Pere C. Prat

EFFECT OF TEMPERATURE AND HUMIDITY ON FRACTURE ENERGY OF CONCRETE

Fracture experiments were conducted at temperatures from 20 to 200 C (68 to 392 F) to determine the dependence of the Mode I fracture energy of concrete on temperature as well as the specific water content. The fracture energy values were determined by testing geometrically similar specimens of sizes in the ratio 1:2:4:8 and then applying Bazants size effect law. Three-point bend specimens and eccentric compression specimmns are found to yield approximately the same fracture energies, regardless of temperature. To describe the temperature dependence of fracture energy, a recently derived simple formula based on the activation energy theory (rate process theory) is used and verified by test results. The temperature effect is determined both for concrete predried in an oven and for wet (saturated) concrete. By interpolation, an approximate formula for the effect of moisture content on fracture energy is also obtained. This effect is found to be small at room temperature but large at temperatures close to 100 C (212 F).

New explicit microplane model for concrete: Theoretical aspects and numerical implementation

Abstract The microplane model is a powerful approach for the representation of the complex triaxial behavior of concrete and other similar materials. However, most efforts in previous formulations were devoted to the development of the model itself and to the experimental data fitting, rather than to a comprehensive theoretical description or to attainment of a modular and computationally efficient implementation in a computer code. In this paper, these objectives are pursued. The formulation of the model has been modified to rationalise the structure of the basic hypotheses, simplify the equations and generalise the concepts whenever possible. The result is a new formulation which, while retaining the favorable properties achieved previously, is also easier to understand, and convenient for computer implementation and large-scale calculations. A computational scheme is presented with the unified structure of a general code serving the double purpose of lest specimen analysis and finite element analysis. In practice, this structure includes two different main programs which call the same set of constitutive subroutines. A salient feature of the new version of the model is that the compulation of the stress corresponding to a prescribed strain increment of finite si/e is fully explicit. Step-by-step numerical integration, usually necessary for the practical use of constitutive models, can be avoided. Consequently, the complexity of the code and the cost of computations can be dramatically reduced. Some examples of applications, used to verify the previous version of the model, are also presented. They demonstrate that this new formulation gives a much better numerical efficiency for code implementation while keeping the same desirable features and accuracy in experimental data fitting.

Tangential stiffness of elastic materials with systems of growing or closing cracks

Although much has been learned about the elastic properties of solids with cracks, virtually all the work has been confined to the case when the cracks are stationary, that is, neither grow nor shorten during loading. In that case, the elastic moduli obtained are the secant moduli. The paper deals with the practically much more important but more difficult case of tangential moduli for incremental deformations of the material during which the cracks grow while remaining critical, or shorten. Several families of cracks of either uniform or random orientation, characterized by the crack density tensor, are considered. To simplify the solution, the condition of crack criticality, i.e. the equality of the energy release rate to the energy dissipation rate based on the fracture energy of the material, is imposed only globally for all the cracks in each family, rather than individually for each crack. Sayers and Kachanovs approximation of the elastic potential as a tensor polynomial that is quadratic in the macroscopic stress tensor and linear in the crack density tensor, with coefficients that are general nonlinear functions of the first invariant of the crack density tensor, is used. The values of these coefficients can be obtained by one of the well-known schemes for elastic moduli of composite materials, among which the differential scheme is found to give more realistic results for post-peak strain softening of the material than the self-consistent scheme. For a prescribed strain tensor increment, a system of N + 6 linear equations for the increments of the stress tensor and of the crack size for each of N crack families is derived. Iterations of each loading step are needed to determine whether the cracks in each family grow, shorten, or remain stationary. The computational results are qualitatively in good agreement with the stress-strain curves observed in the testing of concrete.

Image Analysis for the Quantification of a Developing Crack Network on a Drying Soil

The paper presents a methodology for quantifying surface crack patterns that appear in cohesive soils under drying conditions due to environmental changes, using image analysis techniques. This has practical applications in the study of many geotechnical problems related to soil cracking such as the impact of permeability changes due to cracking in clay barriers, development of preferential flow paths for contaminant transport along cracks, decreasing bearing capacity, and others. The study of soil cracking may become even more relevant with the current climate change that may induce more frequent and severe droughts in many parts of the world, increasing the areas at risk of cracking. Qualitative and quantitative characterization of the crack patterns is needed to study the mechanical behavior of a cracking soil, how cracks generate and propagate. For this purpose a simple laboratory set-up has been developed for continuous monitoring of the processes of formation and propagation of cracks due to desiccation, and to study the final crack pattern. The paper describes a simple technique to process sequences of images obtained during the laboratory tests, and how image analysis can be used to quantify the parameters that characterize the evolving and final crack patterns.

Measurement of mode III fracture energy of concrete

Abstract The test specimens are cylinders with a circumferential notch, loaded in torsion. Maximum loads of geometrically similar specimens of sizes 1 : 2 : 4 are measured and Baǎnts size effect method is used to determine from them the value of fracture energy for Mode III (antiplane shear). This value is found to be about three times larger than the Mode I fracture energy and about nine times smaller than the Mode II fracture energy measured before. These differences appear to be explicable by inclined tensile microcracks in the fracture process zone and a dependence of the fracture energy on the confining normal force across the fracture process zone.

Size effect tests of torsional failure of plain and reinforced concrete beams

The current design code formulae for the torsional failure of plain or longitudinally reinforced beams exhibit no size effect, i.e. the failure of geometrically similar beams of different sizes is supposed to occur at the same nominal stress. Experiments on reduced-scale beams were carried out, and the results confirm that there is a significant size effect, such that the nominal stress at failure decreases as the beam size increases. This is found for both plain and longitudinally reinforced beams. The results are consistent with the recently proposed Bažants size-effect law. However, the scatter of the results and the scope, and range limitations prevent it from being concluded that the applicability of this law is proven.

Size Effect in the Cracking of Drying Soil

Cracking in soils due to water loss is a problem not much studied from a mechanical point of view, despite its environmental implications. For instance, if a clayey soil is used as an impervious barrier in open waste sites, an intense drought may origin cracks and therefore preferential flow paths for polluted water. Cracks produced by environmental agents also reduce the bearing capacity of the soil and increases its propensity to erosion. Previous works have studied the problem either from a Fracture Mechanics perspective (Vallejo [1], Prat et al. [2], Avila [3], Harison et al. [4], [5], Hallet and Newson [6]), analysing the conditions for crack propagation, or from a classical Soil Mechanics approach (Kodikara et al. [7], Abu-Hejleh and Znidarcic [8], Konrad and Ayad [9], Morris et al. [10], Lloret et al. [11], using the effective stress principle. In this case it has been observed that cracks initiate when soil is still close to saturation.

Evaluation of safety factors in discontinuous rock

Abstract Safety factors for kinematically admissible failure mechanisms in jointed rock masses have been defined with linear and nonlinear failure criteria for rock discontinuities. Data required to compute these safety factors are obtained by means of two finite element anlayses of the effects of selfweight and external (structural) loading, respectively. Both types of analysis are closely linked since they share a common geometry. Joint elements are used to simulate the behaviour of rock discontinuities. If kinematically admissible mechanisms are possible under field conditions, the finite element mesh should also allow them to develop. Different aspects of the methodology have been illustrated through the safety evaluation of a 150 m high arch dam and its foundation in fractured cretaceous limestone. Special attention has been paid to the modelling of a realistic geometry including three-dimensional rock blocks and discontinuities. The paper discusses the effect of initial state of stress, the evolution of safety as the external load increases and the relation between the defined safety factors. It also provides practical guidelines for conducting this type of analysis in complex situations.

Origin and Mechanism of Cracks Seen at the Bottom of a Desiccating Soil Specimen

During a laboratory experimental campaign (related to the PhD thesis of the first author) on soil cracking due to drying, several unseen morphological observations were made. Cracks were observed at the bottom of the desiccating specimen apart from the usual cracks at the drying exposed surface. The cracks observed at the bottom are a particularly interesting pattern of fissures, which is different from the one observed at the top specimen. That complicates the analysis of crack development, and obviously has many consequences on the strength properties of the material. In this paper we present the results of the laboratory experiments carried out especially to explore the origins and mechanisms of propagation of the cracks observed at the bottom of the specimen. Those experiments were conducted with the same specimen size but terminating the test at different stages of drying and observing the presence of cracks at the bottom. The data of different sensors at different locations in the specimen for a number of tests at different drying stages, along with the observation from images taken during the tests, lead to the identification of the mechanism of crack initiation at the bottom.

Experimental analysis of 3D cracking in drying soils using ground-penetrating radar

This paper describes the capabilities of a novel technique to investigate crack formation and propagation in drying soils. The technique is a relatively simple, non-destructive indirect technique using a ground-penetrating-radar (GPR) system to detect cracks that form and propagate inside a soil specimen during desiccation. Although GPR devices have been used for multiple applications, their use in soils for the detection of small desiccation cracks has not been demonstrated yet. The experiment and the methodology used to test the accuracy of a small compact commercial GPR device for crack identification are described. The main objective was to identify what type of signals and what crack width and separation between them can be detected using the GPR device. The results indicate that cracks of 1 or 2 mm wide can be detected depending on its position and shape, whereas sub-millimetre cracks are undetectable with the currently existing devices in the market. Regardless of this limitation, the GPR method can be useful to find time-related bounds of when the cracks appear, to point at their location and sometimes at the separation between two of them.Detection of cracks with origin at the bottom or within the specimen was accomplished with this system. Distances of 5 cm or more between cracks can be detected and measured, as well, with accuracy.