Post-peak Strength Research Articles

Tensile Strength of Fiber-Reinforced Soil

AbstractThe tensile strength of soil is an important mechanical parameter that controls the development of tension cracks. In this study, randomly distributed polypropylene fibers were employed to improve soil tensile behavior. Direct tensile tests were conducted on fiber-reinforced soil specimens with different fiber contents and compacted at different water contents and dry densities. Desiccation tests were also performed to evaluate the effectiveness of fiber reinforcement in improving soil tensile cracking resistance. The tensile test results showed that fiber inclusion significantly increased the soil peak strength, reduced the postpeak strength, and changed the brittle tensile failure behavior to a more ductile one. Soil tensile strength increased with the increase in fiber content. The tensile strength of both reinforced and unreinforced specimens decreased with increasing water content and increased with increasing dry density. Moreover, a higher soil dry density showed a more positive effect in m...

Constitutive model selection for unreinforced masonry cross sections based on best-fit analytical moment–curvature diagrams

A significant number of macroscopic constitutive models is available to simulate the behaviour of unreinforced masonry (URM) in compression, but they produce different predictions of sectional response to eccentric compressive loading. In this paper, nonlinear moment–curvature analysis of rectangular URM cross sections is presented to explore the influence of alternative constitutive models and ultimate limit state assumptions in terms of post-peak strength degradation and strain ductility. Theoretical moment–curvature diagrams are quantitatively compared to experimental data at multiple levels of load eccentricity through goodness-of-fit measures. Different constitutive models are found to produce a good simulation of experimental moment–curvature behaviour, depending on whether experimental data are disaggregated in terms of masonry type or not. Moment–curvature diagrams are derived by keeping constant either the magnitude or the eccentricity of axial load. Finally, the effects of tensile properties and model error of theoretical moment–curvature diagrams are assessed.

Thermodynamically consistent elasto-plastic microplane formulation for fiber reinforced concrete

In this work a thermodynamically consistent elasto-plastic microplane constitutive theory, aimed at simulating the failure behavior of Steel Fiber Reinforced Concrete (SFRC), is developed. The continuum (smeared crack) formulation, based on the microplane theory, assumes a parabolic maximum strength criterion in terms of normal and shear (micro-)stresses evaluated on each microplane to simulate the failure behavior of concrete. In the high confinement regime, a non-associated plastic flow rule is also defined in terms of microplane stresses. The well-known “Mixture Theory” is considered to account for the presence of fibers in concrete matrix. The interaction between steel fibers and cracked concrete in the form of fiber-to-concrete bond-slip and dowel mechanisms is taken into account. The complete formulation is fully consistent with the thermodynamic laws. After describing the proposed constitutive theory, numerical analyses at constitutive level of SFRC failure behavior are presented and discussed. Thereby, the variations of the fracture energy, post-peak strength and cracking behavior with the fiber contents are evaluated and compared against experimental data. The attention also focuses on the evaluation of the sensitivity of SFRC failure predictions with the proposed constitutive model regarding fiber orientation on one hand, and the bond-slip bridging actions and dowel mechanism on the other hand.

Study on the efficacy of harmful weed species Eicchornia crassipes for soil reinforcement

Growing awareness of sustainability in construction has increased attention toward increase in use of natural fibers for soil reinforcement applications. Researchers have explored the utility of natural limited life fibers such as jute, reed and sisal for soil reinforcement. In this study, an attempt was made to demonstrate the use of local weed named water hyacinth (Eicchornia crassipes) as soil reinforcement. A series of unconfined compression strength (UCS) tests was conducted on silty sand reinforced with randomly distributed fiber. These series of tests aim to study influence of different fiber content, soil density and moisture content. The stress–strain response of fiber reinforced soil shows the increase in post peak strength and ductility. This was mainly attributed due to presence of cellulose content. The effect of soil density on increased strength due to inclusion of fiber is significant in case of lower moisture content.

Behavior of cement-stabilized fiber-reinforced pond ash, rice husk ash–soil mixtures

In order to study the effect of addition of rice husk ash, pond ash, cement and fiber on the compaction and strength behavior of clay, a series of tests were performed. Modified proctor tests were conducted to evaluate the compaction behavior, while unconfined compression tests (UCS) and split tensile strength (STS) tests were conducted to evaluate the strength properties of clay. For testing purpose specimens were prepared with different amount of admixtures. Pond ash (PA) and rice husk ash were added to clayey soil at ranges of 30–45% and 5–20%, respectively. Polypropylene fibers of length 6 mm and 12 mm are used in this study with different contents as 0, 0.5, 1.0 and 1.5% by dry weight of mix whereas cement contents used are 0, 2 and 4%. To study the effect of curing on strength property, the specimens were cured for 7, 14, and 28 days and tested. Test results have shown that the maximum dry density decreases and optimum moisture content increases with the addition of admixtures, but fiber inclusion have marginal effect. The results shows that the inclusion of fiber reinforcement within un-cemented and cemented soil caused an increase in the UCS, STS and axial strain at failure, decreased the stiffness and the loss of post-peak strength, and changed the cemented soil brittle behavior to a more ductile one. Strength of clay improves by the addition of admixtures and fibers. The study has shown that combination of clay, RHA, PA, cement and fiber can be used as light weight fill material in different structures like embankment, retaining wall etc.

Effects of nanobuds and heat welded nanobuds chains on mechanical behavior of carbon nanotubes

This study reports an investigation on the effects of nanobuds and heat welded nanobuds chains (C60 chains attached or embedded on the surface of a SWCNT) on the mechanical properties of carbon nanotubes by molecular dynamics (MD) simulations. The bond order change and the stress concentration in the distorted connection area of nanobuds when C60 attaches to the surface of the SWCNT are first studied. Then the effect of multiple randomly scattered and attached C60 fullerenes on the ultimate tensile strength of SWCNT is discussed. It is found that both nanobuds and heat welded nanobuds chains lower the ultimate tensile strength of individual defect-free SWCNT. However, the mean post-peak strength of the CNT (after necking) is significantly enhanced by heat welded nanobuds chains. Moreover, the ultimate tensile strength of the SWCNT with vacancy defects does not decrease by heat welded nanobuds chains; interestingly, the maximum tensile strength of the SWCNTs bundles with intertube bridging and vacancy defects even can be enhanced by more than 20% by heat welded nanobuds chains.

The effect of hybridization and geometry of polypropylene fibers on engineered cementitious composites reinforced by polyvinyl alcohol fibers

In the present study, the hybridization effect of polypropylene (PP) with polyvinyl alcohol (PVA) fibers in engineered cementitious composites (ECCs) was investigated. For this purpose, PVA reinforcing fiber was partially replaced by PP fibers with different circular, triangular, and trilobal cross-sectional shapes. The flexural behaviors of the ECCs containing hybrid fibers including first-crack strength, post-crack strength, and toughness were studied under three-point bending test. It was found that hybridization with nonround cross-sectional shape PP fibers had positive effect on the ductility but decreased the flexural strength of resultant ECCs. The greater reduction in toughness and post-peak strength was observed for composite containing circular PP fibers. The results indicated that partially replacement of PVA fibers with nonround cross-sectional shape PP fibers can be considered to be promising method to reduce the costs of ECC production along with attaining improved deformability.

Compressive behaviour of fibre-reinforced cemented paste backfill

Reinforcement of cemented paste backfill (CPB) with polypropylene fibres was investigated as a way of improving the stability of backfilled underground mine stopes. A series of unconfined compressive strength (UCS) tests were carried out on both non-reinforced and fibre-reinforced cemented tailings. Sandy silt tailings from a nickel mine in Western Australia were used in the study. Ordinary Portland cement at concentrations of 3–5% by weight of tailings and 0–0.5% Adfil-Ignis polypropylene fibres by weight of total solids were used for specimen preparation. The stress–strain curves from the UCS tests showed the inclusion of fibres increased the UCS and significantly reduced the post peak strength loss. Accordingly, the fibre-reinforced specimens were found to be much more ductile than unreinforced specimens, which is highly desirable in many backfill applications. Sliced images acquired from X-ray computed tomography (CT-scan) demonstrated that the observed ductile behaviour of reinforced specimens could be explained by the restraint to crack growth provided by the mobilised fibre tensile strength. At large strains, fibre-reinforced specimens had virtually zero dislodged fragments and retained their integrity as shown in both experimental photos and CT sliced images. This was different from unreinforced specimens, which developed large, wide cracks that resulted in fracturing of the tested specimens. The potential for improving the self-supporting capacity of the fill mass using fibre reinforcing but less cement is discussed and potential advantages, such as reduced ore dilution when excavating adjacent stopes are discussed.

A joint shear model incorporating small-scale and large-scale irregularities

The strength and dilation of rock joints in the field cannot be evaluated solely on the basis of parameters scaled from laboratory data, but also requires assessment of large-scale irregularities not present in the laboratory sample. A constitutive model for rock joints has been developed that considers the dilation and strength along both small-scale joint roughness scaled from laboratory data, and large-scale waviness determined from geologic observations. The model’s performance is illustrated by providing its correlation with experimental results taken from literature. The degradation in dilation and post-peak strength along small-scale irregularities is modeled using the plastic work done in shear, and the degradation along large-scale irregularities is modeled using a sinusoidal function. A dimensionless product of plastic work, rock strength, and wavelength of irregularities has been developed which fits the direct shear test results. An approach to scaling shear strength and shear displacement from laboratory to field-scale is also suggested.

Behavior of steel fiber-reinforced high-strength concrete-filled FRP tube columns under axial compression

This paper presents the results of the first-ever experimental study on the axial compressive behavior of steel fiber reinforced high-strength concrete (SFRHSC)-filled FRP tubes. A total of 27 circular unreinforced and steel fiber reinforced high-strength concrete FRP tubes (SFRHSCFFTs) were tested under axial compression. The effects of steel fiber parameters, including fiber shape, fiber aspect ratio and fiber volume fraction were examined. Concrete type was also investigated as a parameter, where two different types of concrete, including a conventional HSC and slurry infiltrated fiber concrete (SIFCON) were considered. The results indicate that both SFRHSCFFTs and SIFCON-filled FRP tubes (SIFCONFFTs) exhibit highly ductile compressive behavior. The results also indicate that the axial stress–strain behavior of CFFTs is influenced by the presence and amount of internal steel fibers, with particularly significant influences noted on the FRP hoop rupture strains and post-peak strength losses. It is found that fiber volume fraction significantly affects the compressive behavior of CFFTs. Concrete type, fiber shape and fiber aspect ratio also have some, but less significant, influence on the behavior of CFFTs. It is observed that the compressive strength and ultimate strain of CFFTs increases with an increase in fiber volume fraction or a decrease in fiber aspect ratio. It is also observed that CFFTs reinforced with hooked end steel fibers exhibit improved compressive behavior compared to the companion CFFTs reinforced with crimped fibers.

Dynamic Shear Strength of a Needle-Punched GCL for Monotonic Loading

This paper presents an experimental investigation of the dynamic internal shear strength of a hydrated woven/nonwoven needle-punched geosynthetic clay liner (GCL) for monotonic (i.e., single direction) loading conditions. Displacement-controlled shear tests were conducted using a large direct shear machine for four normal stress levels ranging from 141 to 1,382 kPa and seven shear displacement rates R ranging from 0.1 to 30,000 mm/min. For each normal stress, peak shear strength first increased and then decreased with increasing displacement rate. Maximum values of peak strength occurred for R=100–10,000 mm/min and were 16–23% higher than corresponding static values measured at R=0.1 mm/min. For each normal stress, residual shear strength first decreased and then increased with increasing displacement rate, with minimum values occurring at R=1 mm/min. On a relative basis, residual strengths show greater dependence on displacement rate than peak strengths. The standard displacement rate for static shear tests of hydrated GCLs (0.1 mm/min) generally yielded conservative values of peak shear strength but unconservative values of residual shear strength, especially for higher normal stress levels. The GCL experienced large post-peak strength reduction for all test conditions.

Compaction and strength characteristics of fly ash and fiber amended clayey soil

An experimental program was undertaken to investigate the effects of discrete polypropylene fibers and Class C fly ash on the stress–strain and shear strength behavior of clayey soil. Two types of fiber (fibrillated polypropylene fiber and multifilament polypropylene fiber) in two different lengths (6.0mm and 19.0mm) and two fiber dosages (i.e. 0.5% and 1.0% by dry weight of soil) were considered. At first, compaction characteristics of the untreated soil and seven different fly ash–soil mixtures (2.5%, 5.0%, 7.5%, 10.0%, 15.0%, 20.0%, and 30.0% by dry weight of soil) were evaluated at standard compaction energy. Considering compaction characteristics, unconfined compression tests of untreated soil and five different fly ash–soil mixtures (5.0%, 10.0%, 15.0%, 20.0%, and 30.0% by dry weight of soil) were carried out after 1-, 7-, 14-, 28- and 90-day curing periods. According to the obtained test results, untreated soil and two different fly ash–soil mixtures (i.e. 10.0, and 30.0% by dry weight of soil) were mixed with fibrillated polypropylene fiber and multifilament polypropylene fiber in two different lengths and in two different fiber dosages, separately. A total of 17 different fiber–fly ash–soil mixtures were composed, and their unconfined compression tests were carried out after 28-day curing period. According to unconfined compression test results, 6 different fiber–fly ash–soil mixtures were selected for UU triaxial tests at the end of 28-day curing period. The obtained test results indicated that the effect of fly ash content on the stress–strain behavior is superior to the effect of fibers. The post-peak strength behavior is strongly affected by fiber type, length and dosage. The magnitude of the effect increases as the fly ash content increases. Inclusion of fiber alone without fly ash decreases unconfined compressive strength (UCS) of the compacted fiber–clay mixtures. On the other hand, when combined with fly ash, fiber inclusion increases UCS depending on fiber type, length and dosage. Moreover, for both fiber types and fiber lengths, the higher the dosage of fiber, the higher the UCS is. The increase in UCS becomes as much as 218% for 30% fly ash content and 1.0% 19mm long fibrillated polypropylene fiber.

Tests on Effects of Loading Rate on Modulus and Strength of Sandstone

For studying on the effects of loading rate on modulus and strength of sandstone, uniaxial compression experiment of the samples, from Balikun mine, was performed at different loading rates. The influence of loading rate on average modulus, uniaxial compressive strength and post-peak strength degradation angle was analyzed, and the results indicate that:: The average modulus of sandstone samples increases with loading rate increasing; the both almost obey linear relationship. The peak strength of sandstone grows with the increase of loading rate; there is an approximately exponential relationship between the two. With loading rate increasing, the post-peak strength degradation angle decreases; the relationship between the both is approximately exponential.

Study of Post-Peak Stress-Strain Relationship of Rock Mass with Joint

The post-peak stress strain relationship expression of intact rock cannot well reflect post peak properties of rock masses with joint, in order to obtain post peak stress-strain relationship suitable for rock masses with joint, based on the test data, based on the Kulun strength criterion, analysis evolution of post peak strength parameters of rock masses with joint. According to rock masses with joint under different confining pressure and fracture dip at different post peak behavior, the post peak stress-strain relationship is simplified as new kind type, regarding maximum principal strain as strain softening parameter, on the assumption cohesion and internal friction angle are the piecewise linear functions of maximum principal strain, the method of solving express of new post-peak stress-strain relationship of rock masses with joint is obtained.

Effect of freeze–thaw cycles on unconfined compressive strength of fine-grained soil treated with jute fiber, steel fiber and lime

This paper presents an attempt about the effect of freeze–thaw cycles on unconfined compressive strength (UCS) of fine grained soil (low-plasticity silt) treated with jute fiber, steel fiber and lime. The stress–strain behavior (post-peak strength, strain hardening, ductility, brittleness index, resilient modulus) of the stabilized soil has also been discussed due to the treatment effects. An extensive laboratory study has been carried out conducting UCS tests under the freeze–thaw cycles of 0, 1, 2 and 3. The mixture proportions of the stabilizers were 2%, 4%, 6%, 8% and 10% for lime and 0.25%, 0.50%, 0.75% and 1% for both of the jute and steel fibers. Considering the UCS performances together with cost–benefit advantages, the effective dosage rates of stabilizers were separately found as 4% for lime, 0.75% for jute fiber and 0.25% for steel fiber. The combinations including these effective rates have been further investigated on the UCS performance and stress–strain behavior under freeze–thaw cycles. The study results that the UCS value of native soil has been performed best due to the combination of effective stabilizer rates all together (i.e., 4% lime+075% jute fiber+0.25% steel fiber). In this effective combination, the UCS value of native soil increases from 220kPa to 1330kPa for non-freeze–thaw cycle, from 205kPa to 1300kPa for one freeze–thaw cycle, from 156kPa to 1100kPa for two freeze–thaw cycles and from 114kPa to 900kPa for three freeze–thaw cycles. The other stabilizer combinations also show good contributions to UCS of native soil. As for the stress–strain responses, the inclusions due to the jute fiber alone in terms of post-peak strength, strain hardening, and ductility are better than the ones of steel fiber and lime at all freeze–thaw cycles. The combination of effective stabilizer rates all together offers a brittleness index with good ductility as well. In regard to the performance of resilient modulus, it is exhibited well by the combination of 4% lime+0.25% steel fiber. The results obtained from the study are fairly promising to employ jute fiber, steel fiber and lime against freeze–thaw resistance.

The influence of percentage of bars lapped on performance of splices

Most design codes encourage the staggering of lapped splices in a section, such that a portion of the bars in the cross section remains continuous throughout the lap zone. A penalty on lap length may be imposed if all bars are lapped at one section. In the literature, however, almost all tested specimens have had all reinforcement lapped at the same section. This paper presents results of tests carried out to shed some new light on the behaviour of lap splices where only a portion of bars is lapped. To this end, four point bending tests were carried out on 24 full-scale beams with all or part of the longitudinal reinforcement lap spliced at mid-span. The beams were reinforced with either 16 or 20 mm diameter rebars, included two grades of concrete and various lap splices configurations, all confined by links. All the beams were designed with the same concrete cover and with the minimum amount of transverse reinforcement permitted by MC2010, equal to 50 % of the area of the lapped bars. The resistance and the residual strength of the splices were measured and compared with the results of specimens with continuous bars and with all bars lapped. The results show that lapping only a portion of bars at a section impairs splice strength, although some post-peak strength is maintained by the continuous bars. These outcomes raise questions over the validity of EC2 and ACI 318-11 provisions which allow a reduction in lap-length when splices are staggered.

New artificial neural networks for true triaxial stress state analysis and demonstration of intermediate principal stress effects on intact rock strength

Simulations are conducted using five new artificial neural networks developed herein to demonstrate and investigate the behavior of rock material under polyaxial loading. The effects of the intermediate principal stress on the intact rock strength are investigated and compared with laboratory results from the literature. To normalize differences in laboratory testing conditions, the stress state is used as the objective parameter in the artificial neural network model predictions. The variations of major principal stress of rock material with intermediate principal stress, minor principal stress and stress state are investigated. The artificial neural network simulations show that for the rock types examined, none were independent of intermediate principal stress effects. In addition, the results of the artificial neural network models, in general agreement with observations made by others, show (a) a general trend of strength increasing and reaching a peak at some intermediate stress state factor, followed by a decline in strength for most rock types; (b) a post-peak strength behavior dependent on the minor principal stress, with respect to rock type; (c) sensitivity to the stress state, and to the interaction between the stress state and uniaxial compressive strength of the test data by the artificial neural networks models (two-way analysis of variance; 95% confidence interval). Artificial neural network modeling, a self-learning approach to polyaxial stress simulation, can thus complement the commonly observed difficult task of conducting true triaxial laboratory tests, and/or other methods that attempt to improve two-dimensional (2D) failure criteria by incorporating intermediate principal stress effects.

Experimental Investigation of Mechanical Properties of Sands Reinforced with Discrete Randomly Distributed Fiber

The addition of cementitious admixtures and/or inclusion of fibers are frequently used in practice to stabilize soils and to improve their mechanical properties. In this study, ring shear tests were conducted to investigate mechanical properties such as shear strength, angle of friction and cohesion values of randomly distributed discrete fiber-reinforced sand mixtures. The length and aspect ratio of the fibers used in the current study were 12 mm and 120, respectively. Specimens were prepared at four different fiber ratios (0.1, 0.3, 0.6, and 0.9 % by weight of sand). A series of ring shear tests were carried out on sand alone and fiber-reinforced sand mixtures at different normal stresses. The test results indicated that the addition of fiber had a significant effect on the shear strength of the sand. Shear stress of the unreinforced sand increases 1.29–2.32, 1.16–1.39, and 1.07–1.5 times at a normal stress of 50, 150, and 250 kPa, respectively with fiber inclusion. Fiber content had positive effects on improving the shear strength parameters (angle of internal friction and cohesion) of the mixtures. The cohesion and angle of internal friction of fiber-reinforced sand prepared at different ratios of fiber increased by 5.3–27.4 kPa and 2.0°–7.3° respectively. The inclusion of fibers improves the ductility of the soil by preventing the loss of post-peak strength.

Analyses of Steel Bridge Circular Columns using Fiber Model considering Local Buckling

One of the most concerned problems for analytical estimation of ultimate strength and ductility of steel bridge columns is the consideration of local buckling. Stress-strain curves with softening behavior to include local buckling in fiber model analysis method were proposed by several researchers. However, accuracy of those models is still relatively low. As a consequence, most steel bridge designs have been done using fiber model analysis method without consideration of local buckling. In order to improve the analytical response estimation of steel bridge columns, a fiber model analysis method considering local buckling is proposed in this study. In this method, multi-fiber elements were used to model the local buckling length to simulate local buckling deformation. The proposed method shows good results not only for estimating peak strength but also post-peak strength compared to previous experimental results. However, further research should be directed to improve the accuracy of the deformation modes.

Lateral Soil-Pipe Interaction in Dry and Partially Saturated Sand

AbstractThis paper describes finite-element (FE) modeling, validated by large-scale tests, to simulate the lateral force versus displacement relationship of pipelines under plane-strain conditions in both dry and partially saturated sand. The FE model is based on an elastoplastic characterization of the soil, with Mohr-Coulomb strength parameters to determine the soil yield surface. Direct shear test data and strain softening models are used to represent peak and postpeak strength behavior. A methodology for defining a strain-compatible secant modulus is also presented. The analytical results are compared with numerous large-scale experimental test results, showing excellent agreement in terms of prepeak, peak, and postpeak performance. The modeling process is used to show the relationship between the maximum lateral force and pipe depth and to explain decreased dimensionless maximum lateral forces mobilized by large diameter pipes at low depth to diameter ratios in dense sand.