Particle flow study on strength and meso-mechanism of Brazilian splitting test for jointed rock mass

A discrete element method （DEM） called particle flow code （PFC2D） was used to construct a model for Brazilian disc splitting test in the present study. Based on the experimental results of intact Brazilian disc of rock-like material, a set of micro-parameters in PFC2D that reflected the macro-mechanical behavior of rock-like materials were obtained. And then PFC2D was used to simulate Brazilian splitting test for jointed rock mass specimens and specimen containing a central straight notch. The effect of joint angle and notch angle on the tensile strength and failure mode of jointed rock specimens was detailed analyzed. In order to reveal the meso-mechanical mechanism of crack coalescence, displacement trend lines were applied to analyze the displacement evolution during the crack initiation and propagation. The investigated conclusions can be described as follows. （1） The tensile strength of jointed rock mass disc specimen is dependent to the joint angle. As the joint angle increases, the tensile strength of jointed rock specimen takes on a nonlinear variance. （2） The tensile strength of jointed rock mass disc specimen containing a central straight notch distributes as a function of both joint angle and notch angle. （3） Three major failure modes, i.e., pure tensile failure, shear failure and mixed tension and shear failure mode are observed in jointed rock mass disc specimens under Brazilian test. （4） The notch angle roles on crack initiation and and joint angle play important propagation characteristics of jointed rock mass disc specimen containing a central straight notch under Brazilian test.

The Effect of Basalt Fiber on Concrete Performance under a Sulfate Attack Environment

To enhance the sulfate attack resistance performance of concrete,Sulfate erosion test was carried out on basalt fiber concrete with different contents,selecting a concentration of 5%sulfate solution and using a dry−wet cycle mechanism attack of basalt fiber-reinforced concrete(BFRC).Every 15 dry−wet cycles,the mass,compressive strength,splitting tensile strength,and relative dynamic elastic modulus of BFRC were tested,and the SO_(4)^(2−)con-centration was measured.This work demonstrates that the mass,relative dynamic elastic modulus,compressive and splitting tensile strength of BFRC reveal a trend of climb up and then decline with the process of the dry−wet cycle.Basalt fiber can enhance the sulfate corrosion resistance of concrete by delaying the erosion of concrete induced by SO_(4)^(2−)and increasing the bearing and anti-deformation capacities of concrete by improving its inter-nal structure.Additionally,when mixing 0.2%basalt fiber into concrete,the strength deterioration rate will be reduced when the peak values of splitting tensile and compressive strength appear at 60 and 75 times the alter-nating dry−wet cycles,respectively.Adverse effects will occur when the fiber volume fraction exceeds 0.2%.The research in this paper can provide a foundation for the engineering applications of basalt fiber concrete.

Assessment of the Mechanical Properties of Carbon-Fiber Heating Cables in Snow and Ice Melting Applications

The use of carbon-fiber heating cables(CFHC)to achieve effective melting of snow and ice deposited on roads is a method used worldwide.In this study,tensile and compressive tests have been conducted to analyze the mechan-ical properties of the CFHC and assess whether the maximum tensile and compressive strengths can meet the pavement design specifications.In order to study the aging produced by multiple cycles of heating and cooling,in particular,the CFHC was repeatedly heated in a cold chamber with an ambient temperature ranging between-20℃ and+40℃.Moreover,to evaluate how the strength of the pavement is affected by its presence,the CFHC was embedded at different depths and concrete blocks with different curing ages were subjected to relevant com-pression and splitting tensile tests.Numerical simulations based on the ANSYS software have also been performed and compared with the outcomes of the static loading tests.The results show that the CFHC embedded in the concrete does not affect the compressive splitting tensile strengths of the pavement.Overall,the CFHC meets the conditions required for continued use in road ice melting applications.

Tensile strength behavior of cement-stabilized dredged sediment reinforced by polypropylene fiber

This study evaluated the feasibility of using polypropylene fiber(PF)as reinforcement in improving tensile strength behavior of cement-stabilized dredged sediment(CDS).The effects of cement content,water content,PF content and length on the tensile strength and stress–strain behavioral evolutions were evaluated by conducting splitting tensile strength tests.Furthermore,the micro-mechanisms characterizing the tensile strength behavior inside PF-reinforced CDS(CPFDS)were clarified via analyzing macro failure and microstructure images.The results indicate that the highest tensile strengths of 7,28,60,and 90 d CPFDS were reached at PF contents of 0.6%,1.0%,1.0%,and 1.0%,exhibiting values 5.96%,65.16%,34.10%,and 35.83%higher than those of CDS,respectively.Short,3 mm,PF of showed the best reinforcement efficiency.The CPFDS exhibited obvious tensile strain-hardening characteristic,and also had better ductility than CDS.The mix factor(C_(C)^(a)/C_(w)^(b))and time parameter(q_(t0)(t))of CDS,and the reinforcement index(k_(t-PF))of CPFDS were used to establish the tensile strength prediction models of CDS and CPFDS,considering multiple factors.The PF“bridge effect”and associated cementation-reinforcement coupling actions inside CPFDS were mainly responsible for tensile strength behavior improvement.The key findings contribute to the use of CPFDS as recycled engineering soils.

Fractal Dimension Analysis of the Fine Aggregate Gradation of Interlocking Skeleton Asphalt Mixture

By analyzing the fine aggregate gradation scales from standards,fine aggregate in the asphalt mixture is regarded as a whole research object and fractal dimensions X of the samples were obtained by linear regression and（AC-13）is 2.43-2.56,Sup-13 is 2.28-2.54,SMA-13 is 2.66-2.88 and SAC-13 is 2.54-2.73.In the dense gradation mixture,there are little different between fractal dimensions of coarse and fine aggregates but it makes sense for skeleton asphalt mixture.For a given coarse aggregate gradation and the same percentage of coarse aggregates,the compressive strengths and splitting strengths of the asphalt mixture are studied when the fractal dimensions are selected as 2.60,2.65 and 2.70,respectively.When asphalt-stone ratio is less than optimal asphalt-stone ratio,the higher compressive strength is,the bigger X can be gotten.When asphalt-stone ratio is larger than optimal asphalt-stone ratio,little difference of compressive strength can be observed under these three conditions.The largest splitting strength can be got when X is 2.65,and larger splitting strength can be observed with the ascending of the asphalt-stone ratio.

Influence of shrinkage-reducing admixture on drying shrinkage and mechanical properties of high-performance concrete

High-performance concrete （HPC） has specific performance advantages over conventional concrete in strength and durability. HPC mixtures are usually produced with water/binder mass ratios （mW/mB） in the range of 0.2-0.4, so volume changes of concrete as a result of drying, chemical reactions, and temperature change cannot be avoided. For these reasons, shrinkage and cracking are frequent phenomena. It is necessary to add some types of admixture for reduction of shrinkage and cracking of HPC. This study used a shrinkage-reducing admixture （SRA） for that purpose. Concrete was prepared with two different mW/mB （0.22 and 0.40） and four different mass fractions of SRA to binder （w（SRA） = 0%, 1%, 2%, and 4%）. The mineral admixtures used for concrete mixes were： 25% fly ash （FA） and 25% slag by mass of binder for the mixture with mW/mB = 0.40, and 15% silica fume （SF） and 25% FA for the mixture with mW/mB = 0.22. Tests were conducted on 24 prismatic specimens, and shrinkage strains were measured through 120 days of drying. Compressive strength, splitting strength, and static modulus of elasticity were also determined. The results show that the SRA effectively reduces some mechanical properties of HPC. The reductions in compressive strength, splitting tensile strength, and elastic modulus of the concrete were 7%-24%, 9%-19%, and 5%-12%, respectively, after 90 days, compared to concrete mixtures without SRA. SRA can also help reduce drying shrinkage of concrete. The shrinkage strains of HPC with SRA were only as high as 41% of the average free shrinkage of concrete without SRA after 120 days of drying.

Experimental Research on the Physical and Mechanical Properties of Concrete with Recycled Plastic Aggregates

In order to study the effect of recycled plastic particles on the physical and mechanical properties of concrete,recycled plastic concrete with 0,3%,5%and 7%content(by weight)was designed.The compressive strength,splitting tensile strength and the change of mass caused by water absorption during curing were measured.The results show that the strength of concrete is increased by adding recycled plastic into concrete.Among them,the compressive strength and the splitting tensile strength of concrete is the best when the plastic content is 5%.With the increase of plastic content,the development speed of early strength slows down.Silane coupling agent plays a positive role in the strength of recycled plastic concrete.The water absorption saturation of concrete has been basically completed in the early stage.The addition of silane coupling agent makes the porosity of concrete reduce and the water absorption of concrete become poor.By summing up the physical and mechanical properties of recycled plastic concrete,it could be found that the addition of recycled plastic was effective for the modification of concrete materials.Under the control of the amount of recycled plastic,the strength of concrete with recycled plastic aggregates can meet the engineering requirements.

Mechanical Properties of Lime-Fly Ash-Sulphate Aluminum Cement Stabilized Loess

Lime-fly ash stabilized loess has a poor early strength,which results in a later traffic opening time when it is used as road-base materials.Consideration of the significant early strength characteristics of sulphate aluminum cement(SAC),it is always added into the lime-fly ash mixtures to improve the early strength of stabilized loess.However,there is a scarcity of research on the mechanical behavior of lime-fly ash-SAC stabilized loess and there is a lack of quantitative evaluation of loess stabilized with binder materials.This research explored the effects of the amount of binder materials,curing time and porosity on the unconfined compressive strength(UCS),splitting tensile strength(STS),cohesion(c)and friction angle(φ)of lime-fly ash-SAC stabilized loess by a series of unconfined compressive tests(UCT)and splitting tensile tests(STT).The results indicate that an increase in curing time and a decrease in porosity lead to a continuous increase in the UCS and STS for lime-fly ash-SAC stabilized loess.The addition of SAC has a prominent enhancement in the early strength of lime-fly ash-SAC stabilized loess.When the curing time,porosity,and binder content were constant,the UCS and STS increase with increasing SAC content;For a stabilized loess with 30%binder content and 5%SAC content after 1 day of curing,the UCS was greater than 0.7 MPa,which meets the requirement of opening traffic,so lime-fly ash-SAC stabilized loess could be used as an excellent maintenance material for road-base;In accordance with the analysis of testing data,empirical relationships between the UCS and STS of lime-fly-SAC stabilized loess and key effect factors(binder materials content,curing time and porosity)were developed,which can provide references for reasonably selecting the amount of binder materials,compaction degree and curing period to meet the required strength of practical engineering.Finally,based on the Mohr-Coulomb theory and the above empirical relationships,a simpler method for calculating the c andφof stabilized loess was proposed,with which,the shear strength parameters can be determined only by UCT or STT.

Relationship between compressive and tensile strengths of roller-compacted concrete

The abstract roller-compacted concrete （RCC） is a zero slump concrete comprising the same materials as that of conventional concrete with different proportions. The RCC must be compacted to reach its final form. The effects of hydration and aggregate interlock on its strength are considerable. For similar binder contents, the compressive strength of the RCC is generally higher than that of the conventional concrete; however, the tensile strength of RCC may not be superior to that of the conventional concrete. Adequate tensile strength is necessary to resist fatigue cracking, particularly in pavement applications. However, the compressive strength is frequently used in assessing the quality control and quality assurance of pavements. Therefore, the relationship between the compressive and tensile strengths of the RCC should be analyzed. Unfortunately, only a few studies have been conducted on this relationship. The objective of this study is to identify the difference between the indirect tensile strengths of the RCC and those of the conventional concrete as well as develop relationship equations to evaluate the compressive and tensile strengths. In this study, regression equations are developed to estimate the indirect tensile strengths, which are known as flexural and splitting tensile strengths, using the compressive strength of the RCC. The results show that the flexural strength of the RCC is within the predicted values obtained from the conventional concrete equations for a given compressive strength. In contrast, the splitting tensile strength of the RCC is relatively lower than that of the conventional concrete for the given compressive strength.

Computer-aided Prediction of the ZrO_2 Nanoparticles' Effects on Tensile Strength and Percentage of Water Absorption of Concrete Specimens

In the present paper, two models based on artificial neural networks and genetic programming for predicting split tensile strength and percentage of water absorption of concretes containing ZrO2 nanoparticles have been developed at different ages of curing. For building these models, training and testing using experimental results for 144 specimens produced with 16 different mixture proportions were conducted. The data used in the multilayer feed forward neural networks models and input variables of genetic programming models were arranged in a format of eight input parameters that cover the cement content, nanoparticle content, aggregate type, water content, the amount of superplasticizer, the type of curing medium, age of curing and number of testing try. According to these input parameters, in the neural networks and genetic programming models, the split tensile strength and percentage of water absorption values of concretes containing ZrO2 nanoparticles were predicted. The training and testing results in the neural network and genetic programming models have shown that two models have strong potential for predicting the split tensile strength and percentage of water absorption values of concretes containing ZrO2 nanoparticles. It has been found that neural network （NN） and gene expression programming （GEP） models will be valid within the ranges of variables. In neural networks model, as the training and testing ended when minimum error norm of network gained, the best results were obtained and in genetic programming model, when 4 genes were selected to construct the model, the best results were acquired. Although neural network have predicted better results, genetic programming is able to predict reasonable values with a simpler method rather than neural network.

Dynamic mechanical behavior of ultra-high strength steel 30CrMnSiNi2A at high strain rates and elevated temperatures

During high speed machining in the field of manufacture,chip formation is a severe plastic deformation process including large strain,high strain rate and high temperature.And the strain rate in high speed cutting process can be achieved to 105 s^（-1）.30CrMnSiNi2Asteel is a kind of important high-strength low-alloy structural steel with wide application range.Obtaining the dynamic mechanical properties of30CrMnSiNi2Aunder the conditions of high strain rate and high temperature is necessary to construct the constitutive relation model for high speed machining.The dynamic compressive mechanical properties of30CrMnSiNi2Asteel were studied using split Hopkinson pressure bar（SHPB）tests at 30-700°C and3000-10000s^（-1）.The stress-strain curves of 30CrMnSiNi2Asteel at different temperatures and strain rates were investigated,and the strain hardening effect and temperature effect were discussed.Experimental results show that 30CrMnSiNi2Ahas obvious temperature sensitivity at 300°C.Moreover,the flow stress decreased significantly with the increase of temperature.The strain hardening effect of the material at high strain rate is not significant with the increase of strain.The strain rate hardening effect is obvious with increasing the temperature.According to the experimental results,the established Johnson-Cook（J-C）constitutive model of 30CrMnSiNi2Asteel could be used at high strain rate and high temperature.

Fresh and hardened properties of high-strength concrete incorporating byproduct fine crushed aggregate as partial replacement of natural sand

This paper presents the fresh and hardened properties of high-strength concrete comprising byproduct fine crushed aggregates(FCAs)sourced from the crushing of three different types of rocks,namely granophyre,basalt,and granite.The lowest void contents of the combined fine aggregates were observed when 40%to 60%of natural sand is replaced by the FCAs.By the replacement of 40%FCAs,the slump and bleeding of concrete with a water-to-cement ratio of 0.45 decreased by approximately 15%and 50%,respectively,owing to the relatively high fines content of the FCAs.The 28 d compressive strength of concrete was 50 MPa when 40%FCAs were used.The slight decrease in tensile strength from the FCAs is attributed to the flakiness of the particles.The correlations between the splitting tensile and compressive strengths of normal concrete provided in the AS 3600 and ACI 318 design standards are applicable for concrete using the FCAs as partial replacement of sand.The maximum 56 d drying shrinkage is 520 microstrains,which is significantly less than the recommended limit of 1000 microstrains by AS 3600 for concrete.Therefore,the use of these byproduct FCAs can be considered as a sustainable alternative option for the production of high-strength green concrete.

Quantification of coarse aggregate shape in concrete

The objective of this study is to choose indices for the characterization of aggregate form and angularity for large scale application. For this purpose, several parameters for aggregate form and angularity featured in previous research are presented. Then, based on these established parameters, 200 coarse quartzite aggregates are analyzed herein by using image processing technology. This paper also analyzes the statistical distributions of parameters for aggregate form and angularity as well as the correlation between form and angularity parameters. It was determined that the parameters for form or angularity of coarse aggregates could be fitted by either normal distribution or log-normal distribution at a 95% confidence level, Some of the form parameters were influenced by changes in angularity characteristics, while aspect ratio and angularity using outline slope, area ratio and radius angularity index, and aspect ratio and angularity index were independent of each other, respectively; and consequently, the independent parameters could be used to quantify the aggregate form and angularity for the purpose to study the influence of aggregate shape on the mechanical behavior of concrete. Furthermore, results from this study＇s in-depth investigations showed that the aspect ratio and the angularity index can further understanding of the effects of coarse aggregates form and angularity on concrete mechanical properties, respectively. Finally, coarse aggregates with the same content, type and surfaces texture, but different aspect ratios and angularity indices were used to study the influence of coarse aggregate form and angularity on the behavior of concrete. It was revealed that the splitting tensile strength of concrete increased with increases in the aspect ratio or angularity index of coarse aggregates.

Effects of CuO Nanoparticles on Microstructure,Physical,Mechanical and Thermal Properties of Self-Compacting Cementitious Composites

In the present study,split tensile strength of self-compacting concrete with different amount of CuO nanoparticles has been investigated.CuO nanoparticles with the average particle size of 15 nm were added partially to self compacting concrete and split tensile strength of the specimens has been measured.The results indicate that CuO nanoparticles are able to improve the split tensile strength of self compacting concrete and recover the negative effects of polycarboxylate superplasticizer on split tensile strength.CuO nanoparticle as a partial replacement of cement up to 4 wt% could accelerate C-S-H gel formation as a result of increased crystalline Ca（OH）2 amount at the early ages of hydration.The increase of the CuO nanoparticles more than 4 wt% causes the decrease of the split tensile strength because of unsuitable dispersion of nanoparticles in the concrete matrix.Accelerated peak appearance in conduction calorimetry tests,more weight loss in thermogravimetric analysis and more rapid appearance of related peaks to hydrated products in X-ray diffraction（XRD） results all also indicate that CuO nanoparticles up to4 wt% could improve the mechanical and physical properties of the specimens.Finally,CuO nanoparticles could improve the pore structure of concrete and shift the distributed pores to harmless and few-harm pores.

Experimental research on the microstructure and compressive and tensile properties of nano-SiO_(2) concrete containing basalt fibers

Urban underground space resources are gaining increasing attention for the sustainable development of cities.Traditional concrete cannot meet the needs of underground construction.High-performance concrete was prepared using varying dosages of nano-SiO_(2)and basalt fiber,and its compressive and tensile strength was measured.The concrete microstructure was analyzed and used to assess the mechanisms through which the nano-SiO_(2)and basalt fibers affect the strength of concrete.The cement hydration productions in concrete produced varied with the dosage of nano-SiO_(2).When the nano-SiO_(2)dosage was between 0 and 1.8%,the mass of the C-S-H gel and AFt crystals increased gradually with the nano-SiO_(2)dosage.When the nano-SiO_(2)dosage was 1.2%,optimum amounts of C-S-H gel and AFt crystals existed,and the compactness of concrete was well,which agreed with the results of the compressive strength tests.When the basalt-fiber dosage was between 3 and 4 kg/m^(3),the basalt fibers and the cement matrix were closely bonded,and the splitting tensile strength of the concrete markedly improved.When the basalt-fiber dosage exceeded 5 kg/m^(3),the basalt fibers clustered together,resulting in weak bonding between the basalt fibers and the cement matrix,consequently,the basalt fibers were easily pulled apart from the cement.When the nano-SiO_(2)and basalt fiber dosages were 1.2%and 3 kg/m^(3),respectively,the compactness of the concrete microstructure was well and the strength enhancement was the greatest;additionally,the compressive strength and splitting tensile strength were 9.04%and 17.42%,respectively,greater than those of plain concrete.The macroscopic tests on the mechanical properties of the nano-SiO_(2)concrete containing basalt fibers agreed well with the results of microstructure analysis.