Crack propagation and size effect in concrete using a non-local damage model

International audienceSize effect is a major issue in concrete structures. Experimental and numerical investigations on the influence of size effect on crack opening, crack length and crack propagation are presented in this paper. An isotropic non-local strain softening damage constitutive law is adopted for the numerical model. The material characteristic length is calibrated using an optimization algorithm and a post-processing method is chosen to obtain information on cracking. Numerical global and local results are compared with the experimental data and conclusions are drawn on the advantages and limitations of the adopted modelling strategy to reproduce size effect in concrete structures

Chemical shrinkage of low water to cement (w/c) ratio CEM I and CEM III cement pastes incorporating silica fume and filler

Chemical shrinkage (CS) is the reason behind early age cracking, a common problem for concrete with low water to cement ratios (w/c < 0.35) known as Ultra-High- and High-Performance Concrete (U-HPC). However, to avoid the crack development initiated by autogenous shrinkage, a precise measurement of CS is required, as the values obtained can determine the correct amount of internal curing agent to be added in the mixture to avoid crack formation. ASTM C1608 is the standardized method for performing CS tests. In this study, recommendations are provided to improve the reliability of results obtained with this standard method, such as good compaction of samples and the use of superplasticizer (SP) for low w/c ratios (≤0.2). Cement pastes with CEM I and CEM III have been tested at different w/c ratios equal to 0.2, 0.3 and 0.4 with and without the addition of superplasticizer. CS results following ASTM-C1608 dilatometry showed that the presence of mineral additions such as silica fume and filler reduced the chemical shrinkage, while CS increased with increasing w/c. Low w/c ratio pastes of CEM III had slightly higher CS rates than CEM I, while the opposite was noticed at higher w/c. SEM images illustrated the importance of a careful compaction and SP use

Measuring crack width and spacing in reinforced concrete members

International audienceCracking behavior of reinforced concrete is usually understood by the cracking of a concrete prism reinforced with a central bar subjected to tension. Bending, which is majority of the real cases, is dealt in the Eurocode by an empirical adjustment of the coefficients. In this paper an experimental program is devised to study the structural size effect of reinforced concrete members on crack width and spacing. Bending tests are performed on three different sizes of beams which are geometrically similar in two dimensions. The main reinforcement ratio is constant in all the beams keeping the same number of bars. The cover to the main reinforcement is also scaled with the beam size. Crack width and cracks spacing are measured using digital image correlation technique. Strain in the main reinforcement is measured using embedded electric strain gauges. It is found that Eurocode underestimates the crack width and crack spacing. Measured values of crack widths show an important structural size effect, which is not accounted by Eurocode crack width formula

Modelling of basic creep effect on concrete damage at a mesoscale level

In its service-life concrete is loaded and delayed strains appear due to creep phenomenon. Some theories suggest that micro-cracks nucleate and grow up when concrete is submitted to a high sustained loading, which contribute to make the concrete weaker. Thus, it is important to understand the interaction between the viscoelastic deformation and damage in order to design reliable civil engineering structures. Several creep-damage theoretical models have been proposed in the literature. However most of these models are based on empirical relations applied at the macroscopic scale. Coupling between creep and damage is mostly realised by adding some parameters to take into account the microstructure effects. In the author opinion, the microstructure effects can be modelled by taking into account the effective interactions between the concrete matrix and the inclusions. In this paper, a viscoelastic model is combined with an isotropic damage model. The material volume is modelled by a Digital Concrete model which takes into account the "real" aggregate size distribution of concrete. The results show that stresses are induced by strain incompatibilities between the matrix and aggregates at mesoscale under creep and lead to cracking

Exploration of the phase composition of activated calcined clay-slag mixtures using 27Al and 29Si MAS NMR spectroscopy

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Study of Evolution of Fracture Process Zone in Concrete by Simultaneous Application of Digital Image Correlation and Acoustic Emission

International audienceIn order to build sustainable structures, the study of mechanical behavior must integrate with local phenomena, e.g. fracture and strain localization. The fracture usually develops in the form of main crack, with branches, secondary cracks and the microcracking zone ahead. Various experimental methods are already employed to detect the fracture process. In this study, digital image correlation is used to measure displacement field in the cracking area. Using displacement field data, crack opening at various locations of crack is measured. The location of crack tip is estimated using crack opening data. Acoustic Emission technique is applied simultaneously with digital image correlation to identify the size of microcracking zone ahead of the propagating crack. It is observed that the two techniques in coupled position proved effective in identifying the fracture process zone and cracking mechanisms of concrete

Study of Evolution of Fracture Process Zone in Concrete by Simultaneous Application of Digital Image Correlation and Acoustic Emission

International audienceIn order to build sustainable structures, the study of mechanical behavior must integrate with local phenomena, e.g. fracture and strain localization. The fracture usually develops in the form of main crack, with branches, secondary cracks and the microcracking zone ahead. Various experimental methods are already employed to detect the fracture process. In this study, digital image correlation is used to measure displacement field in the cracking area. Using displacement field data, crack opening at various locations of crack is measured. The location of crack tip is estimated using crack opening data. Acoustic Emission technique is applied simultaneously with digital image correlation to identify the size of microcracking zone ahead of the propagating crack. It is observed that the two techniques in coupled position proved effective in identifying the fracture process zone and cracking mechanisms of concrete

Failure risk of recycled aggregates concrete

In the sustainable development context, the use of demolition waste increases in the building industry. Recycled aggregates from the demolition of concrete structures are then mixed in new concrete. Furthermore, the performance evaluation of a these recycled aggregate concrete (RAC) mix is not only limited to the determination of its conventional mechanical properties. The failure risk of concrete elements in structures made from RAC needs a comprehensive analysis. For that, a study on the fracture process of RAC has been performed and compared with that of normal concrete of the same structural class. Acoustic emission technique and digital correlation method have been used to follow the cracking process. Also, a new modelling approach for the fracture behaviour of RAC at the mesoscopic scale has been developed. It has taken into account the old attached mortar surrounding recycled aggregates. Results show that RAC has a more brittle behaviour than ordinary concrete and the numerical analysis shows that cracks growth through the recycled aggregates which have brittleness behaviour

Closed Crack Detection in Concrete with Coda Wave Non-Linear Modulation

International audienceConcrete is a widely used construction material by virtue of its cost and mechanical properties. Due to its low tensile strength however, concrete is very sensitive to crack formation. Cracks in concrete are responsible for significant inspection, maintenance and repair costs. In order to optimize structural health management, Non-Destructive Testing (NDT) has been extensively studied. Among all NDT techniques, ultrasonic methods are considered advantageous by providing information on mechanical properties in areas not directly accessible from the surface. Recent studies have led to developing nonlinear ultrasonic methods to increase the sensitivity to damage making possible the detection of large cracks/notches and the monitoring of crack evolution. However, the detection of small cracks in concrete remains a great challenge for NDT techniques. In this study, an ultrasonic method, based on nonlinear acoustic mixing of coda waves by lower-frequency swept pump waves, providing for an efficient global detection of small cracks in concrete is presented. By simultaneous comparison, for uncracked and cracked mortars, of ultrasonic velocity variations and decorrelation coefficient between the unperturbed and the perturbed signals for different pump amplitude, the method allows to accurately detect very small cracks with widths of around 20 mm correlated with velocity variations of approximately 0.01%. This method is reproducible and able to provide a simple means for differentiating damaged and sound concrete. Attention must be paid however to the material evolution during the time span of both a single experiment and the entire experimental campaign as a consequence of the presumed high sensitivity of the observables. Several applications of this technique could be developed in the field of civil engineering, although the power of the pump source would constitute a limitation. For example, the detection of small cracks causing leakage could be performed without any need for percolating fluid

LUCENA GÓMEZ, ANTONIO Y FAMILIA [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte, 201