This study employs a lattice fracture model to simulate static and fatigue fracture behaviour of Interfacial Transition Zone (ITZ) at microscale and mortar at mesoscale. The heterogeneous microstructure of ITZ and mesostructure of mortar are explicitly considered in the models. The initial step involves calibrating and validating the microscopic model of the ITZ through micro-cantilever bending tests. Subsequently, this validated ITZ model serves as a constitutive law to simulate the fracture behavior of mortar at the mesoscale using an uncoupled upscaling method. The influence of microstructural features, such as w/c ratio and microscopic roughness, on the fracture behaviour of ITZ is investigated. Moreover, the effect of ITZ properties and stress level on the fracture performance and fatigue damage evolution of mortar is also studied. The simulation results for both the ITZ and mortar demonstrate good agreement with experimental results. The proposed two models provide insights into the fracture mechanisms and fatigue damage evolution in cementitious materials subjected to static and cyclic loadings.

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This paper presents an experimental study on the interface bonding properties of polyvinyl alcohol (PVA) fiber in alkali-activated slag/fly ash (AASF) pastes. Three interface bonding properties (i.e., the chemical bonding energy Gd, the initial frictional bond strength τ0, and slip-hardening behavior) were determined using single-fiber pullout tests. The microstructure and chemical composition of the reaction products in the fiber/matrix interfacial transition zone (ITZ) and the nearby matrix were also characterized to reveal the influence of PVA fiber to its surrounding matrix. It is found that Gd increases primarily with increasing Ca/(Si+Al) ratio of C-(N-)A-S-H gel. Unlike that in cementitious materials, the inclusion of PVA fiber in AASF pastes promotes the formation of a high-Ca C-(N-)A-S-H phase rather than crystalline portlandite near the fiber surface. This study provides useful guidance for tailoring the interface bonding properties of AASF and also the development of high-performance composites such as strain-hardening geopolymer composites.@en

This study investigates the deformation of free and stress of restrained alkali-activated slag concrete (AASC), respectively, under semi-adiabatic condition. The concrete shows first thermal expansion, which is compensated soon by autogenous shrinkage. The subsequent cooling down of the concrete aggravates shrinkage and development of tensile stress, which eventually results in early cracking of the concrete. The results show that semi-adiabatic condition is severer for AASC than isothermal condition in view of cracking tendency. The evolutions of coefficient of thermal expansion (CTE) and elastic modulus are measured by elaborated experimental methods. Simulating the deformation of AASC by summing thermal and autogenous deformations appears feasible. With the consideration of relaxation, the stress evolution in restrained AASC can be predicted pretty well by the model used in this paper. This study provides insights into the thermal deformation and cracking tendency of AASC in practical circumstances.

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In this study, the flexural strength and fatigue properties of interfacial transition zone (ITZ) were experimentally investigated at the micrometre length scale. The hardened cement paste cantilevers (150 × 150 × 750 μm³) attached to a quartzite aggregate surface were prepared and tested under the monotonic and cyclic load using a nanoindenter. The measured flexural strength of the ITZ (10.49–14.15 MPa) is found to be one order of magnitude higher than the macroscopic strength of ITZ reported in literature. On the other hand, the fatigue strength of the ITZ is lower than that of bulk cement paste at same length scale, measured previously by the authors. The microscopic mechanical interlocking and the electrostatic interaction between aggregate surface and hydration products are thought to contribute to the bond strength of ITZ. This study provides an experimental basis for the development of multiscale analysis of concrete subjected to both static and fatigue loading.

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This study presents an experimental investigation of the rate-dependent mechanical properties of cement paste at the microscale. With the use of a nanoindenter, micro-cantilever beams with the size of 300 μm × 300 μm × 1650 μm were loaded at five different strain rates from around 10⁻⁶/s to 10⁻²/s until failure. It is found that with increasing strain rate, the stress-strain curves show less and delayed pre-peak nonlinearity. Both the flexural strength and the elastic modulus of beams increase with increasing strain rate, while the strain at peak stress exhibits an opposite trend. Examination of the fracture surface indicates that with increasing strain rate the possibility of a crack to pass through stronger components of the hydration products is increased. The experimental observations and possible mechanisms leading to changes in mechanical responses are discussed. It is suggested that at least two micromechanical processes, namely creep and Stéfan effect, are mainly responsible for the rate-dependent behaviour of cement paste within the investigated strain rate range and their dominances seem to vary with the strain rate. At lower strain rate, the strain rate sensitivity of cement paste is thought to be dominated by the creep effect, while at higher strain rate the Stéfan effect appears to be the governing factor.

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This paper presents an experimental investigation on the short-term creep recovery of cement paste at micrometre length scale. Micro-cantilever beams were fabricated and tested with 8 different loading series using the nanoindenter. It is found that cement pastes show high recovery ratios (>80%) even subjected to very high stress levels. Relatively lower recovery ratios and non-linear creep were also observed for w/c 0.4 samples under high stress levels. A good agreement is found between the results predicted using the linear superposition principle and the experimental results except for the measured non-linear creep in w/c 0.4 samples. It is suggested that the short-term creep recovery may be associated with the microscale stress redistribution or the reversible internal water movement. The observed non-linear creep under the highest stress level may be due to the higher density of microcracks generated during the loading stage, which may further promote the water transfer.

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This study presents an experimental investigation of fatigue properties of cement paste at the microscale. A strong size dependence is found for the flexural fatigue life of the cement paste specimen. Microscopic observations on the fractured surfaces suggest that there is a higher density of nano-scale cracks generated during the fatigue loading compared to the static fracture. However, the fatigue damage evolution is found to be very slow and small even under high stress levels. The development of residual deformation for cement paste can be explained by the combined effects of viscoelastic deformation and fatigue cracking growth.

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This paper presents a method to numerically investigate the microstructural effect on the creep behavior of cement paste at the microscale. The lattice fracture model is extended to consider local time-dependent deformations of calcium-silicate-hydrate phases in the cement paste by imposing local forces. The term “experimentally informed model” is used herein as the heterogeneous microstructures of hardened cement pastes were obtained by using the X-ray computed microtomography and directly implemented into the model. The mechanical and creep properties of different constituents at the resolution of 5 µm were inversely identified from the fracture and creep bending tests on cementitious microcantilever beams at the microscale. The model is then validated through the comparison with the testing results of cement pastes with different w/c ratios and microstructures. It is found that the developed model can successfully reproduce experimentally observed behaviors and be applied to explain the experimental results in detail. With the method presented in this paper, the relationship between the volume fractions of different components and the global creep behavior of cement paste can be established. The validation of the model performed at the microscale forms a basis for the multiscale analysis of concrete creep.

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In this study, a numerical model using a 2D lattice network is developed to investigate the fatigue behaviour of cement paste at the microscale. Images of 2D microstructures of cement pastes obtained from XCT tests are used as inputs and mapped to the lattice model. Different local mechanical and fatigue properties are assigned to different phases of the cement paste. A cyclic constitutive law is proposed for considering the fatigue damage evolution. Fatigue experiments performed at the same length scale are used to calibrate and validate the model. The proposed model can reproduce well the flexural fatigue experimental results, in terms of S-N curve, stiffness degradation and residual deformation. The validated model is then used to predict the uniaxial tensile fatigue fracture of cement paste. The effects of microstructure and stress level on the fatigue fracture are studied using the proposed model. This model forms a basis for the multiscale analysis of concrete fatigue.

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In recent decades, several simulation models have been proposed to predict autogenous shrinkage of cementitious systems. In most of these models, however, only the elastic deformation caused by self-desiccation of the hydrating cement paste is considered. In fact, cement paste is not an ideal elastic material. Also the time-dependent deformation, i.e. creep, has been proposed an important component of autogenous shrinkage, especially at the early age. In this study, a simulation model for autogenous deformation is proposed, which includes an elastic part and a time-dependent part. The time-dependent part of this model is based on the activation energy concept. The capillary tension is considered as the driving force of the autogenous shrinkages. In order to evaluate the accuracy of the prediction with the proposed model, CEM I and CEM III/B pastes are studied in this paper. The simulated autogenous shrinkages are compared with experimental results.

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This study proposes an experimental method for studying the short-term creep behaviour of cement paste at micro-scale. The micro-bending tests on miniaturized cantilever beams were used to characterize the viscoelastic properties of cement paste. The effects of w/b ratio, the type of binder and the stress level on the microscopic creep behaviour were investigated. It is found that the short-term creep of cement paste at microscale can be satisfactorily described by a power-law function. A linear viscoelastic behaviour has been observed in different cementitious systems at the microscale with the stress level up to 67.9%. When compared with the creep results in microindentation tests and conventional macroscopic tests, the obtained creep compliance function in this study is found to be both qualitatively and quantitatively representative of the macroscopic results. This experimental study underlines the importance of microstructural effect on the creep behaviours of cementitious materials at microscale.

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In this work, findings of a numerical study performed to investigate the impact behavior of porous concrete, modeled as a four phase cementitious composite consisting of aggregates, cement paste, interfacial transition zones (ITZ) and air, are presented. The numerical analyses contributed to the process of designing a special type of concrete for safety purposes i.e. as a protective building material to be used in safety walls outside important buildings or munition magazines for storing explosives. In case of an explosion, large concrete fragments that are formed, cause a very important threat. Therefore, in the scope of a research project, designing a special type of concrete having sufficient strength, but fracturing into small fragments under impact loading was aimed. In the numerical analyses, model porous concretes, in which the amounts and properties of pores and aggregates could be varied individually, were used to see the sole effect of each parameter. According to the results, it was found that at constant total porosity, the impact strength increased with decreasing pore size while multiple fragmentation was observed. On the other hand, the impact strengths of porous concretes with different size aggregates (with constant total aggregate content and porosity) were approximately the same when no ITZ was defined. However, when ITZ was present, the impact strength was found to decrease as the aggregates were finer. This trend was also valid for the respective full concretes. Representative experimental results of porous concretes were also presented in order to support the numerical results.@en

In this paper the potentiometric response of a Ag/AgCl electrode as a chloride sensor in cementitious materials of different mix design was studied. The chloride sensor’s response was discussed with respect to the presence of hydration products around the sensor. The free chloride content inferred from the sensor’s response was compared to the one obtained from destructive water and acid soluble chlorides. The measured free chloride content, obtained via sensor’s reading, was lower than the obtained water and acid soluble chlorides. The results indicated the influence of the cementitious mix design on the correlation between the free chloride content obtained via sensor’s reading, water and acid soluble chlorides.@en

Nanoindentation is usually used to investigate local elastic properties and hardness of materials. In this paper, the nanoindenter served as a loading tool to perform micro scale bending tests and measure the global response of micro-scale specimens. For testing, cement paste cantilever beams with a square cross-section of 300 μm × 300 μm were fabricated using a micro dicing saw. Monotonic, cyclic and sustained loading bending tests were conducted on the beams using the nanoindenter. The load-deflection behaviours of micro-cantilever beams subjected to various loading conditions are presented in this paper. The interpretation of experimental results regarding the measured deflection of beam is discussed. With this micro-scale test method, the fracture strength as well as time-dependent responses of cement paste at the micrometre level can be assessed in a straightforward manner.@en

Infrastructure is ageing and we are facing a serious challenge on how to deal with it. One possible solution is to repair it, but the life of current concrete repairs, including all types of repairs and application of different materials, is not satisfactory and there is an urgent need for improvement. Understanding the damage development in a repair system, and how to predict, model and prevent its failure is critical to improving performance of concrete repairs. Therefore, the aim of this work was to experimentally and numerically study the interaction between the repair material and existing concrete in two aspects which are so far scarcely understood. Firstly, the dynamics of moisture exchange in repair systems was studied, because this interaction is critical for the development of properties of the repair material and the interface. Secondly, the interaction between the repair material and the substrate, i.e. fracture behaviour of the repair system under different exposure and loading conditions was investigated. The main repair material used in this study is a recently developed fibre reinforced ultra-ductile cement-based material, known as SHCC (Strain Hardening Cementitious Composite). Fine PVA fibres and special material composition ensure that instead of single crack localization, like with conventional concrete, many tiny cracks are generated in SHCC. This is better for durability of repairs and enhances ductility and damage resistant behaviour. Finally, based on acquired knowledge, SHCC and four other repair materials were applied and inspected in a trial repair of the 70-year-old Maastunnel. More details about the presented work can be found in [1].@en

The world’s infrastructure is vital for providing accommodation and mobility for people. Although it is obvious that the construction industry has been crucial for realizing building and civil infrastructures, it is also clear that building activities have a big impact on the environment. Still growing and developing societies and economies do need even more buildings, more roads etc. The question is how all these needs can be accomplished without compromising the ability of future generations to meet their needs (Brundtland Report). In this contribution the urgency of a sustainable construction industry is explained. The need for a change from building in the service of growth to building in the service of sustainability is emphasized. Comprehensive models, with which the entire building cycle can be simulated, would enable engineers to analyse the building and construction process with respect to the demand for raw materials and energy, maintenance and repair, renovation and retrofitting and, finally, recycling and reuse of materials and/or structural components. The option of developing a serious game for sustainable construction is discussed and recommended. With such a game the whole building cycle is simulated, ranging from decision making by stakeholders to execution on the site, curing, maintenance and repair in the service life phase, decommissioning, recycling and reuse.

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This study presents the numerical analyses conducted to investigate the impact behavior of different porous concretes, which have also been cast and tested experimentally. For a realistic representation of the real porous concretes containing arbitrary shaped air pores, a mesh generation code was developed in which the aggregates in the mixtures were directly extracted through computed tomography. In the code, mineralogically different aggregates in porous concretes with gravel could also be individually defined. In the explicit finite element analyses conducted, porous concrete was considered as a four-phase material, consisting of aggregates, interfacial transition zones (ITZ), bulk cement paste and air. The pore size distribution and the fragmentation behavior of the concretes were also numerically analyzed. Among the parameters that have been investigated both numerically and experimentally, aggregate grading, which determines the porosity and pore size distribution of the material, was found to have a dominant effect on the strength as well as the fragmentation properties of porous concretes. Although the amount of ITZ is higher in mixtures containing finer aggregates, those mixtures had higher impact strengths compared to coarser aggregate ones again owing to their much finer pore structures.

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In this work, the different techniques for non-destructive in situ measurement of chloride ion concentration are presented. Non-destructive (ND) in situ measurement is crucial for reliable and continuous determination of chloride ion concentration in concrete. Over the last 20 years, several studies have been performed on ND measurements. These were mainly focused on the application of electrochemical and electromagnetic techniques. Each technique has its advantages and disadvantages. Depending on the requirement of assets managers and constructors and considering the limitations, these techniques can be well applied. The main concepts and comparative analysis, in view of possibilities and limitations, of these non-destructive techniques are presented in this paper.@en

The stability and reproducibility of an Ag/AgCl sensors’ response in an alkaline medium are important for the application of these sensors in cementitious materials. The sensors’ response, or their open circuit potential (OCP), reflects a dynamic equilibrium at the sensor/environment interface. The OCP response in an alkaline medium is affected by the presence of hydroxide ions. The interference of hydroxide ions leads to inaccuracies or a delay in the sensors’ response to a certain chloride content. In this article, the potentiometric response (or OCP evolution) of the chloride sensors is measured in model solutions, resembling the concrete pore water. The scatter of the sensors’ OCP is discussed with respect to the interference of hydroxide ions at varying chloride concentration in the medium. The deviation of the sensor’s response from its ideal performance (determined by the Nernst law) is attributed to dechlorination of the AgCl layer and the formation of Ag2O on the sensor’s surface. Results from the surface XPS analysis of the AgCl layer before and after treatment in alkaline medium confirm these observations in view of chemical transformation of AgCl to Ag2O.@en