Compressive Stress-strain Behavior Research Articles

Effects of saturation degrees, freezing-thawing, and curing on geotechnical properties of lime and lime-cement concretes

There are very limited researches carried out to investigate the influence of saturation degrees, freezing-thawing, and curing times on geotechnical properties of lime concrete (LC) and lime-cement concrete (LCC) due to the capillary action and changes in groundwater table. Subsequently, the primary goal of this research is to investigate the influence of these parameters on mechanical properties of LC and LCC using unconfined compression tests, namely uniaxial compressive strength (UCS), stress-strain behavior, deformability index (ID), secant modulus (ES), failure strain, bulk modulus (K), resilient modulus (MR), brittleness index (IB), and shear modulus (G). At first, the mechanical and chemical characteristics of the utilized materials were measured. Then, samples were made with an optimal amount of cement, lime, coarse-grained soil, fine-grained soil, and water. The samples were then exposed to saturation points extending from 0 to 100% after 14, 28, 45 and 60 curing days. Then, to consider the effect of amount of saturation on the mechanical properties, UCS tests were performed on some of the samples. Other LCC specimens were exposed to freezing-thawing conditions to consider the effect of this phenomenon on the mechanical properties as well. The results of more than 250 UCS tests demonstrated that the curing times significantly affected the strength of LC and LCC specimens. Moreover, it is not ideal and logical to utilize LC and LC columns at a profundity underneath or near the groundwater level, though it is reasonable to adopt LCC and LCC columns at a profundity beneath or near the groundwater level because of the immaterial effect of degrees of saturation on LCC. In addition, this study showed that extending the curing period and diminishing the saturation degree would increase the strength and mechanical properties of the LCC specimens. The results of freezing-thawing demonstrated a negligible increase in the strength of the LCC when the thawing process preceded the freezing process. However, the effect of freeze-thaw cycles on the strength properties of LCC may mostly be neglected.

An empirical approach for the quantification of uniaxial compressive stress-strain of partially saturated granular media under high strain rates

While it is qualitatively known that the dynamic stress-strain response of granular media is highly influenced by the initial density and moisture content, a detailed characterization of such behavior has not been reported in the existing literature. This paper proposes a simplified empirical approach to quantify the uniaxial compressive stress-strain behavior of partially saturated granular media at a given saturation and initial dry density that requires only the response at a reference dry density as input. The relations were formulated based on experimental stress-strain data of Stockton Beach sand retrieved using a modified split Hopkinson pressure bar (SHPB) at a strain rate between 1000 and 1300 s−1. The tested sand was confined within a hardened steel tube, exhibiting initial dry densities of 1.46 g/cm3, 1.57 g/cm3, and 1.69 g/cm3 across water saturations ranging from dry to above 90%. Through a parametric investigation, the resulting empirical relations were shown to be promising in representing the specimen behavior before and after the initiation of water lock-up. This study ultimately illustrates the feasibility of empirical modeling techniques in predicting and characterizing the high strain rate response of partially saturated granular media as a bulk system.

Smart Cement Compressive Piezoresistive, Stress-Strain, and Strength Behavior with Nanosilica Modification

Abstract In this study, smart cement with a water-to-cement ratio of 0.38 was modified with up to 1 % silica nanoparticles (NanoSiO2) to evaluate the effect not only on the sensing properties but also on the compressive stress-strain relationship and strength. The oil well cement (class H) and cement modified with NanoSiO2 were characterized using X-ray diffraction analysis and thermal gravimetric analysis. The smart cement was prepared by adding 0.1 % of the conductive filler based on the percolation theory concept to make the cement piezoresistive yet remain a nonconductive material. Testing evaluated the smart cement behavior with and without NanoSiO2 to verify the sensitivity of electrical resistivity changes with time and compressive loading. The addition of 0.5 and 1 % NanoSiO2 increased the initial electrical resistivity of the smart cement by 17 and 35 %, respectively. In one day, the maximum change in the electrical resistivity (RI24hr) for the smart cement without NanoSiO2 was 364 %. The RI24hr for the smart cement with NanoSiO2 decreased with the amount of NanoSiO2. The addition of 1 % NanoSiO2 increased the compressive strength of the smart cement by 14 and 42 % after 1 and 28 days of curing, respectively. The nonlinear Vipulanandan p-q curing model was used to predict the changes in electrical resistivity with curing time. The piezoresistivity of smart cement with NanoSiO2 was over 500 times higher than the regular cement, depending on the curing time and NanoSiO2 content. The Vipulanandan p-q stress strain and stress change in resistivity models also predicted the experimental results very well. For smart cement modified with NanoSiO2, the resistivity change at peak stress was over 1,250 times higher than the change in the compressive strain. A linear correlation was obtained between the RI24hr and the compressive strength of the modified smart cement based on the curing time.

Compressive behaviour of stub concrete column strengthened with ultra-high performance concrete jacket

This paper firstly performed compressive tests on 18 stub concrete columns with externally confined ultra-high performance concrete (UHPC) jacket. The studied parameters in this testing programme were the thickness of UHPC jacket and the shapes of specimens. The influences of these parameters on the compressive behaviour of stub concrete columns confined by UHPC jacket were reported and discussed. The test results revealed that increasing the thickness of confining UHPC jacket significantly improved the compressive behaviours of concrete stub columns for cylindrical shaped column. However, this influence on that of square shaped stub columns was less significant. An analytical model was proposed to predict the compressive stress-strain behaviour of stub concrete columns with externally strengthening UHPC jacket. The validations of the predictions against the reported test results confirmed the accuracy of the analytical model. The compressive behaviour of stub concrete columns with UHPC jacket was also simulated by 3D nonlinear finite element model (FEM). Validations against the reported test data proved that the FEM simulated well compressive behaviour of concrete stub columns with UHPC jacket.

Stress-strain model for C-shape confined concrete masonry boundary elements of RM shear walls

Reinforced masonry (RM) shear walls with boundary elements have been recently presented as a more ductile alternative to RM rectangular shear walls. Evaluating the complete (i.e. including the post-peak branch) compression stress-strain behavior of the confined and unconfined masonry is essential for predicting the seismic response of the RM walls with boundary elements. Recently, the authors investigated the effect of various volumetric ratios of transverse reinforcement, vertical reinforcement ratios, and grout strength on the axial stress-strain behavior of reinforced masonry boundary elements (RMBEs). However, all the specimens had a specific height to thickness ratio (i.e., AR = 5). This study presents the observed stress-strain relationship of seventeen half-scale fully grouted unreinforced and RMBE specimens, built using C-shape blocks, tested under concentric compression loading up to failure. Thus, quantifying the effect of various aspect (height to thickness) and confinement ratios on the RMBEs peak stress, strain corresponding to peak, and post-peak behavior. The results indicate that, as the hoop spacings and/or aspect ratio decreases, the peak stress and post-peak strains increase. Moreover, this study presents a stress-strain empirical model capable of predicting the RMBE stress-strain response by computing the confined and unconfined masonry stress-strain behavior. The model is calibrated using the experimental data of thirty-three RMBE specimens, tested in this study and literature. The proposed model presents an efficient tool that can be implemented in different analytical/numerical packages.

Effect of Fibers on High-Temperature Mechanical Behavior and Microstructure of Reactive Powder Concrete.

This study was aimed to investigate the effect of steel, polypropylene (PP), and hybrid (steel + PP) fibers on high-temperature mechanical properties of reactive powder concrete (RPC). The mechanical properties considered are cubic compressive strength, axial or prismatic compressive strength, split-tensile strength, flexural strength, elastic modulus, peak strain, and stress-strain behavior. The strength recession due to high temperature was investigated at micro level by scanning electron microscope, energy dispersive X-ray spectroscopy, X-ray diffraction, mercury intrusion porosity, thermogravimetric, and differential scanning calorimetry analyses. The high-temperature tests were carried out at target temperatures of 120, 300, 500, 700, and 900 °C. The hot-state compressive strength of RPC started to decrease at 120 °C; however, a partial recovery at 300 °C and a gradual decrease above 300 °C were observed. The degradation of split-tensile strength, flexural strength, and elastic modulus were gradual with increasing temperature despite the effect of different fibers. Whereas, the peak strain was gradually increasing up to 700 °C. However, after 700 °C, it remained unchanged. Steel fiber reinforced RPC (SRPC) and hybrid fiber reinforced RPC (HRPC) showed a ductile behavior. PP fiber reinforced RPC (PRPC) showed a quite brittle behavior up to 300 °C; however, further heating made the microstructure porous and it became ductile too. Overall the performance of SRPC and HRPC were superior to PRPC because of higher modulus of elasticity, higher strength, and better fire resistance of steel fibers. Fiber reinforced RPC was found to have better fire resistance than traditional types of concrete based on comparative studies with the provisions of design codes and earlier research. The constitutive equations developed can be utilized in computer programs for structural design of RPC structures exposed to fire.

Compressive Stress-Strain Behavior of FeS Sintered bulk at High Temperature

Compressive Stress-Strain Behavior of FeS Sintered bulk at High Temperature

Deformation Behaviour of Single and Gradient Density Natural Rubber Foams

Abstract The deformation behaviour of single and gradient density natural rubber (NR) foams produced from dry natural rubber have been investigated through experimentation and simulation. Different concentrations of a sodium bicarbonate blowing agent (BA) from 4phr to 16phr were utilized to produce different cell sizes and densities. The gradient density foams were fabricated by stacking the ∼ 20 mm layer of single density foam with several layers of foams having different relative densities ranging from 0.341 to 0.185. The mechanical properties and deformation behaviors of the foams were investigated through compression test and digitally-captured progressive deformation images. The models of the foam structures were constructed for simulation study via Finite Element Analysis (FEA) to investigate the cell elastic strain deformation. The results showed that the densities of polymeric foams were crucial in determining the foams compressive stress-strain behaviors. The NR foams with a higher density exhibited the greatest compressive strength properties, and the study also found that the foam cells in the middle section experienced the most cell deformation which expected due to higher stress concentration generated in this section. Further investigation on the gradient density foams structure showed that the foam layer with the lowest density undergoes the most cell deformation, regardless of the layer position in the structure.

Axial stress–strain model for square concrete columns internally confined with GFRP hoops

Good modelling of the axial compressive stress–strain behaviour of confined reinforced-concrete (RC) columns is necessary for the structural analysis and design to assess their strength and ductility capacities. Thus, this paper presents closed-form expressions for predicting the stress–strain response of square concrete columns confined by internal glass-fibre-reinforced polymer (GFRP) hoops and subjected to axial compression loading. The stress–strain proposal is based on test results from the available studies and is mainly composed of a polynomial first branch (pre-peak zone) and a linear second branch (post-peak zone). To ensure a sufficient level of confinement, the thresholds of lightly and heavily confined concrete have been determined based on the proposed model and the compiled database. The model also considers the effects of key parameters such as volumetric ratio, size, configuration and spacing of hoop reinforcement, as well as longitudinal bar type (i.e. steel as opposed to GFRP). Existing models for steel-confined concrete and for concrete in circular cross-section confined by FRP hoops did not accurately predict the test results. Contrary to that, comparisons of the test results with the predictions of the proposed model revealed the model to describe well the stress–strain behaviour of GFRP RC columns with different arrangements of hoops.

Vipulanandan models to predict the electrical resistivity, rheological properties and compressive stress-strain behavior of oil well cement modified with silica nanoparticles

In this study, the effect of silica nanoparticles (NS) and temperature on the rheological properties with ultimate shear stress and weight loss of the oil well cement (class H) modified with NS were investigated. The NS content was varied up to 1% by the weight of the cement. The total weight loss at 800 °C for the oil well cement decreased from 6.10% to 1.05%, a 83% reduction when the cement was mixed with 1% of NS. The results also showed that 1% of NS increased the rheological properties of the cement slurry. The NS modification increased the yield stress (τo) and by 5% to 65% based on the NS content and the temperature of the cement slurry. The addition of 0.5% and 1% NanoSiO2 increased the initial electrical resistivity of the cement by 17% and 35% respectively. The shear thinning behavior of the cement slurry with and without NS has been quantified using the Vipulanandan rheological model and compared with the Herschel-Bulkley model. Based on the Vipulanandan rheological model the maximum shear stress produced by the cement slurry modified with 0% and 1% of NS at the temperature of 25 °C were 148 Pa and 179 Pa respectively hence an increase of 21% in the ultimate shear stress due to the addition of NS. The addition of 1% of NS increased the compressive strength of the cement by 14% and 42% after 1 day and 28 days of curing respectively. Effects of NS content and the temperature on the model parameters have been quantified using a nonlinear model (NLM). The NLM quantified the effect of NS treatment on all the model parameters.

Lightweight Self-Compacting Concrete Incorporating Perlite, Scoria, and Polystyrene Aggregates

Self-compacting concrete (SCC) is a highly fluid form of concrete that can consolidate under its own weight, thereby reducing the need of any mechanical compaction processes. Lightweight concrete (LWC), which incorporates the use of lightweight aggregates, can reduce the density of the concrete and therefore the potential associated dead loads. In recent decades, there have been numerous studies in the production of lightweight self-compacting concrete (LWSCC) that aim to combine the benefits of both SCC and LWC. Due to the various lightweight aggregates and their different material characteristics, it is imperative to assess whether the properties of both concretes still apply when using different lightweight aggregates. This paper aims to develop information about the fresh and hardened properties of lightweight self-compacting concrete using perlite, scoria, and polystyrene lightweight aggregates at varying replacement percentages. The fresh properties were investigated using the slump flow, T500, and J-ring tests. Hardened properties include 7- and 28-day compressive and tensile strengths, and compressive stress-strain behavior at 28 days.

Performance evaluation of glass and rock wool fibers to improve thermal stability and mechanical strength of monolithic phenol-formaldehyde based carbon aerogels

Rock and glass wool fiber reinforced phenol-formaldehyde monolithic composite aerogels (PF/RWF and PF/GWF, respectively) were prepared using solvent saturated sol–gel and impregnation of fibers by initial PF sol. The composite wet gels were dried by using ambient-pressure drying technique. After it, synthesized polymeric aerogels were transformed to rock and glass wool fiber reinforced monolithic carbon composite aerogels (C/RWF and C/GWF) through carbonization at 850 °C. The effect of fiber concentration on the physical and morphological properties of both types of composite aerogels was evaluated. The SEM analysis was also conducted to understand the microstructural changes during the carbonization step. Compressive stress-strain behavior of polymeric and carbon composite aerogels changed drastically by addition of fibers. The Young's modulus of polymeric and carbon composite aerogels increased by increasing of fibers concentration in the aerogel structure and showed a yield stress. The compressive strength of carbon composite aerogels was decreased with respect to the formation of voids and cracks around the fibers through the pyrolysis step. The degradation kinetic parameters of polymeric composite aerogels were evaluated using characteristic kinetic temperature-characteristic kinetic temperature range model (CKT-CKTR). The results showed the higher performance of rock wool fibers to thermal stability enhancement of PF aerogels. By 2.5 wt% wool fibers content, the maximum oxidation rate temperature increased from 594 °C for carbon aerogel to about 620 and 643 °C for C/RWF and C/GWF composite aerogels, respectively. The effect of rock wool fibers on the improvement of thermal oxidation resistance of samples was stronger than glass wool fibers.

Compressive stress strain behavior of crumb rubber concrete (CRC) and application in reinforced CRC slab

This paper presents the stress-strain behavior of CRC with rubber replacement percentages of 6%, 12% and 18%. The peak stress, strain at peak stress, modulus of elasticity, energy dissipation ability and the constitutive model under uniaxial compression were examined. Constitutive models for traditional concrete (TC) were applied to both TC and CRC. Comparisons between experimental results and the models were made. A constitutive model for unconfined CRC was then developed. A numerical study on the flexural behavior of reinforced CRC slabs was conducted with the application of the stress-strain model developed in the paper and the results agreed well with experimental tests.

Measurement of the through thickness compression of a battery separator

The mechanical integrity of the separator is critical to the reliable operation of a battery. Due to its minimal thickness, compression experiments with a single/a few layers of separator are difficult to perform. In this work, a capacitance based displacement set-up has been developed for the measurement of the through thickness direction (TTD) compression stress-strain behavior of the separator and the investigation of its interaction with the electrode. The experiments were performed for a stack of two layers of Celgard 2400 separator, NMC cathode, and separator/NMC cathode/separator stack in both dry and wet (i.e. submersed in dimethyl carbonate DMC) conditions. The experimental results reveal that the separator compression modulus can be significantly affected by the presence of DMC. The iso-stress based rule of mixtures was used to compute the compressive stress-strain curve for the stack from that of the separator and NMC layer. The computed curve agreed with the experimental curve reasonably well up to about 0.16 strain but deviated significantly to a softer response at higher strains. The results suggest that, in the stack, the TTD compressive deformation of the separator is influenced by the NMC cathode.

Development of high-performance self-compacting concrete using waste recycled concrete aggregates and rubber granules

Abstract In recent decades, self-compacting concrete has slowly gained popularity since its inception due to its unique ability to fill formworks with congested steel reinforcement and with little to no use of mechanical compaction required. Due to the environmental impacts associated with the natural aggregates in concrete production, a more sustainable approach in producing self-compacting concrete is to replace natural aggregates with that of recycled concrete aggregates from common construction waste and demolitions. This form of concrete provides a sustainable alternative in minimising the environmental damages associated with the extraction and depletion of natural resources. This experimental research aims to develop information about the fresh and hardened properties of different forms of self-compacting concrete by utilising recycled concrete aggregates in combination with recycled crumb rubber or lightweight scoria aggregates. The fresh properties were investigated in accordance with the guideline provided by the European federation national representing of concrete using the slump flow, T500, and J-ring tests. Hardened properties include 7 and 28 day compressive and tensile strengths, hardened density testing, and compressive stress-strain behaviour at 28 days. Optimal mix design of recycled concrete and crumb rubber aggregates self-compacting concrete are assessed to optimise fresh and hardened properties. The proposed SCC mixes are able to reduce amount of used cement to 40%. Aslo, as the percentage of recycled aggregate replacement increased, developed SCC mixes flowability and passing ability decreased.

Clay-Facilitated Aqueous Dispersion of Graphite and Poly(vinyl alcohol) Aerogels Filled with Binary Nanofillers.

Dispersion of graphite in water was achieved using clay as dispersing aid. In the absence of polymer, the clay/graphite suspensions were sufficiently stable to produce aerogels composed of very thin layers of uniformly dispersed nanoparticles. Poly(vinyl alcohol) (PVOH) aerogels containing binary nanofillers (clay plus graphite) were then fabricated and tested. These composites were found to maintain low thermal and electrical conductivities even with high loading of graphite. A unique compressive stress-strain behavior was observed for the aerogel, exhibiting a plateau in the densification region, likely due to sliding between clay and graphite layers within the PVOH matrix. The aerogels containing only graphite exhibited higher compressive modulus, yield stress and toughness values than the samples filled with binary nanofillers. X-ray diffraction (XRD) spectra for the same composite aerogel before and after compression testing illustrated the compression-induced dispersion changes of nanofillers. Composites containing 50 wt % graphite demonstrated a downshift of its 2D Raman peak implying graphite exfoliation to graphene with less than 5 layers.

Strength and mechanical behaviour of coir reinforced lime stabilized soil

Soil stabilization is an essential engineering process to enhance the geotechnical properties of soils that are not suitable for construction purposes. This study focuses on using coconut coir, a natural fibre to enhance the soil properties. Lime, an activator is added to the reinforced soil to augment its shear strength and durability. An experimental investigation was conducted to demonstrate the effect of coconut coir fibers and lime on the consistency limits, compaction characteristics, unconfined compressive strength, stress-strain behaviour, subgrade strength and durability of the treated soil. The results of the study illustrate that lime stabilization and coir reinforcement improves the unconfined compressive strength, post peak failure strength, controls crack propagation and boosts the tensile strength of the soil. Coir reinforcement provides addition contact surface, improving the soil-fibre interaction and increasing the interlocking between fibre and soil and thereby improve strength. Optimum performance of soil is observed at 1.25% coir fibre inclusion. Coir being a natural product is prone to degradation and to increase the durability of the coir reinforced soil, lime is used. Lime stabilization favourably amends the geotechnical properties of the coir fibre reinforced soil.

Experimental investigation into rubber granules and their effects on the fresh and hardened properties of self-compacting concrete

Self-compacting concrete (SCC) is a highly flowable form of concrete which can be applied in complex formworks consisting of congested reinforcement without the use of any or little mechanical vibration. Over the past decades, the excessive accumulation of waste rubber tyres in Australia has led to significant research in the development of waste rubber materials to be incorporated in the concrete industry. The replacement of natural aggregates in SCC with those of rubber aggregates from waste tyre develops into what is referred to as self-compacting rubberised concrete (SCRC). This form of concrete provides a sustainable alternative which assists in minimising the environmental damages associated with the disposal of waste tyres. This project aims to develop information about the fresh and hardened properties of SCRC at incremental aggregate replacements using three different rubber aggregate sizes. The fresh properties were investigated in accordance with the guidelines provided by the European Federation National Representing of Concrete using the slump flow, T500, and J-ring tests. Hardened properties include 7 and 28 day compressive and tensile strengths, hardened density testing, and compressive stress-strain behaviour at 28 days. In this study optimum crumb rubber aggregates replacement percentage in SCC and optimum crumb rubber aggregates size in SCC have been presented and also optimal mix design of SCRC will be assessed to optimise fresh and hardened properties.

Material Nonlinear Behavior and Microstructural Transition of Porous Polyurethane Foam under Uniaxial Compressive Loads

Porous materials such as polymeric foam are widely adopted in engineering and biomedical fields. Porous materials often exhibit complex nonlinear behaviors and are sensitive to material and environmental factors including cell size and shape, amount of porosity, and temperature, which are influenced by the type of base materials, reinforcements, method of fabrication, etc. Hence, the material characteristics of porous materials such as compressive stress-strain behavior and void volume fraction according to aforementioned factors should be precisely identified. In this study, unconfined uniaxial compressive test for two types of closed-cell structure polyurethane foam, namely, 0.16 and 0.32 g/cm3 of densities were carried out. In addition, the void volume fraction of three different domains, namely, center, surface and buckling regions under various compressive strains (10%, 30 %, 50 % and 70 %) were quantitatively observed using Micro 3D Computed Tomography(micro-CT) scanning system. Based on the experimental results, the relationship between compressive strain and void volume fraction with respect to cell size, density and boundary condition were investigated.

Modeling the axial compressive stress-strain behavior of CFRP-confined rectangular RC columns under monotonic and cyclic loading

This paper presents a stress-strain model for rectangular reinforced concrete (RC) columns confined with carbon fiber-reinforced polymer (CFRP) composites and subjected to cyclic axial compression. The model is mainly based on experimental observations and results from a series of tests reported in the companion study, in which 24 large-size CFRP-confined rectangular unreinforced and reinforced concrete columns under monotonic and cyclic axial compression were tested. Relative experimental results compiled from the available literature are also used. The test database covers unconfined concrete compressive strengths ranging from 17.6 MPa to 51.5 MPa and cross-sectional side lengths ranging from 90 mm to 400 mm. The proposed stress-strain model is composed of three main components: (1) a monotonic stress-strain model for the envelope curve of cyclic response; (2) a parabolic expression for the unloading path; and (3) a straight line for the reloading path. The model also considers the effects of key parameters including cross-sectional size, aspect ratio, effective rupture strain of the CFRP, and internal longitudinal and hoop steel reinforcement. The performance of the model is finally verified through comparison of predictions made by this model with selected test results, as well as the predictions of more than 20 models of other researchers.