Mechanical behavior of concretes damaged by alkali-silica reaction

Alkali-silica reaction (ASR) can induce the premature distress and loss in serviceability of concrete structures. The internal crack pattern produced by ASR affects both transport and mechanical properties. Usually linear expansions are considered as indicative of the grade of damage into the material (internal crack pattern), nevertheless as diverse types of ASR have been recognized (rapid or slow reactive aggregates, fine or coarse aggregates) the effects on strength and rheological properties could be different for a same expansion. This paper compares the mechanical response of a reference concrete (without reactive aggregates) and concretes prepared with three different types of reactive aggregates, with the same mixture proportions. The first concrete incorporated 10% of a highly reactive siliceous orthoquartzite as a part of the coarse aggregate, the second included a highly reactive sand, and the third prepared with a slow reactive granitic migmatite as coarse aggregate. Concretes were moist cured at 38 °C. When linear expansions ranging between 0.11 and 0.18% took place, the stress strain behavior in compression and the load-displacement response in flexure were measured. The same tests were performed on reference concrete at different ages, between 75 and 745 days. Microscopic observations were performed on polished and thin sections in order to analyze concrete microstructure. It appears that the failure mechanism of concrete in compression is clearly affected by ASR, the shape of the stress-strain curves reflects the presence of internal fissures, showing that the capability of controlling crack propagation decreases. Differences in the crack pattern are also reflected in the shape of the load-deflection curves in tension, damaged concretes show an increased non-linearity and a more gradual softening. However, it was found that the modifications in the mechanical properties cannot be directly associated with a level of expansion, as the behavior depends on the component materials and mechanisms involved in the reaction.     

Failure probability prediction of concrete components

In order to predict the probability of failure for brittle fracture of concrete components under multiaxial stress states, the imperfections of concrete components are modeled as cracks with different shapes in this paper. A new probability distribution function for evaluating the failure probability of concrete components is proposed. A simplified measurement method for determining the parameters of the governing Weibull distribution, using the three-point bending test, is presented and discussed. The experimental results of the combined bending/torsion failure tests of concrete components verify that the proposed crack model is more reasonable than the Batdorf's crack model and the proposed prediction formula can evaluate the failure probability of concrete components accurately.     

The determination of the mode II adhesive fracture resistance, GIIC, of structural adhesive joints: an effective crack length approach

This paper reports results from the mode II testing of adhesively-bonded carbon-fibre-reinforced composite substrates using the end-loaded split (ELS) method. Two toughened, structural epoxy adhesives were employed (a general purpose grade epoxy-paste adhesive, and an aerospace grade epoxy-film adhesive). Linear Elastic Fracture Mechanics was employed to determine values of the mode II adhesive fracture energy, G_IIC for the joints via various forms of corrected beam theory. The concept of an effective crack length is invoked and this is then used to calculate values of G_IIC. The corrected beam theory analyses worked consistently for the joints bonded with the epoxy-paste adhesive, but discrepancies were encountered when analysing the results of joints bonded with the epoxy-film adhesive. During these experiments, a microcracked region ahead of the main crack was observed, which led to difficulties in defining the true crack length. The effective crack length approach provides an insight into the likely errors encountered when attempting to measure mode II crack growth experimentally.     

Permeability of concrete under stress

Permeability tests were carried out on concrete specimens subjected to a compressive stress. Special emphasis was placed on understanding the influence of stress application on the permeability on concrete at early ages (1-3 days). It was found that the presence of a compressive stress below a certain threshold value decreased the permeability, but when the applied stress exceeded this threshold, a significant increase in the permeability occurred. Permeability increases due to an applied stress also appear to depend upon the overall stress history.     

Microcracking in electron beam deposited scandia-stabilised zirconia electrolyte

It is the aim of the present work to address some of the aspects of microcracking in electron beam deposited scandia-stabilised zirconia electrolyte applied for solid oxide fuel cells (SOFC) where a thin electrolyte layer is deposited on a relatively thick anode substrate. A model of microcracking for the electrolyte material is proposed which takes into account the statistical distribution of grain sizes, the stress redistribution due to failure of individual structural elements as well as the local criterion of grain fracture. The combination of electron microscopy research with model calculations permits both the specific energy of new surface creation in the electrolyte and critical parameters of the microcracking process to be determined. The annealing-induced electrolyte microcracking discussed in this work corresponds to localised microcracking, where each next structural element fails mainly at an existing microcrack tip. The features of localised microcracking in electron beam deposited scandia-stabilised zirconia electrolyte are analysed.     

Experimental characterization of the self-healing of cracks in an ultra high performance cementitious material: Mechanical tests and acoustic emission analysis

Self-healing of cracks in an ultra high performance concrete, considered as a model material, is investigated in this paper. An experimental program is carried out in order to quantify the phenomenon, which has been mainly highlighted by means of water permeability tests until now. Mechanical behaviour of self-healed concrete under three points bending, and acoustic emission analysis of the cracking mechanisms are reported. The mechanical tests demonstrate a recovery of the global stiffness, depending on the time of healing, for specimens initially cracked and then self-healed, and a slow improvement of structural strength. The acoustic emission (AE) analysis is performed in order to show that the mechanical response is due to new crystals precipitating in the crack. The microcracking of these products during three points bending tests is highlighted and an energy analysis provides insights about the cracking process of healed concrete, including damage of the newly formed crystals and continuation of the crack propagation.     

Quantifying the evolution of static elastic properties as crystalline rock approaches failure

The pervasive damage of rock by dilational microcracks strongly influences rock strength and macroscopic elastic properties. In this study, two types of experiment were performed to investigate and quantify the contribution of microcracking to the static elastic response of Westerly granite as it approaches failure. They are (1) increasing-amplitude cyclic loading experiments and (2) constant-amplitude cyclic loading experiments, some of which incorporated a ‘load-hold’ component to explore the elastic response of time-dependent effects such as stress corrosion. We report values for the tangent moduli in the region of the stress–strain curve where the values are the same for an unloading cycle as they are for the subsequent loading cycle. Hence, although there is hysteresis between these two curves, we assume perfect elasticity in this region. This approach allows the evolution of static elastic properties as rock approaches failure to be documented in great detail, in contrast to previous work that has generally been limited to the linear elastic region of the stress–strain curve. Increasing-amplitude stress cycling causes a gradual reduction in sample stiffness, equating to a decrease in Young's modulus (11%) and an increase in Poisson's ratio (43%) measured at a constant stress level. Elastic properties are also seen to have a strong stress-dependency during loading (46% increase in Young's modulus from 20 to 100 MPa). Experiments devised to promote time-dependent microcracking had a negligible contribution to the evolution of static elastic properties over the timeframe and dry conditions under which our experiments were conducted. Such results can be applied to our understanding of the mechanics, stress distribution and fault displacement models within and surrounding fault zones.