Saturated Concrete Research Articles

A mechanical-diffusive peridynamics coupling model for meso-scale simulation of chloride penetration in concrete under loadings

A coupled mechanical-diffusive peridynamics (PD) model is developed to simulate chloride penetration in saturated concrete under various loadings at meso-scale. PD is implemented in the architecture of finite element method (FEM) and truss elements are used to substitute for the PD bonds. In the coupled model, damage evolution and crack growth in concrete under loadings and chloride penetration into concrete with updated diffusivity caused by cracks can be investigated simultaneously. At each coupling step, the quasi-static mechanical analysis is conducted by solving the equilibrium equations, while the forward differencing method is adopted for chloride diffusion. Several numerical examples are carried out to validate the proposed PD models in terms of addressing mechanical-diffusive problems. The numerical results coincide with the test findings and clearly indicate that the width and depth of cracks can influence the chloride diffusivity when “threshold crack width” is reached. Additionally, the chloride diffusivity increases with the increase of stress level under high compressive, tensile and flexural stress levels.

Differential scheme-based stochastic micromechanical framework for saturated concrete repaired by EDM

A stochastic micromechanical framework is proposed to quantitatively characterize the probabilistic behavior of the saturated concrete healed by the electrochemical deposition method (EDM). Multiphase micromechanical representation for the healed saturated concrete is presented based on the material’s microstructures. Differential scheme-based multilevel homogenization procedures are proposed to quantitatively predict the effective properties of the repaired concrete. The material microstructures are characterized by the non-stationary random process and random variables. The probabilistic behavior for the repaired concrete is reached with high computational efficiency by incorporating the dimensional decomposition method and Newton interpolations. The predictions obtained by the proposed stochastic micromechanical framework are then compared with the available experimental data, existing models, and direct Monte Carlo simulations, which indicates that the presented stochastic micromechanical framework is computationally efficient and capable of characterizing for the probabilistic behavior of saturated concrete repaired by EDM considering the inherent randomness. Finally, the influences of the deposition products and healing degrees on the probabilistic behavior of repaired concrete are discussed based on the proposed models.

Parallel Finite Element Model for Multispecies Transport in Nonsaturated Concrete Structures.

The chloride-induced corrosion of steel reinforcement embedded in concrete is undoubtedly one of the most important durability problems of reinforced concrete structures. The chloride ions as well as other ionic species (Na+, Ca2+, K+, OH−) come from various deicing salts and they are transported from the environment into concrete. To investigate the transport mechanism of the multispecies, complex scientific methods and accurate mathematical models are needed. The purpose of this study is to develop a more robust mathematical model and better computational technique to characterize the coupled effect of ionic transport mechanisms as well as the influence of interaction of ionic species. The new mathematical model was developed based on the Nernst–Planck equation and null current condition to solve the ionic-induced electrostatic potential, and the model was implemented by a parallel finite element algorithm. The verification of mathematical model was done by comparing the model prediction with experimental results for ionic transport in saturated concrete. The comparisons showed good results. The model prediction of the multispecies transport in partially saturated concrete demonstrated that the ionic species dissolved in pore solution could be carried by the moisture movement and pressure gradient. Therefore, the multispecies transport model based on the parallel finite element method is effective, accurate, and can be used for solving the partial differential equations for ionic species transport in concrete.

Stochastic micromechanical predictions for the probabilistic behavior of saturated concrete repaired by the electrochemical deposition method

A stochastic micromechanical framework is proposed to quantitatively characterize the probabilistic behavior of the mechanical performance of the saturated concrete healed by the electrochemical deposition method. Micromechanical model for the healed saturated concrete is presented based on the material microstructures, and new multilevel homogenization procedures are proposed to quantitatively predict the effective properties of the repaired concrete considering the inter-particle interactions. The evolutions of the deposition products are characterized by non-stationary random process, which is represented by Karhunen–Loeve approximations with limited random variables. The probabilistic behavior for the effective properties of the repaired concrete is reached by incorporating the maximum entropy principle and Monte Carlo simulations. The predictions obtained by the proposed stochastic micromechanical framework are then compared with the available experimental data, existing models, and commonly used probability density functions, which indicate that the presented stochastic micromechanical framework is capable of describing the electrochemical deposition method healing process, considering the inherent randomness of the material microstructures. Finally, the influences of the deposition products on the probabilistic behavior of the repaired concrete are discussed on the basis of the proposed models.

Influence of the moisture content on the dynamic modulus of elasticity of concrete made with recycled aggregate

Abstract The elastic behavior of the concrete is estimated from its strength or determined by static or dynamic tests. However, because the codes of practice do not standardize the internal moisture content of the concrete and disregard the use of recycled aggregates when proposing equations for the estimation of the modulus of elasticity, discrepancies between the values measured and estimated are frequent. The influence of the moisture content of concrete containing basaltic coarse aggregates and coarse recycled concrete aggregate in the dynamic modulus of elasticity is discussed in this paper. A basalt coarse aggregate and two recycled coarse aggregates where used. For each type of coarse aggregate, concrete with compression strength between 25 MPa and 55 MPa were produced. The dynamic modulus of elasticity of the saturated samples were determined and range from 26 GPa to 46 GPa. There is a significant difference in the value of the dynamic modulus of elasticity for dry concrete versus saturated concrete, also influenced by the type of aggregate. Estimations of the modulus of elasticity from the compressive strength equations proposed by the codes of practice must be improved considering its characteristics.

Activation Energy of Conduction for Use in Temperature Corrections on Electrical Measurements of Concrete

Abstract The formation factor obtained through electrical resistivity measurements is becoming a popular method to determine transport properties of concrete. Resistivity measurements are dependent on multiple factors, including degree of saturation, pore solution conductivity, and temperature. The Arrhenius equation is used to correct electrical resistivity for temperature effects using an activation energy of conduction (Ea-cond). This parameter has been measured on a wide variety of materials, including pore solutions, pastes, mortars, and concretes (with a variety of saturation states). The reported values of Ea-cond typically range from 9 to 39 kJ/mol. This article examines the factors affecting Ea-cond in order to select an appropriate temperature correction. In this study, Ea-cond was determined from data measured on various concrete mixtures used in transportation infrastructure applications as well as extracted and simulated pore solutions. The Ea-cond of pore solutions remains relatively constant (an average value of 13.9 ± 1.5 kJ/mol) for typical pore solutions and was slightly lower than the Ea-cond of saturated specimens (an average value of 15.8 kJ/mol). It was found that Ea-cond increases as the degree of saturation of the specimen is reduced. Drying increases the ionic concentration of the fluid in the pores; however, this does not explain the changes in Ea-cond. The effects of drying were determined to be primarily due to a change in the volume of the conductive fluid film in the concrete and in the connectivity of the fluid-filled pores. While it is better to directly measure the Ea-cond of a concrete mixture, this is not always feasible or practical. In such cases, for pore solutions, a value of 13.9 kJ/mol can be used, and for saturated concretes, a value of 15.8 kJ/mol can be used. For concretes with a varying degree of saturation, the Ea-cond can be estimated using the developed equation.

Modeling of Chloride Ion Diffusion in Concrete under Fatigue Loading

It is common for reinforced concrete in the saline region to bear fatigue loading and chloride induced corrosion, which has become one of the main causes of structural failure of reinforced concrete. The objective of this paper is to investigate the characteristics of chloride ion transport in concrete under fatigue loading. A new theoretical model describing the chloride ion transport in saturated concrete under fatigue loading is proposed. In this model, the concrete is divided into two parts, matrix and microcrack, to characterize the chloride diffusion coefficient of concrete based on crack area. The influence of fatigue damage on the microcrack area of concrete is quantitatively analyzed and the relationship between fatigue loading and chloride diffusion coefficient is established. Then, based on Fick’s second law, the model is proposed and solved by analytical solution. Some experiments are conducted to verify the proposed model and the simulated and measured results are in good agreement with each other. Finally, the characteristics of chloride ion transport under different influencing factors are analyzed using the proposed model.

Fracture of saturated concrete and rocks under dynamic loading

A dynamic tensile strength of rocks and concrete is studied in a wide range of strain rates using a structural-temporal approach. An idea of a material’s fracture incubation time, taken as a scale dependent material constant and as a primary measure of the reaction of materials subjected to dynamic loading, was applied to explain the instability of the ultimate critical stress values in rock materials and concrete. Temporal dependencies (strength – strain rate; pulse amplitude – pulse duration; maximum stress – pulse duration) of the spall strength for short pulse and high rate loading of dry and saturated concrete are compared and discussed. A phenomenon of the greater ultimate stress of concrete and rocks with the highest relative humidity under dynamic loading is explained. It is shown that conventional fracture criteria based only on the strain rate dependence taken from a usual linear loading law may give ambiguous results being applied for other shapes of the pulse. Application of these criteria to the case of short pulse load indicate that they are not adapted to the estimation of threshold loading parameters in the case of non-linear loading history. On the contrary, it is proved that the incubation time fracture model based on a set of material parameters, invariant to the loading history, is applicable for a broad range of load pulse types and shapes.

Influence of Concrete Carbonation on Electromagnetic Permittivity Measured by GPR and Capacitive Techniques

This paper addresses the effect of concrete carbonation on the propagation and dispersion of electromagnetic (EM) waves and the capability of two EM, non-destructive techniques to detect this pathology. A capacitive technique operating at low frequency (around 33 MHz) and a ground penetrating radar (GPR) with a 1.5 GHz antenna were tested for the monitoring of reinforced concrete structures. To better understand the phenomena involved in concrete carbonation, the results of two complementary experimental campaigns were analyzed for saturated concretes. First, the dispersion curves of complex permittivity were measured for both carbonated and non-carbonated samples by a cylindrical coaxial EM cell. Due to carbonation, the permittivity decreased and the level of dispersion reduced slightly. Second, using GPR (coupled at approximately 900 MHz) and capacitive measurements conducted on controlled slabs, it was confirmed that the real part of the relative permittivity decreased within a range of 2 at 33 MHz and a range of 1 to 900 MHz, while the radar signal amplitude increased.

Analysis and Finite Element Modelling of Water Flow in Concrete

Due to the construction of underground structures and hazardous waste storages, understanding and modelling of seepage in concrete has become an important issue in life-span analyses. The theories and calculation methods of unsaturated soil mechanics provide an opportunity to analyze water flow in other types of porous media (e.g. concrete) as well. This study deals with the determination of the permeability for unsaturated and saturated concrete and modelling the water flow in concrete. The direct measurement of the saturated permeability, the preparation of the drying water retention curve and determination of the depth of penetration of water under pressure are involved in the series of tests. For the fitting method of the experimental water retention curves were used Fredlund and Xing (1994) and van Genuchten (1980) model. The theory of lateral shift was applied to estimate the wetting water retention curve from the drying WRC. Thus, we could calculate the unsaturated permeability functions with Fredlund et al. (1994) and van Genuchten (1980) model. The finite element modelling of the standard test for watertightness were performed with Midas GTS using the measured and calculated unsaturated property functions.

Modeling of water transport in highly saturated concrete with wet surface during freeze/thaw

Wet frost exposure is a usual environmental condition for cement-based materials (CBM) during winter and the most common way of frost testing in the lab. In this study, the temperature gradient, pressure gradient, and the liquid flow under wet frost exposure are modeled for highly saturated CBM with different amount of entrained air. It is found that the water uptake happens at the melting stage, and for non-air-entrained CBM, the hydraulic pumping effect is dominant and will suck the water from wet surface. While for air-entrained CBM, the cryosuction pressure is the main driving force of the inward flow. The results are compared with experimental data from rapid freeze/thaw testing of various types of concrete in water, showing a satisfactory agreement. Sensitivity analysis also indicates that the hydraulic induced flow depends on the amount of entrained air, while the cryosuction induced flow mainly relies on the permeability and temperature gradient.

A critical examination of corrosion rate measurement techniques applied to reinforcing steel in concrete

This paper presents an investigation of five corrosion‐monitoring techniques for reinforced concrete. The techniques studied are the potentiodynamic, galvanostatic, and coulostatic direct‐current techniques as well as electrochemical impedance spectroscopy (EIS), and the connectionless electrical pulse response analysis (CEPRA) technique. The study included monitoring corrosion rates on reinforced concrete specimens with a range of admixed chloride percentages, cover depths, and rebar diameters for 8 months. After this period, the rebars were extracted for mass loss measurements to determine the average corrosion rates. EIS was found to provide accurate measurements of active and passive corrosion rates with a simplified spectrum‐analysis procedure. Galvanostatic and potentiodynamic techniques were able to measure the corrosion rates for actively corroding reinforcements accurately, while the coulostatic technique overestimated it. For passive reinforcements, the coulostatic technique provided reliable corrosion rate estimates, while the potentiodynamic technique provided a minor overestimation, due to the fast scan rate used, and the galvanostatic technique failed in detecting passivity, due to the short measurement duration and confinement failure. Finally, the CEPRA technique provided accurate corrosion rate predictions except for passive rebars with small diameters embedded in saturated concrete.

Modelisation of chloride reactive transport in concrete including thermodynamic equilibrium, kinetic control and surface complexation

In this study, a new physically and chemically based model taking into account thermodynamic equilibrium, kinetics and surface complexation is proposed to predict the ingress of chloride ions into saturated concretes. The numerical results of this multi-ionic model are compared to experimental data such as total and free chloride concentration profiles and chloride binding isotherms. With only one set of parameters, the results show very good agreement for chloride binding isotherm of concretes with different w/b, types of binders (CEM I, CEM III, and CEM I with fly ash) and various exposure conditions (NaCl solution at different concentrations). Such a model overcomes the use of empirical chloride binding isotherms that can be difficult to asses for concretes with supplementary cementitious materials. The very good results underline the need to take into account all the physical and chemical phenomena included in the model.

Prediction of Chloride Ingress in Saturated Concrete Using Formation Factor and Chloride Binding Isotherm

Abstract This article describes an approach to predict chloride ingress in saturated concrete using the formation factor and chloride binding isotherm. The formation factor is calculated as the ratio of the measured electrical resistivity of concrete and the electrical resistivity of the pore solution. The chloride binding isotherms are determined experimentally by placing ground concrete powder in sodium chloride (NaCl) solutions of varying concentrations and using titration to quantify the bound chloride contents. The Nernst-Plank equation is used to predict the chloride ingress with the ionic diffusion coefficients (calculated from the formation factor) and the fitted (Freundlich) chloride binding isotherm. The simulation results correspond well with experimentally obtained chloride profiles of saturated concrete after being ponded in NaCl solution. It is found that 70 % of simulated values for total chloride contents have a relative error of less than 30 % when compared to the experimental data. The approach presented in this article can be considered as an alternative for mixture qualification and quality control, though further work is needed to benchmark this against existing procedures.

Assessment of the electrochemical state of steel reinforcement in water saturated concrete by resistivity measurement

The surface resistivity of concrete with and without reinforcement was measured in the laboratory. Uncarbonated and carbonated concrete specimens were investigated corresponding to a passive and active state of corrosion, respectively. The concrete was water saturated, so the measurement medium was considered to be homogeneous. The measurements were performed directly over the rebar on reinforced specimens in order to minimize boundary effects. The presence of a rebar, with the resulting short-circuit effect due to the large differences of resistivity between concrete and steel, strongly reduced the measured resistivity. The magnitude of this reduction increased as the rebar reached a more active state. Other parameters influencing the short circuit effect, particularly concrete cover depth and concrete resistivity, were studied experimentally as well as numerically. The measurement results were inverted through a numerical simulation using COMSOL Multiphysics software. The Butler-Volmer relation was used to characterise the charge exchange at the steel-concrete interface. This inversion process determined the real resistivity of concrete as well as the electrochemical parameters of the embedded reinforcing steel bar. The methodology combining both measurement and inversion is relevant to evaluate the corrosion state of reinforcement. By comparing the electrochemical parameters of a passive and active rebar, the corrosion state can be determined with the proposed methodology.

Influential Mechanism of Pore Water Pressure on the Compressive Strength of Concrete

The expression of effective stress principle suitable for the micro-structural features of saturated concrete is established. The theoretical equation of pore water pressure coefficient under three-dimensional stress state is proposed. By this model, the compressive strength of concrete under different confining pressures is predicted, and the influential mechanism of pore water pressure on the strength of concrete under complicated stress states is investigated. The results indicate that the existence of pore water pressure changes the loading path of the matrix’s true stress in the principal stress space, therefore it modifies the compressive strength when the stress state of the concrete reaches its limit.

Research on Pump Primers for Friction Reduction of Wet-Mix Shotcrete Based on Precreating Lubricating Layer

Wet-mix shotcrete has lower water content even though looking wet which is generally less than 8% of total content. The inner surface of pipes will absorb water from fresh concrete to wet itself if concrete pump primers is not pumped early; meanwhile, the saturated concrete is transformed into the unsaturated concrete, and pipe blockage often occurs. In order to minimize the loss of water and wet pipes, a new type of pump primers was developed to precreate the lubricating layer that is one of the most dominant factors in determining the pumping capability, which would provide a cost-effective replacement for premium priced cement-based primers. Experiments were conducted to measure the viscosity, stability, and strength of mix solution during development stage. The optimal formula was obtained by orthogonal matching tests with 0.5% sodium stearate, 0.75% xanthan gum, 0.3% polyacrylamide, and 0.4% blender (mass ratio related to mix water). The pressure drop reduction of new primers was larger than that of common primers.

The effect of moisture transport on chloride penetration in concrete

This paper presents results of an experimental investigation on the effect of moisture transport on chloride penetration in partially saturated concrete. The paper includes the development of an experimental setup, analysis of experimental data, and formulation of the governing equations to characterize the moisture-chloride interaction in non-saturated concrete. The experimental setup consisted of two sets of fully and partially saturated concrete specimens that were exposed to chloride solutions. Humidity sensors were embedded in the concrete specimens to measure the internal relative humidity and the variation of humidity profiles over time. The governing equation that describes the chloride transport in saturated concrete was modified to account for the chloride movement induced by the moisture transport. The comparison of the chloride profiles obtained from the partially and fully saturated concrete specimens confirmed the coupling effect between the moisture transport and chloride penetration in concrete. The measured chloride and humidity profiles were analyzed to determine the coupling parameter in the governing equation. The analysis of the experimental data indicated that the coupling term in the governing equation is concentration dependent which means that the governing equation is nonlinear.

Effect of water pressure on mechanical behavior of concrete under dynamic compression state

This paper reports the experimental study of water pressure environment effects on the mechanical behavior of concrete under a dynamic compression state with strain rates ranging from 10−5/s to 10−2/s. The water content of concrete under different water pressure (0, 2, 5, and 10MPa) was obtained, and the failure modes and the strength of concrete were proposed. Results show that strength and failure modes are remarkably affected by water content and strain rate. Under water pressure, the strength of concrete increases nonlinearly as strain rate rises, and the strain rate sensitivity of concrete increases with increasing water content. When the water content of concrete is higher than 1.27 times of wsac, (wsac is water content of concrete which becomes a fully saturated state in atmospheric condition), and concrete under the fast loading results in cone-type failure (CTF). Moreover, the area of crack surface and the volume of debris shedding also decrease as both water content and strain rate increases; however, when the water content of concrete is lower than wsac or the concrete under slow loading, slant shear failure (SSF) occurs. As explained through a basic poromechanics analysis, this dissimilarity is mainly attributed to the excess pore water pressure and the viscous stress inside the saturated concrete during fast (quasi-static or dynamic) experiments.

Internal water pressure development in saturated concrete cylinder subjected to coefficient of thermal expansion tests: Poroelastic model

Coefficient of thermal expansion (CTE) test procedures use saturated concrete samples to (ostensibly) avoid the effect of moisture content. Concrete is a porous material, and internal water pressure develops in saturated concrete when subjected to temperature change due to the difference in CTE between liquid and solid phases. The measured CTE is affected by the internal water pressure development in saturated concrete samples. This paper represents an analytical model that was developed based on the poromechanical phenomenon of concrete to determine the time dependent internal water pressure and axial strain development. Material properties including, CTE, moduli of liquid and solid phase of concrete, as well as porosity and permeability of concrete samples have influence on the internal water pressure development. The predicted axial strain from the proposed numerical model for saturated concrete samples subjected to temperature changes were consistent with the experimental results. Predicted internal water pressure of the hardened concrete cylinder was as high as 700psi (4.8MPa), which is sufficiently high to potentially induce microcracking.