K. van Breugel

Numerical simulation of hydration and microstructural development in hardening cement-based materials (I) theory

This is a report of a study which attempted to develop a simulation model with which adiabatic and/or isothermal hydration curves of portland cement-based materials can be predicted. The simulation model takes into consideration mutual interferences between the hydration process and the development of microstructure. The correlation between structural development and the development of material properties shall be evaluated. Observations are made regarding the accuracy and reliability of the predictions.

Study on the development of the microstructure in cement-based materials by means of numerical simulation and ultrasonic pulse velocity measurement

The formation of microstructure in cementitious materials was simulated with a numerical model. Simulation results have been verified by measuring the evolution of the ultrasonic pulse velocity (UPV). In this contribution, the applied computer-based cement hydration model is presented. The UPV measurements are also presented and evaluated. Experiments were performed on concrete mixtures with water/cement ratio 0.40, 0.45 and 0.55. The concrete was cured isothermally at 10, 20, 30 and 40 °C. Correlations between the development of the microstructure and the evolution of UPV were found. Two critical processes were individuated. The first is the percolation threshold of the solid phase. The second is the full connectivity of the solid phase. Both in the experiments and in the numerical simulations it was possible to distinguish these critical stages. These stages are discussed and conclusions are drawn regarding the potential of numerical simulation models in the study of early age cementitious materials for quantitative analysis of hydration processes.

THREE-DIMENSIONAL MICROSTRUCTURE ANALYSIS OF NUMERICALLY SIMULATED CEMENTITIOUS MATERIALS

In order to predict the transport properties of porous media, such as permeability and electrical conductivity of cementitious materials, a better understanding of the microstructural characteristics, including the geometrical and topological properties, is required. In this contribution, the microstructure of cementitious materials is simulated by using the cement hydration model HYMOSTRUC. In this computer-based numerical model, the hydrating cement grains are modeled as gradually growing spheres, which become in contact while growing. The simulated porous medium can be described as a series of sections taken from three orthogonal directions, in which each unit (pixel) is filled either with a solid or a fluid phase (pores). Various algorithms based on a random walk process are utilized to determine the local geometrical information, such as gravity centers coordinate, perimeter and area of each individual pore. The percolating path of the fluid in three dimensions is traced by using an overlap algorithm. Both three-dimensional (3D) geometrical information and topological space characterization including branch node network and genus of the pores are derived. Calculation results of these algorithms are compared with results obtained by other microstructural models at various degree of hydration.

Numerical simulation of hydration and microstructural development in hardening cement-based materials: (II) applications

Abstract After having outlined the structure of a computer-based simulation program, called HYMOSTRUC (9), which was developed to describe and predict hydration and microstructural development of cement-based materials, characteristic features of the simulation model are discussed. Attention is paid to changes in the number of particles in the early stage of hydration, the development of interparticle contacts and how this affects the rate of hydration and the effect of particle size on hydration characteristics. The correlation between strength and the amount of embedded cement, i.e., cement involved in the formation of interparticle contacts, and the influence of the w c ratio and the curing temperature on this correlation is discussed. Calculated porosity in the matrix-aggregate interfacial zone is shown and compared with experimental results. The potential of the model to describe and predict isothermal and adiabatic hydration curves is illustrated with some examples and the accuracy of the results is discussed.

EXPERIMENTAL STUDY AND NUMERICAL SIMULATION ON THE FORMATION OF MICROSTRUCTURE IN CEMENTITIOUS MATERIALS AT EARLY AGE

Abstract The formation of microstructure in early age cement paste and concrete was examined with an ultrasonic experimental set-up. Research parameters included the influence of curing temperature (isothermal curing at 20, 30 and 40 °C), water/cement ratio (0.40, 0.45 and 0.55) and amount of aggregate. In parallel with the experiments, the cement hydration model HYMOSTRUC was utilized to simulate the formation of the microstructure. In this study, the cement paste was considered as a four-phase system consisting of water, unhydrated cement, hydration products and that part of the hydration product that causes the contact between the hydrating cement grains (so called “bridge volume”). A correlation has been found between the growth of bridge volume calculated with the model and the changes in the pulse velocity. It is believed that ultrasonic pulse velocity (UPV) measurements can represent a valuable tool to investigate the development of the microstructure at early age.

NUMERICAL MODELLING OF AUTOGENOUS SHRINKAGE OF HARDENING CEMENT PASTE

Hardened cement paste is a porous material. As hydration proceeds, pores become emptied and the relative humidity reduces. This reduction of the relative humidity goes along with a reduction of the pressure in the emptied pore space. Thermodynamic equilibrium requires an increase of the surface tension in the boundary layer that develops at the inner pore wall area. For validation of the model, experiments have been carried out. Good agreement is reached between the numerical simulations and the results obtained from experiments.

Investigation of Corrosion and Cathodic Protection in Reinforced Concrete I. Application of Electrochemical Techniques

The electrochemical behavior of steel reinforcement in conditions of corrosion and cathodic protection was studied, using electrochemical impedance spectroscopy (EIS) and compared to reference (noncorroding) conditions. Polarization resistance (PR) method and potentiodynamic polarization (PDP) were employed as well, in addition to ac 2 pin electrical resistance monitoring, thus deriving a comparison of the involved parameters, mainly polarization resistance and bulk electrical properties, obtained by all methods. It was found out that EIS is readily applicable for evaluating electrochemical behavior of the steel surface not only for corroding or passive state, but also in conditions of cathodic protection, although the interpretation of derived parameters is not straightforward and is related to the properties of the product layers, formed on the steel surface in the different conditions. For verification of the latter dependence, EIS, PDP, and PR measurements were performed additionally in cement extract solution, using steel samples from the previously embedded rebars in all technical conditions. The bulk matrix properties in passive, corroding, or under-protection conditions can be well defined by EIS. The evaluation of the electrochemical behavior of the steel surface, in terms of deriving polarization resistance, should take into account the crystallinity, morphology, and composition of the surface layers, which were investigated by scanning electron microscopy and energy dispersive X-ray analysis.

Electrochemical Behavior, Microstructural Analysis, and Morphological Observations in Reinforced Mortar Subjected to Chloride Ingress

The behavior of steel reinforcement was studied using electrochemical impedance spectroscopy (EIS) and polarization resistance (PR) techniques in conditions of chloride-induced corrosion in ordinary Portland cement-mortar specimens immersed in 7% NaCl for a test period of 120 days and compared to specimens immersed in demineralized water for the same period as reference specimens. This study was an initial phase of ongoing research on electrochemical methods for corrosion protection in reinforced concrete structures and aimed at investigating the applicability of widely accepted techniques as EIS and PR and their possible correlation with structural observations of the bulk matrix, relevant to cement-based materials science and product-layers distribution, to corrosion and further protection. The results indicate that the concept of EIS modeling and the components used in the latter correspond well to alterations in structural properties of the bulk matrix, while the electrochemical behavior can be additionally supported by morphological observations of the steel/cement paste interface.

Corrosion Performance of Carbon Steel in Simulated Pore Solution in the Presence of Micelles

This study presents the results on the investigation of the corrosion behavior of carbon steel in model alkaline medium in the presence of very low concentration of polymeric nanoaggregates [0.0024 wt % polyethylene oxide (PEO)113-b-PS70 micelles]. The steel electrodes were investigated in chloride free and chloride-containing cement extracts. The electrochemical measurements (electrochemical impedance spectroscopy and potentiodynamic polarization) indicate that the presence of micelles alters the composition of the surface layers (i.e., micelles were indeed absorbed to the steel surface) and influences the electrochemical behavior of the steel, i.e., the micelles lead to an initially increased corrosion resistance of the steel whereas no significant improvement was observed within longer immersion periods. Surface analysis, performed by environmental scanning electronic microscopy, energy-dispersive x-ray analysis, and x-ray photoelectron spectroscopy, supports and elucidates the corrosion performance. The product layers in the micelles-containing specimens are more homogenous and compact, presenting protective ?-Fe2O3 and/or Fe3O4, whereas the product layers in the micelles-free specimens exhibit mainly FeOOH, FeO, and FeCO3, which are prone to chloride attack. Therefore, the increased “barrier effects” along with the layers composition and altered surface morphology denote for the initially increased corrosion resistance of the steel in chloride-containing alkaline medium in the presence of micelles.

The effect of reinforcement on early-age cracking due to autogenous shrinkage and thermal effects

The effect of reinforcement on early-age cracking in high strength concrete was investigated on laboratory scale. A temperature stress testing machine was used for simulating total restraint and for imposing different curing temperatures onto the concrete. The behaviour of the used high strength concrete was compared to a normal strength concrete. In order to separate thermal effects and autogenous shrinkage specimens were cured isothermally and semi-adiabatically. Further test variables were the reinforcement percentage (0%, 0.75%, 1.34% and 3.02%) and configuration (one reinforcement bar and four reinforcement bars). In order to visualise the crack formation in the early phase, cracks had been impregnated with fluorescent epoxy. The so obtained photos show that reinforcement can induce the formation of smaller cracks. These smaller cracks can postpone the moment at which major cracks are formed. Finally, a procedure is discussed for quantifying the effect of reinforcement decreasing the risk of through-cracking at early age. For that purpose a strain enhancement factor is introduced.