Search Results (48)

The semi-circular bend (SCB) test is widely used to characterize asphalt mixture cracking resistance. However, the practical usefulness of the test depends on the reliability of the measured fracture parameters. This study investigates SCB testing from a reliability perspective, with the aim of identifying the specimen number required for dependable interpretation and the testing conditions that provide the most stable response. The analysis considered nominal maximum aggregate size, notch depth, binder type, aging condition, test temperature, and loading rate. Fracture energy, peak load, and flexibility index, together with their cumulative coefficients of variation, were tracked from n = 3 to n = 6, while six-specimen raw datasets were used for Weibull reliability analysis. The results show that notch depth had the clearest effect on response stabilization, with the 15 mm notch providing the most reliable configuration and reaching the adopted variability limits earlier than the other notch depths. The descriptive Weibull analysis further indicated that the SBS-modified mixture exhibited the highest fracture-energy consistency within the tested dataset, whereas long-term aging, testing at 0 °C, and loading at 50 mm/min were associated with the lowest fracture-energy consistency within the tested dataset. Overall, SCB interpretation should be guided by response reliability, not mean fracture parameters alone. On this basis, a reliability-based SCB framework is proposed to support more dependable mixture comparison and more rational specimen planning. Full article

To reveal the mesoscopic fracture mechanism of fissured concrete, this study employed the discrete element method (DEM) and adopted the parallel bond model (PBM) within the two-dimensional particle flow code (PFC2D) to construct a mesoscopic model of concrete semi-circular bending (SCB) specimens with prefabricated fissures. Three sets of schemes were designed by varying prefabricated fissure angles (0–45°), aggregate contents (30–45%), and porosities (3–6%), and numerical simulations of three-point bending loads were conducted to explore the effects of each parameter on the crack propagation law and load-bearing performance of the specimens. Validation was performed by comparing the simulated load–displacement curves with the typical quasi-brittle mechanical characteristics of concrete (exhibiting “linear elastic rise–pre-peak stress fluctuation–nonlinear decline”) and verifying that the DEM could accurately capture the entire process from microcrack initiation at the aggregate–mortar interface, crack deflection/bifurcation induced by pores, to macroscopic fracture penetration—consistent with the known mesoscopic damage evolution law of concrete. The results indicate that the crack propagation mode evolves from straight extension to tortuous branching as parameters change. Moreover, the peak strength first increases and then decreases with the increase in each parameter: when the fissure angle is 15°, the aggregate content is 35%, and the porosity is 4%, the specimens achieve an optimal balance between crack propagation resistance and energy dissipation, resulting in the best load-bearing performance. Specifically, the prefabricated fissure angle dominates the stress type (tension–shear transition); aggregates regulate crack resistance through a “blocking–diverting” effect; and pores, acting as defects, influence stress concentration. This study verifies the reliability of DEM in simulating concrete fracture behavior, enriches the mesoscopic fracture theory of concrete, and provides reliable references for the optimization of concrete material proportioning (e.g., aggregate–porosity ratio adjustment) and anti-cracking design of infrastructure (e.g., pavement, tunnel linings) in engineering practices. Full article

The uncracked semi-circular bend (SCB) test has recently gained attention as a simple and material-efficient method for determining the tensile strength of brittle geomaterials. However, as reported in the literature and confirmed by our experiments, localized damage at the roller supports remains a critical limitation that may compromise measurement accuracy and test validity. This study addresses this limitation through experimental testing on red and gray sandstone, complemented by numerical simulations to provide deeper insight into stress distribution and fracture mechanisms in the SCB test. Experimental results showed that six out of twelve specimens experienced local damage, ranging from slight crushing and surficial cracking at the base roller zones in red sandstone to rock chipping in gray sandstone. The stiffer sandstone exhibited more severe local damage due to its limited deformability. These damages were attributed to minor geometric imperfections introduced during sample preparation. Nevertheless, all tests yielded valid tensile strength values, with SCB results showing good agreement with Brazilian test outcomes and demonstrating significantly lower coefficients of variation. Finite element simulations confirmed that crack initiation consistently occurred at the middle of the flat edge under pure tensile stress, indicating a mode I fracture mechanism. Numerical analyses further revealed pronounced stress concentrations, particularly compressive stresses, at the roller contact zones, induced by the specimen’s low span-to-depth ratio, which increased the fracture load required for failure. Full article

Microwave-assisted rock fragmentation has been considered as one of the most promising technologies in rock excavation, but due to the fact that excavation is usually carried out in water-rich environments, understanding the dynamic fracture properties of rocks with different water contents after microwave irradiation is thus desirable. This study employed an enhanced split Hopkinson pressure bar (SHPB) system to perform dynamic fracture tests on pre-cracked semi-circular bending (SCB) specimens. It systematically explores the changes in the mechanical properties of sandstone under both dry and saturated conditions after exposure to 700 W of microwave radiation for 10 min. Infrared thermal imaging was utilized to capture the temperature distribution across the specimens, while digital image correlation (DIC) and high-speed photography were used to simultaneously record the crack propagation process. Based on the principle of energy conservation, the analysis of energy dissipation during fracture was performed, and the micro-damage evolution mechanism of the material was revealed through scanning electron microscopy (SEM). The results demonstrated that saturated sandstone exhibited a more rapid heating response and significantly lower dynamic fracture toughness and fracture energy compared to dry samples after microwave irradiation. These findings indicate that water saturation amplifies the weakening effect induced by microwaves, making the rock more susceptible to low-stress fractures. The underlying damage mechanisms of microwave radiation on water-bearing sandstone were interpreted with the theory of pore water pressure and structural thermal stresses. Full article

Salts can have detrimental effects on asphalt pavements, leading to permanent damage that compromises their durability and sustainability. This study investigates the damage mechanisms of asphalt pavements under coupled salt–thermal–mechanical effects using multi-scale modeling. Pull-off and semicircular bending (SCB) tests were conducted to determine material parameters and validate numerical models. Experimental data demonstrated that after 48 h of salt treatment at −10 °C, specimens exhibited reductions in cohesive strength ranging from 23.5% to 26% and adhesive strength decreasing by 25% to 44% compared to untreated controls. More severe degradation was observed at 0 °C, with cohesive strength diminishing by up to 63.8% and adhesive strength declining by up to 71.6%. A multi-scale finite element (FE) pavement model incorporating cohesive zone modeling (CZM) was developed to simulate damage behavior within asphalt concrete. Salt diffusion analysis revealed limited penetration depth within short exposure periods, and results showed that salt penetration reached only about 10 mm into the pavement layers after 48 h. Results indicated significant reductions in adhesive and cohesive strengths due to salt exposure, with damage susceptibility increasing under combined thermal fluctuations and mechanical loading. Additionally, the effects of moving load magnitude and speed on pavement damage were examined, showing higher damage accumulation at lower speeds and heavier loads. This research provides insights into pavement deterioration mechanisms, contributing to improved durability and maintenance strategies for asphalt pavements in salt environments. Full article

The three-point bending test is a key method for determining parameters related to the mechanical fracture properties of rocks. In this study, shale outcrops from Changning County, Sichuan Province, China, were selected. Three-point bending experiments were performed on shale semi-circular disks with a central straight crack, tested both perpendicular and parallel to the bedding direction. The corresponding load–displacement curves and crack opening displacements were obtained. The opening displacements of the specimens were measured through digital image technology, and the tensile strength and stiffness of the specimens were further calculated. A finite element model of the three-point bending test was developed. By integrating the finite element model with the experimentally obtained load–displacement curves, the anisotropic elastic moduli of the shale were inversely determined. Fracture toughness was calculated using two approaches: a formula from the International Society for Rock Mechanics and numerical methods using the finite element model, which was appropriately configured with the previously obtained elastic modulus values. The stress intensity factors for each specimen were calculated and compared. The energy release rate of shale was computed based on the fracture toughness. Results showed that both the fracture toughness and energy release rate of shale were greater in the perpendicular bedding direction than in the parallel direction. As an example, one specimen’s elastic modulus, opening displacement, and energy release rate obtained from experiments were input into the numerical simulation of the three-point bending test. The simulated load–displacement curve matched the experimental results well. This study provides a comprehensive approach to evaluating the anisotropic mechanical fracture properties of shale formations, which is essential for improving the accuracy of hydraulic fracture prediction models and enhancing the efficiency of shale gas extraction. Full article

The semi-circular bending method (SCB) is a useful test for evaluating the cracking resistance of asphalt mixtures with added reclaimed asphalt shingles. A mixture of the asphalt concrete AC 16 with 50/70 paving bitumen was used for the binder course test as a reference mix. The purpose of the paper is to evaluate two aging conditions (short-term and long-term) of the above-mentioned asphalt mixtures in relation to their fracture properties. Laboratory experiments are enhanced with the application of image processing techniques (digital image correlation and image segmentation) that account for the asphalt mixture heterogeneity. Consequently, they can provide a more detailed description of the specimen performance. Statistical analyses of the laboratory results indicate that the best sensitivity in terms of differentiating the tested mixtures, especially taking into account the aging conditions of the mixtures, was observed for the post-peak parameters such as the flexibility index (FI), toughness index (TI), and, above all, cracking resistance index (CRI), for which the average coefficient of the result variability is approximately 10%, while for the FI and TI parameters it is approximately 30%. Digital image correlation analyses provided a confirmative illustration of the aforementioned observation. Full article

This paper presents gradations of both Hot In-place Recycling (HIR) and Cold In-place Recycling (CIR) and analysis of cores collected from CIR/Hot Mix Asphalt (HMA) overlay sections in Iowa. Milling samples were obtained from an HIR recycling project on IA 22 in Wellman, Iowa. It was concluded that the average gradation of HIR millings was coarser than that of CIR millings by retaining the original aggregate gradations. Cores were then extracted from CIR pavement with foamed asphalt overlaid by HMA at the right wheel path and between wheel paths at eight locations on US 34 in Mills and Wapello Counties. The cores were cut into discs to isolate the pavement layers, and the discs were fabricated into semicircular bending test specimens. Based on the Semicircular Bending (SCB) tests performed on both CIR and HMA specimens, CIR specimens with higher asphalt binder contents exhibited higher flexibility index values. Since CIR pavement layers were found to be more flexible than the HMA overlay layers, it can be speculated that CIR layers may serve as a stress-relieving layer and mitigate reflective cracking. Full article

The substitution of virgin asphalt binder with reclaimed asphalt pavement (RAP) has environmental and economic merits, however, cracking susceptibility arises due to the aged asphalt binder within RAP. The objectives of this study are to (1) enhance the cracking resistance of asphalt mixtures containing 50% RAP utilizing recycling agents (RAs) derived from six petroleum-based and bio-based materials, (2) conduct an environmental impact assessment (represented by global warming potential “GWP”) for high-RAP mixtures including RAs, and (3) estimate the cost effectiveness of including high-RAP content in asphalt mixtures. Based on the RAP asphalt binder performance grade (PG), base asphalt binder PG, and RAP content, the RA contents were determined to achieve a target asphalt binder of PG 76-22. A control mixture was benchmarked for comparison, specified for high-traffic volume roads, and contained PG 76-22 polymer-modified asphalt binder. The engineering performance of studied asphalt mixtures was evaluated using the Hamburg wheel-tracking (HWT), semi-circular bend, Illinois flexibility index, Ideal cracking tolerance, and thermal stress-restrained specimen tensile strength tests. It was found that petroleum-derived aromatic oil, soy-based oil, and tall oil fatty acid-based RAs demonstrated a successful restoration of aged RAP asphalt binder without compromising the permanent deformation resistance. The 50% RAP mixtures emitted less GWP by 41% and 42.9% using petroleum- and bio-oil RAs, respectively, and achieved a 31% cost reduction compared to the control mixtures. Full article

This article offers a comprehensive analysis of the impact of MSPF on concrete’s mechanical properties and fracture behavior. Combining findings from numerical simulations and laboratory experiments, our study validates numerical models against diverse fiber percentages and aggregate distributions, affirming their reliability. Key findings reveal that mixed-mode fracture scenarios in fiber-reinforced concrete are significantly influenced by the mode mixity parameter (M^e), quantifying the balance between mode I and mode II fracture components, ranging from 1 (pure mode I) to 0 (pure mode II). The introduction of the effective stress intensity factor (K_eff) provides a profound understanding of the material’s response to mixed-mode fracture. Our research demonstrates that as M^e approaches zero, indicating shear deformation dominance, the concrete’s resistance to mixed-mode fracture decreases. Crucially, the addition of MSPF considerably enhances mixed-mode fracture toughness, especially when Me ranges between 0.5 and 0.9, resulting in an approximately 400% increase in fracture toughness. However, beyond a specific threshold (approximately 4% FVF), diminishing returns occur due to reduced fiber–cement mortar bonding forces. We recommend an optimal fiber content of around 4% by weight of the total concrete mixture to avoid material distribution disruption and strength reduction. The practical implications of these findings suggest improved design strategies for more resilient infrastructure, particularly in earthquake-resistant constructions and sustainable urban development. These insights provide a valuable framework for future research and development in concrete technology. Full article

Cracks in rock and concrete have a great adverse effect on the stability of engineering structures; however, there are few studies on X-shaped fissures which widely exist in rock and concrete structures. Based on this background, three-point bending fracture tests of SCB specimens containing X-shaped fissures are carried out. The momentum equations in the SPH method are improved, and the crack propagations of SCB specimens under three-point bending are simulated. The results show that cracks grow simply along the vertical direction in the sample with no X-shaped fissures, and the existence of an X-shaped fissure changes the crack growth path and final failure modes of the SCB samples. The crack propagation simulation results are consistent with the experimental results, which verifies the rationality of the improved SPH method. The load–displacement curves mainly present three typical stages: the initial compaction stage, linear elastic deformation stage, and failure stage. The peak load decreases first then increases with an increase in eccentricity. With an increase in X-shaped fissure length and decrease in X-shaped fissure angle, the peak load decreases. The damage counts remain at 0 at the initial loading stage, corresponding to the initial compaction stage and the linear elastic deformation stage, and increase sharply at the later loading stage, corresponding to the failure stage, which is consistent with the experimental results. The influence mechanisms of X-shaped fissures on the crack propagation paths are discussed; the existence of different X-shaped fissure morphologies aggravate the tensile stress concentration at specific positions, leading to different crack propagation modes in the experiments. The research results can provide a certain reference for understanding the failure mechanisms of engineering structures containing X-shaped fissures and promote the applications of the SPH method into the simulations of cross-fissure crack propagations. Full article

The effect of moisture on the fracture resistance of asphalt concrete is a significant concern in pavement engineering. To investigate the effect of the water vapor concentration on the fracture properties of asphalt concrete, this study first designed a humidity conditioning program at the relative humidity (RH) levels of 2%, 50%, 80%, and 100% for the three types of asphalt concrete mixtures (AC-13C, AC-20C, and AC-25C).The finite element model was developed to simulate the water vapor diffusion and determine the duration of the conditioning period. The semi-circular bending (SCB) test was then performed at varying temperatures of 5 °C, 15 °C, and 25 °C to evaluate the fracture energy and tensile strength of the humidity-conditioned specimens. The test results showed that the increasing temperature and the RH levels resulted in a lower peak load but greater displacement of the mixtures. Both the fracture energy and tensile strength tended to diminish with the rising temperature. It was also found that moisture had a significant effect on the tensile strength and fracture energy of asphalt concrete. Specifically, as the RH level increased from 2% to 100% (i.e., the water vapor concentration rose from 0.35 g/m³ to 17.27 g/m³), the tensile strength of the three types of mixtures was reduced by 34.84% on average, which revealed that the water vapor led to the loss of adhesion and cohesion within the mixture. The genetic expression programming (GEP) model was developed to quantify the effect of water vapor concentrations and temperature on the fracture indices. Full article

A large number of construction practice projects have found that there are many joints and microcracks in rock, concrete, and other structures, which cause the complexity of rock mechanical properties and are the main cause of geological or engineering disasters such as earthquakes, landslides, and rock bursts. To establish a rock fracture toughness evaluation method and understand the distribution range of fracture toughness of Longmaxi Formation shale, this study prepared three-point bending semi-circular disk shale samples of Longmaxi Formation with different crack inclination angles. The dimensionless fracture parameters of the samples, including the dimensionless stress intensity factors of type I, type II, and T-stress, were calibrated using the finite element method. Then, the peak load of the samples was tested using quasi-static loading, and the load–displacement curve characteristics of Longmaxi Formation shale and the variation in fracture toughness with crack inclination angle were analyzed. The study concluded that the specimens exhibited significant brittle failure characteristics and that the stress intensity factor is not the sole parameter controlling crack propagation in rock materials. With an increase in crack inclination angle, the prefabricated crack propagation gradually transitions from being dominated by type I fracture to type II fracture, and the T-stress changes from negative to positive, gradually increasing its influence on the fracture. An excessively large relative crack length increases the error in fracture toughness test results. Therefore, this paper suggests that the relative crack length a/R should be between 0.2 and 0.6. The fracture load distribution range of shale samples with different crack angles is 3.27 kN to 10.92 kN. As the crack inclination angle increases, the maximum load that the semi-circular disk shale samples can bear gradually increases. The pure type I fracture toughness of Longmaxi Formation shale is 1.13–1.38 MPa·m^1/2, the pure type II fracture toughness is 0.55–0.62 MPa·m^1/2, and the T-stress variation range of shale samples with different inclination angles is −0.49–9.48 MPa. Full article

Fatigue cracking is one of the primary distresses of asphalt pavements, which significantly affects the asphalt pavement performance. The fatigue behavior of the asphalt mixture observed in the laboratory test can vary depending on the type of fatigue test and the dimension and shape of the test specimen. The variations can make it difficult to accurately evaluate the fatigue properties of the field asphalt concrete. Accordingly, this study proposed a reliable method to evaluate the fatigue behavior of the asphalt field cores based on discrete element modeling (DEM). The mesoscopic geometric model was built using discrete element software PFC (Particle Flow Code) and CT scan images of the asphalt field cores. The virtual fatigue test was simulated in accordance with the semi-circular bending (SCB) test. The mesoscopic parameters of the contacting model in the virtual test were determined through the uniaxial compression dynamic modulus test and SCB test. Based on the virtual SCB test, the displacement, contact forces, and crack growth were analyzed. The test results show that the fatigue life simulated in the virtual test was consistent with that of the SCB fatigue test. The fatigue cracks in the asphalt mixture were observed in three stages, i.e., crack initiation, crack propagation, and failure. It was found that the crack propagation stage consumes a significant portion of the fatigue life since the tensile contact forces mainly increase in this stage. Full article

This study experimentally investigates the influence of metal chips and glass fibers on the mode I fracture toughness, energy absorption, and tensile strength of polymer concretes (PCs) manufactured by waste aggregates. A substantial portion of the materials employed in manufacturing and enhancing the tested polymer concrete are sourced from waste material. To achieve this, semi-circular bend (SCB) samples were fabricated, both with and without a central crack, to analyze the strength and fracture behavior of the composite specimens. The specimens incorporated varying weight percentages comprising 50 wt% coarse mineral aggregate, 25 wt% fine mineral aggregate, and 25 wt% epoxy resin. Metal chips and glass fibers were introduced at 2, 4, and 8 wt% of the PC material to enhance its mechanical response. Subsequently, the specimens underwent 3-point bending tests to obtain tensile strength, mode I fracture toughness, and energy absorption up to failure. The findings revealed that adding 4% brass chips along with 4% glass fibers significantly enhanced energy absorption (by a factor of 3.8). However, using 4% glass fibers alone improved it even more (by a factor of 10.5). According to the results, glass fibers have a greater impact than brass chips. Introducing 8% glass fibers enhanced the fracture energy by 92%. However, in unfilled samples, aggregate fracture and separation hindered crack propagation, and filled samples presented added barriers, resulting in multiple-site cracking. Full article