Critical Cracking Research Articles

Shear strengthening of self-compacting lightweight concrete beams with steel fibers and unbonded prestressing bars

This study aims to investigate the shear behavior of self-compacting lightweight concrete (SCLC) beams strengthened with steel fibers and unbonded prestressing bars. For this, the experimental program included the use of hooked-end steel fibers with volume fractions of 0.5 %−0.75 %, and high-strength prestressing bars with varying prestressing forces and transverse reinforcement ratios. The addition of steel fibers in SCLC significantly increased the normalized shear capacity of beams, reaching up to 2.50 times higher values, whereas the substitution of normal coarse aggregates with lightweight aggregate resulted in a decrease of up to 10.1 %. The shear strengthening of SCLC beams with unbonded prestressing bars provided a substantial increase in the ultimate shear capacity up to 2.06 times higher than that of beam without reinforcement. Increasing the prestressing force of bars effectively restrained the occurrence of critical shear cracking, consequently enhancing the shear capacity. Utilizing hybrid reinforcement with fibers and prestressing bars resulted in a transition of failure mode from shear to flexural, attributed to the improved shear capacity. For the SCLC beams reinforced with steel fibers, the shear strengths were predicted by previous prediction models. When modifier for lightweight concrete was considered, the Imam’s model most precisely predicted the shear strengths with an average vu/vn ratio of 1.00 and SD of 0.07. The fib model Code, based on the modified compression field theory, provided accurate predictions for the shear capacities of SCLC beams with unbonded prestressing bars, exhibiting an average Vu/Vn ratio of 1.00 and SD of 0.09. The fiber-resisting component was reasonably considered in the model with an average Vu/Vn ratio of 1.02 and SD of 0.07.

Development and Optimization of a Nanocomposite Gel Foam for Inhibiting Coal Spontaneous Combustion

ABSTRACT A nanocomposite gel foam inhibitor was made by grafting tea polyphenol onto sodium polyacrylate through the graft polymerization reaction, which was then combined with graphene oxide and a foaming agent. Response surface analysis was used to optimize the ratio to improve the water absorption of the inhibitor. The microstructure of the inhibitor was studied using X-ray diffraction and Fourier-transform infrared spectroscopy. The anti-spontaneous combustion performance of the nano-composite gel foam was tested through a leakage wind plugging experiment and a thermogravimetric experiment. The results show that the optimal additions of graphene, tea polyphenol, and potassium persulfate were 0.76%, 2.806%, and 0.727% of acrylic acid, respectively. The ultimate pressures of the gel foam before and after fully forming the gel were 0. 26 kPa and 0. 736 kPa, respectively, which satisfies the demand for leakage wind plugging in a coal mine. The critical temperature and dry cracking temperature of the gel foam were 112.4°C and 201.0°C, respectively, and the quality of the oxygen-absorbing and weight-gaining stage were almost unincreased. The gel foam showed good water retention, oxygen barrier, and antioxidant properties and can play a significant role in inhibiting the coal spontaneous combustion.

Simulation-based Optimal Sensor Placement in Tunnel Structures considering Uncertain in-situ Conditions and Multiple Loading Scenarios

The expansion of the underground infrastructure and the necessity to maintain the full functionality of the existing tunnels highlights the role of structural health monitoring in keeping track of the behaviour of the structure over time. In the study, the focus is on segmental tunnel lining for deep and long tunnels. The aim of this research is to investigate the best position for sensor placement based on the potential loading scenarios that might insist on the tunnel structure. Since for the evaluation of the lining safety we are interested in the maximum stresses reached in the structure, which might lead to durability critical cracking or crushing of concrete, a method is presented to suggest optimal positions for strain gauges. Due to uncertain geological conditions, multiple loading configurations acting on the tunnel structure are simulated using a finite element model. The results are employed to better identify the sensor locations, on the basis of where damage is prone to occur and where maximum information about the stress state in the structure can be inferred. The results obtained by the framework are discussed in light of a practical engineering perspective and the advantages as well as the limitations of the proposed approach are expounded.

Investigation of corrosion resistance and interfacial bonding properties of silver/silicone zinc-rich coating/substrate system in marine environment

This paper aims to study the corrosion resistance and interfacial bonding properties of the silver zinc-rich coating/substrate system and silicone zinc-rich coating/substrate system. The actual working state of the system in marine environment was simulated by the coupled effects of corrosion and pre-stretching. Scanning electron microscope (SEM) with an energy dispersive X-ray spectrometer (EDS) was used in corrosion test to analyze the surface morphology and corrosion products. Meanwhile, the degradation degree of the corrosion resistance was investigated based on the electrochemical impedance spectroscopy (EIS) and water absorption rate. Three-point bending test and tensile test combined with digital image correlation (DIC) were performed to evaluate the interfacial bonding properties of these two systems based on the elastic modulus and critical cracking strain. Results show that the dense accumulation of aluminum and zinc powders makes the silver zinc-rich coating have higher corrosion resistance compared to the silicone zinc-rich coating. The combined effects of corrosion and pre-stretching on the fracture toughness of silicone zinc-rich coating are larger than that of the silver zinc-rich coating. The interfacial bonding properties of the coating/substrate systems gradually degrade as the pre-stretching load approaches the critical cracking load of the coating.

Suppression of cracking in drying colloidal suspensions with chain-like particles.

The prevention of drying-induced cracking is crucial in maintaining the mechanical integrity and functionality of colloidal deposits and coatings. Despite exploring various approaches, controlling drying-induced cracking remains a subject of great scientific interest and practical importance. By introducing chain-like particles composed of the same material and with comparable size into commonly used colloidal suspensions of spherical silica nanoparticles, we can significantly reduce the cracks formed in dried particle deposits and achieve a fivefold increase in the critical cracking thickness of colloidal silica coatings. The mechanism underlying the crack suppression is attributed to the increased porosity and pore sizes in dried particle deposits containing chain-like particle, which essentially leads to reduction in internal stresses developed during the drying process. Meanwhile, the nanoindentation measurements reveal that colloidal deposits with chain-like particles exhibit a smaller reduction in hardness compared to those reported using other cracking suppression approaches. This work demonstrates a promising technique for preparing colloidal coatings with enhanced crack resistance while maintaining desirable mechanical properties.

Fracture characterisation of polymer modified asphalt concrete mixes for applications in diverse climatic regions

ABSTRACT Among various types of polymers used as modifiers, Styrene-Butadiene-Styrene (SBS) is the most commonly used as an elastomeric type of polymer to improve the cracking and rutting resistance of asphalt concrete (AC) mixtures. In this study, we explored low and intermediate-temperature fracture characteristics of mixes prepared with binders modified with incrementally increased percentages of SBS. Low and intermediate-temperature fracture experiments were conducted to compare fracture characteristics of modified and unmodified binders. The application of polymer modification is demonstrated using dense-graded and gap-graded stone mastic asphalt (SMA) type gradations. Based on the fracture properties and trends observed, a performance-based AC mixture grading and selection system for different climatic regions is proposed. Fracture energy converged to a baseline value governed by the low-temperature binder grade regardless of the mix type and polymer modification. Polymer modification in fine-graded mixes was shown to remain effective for testing temperatures of −12°C and above for binders with −22 low PG. The concept of critical cracking temperature is introduced to guide the application of polymer modification in different climatic regions. The findings of this research will support agencies in developing binder selection protocols to optimise the cracking resistance of mixes in different climatic regions.

Study on the change rule of low-temperature properties of asphalt binder with aging time

ABSTRACT Low-temperature cracking is common distress in asphalt pavements. Understanding the change rule of its low-temperature properties is helpful in investigating the optimal maintenance timing of asphalt pavement. In this paper, the change rule of low-temperature properties of asphalt binders with aging time is investigated by various indexes. Four asphalt binders and four anti-aging modified asphalt binders were subjected to the rolling thin film oven (RTFO) test and pressure aging vessel (PAV) test. Dynamic Shear Rheometer (DSR) with the 4 mm parallel plates was used to determine the low-temperature properties of asphalt binders. The results show that aging and low temperature have a negative impact on the low-temperature properties of asphalt binders. The anti-aging agent improves the low-temperature properties and aging resistance of asphalt binders. Compared with other indexes, the critical cracking temperature Tc and the critical temperature difference ΔTc are more suitable for analysing the effect of aging on the low-temperature properties of asphalt binders. Also, the ΔTc shows the three-stage decreasing rule of ‘fast-slow-fast’, with the inflection points. It meant that the low-temperature properties of asphalt binders had been altered at the inflection points, which provided a favourable reference to research on the optimal maintenance timing of asphalt pavement.

Characterizing stress development and cracking of ceramic particulate coatings during drying

AbstractDuring drying, liquid‐applied particulate coatings develop stress and are consequently prone to stress‐induced defects, such as cracking, curling, and delamination. In this work, the stress development and cracking of coatings, prepared from aqueous silica and zinc oxide particle suspensions, were characterized using cantilever beam deflection with simultaneous imaging of the coating surface. Drying uniformity was improved and lateral or edge‐in drying was discouraged by using thin silicone walls around the perimeter of the cantilever. Coatings prepared from larger monodisperse silica particles (D50 ∼ 0.9 µm) dried uniformly but had a high critical cracking thickness (>150 µm) that prevented simultaneous study of stress development and cracking. Coatings prepared from smaller silica particles (D50 ∼ 0.3 µm) cracked readily at low thicknesses but exhibited edge‐in drying that complicated the stress measurement data. This drying nonuniformity was connected to the potential for these small particles to accumulate at the coating surface during drying. Hence, the selection of particle size and density was critical to drying uniformity when characterizing stress development and cracking. Coatings prepared from suspensions of zinc oxide particles (D50 ∼ 0.4 µm) were well‐suited for these studies, with uniform drying stress peaking at ∼1 MPa. Characteristic features in the stress development data above and below the critical cracking thickness (53 µm) were identified, demonstrating that cantilever beam deflection is a useful tool for studying the effectiveness of crack mitigation methods and the fundamentals of coating fracture during drying.

Corrosion-induced cracking behavior of lightweight aggregate concrete: Experimental and numerical study

The corrosion-induced cracking potentially undermines the durability of concrete structures, while the fracture mechanism of lightweight aggregate concrete (LWAC) differs from that of the normal concrete subjected to expansive corrosion products due to the low-strength coarse aggregate. Eleven beams considering different diameters of LWA, the volume fraction of LWA, and surface crack widths were subjected to the accelerated corrosion setup, and the corrosion-induced cracks and steel mass loss were observed. The mesoscopic finite element model was established for the analysis of LWAC, and the model was verified according to the test results. The results of numerical study showed that decrease in the strength of LWA led to the inner damage of LWAC, while the increase in cover thickness made the cracking process of LWAC more rapid. On the basis of mesoscopic simulations, the critical cracking mass loss of steel bar and the empirical cracking parameter were derived, and then the linear equations were adopted for the establishment of relations between mass loss of corroded rebar and the corrosion-induced surface crack width by considering the effect of concrete cover and the rebar spacing.

Bending behavior of strain hardening cementitious composites based on the combined fiber-interface constitutive model

Strain-hardening cementitious composites (SHCC) are a class of fiber-reinforced cementitious composites characterized by strain hardening behavior accompanied by multiple cracking, which is designed by tailoring matrix, fiber, and fiber/matrix interface based on the micromechanics theory. At present, there are several numerical simulation methods to study the multiple cracking behavior, such as discrete crack model, unity finite element method, 3D smeared crack model, multiscale constitutive models, and lattice model. However, it is difficult to provide a good balance between accuracy and computational efficiency due to the vast number of fiber and the complex relationship between fibers, matrix, and interface. This paper developed a combined fiber-interface constitutive model based on the single fiber pull-out theory, and established a combined unit. A three-dimensional two-phase finite element model was established, which had been proved to be effective on simulating the cracking behavior of SHCC. Furthermore, the effects of fiber volume fraction and matrix cracking strength on the four-point bending properties of SHCC were investigated. Further study showed that there was a critical matrix cracking strength for a specified fiber and matrix. This value can optimize the design of SHCC to achieve high strength and high toughness performance.

Strategies and Challenge of Thick Electrodes for Energy Storage: A Review

In past years, lithium-ion batteries (LIBs) can be found in every aspect of life, and batteries, as energy storage systems (ESSs), need to offer electric vehicles (EVs) more competition to be accepted in markets for automobiles. Thick electrode design can reduce the use of non-active materials in batteries to improve the energy density of the batteries and reduce the cost of the batteries. However, thick electrodes are limited by their weak mechanical stability and poor electrochemical performance; these limitations could be classified as the critical cracking thickness (CCT) and the limited penetration depth (LPD). The understanding of the CCT and the LPD have been proposed and the recent works on breaking the CCT and improving the LPD are listed in this article. By comprising these attempts, some thick electrodes could not offer higher mass loading or higher accessible areal capacity that would defeat the purpose.

Cracking and Fiber Debonding Identification of Concrete Deep Beams Reinforced with C-FRP Ropes against Shear Using a Real-Time Monitoring System.

Traditional methods for estimating structural deterioration are generally costly and inefficient. Recent studies have demonstrated that implementing a network of piezoelectric transducers mounted to critical regions of concrete structural members substantially increases the efficacy of the structural health monitoring (SHM) method. This study uses a recently developed electro-mechanical-admittance (EMA)-based SHM system for real-time damage diagnosis of carbon FRP (C-FRP) ropes installed as shear composite reinforcement in RC deep beams. The applied SHM technique uses the frequency response measurements of a network of piezoelectric lead zirconate titanate (PZT) patches. The proposed strengthening methods using C-FRP ropes as ETS and NSM shear reinforcement and the applied anchorage techniques significantly enhanced the strength and the overall performance of the examined beams. The retrofitted beams exhibited increased shear capacity and improved post-peak response with substantial ductility compared with the brittle failure of the non-strengthened specimens. The health condition and the potential debonding failure of the applied composite fiber material were also examined and quantified using the proposed SHM technique. Damage quantification of C-FRP ropes is achieved by comparing and assessing the values of several statistical damage indices. The experimental results demonstrated that the proposed monitoring system successfully diagnosed the region where the damage occurred by providing early warning of the forthcoming critical shear cracking of concrete and C-FRP rope debonding failures. Furthermore, the internal PZT transducers showed sound indications of the C-FRP rope's health condition, demonstrating a direct correlation with the mechanical performance of the fibers.

Critical cracking thickness of drying polymer films.

Polymer coatings are used for a number of applications such as for decorative purposes, to protect surfaces and as functional parts of devices. The mechanical integrity of the coatings is critical to their function and hence it is important that the coatings do not fail during their lifetime. Here, we present a simple model to determine conditions under which drying films of polymer solutions can crack. The model accounts for the properties of the polymer film and substrate and predicts the tensile stress developed in the drying film. As the tensile stress increases and exceeds a critical value, the film relaxes by nucleating a crack. The model predicts a critical thickness below which the film does not crack. The predicted critical cracking thickness is compared with experiments performed on drying films of silicone resin on six different substrates with the value of Young's modulus spanning over six decades. The predicted trend matches the measurements.

Experimental investigation on using antioxidants to control spontaneous combustion of lignite coal

To investigate the effects of antioxidants on the microstructure and characteristic parameters of coal during the oxidation process of coal, three antioxidants (tea polyphenols, resveratrol, and β-carotene) at different mass ratios (1 wt%, 3 wt%, 5 wt%, and 7 wt%) were selected to treat raw coal samples. Fourier transform infrared spectroscopy was conducted to explore the changes in functional group content before and after antioxidant treatment of coal samples. The results show that the content of functional groups in coal had a trend of decreasing and then increasing with a rising mass fraction of antioxidants, and the optimum mass ratio of antioxidants was predicted. On this basis, the effects of antioxidants on characteristic temperature points and gas products of coal samples were determined by the thermogravimetric-infrared experiment. The test results show that the critical temperature and dry cracking temperature of the antioxidant-treated coal samples increased to different degrees compared to the raw coal during the low-temperature oxidation stage. Meanwhile, the concentration and yield of CO, CO2, and CH4 of the antioxidant-treated coal samples showed a decreasing trend compared to raw coal. The chemical kinetics method was used to determine the apparent activation energy in the process of coal oxidation. The apparent activation energies of TP, Res, and β-Car inhibited coal samples in the stage of water evaporation and gas desorption increased by 39 %, 30 %, and 26 %, respectively, compared with the raw coal, while the apparent activation energies in the oxygen absorption and weight gain stage were respectively increased 19.4 %, 12.9 %, and 14.3 % higher than raw coal. The ability of antioxidants to inhibit the spontaneous combustion of coal is TP > Res > β-Car. The results enrich the research on antioxidant inhibition of coal spontaneous combustion and provide a theoretical basis for preventing coal spontaneous combustion.

Evaluation of the Low-Temperature Cracking Performance of Recycled Asphalt Mixture: A Development of Equivalent Fracture Temperature

This study aims to develop a new index to evaluate the low-temperature cracking performance (LTCP) of recycled asphalt mixtures. To achieve this, the assumption that fracture energy has limitation in evaluating the LTCP of mixtures was put forward firstly. To reveal this limitation theoretically, a concept of energy absorption rate that characterized the fracture energy of the mixtures was then given. Thirdly, an equivalent fracture temperature (EFT) corresponding to the critical cracking temperature in a thermal stress restrained specimen test (TSRST) was proposed to evaluate the LTCP of mixtures based on a three-point bending beam (3PBB) test. Finally, some data derived from previous work were collected to verify the proposed assumption, and the 3PBB test and TSRST were conducted to investigate the LTCP of five recycled mixtures with different reclaimed asphalt pavement (RAP) material contents and to verify the rationality of the proposed EFT. The results confirmed that fracture energy had some limitations in predicting the LTCP of mixtures. The EFT was as accurate as the critical cracking temperature in evaluating the LTCP of mixtures. Compared to fracture energy and critical cracking temperature, EFT had bigger absolute change and relative change between different mixtures, meaning it was better to differentiate the LTCP of mixtures. Compared to failure strain, EFT was also more definite and less variable when processed by different data analysts, meaning it was more objective.

RC beams retrofitted by FRP oriented in any direction: Influence of the effectiveness factors

Shear strength of FRP-retrofitted RC beams is usually influenced by different brittle failure modes characterizing the collapse of the FRP reinforcement. The most significant analytical models for assessing the shear strength of FRP-retrofitted RC beams reflect the effect of brittle failure through an effectiveness factor “R”, which reduces the ultimate tensile strength of FRP. The brittle collapse of FRP reinforcement often leads to a lower shear contribution by steel stirrups due to hindering of the yielding of all stirrups involved by critical cracking. Some analytical models consider this phenomenon introducing a further effectiveness factor “r”, which reduces the yielding strength of stirrups. The key differences characterizing many of the shear models are represented by the expressions used for assessment of the effectiveness factors. This paper focuses on the influence of effectiveness factor models in analytical predictions provided by different shear models. The reduction of the shear contribution of steel stirrups due to FRP brittle failure is modeled as dependent on the FRP wrapping scheme, through the ratio between FRP effective strain and steel yielding strain. To highlight the influence of the effectiveness factor for steel stirrups, two analyses are performed through different shear models, considering the effectiveness factor for FRP only, and considering the effectiveness factor for both FRP reinforcement and steel stirrups. The results are discussed considering two databases, one constituted by beams whose FRP reinforcement and steel stirrups are arranged at right angles to the beam axis, and one in which the FRP is arranged at angles different from 90°.

Cracking Resistance of Recycled Rubber Asphalt Binder Composed of Warm-Mix Additives.

Warm-mix asphalt technology has been applied to recycled rubber asphalt binder (RAB), which forms warm-mixed crumb rubber-modified asphalt binder (W-RAB) as a “green” material for environmental conservation and to enhance road performance. Furthermore, low-temperature cracking is one of the major distresses for asphalt pavement, which drastically restricts ride quality and service level. Therefore, the main objective of this study is to comparatively analyze the low-temperature properties of W-RABs based on thermal stress and the simple fractional model. W-RABs were obtained by mixing 60 mesh recycled rubber (CR) and two different types of warm-mix additives, namely viscosity reducer (1, 2, and 3%) and surfactant (0.4, 0.6, and 0.8%). First, Hopkins and Hamming’s numerical algorithm and the Boltzmann superposition principle were used for obtaining thermal stress . Subsequently, critical cracking temperature was derived using the single asymptote procedure (SAP) theory. Second, the simple fractional viscoelasticity model was used to calculate the creep compliance, damping ratio, and dissipation energy ratio, and the results were compared with the Superpave protocol results obtained with bending beam rheometer (BBR) tests. The results showed that a combination of CR and warm-mix additives could slightly improve the thermal crack resistance of the asphalt binder. The addition of 0.6% surfactant yielded the optimum performance, while only a high dosage (3%) of viscosity reducer provided a marked improvement in efficiency, which decreased with a decrease in temperature. This study recommends the use of RAB composited with 0.6% surfactant for areas with extremely low temperature.

Effect of carboxymethyl cellulose and polyvinyl alcohol on the cracking of particulate coating layers

We investigated the effect of polymers on the cracking of particulate coating layers. Carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) were used as contrasting polymer additives due to their different surface charges and film characteristics. Drying stress that generates cracking in particulate layers increased with the addition of both polymers. However, there was a large difference in the critical cracking thickness (CCT) depending on the two polymers, indicating that the structural and mechanical properties of particulate layers varied with the nature of polymer additive. A strong positive correlation was observed between the toughness and CCT of the particulate layer. Compared with particulate layers containing CMC, those containing PVA exhibited 77.8% higher CCT because denser and tougher layers were formed when PVA was added to the coating suspension. Our findings clearly demonstrate that cracking of coating layer can be prevented by improving the toughness of the layer.

Reclamation of waste oils in asphalt modification towards enhanced low-temperature performance of pavement in cold region

ABSTRACT To decrease the usage of petroleum asphalt and improve waste recovery, two different groups of recycled oils were selected to improve the low-temperature performance of asphalt binders in cold region. The multiple stress creeps recovery (MSCR) test, linear amplitude sweep (LAS) test and bending beam rheometer (BBR) test were performed to evaluate the rheological properties of recycled oil modified asphalt (ROMA). The thermal stress and double edge notched tension (DENT) tests were employed to characterise the low-temperature cracking resistance and ductile resistance of the ROMA, respectively. The results of this study showed that recycled oils significantly improved the low-temperature and fatigue performance of asphalt, which is manifested in a drop in the low-temperature critical cracking temperature of asphalt by 3°C, increase in crack tip opening displacement (CTOD) by 72.8%, and extension of fatigue life by 178.9%. The addition of recycled oils reduced the percentage of large molecule size (LMS) while increasing the percentage of medium molecule size (MMS) of the asphalt, which contributes to the better low-temperature performance of the ROMA. The results from this study provide insights into the reclamation of waste oils and developing the low-temperature modifier of asphalt pavements in cold region.

Correlation Analysis between Mechanical Properties and Fractions Composition of Oil-Rejuvenated Asphalt.

To clarify the intrinsic relationship between the mechanical properties of asphalt and its fraction composition, the SARA fraction composition and six macroscopic mechanical properties (critical cracking temperature (TCR), fatigue life (Nf), non-recoverable creep (Jnr3.2), penetration, ductility, and softening point) were investigated for 16 asphalt samples. Fraction contents of asphaltene and aromatic are strongly correlated with TCR and ductility (R2 > 0.92) that characterize the ability of asphalt to adapt to deformation at low and medium temperatures. Heavy fraction (asphaltene and resins) content is also strongly correlated with (R2 > 0.90) penetration and Jnr3.2 that characterize the resistance of the asphalt to overall deformation at medium and high temperatures. To express the changes in the four fractions simultaneously with one indicator, a statistic, average deviation of the fractions between the given asphalt and its original (marked σ), is introduced in this study to characterize the degree of asphalt aging based on the fraction changes. It normalizes the four simultaneous change indicators (percentage of SARA fractions) during asphalt aging into one indicator. This new indicator has a strong correlation with several mechanical performance indicators of asphalt, where it is strongly correlated with TCR (R2 > 0.90), ductility, and penetration, which are also well correlated with Jnr3.2 (R2 > 0.85), Nf (R2 > 0.75), and softening point (R2 > 0.75).