Showing papers on "Embedment published in 2018"

Flexural response of high‐strength steel‐ultra‐high‐performance fiber reinforced concrete beams based on a mesoscale constitutive model: Experiment and theory

Abstract: This paper presents the results of experimental and theoretical studies undertaken to assess the flexural performance of high-strength steel-ultra-high-performance fiber reinforced concrete (HSS-UHPFRC) beams. A total of nine HSS-UHPFRC beams were tested, and the influence of fiber volume fraction, fiber type, longitudinal reinforcement ratio, and concrete strength on the flexural response was evaluated. The results indicate that sufficient longitudinal reinforcement should be provided in a UHPFRC beam to avoid abrupt failure and possible catastrophic collapse. After the loss of the fiber bridging effect, corresponding to the fibers being pulled out from the matrix, which occurs one by one with audible sound which is sizzling, redistribution, and homogenization of the concrete stress beside cracks, induced by the dispersed fibers, takes place and more flexural cracks with small spacing appear besides the existing cracks. The beam stiffness was about 85% of the initial beam stiffness at flexural cracking state and was only approximately 25% of the initial beam stiffness at the ultimate state. A constitutive model is proposed, including a bilinear model for compression and a drop-down model for tension, taking into account uniform distribution, embedment length, and orientation of fibers for the multi-scale mechanics analysis. A flexural strength model was subsequently derived on the basis of the proposed mesoscale constitutive model; strain compatibility and force equilibrium were taken into account. The prediction of the ultimate flexural capacity and the overall post-cracking response with the proposed model show a good agreement with the test results.

Nano-dynamic mechanical analysis (nano-DMA) of creep behavior of shales: Bakken case study

Abstract: Understanding the time-dependent mechanical behavior of rocks is important from various aspects and different scales such as predicting reservoir subsidence due to depletion or proppant embedment. Instead of using the conventional creep tests, nano-dynamic mechanical analysis (nano-DMA) was applied in this study to quantify the displacement and mechanical changes in shale samples over its creep time at a very fine scale. The results showed that the minerals with various mechanical properties exhibit different creep behavior. It was found that under the same constant load and time conditions, the creep displacement of hard minerals would be smaller than those that are softer. On the contrary, the changes in mechanical properties (storage modulus, loss modulus, complex modulus and hardness) of hard minerals are larger than soft minerals. The results from curve fitting showed that the changes in creep displacement, storage modulus, complex modulus and hardness over creep time follow a logarithmic function. We further analyzed the mechanical changes in every single phase during the creep time based on the deconvolution method to realize each phase’s response independently. Two distinct mechanical phases can be derived from the deconvolution histograms. As the creep time increases, the volume percentage of the hard mechanical phase decreases, while this shows an increase for soft phases. The results suggest that nano-DMA can be a strong advocate to study the creep behavior of rocks with complex mineralogy.

Mechanical performance of epoxy coated AR-glass fabric Textile Reinforced Mortar: Influence of coating thickness and formulation

Abstract: The mechanical performance of epoxy coated AR-glass fabric reinforced composite is investigated. A three-stage manufacturing process is considered, which involves fabric surface functionalization, liquid coating deposition and long-term setting and finally fabric embedment in the mortar matrix. Two epoxy coatings are considered, which only differ by the hardening agent. However, coating thickness is significantly diverse as a result of modified viscosity during liquid deposition. Performance is assessed in uni-axial tension as well as in three-point bending and it is expressed in terms of strength curves, data dispersion, crack pattern and failure mechanism. Remarkably, despite being very similar, the analyzed coatings produce a significantly different performance, especially when data dispersion is incorporated and design limits are considered. Indeed, although both coatings are able to consistently deliver fabric rupture at failure, only the thinnest is associated with small data scattering and an almost plastic post-peak behavior in bending. The associated design elongation limit reaches the maximum allowed value according to the ICC guidelines. In fact, it appears that coating thickness plays a crucial role in determining mechanical performance and fabric flexibility. The proposed manufacturing process proves extremely effective at enhancing matrix-to-fabric adhesion and thereby prevent telescopic failure.

Proppant-Conductivity Testing Under Simulated Reservoir Conditions: Impact of Crushing, Embedment, and Diagenesis on Long-Term Production in Shales

Abstract: Hydraulic fractures act as conduits connecting a wellbore to nanodarcy-permeability unconventional reservoirs. Proppants are responsible for enhancing the fracture conductivity, and they help in maintaining high production rates. This study is focused on the measurement of long-term conductivity of proppant packs at simulated reservoir-temperature and pressure conditions. Various conductivity-impairment mechanisms such as proppant crushing, fines migration, embedment, and diagenesis are investigated. Testing was performed with a conductivity cell that allows simultaneous measurement of fracture compaction and permeability. The proppant-pack performance during compression between metal and shale platens was compared. The proppant-filled fracture (concentration of 0.75–3 lbm/ft2) is subjected to axial load (5,000 psi) to simulate closure stress. Brine (3% NaCl + 0.5% KCl) is flowed through the pack at a constant rate (3 cm3/min) at 250°F during an extended duration of time (10–60 days). In this study, Ottawa sand proppant was used between platen facies fabricated from Vaca Muerta and Eagle Ford shales. Testing between metal platens indicated that the reduction in permeability with 20/40-mesh Ottawa sand (≈30% during 12 days) was less than that of 60/100-mesh Ottawa sand, which suffered a 99% reduction in only 4 days. Measurements with 20/40-mesh Ottawa sand between shale platens were conducted at 1.5 lbm/ft2. During a duration of 10 days, the Eagle Ford platens proppant pack exhibits a greater reduction in permeability, in comparison with Vaca Muerta platens. The normalized compaction for Eagle Ford shale platens is 20% more than Vaca Muerta platens because of greater proppant embedment. Particle-size analysis and scanning-electron-microscopy (SEM) images verify proppant crushing, fines migration, and embedment as dominant damage mechanisms. These factors are observed to depend on the testing of shales. The results suggest a substantial degradation of permeability during the initial 5 days of testing, after which the permeability appears to stabilize. Crushed proppant and dislodged shale-surface particles contribute to the fines generated; a greater concentration of fines is observed downstream. In a separate study between Vaca Muerta platens, under similar closure stress and temperature conditions at 2-lbm/ft2 proppant concentration, the permeability reduced by almost three orders of magnitude during a duration of 60 days. It was also observed that growth of diagenetic smectite is accelerated by making the fluid more basic (pH of 10).

Utilizing a novel fiber optic technology to capture the axial responses of fully grouted rock bolts

Abstract: Rock bolts are one of the primary support systems utilized in underground excavations within the civil and mining engineering industries. Rock bolts support the weakened rock mass adjacent to the opening of an excavation by fastening to the more stable, undisturbed formations further from the excavation. The overall response of such a support element has been determined under varying loading conditions in the laboratory and in situ experiments in the past four decades; however, due to the limitations with conventional monitoring methods of capturing strain, there still exists a gap in knowledge associated with an understanding of the geomechanical responses of rock bolts at the microscale. In this paper, we try to address this current gap in scientific knowledge by utilizing a newly developed distributed optical strain sensing (DOS) technology that provides an exceptional spatial resolution of 0.65 mm to capture the strain along the rock bolt. This DOS technology utilizes Rayleigh optical frequency domain reflectometry (ROFDR) which provides unprecedented insight into various mechanisms associated with axially loaded rebar specimens of different embedment lengths, grouting materials, borehole annulus conditions, and borehole diameters. The embedment length of the specimens was found to be the factor that significantly affected the loading of the rebar. The critical embedment length for the fully grouted rock bolts (FGRBs) was systematically determined to be 430 mm. The results herein highlight the effects of the variation of these individual parameters on the geomechanical responses FGRBs.

Influences of Proppant Concentration and Fracturing Fluids on Proppant-Embedment Behavior for Inhomogeneous Rock Medium: An Experimental and Numerical Study

Abstract: Proppant plays a vital role in hydraulic fracturing in tight oil/gas production because it helps to keep the fractures open during the production process. However, it is common for proppant embedment, the main type of proppant degradation, to occur under high compression load, which greatly reduces the fracture conductivity, and consequently reduces the production rate. During the process of hydraulic fracturing, the fracturing fluid only has the chance to contact and infiltrate the fractures that are in the top surface of the rock medium because of ultralow rock permeability and the short time of fluid existence, whereas the condition of other parts of the rock remain unchanged, creating inhomogeneity within the rock medium. Therefore, the present study conducted a comprehensive experimental and numerical evaluation to investigate the behavior of proppant for inhomogeneous rock media, considering the factors (effective stress, proppant concentration, and fracturing fluid) that affect proppant performance. According to the experimental results, increasing the proppant concentration reduces the proppant embedment, and, interestingly, the optimal proppant concentration is approximately 150% coverage. Furthermore, the influence of fracturing fluid on proppant embedment is more significant for high proppant concentrations, and the embedment under water-saturated conditions is higher than that under oil-saturated conditions. The numerical simulation achieved the same result as the experimental study, showing that 150% proppant coverage is the optimal proppant concentration to achieve the minimum proppant embedment. In addition, numerical modeling indicated that the inhomogeneity of the rock formation can also considerably enhance proppant embedment through differential settlement during compression.

Structural behavior of traditional Dieh-Dou timber main frame

Abstract: Under different combinations of horizontal and vertical loads, a total of three quasi-static cyclic tests were conducted to investigate the structural behavior of the Dieh-Dou timber frame. Typical deformation patterns include column rocking, joint rotation around the primary beam-column and column Dou-column regions, vertical shear around the column mortise, embedment around primary beam-column regions, and vertical shearing around the mortise regions of the Dou members. Visible deformation generally began from 1/30 rad onward. The column-restoring force contributed mainly to the frame’s moment resistance when displacement is small. When frame deformation exceeds 1/50 rad, bending moment from the primary beam dominated the global restoring force. Hence, the column-restoring force and the primary beam-column connection generally undertake the primary moment-resisting mechanism while the complex bracket structures above the primary beam play a secondary role. Based on the embedment theory and semi-...

Mechanical properties evaluation of T800 carbon fiber reinforced hybrid composite embedded with silicon carbide microparticles: A micromechanical approach

Abstract: Purpose The purpose of this paper is to study the effect of the addition of silicon carbide (SiC) microparticles and their contributions regarding the tensile and shear properties of the T800 fiber reinforced polymer composite at various fiber volume fractions. The tensile and shear properties of the hybrid composites where continuous T800 fibers are used as reinforcements in an epoxy matrix embedded with SiC microparticles have been studied. Design/methodology/approach The results were obtained by implementing a micromechanics approach assuming a uniform distribution of reinforcements and considering one unit cell from the whole array. Using the two-step homogenization process, the properties of the materials were determined by using the finite element analysis (FEA). The predicted elastic properties from FEA were compared with the analytical results. The analytical models were implemented in the MATLAB Software. The FEA was performed in ANSYS APDL. Findings The mechanical properties of the hybrid composite had increased when compared with the properties of the conventional FRP. The results suggest that SiC particles are a good reinforcement for enhancing the transverse and shear properties of the considered fiber reinforced epoxy composite. The microparticle embedment has significant effect on the transverse tensile properties as well as in-plane and out-of-plane shear properties. Research limitations/implications This is significant because improving the properties of the composite materials using different methods is of high interest in the materials community. Using this study people can work on the process of including different type of microparticles in to their composite designs and improve their performance characteristics. The major influence of the particles can be seen only at lower volume fractions of the fiber in the composite. Only FEA and analytical methods were used for the study. Practical implications Material property improvements lead to more advanced designs for aerospace and defense structures, which allow for high performance under unpredictable conditions. Originality/value This type of study proves that the embedment of different microparticles is a method that can be used for improving the properties of the composite materials. The improvement of the transverse and shear properties will be useful especially in the design of shell structures in the different engineering applications.

Effect of Slope on p - y Curves for Laterally Loaded Piles in Soft Clay

Abstract: Pile foundations are often subject to lateral loading due to various forces on a variety of structures like high rise buildings, transmission towers, power stations, offshore structures and highway and railway structures. The present investigation is to study the effect of slopes on p-y curves (where p is the static soil reaction and y is the pile deflection) due to static lateral loading in soft clay (Consistency index Ic = 0.42). A series of laboratory model tests were carried out on the instrumented model pile on sloping ground (slopes of 1V:1H, 1V:1.5H, 1V:2H, 1V:3H and 1V:5H) and with varying embedment length to diameter ratio (L/D) of 20, 25 and 30. From the experimental results, the bending moment curves along the pile shaft are double differentiated to obtain the soil resistance (p) and double integrated to obtain the deflection (y) using curve fitting method. New p-y curves for piles located on crest of soft clay with different sloping ground surface under static lateral loading are developed. Moreover, the effect of sloping angles on proposed p-y curves was studied.