Microcracks In Concrete Research Articles

A fractal model for simulating the formation of microcracks in the fracture process zone and a theoretical explanation of the size effect of the fracture energy of concrete

A fractal model is presented for simulating the formation of microcracks in the fracture process zone of concrete. The model is used to calculate the fractal dimension D of a network of microcracks. It is found that the branching angle of microcracks can greatly affect the value of D, and a lower value of D can be expected for high-strength concrete than for normal-strength concrete. In the light of fractal theory, a formula for calculating the total area of the fracture surface. including the area of microcracks, is proposed. The size effect of the fracture energy of cement-based materials is explained and the quantitative relationship between the fracture energy Gf, the surface energy of the concrete γs, the fractal dimension of the network of microcracks Dc, and the height of the ligament of the concrete specimen H is established as Gf = 2γsC″HDc−1 (where C″ is a constant). Using this formula. the fra ctal dimension Dc, of the network of microcracks in high-strength and normal-strength concrete was determined experimentally.

Ultrasound tomography applied to AAR-affected concrete structures : Thomas, M.D.A. Proceedings of the 2nd International Conference on Alkali-Aggregate Reactions in Hydroelectric Plants and Dams, Chattanooga, TN (United States), 22–27 Oct. 1995, pp. 359–372. Acres International Corporation (1995)

This paper discusses the potential application of ultrasound tomography and scientific visualization for evaluating the internal condition of massive concrete structures affected by AAR and other deterioration processes. Ultrasound measurements have been used in the evaluation of concrete structures for many decades. Traditional transmission ultrasound measures the average velocity of stress waves between two points on a concrete structure - the velocity being related to concrete quality - but provides no information on the distribution of velocity (and hence quality) between those two points. However, if sufficient measurements are made to form a network of velocities across a single cross-section, tomographic methods can be applied to produce a two-dimensional image of the spatial distribution of velocity data within the section. Results are presented from a preliminary study carried out to assess the potential application of this technique for evaluating the internal condition of large concrete elements. Overall, the initial results are promising, indicating that the presence and location of internal defects can be determined and visualized using this technique. Features of the transmitted wave, other than velocity, are currently being examined to assess their sensitivity to internal micro-cracking of concrete. The frequency content of the received wave may be amore » more sensitive indicator of AAR microcracking and the potential application of spectral analysis in tomographic surveys is discussed.« less

State of the art report—Part 2: Mechanical micro-cracking of concrete

State of the art report—Part 2: Mechanical micro-cracking of concrete

Propagation of microcracks in concrete under uniaxial compression

This Paper reports the findings of a series of short-term sustained loading tests on concrete cylinders in which the extent of microcracking in concrete was continuously monitored using a novel non-destructive method. The approximate crack areas within the cross-sections of these concrete cylinders were estimated from the measured transverse and axial strain readings obtained with electrical resistance strain gauges attached to the surface of the cylinders. The experimental results showed that microcracks were initiated at a stress-strength ratio ranging between 0·15 and 0·44 (rather than at a specific value), but they did not propagate under sustained loads unless the stress–strength ratio exceeded a threshold value of about 0·5. Beyond this threshold level, the rate of microcrack propagation generally increased with increasing stress but, up to a certain stress–strength ratio (ranging between 0·8 and 0·9), decreased with time during sustained loading. It was also observed that this particular stress–strength ratio had the same range of values as the stress at which the volume of concrete became a minimum. Beyond this critical stress–strength ratio, the rate of microcracking increased rapidly with stress and increased with time under sustained loading, indicating that failure of concrete was imminent.

The Characterisation of Microcracking of Concrete at Uniaxial Compression

The constitutive stress-strain behaviour a body of concrete material at monotonic uniaxial compression is not linearly elastic up to the instant of ultimate failure. The non-linear stress-strain responses imply that the concrete material experiences slow progressive microcrack growth before the instant of ultimate catastrophic failure. This paper outlines the application of the continuum micro-fracture mechanics of the strain energy density and the isoenergy density theories to characterise the primordial progressive microcrack growth process within a body of concrete material at global monotonic uniaxial loading in compression to ultimate failure. The body of heterogeneous concrete material is modelled as a continuum comprising a very large assembly of unit-volume isoenergy elements, each of which contains micro-structural material which is linearly elastic, homogeneous and isotropic. The micro-fracture of an isoenergy element is considered as the propagation of an infinitesimal. microcrack.

Localization of Microcracking in Concrete Under Uniaxial Tension

Localization of Microcracking in Concrete Under Uniaxial Tension

Microstructure of Steam Cured Concretes Deteriorated by Alkali-Silica Reaction

AbstractIn order to know the main cause of the cracking of concrete ties we tried to reproduce the deterioration process in the laboratory, using the same thermal cycle and the same materials. Model concretes were first steam cured then examined for ASR using the CSA-A-23-2-14A accelerated test. Linear expansion of concrete prisms were measured and fracture surfaces of concrete after treatment were observed under scanning electron microscope. They showed ASR gels and secondary ettringite. A simultaneous thermal mechanical computation gave the global microcracking of concrete induced by steam curing. A second computation showed the accelerating role of temperature on the local development of ASR products.

Ion diffusion in microcracked concrete

The influence of concrete microcracking state on transport by diffusion was studied for three different species: 3H, 137Cs and Cl −. No influence has appeared for tritium and cesium. A correlation was found between the transfer of chlorides and the specific microcracking surface.

RATE PROCESS ANALYSIS OF ACOUSTIC EMISSION ACTIVITY IN CORE TEST OF CONCRETE

AE activity under uniaxial compression is closely related with the existence of microcracks in concrete. When concrete contains a number of critical microcracks, AE events occur actively from low stress level. In contrast, AE activity in sound concrete is observed just prior to final failure. The discrepancy of AE generating behavior is quantitatively evaluated, introducing the rate process theory. To gauge an applicability of AE rate process analysis to the uniaxial compression tests of core samples, experiments on newly cast concrete and core samples are carried out. The core samples of controlled damage were taken from reinforced concrete slabs which were tested by cyclic fatigue loading. Thus, the correlation with the strength, the age, and the damage level of concrete is clarified.

Microcracking and tension-softening properties of concrete

Tests are reported on the microcracking properties of concrete studied by means of a three-dimensional acoustic emission technique. Observed microcracking properties are discussed in connection with the phase of the fracture process zone of concrete. It is shown that the fracture process zone is already created before the peak load and that the fracture process zone expands widely after the peak load. Moreover, it is revealed that the length of the fracture process zone ahead of the notch tip seems to be independent of the heterogeneity but the width is obviously influenced by the aggregate size. The shape of the tension softening diagram determined by a numerical analysis is discussed in comparison with the acoustic emission behaviour.

Influence of water-cement ratio on micro-cracking of ordinary concrete

The influence of the water/cement ratio on fracture toughness of ordinary concrete has been investigated. The stress intensity factorKIIc and fracture energyJIIc has been tested (Mode II, shearing). The concrete structure was examined by SEM and the influence of water/cement ratio on concrete cracking has been established.

A semi-empirical analysis of micro-cracking in concrete

This paper presents the results of an analysis which considers the interaction between a crack and an array of microcracks. Complex variable techniques are used to derive explicit expressions for the stress intensity factors and energy release rates which arise from the interaction beween a finite length crack and a dislocation dipole. Results are computed and discussed for a variety of geometrical configurations, with the intent of developing an understanding of the effects of position and orientation of the dislocation dipole on crack tip shielding and antishielding. The Green's functions presented in this paper can be employed in semi-empirical analyses, where the effects of micocracking are quantified with the aid of experimentally measured crack densities and openings together with integral representations for the toughening.

The unilateral behaviour of damaged concrete

Abstract This paper addresses the effect of microcracking of concrete on its elastic response under cyclic loading. From the experimental point of view, the unilateral behaviour of damaged concrete is shown to be related to the change of the sign of the load. The experimental results of two particular tests (one uniaxial and the second multiaxial) are analysed here. In the first, a compressive load was applied on the specimen after development of diffused tensile damage. We pointed out the influence of previously-created microcracks on the response of damaged concrete. In the second test, a concrete beam was submitted to a low cycle fatigue test in order to analyse the global stiffness evolution. From the modelling point of view, we consider that a scalar damage variable must be decomposed into two terms related to a change of the sign of the stress tensor. The proposed evolution law and threshold are briefly discussed. Numerical results obtained with this model are given for the beam tests.

The unilateral behaviour of damaged concrete

This paper addresses the effect of microcracking of concrete on its elastic response under cyclic loading. From the experimental point of view, the unilateral behaviour of damaged concrete is shown to be related to the change of the sign of the load. The experimental results of two particular tests (one uniaxial and the second multiaxial) are analysed here. In the first, a compressive load was applied on the specimen after development of diffused tensile damage. We pointed out the influence of previously-created microcracks on the response of damaged concrete. In the second test, a concrete beam was submitted to a low cycle fatigue test in order to analyse the global stiffness evolution. From the modelling point of view, we consider that a scalar damage variable must be decomposed into two terms related to a change of the sign of the stress tensor. The proposed evolution law and threshold are briefly discussed. Numerical results obtained with this model are given for the beam tests.

A holographic real time study of crack propagation in concrete

In this paper the fracture process of concrete is studied on a microscopic level during the bending of notched specimens. A holographic technique called TV-holography is used. This allows real time observation of concrete microcracking, from the first loading until final rupture. The smallest observable crackwidth size is about 0.1 μm. It was found that in this kind of loading, the onset of cracking takes place at about 80% of ultimate load, and that the main crack is not continuous. Further, coarse aggregate particles seemed to arrest propagating cracks and cause crack branching.

Microcracking of High and Normal Strength Concretes Under Short and Long-Term Loadings

The effects of both monotonic and sustained uniaxial compressive stresses on the relationships among microcracking, strength, and deformation of high-strength concrete were studied using x-ray techniques and compared with those of low and medium strength comcretes. The formation, propagation, and stability of microcracks and their contribution to the failure of concrete under both types of loadings are examined and discussed. The stress levels investigated ranged from 40 to 95% of ultimate for the three materials. High-strength concrete showed significntly smaller amounts of cracking at all stress levels than normal strength concretes. As a result, lesser inelastic deformations and higher creep-stress linearity limits were associated with the former rather than with the others.

Modification of the X-Radiography Technique to Include a Contrast Agent for Identifying and Studying Microcracking in Concrete

Abstract The utility of X-radiography for the study of microcracking in concrete is significantly enhanced by impregnating cracks and voids with a lead-nitrate contrast agent. When an X-ray flux is passed through a polished concrete specimen, X-rays are readily attenuated by the contrast agent in the cracks, which produces a sharp tonal contrast in the resulting radiograph between cracks and surrounding concrete. Radiographs obtained using this technique are compared with more conventional X-ray images of the same specimens. This method is similar in concept to neutron radiography, in which a neutron-attenuating contrast agent is used for enhancing crack images.

Characteristics of microcracking in concrete under static and repeated tensile loading

The type and amount of microcracking in normal weight concrete were determined by the direct observation of the slices cut from the necked prismatic specimens subjected to static and repeated tensile loads. It was found from the results of observation under static tensile loading that bond cracks increased almost linearly with increasing load and that mortar cracks developed extensively at about 100% of the ultimate load. The results under repeated tensile loading indicated that the repetitive nature of loading merely conduced to a slight increase in the total amount of internal microcracking. The relations between these findings and two features of the tensile fatigue previously reported were discussed.

The Application of Neutron Radiography to the Study of Microcracking in Concrete

Abstract Radiography of concrete specimens using a neutron flux as the penetrating radiation has been used to identify and study microcracking. The key to the technique is the partial impregnation of the microcracks with a medium with a high neutron attenuation capacity, so as to result in a sharp contrast between the cracks and the surrounding solid mass. Gadolinium has been used as the neutron-attenuating contrast agent, which is applied in the form of an aqueous solution of gadolinium nitrate. The resulting radiographs are compared with more conventional X-ray studies of the same specimens. Neutron radiography appears to be more effective than X-radiography for the identification of cracking.

A non destructive procedure to observe the microcracks of concrete by scanning electron microscopy

The study of microcracks in concrete is very important to understand its behaviour under loading and to improve this material. The direct observation in a scanning microscope is not a suitable method because the concrete is subject to drying shrinkage. Another indirect and non destructive method of observation by SEM is proposed. With this method using a single replica the microcracks of concrete can be observed better than 0.5 μm.