This paper presents the analysis of the deterioration in dam concrete by using acoustic emission (AE). The post-peak cyclic bending tests were carried out. The AE counts and AE hit were used to analyse the damage. The "Kaiser effect" in the post-peak field was observed. However, the results showed a weak corre-lation between the structural deterioration and the AE count, and it was found that the change of loading rate would also make the AE hits and counts change greatly, indicating that it is insufficient to rely only on these traditional AE parameters to characterize the deterioration process of the structure. Then, the frequency analysis based on peak frequency was conducted in order to better characterize the structural deterioration process. The principal component analysis and the K-means clustering method were com-bined to classify the signals. Two types of specific signals which were more sensitive to the structure degradation behavior have been found, and showed promise for structural deterioration characterization. The fracture type analysis indicated that the specific signals with low peak frequency and high RT value may have strong correlation with strong shear behavior, while the signals with high peak frequency and high RT value have strong correlation with micro-crack opening.(c) 2021 Elsevier Ltd. All rights reserved.

采用压汞试验,扫描电镜(Scanning electron microscope,SEM)及X射线衍射仪(X-ray diffractometer,XRD)对3种密度的泡沫混凝土的微观结构及水化产物进行研究。通过压汞试验及SEM得到泡沫混凝土的孔隙率,孔径分布及内部微观形貌。结果表

为探究自密实混凝土(Self-Compacting Concrete,SCC)梁的断裂特性与疲劳断裂行为,针对3种初始缝高比(0.1、0.2和0.3)的SCC梁开展准静态断裂试验。结果表明,SCC梁的临界缝高比和失稳韧度均随初始缝高比的增大而增大。与普通混凝土相比,SC

The freezing/thawing behavior and the frost-resistance of unsaturated porous materials are quite different from those in water saturated condition. Most previously studies have focused on the water saturated porous systems, however, the effect of moisture content on the freezing behavior of unsaturated porous materials is not well understood. Cement paste is a porous material whose pores ranging from nanometers to micrometers. Herein, low-temperature calorimeter (LTC) measurements on porous hardened cement paste of various saturation degrees S-L are performed. It is found that the decrease of moisture content leads to multiple nucleation of ice crystals due to the discontinuity of liquid pores. At the lowest temperature, a large portion of water remains unfrozen because of the block effect of air pores where the propagation of ice crystals into unfrozen pores is cut off. The nucleation temperature decreases with the decreasing S-L due to the small probability of finding appropriate nucleation sites. Ice formed in pores varies from the smallest pore size it can be frozen to a certain big size, and cryo-suction into air pores is limited. This study advances the fundamental knowledge of the influences of moisture content on the freezing nature of unsaturated porous materials. (C) 2020 Elsevier Ltd. All rights reserved.

A microstructure-guided diffusivity model of cement paste is devised to explore the dependence of the relative diffusivity on the microstructure evolutions of cement paste from fresh state via non-spherical cement particles hydration to hardened state. The microstructure-guided diffusivity model contains three main components: (1) the microstructure of fresh cement paste is generated to simulate the initial state of non-spherical cement particles in water; (2) a continuum-based hydration model of non-spherical cement particles (HYD-NSP) is proposed to describe the evolutions of various phases (hydration products and pores) from fresh state to hardened state; (3) a random walk model is implemented to the microstructures of hydrated cement paste digitalized at the resolution of 0.1 μm/voxel for the determination of relative diffusivity of cement paste. Although this study takes five kinds of Platonic particles and sphere as an introductory example, this microstructure-guided model is readily applicable to other complex microstructures induced by the hydration of non-spherical cement particles. Finally, we utilize the framework to evaluate the influence of the geometrical shape of cement particles on the hydrated microstructure and relative diffusivity of cement paste. Results shed light on that the relative diffusivity increases with the increase of sphericity as a shape descriptor of cement particle. This is due to the specific surface area decreasing with the increase of sphericity, resulting in less hydration products and more porosity, which ultimately increases the relative diffusivity.
© 2021 Elsevier Ltd

Magnesium ammonium phosphate cement (MAPC) is commonly applied as a repair material. However, its hydration mechanism remains unclear. A thermodynamic modeling approach is employed to explore the hydration mechanism of MAPC. The results reveal that the hydration products of MAPC include (NH4)(2)Mg (PO4)(2)center dot 4H(2)O, MgHPO4 center dot 3H(2)O and MgNH4PO4 center dot 6H(2)O at 25 degrees C, 0.1 MPa, where both (NH4)(2)Mg(PO4)(2)center dot 4H(2)O and MgHPO4 center dot 3H(2)O are intermediate products that can transform into MgNH4PO4 center dot 6H(2)O with an increase in n(MgO)/n(NH4H2PO4) (M/P). The thermodynamic study of borax-modified and boric acid-modified MAPC indicates that the addition of boric acid does not change the assemblage of the hydration products; however, the addition of borax hinders the formation of MgHPO4 center dot 3H(2)O. According to the thermodynamic simulation, the lunebergite (Mg3B2(PO4)(2)(OH)(6)center dot 6H(2)O) phase, which is commonly thought to coat MgO to delay the hydration of modified MAPC, is not predicted to form. The addition of borax and boric acid can improve the final dissolution of MgO, promoting the formation of struvite.

It is very difficult to predict the compressive strength of grouted hardened cement paste in the fracture areas when grouting technology is used for seepage prevention and reinforcement. In this paper, according to the Powers model of hardened cement paste, combined with indoor and field test data analysis, this study found that: (1) Under the condition that grouting pressure is zero, strength of hardened cement paste decreases with the increase of grout water cement ratio (W/C). Based on Powers model, the relation between grout strength of hardened cement paste and the ratio of paste mixing water is established; (2) The strength of grouting hardened cement paste in cracks increases with the increase of grouting pressure. The mechanism is that with the increase of grouting pressure, the real W/C of cement paste decreases, the paste is denser, as a consequence, the strength of grouting hardened cement paste is higher. (3) The calculation model of compressive strength and grouting pressure of grout hardened cement paste is established, and the compressive strength of grout hardened cement paste can be predicted according to grouting pressure and type of cement. The research results can provide the basis for the prediction and design of the compressive strength of grouting cement paste.

This paper proposes a model to investigate the freezing behavior of cement-based materials of different water saturation degrees. Based on the thermodynamics of phase change of water (crystals-liquid-vapor) in pore and the initial pore size distribution of cement paste, changes in the pore structure and the freezing strains can be obtained by using a poro-elastic-plastic approach. To make it clear to calculate the volumetric change and frost damage, the water saturation degree in the model, covering all the probability of water content in reality, was divided into three categories: (a) smaller than critical saturation degree S-cr, (b) equal to S-cr and (c) higher than S-cr. This model was validated from experimental results in literature, and was applied to investigate the influences of air content, water-to-cement ratio and curing period on cement paste after freeze-thaw cycles (F-T cycles). This framework may be a useful tool to study the frost resistance of unsaturated cement-based materials. (C) 2019 Elsevier Ltd. All rights reserved.

A three-dimensional numerical investigation is presented to study the shape effect of cement particles on the ionic diffusivity of hardened cement paste (HCP), whose microstructure is generated based on the random packing models of spherical and polyhedral particles. The microstructure of HCP composed of four phases is first generated and then converted into a voxel-presented one. Based on the digitized microstructure, the diffusivity of HCP can be obtained by a lattice modelling. The reliability of simulations is validated by comparing to experimental data and the shape effect of cement particles on the diffusivity of HCP is discussed. The results indicate that the shape of cement particles significantly affects the ionic diffusivity of HCP due to its effect on the hydration degree: the cement particles with smaller sphericity contribute to the hydration of HCP, which reduces the ionic diffusivity of HCP. This effect decreases as the hydration age elapses and increases with the increasing water-cement ratio. The intrinsic reason causing this effect is illustrated from the microstructural view of HCP: the shape effect has an influence on the volume fractions of three diffusive phases and pore structures. © 2020