Monitoring the early-age hydration of self-compacting concrete using ultrasonic p-wave transmission and isothermal calorimetry

The early-age hydration (≤48 h) of a series of self-compacting concretes and corresponding mortars and one traditionally vibrated concrete and mortar is monitored in a continuous way using ultrasonic testing and isothermal calorimetry. The mixtures differ in type of mineral addition, superplasticizer, cement, cement-to-powder ratio and water-to-powder ratio. The influence of these different mixture compositions on the kinetics of the hydration during the first days of the hydration is characterized by the heat production rate q and the evolution of the p-wave velocity, which is a consequence of the microstructural changes. The variations in the acceleration caused by mineral additions and the deceleration caused by superplasticizers lead to a significantly different behavior. Separating the impact of each of the affecting factors is not always possible due to their combined actions. The nature of the acceleration due to limestone additions and the deceleration caused by polycarboxylate ether superplasticizers can be distinguished clearly, but cannot be quantified. The correlation between the ultrasonic and isothermal calorimetric results is investigated based on parameters related to the start and the end of the setting and reveals the meaningfulness of these parameters when assessing the hydration of self-compacting mixtures with continuous ultrasonic techniques.     

Identification of viscoelastic behavior for early-age concrete based on measured strain and stress histories

In order to accurately predict the stress of concrete structures that undergo variations of temperature and moisture, a compliance function is required that considers the fast development of material properties in early-age concrete. The purpose of present study is to identify the viscoelastic behavior in actual concrete structures at early ages. To this end, a numerical method to identify the viscoelastic properties from measured strain and stress histories was investigated and a series of full-scale test members was fabricated, from which the behavior of early-age concrete was directly measured. The relaxation function was identified from measured data in the full-scale members and the existing compliance model, which is based on the solidification theory, was modified for incorporation into the early-age behavior. The modified compliance model described the highly viscoelastic behavior of concrete at very early ages and thus allowed more accurate evaluation of deformation and stress in early-age concrete.     

Shrinkage-cracking behavior of OPC-fiber concrete at early-age

The current paper presents the results of early-age restrained shrinkage (RS) tests on Ordinary Portland Cement (OPC) concretes incorporating admixed polypropylene fiber (PP). Four concrete mixtures made with OPC containing various volume fractions of PP fiber were tested. Two identical specimens of each mixture were tested: one subjected to fully restrained conditions and the other allowed to shrink freely, both under the drying conditions of 23°C and 50% relative humidity at the age of 24 h. Direct and indirect tensile tests were also performed in the same concretes to monitor the tensile strength development. With increasing fiber contents in mixture, the tensile strength, creep and elastic modulus characteristics have not significantly changed during the first week of age. Increasing the volume fractions of PP fiber significantly delayed the time of cracking owing to the delayed onset of RS, which is beneficial to crack resistance.     

钢筋混凝土梁动态试验与数值模拟

试验研究、数值模拟地震作用范围内加载速率对钢筋混凝土梁影响。考虑混凝土强度、钢筋强度、剪跨比、加载速率及加载模式等对钢筋混凝土梁力学及变形性能影响;基于ABAQUS有限元软件建立钢筋混凝土梁计算模型,考虑钢筋、混凝土的率敏感性,对梁试件在不同工况下动态性能进行数值模拟;模拟结果与试验结果吻合较好。     

Anisotropic modelling and numerical simulation of brittle damage in concrete

A framework for damage mechanics of brittle solids is presented and exploited in the design and numerical implementation of an anisotropic model for the tensile failure of concrete. The key feature exploited in the analysis is the hypothesis of maximum dissipation, which specifies a unique damage rule for the elastic moduli of the solid once a failure surface is specified. A complete algorithmic treatment of the resulting model is given which renders a useful tool for large-scale inelastic finite element calculations. A rather simple three-surface failure model for concrete, containing essentially no adjustable parameters, is shown to produce results in remarkably good agreement with sample experimental data.     

A preliminary numerical investigation on the influence of material variability in the early-age cracking behavior of restrained concrete

The restraint of drying, autogenous, or thermal shrinkage can result in the development of tensile residual stresses. If the residual stresses that develop are large enough, they may cause cracking in the concrete. Substantial research has focused on the development of test methods to assess stress development and the corresponding potential for cracking. These test methods frequently focus on the determination of material properties that can be used in deterministic computer programs to simulate stress development and cracking. While these models are a great step forward, variability is inherent in the material properties, the construction processes, and the environmental conditions (i.e., temperature and relative humidity). This paper presents results of considering variability in a model for predicting the time of shrinkage cracking. A Monte Carlo simulation procedure has been adopted to account for variability in material properties. It has been found that a log-logistic function can accurately describe variability that can be expected in the time of cracking.     

Cracking behaviour of reinforced concrete subjected to early-age shrinkage

Cracks in freshly cast structures can be the consequence of restrained thermal deformations. Additional to these deformations high strength concrete undergoes autogenous deformations which make this material even more prone to early-age cracking. In order to analyse this behaviour of reinforced concrete structures at early age, an elaborate test series on a high strength concrete (w/c=0.33) was performed. The measured values are: the free deformation of plain and reinforced concrete, stress development in a fully restrained reinforced specimen and in its reinforcement bars, development of bond strength and cube compressive strength. In this paper the experimental results are presented and related to each other. With this information more knowledge is gained concerning the mechanisms leading to stress development and cracking behaviour in reinforced concrete at very early age.     

Dynamic behavior of concrete at high strain rates and pressures: II. numerical simulation

The response of concrete and mortar under high-strain-rate impact loading are analyzed using fully dynamic finite element simulations. The analyses concern the load-carrying capacity, energy absorbency and the effect of the microstructure. The simulations focus on the plate impact configuration used in the experimental part of this research, allowing for direct comparison of model predictions with experimental measurements. A micromechanical model is formulated and used, accounting for the two-phase composite microstructure of concrete. Arbitrary microstructural phase morphologies of actual concrete used in impact experiments are digitized and explicitly considered in the numerical models. The behavior of the two constituent phases in the concrete are characterized by an extended Drucker–Prager model that accounts for pressure-dependence, rate-sensitivity, and strain hardening/softening. Model parameters are determined by independent impact experiments on mortar and through a parametric study in which the prediction of numerical simulations is matched with measurements from experiments on concrete and mortar. Calculations show that significant inelastic deformations occur in the mortar matrix under the impact conditions analyzed and relatively smaller inelastic strains are seen in the aggregates. The influence of aggregate volume fraction on the dynamic load-carrying capacity of concrete is explored. The strength increases with aggregate volume fraction and an enhancement of approximately 30% over that of mortar is found for an aggregate volume fraction of 42%. Numerical simulations also show increasing energy absorbency with increasing aggregate volume fraction and provide a time-resolved characterization for the history of work dissipation as the deformation progresses.     

Effects of nano- and micro-limestone addition on early-age properties of ultra-high-performance concrete

In this study, the effects of micro- and nano-CaCO₃ addition on the early-age properties of ultra-high-performance concrete (UHPC) cured at simulated cold and normal field conditions were investigated. The micro-CaCO₃ was added at rates of 0, 2.5, 5, 10 and 15 %, while the nano-CaCO₃ was added at rates of 0, 2.5, and 5 %, both as partial volume replacement for cement. Results indicate that micro-CaCO₃ acted mainly as an inert filler, creating a denser microstructure and increasing the effective w/c ratio. In addition, nano-CaCO₃ accelerated the cement hydration process through nucleation, and also acted as an effective filling material. Mixtures combining both micro- and nano-CaCO₃ resulted in similar or enhanced mechanical properties compared to that of the control, while achieving cement replacement levels up to 20 %. Thus, through the use of micro- and nano-CaCO₃, more environmentally friendly UHPC can be produced by reducing its cement factor, while achieving enhanced engineering properties.     

Simple lattice model for numerical simulation of fracture of concrete materials and structures

In this paper a numerical model is presented for simulating fracture in heterogeneous materials such as concrete and rock. The typical failure mechanism, crack face bridging, found in concrete and other materials is simulated by use of a lattice model. The model can be used at a small scale, where the particles in the grain structure are generated and aggregate, matrix and bond properties are assigned to the lattice elements. Simulations at this scale are useful for studying the influence of material composition. In addition the model seems a promising tool for simulating fracture in structures. In this case the microstructure of the material is not mimicked in detail but rather the lattice elements are given tensile strengths which are randomly chosen out of a certain distribution. Realistic crack patterns are found compared with experiments on laboratory-scale specimens. The present results indicate that fracture mechanisms are simulated realistically. This is very important because it simplifies the tuning of the model.     

Experimental study on the convective heat transfer coefficient of early-age concrete

During the process of setting and hardening in concrete, the temperature profile shows a gradual nonlinear distribution due to the development of heat of hydration in cement. At early ages of concrete structures, this nonlinear distribution can have a large influence on crack evolution. It is thus important to obtain an accurate temperature history, and to do this, it is necessary to examine the thermal properties of the concrete. In this study, the convective heat transfer coefficient, which represents the heat transfer between a concrete surface and ambient air, was experimentally investigated with test variables such as the velocity of wind, the curing conditions, and the ambient temperature.For analyses using the thermal equilibrium boundary condition, it is generally noted that most of the heat release by the evaporation of moisture occurs at an early stage. To consider this phenomenon, the existing thermal equilibrium boundary condition has been modified so as to consider the evaporation quantity due to the evaporation effect. Convective heat transfer coefficients for a specific case were then calculated from the modified thermal equilibrium boundary condition using experimental results.     

超高钢混烟囱爆破切口角度计算及数值模拟

在超高钢筋混凝土烟囱拆除爆破中,切口角度的计算对于提高过程中的准确性和安全性具有十分重要的意义.为探究应力破坏准则和弯矩破坏准则两种不同爆破切口角度理论计算方法对钢筋混凝土烟囱的适用性,采用理论计算和数值模拟相结合的方法对切口角度理论计算方法进行优选.结果表明:通过应力破坏准则计算烟囱爆破切口角度较为准确,其对应拆除爆...     

Experimental investigation and numerical simulation of post-peak behavior and size effect of reinforced concrete columns

This paper deals with the effect of structural size of reinforced concrete columns with square cross sections on their nominal strength and post-peak behavior. This topic was studied experimentally on three different sizes of geometrically similar specimens. Our attention was focused on the overall performance of columns, in particular: peak strength, post-peak branch, type of a failure, concrete softening and steel buckling. Computational model based on the microplane model M4 for concrete [1,2] was used to simulate experimental results. As the result of experiments, no significant size effect was found in the nominal strength. However, size effect was found in the post-peak behavior. Results of the computer simulation showed good agreement with experimental data and proved the capabilities of the used material model.

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Résumé L'effet de la taille structurale des colonnes en béton armé avec une section carrée sur leur résistance nominale et le comportement après-pic est étudié dans l'article. Cet effet a été étudié à l'aide d'une expérience sur trois échantillons de tailles différentes mais de géométrie ressemblante. L'attention est concentrée sur le comportement global des colonnes et en particulier sur la résistance en pic, la partie de la courbe après-pic, le type de rupture, le ramollissement du béton et le flambement de l'acier. Le modèle numérique basé sur le modèle microplane M4 pour le béton [1, 2] a été utilisé pour la modélisation de l'expérience. Les résultats expérimentaux ont montré qu'aucun effet significatif de l'échelle de taille sur la résistance nominale n'a été trouvé. Cependant, il existe un effet de l'échelle sur le comportement après-pic. Les résultats numériques sont en bon accord avec les données expérimentales et l'utilisation du modèle a été justifiée.

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Editorial Note Prof. Zdeněk Bittnar is a RILEM Senior Member.     

Modeling the elastic properties of concrete composites: Experiment,differential effective medium theory,and numerical simulation

Concrete is a mixture of cement, water and aggregates. In terms of microstructure, besides the cement paste matrix and aggregate inclusions, there is a third phase, which is called the interfacial transition zone (ITZ), which forms due to the wall effect and can be thought of as a thin shell that randomly forms around each aggregate. Thus, concrete can be viewed as a bulk paste matrix containing composite inclusions. To compute the elastic properties of a concrete composite, a differential effective medium theory (D-EMT) is used in this study by assigning elastic moduli to corresponding bulk paste matrix, ITZ and aggregate. In this special D-EMT, each aggregate particle, surrounded by a shell of ITZ of uniform thickness and properties, is mapped onto an effective particle with uniform elastic moduli. The resulting simpler composite, with a bulk paste matrix, is then treated by the usual D-EMT. This study shows that to assure the accuracy of the D-EMT calculation, it is important to consider the increase in the water:cement mass ratio (w/c) of the ITZ and the corresponding decrease in w/c ratio of the bulk matrix. Because of this difference in w/c ratio, the contrast of elastic moduli between the ITZ and the bulk paste matrix needs to be considered as a function of hydration age. The Virtual Cement and Concrete Testing Laboratory (VCCTL) cement hydration module is used to simulate the microstructure of cement paste both inside and outside the ITZ. The redistribution of calcium hydroxide between ITZ and bulk paste regions can further affect the elastic contrast between ITZ and bulk paste. The elastic properties of these two regions are computed with a finite element technique and used as input into the D-EMT calculation. The D-EMT predictions of the elastic properties of concrete composites are compared with the results measured directly with a resonant frequency method on corresponding composites. This comparison shows that the D-EMT predictions agree well with experimental measurements of the elastic properties of a variety of concrete mixtures.     

Identification of material model of TiN using numerical simulation of nanoindentation test

The development of the model of the multistep nanoindentation test with Berkovich indenter, accounting for the residual stress distribution, is one of the aims of the present paper. The specimen is unloaded in the intervals between the deformation steps. Substrate, which is composed of a ferritic steel and biocompatible pulsed laser deposition TiN coating, is considered. The selection of the TiN was inspired by its perspective application as the coating for a constructional element of the heart prosthesis (blood chamber and aortic valves). Sensitivity analysis of the model predictions with respect to its parameters is presented in the present paper. The theory of elastic-plastic deformations is used in the finite element model, which simulates both loading and unloading phases, accounting for the real geometry of the indent. The main goal of the present paper was to inversely analyse the tests for coating/substrate system. Square root error between measured and predicted forces is the objective function in the analysis. Results of the inverse calculations, which are presented in the present paper, may be helpful in simulations of the behaviour of TiN deposited on substrate in various applications as bionanomaterials.     

Modifications of RHT material model for improved numerical simulation of dynamic response of concrete

Sophisticated numerical models are increasingly used to analyze complex physical processes such as concrete structures subjected to high-impulsive loads. Among other influencing factors for a realistic and reliable analysis, it is essential that the material models are capable of describing the material behaviour at the pertinent scale level in a realistic manner. One of the widely used concrete material models in impact and penetration analysis, the RHT model, covers essentially all macro features of concrete-like materials under high strain rate loading. However, the model was found to exhibit undesirable performance under certain loading conditions and some of the modeling issues have been discussed within a recent review paper by the authors. The present paper provides a more in-depth evaluation of the RHT model and proposes modifications to the model formulation to enhance the performance of the model as implemented in the hydrocode AUTODYN. The modifications include Lode-angle dependency of the residual strength surface, tensile softening law and the dynamic tensile strength function. The improvement of the performance of the modified RHT model is demonstrated using numerical sample tests, and further verified via simulations of two series of physical experiments of concrete penetration/perforation by steel projectiles. The results demonstrate an overall improvement of the simulation with the modified RHT model. In particular, the depth of penetration, projectile exit velocity and the crater size are predicted more favourably as compared to the test data. It is also shown that the modeling of the concrete tensile behaviour can affect sensibly the predicted perforation response (e.g., the projectile exit velocity), as is generally expected when the impact velocity exceeds the ballistic limit.     

钢纤维增强超高性能混凝土抗压性能的细观数值模拟

利用LS-DYNA软件在细观层次上建立了三维钢纤维增强超高性能混凝土（Steel fiber reinforced ultra-high performance concrete,SF/UHPC）圆柱体试件有限元模型,对其轴心受压下的力学性能和裂缝发展进行了数值模拟。在验证细观数值模型的有效性和合理性的基础上进行参数分析,着重研究了钢纤维体积率、钢纤维长径比、形状效应和尺寸效应对超高性能钢纤维混凝土抗压强度、韧性和破坏形态的影响。最终,根据模拟结果拟合了超高性能钢纤维混凝土抗压强度计算公式。结果表明：三维超高性能钢纤维混凝土细观模型可以较好地模拟单轴受压应力条件下混凝土的静力性能和损伤破坏机制,所拟合的公式也能较好地预测超高性能钢纤维混凝土的抗压强度。     

Effect of drying conditions on autogenous shrinkage in ultra-high performance concrete at early-age

This experimental study investigated the effects of drying conditions on the autogenous shrinkage of ultra-high performance concrete (UHPC) at early-ages. UHPC specimens were exposed to different temperatures, namely, 10, 20 and 40°C under a relative humidity (RH) ranging from 40 to 80%. The effects of using a shrinkage-reducing admixture (SRA) and a superabsorbent polymer (SAP) as shrinkage mitigation methods were also investigated. The results show that autogenous and drying shrinkage are dependent phenomena. Assuming the validity of the conventional superposition principle between drying and autogenous shrinkage led to overestimating the actual autogenous shrinkage under drying conditions; the level of overestimation increased with decreasing RH. Both SRA and SAP were very effective in reducing autogenous shrinkage under sealed conditions. However, SRA was efficient in reducing drying shrinkage under drying conditions, while SAP was found to increase drying shrinkage. Generally, results indicate that adequate curing is essential for reducing shrinkage in UHPC even when different shrinkage mitigation methods are applied.