Frost damage is one of the main reasons for the decay of concrete in cold regions, which is affected by many factors such as freezing temperature, number of freeze-thaw cycles (FTCs), pore and aggregate structure, and the FTCs damage in concrete presents a certain distribution characteristic. In view of this, a systematic prediction method for FTCs damage distribution was established by the multi-scale method. Firstly, the FTCs damage behavior was simulated by the micro-calculation method. Subsequently, the micro-meso-macro multi-step equivalent method was established for the description of FTCs damage, and the equivalent element of concrete under FTCs was established. The calculation results of the equivalent model show that the characteristic element size should be set as 10 mm to 20 mm, which can satisfy the requirements of the heterogeneous characteristics of FTCs damage and Weibull distribution at the same time. Meanwhile, the effects on the FTCs number and freezing temperature of the FTCs damage distribution were discussed, and the effectiveness of the equivalent element was verified by comparing it with the test results, which the equivalent calculation model can well describe the heterogeneous distribution of FTCs damage.
Interactions between Cl-, SO42- and cementitious materials, reinforcement passive films influence the durability of reinforced concrete structures. Transport of three solutions (NaCl, Na2SO4, mixed) in calcium silicate hydrate (C-S-H) gel, γ-FeOOH nanopores was investigated using molecular dynamics. Solution transport in γ-FeOOH nanopores is slower than in C-S-H gel nanopores because of the lesser hydrophilicity of γ-FeOOH surface. SO42- can form ion clusters to hinder the solution transport and atomic motion, and the ion clusters appear in the solution more frequently than at the interface. Temporary adsorption of Cl-, SO42- on substrate surfaces occurs during transport because of Ca-Cl, Ca-SO4 ionic bonds on the C-S-H surface and Ho (hydroxyl hydrogen atoms) -Cl, Ho-SO4 hydrogen bonds on the γ-FeOOH surface, and these bonds are influenced by the local structure. Two substrates interact with water, Cl-, SO42- via distinct microscopic mechanisms.
For the RC members under cyclic loading, flexural cracks commonly propagate in two opposite transverse directions at the zone under high bending moment and possibly form the cracks penetrating through the entire cross sections (pre-crack), which is considered as a crucial factor for flexure-shear failure of member. This paper clarified the influence of imitation pre-crack on the shear failure behavior of RC slender beams by three point bending test. Shear span depth ratio (a/d=3.14, 4.69) and location and thickness of pre-crack were set as main variables. As the important findings, it was revealed that the influence of pre-crack on the shear strength of RC slender beams is relatively small because it does not affect the formation of the critical diagonal crack and the mode of diagonal tension failure. It was also noted that the pre-crack at 2d (the section of pre-crack is 512 mm, which is twice of the effective depth d, far away from the loading plate center) leads to a reduction of shear strength with increasing pre-crack width, and the maximum reduction is 21.4%. Based on the detailed analysis of crack propagation, it was clarified that pre-crack may result in two patterns of critical diagonal crack, and in the condition that the pre-crack plane vertically intersects with the lateral splitting part of diagonal crack, the lateral splitting part becomes more severe and thereby reduces the shear strength and deformation ability.
A coupling method of finite element and discrete element was used to simulate the process of microwave liberating coarse aggregate under different power conditions, and it deals with the problem that the finite element method can not simulate the cracking process and the discrete element method can not consider the nonuniformity of microwave in the concrete specimens. The liberation ratio was proposed as the quantitative evaluation index, and the influence of microwave power on the liberating effect was analyzed based on the change of the liberation ratio. The mechanism of microwave-assisted liberation was discussed. It was found that the temperature distribution and crack distribution in the test specimens changed with irradiation time and microwave power; the optimal irradiation time and the optimal microwave power were obtained by liberation ratio under different irradiation conditions, and the simulation results were verified by preliminary tests; the coarse aggregate was under tension in both circumferential and radial, while the mortar is under compression in circumferential and under tension in radial under microwave irradiation. Cracks are produced by tensile stress at the junction of coarse aggregate and mortar, then the coarse aggregate are liberated from the mortar.
Concrete aggregate identified as “meta-chert” was irradiated with gamma-rays and neutrons. To identify the volume expansion of the aggregate under neutron irradiation, the following analyses were performed for pristine and irradiated α-quartz and meta-chert: X-ray diffraction (XRD)/Rietveld analysis, dimension change, water pycnometry, He-pycnometry, light optical microscopy (LOM), and scanning electron microscopy (SEM). From the difference of volume expansion observed from dimension change and water / helium pycnometry, the crack opening inside the aggregate subjected to irradiation was elucidated, and this was confirmed by LOM and SEM analysis. The crack contribution to the expansion of the aggregate was significant for neutron fluence > 6.99 × 1019 n/cm2, for E ≥ 0.01 MeV. Based on the XRD analysis, changes in lattice parameters were identified and the cell volume expansion was compared with the data by helium pycnometry. Based on the density change calculation and phase calculation data, the density of X-ray amorphous phase was consistent with that of expanded crystal α-quartz.