Fracture Parameters Of Concrete Research Articles

The microscopic fracture model for determining the fracture behavior of self-compacting lightweight concrete

The fracture toughness (KIC) and tensile strength (ft) of concrete are critical parameters governing crack resistance. Concrete usually exhibits brittleness under flexure and tension, so this brittle fracture first occurs at the micro level. Many researchers have studied its fracture properties at the micro level. However, the theoretical model and calculation formulae do not reflect the influence of micro-material parameters on the corresponding fracture properties. In this paper, the microscopic fracture model of self-compacting lightweight aggregate concrete (SCLC) was developed, considering the parameters of micro-nano silica materials. First, the characteristic parameters, such as particle size, specific surface area, and unit weight of micro silica (MS) and colloidal nano-silica (CNS), were considered, and the calculation equation for fictitious crack growth length was proposed. Then, the micro-fracture model was developed, which was further used to calculate the fracture parameters KIC and ft of SCLC incorporated with micro-nano silica. The results are found to be consistent with the fracture parameters determined by the size effect model. In addition, the failure curves of different structural fracture modes of SCLC were established through the fracture parameters KIC and ft obtained from the microscopic fracture model, and the fracture modes of SCLC incorporated with MS and CNS were identified. The peak load of the minimum specimen size satisfying linear elastic fracture was also predicted. This research provides a new perspective for evaluating concrete fracture parameters, realizes the progressive prediction of concrete fracture performance from macro to micro, and fills in the research gaps in the existing knowledge base on the synergistic effect of MS and CNS to enhance the fracture performance of SCLC.

Age and crack length effects on fracture parameters of dam gallery concrete: Experimental investigation and modeling

Cracks of varying lengths may occur in galleries throughout dam construction, operation, and management processes. This study investigates the influence of age and crack length on the fracture parameters of gallery concrete, which is crucial for understanding the mechanisms and risks associated with gallery cracking and for developing appropriate treatment measures. Wedge-splitting tests were performed on gallery concrete specimens with varying crack lengths and at different ages. It was found that the fracture parameters measured at different ages all decrease with the increase of crack length. Based on the test results, size-independent material constants such as fracture toughness and the effective length of the fracture process zone were derived and modeled using an exponential function to reflect their time-varying characteristics. A fracture model was proposed and validated, which can accurately predict the fracture behavior of gallery concrete across varying ages and crack lengths, and is applicable in practical dam gallery crack analysis for assessing crack stability and evaluating the effectiveness of treatment measures.

Prediction of fracture failure and fracture parameters for dam gallery concrete specimens of arbitrary size and age via boundary effect model and fracture extreme theory

Cracking-associated problems may occur in the construction and operation periods of dam galleries, necessitating attention to the fracture characteristics of gallery concrete. The fracture parameters of concrete exhibit size effects and also change with age. A correct evaluation of the cracking risk or crack stability of gallery structures at any given time requires a comprehensive understanding of both the size effect and time-variant characteristics of the gallery concrete fracture parameters. In the present study, 75 wedge-splitting specimens, with effective heights ranging from 200 mm to 600 mm, were cast using gallery concrete produced by the concrete mixing system at the Baihetan Dam construction site. Subsequently, fracture tests were conducted on these specimens at curing ages spanning from 7 days to 90 days. According to the test results, the fracture parameters of concrete with a specific size exhibited an increasing trend with age within 90 days, while the fracture parameters of concrete at specific ages show a significant size effect, tending to stabilize as the effective depth of specimens gradually increases. The boundary effect model was utilized for analyzing the fracture test results of concrete specimens at different ages, yielding the size-independent double-K fracture parameters and scale parameters for concrete at each age, along with their time-varying patterns. Subsequently, equations for predicting the failure loads of concrete specimens of any age and size were established. Based on the prediction results of failure loads, the double-K fracture parameters of concrete specimens of any age and size were solved by the fracture extreme theory. The predicted failure loads and fracture parameters were in good agreement with the experimental results. The findings of this study can be applied to the full lifecycle crack risk analysis or crack stability assessment of Baihetan Dam gallery structures.

Investigating the fracture parameters of lightweight geopolymer concrete reinforced with steel and polypropylene fibers

In this current paper, the effects of different percentages of scoria (70, 80, and 90) replacing coarse aggregates, steel fiber volume fractions (Vsf = 0.5 %, 1 %, and 1.5 %) merely and in combination with polypropylene fiber volume fractions (Vppf = 0.1 %) on fracture parameters of lightweight geopolymer concrete(LWGC) have been studied using WFM(Work of Fracture Method) and SIZEM(Size Effect Method). The results indicate that: (a) increasing the content of Vsf and Vppf increases the fracture energy and fracture toughness. (b) The content of polypropylene and steel fibers has remarkable effects on fracture parameters in compositions containing 70 % scoria. (c)The composition of 70 % scoria instead of coarse aggregates, containing (Vsf = 1.5 %) and (Vppf = 0.1 %) is more significant in the analysis of fracture parameters than other samples in this research. In general, it can be said that the increase of fracture parameters like Gf, GF, KIC, and Cf is more noticeable in samples containing 70 % scoria. Also, in the samples containing 70 % scoria, containing (Vsf = 1.5 %) and (Vppf = 0.1 %), the Gf, KIC, and GF are almost 1.84, 1.44, and 2.08 times the control samples, respectively. It is also to see an increase in parameters such as Gf, GF, and KIC in samples containing (Vsf + Vppf) compared to (Vsf). This shows the positive effect of adding polypropylene fibers (Vppf = 0.1 %) to the samples. When the content of steel fibers increases by 0.5 %, 1 %, and 1.5 %, the increase in the mentioned parameters can also be observed. The positive effect of increasing the content of steel fibers in increasing the fracture energy obtained from the results is determined.

Effect of notch depth ratio and specimen width on mode II fracture properties of ECC

Engineered Cementitious Composites (ECCs) can be used as repair and reinforcement materials, which can improve the shear performance of the structure. Hence, doubled-notched and unnotched half-symmetric loading tests are used to study the mode II fracture properties of ECC and concrete, and the fracture failure morphology and fracture parameters of ECC and concrete are compared. The effects of the notch depth ratios of 0, 0.1, 0.3, and 0.5 and the specimen widths of 200 mm, 250 mm, and 300 mm on the mode II fracture properties of ECC and concrete are investigated. The mode II crack development of ECC can be divided into four stages: microcrack development stage, multiple cracking stage, main crack formation stage, and crack instability development stage. It can be found that ECC not only has high ultimate tensile strain and energy dissipation capacities, but also has obviously better ability to resist the unstable propagation of shear cracks than concrete. ECC can meet the high shear resistance requirements of some parts of the structure.

Mechanical and Fracture Properties of Air-Entrained FRC Containing Zeolitic Tuff

This study aimed to evaluate the influence of zeolitic tuff, an air-entraining agent, and different types of fibers on the compressive strength and fracture parameters of concrete with increased strength. Notched beams were tested in three-point bending to determine the fracture parameters of concrete. It was established that the partial replacement of Portland cement (10% by mass) with zeolitic tuff, the addition of an air-entraining agent and different types of fibers resulted in the improvement both of the compressive strength (by 3.7% after 28 days of hardening) and fracture properties of concrete (namely, the fracture energy by 35.1% and characteristic length by 61.5%) compared to the reference concrete. The beneficial effects of the air-entraining agent and the mechanisms through which it enhances the properties of concrete by incorporating zeolitic tuff and various types of fibers were explained. It has been demonstrated that the appropriate selection and optimization of various technological factors enable the production of economically effective, high-quality concrete with a 10% lower cement content. As a result, this leads to reduced CO2 emissions, aligning with a sustainable development strategy.

Experimental and meso-scale investigation of size effect on fracture properties in three-point concrete beams (TPB)

In this study, the size effect on concrete fracture parameters has been investigated in three distinct groups. While the geometry is similar across each group, the span-to-height ratio varies among the three groups. The study investigates the impact of three different span-to-height ratios (1.85, 2.65, and 4) and five different notch-to-beam height ratios (0.1, 0.2, 0.3, 0.4, and 0.5) by using three-point notched beams (TPB). Beams with identical spans but different heights are used. A new method is proposed to determine the fracture properties of concrete based on the Boundary Effect Method (BEM). A proposed formula for the fictitious cracks is presented based on the results of numerical analyses at the mesoscale and used in the BEM to determine the values of tensile strength and fracture toughness with the effects of span-to-height ratio. The established mesoscale fracture model reproduces the load-displacement curves of 3 PB specimens concrete, which verifies the rationality of the model with the experimental results. Numerical simulation reveals that the fictitious crack length in concrete is highly sensitive to the size of aggregates, their distribution and spacemen’s dimensions. Discussion on BEM based on principles of mathematical fitting and data processing is provided for more objective quasi-brittle fracture experiments and modeling. The size effect law (SEL) is limited by its requirement for sample geometries to be similar, which introduces some additional error when applied to samples with dissimilar geometries. To minimize this error, this study proposes a new formulation within the BEM method. It is found that the fracture parameters predicted by the BEM method using the proposed formulation for different geometries are consistent with the size effect law (SEL) for similar geometries.

Experimental and numerical investigation on fracture characteristics of self-compacting concrete mixed with waste rubber particles

Replacing natural aggregates in concrete with waste rubber particles is an effective method to solve environmental problems, but the use of rubber due to its unknown characteristics can change the fracture performance and cracking behavior of self-compacting concrete. Hence, through experimental analysis and discrete element method (DEM) simulation, this paper aims to investigate the influence of different rubber contents (0%, 10%, 20% and 30%) on fracture characteristics of self-compacting concrete. Compared with natural river sand, rubber particles have more flexible structure and stronger deformation ability, and these unique properties enhance the bridging effect between matrices and weaken the stress concentration at the crack tip, which shows that the fracture parameters of self-compacting rubberized concrete (SCRC) increase linearly with the increase of rubber content, and this linear growth rate will increase with the increase of beam size. Moreover, the addition of rubber particles absorbs most of the energy generated in the fracture process, which significantly enhances the anti-cracking ability of SCRC, and this ability to prevent crack propagation will be enhanced with the increase of rubber content, which is manifested by the significant decrease of acoustic emission (AE) activity. Based on the shapes of real aggregates and rubber particles, a universal DEM fracture prediction model for SCRC is established, which can better describe the fracture response of SCRC with different rubber contents and sizes under flexural loads. The evaluation of economic and environmental benefits indicates that adding rubber particles into self-compacting concrete can not only save costs, but also produce considerable environmental benefits.

Influence of Nano-TiO2 and water to cement ratio on fracture parameters of concrete

Influence of Nano-TiO2 and water to cement ratio on fracture parameters of concrete

Evaluating fracture parameters of basalt fiber reinforced and pozzolana slurry modified recycled concrete produced from waste

Recycling of construction and demolition waste provides one of the most effective paths for sustainable development, valorising recycled concrete, and environment protection. Unfortunately, due to substandard concrete matrix quality, lower strength and fracture parameters the practical applications of recycled concrete still remain the foremost concerns. To overcome these issues, a new effective strength enhancing approach was investigated in this study. More explicitly, two types of recycled concrete consisting of 20% and 100% of pozzolana slurry encrusted recycled aggregate and strengthened with various dosages of basalt fiber (BF) were evaluated in terms of fracture properties, namely compressive strength, fracture energy, fracture toughness, flexural strength, and elastic modulus. In addition, a digital image correlation (DIC) analysis was established to evaluate the strain contour variation while the microstructure study was conducted to inspect the concrete matrix quality. Experimental result revealed improvement in the ultimate load capacity as well as the overall load-CMOD response of BF modified formulations. For 20TRAC-1BF and 100TRAC-1BF, the fracture energy was improved by 91% and 56% and the fracture toughness was enhanced by 69% and 43%, respectively. Furthermore, the added dosages of BF were found useful in improving the flexural strength, elastic modulus, and toughness indices of both types of recycled concretes. From the DIC analysis, it was found that the BF modified concrete contained continuous spread of strain contours throughout the beam depth and revealed higher strain values than the reference sample. Lastly, the microstructure observation demonstrated better concrete matrix quality and BF-cement matrix interaction which thereby confirms the usefulness of the applied strength enhancing technique for the application of high strength fiber reinforced recycled concrete.

Multigene Genetic Programming Based Prediction of Concrete Fracture Parameters of Unnotched Specimens

This study explores the fracture energy of notched and unnotched concrete specimens subjected to the classical three-point bend test, instantiating a gradational step in the continued development of concrete fracture mechanics. An experimental campaign involving 18 notched test specimens and nine unnotched specimens of three different grades of concrete, an examination of the existing literature models for unnotched specimens, and a novel Multigene Genetic programming (MGGP) based concrete fracture energy model for unnotched specimens are integral to this study. As a salient result, the multiple approaches to quasi-brittle materials adopted in the study, highlighted the criticality of the determination of fracture energy, tensile strength and characteristic length for the crack width study. The failure modes of notched and unnotched specimens were found to be similar. The reported literature has mainly focused on a limited number of fracture energy influencing parameters. Therefore, six impact parameters have been chosen and incorporated into the present study to provide a more acceptable explanation of concrete fracture behaviour. A sensitivity analysis of the parameters and an error analysis of the model undertaken have established the accuracy and robustness of the developed MGGP model. Doi: 10.28991/CEJ-2023-09-02-011 Full Text: PDF

Using beam and ENDB specimens to evaluate fracture characteristics of wavy steel fiber‐reinforced self‐compacting concrete containing different coarse aggregate volumes

AbstractThis research has addressed the effects of coarse aggregate (CA) and wavy steel fiber (WSF) volumes on the self‐compacting concrete (SCC) fracture parameters in beam and edge notched disk bend (ENDB) specimens using size effect and work fracture methods (SEM and WFM). Mix designs were based on 30%–60% CA volumes and 0.15%–0.45% WSF volumes, and 144 different‐size notched beam specimens underwent 3‐point bending tests. Results showed that increasing the CA volume and percent WSFs increased the SCC ductility in both SEM and WFM; increasing the WSF volume affected the fracture energy‐compressive strength relationship negatively at 30% and 40% CA volumes, and increasing the WSF affected the mentioned relationship positively at 50 and 60% CA volumes. Finally, we predicted the relationship between the fracture parameters calculated with ENDB specimens and those of beam specimens calculated with SEM and WFM.

Fracture characteristics of various concrete composites containing polypropylene fibers through five fracture mechanics methods

Abstract This paper investigates and compares the experimental results of fracture characteristics in various polypropylene fiber-reinforced concretes (high strength concrete, lightweight concrete, and engineered cementitious composite) on 90 three-point bend (notched and un-notched) beams. Five widely used fracture mechanics testing methods, such as work of fracture method, stress-displacement curve method, size effect method, J integral method, and ASTM E399, were used to investigate the fracture behavior. Results have demonstrated that fracture energy and fracture toughness improved as the dosage of polypropylene fibers increased in concretes. However, this improvement was different in concretes owing to various results of fracture mechanics testing methods and different properties of each concrete. Aggregates played significant role in the performance of polypropylene fibers on the fracture behavior of concretes. Among testing methods, the ASTM E399 showed the lowest values for the fracture toughness of concretes. Both work of fracture and stress-displacement curve methods exhibited appropriate results for the fracture energy of polypropylene fiber-reinforced concrete composites. The accuracy of size effect method was acceptable for determining size-independent fracture parameters of plain high strength and lightweight concretes. Furthermore, the J integral method showed more relevant results for the fracture toughness of polypropylene fiber-reinforced engineered cementitious composite.

Mechanical and Fracture Parameters of Concretes with Different Water to Cement Ratio in Two Different Conditions of Curing

Concretes with the maximum aggregate size of 8 mm and water to cement ratios 0.50, 0.40 and 0.30 were prepared. Mechanical properties including fracture properties were recorded during the first year of ageing. One set of specimens was stored in water and the others were enveloped by foil and left in the laboratory for 28 days. At the age of 28 days, the specimens were measured by 3D computed tomography (CT). Some microcracks were detected by CT, and their contents were compared with the mechanical properties of the concretes. The results show nearly the same mechanical properties of both the water-cured and the foil-enveloped specimens. Some reasons for this behaviour were discussed. The explanation of this phenomenon can help to understand the influence of self-healing on cracks in high-performance concrete.

Determination of concrete fracture parameters using inverse analysis: Influence of the tensile softening model

The paper is focused on the identification of selected mechanical fracture parameters of concrete. An inverse analysis based on an artificial neural network is used for this purpose. In this approach the laboratory measurements are matched with the results gained by reproducing the same test numerically. The identification of mechanical fracture parameters is carried out from the records of three-point bending and wedge-splitting tests performed using three specimen sizes. The ATENA software is employed for the numerical simulation of the fracture tests. The material model with the exponential and bilinear tensile softening law is selected to govern the gradual evolution of localized damage. The obtained parameters are finally analyzed and discussed in terms of their dependence on the size of the initial uncracked ligament. The results show that both tensile softening models are able to capture the behavior of the specimens in the softening phase reasonably well. The tensile softening model does not affect the modulus of elasticity values but has a slight effect on the tensile strength and fracture energy. For the latter two parameters, both models detected the influence of specimen size on their values.

Discrete modeling of concrete failure and size-effect

Size-effect in concrete and other quasi-brittle materials defines the relation between the nominal strength and structural size when material fractures. The main cause of size-effect is the so-called energetic size-effect which results from the release of the stored energy in the structure into the fracture front. In quasi-brittle materials and in contrast to brittle materials, the size of the fracture process zone is non-negligible compared to the structural size. As a consequence, the resulting size-effect law is non-linear and deviates from the response predicted by linear elastic fracture mechanics. In order to simulate the size-effect, one needs to rely on numerical modeling to describe the formation, development and propagation of the fracture process zone. Although a number of models have been proposed over the years, it transpires that a correct description of the fracture and size-effect which accounts for boundary effects and varying structural geometry remains challenging. In this study, the Lattice Discrete Particle Model (LDPM) is proposed to investigate the effects of structural dimension and geometry on the nominal strength and fracturing process in concrete. LDPM simulates concrete at the aggregate level and has shown superior capabilities in simulating complex cracking mechanisms thanks to the inherent discrete nature of the model. In order to evaluate concrete size-effect and provide a solid validation of LDPM, one of the most complete experimental data set available in the literature was considered and includes three-point bending tests on notched and unnotched beams. The model parameters were first calibrated on a single size notched beam under three-point bending and on the mechanical response under unconfined compression. LDPM was then used to perform blind predictions on the load-crack mouth opening displacement curves of different beam sizes and notch lengths. Splitting test results on cylinders were also predicted. The results show a very good agreement with the experimental data. The quality of the predictions was quantitatively assessed. In addition, a discussion on the fracturing process and dissipated energy is provided. Last but not least, the Universal Size-Effect Law proposed by Bažant and coworkers was used to estimate concrete fracture parameters based on experimental and numerical data.

Fracture model for predicting tensile strength and fracture toughness of concrete under different loading rates

For the sake of safety evaluation of concrete structures under dynamic loading conditions, the rate dependences of concrete fracture parameters are necessary to be elucidated rationally first. Therefore, the aim of this study is to develop a fracture model for determining the size-independent tensile strength (ft), the fracture toughness (KIC) and the fracture energy (GF) under different loading rates. The fracture mechanism was analyzed comprehensively using three-point bending tests on concrete beams with varied depths (h) and ratios of the initial crack length to the beam depth (a0/h). As the loading rate increases, the number of fractured coarse aggregates is increased, and the shape of the load–displacement curve around the maximum fracture load (Fmax) becomes sharper. Subsequently, a fracture model was proposed to predict the size-independent parameters ft, KIC and GF by incorporating the average aggregate size (davg) and a discrete coefficient (β) to indicate material heterogeneity and discontinuity, respectively. The critical effective crack length (Δac) was discretized and quantified as davg multiplied by β. Closed-form solutions of ft, KIC and GF were obtained using Fmax based on the boundary effect model. ft, KIC and GF can be explicitly predicted after determining Fmax from the fracture tests, and were generally irrelevant to h and a0/h under individual loading rates, indicating their size-independence. The scatters in the ft, KIC and GF values were determined using a statistical analysis. The rate dependences of the fracture parameters were clarified quantitatively through elucidating the variations in ft, KIC and GF with respect to the loading rate. Moreover, the predicted fracture parameters based on the proposed model were insensitive to the possible variations of davg and β. This study brings a new insight into the fracture behaviors of concrete under different loading rates and provides scientific guidance for the disaster prevention of concrete structures.

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.

Fracture properties of concrete under freeze–thaw cycles and sulfate attack

• The fracture properties of concrete exposed to freeze–thaw cycles in different sulfate solutions were investigated. • The visual change, weight loss, relative dynamic elastic modulus, compressive strength and fracture parameters of concrete were studied. • The evolutions of the fracture process zone and damage scale of the specimens were analyzed. The fracture properties of concrete exposed to freeze–thaw (F-T) cycles in different sulfate solutions were investigated using pre-notched three-point bending beams. 5 % Na 2 S0 4 , 5 % MgS0 4 (by mass) and water, i.e. three exposure conditions were considered. The visual change, weight loss, relative dynamic elastic modulus, compressive strength and fracture parameters of concrete were measured at every 25F-T cycles. The evolutions of the fracture process zone (FPZ) and damage scale of the specimens were studied. Results indicate that (1) before 100F-T cycles, the specimens in water suffer severer damage than those in sulfate solutions, and the deterioration degree of the specimens in 5 % Na 2 S0 4 is the greatest after 200F-T cycles. (2) With increasing the F-T cycles, the obtained double- K fracture parameters, microcracking zone length and aggregate occlusion zone decrease linearly. Morever, the maximum FPZ length basically decreases, and a small increase tendency is observed at 125 and 150F-T cycles due to the significant increase in the deformation capacity of the specimens.

An extensive investigations on fracture parameters of concretes based on quaternary binders (QBC) by means of the DIC technique

• Determination of the fracture toughness K Ic S and CTO D c in quaternary binder concretes (QBC). • Application of Digital Image Correlation (DIC) measurements to QBC. • Two measuring devices were used in the studies, i.e. MTS 810 press and DIC system. • Linear and nonlinear fracture mechanics parameters were analyzed. • There is a correlation between K Ic S and CTO D c obtained by using two measuring devices. The following article presents an extensive investigations of the fracture mechanics parameters of concretes made of quaternary binders (QBC). A composition of the two most commonly used mineral additives, i.e. fly ash (FA) and silica fume (SF) in combination with nanosilica (nS), has been proposed as a partial replacement of the ordinary Portland cement (OPC) binder. Four series of concretes were made, one of which was the reference concrete (REF) and the remaining three were QBC. During the studies K Ic S and CTOD c were investigated. In addition, digital image correlation (DIC) technique was used. Based on the conducted studies it was found that the QBC containing: 80% OPC, 5% FA, 10% SF, and 5% nS have shown the best results of K Ic and CTOD c, whereas the worst parameters were characterized by concrete with more content of FA additive in the concrete mix, i.e.,15%.