Yang Ju

Abstract: A number of three-point bending and fracture tests of 200 MPa-level reactive powder concrete (RPC) with the various fiber contents have been conducted to probe the nature and characteristics of tough-ness of RPC200. The contribution of the embedded fibers to improving the crack-resistant capacity, energy absorption capacity and toughness with various deformation mechanisms has been analyzed. Taking account of that the first-crack deformation, peak-load deformation and their improvement varied with the fiber contents and that the deformation mechanism affected differently the performance at the first crack and the peak load, we took the peak-load deformation of plain RPC200 as the reference de-formation to measure the toughness of fibered RPC200. Two toughness indices T2(n-1)(n) and FT2(n-1)(n) have been formulated based on P-δ responses and P-CMOD responses. The indices quantify the tough-ness of RPC200 with the various deformation mechanisms relative to perfectly elastoplastic materials by setting the toughness level 2(n-1) as the initial reference. It is shown that the toughness index T2(n-1)(n) reflects the function of fibers to improve the toughness of RPC with the deformation throughout specimens, but overestimates the contribution to enhancing the toughness in post-peak periods. It underestimates, on other hands, the contribution to improving the toughness in the period from the first crack to the peak load. In contrast, the toughness index FT2(n-1)(n) properly presents the capability that fibers absorb energy and constrain crack propagation in the matrix when the deformation is con-centrated on the open crack. The proposed index unveils the contribution of fibers to toughening RPC200 both in the period from the first-crack to the peak load and in the period of post peak. This characterization method not only reveals the nature of toughness but also levels the toughness of RPC200. It could provide a way to establish an objective toughness characterization for RPC200 and facilitate its applications.

Abstract: A number of porous models having the similar statistical characteristics of pores and physical properties with natural sandstones have been produced using reactive powder concrete (RPC) and polystyrenes. Spit-Hopkinson-Pressure -Bar tests and CT scans have been carried out on the models with the various porosities to probe the performance of wave propagations and the responses of pores and the matrix during wave propagations. It is shown that porosities significantly influence wave propagations. For an identical impact strain rate, the greater the porosity is, the larger the amplitude of the reflected wave appears, the more the peak in the reflected wave presents, and the smaller the amplitude of the trans-mitted wave turns out. A single peak emerges in the reflected wave when the porosity falls down to 5%. The larger the impact strain rate, the much remarkable the phenomena. The energy-dissipated ratio of porous models, i.e., WJ /WI, linearly increases with the increment of porosities. The ratio is sensitive to the impact strain rate. Differences in the performance of wave propagations and energy dissipation result from the varied mechanisms that pores response to impacts. For the porosity less than 10%, the mechanism appears to be a process fracturing the matrix to generate new surfaces or pores. Energy has primarily been dissipated in creating new surfaces or pores. No apparent pore deformation takes place. The impact strain rate takes little effect on pore geometry. For the porosity of 15% or more, the mechanism works depending on the impact strain rate. When a low impact strain rate applies, the mechanism still appears to crack the matrix to generate surfaces or pores, but the amount is lower as compared to the case with a low porosity. If a large impact stain rate applies, the mechanism combines both fracturing the matrix and deforming the pores, with the deforming pores predominating. The vast majority of energy has been dissipated to deform pores. Only high porosity and impact strain rate can bring significant deformation to the pores. The proposed eccentricity of pores is capable of character-izing the geometry of pores and its change during wave propagations.

Abstract: The geometric features and the distribution properties of pores in rocks were in-vestigated by means of CT scanning tests of sandstones. The centroidal coordi-nates of pores, the statistic characterristics of pore distance, quantity, size and their probability density functions were formulated in this paper. The Monte Carlo method and the random number generating algorithm were employed to generate two series of random numbers with the desired statistic characteristics and prob-ability density functions upon which the random distribution of pore position, dis-tance and quantity were determined. A three-dimensional porous structural model of sandstone was constructed based on the FLAC3D program and the information of the pore position and distribution that the series of random numbers defined. On the basis of modelling, the Brazil split tests of rock discs were carried out to ex-amine the stress distribution, the pattern of element failure and the inosculation of failed elements. The simulation indicated that the proposed model was consistent with the realistic porous structure of rock in terms of their statistic properties of pores and geometric similarity. The built-up model disclosed the influence of pores on the stress distribution, failure mode of material elements and the inosculation of failed elements.

Abstract: This paper presents the experimental and theoretical investigation of property of stress wave propagation in jointed rocks by means of SHPB technique and fractal geometry method. Our aim focuses on the influence of the rough joint surface configuration on stress wave propagation. The comparison of behavior of reflection and transmission waves, deformation and energy dissipation of a rough joint surface characterized by its fractal feature with that of a smooth plane joint has been carried out. It has shown that the rough joint surface distinctly affects the stress wave propagation and energy dissipation in the jointed rocks. The rougher the joint surface was, the more permanent deformation occurred and the more attenuation stress wave took place as well. A nonlinear relationship between the normalized energy dissipation ratio WJ/WI of the jointed rock and the joint roughness in terms of the fractal dimension has been formulated. It seems that the ratio WJ/WI, presenting how much energy has been dissipated in the joint, nonlinearly increased with the increment of the fractal dimension D of the jointed surface. The ratio WJ/WI of a roughly jointed rock, however, tends to be the same as that of a smoothly jointed rock if the fractal dimension is less than a critical value Dc = 2.20. The energy dissipation ratio at the critical point Dc seem to be a constant, not dependent of rock type but fractal joint configuration

Abstract: Reactive powder concrete (RPC) is a novel cement-based composite material with ultra-high strength. Embedding a certain amount of short steel fibers in the matrix can improve the RPC�s toughness and overcome the disadvantage of high brittle-ness. In this paper, a number of direct uniaxial tension tests have been carried out with �8-shape� RPC200 specimens. The bond-slip process, mesoscopic structural variation and mechanical characteristics of a fiber pullout of the matrix have been investigated using the real-time SEM loading system and CCD observation tech-niques. The influence of the volume of embedded short steel fibers in matrix on the mesoscopic morphology of attachments on the surface of a pulled individual fiber, the initial cracking force, the ultimate pullout force, interfacial bond strength and the pullout rupture energy have been analyzed. A general formulation relating these quantities to the volume of fibers in matrix has been proposed. The components comprising the interfacial bond strength have been outlined. In addition, the con-tribution that fibers make to enhance and toughen the reactive powder concrete has been discussed. It is shown that there exists an optimal threshold of fiber volume �v, opt =1.5% at which the bond performance of a fiber pullout of RPC behaves best.