Study of the mechanical properties and propagation mechanisms of non-coplanar and discontinuous joints via numerical simulation experiments.

Non-coplanar and discontinuously jointed rock masses are more complex than coplanar and discontinuously jointed rock masses. The mechanical properties and propagation mechanisms of non-coplanar and discontinuous joints were studied via direct shear tests with microscopic numerical simulation experiments. The numerical simulation tests were performed under different normal stresses, joint inclination angles, and shear rates. The numerical experimental results show that the microscale failure of non-coplanar and discontinuously jointed rock masses is mainly caused by tensile cracks. Additionally, when the peak shear stress is reached, the growth rate of cracks increases rapidly, and the number of cracks increases with increasing normal stress. The shear strength of non-coplanar and discontinuously jointed rock masses increases with increasing normal stress. Under the same normal stress, the variation curves of the shear strength of non-coplanar and discontinuously jointed rock masses with respect to the dip angle exhibit an "S"-shaped nonlinear pattern. Rock masses with joint inclination angles of approximately 15° and 65° have minimum and maximum shear strengths, respectively. The joint dip angle has a significant impact on the final failure mode of rock bridges in the rock mass. As the joint dip angle increases, the final failure modes of rock bridges change from "end-to-end" connection to a combination of "head-to-head" and "tail-to-tail" connections. The shear rate has a certain impact on the peak shear stress, but the impact is not significant. The spatial distribution of the tensile force chains changes as shearing progresses, and stress concentration occurs at the tips of the original joints, which is the reason for the development of long tensile cracks in the deeper parts.

Simplified Procedure to Determine the Cohesive Material Law of Fiber-Reinforced Cementitious Matrix (FRCM)-Substrate Joints.

Fiber-reinforced cementitious matrix (FRCM) composites have been largely used to strengthen existing concrete and masonry structures in the last decade. To design FRCM-strengthened members, the provisions of the Italian CNR-DT 215 (2018) or the American ACI 549.4R and 6R (2020) guidelines can be adopted. According to the former, the FRCM effective strain, i.e., the composite strain associated with the loss of composite action, can be obtained by combining the results of direct shear tests on FRCM-substrate joints and of tensile tests on the bare reinforcing textile. According to the latter, the effective strain can be obtained by testing FRCM coupons in tension, using the so-called clevis-grip test set-up. However, the complex bond behavior of the FRCM cannot be fully captured by considering only the effective strain. Thus, a cohesive approach has been used to describe the stress transfer between the composite and the substrate and cohesive material laws (CMLs) with different shapes have been proposed. The determination of the CML associated with a specific FRCM-substrate joint is fundamental to capture the behavior of the FRCM-strengthened member and should be determined based on the results of experimental bond tests. In this paper, a procedure previously proposed by the authors to calibrate the CML from the load response obtained by direct shear tests of FRCM-substrate joints is applied to different FRCM composites. Namely, carbon, AR glass, and PBO FRCMs are considered. The results obtained prove that the procedure allows to estimate the CML and to associate the idealized load response of a specific type of FRCM to the corresponding CML. The estimated CML can be used to determine the onset of debonding in FRCM-substrate joints, the crack number and spacing in FRCM coupons, and the locations where debonding occurs in FRCM-strengthened members.

Study on the influence of pore water pressure on shear mechanical properties and fracture surface morphology of sandstone.

To further investigate the weakening effect of pore water pressure on intact rock mechanics properties and characteristics of fracture surface after failure, direct shear tests of sandstone were conducted under different pore pressure. A 3D scanner was employed to digitize the morphology of the post-shear fracture surface. The variogram function was applied to quantify the anisotropic characteristics of post-shear fracture surface. The relationship between deformation during shear failure of intact rock and quantitative parameters of fracture surface after shear failure was initially established. It can be found that amplitudes of the sinusoidal surface determine the maximum value of variogram, and period affect lag distance that reach the maximum value of variogram. Test results revealed that the increase of pore pressure has obvious weakening effect on shear strength and deformation of rock. Moreover, the increase of pore pressure makes the shear fracture surface flatter. It can be obtained that both Sillmax and Rangemax are positively related to shear strain, but negatively related to normal strain.

Effect of Freeze-Thaw Cycles on the Shear Strength of Root-Soil Composite.

A large alpine meadow in a seasonal permafrost zone exists in the west of Sichuan, which belongs to a part of the Qinghai-Tibet Plateau, China. Due to the extreme climates and repeated freeze-thaw cycling, resulting in a diminishment in soil shear strength, disasters occur frequently. Plant roots increase the complexity of the soil freeze-thaw strength problem. This study applied the freeze-thaw cycle and direct shear tests to investigate the change in the shear strength of root-soil composite under freeze-thaw cycles. This study examined how freeze-thaw cycles and initial moisture content affect the shear strength of two sorts of soil: uncovered soil and root-soil composite. By analyzing the test information, the analysts created numerical conditions to foresee the shear quality of both sorts of soil under shifting freeze-thaw times and starting moisture levels. The results showed that: (1) Compared to the bare soil, the root-soil composite was less affected by freeze-thaw cycles in the early stage, and the shear strength of both sorts of soil was stabilized after 3-5 freeze-thaw cycles. (2) The cohesion of bare soil decreased more than that of root-soil composite with increasing moisture content. However, freeze-thaw cycles primarily influence soil cohesion more than the internal friction angle. The cohesion modification leads to changes in shear quality for both uncovered soil and root-soil composite. (3) The fitting equations obtained via experiments were used to simulate direct shear tests. The numerical results are compared with the experimental data. The difference in the soil cohesion and root-soil composite cohesion between the experiment data and the simulated result is 8.2% and 17.2%, respectively, which indicates the feasibility of the fitting equations applied to the numerical simulation of the soil and root-soil composite under the freeze-thaw process. The findings give potential applications on engineering and disaster prevention in alpine regions.

Experimental Study on the Interface Characteristics of Reinforced Crushed Rock Cushion Layer Based on Direct Shear Tests.

Through indoor large-scale direct shear tests, the interface characteristics of the crushed rock cushions layer reinforced with ParaLink geogrid were studied. The test results indicate that the shear strength of the crushed rock aggregate and the interface strength parameters have a non-linear relationship with the normal stress. The addition of the geogrid reduces the shear strength of the crushed rock aggregate and the interface strength parameters, which is mainly due to the relatively large size, small thickness, and high smoothness of the geogrid. The reinforced geogrid has a significant impact on the deformation and fragmentation characteristics of the crushed rock aggregate. It effectively suppresses the shear contraction and shear dilation effects of the crushed rock aggregate, reducing its peak compression and peak dilation angle. Furthermore, it inhibits the tendency of particle fragmentation in the crushed rock aggregate.

A Simplified Model for Shear Behavior of Mortar Using Biomimetic Carbonate Precipitation.

As a common molecule in biomineralization, L-aspartic acid (L-Asp) has been proven to be able to induce in vitro CaCO3 precipitation, but its application in sand reinforcement has never been studied. In this study, L-Asp was employed in sand reinforcement for the first time through the newly developed biomimetic carbonate precipitation (BCP) technique. Specimens with different number of BCP spray cycles were prepared, and a series of direct shear tests were conducted to investigate the impact of spray number on shear strength, critical displacement, and residual strength. Then a simplified power model for shear stress-displacement behavior was established and calibrated with the measured data. The results show that BCP can significantly improve the shear strength of sand. As the number of spray cycles increases, both the shear strength and residual strength increase, while the critical displacement decreases. Such variations can be described with two sigmoid models and a linear model, respectively. The simplified power model performs well in most cases, especially at higher spray numbers. This study is expected to provide a practical model for the shear behavior of BCP-treated mortar.

Sustainable utilization of chemically depolymerized polyethylene terephthalate (PET) waste to enhance sand-bentonite clay liners.

Polyethene terephthalate (PET) waste poses major environmental harm which can be minimized by reusing it in clay soil stabilization. In general, various polymers are known to reduce hydraulic conductivity and increase the shear strength of clays. However, the application of the effect of a chemically depolymerized form of PET, i.e., Bis (2-Hydroxyethyl) terephthalate (BHET) has not been performed as an additive in Compacted Clay Liners (CCLs) for landfills. This research focuses on the effect of the air curing period (1 and 28 days) on the hydromechanical behavior of BHET-treated SBM (0, 1, 2, 3, and 4 % by dry weight). Results from One Dimensional Consolidation tests showed that an increase in BHET content reduced both compressibility and hydraulic conductivity of SBM due to pore clogging mechanism of swollen BHET hydrogel, however, hydraulic conductivity reduced over 28 days of curing due to loss in re-swelling availability of the hydrogel, thereby allowing less tortuous paths to flow. Results from Consolidated-Drained Direct Shear tests showed that for 1 and 28-days curing, BHET treatment to SBM increased the cohesion (c') due to strong polymer interparticle bridging, however, polymer coating over the sand grains causes a reduction in its surface roughness to decrease the frictional angle (ϕ'). SEM (Scanning Electron Microscopy) and EDX (Energy-dispersive X-ray spectroscopy) analysis on BHET-treated specimens support the flocculation of bentonite, polymer bridging of sand and clay-sand polymer links. A significant Pb2+ removal capacity was also observed with BHET-treated SBM from the batch tests. FTIR (Fourier Transform Infrared Spectroscopy) analysis on batch sorption specimens confirms the role of the carbonyl groups (C = O) and hydroxyl groups (OH) present in the BHET structure indicating the possibility to adsorb Pb2+. The findings of the study suggested that a mechanism of interaction exists between sand-bentonite and BHET polymer and it can be adopted in CCLs design.

Effects of moisture content and landfill age on the shear strength properties of municipal solid waste in Xi'an, China.

With the continued expansion of waste landfills, accidents may occur if the landfills are not properly stabilized. In this study, samples of municipal solid waste (MSW) from a waste landfill in Xi'an, China were collected through on-site drilling. Considering the effects of nine landfill ages (1, 2, 3, 11, 12, 13, 21, 22, and 23 y) and six moisture contents (natural, 20, 40, 60, 80, and 100%), 324 groups of MSW were tested in the laboratory using a direct shear test apparatus. The results indicate the following: (1) with an increase in horizontal shear displacement, the shear stress of MSW gradually increases without a peak stress phenomenon, which is a displacement hardening curve; (2) with an increase in landfill age, the shear strength of MSW increases; (3) with an increase in moisture content, the shear strength of MSW increases; (4) with an increase in landfill age, the cohesion (c) decreases and the internal friction angle (φ) increases; and (5) with an increase in moisture content, the c and φ of MSW increases. The c range found in this study was 6.04-18.69 kPa, while the φ was 10.78-18.26°. The results of this study can provide a reference for stability calculations for MSW landfills.

Study on the improvement of clay properties by xanthan gum and its application on ecological slope protection engineering.

Traditional substrate binder releases greenhouse gases during the production and application processes, and is detrimental to the vegetation restoration on slopes. To develop a new environmentally friendly soil substrate, this paper conducted a serial of experimental studies on the ecological function and mechanical properties of the xanthan gum (XG)-amended clay by plant growth tests and direct shear tests. The improvement mechanism of the xanthan gum (XG)-amended clay has also explored through microscopic examinations. Experimental results of plant growth tests show that the germination of ryegrass seeds and growth of seedlings can be effectively promoted by adding a proper content (≤2%) of XG into clay. Plants in substrates with 2% of XG grew best, while a high content (3-4%) of XG has an inhibitory effect on the plant growth. The results of direct shear tests illustrate that the shear strength and cohesion both increase with the increase of XG contents, while the internal friction has an opposite trend. The improve mechanism of the xanthan gum (XG)-amended clay were also explored by XRD tests and micro-scopic examinations. It is found that shows that XG does not react chemically to form new mineral components after mixing with clay. The mechanism of XG improving clay is mainly because the XG gel can fill the pores between clay particles, and enhance the cementation between clay particles. XG can enhance the mechanical properties of clay and offset the deficiencies of traditional binder. It can play an active role in the ecological slope protection project.

Effect of Sustained Loading on the Direct Shear Behaviour of Recycled C&D Material-Geosynthetic Interfaces.

Recycled construction and demolition (C&D) wastes have been pointed out as a feasible alternative to traditional backfill materials of geosynthetic-reinforced structures, but the current knowledge about the interface behaviour between these unconventional (recycled) materials and the reinforcement is still limited, particularly as far as the time-dependent response is concerned. In this study, a series of large-scale direct shear tests was conducted using an innovative multistage method to evaluate the influence of shear creep loading on the direct shear response of the interfaces between a fine-grained C&D material and two different geosynthetic reinforcements (high-strength geotextile and geogrid). The peak and large-displacement interface shear strength parameters obtained from tests involving sustained loading were compared with those from conventional interface tests. Test results have shown that the shear creep deformation of the interfaces increased with the magnitude of sustained loading. The test specimens experienced additional vertical contraction during the creep stage, which tended to increase with the applied normal stress. For the recycled C&D material-geotextile interface, the sustained loading induced a reduction in the apparent cohesion and a slight increase in the friction angle, when compared to the values estimated from conventional tests. In turn, for the geogrid interface, the apparent cohesion values increased, whereas the friction angle did not significantly change upon shear creep loading.

Effects of Moisture and Stone Content on the Shear Strength Characteristics of Soil-Rock Mixture.

Soil-rock mixture is a commonly used geotechnical material used in many construction projects, such as slopes, tunnels, and dams. The shear strength of soil-rock mixture is its key property and is affected by many factors. This study aimed to investigate the shear strength characteristics of soil-rock mixture and the influences of moisture and stone content on shear strength parameters. Soil-rock mixture samples with four different stone and moisture contents were fabricated and tested using a large-scale direct shear test apparatus under four vertical pressures. The results demonstrated that the shear properties of the soil-rock mixture showed significant Mohr Coulomb failure criteria for all stone contents. As the moisture content increased, the shear strength of the soil-rock mixture first increased by 10~18% and then decreased after w = 12% to the residue value. The change in cohesion and internal friction angle of soil-rock mixture with different moisture contents shared a similar trend. For w < 12%, the cohesion and internal friction angle increased with moisture content, and for w > 12%, the two indexes obviously decreased. As the stone content increased from 30% to 60%, the shear strength of the soil-rock mixture increased by 82~174%. The internal friction angle increased linearly with stone content, while the cohesion of the mixture first increased and then decreased after the stone content reached 50%. The results can help in the designation and application of soil-rock mixture.

Shear Behavior of Recycled Fine Aggregate Reinforced by Nano-MgO Modified Cement.

In order to study the mechanical modification effect of nano-MgO on cement-reinforced recycled fine aggregate (CRA), direct shear tests and triaxial shear tests were carried out. In the test of recycled fine aggregate reinforced by nano-MgO modified cement (MCRA), the cement content was fixed at 2%, and the nano-MgO content varied between 0%, 0.5%, 1.0%, 1.5% and 2.0%. The test results showed that adding nano-MgO can greatly increase both the direct shear strength and triaxial shear strength of MCRA. This increase in direct shear strength was mainly attributed to the increase in cohesion. However, this increase in triaxial shear strength was attributed to the simultaneous increase in the cohesion and friction angle.

Experimental Investigation of the Mechanical Properties of the Sand-Concrete Pile Interface Considering Roughness and Relative Density.

The aim of this study was to investigate the effect of roughness and relative density on the mechanical properties of sand-concrete pile interface. A series of direct shear tests were carried out on the interface using a large-scale direct shear apparatus with various relative densities of sand (73%, 47%, and 23%) and concrete blocks with four roughness values (I = 0, 10, 20, and 30 mm). Various mechanical properties (such as shear stress, volume change, peak shear strength, secant friction angle, and normalized friction coefficient) from the interface tests were compared with trends obtained from the pure sand direct shear test. For the smooth interface, the shear stress-horizontal displacement curves of the dense sand specimen exhibited a slight softening response, which became more apparent as the roughness increased. The curves of the loose sand specimen demonstrated a hardening response. The volumetric response was influenced by the combination of normal stress, relative density, and roughness. The peak shear strength demonstrated a nonlinear increasing trend as the normal stress increased. With an increase in the normal stress, the secant friction angle and peak friction coefficient decreased as exponential and power functions, respectively. Additionally, a critical roughness value Icr resulted from the tests, which halted the upward trend of the peak friction coefficient and normalized the secant friction angle when I exceeded Icr.

Mechanical Properties and Acoustic Emission Characteristics of Anchored Structure Plane with Different JRC under Direct Shear Test.

Rock mass, the heterogeneous natural material composed of rock and discontinuities, is an important part of engineering construction. Discontinuities affect the mechanical properties of natural rock mass and further threaten the stability of rock engineering. To study the failure characteristics of anchored structure plane with different JRC, jointed specimens with four different JRC were fabricated by pouring cement mortar. Specimens were tested under four different normal loads to figure out how JRC and anchorage angle affect the mechanical properties of anchored structure plane. Besides, acoustic emission (AE) testing technology was adopted to explore the AE characteristics of anchored structural plane under shearing. The results showed that there exists a positive correlation between the peak shear strength and JRC. The undulation shape of structural plane led to an obvious downward trend in the strain softening stage of the structural plane with JRC of 6-8 and 18-20. When the anchorage angle ranged from 45° to 60°, the potentiation of bolt was the most significant. Based on the AE results, the larger the normal stress, the more likely the cumulative count curves were to enter the fast growth phase before the peak. The characteristics of b-value curves are mainly related to the topography of structural planes and whether the bolt is deformed. In the direct shear test, the cumulative proportion of shear cracks was more than 85%, which is much higher than that of tensile cracks. The variation of cumulative tensile cracks goes through three stages: slow growth, rapid growth, and slow growth. Compared with the unanchored structural plane, the variation range of real-time tensile cracks of the anchored structural plane is large, and sometimes the proportion of real-time tensile cracks may reach 80%.

Effects of the reinforcement content and reinforcement scale on the shear strength characteristics of mechanically biologically treated waste.

Mechanical and biological treatment (MBT), which can be used for waste reduction and for the burning of methane from biological treatments to generate electricity and heating, has become a popular research topic in environmental geotechnical engineering. This study investigated the mechanical behaviour of MBT waste and the effects of different reinforcement contents and reinforcement scales on its shear strength characteristics, and 68 groups of MBT waste samples from the Hangzhou Tianziling landfill were tested in the laboratory with a direct shear test apparatus. The samples exhibited displacement hardening behaviour in their mechanical response. The results show that the content and scale of the reinforced materials in MBT waste play an important role in the strength characteristics of MBT waste, and graphs showing the variation of the MBT waste shear strength and shear strength parameters with different reinforcement contents and reinforcement scales are shown. The range of cohesion c is 6.0-12.0 kPa, and the internal friction angle φ is 15.6-26.6°, respectively. The results of this study provide a reference for the assessment of slope stability at MBT landfills.

Effect of Wet-Dry Cycles on the Bond Behavior of Fiber-Reinforced Inorganic-Matrix Systems Bonded to Masonry Substrates.

In recent years, inorganic-matrix reinforcement systems, such as fiber-reinforced cementitious matrix (FRCM), composite-reinforced mortars (CRM), and steel-reinforced grout (SRG), have been increasingly used to retrofit and strengthen existing masonry and concrete structures. Despite their good short-term properties, limited information is available on their long-term behavior. In this paper, the long-term bond behavior of some FRCM, CRM, and SRG systems bonded to masonry substrates is investigated. Namely, the results of single-lap direct shear tests of FRCM-, CRM-, and SRG-masonry joints subjected to wet-dry cycles are provided and discussed. First, FRCM composites comprising carbon, polyparaphenylene benzobisoxazole (PBO), and alkali-resistant (AR) glass textiles embedded within cement-based matrices, are considered. Then, CRM and SRG systems made of an AR glass composite grid embedded with natural hydraulic lime (NHL) and of unidirectional steel cords embedded within the same lime matrix, respectively, are studied. For each type of composite, six specimens are exposed to 50 wet-dry cycles prior to testing. The results are compared with those of nominally equal unconditioned specimens previously tested by the authors. This comparison shows a shifting of the failure mode for some composites from debonding at the matrix-fiber interface to debonding at the matrix-substrate interface. Furthermore, the average peak stress of all systems decreases except for the carbon FRCM and the CRM, for which it remains unaltered or increases.

Physical and mechanical properties of waste ground at inert waste landfills.

The physical and mechanical properties of waste ground were examined at 14 locations across 4 inert waste landfills in Japan with the goal of establishing a safe and cost-effective design methodology specific to inert waste landfills. Composition analysis, basic physical properties, angle of repose, CASPOL impact value tests, and in situ direct shear tests were conducted. Inert wastes were comprised of three main components: fibrous, granular, and soil-like content, and their compositions varied between 3.6-54%, 13-45%, and 43-74%, respectively. As the fibrous content and age after reclamation increased, the water content increased but the percentage air voids decreased. The impact value (Ia), which is an indicator of the bearing capacity, increased as the dry density increased. For all locations, the angle of repose after avalanche (αa) was found between 34 and 44°. In direct shear tests, the cohesion (c) and internal angle of friction (φ) ranged from 2 to 21 kN/m2 and 22-59°, respectively. The shear stresses obtained from these c and φ values were higher than those for the municipal solid wastes, particularly for landfills with fibrous fractions ranging 14-31% under a normal stress of 25.55 kN/m2. c increased and φ decreased as the dry density increased. The correlation calculated for c and φ with Ia for inert waste landfill were c = 4.10Ia - 21.32 and φ = -4.61Ia + 82.37. Finally, the utilization of the results obtained in this study is discussed in three design stages: planning, landfilling, and future expansion.

Experimental and Numerical Studies on the Direct Shear Behavior of Sand-RCA (Recycled Concrete Aggregates) Mixtures with Different Contents of RCA.

Recycled concrete aggregate (RCA) is a typical construction and demolition (C&D) material generated in civil engineering activities and has been widely used as the coarse-grained filler added to sand for roadbed fillings. The effect of RCA content on the mechanical behavior of sand-RCA mixtures is complicated and still not fully understood. To explore the effect of RCA content on the macroscale and microscopic behavior of the sand-RCA mixtures with various RCA contents, laboratory direct shear tests and numerical simulations using the 3D discrete element method were performed. Experimental direct shear tests on sand-RCA mixtures with different contents of RCA were first carried out. Numerical direct shear models were then established to represent the experimental results. The particle shape effect was also considered using a new realistic shape modeling method to model the RCA particles. Good agreement was observed between the DEM simulation and experimental results, verifying the ability of the numerical direct shear models to represent the direct shear behavior of sand-RCA mixtures. The macroscopic responses of both experimental and numerical tests showed that all samples presented an initial hardening followed by a post-peak strain softening. The peak-state friction angles increased with the RCA content for samples under the same vertical stress. The effect of RCA content on the microscopic behavior based on DEM simulation was also found. The microscopic properties of RCA-sand mixtures, such as coordination numbers, PDFs and contact force transformation features, were analyzed and related to the macroscopic results.

Validación de una metodología para obtener la envolvente de ruptura al esfuerzo cortante en suelos no saturados compactados / Validation of a methodology to obtain the shear failure envelope of unsaturated compacted soils / Validação de uma metodologia para obter a envoltória de ruptura por cisalhamento em solos não saturados compactados

RESUMEN Obtener la envolvente de ruptura de suelos no saturados requiere de equipos especializados, de elevado costo y difícil acceso. Sin embargo, a través de ensayos de laboratorio simples combinados como corte directo, tracción indirecta y succión, es posible determinar la envolvente. La presente investigación busca, por tanto, validar dicha metodología, en un material limo de origen residual de la ciudad de Medellín-Antioquia, Colombia, compactado a la máxima densidad a través del ensayo Proctor normal. La validación se efectúa por medio de la repetitividad de una serie de ensayos de corte directo para diferentes valores de succión; comprobando que, es posible encontrar la envolvente de ruptura al corte de un suelo fino residual compactado, para distintas condiciones de succión, por medio de los ensayos de laboratorio de corte directo en condición consolidada y drenada, succión por el método del papel filtro y tracción indirecta, cuando no se cuenta con equipos de corte con succión controlada.

ABSTRACT Obtaining the failure envelope of unsaturated soils requires specialized equipment, which is expensive and difficult to access. Nevertheless, by combining simple laboratory tests like direct shear test on saturated samples, indirect tension test, and soil suction measurements by filter paper it is also possible to determine the envelope. This research aims to validate this latter methodology on a residual origin silt material from the city of Medellin, Colombia, which is compacted to the maximum dry density through the standard proctor test. The validation is carried out by repeating a series of direct shear tests for different suction values. It was verified that it is possible to find the shear failure envelope of a compacted fine-grained residual soil for different suction conditions, using the direct shear test under consolidated drained conditions, suction by the filter paper method, and the indirect tension test, when no suction controlled shear equipment is available.

RESUMO A obtenção da envoltória de ruptura de solos não saturados com controle de sucção requer equipamentos especiais, que são caros e de difícil acesso. No entanto, através de ensaios simples laboratoriais combinados, tais como os de cisalhamento direto, tração indireta e sucção, é possível determinar a envoltória de resistência. A presente investigação procura validar esta metodologia, usando um material siltoso de origem residual da cidade de Medellin-Antioquia, Colômbia, compactado na condição de densidade máxima obtida no ensaio de compactação Proctor normal. A validação é feita por meio da realização de uma série de ensaios de cisalhamento direto para diferentes valores de sucção; mostrando que é possível encontrar a envoltória de ruptura de um solo fino residual compactado, para diferentes condições de sucção, por meio de ensaios laboratoriais de cisalhamento direto consolidados drenados, sucção determinada pelo método do papel de filtro e ensaio de tração indireta, quando não se dispõe de equipamento de cisalhamento direto à sucção controlada.

Study on the Strength Performance of Recycled Aggregate Concrete with Different Ages under Direct Shearing.

In order to study the mechanical properties of recycled aggregate concrete (RAC) at different ages, 264 standard cubes were designed to test its direct shear strength and cube compressive strength while considering the parameters of age and recycled aggregate replacement ratio. The failure pattern and load-displacement curve of specimens at direct shearing were obtained; the direct shear strength and residual shear strength were extracted from the load-displacement curves. Experimental results indicate that the influence of the replacement ratio for the front and side cracks of RAC is insignificant, with the former being straight and the latter relatively convoluted. At the age of three days, the damaged interface between aggregate and mortar is almost completely responsible for concrete failure; in addition to the damage of coarse aggregates, aggregate failure is also an important factor in concrete failure at other ages. The load-displacement curve of RAC at direct shearing can be divided into elasticity, elastoplasticity, plasticity, and stabilization stages. The brittleness of concrete decreases with its age, which is reflected in the gradual shortening of the elastoplastic stage. At 28 days of age, the peak direct shear force increases with the replacement ratio, while the trend is opposite at ages of 3 days, 7 days, and 14 days, respectively. The residual strength of RAC decreases inversely to the replacement ratio, with the rate of decline growing over time. A two-parameter RAC direct shear strength calculation formula was established based on the analysis of age and replacement rate to peak shear force of RAC. The relationship between cube compressive strength and direct shear strength of recycled concrete at various ages was investigated.