Seismic Performance of Pre-Fabricated Segmental Bridges With An Innovative Layered-UHPC Connection

Ultra-high-performance concrete (UHPC) has been regarded as promising alternative to provide reliable connections between difference segments (e.g., columns and pier footing/cap) during accelerated bridge construction (ABC) procedures. This paper proposes an innovative layered-UHPC connection for the pre-fabricated segmental (PFS) pier, whose seismic performance was validated through quasi-static experiment. The corresponding design procedure for PFS pier with this type of connections is presented based on the test results. The layered-UHPC connection ensures the emulative performance of pre-fabricated bridge as cast-in-place (CIP) ones, as well as provides greater economic efficiency than traditional UHPC connections. Based on experimental results, key issues concerning this connection, including the tensile behavior of UHPC, height of connection region, thickness of UHPC layer and steel bars in grouting bed, are presented and discussed. Then a seismic design procedure is proposed utilizing the capacity protection philosophy widely adopted in design specifications. The layered-UHPC connection is expected as capacity-protected component without damage, since it provides anchorage for steels extended from columns and pier cap/footing. While the pre-fabricated region is designed as ductile component undergoing nonlinearity during strong earthquakes. Following the detailed elaborations about the design philosophy, requirement and implementation steps, this procedure is further presented through illustration examples using PFS piers with various heights. The results show that PFS piers designed according to this procedure could meet the requirement under both frequent and rare earthquakes. Note that the PFS piers with this layered-UHPC connections could be designed similar to and emulative as CIP ones, which is believed friendly to designers in engineering practice.

Rapid Consolidation of WC-ZrSiO4 Hard Materials by Spark Plasma Sintering: Microstructure, Densification, and Mechanical Properties

Densely consolidated WC-based hard materials with 5–20 vol% ZrSiO4 was fabricated by spark plasma sintering at 1400 ℃ at a constant heating rate of 70 ℃/min−1. To achieve mechanical alloying of WC-ZrSiO4, planetary ball milling was carried out for 12 h, during which the brittle-brittle components (WC-ZrSiO4) became fragmented and their particles became refined. It was observed that certain, specific, non-isothermal sintering kinetics, such as apparent activation energy, sintering exponents, and densification strain, affected the densification behavior. The evolution of phase structure from powder to compact was found to be related the lattice distortion and micro-strain in the basal planes of WC. By examining the mechanical properties of the samples, it was that the added zircon content leads to enhanced fracture toughness (12.9 MPa m1/2) owing to the presence of WC-ZrSiO4 in the cemented carbide. In fact, the microcrack propagation of the fracture passed through zircon from a transgranular to a ductile component (fcc) where the crack tips could be absorbed. Graphic Abstract

Fractographic Study of the Mechanism of Destruction of the Nitrocarburized Layer

In this paper we investigate the nature of the impact fracture of steels 20 and 20Cr specimens in the nitrocarburized layer and in the core. The object of the study were the samples after thermocycling and isothermal nitrocarburizing. As the results showed, the greatest increase in impact ductility is achieved in five cycles of nitrocarburizing. It is shown that the destruction of the hardened layer and the steel core after the isothermal process is quasirectangular in nature. The presence of the diffusion layer treated by modes of thermocycling nitrocarburizing, areas of ductile fracture and quasi-cleavage in the fracture indicates greater intensity of the process of destruction in comparison with the isothermal process, in which areas of intergranular fracture are present and ductile fracture elements are not present in the fracture. Thus, the fractographic study revealed some features of the mechanism of steel destruction after chemical-thermal nitrocarburizing in comparison with the isothermal process. During thermal cycling of steels, a large amount of the ductile component is observed in the fracture. As the results showed, the greatest increase in impact ductility is achieved in five cycles. In steel 20Cr, the impact ductility increases by 2 times, and in steel 20 by 2.6 times. Increasing the number of cycles to 9 leads to a significant reduction in impact ductility. So in steel 20Cr after chemical-thermal nitrocarburizing, the impact ductility values become less than after classical processing. A further increase in the number of cycles leads to an even greater decrease in the impact ductility values.

Single-event throw distribution along the Delta fault (Baikal rift) from geomorphological and ground-penetrating radar investigations

<div> <p>Tectonic displacement is one of the important parameters in determining the seismic potential of an active fault. Its distribution along the fault strike is highly variable; therefore, when assessing seismic hazard, both the quality and the number of measurements of single-event throws are essential. We reconstructed and studied peculiarities of distribution of vertical displacements, which occurred on the land-based part of the Delta fault during the devasting M~7.5 Thagan earthquake of 12 January 1862. Morphologically, the seismogenic structure is expressed by the fault scarp in unconslolidated Holocene sediments, which underwent significant liquefaction and fluidization during the seismic event. In space, the fault scarp coincides with the lacustrine-deltoid and alluvial-deltoid terraces of Lake Baikal and the Selenga river and complicated by eolian deposits.</p> <p>As a basic method, we used ground-penetrating radar (GPR) in combination with data from shallow drilling, trenching and analysis of seven topographic profiles. By measuring near-field displacements at the fault planes (brittle component) and far-field displacement at a distance from the fault plane (sum of brittle and ductile components according to Homberg et al. (2017)) on GPR sections, we subtracted folding component of the total throw. Besides, we considered a number of other parameters in relation with the value of the last single-event offset in the upper sedimentary layer at a depth of the first meters. As a result, it was found that the displacement during the Tsagan earthquake occurred under NW-SE extension as motion on a stepped system of normal faults with a dip of the major plane to the NW at angles 56–77°. The total throws from GPR data on each of seven profiles were 3.83 m, 9.59 m, 2.4 m, 4.27 m, 9.28 m, 5.23 m, and 1.81 m, which are aligned with vertical fault displacements H1 with an error from 0.03 to 0.47 m. H1 was defined as a vertical distance between the intersections of the fault plane, and planes formed by the displaced original geomorphic surfaces (McCalpin, 2009). The brittle components were 2.32 m, 5.54 m, 1.93 m, 3.0 m, 6.07 m, 3.2 m, and 1.58 m, respectively. The contribution of the ductile component to the total displacement varies from 13% to 42%, the visible fault damage zone widths are from 2.55 m to 20 m. The maximal contributions of the ductile component correspond to minimal fault dips of the major fault plane and, as a whole, to the largest fault damage zone widths, which also correlate well with the offset values.</p> <p>The structural features of the rupture zones and peculiarities of throw distribution in unconsolidated sediments should be taken into account in order to avoid underestimating the magnitudes of the normal fault earthquakes and their seismic effect. In the case of soft sediments of mixed rheology (competent and incompetent), obviously, one should expect large values of total displacements and wider zones of deformations, in comparison with homogeneous sections. Acknowledgments: The reported study was partly funded by RFBR, project number 19-35-90003.</p> </div>

Effect of Micro-Alloying with Boron and Niobium on Properties of Cr-Mn-Mo Steels

Economically alloyed steels for critical details of drill pipes have been studied. Effect of microalloying on structure and properties has been investigated. The article shows that boron and niobium additives change the structure and properties of Cr-Mn-Mo steels after quenching and high tempering. Methods of optical and electron microscopy have been used. Basic mechanical properties and impact strength of investigated steels are determined. Optical and electronic fractography has been carried out. The quantitative content of the ductile component is determined in steel fracture. It is shown that steel microalloying leads to a substantial structure refinement. This is due to the influence of niobium on the austenite grain value. An increase in the amount of carbide particles leads to structure refinement with an increased molybdenum content. Boron microadditives allow obtaining the tempered martensite structure throughout the product section. This provides an increase in both the strength and ductile properties. Combined microalloying of chromium-manganese-molybdenum steel with additions of boron up to 0.005 % and niobium up to 0.05 % makes it possible to increase the strength and reduce the tendency to brittle fractures significantly. The nature of the fracture becomes completely ductile. Distinct cleavage fracture surface feature “river patterns” are observed in unmodified steels. Сleavage facetes are large enough, it proves the presence of large grains in the steel. Microalloying changes the destruction mechanism, it becomes a ductile “dimple rupture”. An increase in the molybdenum content to 0.6 % makes it possible to obtain strength above 1100 MPa in microalloyed steel.

Polymer Microlayer Composites

Continuous microlayer composites of polycarbonate (PC), a ductile glassy polymer, and styrene-acrylonitrile copolymer (SAN), a glassy relatively brittle polymer, were discussed. Microlayered systems composed of 49 to 776 continuous layers, in which the primary variable was the variation in thickness of the continuous layers between 30 and 2μm, were emphasized. The bulk properties of these microlayered composites showed a dramatic improvement in the toughness or ductility and in the fatigue properties as the layer thickness decreased. Investigation of the irreversible deformation processes revealed that when the layers were thicker (30μm, 49 layers) the SAN layers crazed and the PC shear banded in the usual manner. Subsequently the composite system fractured in a relatively brittle manner due to the development of large voids in the SAN layers. When the layer thickness was reduced to 2μm (776 layers) the entire system behaved in a ductile manner and both the PC and SAN shear banded due to a new cooperative process. This was analyzed by considering the micromechanics of these irreversible processes at the PC-SAN interface. As a result of this work, it is expected that ultra-thin layered structures of other alternating composite systems will reveal synergistic properties if the interfacial properties are designed so that the ductile component will dominate the yield and failure characteristics of the entire system.