Splitting Failure Mode Research Articles

Pre-and post-heating bar-concrete bond behavior of CFRP-wrapped concrete containing polymeric aggregates and steel fibers: Experimental and theoretical study

The present research attempted to explore the impact of steel fibers and carbon fiber-reinforced polymer (CFRP) sheets on the pre-and post-heating bond behavior of steel bars in high-strength concrete incorporating waste polyethylene terephthalate (WPET) by conducting the pullout and beam tests and in both pullout and splitting failure modes. To achieve this aim, one hundred and forty-four cubes and forty-eight prisms were cast for the pullout and beam tests, respectively. The variable parameters included the replacement ratio of the fine aggregates with waste polyethylene terephthalate (WPET) particles (0 % and 10 % by volume), steel-fiber content (0 % and 1 % by volume), bar diameter (12-and 16-mm), target temperature (25, 200, and 600 °C), and confinement level (no confinement or CFRP wrapping applied after the thermal regime). Bond strength, bond stress-slip behavior between rebar and concrete, failure mode, and rebar debonding energy were among the parameters investigated after cooling. The results showed that in contrast with the positive effect of CFRP sheets on debonding energy, steel fibers have a positive effect on debonding energy only at 600 °C. Steel fibers did not offset the negative effect of polymeric aggregate, negatively changed the pullout mode to a mixed pullout/splitting failure at 200 °C, and tended to favor splitting failure mode to mixed mode at 600 °C. Moreover, steel fibers negatively affected the bond strength of pullout specimens in both confined and unconfined conditions, especially at 25 and 200 °C, and were less effective than CFRP sheets in avoiding splitting. Small-diameter bars reduced the negative effect of steel fibers on the bond, while polymeric aggregate tended to enhance the bond strength differences between large- and small-diameter bars. The effect of increasing the bar diameter on the effectiveness of CFRP sheets on the bond strength was not clear, but as a general trend, the effectiveness increased with the increase of the bar diameter. A predictive model based on the experimental results of this study and other studies was proposed as well to introduce the positive effect of the CFRP confinement and the dubious effect of steel fibers and polymeric aggregate on bond strength.

Influence of chemical composition and discontinuities on energy transformation and rock mass behaviour: Insights into geological dynamic

AbstractIn this study, efforts were made to incorporate the influence of discontinuities and failure modes of rock into the classification of rock masses. The past tectonic activities may create microfractures in the rock body therefore the failure moods have been determined carefully under uniaxial compression. The results of the discontinuity analysis, conducted through kinematic study, highlighted the significant impact of wedge failure on the failure of the rock mass. In correlating the geological strength index with rock mass rating, it was observed that joint volume played a negative role, whereas compressive strength played a positive role. These correlations are particularly applicable for a certain rock type, as the compressive strength is inherently dependent on the type of rock. The analysis of failure modes under uniaxial compression reveals that the dissipation energy coefficient initially undergoes rapid increase before reaching its minimum value at the failure stage. The microstructures of the rock effect significantly the elastic and dissipation energy characteristics. Specifically, the axial splitting failure mode emerges as predominant. Given the area's past tectonic activity, these results emphasize the impact of microfractures within the rock body. Relating the failure criteria with the chemical composition of rock types reveals that rocks abundant in SiO2, such as gabbronorite, tend to exhibit brittle failure. Additionally, a dominance of Al2O3 over Fe2O3 suggests a predisposition towards brittle failure, while an increased ratio of CaO to MgO implies increased susceptibility to compression.

Effect of density value and gradient distribution on the deformation mechanism of foamed concrete

Foamed concrete is an essential material in engineering that can be categorized into two types based on density distribution, namely uniform foamed concrete (UFC) and gradient foamed concrete (GFC). However, there exists a research gap concerning the mesoscopic deformation mechanism of UFC and GFC. The objective of this research is to bridge this gap by examining the quasi-static compression characteristics of UFCs with three distinct densities and GFCs with different density sequences. The results reveal that the strength of pore walls significantly influences the failure mechanism of UFCs with varying densities. Specifically, UFCs with low density exhibit weak pore-wall strength, leading to stress concentration at the pore-wall junction. During compression, these weak pore walls are widely dispersed within the specimen, resulting in a powdering failure mode. Conversely, UFCs with high density possess stronger pore walls, which prevent the powdering failure mode by maintaining adequate pore-wall strength. Nevertheless, the existence of a dominant crack within the specimen results in a splitting failure mode. In the context of GFCs, deformation occurs in a sequence from low to high density, with each layer exhibiting a failure mode corresponding to its density. Note that the last-deforming layer in this brittle gradient foam cannot attain the strength of the corresponding uniform foam. This is due to the failure of the second layer, which results in uneven contact surfaces and prompts the third layer to crack simultaneously. Finally, a statistical model is developed to forecast the compressive Stress–strain curve of foamed concrete, demonstrating remarkable agreement with experimental data.

Experimental and numerical study of disk specimen with rough structural plane under splitting test

To study the effects of the anisotropic matrix and structural planes on the splitting strength and failure mode of rocks, Brazilian splitting tests were carried out with seven different loading angles on specimens of rock-like materials with rough structural planes. The surface strains of the samples during the failure process were monitored and analysed with the help of a high-speed camera and digital image correlation (DIC) technology. The test results showed that the Brazilian splitting strength (BSS) decreased gradually with an increased loading angle. According to the crack morphology, the samples showed three failure modes, and the structural plane and the loading angle (θ) had an important effect on the failure mode. When θ < 75°, the sample failure was mainly affected by the matrix, and when θ > 75°, the sample failure was mainly controlled by the structural plane. The numerical simulation of the sample with a structural plane was carried out by the PFC2D particle flow program, the micro parameters were calibrated using a back propagation (BP) neural network model. The internal cracks of the sample under a splitting load were mainly matrix tensile microcracks and structural plane shear microcracks, and the tensile microcracks in the side with the weak matrix appeared significantly earlier than those in the side with the strong matrix. With increasing loading angle, the proportion of tensile microcracks in the matrix increased, while the proportion of shear microcracks in the matrix decreased, especially in the weak matrix. The microcracks at the structural plane mainly changed from tensile microcracks to shear microcracks, and the development degree of microcracks along the structural plane was more significant than that on the weak matrix with increasing loading angle. The results of the study can provide a reference for rock stability evaluation and utilization.

Study on failure mechanism of tight sandstone based on moment tensor inversion

Understanding deep rocks' mechanical properties and failure evolution is crucial for efficient resource development. This study investigates the mechanical properties of tight sandstone and analyzes its acoustic emission (AE) characteristics using a combined discrete element model and moment tensor inversion. The AE activity during loading is categorized into three stages: crack initiation, stable crack propagation, and unstable crack propagation. Confining pressure loading suppresses AE activity during the crack initiation stage due to damage healing phenomenon. Moment tensor inversion reveals that tensile failure is the primary AE failure source, despite samples exhibiting splitting and shear failure modes. The proportion of AE failure types varies with stress levels and depends on the mechanical environment. Microcracks initiate at the ends of the sample and propagate inward along the loading direction, resulting in a blank area of AE events in the middle. This blank area can be utilized to predict specimen failure mode. The b value, representing the ratio of small to large magnitude events, decreases with increase of the confining pressure, indicating higher energy release during specimen failure under high confining pressure. The research results can provide a reference for predicting the failure of tight sandstone.

Post-heating flexural behavior of reinforced concrete beam with lap-spliced bar and feasibility of improving flexural performance by adding hybrid fibers

Using bar splices in reinforced concrete structures is inevitable due to transportation and construction limitations. On the other hand, executing lap splices in flexural members such as beams allows these members to potentially fail at the splice zone before reaching their ultimate flexural capacity. The failure of the beam at the splice location results from different factors, one of which is the exposure of concrete members to elevated temperatures. To address this, the flexural behavior of beams with lap-spliced bars after exposure to heat was examined, and the feasibility of improving their flexural performance by incorporating hybrid steel-polypropylene fibers was studied in the work. Here, 24 reinforced concrete beam specimens were constructed in 12 groups and exposed to the four-point bending test. Variables considered in the manufacturing and testing of these specimens included the splicing method (continuous reinforcement, simple lap splice, and hooked-end lap splice), concrete mix type (plain and fiber-reinforced), and exposure temperature (25 and 600 °C). Parameters under study included the load–deflection behavior, crack pattern, cracking load, ultimate load, failure mode, initial stiffness, effective stiffness, energy-absorption capacity, and ductility of the beams. The results indicated that exposing the spliced concrete beams to elevated temperatures changed the failure mode from flexural to splitting; however, adding hybrid fibers to the concrete mix improved the behavior of these members, with a more pronounced improvement in the specimens with the hooked-end splice. In addition, the simultaneous use of the hooked-end splice and hybrid fibers in the beams that had been exposed to heat prevented the splitting failure mode in these members. Furthermore, splicing the bar in the heated and unheated beam specimens lowered the energy-absorption capacity and ductility of the beams. The addition of fibers improved the energy absorption and ductility of the heated and unheated specimens, and the beams with the hooked-end splice showed a more significant improvement.

Investigations on influences of Engineered Cementitious Composites (ECC) on pull-out properties of studs in steel-concrete composite bridge

Stud plays an important role in the connection of steel-concrete composite bridge (SCCB). The stud is under pull-out load in the negative moment zone of SCCB. Engineered Cementitious Composites (ECC) are a new type of materials with tensile strain hardening and multiple cracking behavior to strengthen SCCBs. In this study, the cyclic pull-out effect on the ECC-stud connection was studied through pull-out tests. The stud diameter (d=13/16 mm) and the embedment depth (hef=40/60/80 mm) are included as design parameters, resulting in a total of 18 specimens of six scenarios. The failure mode, load-bearing capacity, and ductility factor of the specimens under cyclic loading are compared with those under monotonic loading, and the stiffness, energy dissipation capacity, and residual displacement of the specimens under cyclic loading are analyzed. Based on the experimental results, under cyclic loading, the concrete cone failure and splitting failure occur, and the threshold of hef/d between the two failure modes is 3.75–4.62, which is smaller than the threshold of the monotonic scenarios. Both concrete and splitting failure modes show ductile failure characteristics because of the bridging effect of fibers in ECC. The load-bearing capacity and ductility factor under cyclic loading are lower than those under monotonic load. This is because, under cyclic loading, the specimens show cumulative damage, which, however, is weakened due to the strengthening effect of ECC. As d and hef increase, the load-bearing capacity, ductility factor, stiffness, and energy dissipation capacity increase correspondingly, and the residual displacement decreases. The stud with a larger d shows a more obvious ductility holding stage.

Failure Behavior and Failure Locations of Oxytenanthera Abyssinica Bamboo Culms under Bending Load

Bending failure modes in solid stem wood and solid culm bamboo varies depending on material and geometric properties. Solid culm cross-sections of Oxythenantera Abyssinica round bamboo resemble to wood, but are anatomically different in their mode of growth and tissue organization. The bending stress gradient and failure behavior has highly related with the culm internal voids termed as hollowness which depends on age of bamboo. Hollowness (k) refers the relative proportion of void volume to solid volume with in a culm. Full culm beam specimens with length of 3.5 m were subjected to bending under 4-point static loading according to ISO 22155. Pattern of vertical deflection varies depending on the maturity of the culm. A statistical and experimental results showed that, for K values between 12-15% a mixed mode of local buckling with a longitudinal shear splitting failure mode was resulted in 4-year age bamboo specimens with a slight (14 mm) shift from the shear center inducing large vertical deflection (142 mm) at midspan. A kink buckle and green stick modes were observed in 2-year and 3-year ages culms at failure point of 124 and 158mm at length ‘L’ (L/2.5 to L/4.5) from shear center with a k value 25-27% and 18-22% respectively.

Splitting failure and deformation evolution of the red argillaceous siltstone disc in the high humidity environment

The failure mechanism of weakly cemented soft rock under water-rock coupling effect is extremely complex. To reveal the tensile failure of red argillaceous siltstone in high humidity environment, five kinds of disc samples with different water contents were obtained through maintenance in constant temperature and humidity oven. The Brazilian test was carried out to reveal the variation characteristics of splitting behavior of rock samples in high humidity environment. Detailed analysis was conducted on the evolution law of stress-strain relationship and mechanical parameters under five different water contents. Combined with the DIC (Digital Image Correlation) investigation, the displacement evolution patterns of samples with different water content and loading stages were analyzed. Results show that he macroscopic mechanical parameters such as tensile strength and elastic modulus of weakly cemented red sandstone samples under water environment are negatively parabolic correlated with the increase of water content. The moisture content has an important influence on the disc splitting failure mode, forming the main fracture along the radial direction of loading, and showing a local crushing area in the loading area, and the crushing area increases with the increase of water content. Microstructural variation of red sandstone under a high humidity environment is the main reason for its deterioration of macroscopic mechanical properties.

Enhancing the mechanical and energy release performance of nano-aluminum@fluororubber (nAl@F2311) core–shell microstructured composite materials

Aluminum (Al)-based reactive materials have recently attracted much attention due to their excellent chemical energy release characteristics. However, there still exists a great challenge to improve the mechanical properties and energy density of Al-based reactive materials. In this work, we reported that core–shell nano-aluminum@fluororubber (nAl@F2311) composites with good mechanical properties and high energy release characteristics were designed and fabricated by the electrical exploding wires method. The results showed that Al nanospheres were coated by F2311 uniformly to form the nAl@F2311 core–shell microstructure with high Al contents. Quasi-static/Split Hopkinson Pressure Bar dynamic compression test results showed axial splitting failure mode of nAl@F2311 composites. nAl@F2311-10 composites with 90 wt. % Al contents had higher compressive strength, with quasi-static and dynamic compressive strength of 117.6 and 304.6 MPa, respectively. nAl@F2311-15 composites with 85 wt. % Al contents had a lower ignition threshold. Furthermore, the impact-induced energy release test showed higher fluorine contents will accelerate energy release, reduce impact ignition threshold, and improve the reaction efficiency of nAl@F2311 composites. The high reaction efficiency (97.79%) of the nAl@F2311-15 composites was obtained at an impact velocity of 1090 m/s. This offered a concept-of-proof work to design and fabrication of nanostructured reaction materials, which had high strength and energy release performance.

The combined effect of temperature and seawater on the compression properties of carbon fiber vinyl ester composites for sandwich structures

For naval applications, composite sandwich structures are of significant interest, and are often manufactured using thin (2 to 4 mm) composite facings made from carbon or glass fiber reinforcement, attached to a thick (25 to 50 mm) section of PVC cellular foam or balsa wood-based core materials using a suitable polymeric resin. In the present study, we focus on the hygrothermal effect on the fiber-dominated compression properties of carbon fiber reinforced vinyl ester resin based polymeric composite (CF/VE), used as “skin” for a polymeric composite sandwich material. Hygrothermal conditioning is achieved by saturating samples in simulated seawater at 40°C. Compression properties are evaluated for coupons extracted along warp and fill undergoing- no conditioning, conditioning till saturation (up to 6 months), and long-term conditioning (2 years). Sea-water saturation yields in up to 12% drop in compression strength with a further 3–4% drop resulting from long-term conditioning. No statistically significant modulus degradation is noticed due to short or long-term hygrothermal exposure. The failure mechanism of the warp extracted coupon, which fails in a splitting failure mode originating due to the delamination between the 0/90 interface, or the fill extracted coupon, which fails due to the instability caused by tow micro-buckling, remains unchanged due to combined exposure (short or long-term) of seawater and temperature. The loss in strength is attributed to the degradation of the fiber-matrix interface, which is validated via conducting single fiber push-in tests with a nominal diameter of 7 micron for conditioned and unconditioned coupons.

A double-cylinder model for bond analysis of deformed bars with eccentric concrete confinement

Bonding between deformed bar and concrete is critical for the load capacity of reinforced concrete structure. In practical engineering, the deformed bar is always eccentrically embedded in a concrete member instead of concentrically installed in it. In this paper, a double-cylinder model, which considers the additional concrete confinement outside the conventional cylinder of concentric embedment case as an extension of cylinder diameter, is first proposed to model overall concrete confinement effect on the deformed bar for the eccentric embedment case. Thereafter, combining the eccentric concrete confinement with other main influencing factors on the bond, such as friction coefficient, rib height and rib spacing, the ultimate bond strength is predicted for the concrete splitting failure mode. By comparing with the prediction bond strengths and experimental results from previous tests, a series of confinement adjustment factors based on the concentric cases are confirmed for various eccentric situations. As a result, a formula of the concrete confinement transition between the eccentric and concentric cases is obtained based on the least square method. Besides, parametric studies are conducted to study the effects of cover thickness, reinforcing bar diameter, concrete tensile strength on the bond strength between deformed bar and concrete.

The importance of accounting for large deformation in continuum damage models in predicting matrix failure of composites

The work presented in this paper investigates the ability of continuum damage models to accurately predict matrix failure and ply splitting. Two continuum damage model approaches are implemented that use different stress–strain measures. The first approach is based on small-strain increments and the Cauchy stress, while the second approach account for large deformation kinematics through the use of the Green–Lagrange strain and the 2nd Piola–Kirchhoff stress. The investigation consists of numerical benchmarks at three different levels: (1) single element; (2) unidirectional single ply open-hole specimen and (3) open-hole composite laminate coupon. Finally, the numerically predicted failure modes are compared to experimental failure modes at the coupon level. It is shown that it is important to account for large deformation kinematics in the constitutive model, especially when predicting matrix splitting failure modes. It is also shown that continuum damage models that do not account for large deformation kinematics can easily be adapted to ensure that the damage modes and failure strength are predicted accurately.

Effect of natural defects on the fracture behaviors and failure mechanism of basalt through mesotesting and FDEM modeling

As an important geological medium for engineering construction in Southwest China, basalt typically contains natural defects, such as hidden joints and amygdales, which significantly affect its mechanical properties. In this work, the fracture behaviors of basalt with different defects were studied in terms of its strength, deformation and failure characteristics, and crack evolution laws using a comprehensive mesotesting scheme and finite-discrete element method (FDEM) modeling, revealing the failure mechanism of basalt. The results showed that: (1) Intact specimens exhibiting high strength and smooth stress–strain curves burst instantly when loaded to the peak strength, and the corresponding acoustic emission events are abnormally concentrated, indicating a fragmentation failure mode. (2) The strength of fractured samples decreases significantly, the stress–strain curve is bimodal or multimodal, and the acoustic emission events are very active during the entire failure process, which is closely related to the local damage generated during each significant stress drop, with tension dominant, shear dominant, or tension–shear composite failure modes. (3) For amygdaloidal basalt samples with a serrated stress–strain curve, each small-scale stress drop is accompanied by the initiation, propagation, and coalescence of microcracks, resulting in relatively scattered acoustic emission events, indicating a splitting failure mode. (4) Because of the small particle size, dense arrangement, good uniformity, and high strength at the grain scale, intact basalt exhibits a relatively uniform stress field easily induced under loading, leading to an extremely high strength, fragmentation failure mode, and significant brittleness. However, a heterogeneous stress field near the original defects, such as hidden joints or amygdales, is prominent, and it significantly affects the fracture paths, which is the main reason for the strong randomness of the mechanical properties, evident strength reduction, and significant change in the failure modes. The research results can provide reference for revealing the occurrence mechanism, guiding warning systems, and formulating control methods for high-stress disasters in hard brittle rocks.

Comparative Study of Marine and Lacustrine Shale Reservoirs from the Viewpoint of Rock Mechanics

Marine and lacustrine shales are two major important types of organic-rich shales that are usually deposited in the stable ocean basin and deep lake/swamp settings, respectively. In the past decade, commercial development of marine shale gas resources has been achieved. The underdeveloped lacustrine shale also showed great oil and gas potential. Is it feasible to simply copy the development mode of marine shale gas to lacustrine shale? This is what this paper would attempt to answer from the viewpoint of rock mechanics. Using borehole cores from two typical marine and lacustrine shale gas wells, a series of experiments, including X-ray diffraction (XRD), triaxial compression, fracture surface three-dimensional (3D) scanning, and scanning electron microscopy (SEM), were implemented to comprehensively reveal the mechanical difference between marine and lacustrine shales. Postfailure fracture complexity and surface roughness were quantified by fractal dimension and morphology reconstruction, respectively. A new index BInew, considering the influences of the complete stress–strain curve and fracture characteristics, was proposed to evaluate the brittleness of these two types of shales. Results showed that the mechanical properties of marine and lacustrine shales were quite different and should be exploited by following their own characteristics. Mechanical parameters of lacustrine shale, such as compressive strength, Young’s modulus, cohesive strength, and internal friction angle, were much lower than those of marine shale. Different from the typical shear plus bedding splitting failure mode of marine shale, lacustrine shale usually formed only one smooth shear fracture. Based on the newly proposed index, the brittleness of lacustrine shale was significantly lower than that of marine shale. Huge discrepancy in quartz and clay mineral contents and the consequent petrophysical structure divergence were the reasons for the marked mechanical difference between marine and lacustrine shales. Differentiation strategies and effects of hydraulic fracturing for these two kinds of formations were also discussed.

Evaluation of post-fire pull-out behavior of steel rebars in high-strength concrete containing waste PET and steel fibers: Experimental and theoretical study

The present research addressed the post-fire bond performance between high-strength concrete incorporating waste polyethylene terephthalate (WPET) and steel reinforcement; a subject that had not been previously addressed. Further, the impact of incorporating steel fiber, as a commonly used material, into the mixture on the bond improvement was assessed. To this end, the content of WPET substituting for the fine aggregate by volume (0, 5, and 10%), volume fraction of steel fibers (0, 0.5, and 1%), applied temperature (25, 200, 400, and 600 °C) were considered as variables in a total of 108 samples that were produced. Afterward, the samples were subjected to the pullout test to examine different parameters. These parameters included the bond behavior, failure mode, bond stress-slip response, and bond strength-compressive strength behavior. Based on the results, with the incorporation of 5 and 10% volume content of WPET replacing fine particles, the bond strength declined by 4 and 26%, respectively, and as the temperature was raised, this reducing effect increased. Furthermore, the presence of steel fiber had a positive impact in the samples with the splitting failure mode, while it had a negligible or sometimes even negative impact in the samples with the pull-out failing mode. Lastly, multivariate prediction models were proposed for the bond behavior and bond-slip response as a function of the concrete compressive strength, WPET content, content of steel fiber, and the target temperature. Afterward, a comparison was made between the empirical results and the predictions of models developed by other researchers and the fib Model Code 2010 and ACI 408-12 codes.

Numerical and Experimental Investigation on Concrete Splitting Failure of Anchor Channels

Concrete splitting failure due to tension load can occur when fastening systems are located close to an edge or corner of a concrete member, especially in thin members. This failure mode has not been extensively investigated for anchor channels. Given the current trend in the construction industry towards more slender concrete members, this failure mode will become more and more relevant. In addition, significantly different design rules in the United States and Europe indicate the need for harmonization between codes. Therefore, an extensive numerical parametric study was carried out to evaluate the influence of member thickness, edge distance, and anchor spacing on the capacity of anchor channels in uncracked and unreinforced concrete members. One of the main findings was that the characteristic edge distance depends on the member thickness and can be larger than 3hef (hef = embedment depth) for thin members. Based on the numerical and experimental test results, modifications of the design recommendations for the splitting failure mode are proposed. Overall, the authors recommend performing the splitting verification separately from the concrete breakout to design anchor channels in thin members more accurately.

Bond strength prediction of spliced GFRP bars in concrete beams using soft computing methods

The bond between the concrete and bar is a main factor affecting the performance of the reinforced concrete (RC) members, and since the steel corrosion reduces the bond strength, studying the bond behavior of concrete and GFRP bars is quite necessary. In this research, a database including 112 concrete beam test specimens reinforced with spliced GFRP bars in the splitting failure mode has been collected and used to estimate the concrete-GFRP bar bond strength. This paper aims to accurately estimate the bond strength of spliced GFRP bars in concrete beams by applying three soft computing models including multivariate adaptive regression spline (MARS), Kriging, and M5 model tree. Since the selection of regularization parameters greatly affects the fitting of MARS, Kriging, and M5 models, the regularization parameters have been so optimized as to maximize the training data convergence coefficient. Three hybrid model coupling soft computing methods and genetic algorithm is proposed to automatically perform the trial and error process for finding appropriate modeling regularization parameters. Results have shown that proposed models have significantly increased the prediction accuracy compared to previous models. The proposed MARS, Kriging, and M5 models have improved the convergence coefficient by about 65, 63 and 49%, respectively, compared to the best previous model.

Depositional and Diagenetic Controls on Macroscopic Acoustic and Geomechanical Behaviors in Wufeng-Longmaxi Formation Shale

This integrated study provides significant insight into parameters controlling the dynamic and static elastic behaviors of shale. Acoustic and geomechanical behaviors measurement from laboratory have been coupled with detailed petrographic and geochemical analyses, and microtexture observations on shale samples from the Wufeng−Longmaxi Formation of the southeast Sichuan Basin. The major achievement is the establishment of the link between depositional environment and the subsequent microtexture development, which exerts a critical influence on the elastic properties of the shale samples. Microtexture and compositional variation between upper and lower sections of the Wufeng−Longmaxi Formation show that the former undergoes normal mechanical and chemical compaction to form clay supported matrices with apparent heterogonous mechanical interfaces between rigid clasts and the aligned clay fabric. Samples from lower sections exhibited a microcrystalline quartz-supported matrix with a homogeneous mechanical interface arising from syn-depositional reprecipitation of biogenic quartz cement. This type of microtexture transition exerts primary control on elastic behavior of the shale samples. A clear “V” shaped trend observed from acoustic velocities and static Young’s moduli document contrasting roles played by microtexture, porosity and organic matter in determining elastic properties. Samples with a quartz-supported matrix exhibit elastic deformation and splitting failure modes. The increment of the continuous biogenic quartz cemented medium with limited mechanic interface. By contrast, samples showing a predominantly clay-supported matrix exhibited more signs of plastic deformation reflecting heterogeneous mechanical interfaces at grain boundaries.

High-cycle fatigue of bond in reinforced high-strength concrete under push-in loading characterized using the modified beam-end test

In this paper, combined experimental–numerical investigations of the bond behavior between high-strength concrete and steel reinforcement under monotonic, cyclic and fatigue loading are presented. A modified beam-end test specimen is used to study the bond behavior under push-in compressive loading. The bond length, the concrete strength as well as the bar diameter have been varied in the test program consisting of 56 beam-end test specimens. The bond behavior has been tested under several loading scenarios and the results have been compared with the results of the pull-out tests. Push-through and combined push-through × splitting failure modes have been observed in the test program. Moreover, a bond fatigue model based on a cumulative measure of inelastic slip developed by the authors has been used to demonstrate how to identify the model parameters for a test exhibiting a pure push-through failure. The tests including three design parameters and four distinct loading scenarios will serve for further detailed theoretical research on the bond fatigue phenomenology governing the behavior of reinforced concrete structures.