Coral Sand Research Articles

Three-dimensional mesoscale modelling of the compressive behaviors of coral sand

Three-dimensional mesoscale modelling of the compressive behaviors of coral sand

Dynamic Characteristics of Coral Sand in the Condition of Particle Breakage

To study the influence of coral sand particle breakage on its dynamic characteristics, taking the South China Sea coral sand as the research object, one-dimensional consolidation test was conducted to prepare different particle breakage degree samples, and then, the GCTS resonant column test was carried out on these samples. The results show that with the degree of coral sand particle breakage increases, the maximum dynamic shear modulus of coral sand first increases and then decreases, and the minimum damping ratio also shows a similar trend. In addition, based on Hardin’s theory of the relative breakage, this study proposes a new modified relative breakage B r ′ for describing particle breakage and establishes a relationship between the maximum dynamic shear modulus and the modified relative breakage B r ′ .

Damage and deterioration characteristics of basic magnesium sulfate cement-coral aggregate concrete exposed to elevated temperature

Different proportions of coral aggregate, coral sand and basic magnesium sulfate cement (BMSC) were used to prepare the basic magnesium sulfate cement-coral aggregate concrete (BMSC-CAC), which was applied in the island and marine engineering as a brand new type of concrete. The specimens of BMSC-CAC exposed to elevated temperature (100°C, 300°C, 500°C, 700°C, and 900°C) were tested by the non-destructive method to obtain the surface characteristics at different temperatures. A new parameter, elevated temperature damage (D), derived from continuous damage mechanics (CDM) was applied to indicate the degree of concrete damage after being exposed to elevated temperature. The surface characteristics of the specimen were changed significantly at different temperatures, especially the aggregate colours. The fitting relationship between ultrasonic velocity and temperature was established, which can be well used to fit different types of concrete. The elevated temperature damage degree (D) was defined by damage mechanics, and the mathematical relationship between D and the residual compressive strength was discussed.

The Effect of Slag on the Mechanical Properties of Coralline-Activated Materials and the Formation and Transformation of Mineral Crystals

In this study, coralline-activated materials were prepared using ball-milled coral powder as cementitious material and coral sand as fine aggregate. XRF (X-ray fluorescence) and chemical dissolution tests were carried out to determine the content and reactivity of various elements in coral powder. The compressive strength of the developed composites was evaluated at different ages, and the formation and transformation of mineral crystals in coralline-activated samples were further analyzed by XRD (X-ray diffractometer). The results show that the calcium content in coral powder was as high as 89.5% (loss on ignition). However, only 56% of the active calcium could participate in the polymerization reaction. The silicon and aluminum content was too low, and the slag addition could improve the deficiency of silicon and aluminum in coral powder. With the increase in slag content (from 0% to 50%), the compressive strength of the composites increases significantly. Nevertheless, the enhancement is not pronounced when the slag content exceeds 50%. The increase in slag amount can stimulate the transformation of calcium minerals, e.g., aragonite and calcite, into hydrated calcium silicate and calcium aluminosilicate gels, which significantly enhances the resulting compressive strength.

Application of the Grey Model in One-Dimensional Long-Term Creep Analysis of Coral Mud From the South China Sea

Coral mud is widely distributed in the South China Sea. Studying deformation laws of coral mud is important to the infrastructure development in the South China Sea. In this article, a one-dimensional oedometer test of coral mud under near-natural conditions is performed. The exponential fitting model, Burgers model and ODGM model are used to analyse the time-strain relationship of coral mud under different vertical pressures. The self-fitting accuracy and prediction accuracy of the three models under different vertical pressures are compared. Then, the number of data points used in the model was reduced, and the associated change in accuracy and model stability of the three models were analysed. The study found that the ODGM model has high self-fitting, high prediction accuracy, and a high model stability when the number of data points is reduced, which describe the long-term nonlinear deformation of coral mud more accurately.

Effects of particle shape on dynamic mechanical behaviours of coral sand under one-dimensional compression

Coral sand is characterized by high porosity, high compressibility and high crushability mainly due to its irregular particle shape and internal pore structure. Single coral particles in a size fraction tend to have a variety of shapes from bulky to flaky, which emerges as one of the most pivotal factors affecting the macro-scale performances of coral sand. A series of confined one-dimensional dynamic compression tests were conducted on coral sand with four classified shapes (e.g., blocky, dendritic and rodlike, flaky particles, and coral debris) via a modified 37-mm-diameter split Hopkinson pressure bar (SHPB) apparatus. The effects of particle shape on the dynamic mechanical behaviours of coral sand at high strain rates were assessed in terms of the stress-strain response, compressibility, particle breakage and the change in particle shape measures. It is revealed that coral sand exhibits high small-strain stiffness, a low crushing strength, a notable strain hardening and a high compressibility behaviour as subjected to dynamic compression at high strain rates. In the low-stress level, it is the high interparticle friction and interlocking induced by the irregular shape form and rough surface texture that lead to a gentle descending tendency in the void ratio. While in the high-stress level, it is the particle crushing along with its rearrangement that results in the high compressibility of the coral sand assemblage. In general, the particle crushing strength tends to decrease as the particle shape shifts from bulky to elongated and to flaky. The large-strain stiffness decreases and the compressibility increases with an increase in the overall particle irregularity. Coral particles are prone to be more spherical and smoother as the average degrees of sphericity, convexity and aspect ratio approach to unity, due mainly to the intense asperity damage and primary fracture under dynamic tests. Furthermore, the percentage in the degrees of sphericity, convexity, aspect ratio and flatness increases with respect to the initial degrees of themselves. It can be further postulated that the extent of breakage could be quantified by the statistical change in the particle measures such as sphericity and convexity aside from particle dimensions for granular materials with irregular shapes.

Improvement of Coral Sand With MICP Using Various Calcium Sources in Sea Water Environment

Microbial-induced calcium carbonate precipitation (MICP) is a promising technique for strengthening soil. The influences calcium sources (calcium chloride, calcium acetate, and calcium nitrate) and seawater/freshwater environment on the improvement of coral sand were studied. A microbial-induced calcium carbonate precipitation (MICP) aqueous solution experiment was carried out to explore the influence of the different calcium source and seawater/freshwater on the relationship between calcium carbonate precipitation and time. Coral sand column treatment tests were conducted to investigate the environmental impact on soil strengthening. The results show that the actual production of calcium carbonate is 20% lower than the theoretical value. Calcium chloride was found to be the optimal calcium source in terms of the seawater environment affect 2% of calcium carbonate production compared with the freshwater environment. The results of tests on three different calcium sources (calcium chloride, calcium acetate, and calcium nitrate) showed that seawater adversely affected the unconfined compression strength (UCS) and the tensile splitting strength (TSS) of strengthened specimens compared with freshwater. The UCS and TSS values in the freshwater environment were approximately 30–45% higher than those in the seawater environment. Nevertheless the strengthened specimens in the seawater environment exhibited high strength, and the MICP improvement effect can be guaranteed. Therefore, The MICP technology can be used to improve coral sand for construction work in seawater environments.

Biochemical, Strength and Erosional Characteristics of Coral Sand Treated by Bio-Stimulated Microbial Induced Calcite Precipitation

Biochemical, Strength and Erosional Characteristics of Coral Sand Treated by Bio-Stimulated Microbial Induced Calcite Precipitation

Numerical assessment of effects of strain rate on the critical state of crushable sand

This study highlights the effects of strain rate on the critical state (CS) response of crushable sand. A set of drained triaxial tests is simulated using the discrete element method (DEM) to understand the rate effects on the stress–strain and volumetric behaviour of the granular sample. The DEM parameters are obtained by comparing the stress–strain and particle crushing behaviour of in-house experimental analysis on crushable coral sand under a slow strain rate. In DEM, the particles are subjected to varied strain rates under different initial confining pressures and initial densities to capture the rate effects on macroscopic responses until the CS. It is observed that crushing increases with an increase in confining stress. However, a higher strain rate induces relatively lower crushing and higher strength in terms of both peak stress and residual stress. It is observed that in pressure–volume space, the CS line alters with the increasing strain rate of the crushable samples, especially under high-confining conditions, whereas the effect of strain rate on CS seems to be negligible under low-confining conditions due to the absence of particle crushing.

Utilizing of coral/sea sand as aggregates in environment-friendly marine mortar: Physical properties, carbonation resistance and microstructure

The utilization of marine waste and resources to produce eco-friendly building materials is essential for the sustainable development of the construction industry on islands and coastal areas. The sea sand and coral sand sourced from coral waste are the most promising. Therefore, the objective of this work is to investigate the feasibility of coral/sea sand as fine aggregate to prepare an environment-friendly marine mortar. The effects of various volume fractions of coral sand intended to replace sea sand on the flowability, physical performances, carbonation resistance, and microstructure of the prepared marine mortars were evaluated. The experimental results showed that as the sea sand replacement by coral sand was increased, the flowability and dry bulk density decreased, while the water absorption increased. Encouragingly, usage of a combination of coral and sea sands enhanced the flexural strength of marine mortars. A mixture containing 80 vol% of coral sand showed the best effect. The carbonation depth of marine mortar was found to noticeably increase with increasing replacement ratios of sea sand by coral sand, closely relating to the increase in the volume of capillary and large pores. In particular, the combined utilization of coral sand and sea sand resulted in a compacted interfacial transition zone (ITZ) between fine aggregates and the matrix.

Particle size and confining-pressure effects of shear characteristics of coral sand: an experimental study

Particle size and confining-pressure effects of shear characteristics of coral sand: an experimental study

Study on the mechanical properties of coral sands with different particle gradations

The properties of coral sands are significantly affected by the gradation changes caused by the particle breakage. In this paper, a series of triaxial consolidated drained (CD) shear tests were carried out to investigate the influence of gradation on the particle breakage and mechanical properties of coral sands. The results showed that the softening and dilatancy during shearing are more significant for coral sands with smaller average particle sizes under low confining pressure. The peak friction angle decreases with the confining pressure and can be fitted by a power function for different gradations. The particle size distribution of the “hyperbolic” or “straight” curve has a trend to transform into a “sigmoidal” type after shearing; and the variation is affected by the confining pressure and the initial gradation. The critical state lines of the coral sands with different gradations are parallel straight lines on the plane. In addition, the relative breakage of coral sand particles increases with the confining pressure and the mean particle size. Based on the triaxial compression test results, the expressions relating relative breakages to the confining pressure and the gradation parameters were proposed.

Research on improvement of Terzaghi’s bearing capacity equation for coral sand foundation

The mechanical characteristics of coral reef sandy soil is significantly different from the sand and clay soils. Currently, Terzaghi's bearing capacity equation is still commonly applied to evaluate the bearing capacity of coral reef, while the calculation result is not consistent with the actual condition. Based on a port project in east Africa, plate loading test of the coral reef sandy soil has been conducted and the P-S curves are obtained, and the bearing capacity values obtained from the plate loading test results are compared with the calculated results based on the Terzaghi’s bearing capacity equation. According to multiple linear regression and the optimal regression equation selected by the significance test, the modified Terzaghi foundation equation of bearing capacity in coral reef sandy soil area is proposed, and the modified Terzaghi foundation equation has important reference value for the calculating of the bearing capacity of the coral reef sandy soil foundations.

Analysis of Early-Age Hydration Behavior and Micro-Mechanism of Coral Sand Cement Mortar.

Coral sand cement (CSC) mortar is increasingly used in reef projects, which is prepared by mixing coral sand with cement and water in certain proportions. Considering that early-age hydration behavior is closely related to the strength and durability of the mortar, the early-age hydration process and micro-morphology of CSC mortars with various water–cement ratios (W/C) and sand–cement ratios (S/C) were studied. A monitoring system based on FBG is proposed in this paper, which uses the high sensitivity and conformability of optical fiber to measure the hydration temperature and internal shrinkage strain simultaneously and continuously. The standard sand cement (SSC) mortar with the same sand gradation and mix proportion is also prepared for comparison. The micro-morphology is observed by a scanning electron microscope (SEM) for measurement results’ explanation. The results show that the variation of the hydration temperature and shrinkage strain with hydration time of both CSC mortars and SSC mortars follow a unimodal function. Differently, the peak hydration temperature for CSC is obviously lower than that of SSC. The peak temperature of CSC mortar decreases linearly with the increase in S/C, and the decrease rate of the peak temperature is higher for CSC with small W/C than that with higher W/C. For mortars with lower W/C, the peak shrinkage strain of CSC is larger than that of SSC. Meanwhile, for mortars with higher W/C, the peak shrinkage strain of CSC changes to be lower than that of SSC, which is attributed to the significant water absorption characteristic of CSC. Therefore, as an eco-friendly lightweight aggregate, CS is more suitable than SS for the design of high W/C and alleviating the hydration heat of mass concrete under the meeting of strength.

Beachrock Formation Mechanism Using Multiproxy Experimental Data from Natural and Artificial Beachrocks: Insights for a Potential Soft Engineering Method

Beachrocks are a window to the past environmental, geological, sedimentological and morphological conditions that were dominant in the coastal zone during their formation. Furthermore, beachrocks have the ability to reduce coastal erosion impact on sandy beaches. This study focuses on the beachrock formation mechanism through the comparison of cement characteristics, mineral chemistry and sedimentology of beachrock occurrences from two different geological and geographical localities: Diolkos, Corinth, Greece and Sumuide, Okinawa, Japan. In addition, in order to investigate a potential soft engineering method to protect coasts from erosion, artificial beachrock samples were created in vitro using sand samples and ureolytic bacteria from both areas under accelerating conditions. For Okinawa artificial beachrock experiments, the bacteria Pararhodobacter sp. was used, and for Diolkos, it was the bacteria Micrococcus yunnainensis sp. For the natural beachrocks, a multi-analytical approach was accomplished with the use of microscopic investigation, a scanning electron microscope, energy-dispersive X-ray spectroscopy, X-ray diffraction and X-ray fluorescence. Correlations were made between natural and artificial beachrocks. Results have shown that Diolkos beachrock was formed in the upper part of the intertidal zone, consisting of detrital material originating from the local bedrock, while Sumuide beachrock formed in the low intertidal–upper subtidal zone, consisting of coral sand and foraminifera fragments. For the artificial beachrocks, three samples were created using the microbial-induced carbonate precipitation (MICP) method, one from Diolkos (Corinth, Greece) and two from Sumuide (Okinawa, Japan). Diolkos artificial beachrock was better consolidated in comparison to Sumuide. Our investigation has shown that bacterial density was the key factor for the creation of the artificial beachrocks, while the samples’ granulometry played a secondary role in the process. The laboratory artificial beachrocks show encouraging results for a new soft engineering method to encounter beach erosion while keeping an ecofriendly character by saving energy, material resources and gas emissions. Artificial beachrocks can share the same properties of a natural beachrock and can contribute positively to marine biodiversity as a natural rocky habitat.

Biocementation of Coral Sand Under Seawater Environment and an Improved Three-Stage Biogrouting Approach

Biocementation of Coral Sand Under Seawater Environment and an Improved Three-Stage Biogrouting Approach

Experiment and discrete element modeling of particle breakage in coral sand under triaxial compression conditions

Particle breakage is a major factor that determines the strength and deformation of coral sand. Consolidated drained triaxial compression tests under different confining pressures have been conducted on coral sands as a basis for 3D discrete element simulations, focusing on the particle breakage characteristics. Considering angular breakage and overall breakage in the numerical model, particle breakage is simulated with a fragment replacement method, and the particle size and the equivalent stress are combined as the criterion to judge the occurrence of particle crushing. The validation of the numerical model is proved by reproducing comparable stress-strain relations, evolutions of grain size distribution and relative breakage rate. The overall behavior of the specimen transit from strain softening to strain hardening with the increase in confining pressure from 100 kPa to 400 kPa. Meanwhile, the number of angular breakages increases at a decreasing rate, while the number of overall breakages increase at an increasing rate. This phenomenon implies that overall breakage overwhelms angular breakage and becomes the dominating particle breakage mechanism gradually with increasing confining pressure. Microscopic analyses reveal that higher confining pressure increases the overall contact force, the portion of strong contact force and the coordination number.

Cover Picture and Issue Information

People and NatureVolume 3, Issue 6 p. 1127-1129 COVER PICTURE AND ISSUE INFORMATIONFree Access Cover Picture and Issue Information First published: 30 November 2021 https://doi.org/10.1002/pan3.10105AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Cover image: Booby on a camera, taken on the French Frigate Shoals, Hawaii, USA. Human influence on the natural world extends even to the most remote places, such as French Frigate Shoals in the Pacific Ocean. Seabirds here are unafraid of people but they are exposed to discarded plastic waste and to the effects of climate change. The low-lying coral sand island where many of these birds were filmed nesting has since been erased by a severe storm. Including the environmental context in pure wildlife films has never been more important. See pages 1138–1146. Photo credit: John Aitchison. Volume3, Issue6Special Issue ‐ Nature on screen: The implications of visual media for human–nature relationshipsDecember 2021Pages 1127-1129 RelatedInformation

Effects of groundwater level on the seismic responses of coral sand ground and superstructure by shaking table tests

Effects of groundwater level on the seismic responses of coral sand ground and superstructure by shaking table tests

Study of the effects of fine coral powder and salinity on the mechanical behaviour of coral sand-seawater cement mortar

The coral sand and seawater are widely used as construction materials. The coral powder and salt contained in the coral sand-seawater cement mortar have a significant effect on the void distribution, cemented effect and mechanical behaviours. Thus, the mechanisms for the influences of coral powder content and salinity were investigated by a series of piezoelectric transducer (PZT) tests and uniaxial compressive strength tests. The cement mortar specimens with various coral powder contents and salinities were prepared and examined. The microstructure and pore characteristics of mortar under different conditions were analysed by scanning electron micrograph (SEM). The results show that the main ingredient in coral powder (i.e., CaCO3) would accelerates the cement hydration. The fine particles could fill the skeletal pores. Therefore, an appropriate content of coral powder can improve the strength and deformation properties of mortar. Salt had the dual effect of decreasing the stiffness of the cemented mortar specimen and intensifying the aggregation of coral powder particles. The aggregation of the coral powder particle in coarse skeleton is the key factor affecting the overall mechanical properties of the seawater-sea sand cement mortars. The coupling effects of coral powder and salt influence the microstructure and macroscopic mechanical properties of the mortar, and the effects of the two factors are mutually restricted. These research results provide a reference for guiding the optimisation of the mechanical properties of coral sand-seawater cement mortars and improving the safety and stability of structures engineered with these mortars.