Papers by Author: Xin Cheng

Abstract: Chemical admixtures are frequently used to regulate the setting and strength development of concrete materials. In this study, tricalcium silicate (C₃S) was used as a model of the cement system, and the influence of calcium chloride, an extremely useful accelerator, on C₃S hydration and the pore structure of hardened C₃S paste were investigated by the combination of the techniques of differential scanning calorimetry (DSC) and the N₂ adsorption (BET). The results indicated that the addition of calcium chloride would significantly shorten the pre-induction and induction periods and enhance the specific surface area and porosity of hardened C₃S paste. However, the presence of CaCl₂ has little effect on the pores, with a width ranging from 2.5 nm to 5 nm. DSC technique has an advantage of measuring continuously the process of C₃S hydration by changes of free water in hydrated C₃S.

Abstract: The super-softening property of redispersible polymer powder can improve the flexibility of cement-based materials while guarantees other work performances. In this study, the influences of different mixing amounts of redispersible polymer powders on the compressive strength, flexural strength, flexural compression ratio and bonding strength of sulphoaluminate cement mortar were investigated. The microscopic morphological structure of redispersible polymer powder-modified sulphoaluminate cement mortar was also observed through SEM to analyse the influence mechanism. Results show that, with the mixing amount of 2.0%, the compressive strength of cement mortar was improved. This mixing amount also increased the 28d flexural strength to 8.21 MPa (5.0% improvement) and 14d bonding strength to 5.6 MPa (27.3% improvement). Addition of more redispersible polymer powder resulted in the increase in flexural compression ratio because the powder particles formed water-insoluble continuous polymer colloidal film in hardened paste. This film generated a network with cement dydration products throughout the paste, consequently promoting the flexibility of cement.

Abstract: It is vital to use acoustic emission technology to study on health monitoring of concrete. The aim of this study is to investigate the ability of the AE in order to test the effect of the stepwise displacement load on the damage mechanism of the concrete beams that embedded sensors under three-point bending. The result showed that as the damage increased, amplitude and energy increased. At the same time, embedded sensors can locate the position of the crack.

Abstract: High performance concrete was prepared by using iron tailings mixed with manufactured sand. Effects of the content of iron tailings on the performance of concrete were studied. The results show that the slump of concrete containing iron tailings is equal to concrete with river sand when the content of iron tailings is 60%. With the increase of the content of iron tailings, compressive strength of concrete shows a trend of decrease at same age; meanwhile drying shrinkage decreases and is less than the concrete prepared river sand. After 200 times of freeze-thaw cycle, mass loss of concrete with mixed sand is less than 5% and compressive strength loss is less than 25% which are close to concrete with river sand.

Abstract: The surface of nanoFe₂O₃ was modified with silane coupling agent 3-(methacryloyloxy) propyltrimethoxysilane (KH-570) under acidic conditions in order to improve the dispersity of particles in organic polymer and to enhance the interfacial bonding with the matrix. Contact angle measurement, thermogravimetric analysis (TG), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) were adopted to characterize and analyze the unmodified and modified nanoFe₂O₃. Through TG curves, it was calculated that when the KH-570 content was 25%, the grafting rate of Fe₂O₃ reached a maximum value at 7.28%. The FT-IR and XRD results indicated that KH-570 successfully modified the nanoFe₂O₃ with its organic functional groups chemically bonded on the surface of nanoFe₂O₃ which contributed to prevent particles from conglomerating. The surface treatment of nanoFe₂O₃ had no significant effect on the nanoparticles crystalline form. Contact angle measurement investigated that the hydrophile of modified nanoFe₂O₃ was decreased.

Abstract: A new kind of embedded acoustic emission (AE) sensor has been developed and it was made up of 1-3 cement-based piezoelectric composites. Compared with those sensors which were traditionally affixed to concrete structures, the embedded AE sensors could minimize the leakage of AE hits and improve the accuracy of data acquisition. The AE thresholds of the sensors are 45dB and they can perceive the position of source and crack extending areas in concrete. Because of the low operation cost, the admirable compatibility with concrete and the excellent durability, the sensors could be widely used in the monitoring of civil structure.

Abstract: Frost Resistance of rapid hardening sulphoaluminate cement was modified with redispersible powders. Frost resistance and strength of sulphoaluminate cement mortar were tested. With the help of SEM and mercury porosimetry, the morphology of the hydrates and the micro-structure of the mortar were observed. The frost resistance mechanism of redispersible powders in the mortar is analyzed. The results show that the frost resistance of sulphoaluminate cement mortar could be improved significantly by mixing redispersible powders. The optimal content of redispersible powders is 0.9%.

Abstract: The standard diffusion calculating model is one of the models widely used to predict the concrete construction life in chloride environment. According to test results, the surface concentration and the initial concentration of chloride ion on sulphoaluminate cement sample was analyzed by mathematical method. Life prediction model in chloride environment was built. According to the model, the sulphoaluminate cement construction life will be 51.5 years when the thickness of cement protective layer is 40mm.

Abstract: 1-3 polymer-based piezoelectric composites were fabricated using epoxy as matrix by the cut-filling method. The influences of shape parameter on properties of the piezoelectric composite, which include the unit cross-sectional area and the aspect ratio w/t were analyzed. The results indicate that with the increasing of the unit cross-sectional area, the quality factor value Q_m increases and the hydrostatic piezoelectric voltage g_h increases and then goes down rapidly while the PMN volume fraction φ(PMN) is kept under the 50%. When the φ(PMN) is 60%, g_h is decreased. The trend of the hydrostatic figures of merit d_h·g_h is similar with g_h as the change of the unit cross-sectional area, but the value is different. In the 60% PMN volume fraction, the optimal value of the d_h·g_h is chosen. With the increasing of the w/t, the hydrostatic pressure sensitivity M_h, the d_h·g_h values and the Q_m values are all decreased rapidly, and the thickness electromechanical coupling factor k_t is increased. In other words, the test results show that the smaller of unit cross-sectional area and thinner of thickness, the more helpful for frequency bandwidth and sensitivity when it is used in transducer.

Abstract: 1-3 type cement/epoxy resin based piezoelectric composite was designed and fabricated aiming at providing a new method for cement hydration monitoring. Combining with piezoelectric impedance technology, the cement hydration reaction process was monitored by using the composite. The research results show that in the initial cement hydration period, the resistance-frequency curves of the sensor drift toward low frequency direction, while the anti-resonance resistance value decreases gradually. With increasing cement hydration time, the resistance-frequency curves of the sensor drift toward high frequency direction and the anti-resonance resistance value shows fluctuation changes. The cement hydration reaction process can be divided into different periods according to changes of anti-resonance frequency and anti-resonance resistance value of the sensor.