Sand Research Articles

Vortex Trapping of Suspended Sand Grains Over Ripples

AbstractCoastal hydrodynamics and morphodynamics integrate the effects of small‐scale fluid‐sediment interactions; yet, these small‐scale processes are not well understood. To investigate sediment trapping by turbulent coherent structures or vortices, the transport of coarse sand over ripples was analyzed in a small‐oscillatory flow tunnel with phase‐separated Particle Image and Tracking Velocimetry. Results from one of the first direct measurements of vortex‐trapped sand grains under oscillatory flows are presented. The vortices mobilized sand grains along the ripple slopes just prior to flow reversal and transported the suspended sediment grains. During several flow cycles, some sand grains were temporarily trapped in the vortex, prescribing semi‐circular trajectories off‐center from the vortex core in quadrants of the vortex that were closest to the ripple slope, as illustrated by Nielsen (1992, https://doi.org/10.1142/1269). Comparisons of the horizontal sediment grain velocity with the horizontal fluid velocity yielded a linear relationship with a slope of 0.87. The vertical grain velocities also varied linearly with the vertical fluid velocity with a slope of approximately 1 and an offset of −0.08 m . The offset is close to the still water settling velocity for coarse sand grains, as hypothesized during vortex trapping. Additionally, estimates of the off‐center distance, between the centers of the semi‐circular sediment paths and vortex cores, compared well with the ratio of the settling velocity to the radian frequency of the vortex yielding a linear regression slope of 0.99. Improved understanding of vortex trapping effects on sediment dynamics may decrease uncertainty in model predictions of large‐scale coastal hydrodynamics and sediment transport.

Survival strategies of Bar-tailed Godwit in tropical sand beaches: Straight-line and zigzag foraging techniques

The near-threatened Bar-tailed Godwit (Limosa lapponica) makes the longest non-stop endurance flights among shorebirds. They are regular winter visitors to the Kadalundi-Vallikkunnu Community Reserve and adjoining sand beaches along the west coast of India. The research investigates their foraging strategies within sand beaches, observed over six years (2016-2022). Extensive field observations across major sand beaches documented two distinct modes of continuous stitching behaviour; a straight-line mode and a zigzag mode. Despite predation pressure and human disturbances, Bar-tailed Godwits achieved high prey capture rates, likely due to the abundance of benthic invertebrates. The study also revealed the prevalence of this behaviour and its association with rate of prey consumption, particularly focusing on the availability of Cosmonotus crabs, the dominant prey species. The zigzag pattern, was associated with higher prey capture rates, suggesting an adaptive response to prey distribution. Our findings suggest that, this unique foraging strategy allows Bar-tailed Godwits to optimize prey capture while conserving time and energy, especially in resource-scarce environments. Furthermore, the implications of these behaviours are discussed in the context of prey availability, nutrient content, and the energetic demands of long-distance migration, shedding light on to the adaptive mechanisms employed by shorebirds in challenging habitats.

The variation of particle concentration with height of wind-blown coral sand

Wind-blown coral sand movement is common in the marine coral sand island environment but received much less research attention compared to desert and coastal sands. We used the particle image velocimetry technique with wind tunnel experiments to determine the decay trends of the particle number density, nominal particle area density, and actual particle area density with height for wind-blown coral sands from the South China Sea. Then, a new morphology factor FM that consists of volume V, density ρs, drag coefficient CD, and projected area A of coral sands, was defined to evaluate the influences of particle characteristics on wind-blown sand movement and the results were compared with those of quartz sands from an inland desert. We found that the average FM of coral sands is more comparable to that of coarse quartz sands than smaller size groups. Coral sands tend to move nearer the surface during aeolian processes compared to smaller quartz ones due to their larger FM. The decay rate of particle number density of coral sands with height is similar to that of coarse (0.8–1 mm) quartz sands, but significantly larger than that of smaller quartz ones. The decay rate of the actual particle area density of coral sands with height is larger than that of their nominal particle area density, so that significant deviations may exist if a fixed particle size and spherical shape are assumed to study wind-blown particle movement. The present work contributes to understand the effect of particle characteristics on the wind-blown sand movement from a physical mechanism perspective for both desert quartz sands and marine coral sands.

Zircon Xenocrysts From Easter Island (Rapa Nui) Reveal Hotspot Activity Since the Middle Jurassic

AbstractWe report the finding of mantle‐derived zircon grains retrieved from red soils, regoliths, and beach sands from Easter Island, that are much older than the island volcanism (0–2.5 Ma) and its underlying lithosphere (Pliocene, 3–4.8 Ma). A large population of 0–165 Ma old zircons have coherent oxygen (δ18O 3.8–5.9‰) and hafnium (εHf(t)+3.5–+12.5) mantle isotopic signatures. These results are consistent with the crystallization of zircon from plume‐related melts. In addition, a chemically diverse population with ages from the Paleozoic to the Archean was found. These older populations are enigmatic but they could represent remnants of ancient subducted sediments. Meanwhile, the ∼0–165 Ma population is interpreted as plume‐derived, suggesting that the hotspot started at least ∼165 Ma ago. A spike of ∼164–160 Ma zircons could represent a Large Igneous Province (LIP) stage upon the first arrival of the plume. We use plate reconstructions to show that such a LIP would have formed on the Phoenix Plate and would have subducted below the Antarctic Peninsula around 100–105 Ma. There, LIP subduction would offer a solution for the enigmatic Palmer Land deformation event, previously proposed to result from a collision with an unknown indenter. The here‐reported “ghost” of a prolonged hotspot activity suggests that fragments of the Easter plume and of the ambient sub‐lithospheric mantle stored and re‐sampled zircon xenocrysts due to convective (re)circulation at the scale of the plume head. Our study demonstrates how zircon geochronology and geochemistry provide novel insights into global‐scale geodynamics, offering new perspectives on the dynamics of mantle plumes and hotspot activity.

Multivariate Analysis of Geochemical Data to Determine Critical Minerals in Prigi Beach Sand, Trenggalek

Abstract Critical minerals are essential elements mainly for industry. As technological advancements, the demand for critical minerals also increases. Critical minerals include Rare Earth Elements (REE), Lithium (Li), Gallium (Ga), Germanium (Ge), Platinum Group Metals (PGM), Indium (In), Beryllium (Be), Tungsten (W), Cobalt (Co), Molybdenum (Mo), Niobium (Nb), Antimony (Sb), Vanadium (V), Nickel (Ni), Chromium (Cr), Tellurium (Te), Tin (Sn), Thorium (Th), Zirconium (Zr), Hafnium (Hf), Selenium (Se), Rhenium (Re), Phosphate (P), Potassium (K), and others. This study aims is to figure out the distribution of critical minerals in the sand of Prigi Beach, Trenggalek. Chemical elements are determined using X-Ray Fluorescence (XRF). Based on data analysis, critical minerals were detected with the presence of elements such as Al, K, Ca, Si, Ti, P, V, Mn, Cr, Zn, Fe, Sr, Eu, and Cu. Using the Pearson Correlation and Principal Component Analysis (PCA) methods. Significant correlations were found between (Al-Zn), (Si with K-Ti-V-Fe-Cu-Eu), (K with Ti-V-Fe-Cu-Eu), (Ca-Sr), (Ti with V-Mn-Fe-Cu-Eu), (V with Fe-Cu-Eu), (Fe-Eu), and (Cu-Eu). The elements that dominate the critical minerals based on geochemical data analysis of the sand revealed three highest concentrations consisting of Fe (41.44%), Si (35.32%), and Ca (10.96%). Base on several previous studies on critical minerals in beach sands, including a study by Endang (2023) it is know that Fe is a dominant element compared to others such Ca, Al, and K. This is due to the influence of various geological conditions and human activities.

Green Compression-Cast Concrete Material and Its Fiber-Reinforced Polymer (FRP)-Reinforced Concrete Structures

Concrete manufacturing is a significant contributor to carbon emission. This paper presents an extensive study on compression-casting, which is a newly developed and highly efficient concrete manufacturing technology. Compression-casting is a mechanical process that improves the properties of all types of concrete materials without using chemical additives or mineral admixtures. It is particularly suitable for producing high-quality concrete from secondary/substandard or waste/recycled aggregate materials, thereby promoting green, low-carbon, and sustainable construction. This paper introduces compression-casting and presents the experimental results of the material and relevant fiber-reinforced polymer (FRP)-reinforced structures. Various applications of the compression-casting technology on inferior concrete materials, including rubber, recycled aggregate, desert sand, and recycled powder concrete, have also been introduced. The behavior of FRP-reinforced structural members of compression-cast concrete (CCC) is evaluated and compared with that of normal concrete (NC). It is concluded that compared to NC, CCC has the following advantages: improved mechanical properties, microstructure, durability, cement saving, economy, and reduced carbon dioxide emissions. Finally, we provide recommendations for addressing CCC structure brittleness. As a result, this method can reshape the future of concrete manufacturing.

Assessment of structural concrete made by sustainable sand at high temperatures: An experimental research

This study assesses the mechanical properties of sustainable sand concrete mixes of DM1*, DM2, DM3, and DM4 by absolute volume method when exposed to elevated temperatures. The concrete specimens were placed in a muffle furnace for two hours at (150–600 °C). Experimental investigations show that the mix strength increases (up to 300 °C), a sudden fall in strength at 450 °C, and a sharp decline at 600 °C was seen. At 600 °C, the highest reduction in compressive and flexural strength of the mixes was reported (21.2, 11.2, 16.6, and 20.4 MPa) and (2.87, 1.41, 2.42, 1.95 MPa), respectively. The concrete mix DM2 with (100 % recycled crushed sand) shows a more significant loss than other mixes. The concrete mix DM4 (50 % recycled crushed sand + 50 % desert sand + 12.5 % silica fume) performs significantly better than other mixes at elevated temperatures. Hence, 100 % recycled crushed sand in structural concrete is not recommended for engineering work.

Effect of particle size on the transport of polystyrene micro- and nanoplastic particles through quartz sand under unsaturated conditions

Micro- and nanoplastics (MNPs) are contaminants of emerging concern recently found in soil ecosystems. Their presence in terrestrial environments and their migration to aquatic environments may become a risk for the health of ecosystems and, through them, of humans. Understanding the interaction between particle properties and physicochemical and hydrodynamic factors is crucial to evaluate their fate and their potential infiltration towards groundwater. This study investigates the impact of particle size on MNPs transport through sand under unsaturated conditions. Infiltration column experiments with polystyrene MNPs ranging from 120 to 10,000 nm were conducted and supported by numerical modelling to derive reactive transport parameters. Results show a significant effect of particle size on the transport of MNPs, with higher recovery values observed for smaller particles (120 nm; 95.11%) compared to larger particles (1,000 nm; 71.44%). No breakthrough was observed for 10,000 nm particles, indicating a complete retention within the quartz sand matrix. DLVO theory confirmed the dominance of electrostatic repulsive forces between MNPs and sand grains, suggesting an unfavourable environment for MNPs to adhere to quartz sand. Consequently, particle retention in the sand matrix occurs predominantly by physical processes. Equilibrium sorption modelling reveals that larger particles (1,000 nm) tend to be immobilized in small pores throats due to straining, resulting in lower recoveries. When they are not trapped, particles tend to travel faster through preferential flows due to a size exclusion effect, evidenced by shorter arrival times at the column outlet compared to tracers. These findings highlight the influence of particle size on the transport and retention of MNPs in quartz sand under unsaturated conditions and contribute to a better understanding of their transport dynamics and environmental fate.

Effects of soil composition and curing conditions on the strength and durability of Cr3+-crosslinked biopolymer-soil composites

Stabilized soil composites incorporating Cr3+-crosslinked xanthan gum (CrXG), a self-stiffening cation-crosslinked biopolymer, have recently emerged as sustainable construction materials for earthen structures. However, the influence of curing conditions and soil composition in altering the mechanical properties of CrXG–soil composites has so far received limited attention. This study investigates the effects of fine contents and curing conditions on the time-dependent strength development and durability of CrXG-soil composites. CrXG-soil composites, ranging from poorly graded sand to clayey silty sand, are subjected to unconfined compressive strength (UCS) and durability tests under various curing conditions, including wet, submerged, and dry conditions. Microscopic structural changes are characterized using scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). The results showed that the UCS of CrXG-soil composite increases nonlinearly, reaching up to 4.8 times the initial wet UCS after 28 days of curing, closely aligning with predictions from a hyperbolic model. Notably, CrXG-soil compositions with a clay-sand mixture (CSM) containing 15 % fine content (CSM15) demonstrated consistent strength parameters across all curing conditions in UCS tests. CSM15 also maintains a 90 % of durability index after eight dry-wet cycles and a dry UCS of 300 kPa after 130 days of atmospheric weathering. Microscopic-scale analysis confirms the stable agglomeration of CrXG-clay matrices between sand grains, with the peak wavelength of the major functional group remaining constant, even under multiple cycles. These findings contribute to a deeper understanding of CrXG–soil composites, offering valuable insights into optimizing soil compositions and enhancing the technical feasibility of applying these composites as a sustainable surface protection strategy for earthen structures, such as levees and road slopes.

Stranger things: on the novel buccopharyngeal anatomy and functional morphology of ‘sand-eating’ Malagasy tadpoles (Anura: Mantellidae: Mantidactylus)

Abstract Anuran larvae are characterized by an extensive array of specialized oral structures that allow them to both graze on substrates and suspension feed with great efficiency. Diversity in these feeding structures accounts for significant diversity of anurans. Herein we describe an astonishing novel buccopharyngeal morphology in six larvae of ‘sand-eating’ tadpoles of the Mantellidae genus Mantidactylus. The buccopharyngeal cavity of these tadpoles is characterized by the presence of ruffled ridges not seen previously in any other anuran larva. Extensive review of the literature and of new empirical data demonstrates the uniqueness of this morphology. We present both CT scans and ergonomic arguments in support of the hypothesis that the ruffled ridges are employed as scrubbing pads, allowing these tadpoles to remove food particles from sand grains. In addition, we explore how the ridges may have developed and evolved.

Microplastic Abundance, Characteristics, and Heavy Metal Contamination in Coastal Environments of Western Sri Lanka

This study was conducted to assess the abundance of microplastics and associated metal contamination from selected beaches in the Western Province of Sri Lanka. Beach sand samples were collected from four beaches: Modera, Negombo, Mount Lavinia, and Panadura. Microplastics were extracted from dried sand samples using a saturated NaCl solution and sieved. Particles were identified using Fourier Transform InfraRed spectrophotometer, and associated heavy metals; Cr, Pb, Cu, Zn, and Ni were subjected to acid digestion for 24 hours before analysis using Microwave Plasma Atomic Emission spectrometry. More than half of the extracted plastics (56.31%) were identified as microplastics. The average microplastic abundance in beach sand samples ranged from 42.0-91.3 items/kg. The lowest and highest abundances were reported from the sand collected at Mount Lavinia and Panadura beaches, respectively. Hydrodynamic factors like ocean currents, wave patterns, associated Southwest monsoon period, and human activities may have caused the variability in microplastic abundances and metal contamination. The majority of the microplastics (52.29%) were polyethylene, whereas the second most abundant microplastic type was polypropylene (35.18%), resembling the records of the most common plastic waste types in the country. The majority of microplastics were fragments (87.95%) while white was the prominent color (53.49%). The toxic trace element concentration ranged from 5.0×10-3 to 1.8×102 μg/g in beaches. Furthermore, this study establishes a baseline for the west coastline prior to the X-press Pearl Ship Disaster in 2021. Future studies are encouraged to assess the beach microplastic pollution across the entire Sri Lankan coastline.

Coevolution of the holocene coastal barrier and lagoon terraces and their implications on the record of relative sea-level change in southernmost Brazil

This paper presents the coeval history of a sandy coastal barrier and four lagoon margins located in the Rio Grande do Sul State, southernmost Brazil. To achieve this objective we compared the Holocene relative sea-level changes recorded along a regressive coastal barrier with the register of base-level fluctuations preserved in the lagoon terraces. We used geomorphology, geochronology, geophysics (GPR), and high-resolution sequence stratigraphy to evaluate changes on the relative sea- and base-level recorded in the barrier and lagoon terraces, respectively. The record of the relative sea-level fall (forced regression) began at ∼4 ka in the coastal barrier and around 3.4 ka in the lagoon system. Maximum altitudes of the eolian/beach deposits boundary are similar on both barrier and lagoon settings, hence pointing out that relative sea-level controlled lagoon base-level. The forced regression stage is more accurately preserved in the margin of the lagoons than in the coastal barrier. Along the former, four progradational sets related to stepped terraces record pulses of relative sea-level fall at 3.4, 1.4, 0.35, and 0.27 ka. The pulses that took place from 1.4 ka onwards are recorded as low terraces in the lagoon but appear in the coastal barrier as an acceleration in the rate of the relative sea-level fall. Each relative sea-level fall is supposed to have liberated beach sands for eolian reworking during the subsequent stillstand phase and allowed the development of transgressive dunes. Therefore, we conclude that lagoonal terraces vertical displacements are adequate proxies for high-resolution analysis of relative sea-level changes.

Mechanical properties of aeolian sand cemented via microbially induced calcite precipitation (MICP)

The cementation of desert aeolian sand is a key method to control land desertification and dust storms, so an economical, green and durable process to reach the binding between sand grains needs to be searched. The method based on the microbially induced calcite precipitation (MICP) appeared in recent years as a promising process that proved its efficiency. The feasibility of the MICP technique to treat aeolian sand composed by low clay content, fine particles, low water content and characterized by weak permeability was demonstrated in the present paper. The effects of initial dry density, cementation number and curing time on the permeability and strength of MICP-treated aeolian sand were investigated using permeability tests and unconfined compressive strength (UCS) tests. The microstructure of aeolian sand was observed by scanning electron microscopy (SEM) tests and X-ray diffraction (XRD), aiming to reveal the solidification principle of MICP. The tests result indicated that when the initial dry density and the cementation number rose, the hydraulic conductivity of aeolian sand decreased while the mechanical strength given by UCS values improved. When the initial dry density was 1.65 g/cm3, the curing time was 3 h and the cementation number reached 20, the hydraulic conductivity and UCS reached 0.00151 cm/s and 1050.30 kPa, respectively. With increasing curing time, the hydraulic conductivity first decreased, followed by an increase, while the UCS exhibited an up and then a downtrend. Furthermore, the correlation between UCS values and the CaCO3 content reached a high R2 value equal to 0.912, which confirmed that the cementation occurred in sandy material and governed the soil strengthening. Indeed, the calcium carbonate crystals observed by SEM and XRD enhanced the friction between particles when they wrapped around the sand grains surface, while carbonates reduced the soil permeability when filling the pores and sticking the sand particles together. Finally, the theoretical and scientific knowledge brought by the present study should help in managing sand in desert areas.

OS FULGURITOS DE ARACATI, NO ESTADO DO CEARÁ

This work describes the first known occurrence of fulgurites in Ceará coastline found in the Aracati county. Fulgurites are the result of the natural process of the incidence of electrical discharges of lightning on sand, soil and rocks. The product of this process in sand and soil is a new rock with unique recrystallization characteristics of the material exposed to the discharge. In some occurrences around the world, there is a record of vitrification observed with the naked eye, which was not possible in Aracati. Here, the samples collected are representative of two predominant shapes: oblong and cylindrical. The first form presents cohesion of sand grains in the central part of the sample and friability on the crests, without presenting the cavities characteristic of the cylindrical shape. This, in turn, presents differences in density, fragility and color in relation to the oblong shape. These characteristics are very similar to those of the petrified roots described in the municipality of Itarema as rhizoconcretions, however, Prof. Dr. Marcondes Lima da Costa in Aracati, in March 2024, made it possible to investigate the material with a focus on fulgurites.

Positioning of New Beaches in Bali: The Perspective of Tourist

Beach in Bali are one of the main attractions of the island's tourism. The beauty of Bali's beaches, with their clean white sand, clear sea water, and ideal surfing waves, makes them famous. Each beach has unique characteristics that can be used as a competitive advantage to help tourists choose their destination in Bali. This study uses Multidimensional Scaling and Correspondence Analysis techniques to identify the advantages of each attribute for tourists. The results show that Diamond Beach's position as a tourist destination is similar to Green Bowl Beach. Based on the evaluation of fifteen items by respondents, it is evident that Diamond Beach has a diverse range of positive perceptions. Diamond Beach excels in attributes such as beach safety, sand color, and cleanliness.

Mineralogy and Major Element Geochemistry of the Oligocene Barail Group Sandstones from the Sylhet Trough, Bengal Basin: Provenance and Tectonic Implications

The origin of Oligocene sediments in the Bengal Basin and associated tectonic setting remain poorly understood. This study investigates the framework mineralogy and major element geochemistry of the Barail Group sandstones from the Sylhet Trough within the Bengal Basin to clarify the provenance and tectonic history of the Oligocene. Modal analysis (Q83F7L10) and geochemical data support a classification of sublitharenite to subarkose, some with Fe enrichment. The heavy mineral assemblage is dominated by opaque minerals, followed by ultrastable minerals with zircon > tourmaline > rutile. The sub-angular to sub-rounded sand grains with a compositionally moderate mature nature suggest that the sediments were deposited close to the source area. The mineralogical and geochemical provenance discrimination diagram suggests contributions from felsic igneous, sedimentary/metasedimentary, and low-grade metamorphic sources, with detritus derived from the Indian craton and proto-Himalaya region. Data suggest moderate to intense chemical weathering, indicative of low relief and a sub-humid to humid climate in the source area. The tectonic analyses indicate that the Bengal Basin transitioned from a predominantly passive margin to an active tectonic margin setting during the Oligocene.

Research on the Management of Residents in Resettlement Communities in the Yellow River Beach Area: A Case Study of H Community in Jinan City

In recent years, China has attached great importance to the development of the Yellow River coastal areas and formulated many promotion policies for this purpose. The relocation and resettlement of residents in the Yellow River beach area is one of them. The implementation of this measure has enabled the people in the beach area to move out of the Yellow River coast in an orderly manner, ensuring the safety of residents and making their lives more convenient. At the same time, it also promotes the construction of new rural communities. H community is a typical community for relocation in Huanghe Beach District, Jinan City. This study conducted semi-structured interviews with residents of different identities and occupations in the H community, and conducted field research for a period of time to point out the governance status and difficulties of the H community. Based on the research, suggestions were proposed for similar communities and problems.

Experimental and numerical study on mechanical properties of sand-contaminated ballast aggregates in confined condition

This study investigates the impact of sand contamination on the mechanical properties of railway ballasted tracks in confined condition. A combination of experimental confined uniaxial compression tests and discrete element method (DEM) simulations was employed. The experiments assessed the bulk density and elastic modulus of ballast aggregates, while the DEM simulations focused on sand movement, coordination numbers, and contact forces to elucidate the mesoscopic behavior. Findings from both experiments and simulations consistently demonstrate that sand contamination linearly increases the bulk density and causes a non-linear increase in the elastic modulus of ballast aggregates. With increasing degrees of sand contamination under vertical loading, both coordination numbers and contact forces are reduced. Sand intrusion initially leads to an uneven distribution of sand grains, primarily in the lower layer of the ballast aggregate; however, this distribution becomes more uniform when contamination exceeds 62.5%. The presence of sand particles diminishes the contact forces between ballast particles, thereby escalating the challenges associated with maintenance and repair.

Study of Structural Defects Evolution in Fine-Grained Concrete Using Computed Tomography Methods

Introduction. When studying composite materials for construction purposes, it is needed to consider the mechanisms of formation of the structure and properties of modern concretes in the process of strength development. In studies of modern composite materials based on cement binder, there is no information about the development of structural defects and destruction of the material at the initial stages of strength development. This information can be obtained using X-ray computed tomography, a promising method of nondestructive testing of the state of the material. Therefore, the objective of this work was to study the formation and propagation of cracks in samples of fine-grained concrete with different fractional composition of sand due to natural processes of cement shrinkage, as well as the mechanics of destruction of samples of modified fine-grained concrete when applying a compressive load at the early stages of strength development. Materials and Methods. The study used fine-grained concrete mixtures of three compositions with different sand gradation. The tomography samples were made by placing fresh mixtures in polymer cylindrical containers. Tomography of the samples immediately after manufacture, as well as after 8 and 51 days, was performed in a YXLON Cheetah microfocus X-ray machine. The composition with two-fraction sand was modified by mechanical activation of the components, 20×20×20 mm cube samples were made. Further, compression tests were performed at the Instron installation after 3 and 7.5 hours, and then — tomography of the destroyed samples.Results. It was established that the destruction of contact zones depended on the ratio of the size of the fractions. In the presence of a bulk of coarse sand grains in concrete, the destruction of contact zones was more pronounced and had a main mode. When using fine or polyfraction sand, contact zones were destroyed locally and had a visually smaller area. The images of the destroyed modified sample, tested 3 hours after manufacturing, showed clear cracks and indents on the edges, which indicated the elastic-plastic nature of the destruction. In 7.5 hours, the edges of the sample upon destruction were covered with a network of small cracks; inside the sample there were also numerous cracks and microcracks, which indicated brittle fracture. Based on the obtained images of the deformed structure of modified concrete, the mechanism of transition from elastic-plastic destruction of the material to brittle one was clearly visible.Discussion and Conclusion. The studied dependences of the influence of the size of fine aggregate on the mechanisms of formation and propagation of structural defects contribute to the theory of the processes of destruction of fine-grained concretes. The results obtained prove the prospects of using X-ray computed tomography as a method of nondestructive testing of the internal structure of fine-grained concrete, including at the early stages of strength development.

Mechanical properties investigation on recycled rubber desert sand concrete

The mechanical properties of recycled rubber desert sand concrete were studied to determine the effect of the replacement rate of recycled rubber and desert sand. The replacement rates of recycled rubber aggregates were set to 0 %, 5 %, 10 %, and 15 %, respectively, and for dessert sand aggregates were set to 0 %, 10 %, 20 %, and 30 %, respectively, during concrete preparation. The concrete's compressive, tensile, and freeze-thaw mechanical properties were tested at each replacement rate. The experimental results show that when the replacement rate of desert sand increases from 0 % to 30 %, the replacement rate of recycled rubber is 15 %, and the splitting tensile strength of concrete shows a pattern of first increasing and then decreasing. When the replacement rate for desert sand is 30 %, the replacement rate for recycled rubber rises from 0 % to 15 %, and the splitting tensile strength shows a pattern of first decreasing. With replacement rates of 10 percent for recycled rubber aggregate and 20 percent for desert sand aggregate, the damage to the concrete from external forces is relatively ideal. The compressive strength values are the highest, and the split tensile strength is the best, demonstrating good compressive and tensile strength. The results of CO2 emission analysis show that as the amount of recycled rubber increases, the CO2 emissions per unit volume of concrete also increase. The interface of the micro-optical structure in the concrete is straightforward, the pores are few, and it exhibits good mechanical properties. The concrete undergoes freeze-thaw cycles with relatively stable changes in compressive strength and split tensile strength, demonstrating a solid freeze-thaw resistive mechanical property.