigate the publication trends, contributions, collaborations, and challenges on a global scale. Furthermore, collaboration analysis was performed to detect collaboration networks within the flash drought research field. A total of 76 studies were published during the study period. The research output grew exponentially with an average growth rate of 30% per year. The challenging issues in the field of flash drought research are the search for appropriate definition of flash drought, the development of effective early warning systems and the scarcity of high-resolution data. By presenting the details of the evolution of this new conceptualization in drought research, our study highlights the main pathways of scientific progress and stimulates future research.
Significant buildings, transportation hubs, protective or defence structures are at exposure risk to extreme load event such as blast loading or direct armed attack. This kind of loading is typical ...by its rapid increase in release of energy in a very short time. It is believed that emerging cementitious materials such as Engineered Cementitious Composite (ECC) or Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) provide better passive protection to significant buildings compared to High-Strength Concrete (HSC) or conventional Fibre Reinforced Concrete (FRC). In this study, response of several UHPFRC slabs to projectile impact was examined in order to simulate the effects of possible damage by small fire arms or by fragments generated by an explosion. Real ammunition was used with impact velocity in the range of 691–720 m/s and fibre content of the UHPFRC was selected as the main test variable. The optimal fibre content in the UHPFRC mixture was determined using damage parameters such as crater diameter, penetration depth, debris fragment mass and residual penetration potential of the bullet. It was verified experimentally that the optimal fibre content in the UHPFRC mixture is 2% by volume. Using less than 2% of fibre volume fraction might be unsafe in the slim UHPFRC targets due to increased volume of secondary fragments generated from the back side of the slab and also due to the higher residual penetration potential of the bullet exiting the back side of the slab. Using more that 2% of fibre volume fraction could be inefficient. No improvement in all damage parameters was observed when the fibre volume fraction was changed from 2% to 2.5% or 3%.
•UHPFRC slabs were subjected to projectile impact.•Projectile impact tests were carried out by using a real ammunition.•The fibre content in the UHPFRC was set as the main test variable.•Established damage parameters included penetration depth, crater diameter and residual penetration potential of the bullet.•The optimal fibre content in the UHPFRC mixture was determined to be 2% by volume.
•We research the issues of dynamic bond for reinforced concrete both experimentally and numerically.•We highlight how stress waves propagate in the system after the impact event and thus how dynamic ...experimentally recorded signals (strain gauge signals) are influenced by the chosen experimental set-up.•Numerical simulations enable insights into local structural phenomena of bond behaviour and crack formation.•The results are in good qualitative accordance with data from literature.
Finite element analysis of concrete-to-rebar bond specimens under dynamic loading was conducted. This involved detailed modelling of the reinforcing steel bar ribs and the concrete keys in between. The modelling aimed to realise insight into the local structural phenomena and create a solid base for the prediction of bond behaviour under varying conditions. The analysis was performed in 3D with the explicit finite element code LS-Dyna. The proposed Soil and Foam Failure material model for concrete is described and definition of the material parameters is discussed. The numerical results are compared to experimental data obtained during dynamic push-in bond experiments carried out at the Technische Universität Dresden. Strain signals, slip measurements, bond stresses and crack patterns are analysed. The obtained signals are decomposed based on fundamentals of wave propagation. Significant influential factors like the experimental set-up geometry are hence identified. The capability of the model to predict key aspects of bond behaviour under dynamic loading is demonstrated and its applicability for future parametric studies is highlighted.
Detailed investigations of time series features across climates, continents and variable types can progress our understanding and modelling ability of the Earth’s hydroclimate and its dynamics. They ...can also improve our comprehension of the climate classification systems appearing in their core. Still, such investigations for seasonal hydroclimatic temporal dependence, variability and change are currently missing from the literature. Herein, we propose and apply at the global scale a methodological framework for filling this specific gap. We analyse over 13,000 earth-observed quarterly temperature, precipitation and river flow time series. We adopt the Köppen–Geiger climate classification system and define continental-scale geographical regions for conducting upon them seasonal hydroclimatic feature summaries. The analyses rely on three sample autocorrelation features, a temporal variation feature, a spectral entropy feature, a Hurst feature, a trend strength feature and a seasonality strength feature. We find notable differences to characterize the magnitudes of these features across the various Köppen–Geiger climate classes, as well as between continental-scale geographical regions. We, therefore, deem that the consideration of the comparative summaries could be beneficial in water resources engineering contexts. Lastly, we apply explainable machine learning to compare the investigated features with respect to how informative they are in distinguishing either the main Köppen–Geiger climates or the continental-scale regions. In this regard, the sample autocorrelation, temporal variation and seasonality strength features are found to be more informative than the spectral entropy, Hurst and trend strength features at the seasonal time scale.
PDF
