Mix design and strength of soil–cement concrete based on the effective water concept

Saturated surface-dry condition of fine aggregate has been well defined in mortar and concrete production as the dry mass percentage moisture content where aggregate particles are saturated. However in soil–cement concrete construction, no mix design based on the saturated surface-dry condition of soil was attempted partly because the control of soil moisture on site is difficult. We have proposed the definition and testing methods of the saturated surface-dry condition of soil in the previous papers. In this paper, we introduce the concept of effective unit water to the soil–cement concrete mix design and propose a mix design method for soil-sand-cement systems. Strength and density of the soil–cement concretes were determined and discussed. A number of mixes were proportioned with accurate control of composition (C/W, soil/sand ratio, free water, etc.) and consistency. It was found that strength can be determined by C/W for a wide variety of clayey soils in various soil/sand ratios except for a soil with very high content of organic material. A set of nomograms for C/W versus strength and soil/sand versus strength were presented for future proportioning.     

Microstructure and mechanical properties of Mg – Al9Zn/SiCp composite produced by vacuum stir casting process

Abstract

15 vol.-% SiC particle reinforced cast Mg – 9AlZn (AZ91C) composite was produced by a vacuum stir casting process, and the microstructure and mechanical properties of the composite investigated. The stirring process was carried out at a speed of 750 – 1500 rev min^-1 with a stainless steel impeller for 25 min in a vacuum of 20 – 40 mbar. SiC particles in the composite exhibited a reasonably homogeneous distribution and were well wetted by magnesium. The Mg – Al9Zn/15SiC_p composite showed significant improvement in yield strength and elastic modulus following T4 heat treatment. The ultimate tensile strength of the composite was low, but close to that of unreinforced magnesium alloy. Mg/SiC interfacial reactions and reaction mechanisms are discussed. No evident interfacial products were found at a low process temperature of 700°C. However, significant chemical reactions at the Mg/SiC interface occurred when the composite melt was maintained at 750°C, and complex reaction products were formed. The fluidity of the composite melt deteriorated seriously after the interfacial reactions occurred.     

Investment casting – developments in microstructural control and mechanical performance

Abstract

The evolution of the ancient craft of lost wax casting into present investment casting practice, associated with a trend from the production of artistic artefacts to the manufacture of critical load bearing components, has resulted in progressive improvements in the mechanical integrity of the castings through systematic microstructural control. In this paper, the factors which influence the principal microstructural characteristics (porosity, grain size and shape, segregation, crystal texture) and the effects of those features on various mechanical properties are examined.

MST/749     

Development of cement-based lightweight composites – Part 1: Mix design methodology and hardened properties

This article aims at the development of durable cement-based lightweight aggregate composites, with a good balance between the thermal and mechanical properties. The mixtures are developed with the optimized packing applying the modified Andreasen and Andersen model, to obtain the optimal target grading curve of all the solids in the mixture. A lightweight material produced from recycled glass is used as the lightweight aggregates (LWA) in order to obtain the desired low thermal conductivity.The properties of the designed composites, including the flowability and relative viscosity in fresh state, and the porosity, strength and thermal properties in hardened state are investigated. The porosity of the developed composites is studied by both modeling and experiments. Results indicate that there is a certain amount of closed internal LWA pores in the composites, which contributes positively to a better thermal insulation property. The developed composites have a low thermal conductivity while still retaining sufficient strength. Therefore, the designed composite can be used monolithically as both load-bearing element and thermal insulator.     

Evaluation of performance and design of GFRP dowels in jointed plain concrete pavement – part 2: numerical simulation and design considerations

The article presents a numerical investigation using three-dimensional finite element analysis (FEA) to compare the performance of glass fibre reinforced polymer (GFRP) and steel dowel bars as load transfer devices across the transverse joints of jointed plain concrete pavements (JPCP). The FEA model used concrete damaged plasticity formulation to characterise the concrete pavement; elastic, transversely isotropic material characteristics were assumed for the GFRP dowels and classical metal plasticity formulation was used for the steel dowels. The numerical results were validated with the experimental results, and a good agreement was achieved. The results showed less stress concentration in the concrete underneath the GFRP dowels (38 mm diameter) compared with the steel dowels (25 mm diameter) of similar flexural rigidity. Finally, on the basis of a detailed parametric study, design considerations for the GFRP dowels in JPCP are suggested.     

Processing,microstructure, and properties of Mg–SiC composites synthesised using fluxless casting process

Abstract

In the present study, elemental magnesium and magnesium–silicon carbide composites were synthesised using the methodology of fluxless casting followed by hot extrusion. Microstructural characterisation studies revealed low porosity and a completely recrystallised matrix in every material. The average size of the recrystallised grains was found to decrease with an increasing presence of SiC particulates. For the reinforced magnesium, fairly uniform distribution of SiC particulates and good SiC–Mg interfacial integrity was realised. The results of X-ray diffraction studies indicated the absence of oxide phases and no evidence of interfacial reaction products except in the case of Mg–26.0 wt-%SiC sample. Results of physical and mechanical properties characterisation revealed that an increase in the amount of SiC particulates incorporated leads to an increase in macrohardness and elastic modulus, which does not affect the 0.2% yield strength and reduces the ultimate tensile strength, ductility, and coefficient of thermal expansion. The weight percentage of SiC particulates when plotted against hardness and 0.2% yield strength revealed a linear correlationship. An attempt is made to investigate the effect of increasing amount of SiC particulates on the microstructural features, and physical and mechanical properties of the magnesium matrix.     

Evaluation of performance and design of GFRP dowels in jointed plain concrete pavement – part 1: experimental investigation

Dowel bars are provided at the transverse joints of the jointed plain concrete pavement to allow for expansion and contraction of the pavement due to moisture and temperature changes. This paper presents experimental and analytical investigations for the deflection response of glass fibre-reinforced polymer (GFRP) dowels for different joint widths and concrete grades. The results were compared with those obtained from investigations into the conventional epoxy-coated steel dowel bars of similar rigidity. The experimental results showed that the 38 mm (1.5 in.) GFRP dowels perform better in terms of joint face deflection compared with 25 mm (1 in.) epoxy-coated steel dowel bars. In addition, these results showed that the deflection of the GFRP dowel was significantly affected by changing the concrete compressive strength and the joint widths.     

Effect of dilute HF solutions on chemical, optical, and mechanical properties of soda–lime–silica glass

Commercially and laboratory prepared soda–lime–silica (SLS) glasses compositions were investigated for corrosion behavior toward varying dilute HF solutions. The corrosion is found to be low in 0.05 M HF solution and relatively high with 0.5 M HF. Infrared reflectance studies were measured to monitor the surface changes after etching process. The corrosion data are explained on the basis of selectivity of HF toward the bridging and nonbridging oxygens. Optical transmittance, impact toughness, and density were measured after corrosion; the data are used to verify the suggested corrosion mechanism. Significant changes were reached on the remeasurement of previous collective studied properties when the commercial glass was irradiated.     

Compressing fresh concrete technique and the effect of excess water content on physical–mechanical properties of compressed concrete

This paper offers an innovative practical technique for applications in which high workability concrete is needed. In this technique, concrete is produced by compressing the fresh concrete through a fabricated pressure apparatus without incorporating additives and no need for external vibration and workability control. Applying this technique, excess water is completely expelled out from the fresh concrete and porosity is remarkably decreased. In this study, several mixes having different excess water contents with the same cement and aggregates were prepared to attain different workability levels. To evaluate the effect of excess water content on properties of hardened concrete, the physical and mechanical properties of both compressed and uncompressed concrete were determined, including compressive strength, modulus of elasticity, strain at peak stress, stress–strain curve, failure mode, water absorption, density and ultrasonic pulse velocity. The results obtained from this study showed that the excess water content added to the fresh concrete does not influence the physical–mechanical properties of the compressed concrete while those of the uncompressed concrete are significantly degraded. Moreover, compressing the fresh concrete dramatically improves the properties of the compressed concrete, as compared to the corresponding uncompressed concrete.     

Effect of rolling speed on microstructure and age-hardening behaviour of Al–Mg–Si alloy produced by twin roll casting process

In the present investigation the microstructure and age-hardening behaviour of Al–Mg–Si alloy prepared by twin roll casting (TRC), varying rolling speed (i.e., 3, 4, and 5 rpm), were studied. The as-cast samples were subjected to optical microscopy (OM) to understand the effect of rolling speed on the alloy microstructure. Significant difference in grain size and shape was observed for all the alloys in as-cast condition. The as-cast samples were solutionized at 540 °C for 2 h followed by isothermal heating at 180 °C for different time intervals. Thereafter, the as-cast and solutionized samples were subjected to scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). Segregation of solute atoms at grain boundaries were observed for both as-cast as well as solutionized samples. Age-hardening results show that time to attain peak hardness decreases for the alloy produced by higher rolling speed.     

Strength and mechanical behavior of soil–cement–lime–rice husk ash (soil–CLR) mixture

This study presents results of geotechnical investigations on treated silty sand soil with cement, lime and rice husk ash (CLR) and cement-lime (CL) admixture. Consolidated undrained triaxial test and unconfined compressive test were performed to estimate the potential of CLR and CL. The study investigates the influence of the amount of CLR%, main effective stress and curing days on soil strength, deformation, post peak behavior and brittleness. The percentages of the additives of CLR and CL varied from 2.5 to 12.5 % by dry weight of the soil with dry densities of 14.5 kN/m³ and the curing times of 3, 7, 28 and 60 days were examined. From the results, the stress–strain response is strongly influenced by the CLR contents and effective confining pressure. Strength and post peak strength of the CLR–soil are greatly improved by an increase in binder content. An increase of the effective cohesion c′ (kPa) and effective friction Φ′ (degree) is observed with increasing the CLR content, consistently. Brittle behavior observed at lower confining pressures and high CLR content. For both CLR and CL additives, linear trend was observed for variation of the q _u (kPa) with respect to the additives percentages. RHA was also found to be effective in increasing the shear strength of CLR–soil mixture.     

Effect of thermal ageing on microstructure,mechanical properties,and fracture behaviour of 50 % cold rolled duplex stainless steel (alloy 2507)

The paper deals with cold rolling and ageing on microstructure and mechanical properties of 2507 duplex stainless steel. Microstructure depicts acicular/Widmanstätten austenite and δ-ferrite with dissimilar volume fraction (∼0.55 for ferrite and ∼0.45 for austenite). Cold rolling and ageing at 950 °C, 1000 °C and 1050 °C result in equiaxed austenite for samples solution treated at 1040 °C and elongated at 1300 °C. By lowering ageing temperature from 1050 °C to 950 °C, structure becomes finer from ∼20 μm to <10 μm grain size. The sigma (σ) phase appears after ageing at 950 °C. Micro-hardness reveals maximum hardness for the hot rolled, solutionized (1040 °C) water quenched, and cold rolled (50 %) sample (380 HV_δF 100 and 430 HV_γ 100), whereas the tensile results reveal the hot rolled, solution treated (1300 °C, 1040 °C), cold rolled and aged at 950 °C samples show higher strength (yield strength=625 MPa, 567 MPa and ultimate tensile strength=892 MPa, 826 MPa) and lower ductility (23 %, 32 %) due to the σ-phase. The solution treated (1040 °C), cold rolled, aged at 1050 °C sample exhibits attractive strength and ductility combination (∼30 GPa %). Fractography supports the tensile results.     

Thermal–mechanical fatigue behaviour of P92 T-piece and Y-piece pipe

This paper presents a study on thermal–mechanical fatigue (TMF) behavior of P92 T-piece and Y-piece pipe at the most critical working fluctuations. Pressure and temperature in isothermal, in-phase (IP) and out-of-phase (OP) loading conditions were taken into account. Cyclic plasticity model considering the effect of temperature was used, in which both kinematic hardening variable and isotropic hardening variable are included. All the parameters used in the simulation were obtained from low cycle fatigue (LCF) tests at different temperatures. These parameters have been validated through the comparison of experimental data with the simulated data. Then, finite-element models (FEM) of P92 T-piece and Y-piece pipe were developed to investigate the location of the most critical region at typical thermal-mechanical loading. Simulated results reveal that the most dangerous position occurs at the region where the inner surface of horizontal pipe and branch pipe crossed for both T-piece and Y-piece pipe which is irrelevant to the types of loading. IP loading is the most serious working condition for both T-piece and Y-piece pipe. Comparing with T-piece pipe, Y-piece pipe at IP loading is the most dangerous condition.     

A review of out-of-autoclave prepregs – Material properties,process phenomena,and manufacturing considerations

Out-of-autoclave (OoA) prepreg materials and methods have gained acceptance over the past decade because of the ability to produce autoclave-quality components under vacuum-bag-only (VBO) cure. To achieve low porosity and tight dimensional tolerances, VBO prepregs rely on specific microstructural features and processing techniques. Furthermore, successful cure is contingent upon appropriate material property and process parameter selection. In this article, we review the existing literature on VBO prepreg processing to summarize and synthesize knowledge on these issues. First, the context, development, and defining properties of VBO prepregs are presented. The key processing phenomena and the influence on quality are subsequently described. Finally, cost and environmental performance are considered. Throughout, we highlight key considerations for VBO prepreg processing and identify areas where further study is required.     

Microstructure,mechanical property and corrosion behavior of co-continuous β-TCP/MgCa composite manufactured by suction casting

The co-continuous β-TCP/MgCa composite was fabricated by infiltrating MgCa alloy into porous β-TCP using suction casting technique. The microstructure, mechanical property and corrosion behaviors of the composite have been evaluated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), mechanical testing, electrochemical and immersion tests. It was shown that the composite structure was compact and the interfacial combination between MgCa alloy and β-TCP scaffold was very well. The composite had an ultimate compressive strength of (147 ± 13) MPa, which was near with the natural bone (2–180 MPa) and about 1000-fold higher than that of the original porous β-TCP scaffold, but it still retained over half of the strength of the MgCa bulk alloy. The electrochemical and immersion tests indicated that the corrosion resistance of the composite was better than that of the MgCa bulk alloy, and the corrosion rate of the MgCa matrix alloy was quicker than that of the porous scaffold for the composite. The corrosion products of the composite surface were mainly Mg(OH)₂, hydroxyapatite (HA) and Ca₃(PO₄)₂.     

Effect of forging process on microstructure,mechanical and corrosion properties of biodegradable Mg–1Ca alloy

The performance of Mg–1Ca alloy, a biodegradable metallic material, may be improved by hot working in order that it may be of use in bone implant applications. In this study, Mg–1Ca cast alloy was preheated to different temperatures before undergoing forging process with various forging speeds. Macro- and microstructure of the samples were examined by stereo and scanning electron microscopes (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), respectively. To determine the mechanical properties of the alloy, hardness value and plastic deformation ability of the samples were measured. To investigate the corrosion behaviour of the alloy, immersion and electrochemical tests were performed on the samples in simulated body fluid and the corrosion products were characterized by SEM/EDS. The results showed that increasing forging temperature decreased grain size led to improved hardness value and plastic deformation ability of the alloy, whereas no significant effect was observed by changing forging speed. Moreover, forging at higher temperatures led to an increase in the amount of Mg₂Ca phase at grain boundaries resulted in higher corrosion rates. It can be concluded that although forging process improved the mechanical properties of the alloy, it does not satisfy the corrosion resistance criteria required for bone healing.     

Manufacture and mechanical characterisation of high voltage insulation for superconducting busbars – (Part 1) Materials selection and development

It is planned that the high voltage electrical insulation on the ITER feeder busbars will consist of interleaved layers of epoxy resin pre-impregnated glass tapes (‘pre-preg’) and polyimide. In addition to its electrical insulation function, the busbar insulation must have adequate mechanical properties to sustain the loads imposed on it during ITER magnet operation. This paper reports an investigation into suitable materials to manufacture the high voltage insulation for the ITER superconducting busbars and pipework. An R&D programme was undertaken in order to identify suitable pre-preg and polyimide materials from a range of suppliers. Pre-preg materials were obtained from 3 suppliers and used with Kapton HN, to make mouldings using the desired insulation architecture. Two main processing routes for pre-pregs have been investigated, namely vacuum bag processing (out of autoclave processing) and processing using a material with a high coefficient of thermal expansion (silicone rubber), to apply the compaction pressure on the insulation. Insulation should have adequate mechanical properties to cope with the stresses induced by the operating environment and a low void content necessary in a high voltage application. The quality of the mouldings was assessed by mechanical testing at 77 K and by the measurement of the void content.