Fracture Toughness Of Concrete Research Articles

The discrete element method simulation and experimental study of determining the mode I stress-intensity factor

The present study addresses the direct and indirect methods of determining the mode-I fracture toughness of concrete using experimental tests and particle flow code. The direct method used is compaction tensile test and the indirect methods are notched Brazilian disc test, semi-circular bend specimen test, and hollow center cracked disc. The experiments were carried out to determine which indirect method yields the fracture toughness closer to the one obtained by the direct method. In the numerical analysis, the PFC model was first calibrated with respect to the data obtained from the Brazilian laboratory test. The crack paths observed in the simulated tests were in reasonable accordance with experimental results. The discrete element simulations demonstrated that the macro fractures in the models are caused by microscopic tensile breakages on large numbers of bonded particles. The mode-I fracture toughness in the direct tensile test was smaller than the indirect testing results. The fracture toughness obtained from the SCB test was closer to the direct test results. Hence, the semi-circular bend test is recommended as a proper experiment for determination of mode-I fracture toughness of concrete in the absence of direct tests.

Determination of fracture toughness parameters of concrete using compact pressure test

Fractures start with the formation of cracks and occur as the cracks spread in building materials. This causes significant damage to the buildings. Fracture mechanics is a science that investigates crack behavior, crack analysis and what to do for prevention of cracks. In the present study, concrete fracture toughness parameters were investigated by compact pressure test. In this context, research and a series of experiments were conducted on fractures, types of fractures, fracture mechanics, and compact pressure test. In the present empirical study, 5 unnotched and 15 notched, a total of 20 samples were used. 40 mm notches were created on notched samples. Cube splitting experiments were conducted using 10 mm wide strips on unnotched samples. For Type-I, Type-II and Type-III that were used in the calculation of the fracture toughness parameters of the said samples, fracture loads, tensile values and fractured sample details are presented. Since the concrete is a semi-brittle material, compact pressure samples were analyzed according to the principles of linear elastic fracture mechanics. It was concluded that the standard deviation for the critical fracture parameter (KIc) of the sample, which was naturally more stable than the present study, was smaller and as the structure size increased, the fracture toughness value increased as well.

Generalized Fracture Toughness and Compressive Strength of Sustainable Concrete Including Low Calcium Fly Ash.

The paper presents the results of tests on the effect of the low calcium fly ash (LCFA) addition, in the amounts of: 0% (LCFA-00), 20% (LCFA-20) and 30% (LCFA-30) by weight of cement, on fracture processes in structural concretes. In the course of the experiments, compressive strength of concrete and fracture toughness for: I (tensile), II (in-plane shear) and III (anti-plane shear) models of cracking were measured. The tests determined the effect of age of concretes modified with LCFA on the analyzed parameters. The experiments were carried out after: 3, 7, 28, 90, 180 and 365 days of curing. Fracture toughness of concretes was determined in terms of the critical stress intensity factors: , , and then a generalized fracture toughness was specified. The obtained results are significant for the analysis of concrete structures subjected to complex loading. The properties of composites with the additive of LCFA depend on the age of the concrete tested. Mature concretes exhibit high fracture toughness at 20% additive of LCFA, while the additive of LCFA in the amount of 30% weight of cement has a beneficial effect on the parameters of concrete only after half a year of curing.

Effect of curing time on the fracture toughness of fly ash concrete composites

This paper presents the results of an experimental investigation carried out to evaluate the compressive strength and fracture toughness of concrete mixtures in which main binder (Ordinary Portland Cement – OPC) was partially replaced with Class F fly ash (FA). OPC was replaced with two percentages (20% and 30%) of FA by weight. Compressive strength and fracture toughness under mode III – KIIIc (torsional loading), were determined at: 3, 7, 28, 90, 180 and 365 days. Test results indicate significant improvement in the strength properties and fracture toughness of mature concrete, by the inclusion of 20% FA as partial replacement of OPC. On the other hand, the additive of FA in the amount of 30% weight of OPC has a beneficial effect on the mechanical parameters of concrete only after half a year ofcuring. The obtained results are significant in the analysis of concrete structures subjected to complex loading, or structures where torsional moment is the basic load.

Wedge-splitting tests for tensile strength and fracture toughness of concrete

This study presents a new approach for determination of tensile strength ft and fracture toughness KIC of concrete using wedge-splitting (WS) specimens as neither property can be easily measured because of the heterogeneous material structures. The Boundary Effect Model (BEM) was extended to WS geometry and relevant equations were derived so that more concrete properties can be determined by the load and displacement measurements. The new model considers the influence of coarse aggregate structures on concrete fracture by linking the fictitious crack growth Δafic at peak load Pmax to the maximum aggregate size dmax. The random aggregate distribution and inevitable variations in Δafic and Pmax are considered by a discrete number β. WS results of concrete specimens with dmax from 10mm to 25mm and specimen size W from 150mm to 1200mm reported in literature are analyzed for a range of β from around 0.2 to 2.0. It is found that on average β≈1 is a good approximation for common WS specimens with W/dmax<20. Then reliable ft and KIC, which is the asymptotic limit for a large structure with a long crack, can be obtained, regardless whether WS specimens are geometrically similar, or have the same size but different notch lengths. Finally, it is concluded that quasi-brittle fracture of concrete is fully predicted if both KIC and ft are known.

Photonics and fracture toughness of heterogeneous composite materials

Fracture toughness measures the resistance of a material to fracture. This fundamental property is used in diverse engineering designs including mechanical, civil, materials, electronics and chemical engineering applications. In spite of the advancements made in the past 40 years, the evaluation of this remains challenging for extremely heterogeneous materials such as composite concretes. By taking advantage of the optical properties of a thin birefringent coating on the surface of opaque, notched composite concrete beams, here we sense the evolution of the maximum shear stress distribution on the beams under loading. The location of the maximum deviator stress is tracked ahead of the crack tip on the experimental concrete samples under the ultimate load, and hence the effective crack length is characterised. Using this, the fracture toughness of a number of heterogeneous composite beams is evaluated and the results compare favourably well with other conventional methods using combined experimental and numerical/analytical approaches. Finally a new model, correlating the optically measured shear stress concentration factor and flexural strength with the fracture toughness of concretes is proposed. The current photonics-based study could be vital in evaluating the fracture toughness of even opaque and complex heterogeneous materials more effectively in future.

Mode I fracture toughness of fibre reinforced concrete

Low fracture toughness of concrete represents a serious shortcoming. An effective way to improve the concrete toughness is represented by the dispersion (during mixing) of discontinuous fibres into the concrete mix. The principal beneficial effect of fibres is the crack bridging in the cementitious matrix, providing resistance to crack propagation before fibre debonding and/or pulling out or failure. In the present paper, the fracture behaviour of FRC (fibre reinforced concrete) specimens is examined, with micro-synthetic polypropylene fibrillated fibres being randomly distributed in concrete. The modified two-parameter model, proposed by the authors to calculate Mode I plain-strain fracture toughness for quasi-brittle material, is able to take into account the possible crack deflection (kinked crack) during stable crack propagation.

Quantification of thermally induced damage and its effect on dynamic fracture toughness of two mortars

Concrete structures have a risk of simultaneous exposure to high temperature and blasting or impact load. Influences of thermally induced damage on the dynamic fracture toughness of concretes are thus significant to the safety design of concrete structures. We study the effect of thermally induced damage on the dynamic fracture toughness of two mortars. The notched semi-circular bend (NSCB) specimens were heat-treated at 150°C, 250°C, 350°C, 450°C and 600°C, and then tested at room temperature of 25°C with a split Hopkinson pressure bar (SHPB) apparatus. Before the dynamic fracture tests, X-ray Computer Tomography (CT) scanning was utilized to quantify the thermally induced micro-cracks and chemical changes of two mortars in terms of the CT value. In addition, densities and P-wave velocities of two mortars were also measured. The CT value, density and P-wave velocity decrease with the increase of the heat-treatment temperature, which can be attributed to the evolution of micro-structures due to the undesirable chemical reactions and the increase of micro-cracks. The results of dynamic tests show that the dynamic fracture toughness increases with the loading rates and deceases with the heat-treatment temperatures. The thermally induced damage is a combination of (a) the thermally induced micro-cracks and various chemical changes (which is characterized by the CT value), and (b) the deterioration of the binding property of reaction products. Thus the damage variable is introduced to represent the thermally induced damage for two mortars. A formula for describing the dependence of the dynamic fracture toughness on the loading rate effect and the thermal effect is developed by using the damage variable. The formula can predict the trend of the fracture toughness of two mortars well.

Effect of tensile strength of rock on tensile fracture toughness using experimental test and PFC2D simulation

The effect of tensile strength on the tensile fracture toughness of rock like specimen was studied in this paper. Brazilian test was done to determine tensile strength of material. A compression to tensile load transforming (CTT) device was developed for determination of mode I fracture toughness of concrete. Also particle flow code (PFC) was used for validation of the experimental outputs. Three concrete slabs with different tensile strength were prepared for investigation of the effects of tensile strength on the fracture toughness. The samples were made from a mixture of water, fine sand and cement with different ratio. These samples were installed in CTT device. A 30-tons hydraulic load cell applied compressive loading to CTT end plates with a constant pressure of 0.02 MPa per second. Compressive loading was converted to tensile stress on the sample because of the overall test design. The results show Fracture toughness has a close relationship with tensile strength of concrete so it increases with increasing the tensile strength. In constant join length, the angle of crack growth related to normal load was decreased with increasing the grain size. Numerical simulation shows that failure pattern and fracture toughness was nearly similar to experimental results. Finally, it can be concluded that CTT device was capable for determination of fracture toughness of concrete.

Effect of basalt fibers on fracture energy and mechanical properties of HSC

Fracture energy is one of the key parameters reveal cracking resistance and fracture toughness of concrete. The main purpose of this study is to determine fracture behavior, mechanical properties and microstructural analysis of high strength basalt fiber reinforced concrete (HSFRC). For this purpose, three-point bending tests were performed on notched beams produced using HSFRCs with 12 mm and 24mm fiber length and 1, 2 and <TEX>$3kg/m^3$</TEX> fiber content in order to determine the value of fracture energy. Fracture energies of the notched beam specimens were calculated by analyzing load versus crack mouth opining displacement curves by the help of RILEM proposal. The results show that the effects of basalt fiber content and fiber length on fracture energy are very significant. The splitting tensile and flexural strength of HSFRC increased with increasing fiber content whereas a slight drop in flexural strength was observed for the mixture with 24mm fiber length and <TEX>$3kg/m^3$</TEX> fiber content. On the other hand, there was no significant effect of fiber addition on the compressive strength and modulus of elasticity of the mixtures. In addition, microstructural analysis of the three components; cement paste, aggregate and basalt fiber were performed based on the Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy examinations.

Determination of tensile strength and fracture toughness of concrete using notched 3-p-b specimens

Maximum loads of notched three-point-bend specimens with constant height are used to determine the tensile strength and fracture toughness of concrete. Normally the tensile strength and fracture toughness cannot be obtained from small three-point-bend specimens because of the highly heterogeneous composite structures of concrete and formation of fictitious crack prior to the maximum load. A simple fracture model is established, linking fictitious crack to the maximum aggregate size so that the concrete structure and fictitious crack formation can be considered together. Maximum loads of three-point-bend specimens with different initial notches containing different influence of the tensile strength and fracture toughness are then analyzied.

Mode I fracture toughness of fibre-reinforced concrete by means of a modified version of the two-parameter model

The present paper proposes a method to calculate Mode I plane-strain fracture toughness of concrete, by taking into account the possible crack deflection (kinked crack), even in the case of a far-field Mode I loading. As a matter of fact, during fracture extension, cracks may deflect as a result of microstructural inhomogeneities inside the material. Concrete is an inhomogeneous mixture due to aggregates embedded in the cementitious matrix, but additional inhomogeneities may be represented by fibres. Firstly, a two-parameter fracture model based on Mode I analytical expressions of the linear elastic fracture mechanics is employed. Then, in order to take into account the possible crack deflection as a result of the above inhomogeneities, a modified version of such a model is here discussed. Three-point bending tests on both plain concrete specimens and concrete specimens reinforced with micro-synthetic polypropylene fibrillated fibres are experimentally performed, and the modified model is applied.

Experimental validation and fracture properties analysis on wedge splitting concrete specimens with different initial seam-height ratios

In order to study the fracture properties of non-standard concrete wedge splitting tensile specimens with different initial seam-height ratios and the impact on concrete fracture toughness based on aggregates cohesive force, five groups non-standard concrete wedge splitting specimens with different initial seam height ratios as 0.2, 0.25, 0.3, 0.35 and 0.4 were designed and constructed for the experiment. Based on double-K fracture model and fictitious crack model, a new fracture toughness calculation model for non-standard concrete wedge splitting tensile specimen with different initial seam height ratios was built. The results showed that: the value of initial cracking load Pini and unstable fracture load Pun decreased gradually with the increase of specimens’ initial seam height ratio. With the increase of seam height ratios, the value of the critical effective crack length ac showed a linearly increasing trend, but the subcritical crack propagation length Δac showed a decrease trend. The unstable fracture toughness KunIC and the cohesive toughness KcIC were affected by seam height ratios, decreased with initial seam height ratios increasing, and the initial fracture toughness KiniIC was independent of it that could be treated as material constant. When calculating the initial fracture toughness KiniIC, the model in this paper could eliminate the errors caused at the moment of determining initial cracking load Pini, which proved its advantage. For the specimens with smaller initial seam height ratio, its fracture features was influenced by maximum tension stress and showed some shearing failure.

Effect of T-stress on the mode-I fracture toughness of concrete

T-stress expressions are provided for three-point bending (TPB) beams and compact tension (CT) specimens and then its influence on mode I fracture toughness of concrete is investigated. The study shows that T-stress is dependent on the specimen's geometry and the material's property as well, and for TPB and CT specimens of regular size, T-stress is so small that its consequences can be neglected. The study also indicates that concrete specimen size should be carefully chosen to make sure the existence of K-dominance ahead of the crack tip, thus fracture toughness extracted from these specimen configurations can be reliable.

Specimen size effects on mode II fracture toughness of concrete

The size of concrete specimen is the main reason affecting the shear fracture toughness, so this research carried out five groups of experiments including 40 symmetrically loading specimens with different lengths and heights. Through P-CTSD (loading and crack tip sliding displacement curve), P–ϵ (loading and strain curve) and P–t (loading and time curve), the effects of length and height of specimens on shear fracture toughness were studied. Stability of specimen is strengthened by increasing length and weakened by increasing height. Size effect on fracture toughness is weakened by increasing of length and strengthened by the increasing of height. Fracture toughness will increase with the increasing of length and decrease with the increasing of height.

Numerical Simulation of Influence of Aggregate-Mortar Interface on Fracture Toughness of Concrete

To explore the feasibility of the approach of "interfacial toughening in concrete", the 2D concrete model which possesses the characteristic of random distribution of spherical aggregates was used to simulate the relations between interface material and toughness of concrete containing flexible interfacial material. The influence of interfacial material on the toughness of concrete was calculated with elastic modulus, thickness and the displacement at peak load for variables. And the results show that: The peak width of the concrete's load-displacement curve is increased evidently with the ultimate displacement also improved evidently upon replacing the normal ITZ with flexible interface phase material; the concrete's peak load, the displacement at peak load and the fracture energy increase gradually with the increase of the elastic modulus of interface phase material. When the elastic modulus exceeds 50MPa, the concrete's peak load will exceed that of normal concrete promptly with the displacement at peak load and the fracture energy all reaching the maximal value; With the gradual increase of the thickness of interface phase material, the concrete's peak load declines gradually while the displacement at peak load increases gradually; The concrete's fracture energy and ductility coefficient approximately increase first and then decline with the change of thickness. When the thickness is 0.2mm, the concrete's fracture energy and ductility coefficient reach the optimal value in setting scope. With the gradual increase of the interface phase material's displacement at peak load, the concretes peak load increases gradually while its fracture energy and ductility coefficient increase first and then decline. When the displacement at peak load is 0.05, the two reach the optimal value; when the displacement at peak load exceeds 0.05, the cracks will extend from the aggregates-mortar base debonding to the mortar matrix. The results have certain exploration value for developing concrete toughening technology.

Experimental Research on Size Effect of Mode II Fracture Toughness of Concrete

Based on specimen size, which is the main reason of the shear fracture toughness of concrete, experimental research was carried out by 5 groups including 40 symmetrically loading specimens with different length and height. Through load and crack tip sliding displacement curve P-CTSD, load and strain curve P-ε and load and time curve P-t, the effects of length and height of specimens to shear fracture toughness were studied. Specimen stability is strengthened with increasing of length and weakened with increasing of height. Size effect of fracture toughness is weakened with increasing of length, is strengthened with the increasing of height. Fracture toughness increases with the increasing of length, decreases with the increasing of height. Research Background The size effect exists in parameters of concrete, such as concrete strength, modulus of elasticity, fracture toughness, fracture energy and so on [1-. In 1961, the theory of fracture mechanics was applied to concrete structure for the first time by Kaplan [. A vast majority of research work about concrete fracture mechanics was carried out by international scholars [6-. As the development of fracture theory of concrete, the size effect of fracture parameters became the focal point in theory study. Karihaloo [ pointed out that the size effect of concrete strength strengthens with the increasing of components size, however, the size effect weakens when crack length decreased relative to the size of specimens. Hu [3, 10, 11] accounted for the size effect by applying the theory of boundary effect and carried out the concept of local fracture energy which changes with width of fracture process zone. Based on the fictitious crack model, an analytical method [12, 13] for predicting the effective fracture toughness of concrete of three-point bending notched beams is proposed and the effects of initial seam height ratio and height on fracture parameters were carried out by Wu and Xu. At present, research on shear fracture toughness of concrete is immature and there are almost no papers about the size effect of shear fracture toughness of concrete. Aiming at the issue, this paper conducts a study on the size effect of shear fracture toughness of concrete by using symmetrically single-edge notched specimen.

Waste Fiber Recycled Concrete Performance Based on Fracture Mechanics Research

This paper studies on the waste fiber strength test of 88 concrete beams crack , the beams section-dimension are 100mm×100mm and length 500mm. Research on recycled aggregate replacement ratio, the waste fiber length and incorporation, the crack height-ratio effect on fracture toughness of concrete. The result indicates that the waste fibers can effectively limit extension and perforation of crack of recycled aggregate, increasing the ductility of concrete, enhancing fracture properties of concrete and conducive to the improvement of fracture toughness of concrete.

A simple fracture mechanical interpretation of size effects in concrete fracture toughness tests

Concrete is a so called quasibrittle material which, despite predominantly elastic material response, exhibits in tension loading a stable non-linear fracture response, when tested under displacement control. The reason for the non-linearity is the development of a fracture process zone, in front of the crack, due to micro-cracking and crack bridging. The effect of the fracture process zone is to make the specimen sense the crack as being longer than a0+Δa. The fracture process zone causes thus an effective increase in the crack driving force and apparent fracture resistance. Here, a novel LEFM based estimate of the effective stress intensity factor and the effective crack growth at maximum load in a fracture mechanics test is used to obtain a simple power law approximation of the effective K–R curve. It is shown that it is applicable to the description of not only different size specimens, but also specimens with varying geometry. The method is based on a new theoretical estimate of the effective crack growth corresponding to maximum load.

Variations in fracture energy of concrete subjected to cyclic freezing and thawing

Fracture energy is one of the fundamental parameters representing cracking resistance and fracture toughness of concrete. The paper deals with the effect of internal frost damage on the fracture energy of concrete. The fracture energy value was assessed on beams with initial notches in three-point bend test assuring stable failure of the specimen. It was found, that the internal damages due to cyclic freezing and thawing have a significant effect on variations in fracture energy, related to changes in destructive load value as well as in deformability of material. The analysis of load-deflection curves obtained made it possible to fit the simple function, describing the post-peak behaviour of concrete subjected to frost damages, which can be useful for the calculation of GF value. It was proved that it is reasonable and feasible to study the freeze-thaw damage process of concrete using fracture mechanics methods.