Initial Widths Research Articles

Autolytic capsules incorporating alkali-activated slag system for self-healing cementitious composites

The optimization of healing materials used in capsules for effective crack self-healing has garnered significant attention. This study focused on the development of a novel capsule using a full-solid-waste alkali-activated slag system. Instead of traditional cement clinker and chemical agents, healing materials such as red mud, calcium carbide residue, phosphogypsum, and ground granulated blast furnace slag were used. The capsules were autolytic due to the retardation and weak expansion properties of gypsum. The healing mechanisms were mainly attributed to the formation of ettringite and calcium silicate hydrate. Crack width, crack area, and strength recovery tests were used to thoroughly analyze the self-healing behavior of the capsules. Detailed characterizations were conducted to understand the healing products and the evolution process. The results showed that cracks with initial widths less than 350 μm were well-healed, with a healing ratio ranging from 55% to 100% for all cracks (initial widths <600 μm). The cementing characteristics of the healing products were significantly increased by the formation of substantial ettringite, which led to an enhanced strength recovery ratio. Crucially, the composition of the healing products closely resembled the hydrates in the matrix, suggesting that the healed mortar had a high level of system compatibility. These results highlighted the potential of the developed alkali-activated slag capsule in promoting self-healing of cracked cementitious composites.

Assessment of self-healing behavior of polypropylene fiber-reinforced cement mortar with crystalline admixture: the effects of crack widths, cracking ages, and external conditions

Crystalline admixture (CA) is an effective self-healing agent for mortar. However, the effects of crack parameters (i.e. crack width and cracking age) and the service environment on the self-healing behavior of CA-containing mortar are not well understood. Herein, the self-healing behavior of mortar containing a self-developed CA was assessed by testing strength recovery, impermeability recovery, and crack closure in pre-cracked specimens. Three initial crack widths (0.2, 0.3, and 0.4 mm), five cracking ages (3, 7, 14, 28, and 56 days), and four external exposure conditions (humidity chamber, air exposure, water immersion, and wet-dry cycles) are investigated. Furthermore, the influence of different external conditions on the healing products at the region of crack and the pore structure of hardened paste containing CA are studied. The results show that adding 4.54% CA into mortar allows rapid healing of 300 μm-wide cracks. Although wider cracks (400 μm) are more difficult to heal, the sorptivity coefficients of the mortars with 400 μm-wide cracks after healing decrease. When the cracks are produced at an earlier age, the pre-cracked specimens have higher recovery ratios of strength and impermeability after healing, and the specimens pe-cracked at a later age still have acceptable compressive strengths after healing. The analysis shows that the strengths and impermeabilities of pre-cracked mortars containing CA exposed to the four external conditions are all recovered. The best self-healing performance is observed for the specimens exposed to water immersion and wet-dry cycles conditions. Somewhat less good self-healing was observed in the specimens exposed to humid chamber condition, while the worst self-healing performance was in the specimens exposed to air exposure condition. This study provides a theoretical basis for the application of novel CAs in cement-based materials.

Cost-sensitive continuous ensemble kernel learning for imbalanced data streams with concept drift

In stream learning, data continuously arrives over time, often at a very high rate. For imbalanced data streams with concept drift, it becomes essential to simultaneously address classification accuracy and time efficiency. However, existing algorithms currently either demand excessive computational time or exhibit relatively lower classification performance. To address these issues, in this paper, a cost-sensitive continuous ensemble kernel learning method (CCEKL) is proposed to deal with the imbalanced data streams with concept drift. Firstly, for two-class data streams, a novel misclassification cost is proposed to adapt to the changing imbalance rate. Secondly, based on the modified loss function, a continuous kernel learning method is applied to adapt to the changing data distribution. Furthermore, the two-class method is extended to multi-class classification to deal with multi-class dynamic imbalance problems. Finally, we use an ensemble approach to address the proposed algorithm sensitivity to the initial kernel width. To further improve efficiency, a parallel approach is applied to simultaneously train multi-classifiers with different initial kernel widths. Experimental results on 30 data streams reveal that, for binary classification, CCEKL achieves superior balanced accuracy in a shorter training time. For multi-class classification, CCEKL attains better balanced accuracy without requiring excessive training time, surpassing the most of the state-of-the-art baseline algorithms.

Healing outcomes of open versus closed flap procedures for collagen membrane coverage following immediate dental implant placements with simultaneous guided tissue regeneration

ObjectiveTo compare the outcomes of open healing to complete closure for collagen membrane coverage for immediate implant placements with simultaneous guided bone regeneration (GBR) in two retrospective cohorts. MethodsThe subjects included 118 patients who received Bio-Gide® collagen membrane coverage for immediate implant placements and GBR in 20 anterior and 98 posterior teeth. For 58 patients, gingival flaps were released to achieve full coverage of collagen membrane (CC group). For 60 patients, no efforts were made to release the gingival flaps and collagen membrane was left exposed for open healing (OH group). Antibiotics and analgesics were prescribed for 7 days after surgery. The width of crestal open wounds were measured after surgery (W0), and at 1, 2 and 16 weeks (W16). Changes in bone mass were assessed by cone-beam computed tomography after implant placement and again at W16. Gingival and bone tissues over the implant cover screws were harvested and assessed for 16 patients in the OH group at W16. ResultsNo wound dehiscence occurred in the CC group from W0 to W16. Both the vertical and horizontal bone dimension changes were not significantly different between the OH and CC group. For the OH group, soft tissue was completely healed at W16 when the initial wound widths were ≤6 mm. For those with initial wound widths ≥ 7 mm, the cover screws were exposed in 5/16 patients at W16 but did not affect the final restorations. Tissue staining showed keratinized mucosa and new bone formation above the dental implant in the OH group. ConclusionOpen healing achieved healing outcomes similar to those of complete closure for collagen membrane coverage following immediate implant placements. Clinical significanceFor immediate implant placement requiring bone grafting and collagen membrane coverage, it is unnecessary to release the gingival flaps or use tissue grafts to achieve full coverage of the crestal wounds. Open healing with exposed membrane could achieve similar outcomes with less pain and swelling.

Three-dimensional magnetohydrodynamic modeling of auto-magnetizing liner implosions on the Z accelerator

Auto-magnetizing (AutoMag) liners are cylindrical tubes that employ helical current flow to produce strong internal axial magnetic fields prior to radial implosion on ∼100 ns timescales. AutoMag liners have demonstrated strong uncompressed axial magnetic field production (&amp;gt;100 T) and remarkable implosion uniformity during experiments on the 20 MA Z accelerator. However, both axial field production and implosion morphology require further optimization to support the use of AutoMag targets in magnetized liner inertial fusion (MagLIF) experiments. Data from experiments studying the initiation and evolution of dielectric flashover in AutoMag targets on the Mykonos accelerator have enabled the advancement of magnetohydrodynamic (MHD) modeling protocols used to simulate AutoMag liner implosions. Implementing these protocols using ALEGRA has improved the comparison of simulations to radiographic data. Specifically, both the liner in-flight aspect ratio and the observed width of the encapsulant-filled helical gaps during implosion in ALEGRA simulations agree more closely with radiography data compared to previous GORGON simulations. Although simulations fail to precisely reproduce the measured internal axial magnetic field production, improved agreement with radiography data inspired the evaluation of potential design improvements with newly developed modeling protocols. Three-dimensional MHD simulation studies focused on improving AutoMag target designs, specifically seeking to optimize the axial magnetic field production and enhance the cylindrical implosion uniformity for MagLIF. By eliminating the driver current prepulse and reducing the initial inter-helix gap widths in AutoMag liners, simulations indicate that the optimal 30–50 T range of precompressed axial magnetic field for MagLIF on Z can be accomplished concurrently with improved cylindrical implosion uniformity.

Statistical and experimental study on microcrack of mortar by a self-healing agent of bacteria adsorbed by expanded perlite

PurposeThe purpose of this paper is to add expanded perlite (EP) immobilized microorganisms that replace part of the standard sand in mortar to improve the self-healing ability of mortar cracks and reduce the water absorption of mortar after healing.Design/methodology/approachBacillus pseudofirmus spores were immobilized with EP particles as self-healing agents. The effects of adding self-healing agents on the compressive strength of mortar specimens were observed. The ability of mortar specimens to heal cracks was evaluated using crack microscopic observation and water absorption experiments. The filler at the cracks was microscopically analyzed by scanning electron microscope and X-ray diffraction experiments.FindingsFirst, the internal curing effect of EP promotes the hydration of cement in mortar, which generates more amount and denser crystal structure of Ca(OH)2 at mortar cracks and improves the self-healing ability of mortar. Second, the self-healing ability of mortar improves with the increase of self-healing agent admixture. Adding a self-healing agent of high admixture makes the planar undulation of calcite crystal accumulation at mortar cracks more significant. Finally, the initial crack widths that can be completely healed by adding EP and self-healing agents to the mortar are 200 µm and 600 µm, respectively.Originality/valueThe innovation points of this study are as follows. (1) The mechanism of the internal curing effect of EP particles on the self-healing ability of mortar cracks was revealed by crack microscopic observation tests and microscopic experiments. (2) The effect of different self-healing agent amounts on the self-healing ability of mortar cracks has been studied. (3) The effects of EP particles and self-healing agents on healing different initial widths were elucidated by crack microscopic observation tests.Graphical abstract

Mode-II progressive fracture of sawn timber and timber-adhesive bond: experimental evaluation of end notched flexure (ENF) testing using DIC

The end-notched flexure (ENF) testing technique has been used in this paper to investigate the Mode II fracture behaviour of sawn timber and timber-adhesive bonds. Mode-II strain energy release rates were obtained by using classical data reduction schemes based on actual crack lengths captured using digital image correlation (DIC) technique. The compliance-based beam method (CBBM), which does not require the actual crack length measurements, was also evaluated for its suitability for Mode-II fracture of timber. Utilizing forward shear slip measurement from DIC at every millimetre of crack length, a simple yet effective and innovative technique to identify crack tip opening and to monitor crack length has been proposed herein. Three different initial notch widths were used in the current study to examine their suitability in producing reliable crack initiation and consistent bond delamination in solid timber and glued timber. The strain energy release rates calculated from the actual crack and equivalent crack length values showed similar trends in fracture resistance curves, which are commonly known as R curves, but the initial strain energy release rate (GII0) and the critical strain energy rate (GIIC) obtained from the two methods showed some discrepancies. Equivalent crack method CBBM was found to be inadequate in determining a consistent initial strain energy release rate despite producing a stable and steady fracture energy after crack initiation. Experimental results showed that Mode-II timber-adhesive bond line fracture is relatively more brittle than timber fracture, and the average critical strain energy release rate of timber-adhesive bond (GIIc) was 14% greater than that of sawn timber. Based on experimental observations, specific test criteria for ENF testing of timber structures are proposed in this paper.

Investigating the potential for porous ceramics as bacterial carrier in self-healing cemented paste backfill

Waste porous ceramic (WPC), which remains in the industrial manufacturing process, is difficult to be disposed of. This study proposes a new idea for resource utilization of WPC, which means that WPC as a bacterial carrier applies in self-healing cemented paste backfill. The authors investigated the effects of WPC pore size, initial crack size, and bacterial concentration on the crack self-healing ratio of the backfill, analyzed the crack self-healing evolution process by photographing, and finally used a series of instruments to characterize the physical phase at the sample fracture and explain the mechanism. The results showed that the crack self-healing ratio of WPC with small pore size (pore size distribution of 1–2 mm) as the bacterial carrier was 13–22% higher than that of WPC with large pore size (pore size distribution of 3–4 mm) at 3–7 days. Also, the strength recovery ratio of the cemented paste backfill can be improved to 10–23% when adding bacteria-WPC. The self-healing ratio can reach 100% in 28 days for initial crack widths less than 1 mm, while the self-healing ratio decreases to 98% for widths near 2 mm. Additionally, the biomineralization ratio of the 1.5 mol/L embedded bacterial solution was 2.77–4.68% higher than that of 1.0 mol/L. WPC can effectively release the embedded bacterial solution once the backfill cracks appear, which proves the feasibility of WPC as a bacterial carrier. This study also lays the experimental and theoretical foundation for porous ceramic management.

Statistical investigation of the widths of supra-arcade downflows observed during a solar flare

ABSTRACT Supra-arcade downflows (SADs) are dark voids descending towards the post-reconnection flare loops and exhibit obvious variation in properties like width. However, due to the lack of further statistical studies, the mechanism behind such variations hitherto remains elusive. Here, we statistically investigated widths of 81 SADs observed in one flare by the Solar Dynamics Observatory (SDO). For each of SADs, six moments were selected with equal time intervals to measure their widths at different stages of their evolution. It is found that most SADs show a roughly monotonous width decrease during their descents, while some SADs with small initial widths can have complex evolutions. 3D reconstruction results based on SDO and Solar Terrestrial Relations Observatory Ahead (STEREO-A) images and thermal properties analysis reveal that differences in magnetic and plasma environments may result in that SADs in the north are overall wider than those in the south. Additionally, correlation analysis between the width and other parameters of SADs was further conducted and revealed that (1) SADs with different initial widths show no significant differences in their temperature and density evolution characteristics; (2) SADs with small initial widths usually appear in lower heights, where more frequent collisions between SADs could lead to their intermittent acceleration, width increment, and curved trajectories. These results indicate that SADs with different initial widths are produced the same way, while different environments (magnetic field or plasma) could affect their subsequent width evolutions.

Experimental investigation of the quantified influence of gypsum plasterboard joints on the fire performance of cold-formed steel walls

The fire-induced width development of a joint and the corresponding impact on the fire performance of cold-formed steel (CFS) walls were not quantified in previous investigations. Six non-cavity-insulated mid-scale CFS walls lined with artificial board joints were tested under ISO 834 fire conditions. Specimens were sheathed with double layer gypsum plasterboard (GP) on both sides and contained three types of GP board joints (face layer joint, base layer joint and overlapped joint). Moreover, 5 mm and 30 mm initial joint widths were set for each type of board joint to quantitatively investigate the negative impact of joints on the fire performance of walls and identify the positions on the wall that are vulnerable to fire-induced structural failure. Based on pixel comparison, the curves of the time-joint width are given, which can provide supporting data for investigating the heat transfer numerical simulation of CFS walls with joints in fire. In addition, the conventional numerical heat transfer model of CFS walls assigns the thermal boundary to the GP board joint surface. However, during the initial 30 min of fire exposure, temperatures at both the face layer joints and overlapped joints were significantly lower than the fire-side temperatures of the walls. Hence, the temperature results at the board joint of conventional numerical heat-transfer simulation results tend to be too conservative. It is suggested that the numerical heat transfer model of CFS walls should consider the development of GP board joint width and the flow-thermal coupling effect in the joint area. In addition, for a typical CFS load-bearing wall lined with double layers of GP boards on both sides, although each wall stud has the same vertical load (load ratio 0.65), fire-induced structural failure can occur when the HF critical temperature reaches 453 °C. Therefore, the wall stud adjacent to the overlapped joints will be the first to display fire-induced structural failure. Subsequently, the studs near the face layer joints will fail, and the wall studs near the base layer joints will fail last.

Width of a beta-decay-induced antineutrino wave packet

The time evolution of a neutrino is dependent on its initial properties at creation including flavor, energy, and wavepacket size. There exists no solid theoretical prediction for the latter property in the context of nuclear beta decay, despite the importance of this process for the past, present, and future of neutrino experimentation. In this paper, we provide a quantitative prediction for the size of a beta-decay-induced electron antineutrino wavepacket by treating the parent nucleus decaying to an entangled antineutrino-recoil system using the formalism of open quantum systems. Of central importance is the delocalization scale of the parent particle. We construct a systematic description of the hierarchy of localizing entanglements that provides an unambiguous statement of the relevant localization scale, found to be closely related to the diameter of the parent nucleus (e.g. $\sim$5-6~fm for beta-decaying fission daughters) and as low as the typical nucleon-nucleon correlation distance ($\sim$1~fm). Inside a nuclear reactor, for example, this translates to initial electron antineutrino wavepacket widths in the $\sigma_{\nu,x}\sim10\mathrm{-}400$~pm range for $E_{\overline{\nu}_e}>1.8$~MeV, with dependencies on decaying nucleus size, the emitted antineutrino energy, and the kinematics of the recoiling system. Wavepacket sizes in this envelope do not produce an observable effect on oscillation probability in foreseeable reactor experiments in the standard three-neutrino model, including JUNO which is expected to be sensitive to $\sigma_{\nu,x}\lesssim3$~pm.

Effect of concrete workability on bond properties of steel rebar in pre-cracked concrete

Although research has shown a considerable influence of the pre-cracking phenomenon on steel-congested concrete members, only normal concrete (NC) has been considered in the literature. The intention in this paper is thus to study the effect of the pre-cracking phenomenon on the bond response of pre-cracked NC with different slump flow values and self-consolidating concrete (SCC). Initial crack widths ranging from 0.0 to 0.5 mm are studied. Results show that initial crack widths larger than 0.10 mm have a significant influence on bond properties, such that reduction factors greater than 30% and 50% are obtained for the maximum bond strength of concrete specimens exposed to initial crack widths of 0.2 mm and 0.4 mm, respectively. Results show that concrete mixtures with higher workability are less sensitive to the pre-cracking phenomenon as compared to NC mixtures. The average bond stress of steel rebar in the pre-cracked SCC is found to be similar to that of the NC with a slump flow of 200 mm, which is considerably better than for NC with a slump flow of 97 mm. Moreover, results show that 65.8, 80.6, 88.5 and 93.1% fracture energy reductions are obtained for crack widths of 0.20, 0.30, 0.40 and 0.50 mm, respectively, as compared to the small crack width of 0.15 mm.

Self-healing of high-performance engineered cementitious materials with crystalline admixture in the seawater environment

The present paper studies the crack healing effect of high-performance cement-based materials mixed with crystalline admixture (CA) in seawater environments. Specifically, the healing effects of specimens with different initial crack widths in tap water and seawater environments were investigated through the seepage healing test (simulating a structure that a side contact with water and the other side contact with air). The variations of seepage velocity and water flow in the early 24 h of healing were mainly investigated. It was found that crystalline materials delay the healing of cracks in the initial stage of healing and this defect is more obvious under the seawater healing condition. Inductive Coupled Plasma (ICP) test was used to detect the variation of ion concentration in seawater; XRD, FTIR, and TG tests were carried out for characterizing the composition and relative content of healing products in the crack under the seawater and tap water healing conditions. Moreover, the effect of CA on the composition of healing products and the effect of crack width on healing products were quantified. The distribution characteristics of healing products along the crack depth under seawater seepage and seawater immersion healing conditions were then obtained.

An enhanced artificial bee colony algorithm using radial bases function neural networks for color image segmentation

The aim of segmentation process is to partition the image into homogeneous regions or objects. The proposed segmentation algorithms using adaptive feature weighting to distinguish the significance of various features. To extract effective feature extraction could be used for establishing regions in region based image segmentation. The main goal of this exploration work is to segment objects that separating the foreground and background of the color images. The segmentation for color images is focused on the Enhanced Artificial Bee Colony segmentation algorithm that utilizes the Radial Basis Function (EABCRBF) neural network algorithm. This work arranges RBF neural networks to predict the end limits of the segmentation, which are formed from the artificial bee colony province made in the image histogram topography. The radial basis function initial parameters, for instance, centers and widths are automatically set upon the histogram peaks and minima. The proposed EABCRBF strategy is compared with two existing techniques of Artificial Bee Colony (ABC) and Fuzzy C-Means (FCM). The EABCRBF method gives great outcomes for color image segmentation as it extracts additional information from color images. The experimentation results are shown through Mat Lab R2013a.

Physics-based deep learning for modeling nonlinear pulse propagation in optical fibers.

A physics-based deep learning (DL) method termed Phynet is proposed for modeling the nonlinear pulse propagation in optical fibers totally independent of the ground truth. The presented Phynet is a combination of a handcrafted neural network and the nonlinear Schrödinger physics model. In particular, Phynet is optimized through physics loss generated by the interaction between the network and the physical model rather than the supervised loss. The inverse pulse propagation problem is leveraged to exemplify the performance of Phynet when in comparison to the typical DL method under the same structure and datasets. The results demonstrate that Phynet is able to precisely restore the initial pulse profiles with varied initial widths and powers, while revealing a similar prediction accuracy compared with the typical DL method. The proposed Phynet method can be expected to break the severe bottleneck of the traditional DL method in terms of relying on abundant labeled data during the training phase, which thus brings new insight for modeling and predicting the nonlinear dynamics of the fibers.

Aggregate interlock in fractured concrete mesoscale models: a novel finite element modelling approach

Aggregate interlock is a stress transfer mechanism in cracked concrete. After concrete cracks under tensile loading, crack interfaces can experience significant slip deformation due to the applied crack kinematics. Upon rising slip along crack interfaces, aggregate interlock stresses are generated which transfer shear stress and normal stress. Many experimental programmes and analytical expressions have been developed for several decades. However, a finite element model considering realistic crack surfaces was still not developed. The complexity of developing a FE model lies due to the mesoscopic nature of the problem. In this study, concrete mesoscale models were employed to generate realistic cracked concrete surfaces. Uniaxial tensile fracture propagation in concrete mesoscale models were achieved using Zero-thickness cohesive elements approach. Once cracked concrete FE models are developed, validation of the proposed FE models was conducted against two experimental campaigns. The study comprises the evaluation of the surface roughness index of the cracked concrete surfaces. The FE model predicts secondary cracking under low initial crack widths and mixed mode angles. FE predictions were further compared with Walraven’s simplified formulae, Bažant’s rough crack model, Cavagnis’s aggregate interlock formulae and contact density model and consistence agreement was observed. Finally, strengths and weaknesses of the proposed FE modelling approach for aggregate interlock was discussed and further implementations were also highlighted.

Effects of initial and boundary conditions on gravel-bed river morphology

Assuming that the equilibrium geometry of river channels does not depend on their initial state but solely on boundary conditions, several formulas have been derived that relate the channel depth and width to the river bankfull discharge and bed material. However, due to the existence of a threshold for sediment motion and the strong non-linearity between sediment transport and flow rate, this assumption might not be generally valid for gravel-bed rivers. This research clarifies the role of the initial conditions, more specifically the initial channel width, on the geometry of gravel-bed rivers considering a variety of boundary conditions. The approach includes laboratory experiments and two-dimensional modeling, reproducing the evolution of alluvial channels with different starting widths, discharge regimes and sediment input rates. The experiments represent the Arc River (France). Thus, the characteristics of this river were used in the numerical model to obtain a realistic virtual case complementing the experiments. Different boundary and starting conditions resulted in either braided or single-thread channels. We found that the initial width strongly influences the evolution process and leaves a footprint on the river braid-belt extension. The active width of braided systems and the width of single-thread channels do not depend on the starting condition. They depend on sediment input rather than on discharge variability. Different initial widths result in different final bed levels. This indicates that the initial channel width may affect the degree of channel incision or aggradation. The results of this study justify the use of equilibrium formulas for single-thread rivers.

Use of recycled concrete aggregates as carriers for self-healing of concrete cracks by bacteria with high urease activity

Microbial induced carbonate precipitation (MICP) has become one of the most attractive strategies to achieve self-healing of concrete cracks in recent decades. In order to provide a moderate space for bacteria in the harsh environment of concrete, the incorporation of protective carriers is a necessity. In this study, the microbial urease activity at different generation of propagation in the selected culture medium was first investigated, followed by exploring the potential of using recycled concrete aggregates (RCAs) as microbial carriers, considering the porous nature of RCAs. The specific urease activity of bacteria at 48 h kept as high as 2.4 × 10−8 mM·min−1·cells−1 even after 5 generations of propagation in the selected culturing medium. RCAs would not result in a loss of ureolytic activity after immobilization and were capable of retaining bacterial viability during concrete mixing. Compared with concrete without any healing agents or with only substrates and calcium source, the one with bio-agents loaded in RCAs completed most of the crack healing in the first week. When the initial crack widths were less than 0.6 mm, the average crack healing ratio and the crack area healing ratio were 71% and 84%, respectively, for microbial self-healing. The depth of crack healing was 17.8 mm as indicated by the microbial precipitates on the crack wall. After microbial self-healing, the compressive strength regain ratio achieved 99.7% while the water tightness regain ratio attained 99.2%, and the cracks were filled with densely packed irregular rhombohedral-shaped crystals consisting of calcite and a small amount of vaterite.

Triggering tearing in a forming current sheet with the mirror instability

We study the time-dependent formation and evolution of a current sheet (CS) in a magnetised, collisionless, high-beta plasma using hybrid-kinetic particle-in-cell simulations. An initially tearing-stable Harris sheet is frozen into a persistently driven incompressible flow so that its characteristic thickness gradually decreases in time. As the CS thins, the strength of the reconnecting field increases, and adiabatic invariance in the inflowing fluid elements produces a field-biased pressure anisotropy with excess perpendicular pressure. At large plasma beta, this anisotropy excites the mirror instability, which deforms the reconnecting field on ion-Larmor scales and dramatically reduces the effective thickness of the CS. Tearing modes whose wavelengths are comparable to that of the mirrors then become unstable, triggering reconnection on smaller scales and at earlier times than would have occurred if the thinning CS were to have retained its Harris profile. A novel method for identifying and tracking X-points is introduced, yielding X-point separations that are initially intermediate between the perpendicular and parallel mirror wavelengths in the upstream plasma. These mirror-stimulated tearing modes ultimately grow and merge to produce island widths comparable to the CS thickness, an outcome we verify across a range of CS formation timescales and initial CS widths. Our results may find their most immediate application in the tearing disruption of magnetic folds generated by turbulent dynamo in weakly collisional, high-beta, astrophysical plasmas.

DEFORMABILITY AND CRACK RESISTANCE OF AIRFIELD SLABS

Abstract. The results of experimental studies of deformability and crack resistance of models of airfield slabs made of reinforced concrete and steel fiber concrete are presented. Two series of plates were tested ‒ three models of reinforced concrete and three models with steel fiber added to the concrete mixture in amount of 1% of the total volume of the product. The load was applied in small steps, the instrument readings were recorded twice at each step, and the crack opening width was measured starting from the moment of the first crack formation. Dial gauges and deflectometers were used as measuring instruments. According to the normative documents acting in Ukraine, one of two possible loading schemes was considered ‒ with the loading by the concentrated force applied on the cantilever part of a plate. The plate models were tested on a specially made stand which consisted of four supporting struts connected in pairs by beams. The airfield slab was supported by the beams. The load was applied along the width of the plate in steps ‒ 0.05 of the destructive load, along two concentrated vertical strips. Each degree of load ended with a five-minute dwell time, at the beginning and end of which readings were taken on the measuring instruments. The deformations at the same levels were measured with dial gauges. The process of crack formation was observed with a Brinell tube in the places of the greatest crack opening. It follows from the obtained results that the process of cracking in the fiber concrete slab begins at higher loads than in the reinforced concrete slab. The final and initial crack opening widths of all cracks in the fiber concrete slab are significantly lower than in the reinforced concrete slab. The deformations in steel-fiber concrete slabs during the application of load in the cantilever part, both for compressed and stretched fibers are higher than in reinforced concrete slabs. At the initial stages of load application in the cantilevered part of the slabs, the deflections increase in a linear relationship. The curves get non-linear character for airfield slabs made of reinforced concrete when the load reaches the level of 10÷25 kN, for steel-fiber-concrete slabs ‒ 15÷30 kN. In reinforced concrete slabs, the non-linearity starts a little earlier and is expressed more clearly. Experimental studies show that dispersed reinforcement of airfield slabs with steel fiber leads to their higher crack resistance.