Kinds Of Aggregates Research Articles

Research on the evolution law of aggregate micro-texture during long-term wearing of asphalt pavement

The micro-texture of aggregates plays a very important role in anti-skid performance of asphalt pavement. Three kinds of aggregates, limestone, basalt and sandstone, were used to prepare asphalt mixture to study the evolution law of micro-texture of aggregates under long-term wear. One Third Scaled Model Mobile Load Simulator (MMLS) was used to conduct wear tests on asphalt mixtures under two working conditions: drying and water immersion. Laser confocal scanning microscopy (LCSM) was used to test and analyze the micro-texture evolution of the original aggregate, mixed aggregate, and accelerated loaded aggregate surfaces. The mechanism of texture evolution was analyzed based on the mineral composition, Vickers hardness, and micro-structure of the aggregate. The results show that wear can lead to varying degrees of polishing of the aggregate micro-texture, with limestone>basalt>sandstone, and the drying condition is more severe than the water immersion condition. The wear degree of the texture is closely related to the mineral composition and hardness of the aggregate. The more complex the mineral composition, the greater the hardness and hardness dispersion, the less likely the texture will be polished. The micro-texture indexes Ra and Rq have the highest correlation with anti-skid performance, and the relationship between them is obtained by fitting. Scanning electron microscopy (SEM) shows that the fine particles in sandstone are easy to fall off, which is conducive to the regeneration of texture, and is also the internal reason for its best anti-skid durability.

Improving the early-age behavior of concrete containing coarse recycled aggregate with raw-crushed wind-turbine blade

Recycled aggregate used in replacement of coarse natural aggregate can lower the quality of both fresh and hardened concrete properties. There are many methods to reduce the negative effects of recycled aggregate in the literature, although most involve unsustainable solutions that jeopardize any environmental benefits associated with their use. In this study, a novel approach is presented for mitigating the adverse impacts of coarse recycled aggregate used in concrete mixes. Furthermore, it allows solving the current issue of the recycling of wind-turbine blades at the same time. Raw-crushed wind-turbine blade, a waste from the crushing of decommissioned wind-turbine blades with a high proportion of glass fiber-reinforced polymer fibers, was added to concrete mixes containing coarse recycled aggregate. As a main outcome, additions of that waste were found to enhance the early-age properties of concrete containing that kind of aggregate. Plastic shrinkage was reduced, and flexural strength increased by as much as 47 % and 14 %, respectively, while compressive strength remained approximately constant.

Federated Constrastive Learning and Visual Transformers for Personal Recommendation

This paper introduces a novel solution for personal recommendation in consumer electronic applications. It addresses, on the one hand, the data confidentiality during the training, by exploring federated learning and trusted authority mechanisms. On the other hand, it deals with data quantity, and quality by exploring both transformers and consumer clustering. The process starts by clustering the consumers into similar clusters using contrastive learning and k-means algorithm. The local model of each consumer is trained on the local data. The local models of the consumers with the clustering information are then sent to the server, where integrity verification is performed by a trusted authority. Instead of traditional federated learning solutions, two kinds of aggregation are performed. The first one is the aggregation of all models of the consumers to derive the global model. The second one is the aggregation of the models of each cluster to derive a local model of similar consumers. Both models are sent to the consumers, where each consumer decides which appropriate model might be used for personal recommendation. Robust experiments have been carried out to demonstrate the applicability of the method using MovieLens-1M, and Amazon-book. The results reveal the superiority of the proposed method compared to the baseline methods, where it reaches an average accuracy of 0.27, against the other methods that do not exceed 0.25.

A multiscale study of the influence of aggregate type on adhesion at the asphalt–aggregate interface

In order to examine the influence of aggregate type on the asphalt–aggregate interface, the adhesion performance between asphalt and aggregate was investigated at three scales. The pull-out test was conducted on asphalt and three kinds of aggregates at the macroscale to directly estimate the adhesion performance between them. At the microscale, the surface energies between asphalt and limestone, basalt, and andesite were examined through a surface energy experiment, and then the adhesion and debonding work were assessed. At the nanoscale, the interfacial behavior between six mineral compositions (SiO2, CaO, MgO, Al2O3, Fe2O3, and Na2O) of these aggregates and asphalt was determined using molecular dynamics simulation. The correlation of adhesion work at the microscale and nanoscale was further explored to unveil the bonding mechanisms between asphalt and aggregate. The macroscale results indicate that the adhesion force of andesite is slightly greater than that of limestone, probably because andesite has a greater surface roughness than limestone. The microscale results show the best adhesion and water stability between asphalt and limestone. The finding further explains that alkaline aggregates exhibit superior interfacial adhesion to asphalt. The nanoscale results show that the adhesion work of alkaline oxides with asphalt is more significant than that of acidic oxides with asphalt. Limestone and basalt have better adhesion to asphalt due to higher alkaline oxide content, while andesite has poorer adhesion to asphalt due to lower alkaline oxide and higher SiO2 content. The interfacial adhesion work increases from the microscale to the nanoscale by a factor of 2.91–4.75.

Research on Statistical Characteristics and Prediction Methods of Ferronickel Slag Pervious Concrete Performance with Different Sizes of Aggregate and Mixtures

In the exploration of sustainable construction materials, the application of ferronickel slag (FNS) in creating pervious concrete has been investigated, considering its potential to meet the dual requirements of mechanical strength and fluid permeability. To elucidate the statistical properties and models for predicting the performance of FNS-composited pervious concrete with different sizes of aggregates and mixtures, a series of experiments, including 54 kinds of mixtures and three kinds of aggregate, were conducted. The focus was on measuring the compressive strength and the permeability coefficient. The results indicate that the compressive strength of pervious concrete decreases with the increase in aggregate size, while the permeability coefficient increases with the increase in aggregate size. Through normalization, the variability of these properties was quantitatively analyzed, revealing coefficients of variation for the concrete’s overall compressive strength and the permeability coefficient at 0.166, 0.132, and 0.150, respectively. Predictive models were developed using machine learning techniques, such as Linear Regression, Support Vector Machines, Regression Trees, and Gaussian Process Regression. These models demonstrated proficiency in forecasting the concrete’s compressive strength and permeability coefficient.

Evaluation for water stability of bitumen mixture with multi-scale method

The interaction forces were investigated between matrix bitumen and four kinds of aggregates, including basalt, granite, limestone A, and limestone B in various pH solutions. The aim was to accurately evaluate the water damage resistance of bitumen mixtures at macroscopic and microscopic levels with a conjunctive method of boiling water tests, atomic force microscopy (AFM) colloidal probe technique, zeta potential measurements, and molecular dynamics simulation. Macroscopic evaluation was conducted through indoor water-boiled flaking rate tests to analyze the flaking rate and adhesion class of bitumen and aggregate. The microscopic forces were measured by AFM probe technique between bitumen and aggregate, zeta potential measurements were carried out and the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was employed to interpret the bitumen–aggregate interaction behaviors. Molecular dynamics simulations were considered to explain the interface binding tendency between bitumen and aggregate. The AFM force measurements showed that the long-range repulsion forces changed to attractive forces and high adhesive forces with the pH change from 4 to 6. However, the strong long-range repulsive forces were decreased, compounded with a relative low adhesion in the alkaline solution. The long-range force between granite and bitumen was a strong attraction, and adhesion was strong when the solution pH was 4, which is a more special phenomenon. It may be due to the electrostatic attraction between the positive charge on the granite surface and the negative charge on the bitumen surface. Zeta potential values of bitumen droplets and aggregate particles exhibited an overall decreasing trend with increasing solution pH, and the variation of surface potential value is consistent with the variation trend of long-range force between aggregate and bitumen. The microscopic forces between bitumen and aggregate measured by AFM were validated by molecular dynamics simulations, revealing a strong correlation between debonding work and interfacial forces. The results indicate that the adhesion between aggregates and bitumen in aqueous solution followed the order: limestone A > limestone B > basalt > granite, and alkaline conditions are not conducive to preventing water damage in bitumen mixtures. It provides insights into the water stability mechanism of bitumen-aggregate at macroscopic and microscopic scales, offering theoretical support for preventing water damage in bitumen pavements.

Pull-out behaviour of steel and carbon fibers in 3D-printable cement matrices of various compositions

The aim of this study was to investigate some of the factors which affect the fiber–matrix pull-out behaviour and bond strength. 3D-printable cement mixtures with various compositions were used as matrices. The effect of parameters such as type of fiber (steel or carbon), kind of aggregates (sand or limestone powder), and kind of modifier additives (metakaolin or complex nano-sized additive based on SiO2 (CNA)) on fiber–matrix pull-out behaviour were determined. The results demonstrated that the bond strength of a fiber-matrix mainly depends on the fiber type. The bond strength of a “steel wire matrix” was 2–4 times higher than the bond strength of a “carbon fiber matrix”. The fiber–matrix bond strength increased as the conditions for the wetting of a fiber’s surface improved, especially for carbon fiber. Microstructural investigation revealed that the congestion of hydration products in a fiber–matrix interface significantly improved the pull-out behaviour. The use of sand as an aggregate for a cement matrix enhanced the bond strength by 10–15% in “steel wire matrix” systems. The introduction of metakaolin and CNA into a cement matrix increased the bond strength by 15–20% for sand-based and powder-based cement matrices.

Effect of protein concentration on thermal aggregation and Ca2+-induced gelation of soymilk protein

The concentration of protein in soymilk is a key element in the production of soy products. In this study, the thermal aggregation behavior of soymilk proteins and the properties of aggregates formed at different concentrations (33 mg/g-56 mg/g) were analyzed. Results showed that the soymilk proteins formed two kinds of aggregates which demonstrated different properties at particular concentrations. In the low concentration range (<40 mg/g), rod-like aggregates were formed. The content and size of the aggregates increased with growing protein concentration, but the morphology didn't change. Additionally, the surface hydrophobic groups, sulfhydryl groups(-SH) and electrostatic charge of the protein aggregates exposed to the outside also increased, which drastically (20%) reduced the activation energy of the Ca2+-induced gelation and led to the cross-linkage of protein aggregates to form a dense, cluster-like network structure, which facilitated the formation of gels with high levels of hardness and increased water holding capacity (WHC). At protein concentrations greater than 40 mg/g, however, the protein subunits formed aggregates, which also combined with each other along an axial direction through the hydrophobic groups and -SH on the surface, thereby forming linear or even micro-network agglomerates. In this way, the surface hydrophobicity(S0) and -SH content decreased linearly with the rising protein concentration, while the surface charge density increased, the activation energy required for soymilk gelation increased, and the protein aggregates crosslinked to form a coarse network structure with large voids. Hence, the hardness, elasticity, toughness and WHC of the gel decreased with the increase in protein concentration. This study reveals the mechanism by which protein concentration changes the gel-forming ability of soymilk, and have important theoretical reference for enterprises to select the appropriate protein concentration for soymilk preparation, or to optimize the “concentration dilution method” to meet the processing requirements of different types of soy products.

Adhesion Characteristics of Graphene-Modified Asphalt Using Surface-Free Energy Method

Graphene is an auspicious nanomodified material for asphalt pavements. This study aims to evaluate the effect of graphene composites (GCs) on the adhesion characteristics of asphalt based on surface-free energy (SFE) theory. The surface-free energy of graphene-modified asphalt was measured by the sessile drop method. The work of adhesion and stripping between asphalts and two kinds of aggregates were calculated through using their surface-free energy components. The macroadhesion property between asphalts and aggregates was obtained by the improved boiling method. Immersion Marshall test and freeze–thaw splitting test were conducted to verify the effectiveness of surface-free energy indexes. The results demonstrated that GC significantly increased total surface-free energy and dispersion components of asphalt but reduced the polarity components. The adhesion work between asphalt and both aggregates increased after modification, whereas the stripping work decreased after modification. GC can substantially improve the adhesion and resistance to moisture damage at the asphalt–aggregate interface, and the effect on asphalt-limestone is more obvious. There are strong correlations between macro evaluation indicators and surface-free energy calculated indexes, where the macro indicators are more strongly correlated with stripping work. Limestone is generally better at resisting moisture damage than granite. GC remarkably improved the water stability of the mixture, which further indicates that energy indexes are a good method to quantize the moisture damage resistance of hot mix asphalt (HMA). The high dosage of graphene-modified asphalt is potentially applicable to porous asphalt concrete (PAC) pavement with extra-large voids.

Assessment of whole egg fractionation by tangential flow filtration: The problem of low-density lipoprotein aggregates

Eggs are a compelling source of bioactive compounds. Membrane separation is a green technology that allow components to be obtained while preserving their biological activity. The objective of this study was to assess the application of membrane separation technology to egg products. Whole egg plasma, egg yolk plasma and egg white were tangential flow filtrated evaluating the effect of the pore size and membrane materials. Transmission of dry matter and proteins from feed to filtrate was noticed in all the processes. Nevertheless, cholesterol and carotenoids were not detected at filtrates from 750 kDa to 0.2 µm when mixed ester or polyethersulfone were the membrane materials. The interactions between egg yolk and egg white components difficult the separation. Cholesterol was part of low-density lipoproteins. These lipoproteins together with carotenoids were filtrated with modified polyethersulfone at a minimum pore size of 0.2 µm. High uncertainty was obtained due to low-density lipoproteins aggregation. Three kinds of aggregates can justify these results: low-density lipoproteins aggregated themselves, low-density lipoproteins bound to ovomucin and other smaller egg white proteins (<750 kDa), and low-density lipoproteins bound only to smaller egg white proteins. The detected chelate effect of egg white proteins could be important in vivo. Therefore, research should be led to assess the influence of cooking method and digestion of the different egg components on the bioaccessibility of nutrients and the generation of bioactive peptides. Moreover, the filtration processes analysed herein may be useful techniques for liquid food matrix studies.

Full-scale sustainable structural concrete containing high proportions of by-products and waste

The construction industry in general is, through minor low-cost processing methods, converting several of its by-products into viable materials; furthermore, some siderurgic sector by-products are likewise of use. In this context, large-scale batches (mix volumes over 0.5 m3) of good quality structural concrete are proposed, in which two kinds of binder and two kinds of aggregate (steel slag and recycled concrete) are used to perform four concrete mixtures, containing more than 80 % in mass of good-quality recycled materials. A batch of tests, both in the fresh and in the hardened state, are performed, covering on-site placement and long-term properties, to guarantee the suitability and the quality of the mixtures as structural concretes. Most of the results were encouraging, mainly depending on the aggregate and the binder types that were used. The fresh-state workability of all the test mixtures was good. All the results in terms of hardened properties, strength (42 MPa in type I cement mixtures, and 32–38 MPa in type III cement mixtures), stiffness, long-term shrinkage, and microstructural state (porosity, permeability) were acceptable, their quality depending on the type of each component. The good results of the mixtures based on the slag-based binder deserve attention. Some weak points found were the slightly higher specific weight of the slag aggregate mixes (amounting to more than 2.7 Mg/m3), plastic shrinkage rates (in some cases greater than 1.2–1.5 thousand), and loss of resistance against chlorine penetration in recycled concrete mixes. However, drawbacks of that sort are no obstacle to their use in most structural applications.

Evaluation of Adhesive Properties of Different Mineral Compositions in Asphalt Mixtures with Experimental and Molecular Dynamics Analyses

The adhesion between bitumen and mineral composition plays a vital role for the performance of asphalt mixtures. This study compares the adhesion of limestone, dolomite, and granodiorite to bitumen and evaluates the effects of different mineral components on adhesion. Three kinds of aggregates were tested through rolling-bottle tests. Afterwards, the respective fillers were integrated into asphalt mastic in a 1.6:1 mass ratio with bitumen and were subjected to frequency scan tests separately. A modified Luis Ibrarra-A model, K. Ziegel-B model, and K-B-G model were used to evaluate the bitumen–filler interactions based on the rheology of the asphalt mastic. In addition, the interface behavior between eight mineral components from these fillers/aggregates and bitumen were investigated by molecular dynamics (MD) simulations. The work of the adhesion and molecule concentration profiles were obtained from MD simulations. The results showed that the limestone and dolomite had better interfacial adhesion to the bitumen than the granodiorite. The calcium oxide and titanium oxide had the highest potential adsorption effect on the bitumen. Moreover, the high calcium oxide content contributed to better bitumen adhesion with the limestone and dolomite than with the granodiorite, which was further confirmed by additional molecule concentration profile analysis. This research contributes to the in-depth understanding of the effect of different chemical properties on the performance of asphalt mastic and the selection of suitable mineral components to improve the bitumen–filler/aggregate interface and asphalt mixture performance in general.

Disruption of aggregates of a Zn2+-complex of a schiff base in water by surfactants: Insights from fluorescence spectroscopy in ensemble and single molecule levels

Schiff bases and their metal ion complexes have potential biological applications. It is important to study their interactions with biomimemtic assemblies in order to design such applications. To this effect, the nature and interaction between the aggregates of an organic fluorophore (Znsalampy) and surfactants viz. sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) have been monitored using bulk macroscopic techniques like steady state and time resolved fluorescence as well as single molecule techniques like Fluorescence Correlation Spectroscopy (FCS). The presence of more than one kind of aggregates is observed in neat aqueous solutions is confirmed by the complementary techniques. The more rigid aggregates are found to be less prone to disruption by surfactants. SDS is found to be significantly more effective than CTAB in breaking the aggregates of Znsalampy.

Effects of coarse aggregate morphology on concrete mechanical properties

The morphology of coarse aggregate can significantly affect the mechanical performance of concrete. In this study, the correlation between the compressive properties of concrete and the coarse aggregate morphology was systematically explored. Three kinds of aggregates with significant morphology differences were selected: Spherical Aggregates, Flaky Aggregates, and Elongated Aggregates. Three-dimensional morphological information was obtained by 3D Structured-Light Scanner, and quantitative statistical analyses were conducted to assess the distribution of morphological characteristics for these three-types aggregates. It is found that the three-type aggregates with the same mesh size can be properly discriminated through the axis length, flatness, elongation, the product of flatness and elongation, and sphericality. The simplex-centroid design was used to build the correlation between aggregate morphology and the compressive strengths of concrete with different water-cement ratios. The concrete with 100% spherical aggregates owns the highest compressive strength, which is higher than that of concrete mixed with other two aggregates by 9.4%–36.2% and 5.2%–28.8% at the curing of 3-day and 28-day, respectively. When the ratio of spherical aggregates reaches above 50%, the strength loss caused by the elongated aggregates and flaky aggregates content is not obvious. Further, the failure mechanism of concrete with 100% spherical aggregates, 100% flaky aggregates, and 100% elongated aggregates under cyclic loadings were investigated by X-ray computed tomography scanning. It is found that the cracks inside spherical aggregates concrete mainly initiate from the edges and the core section remains almost intact before the destruction by a major oblique crack. Meanwhile, the high residual strain after cyclic loadings can originate from the generated vertical cracks under compression. The research findings in this study can serve as a solid base for the designing of concrete structures.

Rapid Test Method for Evaluating Inhibiting Effectiveness of Supplementary Cementitious Materials on Alkali-Silica Reaction Expansion of Concrete.

At present, there are many problems in various tests when judging the alkali activity of aggregates. The most practical engineering concrete prism test (CPT) takes one year, and the concrete suppression method needs two years. The aim of this paper is to discuss inhibiting effectiveness of supplementary cementitious materials (SCMs) on alkali–silica reaction (ASR) expansion of concrete and evaluate this rapid test method. Three kinds of aggregates were selected by chemical analysis, XRD and petrographic analysis. The high alkali–silicic acid activity of three aggregates was determined by accelerated mortar bars, concrete microbars and CPT. The expansion of concrete specimens made of three kinds of aggregates was measured and analyzed by using the method of length measurement. By changing the curing temperature to 40 °C, 60 °C and 80 °C, the test period of CPT is accelerated. It proved that the expansion of CPT is larger at 60 °C and smaller at 40 °C. The inhibition test was also accelerated by adding different proportion of SCMs (fly ash or blast furnace slag) and adjusting the curing temperature to 60 °C and 80 °C. On this basis, the inhibition test was accelerated by changing NaOH solution instead of moist air curing. The test period of the accelerated inhibition test can be effectively shortened from two years to 4 months, The expansion trend of each parameter and specimen is evaluated, the evaluation cycle can be adjusted to 5–6 months. The microscopic reaction characteristics of concrete specimens were investigated by means of SEM. According to each parameter and criterion, the judging standard of concrete rapid test and rapid restraint test is given in this paper.

Application of Pre-Wetted High Titanium Heavy Slag Aggregate in Cement Concrete.

High titanium heavy slag is one kind of solid waste that exists in large amounts in the southwest of China. In this paper, this high titanium heavy slag is used as natural pre-wetted material in concrete because of its porous structure. Three kinds of aggregates are used in this concrete. The first one is natural limestone and river sand. The second one is dry slag fine aggregate and coarse aggregate. The third one is pre-wetted coarse slag aggregate and dry slag fine aggregate. The strength, dry shrinkage, autogenous shrinkage, relative humidity, pore size distribution, stress–strain relationship, micro-hardness and chloride penetration of concrete composed of the above three aggregates are tested in this study. The results show that pre-wetted slag aggregate is a suitable internal curing material. The concrete with pre-wetted slag aggregate shows higher strength, lower shrinkage and smaller porosity. The water absorbed in the slag aggregate can be released effectively to increase the relative humidity, accelerate hydration, improve porosity and increase the interface strength.

INFLUENCE OF THE TYPE AND SIZE OF GRINDING MEDIA ON THE PROPERTIES OF REACTIVE ALUMINA FOR REFRACTORY CONCRETE

Refractory concrete products are used in various fields of production, such as metallurgy, ceramics, glass, cement and even nuclear industry. In any places where high temperature is used, a kind of "protector" is required from aggressive media, such as liquid, gas, solid materials. Just such a material is monolithic refractory, or simply refractory concrete. Such concrete, as well as ordinary construction, consists of various kinds of aggregates that have both high melting temper-atures and strong mechanical properties that give the final composite the necessary properties. This article presents the results of a study ofthe effect of the type and size of grinding media in a vibrat-ing mill on the properties of reactive alumina for refractory concrete. Studies have shown that the use of Ø12-18 balls as grinding media is the most optimal. Microscopic analysis showed that the shape and size of the grinding media affects the size and shape of the particles of reactive alumina. Since it allows you to reduce the grinding time from 30 min (workshop) to 20 min, which from an economic point of view reduces production costs by reducing energy costs and increasing the ser-vice life of the vibrating mill. Also, by increasing the specific surface area to 11838 cm2/g compared to the workshop (8500 cm2/g), it is possible to achieve better packing of particles in concrete. Thus, high indicators of reactive alumina were obtained, which will improve the construction and tech-nical properties of refractory concrete.

New test methods to determine water absorption by capillarity. Experimental study in masonry mortars

In this work, two alternative methods are presented to evaluate the absorption of water by capillarity in masonry mortars. For this, two different types of sensors of our design have been developed, one capacitive and the other resistive. The measuring equipment has been tested in mortar samples made with three kinds of aggregates: natural, recycled ceramic, and recycled concrete. The results obtained employing the capacitive sensor show how there is a good correlation between the mass of water absorbed by the mortars and the variation in capacitance, taking into account that mortars made with recycled ceramic aggregate were the ones that present a higher capillary absorption coefficient and lower mechanical resistance. On the other hand, with the help of the resistive sensor, it has been possible to monitor in real-time the rise of water by capillarity inside the samples, determining how the percentage of humidity measured at different heights of the samples evolves. As a result, it has been observed that water experiences a greater rise due to capillarity in mortars made with natural aggregate.

Direct comparison between single-scattering properties of ordered and disordered aggregates of nano-sized scattering centers.

In this study, we examine the single-scattering properties of aggregates of nano-sized spherical scattering centers (spherules) of different sizes and refractive indices, with an emphasis on contrasting the single-scattering properties of ordered and disordered aggregates, holding all other parameters constant. The ordered aggregates are constructed by arranging the spherules in a simple cubic configuration, while the disordered aggregates are constructed using an ideal amorphous solid algorithm. The single-scattering properties of both kinds of aggregates are computed using the superposition T-matrix method. We find that, in most cases, the scattering and absorption and hence extinction of radiation by ordered aggregates is stronger than for disordered aggregates: for the cases we examine, the average percent difference in scattering efficiency is 14%, and the maximum percent difference in scattering efficiency is 44%; the average percent difference in absorption efficiency is 5.3%, and the maximum percent difference in absorption efficiency is 18%; the average percent difference in extinction efficiency is 12%, and the maximum percent difference in extinction efficiency is 40%. These differences have implications regarding radiative transfer in systems of amorphous particles in general.

Changes in soil aggregate stability and aggregate-associated organic carbon during old-field succession in karst valley.

Soil is the largest carbon pool whose change will have an impact on the terrestrial carbon cycle in the terrestrial ecosystem. Old-field succession on abandoned farmland, which usually has a noticeable effect on soil status, is a common phenomenon in karst valley where human activity alters frequently. In order to understand the changes in the accumulation of organic carbon (OC) in aggregates and bulk soil in different stages of old-field succession on abandoned farmland in the karst valley area, soil samples were collected at 0-10-cm and 10-20-cm depth representing three typical stages of old-field succession, i.e., abandoned farmland, secondary grass, and secondary shrub in Qingmuguan karst valley area, Chongqing City, Southwest China. Results displayed that during old-field succession (1) the mean weight diameter and geometric mean diameter of the aggregates increased and the fractal dimension decreased; (2) OC content within aggregates and bulk soil had no significant change in topsoil (0-10cm); OC content within microaggregates and bulk soil had a significant reduction in subsoil (10-20cm); the OC content within silt and clay fractions was significantly higher than that within the other two kinds of aggregates; (3) bulk-soil OC storage had no significant change but its accumulation relied more on the increase in the number of larger aggregates. It is concluded that the old-field succession in karst valley was beneficial to protect against soil erosion by improving soil aggregate stability, but had a limited effect on soil organic carbon sequestration.