Fracture Properties Of Concrete Research Articles

Crack Resistance of Lightly Reinforced Concrete Structures.

The crack resistance of concrete structures with low reinforcement ratios requires a broader examination. It is particularly important in the case of foundations working in changing subsoil conditions. Unfavorable phenomena occurring in the subsoil (e.g., ground subsidence, landslips, non-uniform settlement) can lead to unexpected cracking. Therefore, it is necessary to check the effectiveness of the low reinforcement provided. As there are limited studies on lightly reinforced concrete structures, we performed our own experimental investigation and numerical calculations. In the beams analyzed, the reinforcement ratio varied from 0.05% to 0.20%. It was found that crack resistance in concrete members depends on the reinforcement ratio and steel bar distribution. A comprehensive method was proposed for estimating the crack resistance of lightly reinforced concrete members in which both the reinforcement ratio and the reinforcement dispersion ratio were taken into account. Furthermore, the method considered the size effect and the fracture properties of concrete. The proposed method provides the basis for extrapolation of the test results obtained for small elements and conclusions for members with large cross-sections, such as foundations, which frequently use lightly reinforced concrete.

Review of Fracture Properties and Research Methods of Fiber Recycled Concrete

Aiming at the background of the application of fracture mechanics in fiber recycled concrete as well as the shortcomings of the current research, the relevant results and progress are summarized from three aspects, namely, the basic theory of fracture mechanics of fiber recycled concrete and the research progress of the fracture properties of fiber recycled concrete as well as the testing methods of the fracture of fiber recycled concrete, respectively. The development history of the application of fracture mechanics in concrete is described, three fracture forms, fracture process zones and three softening curve models of concrete are introduced, and it is proposed to combine the bilinear softening curve and the virtual crack zone model to construct the ontological relationship of the whole process of concrete cracking. The current research status of recycled brick aggregate concrete and polypropylene coarse fiber concrete is introduced. The incorporation of recycled brick aggregate adversely affects the workability, basic mechanical properties and fracture properties of concrete. The reinforcing effect of polypropylene fibers on concrete can effectively improve the adverse effects of brick aggregate on concrete, which opens up the research prospect of green concrete and has high research value. The applications of three-point bending test, four-point bending test and DIC technique in the study of concrete fracture characteristics are introduced.

Simulation of four-point bending tests for the viscoelastic fracture properties of concrete

Understanding the fracture properties of concrete, such as crack propagation behavior and fracture energy, is crucial for designing and evaluating concrete structures. Experimental results are insufficient and cannot be directly employed for a comprehensive analysis of the fracture behavior of concrete structures under load, particularly when considering concrete as viscoelastic with the presence of cracks. Recognizing the time and cost constraints of traditional experimental testing, this research leverages numerical simulations as a cost-effective alternative to determine viscoelastic material parameters. Thus, the critical evaluation of concrete fracture properties, fundamental for the design and assessment of concrete structures, is addressed. Employing a finite element method for four-point bending tests, the study systematically investigates parameters such as initial crack depth, displacement acceleration, and time step. The material properties of concrete are described using viscoelastic models. The findings provide valuable insights into crack propagation behavior and deformation characteristics, emphasizing the significant influence of the modulus of elasticity on both maximum load values and displacement. These findings contribute to a deeper understanding of the structure's response and underscore the importance of considering these parameters in similar simulations. The study highlights the importance of considering these parameters in simulations to enhance the understanding of concrete fracture behavior. The paper's contributions can extend to optimizing concrete mixtures, formulating repair strategies, and improving structural assessments. Further research is suggested to improve the accuracy of simulations and investigate material properties under various conditions.

Recycled Fine and Coarse Aggregates' Contributions to the Fracture Energy and Mechanical Properties of Concrete.

This paper investigates the fracture mechanical properties of concrete, using crushed concrete aggregates (CCA) and granulated blast furnace slag (GGBS) for partial cement replacement. CCAs made from prefabricated concrete replace 100% of the fine and coarse fractions in concrete recipes with w/c ratios of 0.42 and 0.48. Two pre-treatment methods, mechanical pre-processing (MPCCA) and accelerated carbonation (CO2CCA), are investigated for quality improvements in CCA. The resulting aggregates show an increased density, contributing to an increase in the concrete's compressive strength. The novelty of this paper is the superposition of the effects of the composite parts of concrete, the aggregate and the cement mortar, and their contributions to concrete fracture. Investigations are directed toward the influence of fine aggregates on mortar samples and the influence of the combination of coarse and fine aggregates on concrete samples. The physical and mechanical properties of the aggregates are correlated with mortar and concrete fracture properties. The results show that CCA concrete achieves 70% of the fracture energy values of concrete containing natural aggregates, and this value increases to 80% for GGBS mixes. At lower w/c ratios, MPCCA and CO2CCA concretes show similar fracture energies. CO2CCA fine aggregates are the most effective at strengthening the mortar phase, showing ductile concrete behavior at a w/c ratio of 0.48. MPCCA aggregates contribute to higher compressive strengths for w/c ratios of 0.42 and 0.48. Thus, mechanical pre-processing can be improved to produce CCA, which contributes to more ductile concrete behavior.

Fracture properties of extruded fiber-reinforced mortar with preferentially aligned fibers

The fracture properties of fiber-reinforced mortar were determined in this paper using three-point bend tests on notched beams and employing the two-parameter fracture model (TPFM). A comparison was made between two casting methods: conventional casting and pump-driven extrusion. The hypothesis suggested that the extrusion process would preferentially align fibers parallel to the tensile stresses, thereby enhancing the concrete’s fracture properties. The results corroborated the enhancement of concrete's ductility and post-peak behavior when fibers were added, consistent with previous studies. However, this study revealed a novel insight that preferential fiber alignment achieved through extrusion could further enhance the fracture properties of concrete. Furthermore, digital image correlation was used to obtain the entire displacement field during testing. The crack propagation process and the strain localization were investigated, and the fracture toughness considering crack deflection was calculated using a modified two parameter model (MTPM). The results showed that extrusion-based specimens exhibited more deflected cracks, affirming the hypothesis that fiber alignment via extrusion influenced the crack propagation, thereby enhancing the fracture properties of the composite.

The Effect of Polypropylene Fibers on the Fracture Characteristics of Lightweight Aggregate Crumb Rubber Concrete Composites

The increasing use of rubber tires and their low recycling ratio have made them a serious environmental problem. This work aims to develop and investigate enhanced lightweight aggregate crumb rubber concrete (PFLWACRC) composites regarding the fracture properties of concrete. Polypropylene (PP) fibers are commonly familiar with increased crack growth endurance of concrete. On the other hand, the reuse of waste rubber in concrete plays a major role in the mitigation of the effects of climate change. Various concrete mixtures were designed with conventional Portland cement and common lightweight coarse aggregates. The variables considered in this study are PP fibers in different percentages (0.2%, 0.4%, and 0.6% volume fraction), and crumb rubber with various substitution proportions (0%, 5%, 10%, 15%, 20%, and 25% of fine aggregates). Cement with 450kg/m3 density with 10% silica fume was used. The fracture characteristics, which involve fracture toughness (KIC) and fracture energy generation (GF), of all concrete mixtures were evaluated by testing two types of samples, i.e. 54 notched concrete beams with dimensions of 10×10×52cm and 54 cylinders with diameter×height equal to 15×30cm. The results showed that the fracture toughness generation addresses the energy scattering limit of concrete mixtures. The findings showed that the existence of PP fibers increased the fracture energy and critical Crack Mouth Opening Displacement (CMOD)c. The PP fibers had a limited effect on the compressive strength and may even reduce it, but a remarkable enhancement of the energy absorption was observed.

An improved understanding of the influence of w/c ratio and interfacial transition zone on fracture mechanisms in concrete

The interfacial transition zone (ITZ) is significantly influenced by the water/cementitious materials (w/cm) ratio and governs the overall strength and fracture properties of concrete. Even though an ample number of studies on this topic are available in the literature, there is a lack of unified conclusions pertaining to the influence of w/cm ratios on various fracture characteristics and the underlying mechanisms. In this study, an attempt was made to investigate fracture characteristics (such as the size of the fracture process zone (FPZ), fracture energy, traction-free cracks (TFCs), toughening mechanisms) for varying ITZ properties and water/cement (w/c) ratios. Plain concrete beams of size 700 × 150 × 80 mm (l × d × b) with varying w/c ratios were tested under centre-point loading by controlling the crack mouth opening displacement. The acoustic emission (AE) technique was used to assess the internal damage parameters. A novel method was developed for identifying the TFC tip and the FPZ size using AE event location data at different stages of loading. The role of the ITZ on the fracture mechanisms of different types of concrete was also assessed. Evaluation of the fracture energy revealed that its relation with the w/c ratio is affected by the type of concrete.

Synergistic performance of steel-brass hybrid fibres on the concrete fracture properties

Synergistic performance of steel-brass hybrid fibres on the concrete fracture properties

Non-fitting theoretical models for the fracture properties of concretes subjected to high temperature

The fracture properties of concretes, which are often characterized by the tensile fracture strength, compressive fracture strength, fracture energy and characteristic length are closely related to temperature. However, there are still few systematic theoretical models of temperature-dependent fracture properties of concretes. In this work, a series of novel temperature-dependent theoretical models are developed to determine the fracture properties of concretes based on the force-heat equivalence energy density principle and classical concrete fracture theories. The models establish the quantitative relationship between the high-temperature tensile fracture strength, compressive fracture strength and characteristic length, and the basic material parameters including the Young's modulus and melting point. It is worth noting that this work quantitatively characterizes the fracture properties of concretes subjected to high temperatures using the simple material parameters, without the need to carry out any data fitting. The fracture properties of concretes with different aggregate types, different fiber-reinforced concretes, and ordinary Portland cement-based concretes are predicted and systematically analyzed using the proposed models. The model-predicted tensile fracture strength, compressive fracture strength and characteristic length of materials up to 800 °C agree well with the experimental measurements without using any fitting parameters. The coincidence rate at many temperature points could reach up to and above 90%, even approaching about 100%. The fracture properties of concretes and their main mechanisms at various temperatures thus can be depicted by using the developed models. Additionally, this work offers a new and simple testing method to determine the characteristic length of concretes and its change with temperature.

Strain rate effect on the fracture parameters of concrete three-point bending beam with a small span-to-height ratio

The fracture properties of concrete are greatly affected by strain rate, and changes in fracture parameters can reflect the crack resistance and failure mechanism of concrete. In this study, fracture tests were carried out on samples of concrete three-point bending beam (TPB) by considering 3 small span-to-height ratios (2, 2.5 and 3) and 4 strain rate grades (10−5/s, 10−4/s, 10−3/s and 10−2/s), in order to examine the strain rate effect on the fracture parameters of TPB. From the test data, the crack propagation rate, fracture toughness, length of fracture process zone (FPZ) and determination of damage scale were calculated. The results showed that the fracture parameters of TPB with a small span-to-height ratio were subjected to an obvious strain rate effect. With the increase of span-to-height ratio, the strain rate effect on the crack propagation rate, fracture toughness and determination of damage scale was gradually weakened, while that on the length of fracture process zone (FPZ) was gradually strengthened. Overall, changes in strain rate can significantly affect the fracture parameters of TPB with a small span-to-height ratio. This paper provides a reference for further research on the strain rate effect and failure mechanism of TPB with a small span-to-height ratio.

Tensile Fracture Property of Concrete Affected by Interfacial Transition Zone

As a weak link between aggregate and mortar in concrete, interfacial transition zone (ITZ) usually plays a key role in concrete fracture. To investigate the tensile fracture property of concrete affected by the mechanical properties of ITZ numerically, the geometrical models of heterogeneous concrete were established with the parameterization modeling. They include three phases, namely, mortar, ITZ, and randomly distributed aggregates with distinct sizes and orientations. The cracking behaviors of mortar and ITZ were characterized by the bilinear cohesive zone constitutive model. Based on the experiments, the mechanical properties of ITZ were mediated by changing the water–cement ratio of mortar, the aggregate surface roughness and the content of silica fume in interfacial agent. A series of numerical simulations were conducted on the concrete models in tension after the numerical modeling method was validated. The macroscopic tensile fracture properties of concrete were quantitatively connected with some microscopic variables, including the water–cement ratio of mortar, the aggregate surface roughness and the silica fume content in interfacial agent. It was found that the tensile fracture properties of concrete have negative linear correlations with the water–cement ratio of mortar, while the effects of the aggregate surface roughness and the silica fume content in interfacial agent are very complex. The tensile fracture mechanical properties of concrete have a bilinear relationship with the aggregate surface roughness and an approximate quadratic parabola relationship with the content of silica fume in the interfacial agent. This study is beneficial to improve the fracture resistance of concrete by some interface handling measures.

Effect of calcium leaching on the fracture properties of concrete

In this work, acceleration leaching and three-point bending (TPB) tests were performed on single-notched TPB beam specimens to explore the evolution of the fracture properties of concrete subjected to calcium leaching. The load-crack mouth opening displacement (P-CMOD) curves obtained for concrete at various leaching times were adopted to analytically calculate parameters that can reflect the fracture properties of concrete, such as the crack extension resistance (KR), fracture toughness (KIcini and KIcun), elastic modulus (E), tensile strength (ft), and fracture energy (Gf). Furthermore, a phase-field fracture model for concrete that considers calcium leaching effects is established by incorporating the time-dependent damage variables E, ft, and Gf. The results reveal that the KR curves of the leached concrete increase with crack propagation but lower and flatten with more leaching time. The fracture parametersKIcini, KIcunE, ft, and Gf rapidly decrease with leaching time in the first 60 days. However, with more leaching time, the impact of calcium leaching on the concrete fracture properties gradually diminishes. The variation in fracture parameters of leached concrete can be described by a series of exponential functions. The residual value fracture parameters of the fully leached concrete are estimated to be approximately 60–80 % of those of sound concrete. The established fracture model reproduces the P-CMOD curves of concrete at each leaching time, which verifies the rationality of the model. The numerical simulation reveals that crack propagate faster in leached concrete than in sound concrete, and leached concrete is more prone to cracking.

Study on the dynamic fracture properties and size effect of concrete based on DIC technology

The fracture properties of concrete are highly influenced by the size effect and strain rate effect. To explore the size effect on the dynamic fracture properties of concrete, a fracture experiment on the three-point bending beam was carried out in this paper under four strain rates (10−5/s, 10−4/s, 10−3/s and 10−2/s) and three span to height ratios (2, 2.5 and 3). Then, the fracture characteristic parameters and mechanism were analyzed based on the experimental data combined with the digital image correlation (DIC) technology. The results indicate that the fracture properties of concrete have an obvious strain rate effect and size effect. The fracture load, fracture energy and crack initiation toughness all increase with an increasing strain rate, but decrease with an increasing span to height ratio. On the contrary, the unstable fracture toughness shows no obvious size effect or strain rate effect. Based on the DIC technology, the evolution of crack propagation length in different loading stages was calculated. It is shown that, under the strain rate of 10−5/s, 10−4/s, 10−3/s and 10−2/s, the rapid propagation of crack occurs in the period from Pre-70% to Pre-90%, from Peak load to Post-90%, from Post-90% to Post-80%, and from Post-90% to Post-80%, respectively. More specifically, the crack propagates most rapidly under the strain rate of 10−2/s. The rapid propagation period tends to be delayed with the increase of strain rate, and the strain rate effect is gradually weakened with the increase of span to height ratio. By carrying out in-depth research on the dynamic fracture failure mechanism of concrete, this paper provides reference for the future studies on the dynamic fracture properties of concrete.

Experimental investigation and modified calculation model of critical crack propagation length of concrete

The critical crack propagation length is essential to calculate the fracture toughness of concrete and the further safety evaluation of the cracked concrete structures. In this study, the critical crack propagation lengths of the notched three-point bending (TPB) beams with various geometric dimensions and concrete strength grades are measured with two experimental methods. One way is to paste strain gauges on the sides of the crack propagation path, and another way is to arrange clip gauges along the depth direction. The results indicate that the effective crack propagation lengths derived based on the existing fracture model are smaller than the experimentally measured critical crack propagation lengths with the maximum relative error of 45%. To obtain the critical crack propagation length with the simple theoretical model accurately, a modified analytical method is proposed, where the elastic equivalent crack propagation length corresponding to 95% of the peak load in the post-peak section is treated as a good approximation of the critical crack propagation length. With the modified method for determining the critical crack propagation length, the fracture toughness of concrete can be derived reasonably. It is expected that the modified method would provide a valuable reference for determining the fracture property of concrete using the small size TPB beams in practical engineering.

Validation of two different analysis techniques to obtain dynamic mechanical properties of concrete using a modified Hopkinson Bar

In the present work, two different approaches for obtaining dynamic fracture properties of concrete are analyzed. Spalling tests over cylindrical samples using a modified Hopkinson Bar were carried to measure the tensile strength and the fracture energy of conventional concrete. Results for strain rates ranging from 60 to 130 s−1 are presented for both techniques and compared to the respective quasi-static values. Moreover, two different projectile shapes (cylindrical and conical) have also been evaluated. In this sense, a deep qualitative and quantitative analysis is also performed, regarding the variations in tensile stress evolution of the pulses. Validation of both methodologies is achieved by comparing the obtained results.

Synthesis of pyrolytic carbonized bagasse to immobilize Bacillus subtilis; application in healing micro-cracks and fracture properties of concrete

Growing amount of agricultural waste and its burning in open environments is contributing towards the carbon emissions and triggering serious health hazards. It can be reduced by beneficially employing the wastes, as carrier media of calcite (CaCO3) precipitating microbes, into the concrete. Instant formation of CaCO3 in micro-cracks using bio-inspired concrete prevents aggressive ions to penetrate into the inside concrete, hence boosting the durability. In the present study, Bacillus subtilis (BS) were immobilized with nano-micro sized carbonaceous solid material, bagasse ground biochar (GBC), to enhance the CaCO3 precipitation. The mechanical behavior of the samples was investigated in terms of bending and compression. Biochar immobilized BS concrete (BSCM) exhibited promising flexural behavior, higher strain energy storing capability, and higher modulus of fracture toughness. Moreover, 23.18% enhancement in compressive strength was achieved after 56 days of curing in comparison to the control samples. Furthermore, autonomous cracks closure mechanism was monitored as the function of time, the BSCM showed effective crack healing with maximum 100% and 68% sealing of 500 μm and 800 μm wider cracks respectively, in the selected time frame. The BSCM samples revealed higher ultrasonic pulse velocities and lesser sorptivity due to the densified microstructure of concrete by bacterial precipitated CaCO3. The x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy and thermal gravimetric analysis confirmed the existence of CaCO3 inside the cracks. Consequently, immobilizing BS with bagasse GBC could be considered as a promising solution for prompt cracks repairing and enhancing the mechanical properties of concrete.

The role of aggregate granulation on testing fracture properties of concrete

Concrete is a porous material containing aggregate of different sizes, hardened cement matrix with air pores, microcracks and water. Concrete internal structure is different from that of other engineering materials. Furthermore concrete is described as quazi-brittle material. Fracture processes in it form in a way that does not fit within classical theories. Therefore, to describe failure of concrete structures nonlinear fracture mechanics is often applied with success. Basic concrete parameters, like compressive and tensile strength, and modulus of elasticity, are not enough to analyze fracture processes in concrete structures. Additional fracture properties should be tested, among them fracture energy, complete diagram of stress-deformation under axial tension and the width of fracture process zone. Recognizing and testing fracture parameters is of paramount importance when analysing fracture process in concrete structures. The correct data of material’s properties and the adequate fracture model applied in numerical simulations influence final results. In the paper the findings reported in the professional literature are summarized and obtained results of the own numerical simulation are reported in order to give a deeper knowledge on the role of aggregate on fracture properties of concrete.

Size effects in unreinforced and lightly reinforced concrete beams failing in flexure

Fracture-based models commonly use a characteristic length as the basis for determining size effects in concrete beams. The characteristic length is related to the concrete fracture process zone and defined in terms of the concrete fracture properties. Semi-empirical constants are then developed to accommodate any unidentified (geometric or crack bridging) parameters. However, a reliance on semi-empirical factors can limit the applicability to different systems, concretes and reinforcing materials. The aim of the current work is to formulate an analytical size effect model based solely on fundamental material and geometric properties. The particular focus is unreinforced and lightly reinforced concrete beams that fail in flexure due to unstable crack propagation. The proposed ’generalised’ characteristic length approach is based on the mode-I fracture behaviour of concrete and includes crack bridging forces due to the presence of longitudinal reinforcement. The theoretical expressions suggest that the geometric shape of a beam, the fracture properties of the concrete and the crack bridging forces (where present) significantly influence the characteristic length. Experimental investigations on geometrically similar unreinforced and lightly reinforced concrete beams are undertaken as a means for initial validation. The validation is then extended to a wider dataset of existing experimental results in the literature. The generalised characteristic length approach is able to capture both the influence of the concrete strength and the size effect mitigation due to the inclusion of longitudinal reinforcement. This confirms that the generalised approach holds promise and could be expanded to other quasi-brittle materials and non-conventional reinforcing materials.

Experiment Study and Finite Element Analysis of the Coupling Effect of Steel Fiber Length and Coarse Aggregate Maximum Size on the Fracture Properties of Concrete

The effects of steel fiber length (lf = 30 mm, 40 mm, 50 mm and 60 mm) and coarse aggregate maximum size (Dmax = 10 mm, 20 mm, 30 mm and 40 mm) on fractural properties of steel fiber reinforced concrete (SFRC) was investigated. The results show that the fracture energy (Gf) of SFRC reaches its maximum when Dmax increases to 30 mm, and it increases first and then decreases as lf increases, but it still has a significant increase compared to the control concrete. The Gf ratio increases first and then decreases as the lf/Dmax increases. The Gf of the SFRC fracture surface follows the same trend as the fractal dimension. The rational range of the lf/Dmax is 2.5–4 for the considerable strengthening effect of steel fiber on fracture performances of concrete with the Dmax of 10 mm and 20 mm and 1.5–2.33 for that concrete with the Dmax of 30 mm and 40 mm. The finite element analysis results are compared with the experimental results, and the results show that the fracture process of the finite element model is consistent with the experiment.

Design of concrete fracture property by average aggregate size

This study presents a design methodology for concrete fracture properties, linking the average aggregate size dav to the tensile strength ft and fracture toughness KIC for a given cement grade and water/cement (W/C) ratio. Two different concrete mixes with dav = 3.5 and 7 mm were tested and analyzed. Similar peak load Pmax values were obtained under three-point-bending conditions, but with different “ductility”. Variations of ft &KIC with dav were established by a simple design formula, which can be used to tailor bulk concrete fracture properties for specific purposes. A statistical method was introduced to estimate the effective dav if only the maximum aggregate size dmax was given. Comprehensive fracture data in literature were analyzed to demonstrate the estimation of dav from dmax. The purposely designed concrete mixes and data in literature show the significance of dav design even after the cement grade and W/C ratio are fixed.