Analytical formulas for interfacial transition zone properties
Abstract
Two analytical results are presented that are of use to concrete material technologists. Using a model of concrete in which the aggregates are spherical, but with an arbitrary size distribution, a result from statistical geometry can be used to accurately give the total interfacial transition zone (ITZ) volume for any width ITZ and any volume fraction of aggregates. In reality, the ITZ contains a gradient of porosity and therefore a gradient of properties. When only a small volume fraction of aggregates is present (called the dilute limit), it is possible to analytically solve for the effect of the ITZ on the overall concrete properties. This calculation can be carried out for the effective linear elastic moduli, linear electrical conductivity/ionic diffusivity, and linear thermal/moisture shrinkage/expansion. The details of the calculation are summarized and applications described.
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A review of the interfacial transition zones in concrete: Identification, physical characteristics, and mechanical properties
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Mesoscopic characteristics and spatiotemporal variability of chloride transport in concrete
2024, Construction and Building MaterialsChloride-induced corrosion is the leading cause of structural performance deterioration and life decay in marine environments. This paper delves into the mesoscopic characteristics and spatiotemporal variability of chloride transport in concrete. We conducted extensive mesoscopic simulations to reveal the effects of ITZ and aggregate volume fraction, morphology, size, and particle quantity on chloride transport. We identified the probability characteristics of chloride concentration and diffusion coefficient based on statistical analysis of the mesoscopic simulation results. We proposed a novel probabilistic framework and predictive model for chloride diffusion coefficients incorporating aggregate parameters like volume fraction and particle size. The findings of this study demonstrate that the chloride transport shows noteworthy spatiotemporal variability; the aggregate size effect, aggregate particle quantity effect, and the local effect of large aggregates are all significant characteristics of the chloride transport in addition to the pre-discovered dilution effect, tortuosity effect, and ITZ effect. The chloride diffusion coefficient approximately follows a log-normal distribution, whereas the chloride concentration roughly follows a normal or log-normal distribution. The findings in this work are instrumental in predicting the probabilistic chloride transport and service life of reinforced concrete structures.
Research on the effect of lithological characteristics of manufactured sand on the strength of mortar
2023, Journal of Building EngineeringThe excessive extraction and utilization of river sand may lead to the destruction of river ecosystems. The use of manufactured sand as a substitute for river sand can effectively avoid the aforementioned issues. However, there are significant differences in the strength of mortar or concrete prepared using different types of sand. The influence law for lithological characteristics of three types of manufactured sand on mortar strength was investigated in depth. The core causes of strength variation influenced by different parent rocks were explained based on various macroscopic and microscopic test methods. The results of the flexural and compression tests showed that the different parent rock types would have a significant effect on the mortar strength, where molybdenum tailing sand had the largest increase in 28d strength of 17%. In addition, as decreasing the interfacial transition zone (ITZ) volume fraction and increasing the elastic and hardness moduli of ITZ, the mortar strength gradually increased. This study showed that using manufactured sand as a substitute for river sand reduced extraction and improved mortar strength while maintaining workability. Molybdenum tailings sand performed the best, which was significant for sustainable construction and environmental protection.
A numerical framework for the ITZ percolation, effective fraction and diffusivity of concrete systems considering the nonuniform ITZ
2023, Journal of Building EngineeringThe critical percolation and effective fraction of interfacial transition zone (ITZ) around the aggregates are generally considered as two important structural factors in the prediction of macroscopically transport properties in concrete, and their solutions are very susceptible to the geometric features of both aggregate particles and ITZs. Understanding the influence of aggregate features and ITZ thickness on the variation of these ITZ configurations and their effects on the macroscopic performance of materials would have great significance for the optimization and adjustment of concrete materials. However, in most existing numerical studies, both the coarse and fine aggregates are generally simplified as the hard particles of the same morphology and the ITZ thickness around them is also set to be uniform, which don’t reflect the reality perfectly and may lead to the great deviation. To clarify the variation of ITZ features and ionic diffusivity with the ITZ effect in a more realistic way, the polydispersity of aggregate shapes is taken into consideration by assuming coarse and fine aggregates as polygons and ovals, respectively. Moreover, a differential ITZ thickness is also assigned for the aggregates according to their sizes and W/C ratio. Based on the above premises, a numerical framework for the prediction of the effect of individual phases on the ITZ percolation, effective fraction and diffusivity of concrete is developed sequentially in this paper. The whole framework can be divided roughly into three parts: (1) the numerical simulation and quantification of the critical percolation of nonuniform ITZs; (2) the statistical evaluation and quantitative analysis of the effective ITZ fraction by statistical geometry; (3) the continuum percolation-based effective medium prediction for the relative diffusivity of polyphase concrete systems. According to the systematically numerical investigation, it can be found that the oversimplification of both ITZ thickness and aggregate shape may result in the serious deviation for the evaluation of ITZ percolation, effective fraction and macroscopic properties, especially the ITZ thickness. The numerical results show that when the ITZ thickness is assumed to be uniform for all the aggregates, the ITZ percolation threshold ϕagg,c, its effective fraction ϕITZ and ionic diffusivity Dcon of concrete system would generally be overestimated, underestimated and underestimated compared with the ones with the nonuniform ITZs, respectively. The proposed models here may give more realistic results than the previous prediction models in 2D case.
Multiscale creep model for concrete considering from C-S-H gel scale to mesoscale with ITZ and irregular-shaped aggregates
2023, Cement and Concrete CompositesA multiscale creep model of concrete is proposed from C–S–H gel scale to mesoscale considering the effects of ITZ and aggregates with irregular-shaped aggregates. A novel averaged Eshelby tensor-based homogenization method is devised to obtain the effective creep of concrete with realistic-shaped aggregates covering from ellipsoidal-like gravels, polyhedral-like crushed stones, to arbitrary concave aggregates. This method is generic that overcomes the constraints of the traditional Eshelby's solution. In addition, ITZ is regarded as a homogeneous viscoelastic layer, its volume fraction and viscoelasticity are derived. The proposed multiscale model is verified by comparing with finite element simulations and experimental data. The influences for a wide spectrum of aggregate and ITZ configurations on concrete creep are investigated. Moreover, the scale-transfer effect of microscale parameters on cement paste creep and on concrete creep is quantitatively characterized for the first time.
3D mesoscale investigation on the compressive fracture of concrete with different aggregate shapes and interface transition zones
2023, Construction and Building MaterialsThe purpose of this work is to investigate the effect of aggregate shape parameters and interface transition zone (ITZ) thickness on the mechanical properties and failure behavior of concrete under uniaxial compression. A 3D mesoscopic model including mortar, shape-alterable aggregate and thickness-controllable ITZ was established in ABAQUS software by programming. Based on the concrete damaged plasticity (CDP) method and the cohesive element approach, the simulation analysis was carried out from the aspects of macroscopic mechanical response, macroscopic failure morphology, mesoscale fracture propagation, and energy dissipation. Results indicate that polyhedral aggregate and spherical aggregate have no significant effect on the macroscopic mechanical properties of concrete. Aggregates with sharp edges and corners may cause concrete to be destroyed into more fragments. The ITZ with 0 thickness and medium thickness can predict the compressive strength of concrete, ITZ model with solid thickness is more suitable for meso-crack research. Thinner solid ITZ fits poorly with the experimental value, and the change of ITZ has little influence on polyhedral aggregates. It is proposed to establish a polyhedral aggregate and medium thickness ITZ model for mesoscale concrete numerical simulation.