Huisu Chen

The effect of hybrid fibers and expansive agent on the shrinkage and permeability of high-performance concrete

Abstract In this paper, high-performance concrete (HPC) incorporated with expansive agent and hybrid fibers, i.e., steel fibers, polyvinyl alcohol fiber (PVA fiber), and polypropylene fiber (PP fiber), was produced. The properties measured included shrinkage and water permeation of the concrete. The effect of hybrid fibers and/or expansive agent on the shrinkage and water permeation properties was investigated. Test results indicated that the hybrid fibers of different types and sizes could reduce the size and amount of crack source at different scales; hybrid fibers combined with expansive agent provided better enhancement for shrinkage resistance and impermeability of HPC than monoincorporation of hybrid fibers or expansive agent; the improvement of the shrinkage resistance and the impermeability of the concrete resulted from the combined use of expansive agent and hybrid fibers, which was dependent on the amount of expansive agent, types and sizes of hybrid fibers, total volume fraction of fibers, proportions of hybrid fibers, and so on. The relevant mechanisms were also discussed based on the analysis of the test results of pore structure of the concrete.

The effect of silica fume and steel fiber on the dynamic mechanical performance of high-strength concrete

The impact and fatigue performance of high-strength concrete (HSC), silica fume high-strength concrete (SIFUHSC), steel fiber high-strength concrete (SFRHSC), and steel fiber silica fume high-strength concrete (SSFHSC) under the action of repeated dynamic loading were studied. The mechanisms by which silica fume and steel fiber reduce damage were also investigated. The results indicate that steel fiber effectively restrained the initiation and propagation of cracks during the failure of an HSC structure, mitigated the stress concentrations at the tips of cracks, and delayed the damage process under impact and fatigue. Silica fume effectively improved the structure of the interface, eliminated the weakness of the interfacial zone, reduced the number and size of cracks, and enhanced the ability of steel fibers to resist cracking and restrain damage. As a result, the corporation of steel fibers and silica fume can increase greatly the performance of HSC subjected to impact and fatigue.

Influence of Boundary Conditions on Pore Percolation in Model Cement Paste

Fresh model cement mixtures, with the same w/c ratio and particle size distribution, were simulated by the SPACE system that is based on a dynamic mixing algorithm. Thereupon, they were hydrated by the HYMOSTRUC 3D system. Boundary conditions were varied, rendering possible assessment of their influence on percolation of capillary porosity by serial sectioning and using the overlap of slices. Simulation results revealed increases in total porosity and in connected fraction of capillary pores due to the existence of aggregate. The de-percolation threshold of capillary porosity was found not only related to total porosity and image resolution, but also governed by the spatial distribution of capillary pores.

Analytical and modeling investigations of volume fraction of interfacial layers around ellipsoidal aggregate particles in multiphase materials

The determination of the volume fraction of interfacial layers is very significant for assessing the quantitative relationship between the microstructure and macroscopic physical properties of complex multiphase materials. In this work, based on a three-phase composite structure, an approximate analytical model for the volume fraction of interfacial layers around ellipsoidal aggregate particles is presented in detail. To verify the accuracy and reliability of the derived analytical model, a numerical model is introduced by means of random packing of polydispersed ellipsoidal aggregate particles, in which the relative spatial position between an arbitrary point and an ellipsoidal particle is precisely and conveniently determined. With the analytical and numerical models applied, the dependence of the volume fraction of interfacial layers on various factors, such as the particle shape, the volume fraction and the maximum particle size of aggregates, and the thickness of the interfacial layers, is evaluated. Furthermore, the results from the analytical model and the numerical model with these factors are compared. It is found that the theoretical results are favorably consistent with the simulated results.

Modeling of soft interfacial volume fraction in composite materials with complex convex particles

The influence of the soft interfacial volume fraction on physical properties of composite materials has been found to be significant. However, the soft interfacial volume fraction is difficultly determined by traditional experimental methods and simple models proposed so far. This article addresses the problem by means of theoretical and numerical approaches that start at a microscopic scale of composite materials, which are regarded as a three-phase composite structure with polydisperse convex particles, soft interfaces, and a matrix. A theoretical scheme for the soft interfacial volume fraction is proposed by a theory of the nearest-surface distribution functions and geometrical configurations of polydisperse convex particles. The theoretical scheme represents a generalized model for the soft interfacial volume fraction in that it cannot only determine the interfacial volume fraction around convex polyhedral particles but also to derive that around ellipsoidal and spherical particles. In order to test the theoretical scheme, a numerical model that adopts the three-phase composite structure and a numerical Monte Carlo integration scheme is presented. Also, theoretical and numerical results of the soft interfacial volume fraction around ellipsoidal and spherical particles in the literature are further compared. By way of application, it is shown that the developed model provides a quantitative means to evaluate the dependence of the soft interfacial volume fraction on various factors, such as geometrical configurations of particles and the interfacial thickness.

Parking simulation of three-dimensional multi-sized star-shaped particles

The shape and size of particles may have a great impact on the microstructure as well as the physico-properties of particulate composites. However, it is challenging to configure a parking system of particles to a geometrical shape that is close to realistic grains in particulate composites. In this work, with the assistance of x-ray tomography and a spherical harmonic series, we present a star-shaped particle that is close to realistic arbitrary-shaped grains. To realize such a hard particle parking structure, an inter-particle overlapping detection algorithm is introduced. A serial sectioning approach is employed to visualize the particle parking structure for the purpose of justifying the reliability of the overlapping detection algorithm. Furthermore, the validity of the area and perimeter of solids in any arbitrary section of a plane calculated using a numerical method is verified by comparison with those obtained using an image analysis approach. This contribution is helpful to further understand the dependence of the micro-structure and physico-properties of star-shaped particles on the realistic geometrical shape.

Quantitative characterization of the microstructure of fresh cement paste via random packing of polydispersed Platonic cement particles

On a microscopic scale, fresh cement paste is composed of random packing of irregular cement particles, and their initial packing behavior plays an important role in microstructural evolution. The preponderance of previous works has focused on the microstructure model by random packing of three-dimensional spheroidal particles, and little is known about non-spheroidal particles. In this paper, a modified cement particle size distribution function is used to facilitate the particle size distribution of convex polyhedral cement particles. Based on an overlapping detection algorithm, the microstructure model of fresh cement paste is simulated by the random sequential packing of Platonic cement particles of various sizes. Applying stereological tools and the serial sectioning analysis technique, the modeling microstructure composed of polydispersed Platonic cement particles is characterized and compared with that of ellipsoidal cement particles with various aspect ratios. The statistical results are investigated to evaluate the influence of cement particle shape on the microstructure of fresh cement paste. Finally, with the derived experimental and numerical results of microstructural parameters, the reliability of the statistical results is verified.

Interfacial effect on physical properties of composite media: Interfacial volume fraction with non-spherical hard-core-soft-shell-structured particles

Interfaces are known to be crucial in a variety of fields and the interfacial volume fraction dramatically affects physical properties of composite media. However, it is an open problem with great significance how to determine the interfacial property in composite media with inclusions of complex geometry. By the stereological theory and the nearest-surface distribution functions, we first propose a theoretical framework to symmetrically present the interfacial volume fraction. In order to verify the interesting generalization, we simulate three-phase composite media by employing hard-core-soft-shell structures composed of hard mono-/polydisperse non-spherical particles, soft interfaces, and matrix. We numerically derive the interfacial volume fraction by a Monte Carlo integration scheme. With the theoretical and numerical results, we find that the interfacial volume fraction is strongly dependent on the so-called geometric size factor and sphericity characterizing the geometric shape in spite of anisotropic particle types. As a significant interfacial property, the present theoretical contribution can be further drawn into predicting the effective transport properties of composite materials.

Influence of 3D spacer fabric on drying shrinkage of concrete canvas

Concrete canvas has taken attention for their rapidly deployable hardened characteristic property in civil engineering. However, the drying shrinkage of concrete canvas has not been addressed yet in the literatures. In this study, a theoretical model was presented for studying influences of 3D spacer fabric on drying shrinkage of concrete canvas. The model was based on assumption that drying shrinkage restraint provided by 3D spacer fabric is joint action of each component of 3D spacer fabric separately. To calibrate this model, the drying shrinkage of two concrete canvases reinforced by PET-based 3D spacer fabric with one solid outer textile substrate was experimented. Moreover, a simplified expression of maximum tensile stress generated in the matrix of both concrete canvases was obtained for evaluating their risk of drying shrinkage-induced cracking. The results showed that drying shrinkage strain of concrete canvas samples became lower due to the restraint provided by 3D spacer fabric and a satisfactory correlation between model predictions and experimental results was found at later age. For both concrete canvases, a greater restraint was found in warp direction, thereby resulting in a larger tensile stress generated in the matrix. Furthermore, the restraint on the drying shrinkage of concrete canvas was provided mostly by spacer yarns and thereby it contributed to the most of maximum tensile stress generated in the matrix of concrete canvas.

Improvement of mechanical properties of concrete canvas by anhydrite-modified calcium sulfoaluminate cement

A carefully designed formulation of dry cement powder for concrete canvas, which is expected to have both high mechanical strengths and short setting times, is obtained by partially replacing calcium sulfoaluminate cement (CSA) with anhydrite at four levels (0%, 10%, 20% and 30% by mass of CSA cement). The influence of anhydrite fineness on the mechanical properties of concrete canvas and its mechanical anisotropy are both investigated. X-ray diffraction analysis and isothermal calorimetry are used to investigate the underlying mechanism. Results reveal that increasing anhydrite content or fineness improve the mechanical strengths of concrete canvas and shortened its setting times. However, a slight decrease of mechanical strength occurs at the later age when the replacement level was 30 wt%. A large amount of unhydrated particles is found in hardened specimens. The concrete canvas shows higher mechanical strengths in the warp direction than in the weft direction, and it exhibits the lowest compressive strength in the through-the-thickness direction.