The effect of meta-halloysite on alkali–aggregate reaction in concrete

Damage to concrete structures may occur as a result of internal effects. Alkali silica reaction (ASR) is a long term reaction between alkalis and reactive aggregate present in the concrete. The reaction product is sodium–potasium–calcium silica gel, able to absorb water, resulting in the expansion and cracking of concrete. The key problem is to find the right method for mitigating the internal damage. This paper presents the results of an investigation into the effectiveness of calcined halloysite (meta-halloysite) in improving the resistance to alkali-silica reaction (ASR). The pozzolanic reactivity of meta-halloysite was also evaluated using Thermo-Gravimetric Analysis. Microstructures of mortar bars were observed by Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (EDS) to investigate the location and chemical composition of ASR gel. The results from this study showed satisfactory level of pozzolanic reactivity when cement was partially replaced by meta-halloysite. It was demonstrated that a 20% addition of meta-halloysite are able to mitigate ASR and lower expansion of mortar bars with reactive aggregate to a safe level of not more than 0.1% at 14 days. Microstructural observations of the specimens containing meta-halloysite indicated the presence of a calcium–alkali–silicate–hydrate gel. But fewer reaction products and with different composition than those forming in the pastes without mineral additives are present.     

Rate dependence of the tensile and flexural strengths of glass–ceramic Macor

Understanding of the tensile and flexural strengths of the glass–ceramic Macor bears important applications in materials science, aerospace, defense, and other engineering disciplines. In this article, we systematically investigate the rate dependence of the tensile strength and the flexural strength of Macor utilizing two methods: the Brazilian disk (BD) test and semi-circular bend (SCB) test. Both static tests and dynamic tests are conducted to explore the rate dependence of tensile and flexural strengths of Macor. The static measurement is conducted with a servo-controlled material testing machine, and the dynamic experiment is carried out with a 6.35-mm diameter split Hopkinson pressure bar (SHPB) system. The pulse-shaping technique is used to achieve dynamic force balance, and thus eliminates the loading inertial effect and enables quasi-static stress analysis. The experimental results show that both the tensile strength and the flexural strength of Macor are loading rate dependent. The flexural strength is observed to be consistently higher than the tensile strength.     

Reinvestigation of the tensile strength and fracture property of Ni(1 1 1)/α-Al2O3(0 0 0 1) interfaces by first-principle calculations

First-principle calculations are performed to reinvestigate the mechanical tensile property and failure characteristic of Ni/Al₂O₃ interfaces, in order to clear the inconsistence existed in the literatures. Four types of interface models without initial lateral stresses are used, i.e., Al-terminated O-site, O-terminated Al-site, Al-terminated Al-site and Al-terminated H-site models. Two kinds of tensile methods, viz., uniaxial extension and uniaxial tension, are adopted to check the mechanical responses of these interface models. It is found that the results under uniaxial extension are generally consistent with those under uniaxial tension, including the overall shapes of stress–strain curves and the values of tensile strengths. Moreover, the initial lateral stresses have an apparent influence on the mechanical properties of the interfaces during the loading process, such as tensile strength, fracture strain and the work of separation. Our simulation results also clarified that, under tensile loading, the most stable O-terminated Al-site interface model tends to fracture in a brittle way along the sublayer between in-plane Ni–Ni atomic bonds, while all of the Al-terminated interface models will fail in a ductile fracture manner with relatively lower stress levels, breaking along the interlayer between the Ni(1) and Al(1) layers.     

The effect of Zr on the microstructure and tensile properties of hot-extruded Al–Mg2Si composite

This work was carried out to investigate the effect of different amounts of Zr on the microstructure and tensile properties of homogenized and hot extruded Al-15% Mg₂Si composite using optical microscopy and scanning electron microscopy (SEM). The results showed that Zr addition has no significant effect on the morphology of both primary and eutectic Mg₂Si phase in as-cast condition. But, applying homogenizing and extrusion processes changed the morphology of Mg₂Si phases from irregular to a more spherical shape. Further results demonstrated that the average size of primary Mg₂Si decreases with the addition of Zr up to 0.1% from 56 μm to 24 μm in hot-extruded condition. As the mount of Zr increased up to 0.1 wt.%, ultimate tensile strength (UTS) and elongation values were also increased from 160 MPa and 3.2% to 292 MPa and 9.5%, respectively. Fracture surface examinations revealed a transition from brittle fracture mode in as-cast composite to ductile fracture in hot-extruded Zr-modified specimens. This can be attributed to the changes in size and morphology of Mg₂Si intermetallic and porosity content.     

The effect of annealing on the transformation and the microstructure of Mn1 − xCrxCoGe alloys

In this work the effect of different thermal treatments on the transformation behavior of Mn_1 − xCr_xCoGe alloys, with x = 0.15 and 0.20 has been analyzed. The changes in the transformation temperatures have been studied by differential scanning calorimetry (DSC). The results show that the structural transition temperature depends on the previous annealing. However, under the same heat treatment no significant change is observed on the transformation temperatures when replacing Mn by Cr. The microstructural evolution has been monitored using in-situ X-ray diffraction and transmission electron microscopy. The effect of an applied magnetic field on the transformation has been explored by SQUID magnetometry. Two different magnetic transitions are found: a first order paramagnetic (PM) to ferromagnetic (FM) corresponding to the transformation observed by calorimetry and a re-entrant spin glass to ferromagnetic transition.     

Effects of extrusion temperature on the microstructure and tensile properties of Al–16 wt% Al4Sr metal matrix composite

This study was undertaken to investigate the effect of extrusion temperature on the microstructure and tensile properties of Al metal matrix composite (MMC) containing 16 wt% Al₄Sr intermetallic. Microstructural examinations were assessed by the use of optical microscope, scanning electron microscope (SEM) and X-ray diffractometry (XRD). The results showed that hot extrusion with the ratio of 18:1 at 420 °C reduces the maximum length of Al₄Sr particles from 222 μm to 35 μm. It was found that by applying extrusion parameters in optimum conditions, uniform distribution of fine Al₄Sr intermetallic in Al matrix is obtained. Microstructural evolution also intensified the ultimate tensile strength (UTS) values of the MMC from 54 MPa to 145 MPa. Remarkable result of this study revealed that hot extrusion improves the ductility of the MMC significantly. Fractographic examinations of the composite in as-cast condition showed a complete cleavage fracture surface that changes to more homogenous dimples after hot extrusion process.     

The influence of freeze–thaw cycles on the unconfined compressive strength of fiber-reinforced clay

Freeze–thaw cycling is a weathering process that frequently occurs in cold climates. In the freeze state, thermodynamic conditions at temperatures just below 0 °C result in the translocation of water and ice. Consequently, the engineering properties of soils such as permeability, water content, stress–strain behavior, failure strength, elastic modulus, cohesion, and friction angle may be changed. Former studies have been focused on changes in physical and mechanical properties of soil due to freeze–thaw cycles. In this paper, the effect of freeze–thaw cycles on the compressive strength of fiber-reinforced clay is investigated. For this purpose, kaolinite clay reinforced by steel and polypropylene fibers is compacted in a laboratory and exposed to a maximum of 10 closed-system freezing and thawing cycles. The unconfined compressive strength of reinforced and unreinforced specimens is then determined. The results of the study show that for the soil investigated, the increase in the number of freeze–thaw cycles results in the decrease of unconfined compressive strength of clay samples by 20–25%. Moreover, inclusion of fiber in clay samples increases the unconfined compressive strength of soil and decreases the frost heave. Furthermore, the results of the study indicate that fiber addition does not decrease the soil strength against freeze–thaw cycles. Moreover, the study shows that the addition of 3% polypropylene fibers results in the increase of unconfined compressive strength of the soil before and after applying freeze–thaw cycles by 60% to 160% and decrease of frost heave by 70%.     

The role of Sr<Superscript>2+</Superscript> on the structure and reactivity of SrO–CaO–ZnO–SiO<Subscript>2</Subscript> ionomer glasses

The suitability of Glass Polyalkenoate Cements (GPCs) for use in orthopaedics is retarded by the presence in the glass phase of aluminium, a neurotoxin. Unfortunately, the aluminium ion plays an integral role in the setting process of GPCs and its absence is likely to hinder cement formation. However, the authors have previously shown that aluminium free GPCs may be formulated based on calcium zinc silicate glasses and these novel materials exhibit significant potential as hard tissue biomaterials. To further improve their potential, and given that Strontium (Sr) based drugs have had success in the treatment of osteoporosis, the authors have substituted Calcium (Ca) with Sr in the glass phase of a series of aluminium free GPCs. However to date little data exists on the effect SrO has on the structure and reactivity of SrO–CaO–ZnO–SiO₂ glasses. The objective of this work was to characterise the effect of the Ca/Sr substitution on the structure of such glasses, and evaluate the subsequent reactivity of these glasses with an aqueous solution of Polyacrylic acid (PAA). To this end ²⁹Si MAS-NMR, differential scanning calorimetry (DSC), X-ray diffraction, and network connectivity calculations, were used to characterize the structure of four strontium calcium zinc silicate glasses. Following glass characterization, GPCs were produced from each glass using a 40 wt% solution of PAA (powder:liquid = 2:1.5). The working times and setting times of the GPCs were recorded as per International standard ISO9917. The results acquired as part of this research indicate that the substitution of Ca for Sr in the glasses examined did not appear to significantly affect the structure of the glasses investigated. However it was noted that increasing the amount of Ca substituted for Sr did result in a concomitant increase in setting times, a feature that may be attributable to the higher basicity of SrO over CaO.     

The effect of shale in quartzite aggregate on the creep and shrinkage of concrete—A comparison with RILEM model B3

This paper presents the results of an investigation into the effects of increasing shale content in nominally “quartzite” aggregates on the creep and shrinkage behaviour of concretes made with such aggregate. The investigation was prompted by concern because of the increasing presence of the weaker and poorly shaped shale particles appearing in the quartzite aggregates being used in the Gauteng region of South Africa. In this investigation, two strength grades of concrete were prepared using varying proportions of shale in the stone and sand fractions of the quartzite aggregates. A secondary outcome of this investigation was the opportunity to assess the effect of aggregate type on the prediction of concrete creep and shrinkage using Model B3. The results indicate that increasing shale content in concrete has a significant effect in increasing both creep and shrinkage strains. Shrinkage increases almost linearly with increasing shale content of the total aggregate. On the other hand, shale in the sand fraction appears to have a larger effect in increasing creep than shale in the stone fraction. An assessment of the need for an aggregate-based adjustment to Model B3 is also presented.

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Résumé Cet article présente les résultats d’une recherche relative aux effects que peut avoir l’augmentation de la teneur en schiste dans des granulats nominalement “quartzites” sur le comportement des bétons faits avec de tels granulats. La présente recherche est née du souci de comprendre la teneur croissante des particules de schiste plus faibles et mal formées apparaissant dans les granulats quartzites utilisés dans la région de Gauteng en Afrique du sud. Au cours de cette recherche, deux gammes de résistance de béton ont été préparées en utilisant des proportions variables de schiste dans les fractions de pierre et de sable des granulats quartzites. La recherche a aussi été l’occasion d’évaluer les effets du type granulat sur la prévision du fluage et du retrait du béton en utilisant le modèle B3. Les résultats indiquent qu’une augmentation de la teneur de schiste dans du béton a un effet significatif sur l’augmentation des contraintes du fluage et du retrait. Le retrait augmente presque linéairement avec l’augmentation de la teneur en schiste du granulat total. D’autre part, le schiste dans la fraction de sable semble avoir un plus grand effet sur l’augmentation du fluage que le schiste dans la fraction en pierre. Une évaluation du besoin d’un réglage basé sur le granulat par rapport au modèle B3 est également présentée.

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Editorial Note Dr. Yunus Ballim is a RILEM Senior Member.     

Atomic diffusion in the Fe [0 0 1] ∑ = 5 (3 1 0) and (2 1 0) symmetric tilt grain boundary

Both the formation and diffusion activation energies of single vacancy migrating intra-layer and inter-layer near the Fe [0 0 1] Σ = 5 (3 1 0) and (2 1 0) symmetric tilt grain boundaries have been calculated by using the MAEAM and a MD method. From energy minimization, the vacancy concentration in the second layer is higher than the one in the other layers for both (3 1 0) and (2 1 0) STGBs. By the diffusion activation energies of the vacancies migrating intra-layer and inter-layer, the vacancies located from the first to the eighth layers of (3 1 0) STGB as well as the ones located from the first to the tenth layers of (2 1 0) STGB are favorably migrated to the second layer. Thus there is a vacancy aggregation tendency to the second layer near the grain boundary. For the vacancy migrating intra-layer and inter-layer, the influences of the grain boundary are respectively as far as to the fifth and eighth layers for (3 1 0) STGB as well as to the sixth and tenth layers for (2 1 0) STGB.     

The effect of bone ingrowth depth on the tensile and shear strength of the implant–bone e-beam produced interface

New technologies, such as selective electron beam melting, allow to create complex interface structures to enhance bone ingrowth in cementless implants. The efficacy of such structures can be tested in animal experiments. Although animal studies provide insight into the biological response of new structures, it remains unclear how ingrowth depth is related to interface strength. Theoretically, there could be a threshold of ingrowth, above which the interface strength does not further increase. To test the relationship between depth and strength we performed a finite element study on micro models with simulated uncoated and hydroxyapatite (HA) coated surfaces. We examined whether complete ingrowth is necessary to obtain a maximal interface strength. An increase in bone ingrowth depth did not always enhance the bone–implant interface strength. For the uncoated specimens a plateau was reached at 1,500 μm of ingrowth depth. For the specimens with a simulated HA coating, a bone ingrowth depth of 500 μm already yielded a substantial interface strength, and deeper ingrowth did not enhance the interface strength considerably. These findings may assist in optimizing interface morphology (its depth) and in judging the effect of bone ingrowth depth on interface strength.     

The effect of solution temperature on the microstructure and tensile properties of Al–15%Mg2Si composite

The effect of different solution temperatures has been investigated on the microstructure and tensile properties of in situ Al–Mg₂Si composite specimens were subjected to solutionizing at different temperatures of 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C and 580 °C for holding time of 4 h followed by quenching. The microstructural studies of the polished and etched samples by scanning electron microscopy (SEM) in the solution condition indicated that the increase in the temperature changes the morphology of both the primary and secondary Mg₂Si phases. Solutionizing led to the dissolution of the Mg₂Si particles and changed their morphology. Tensile test results indicated that ultimate tensile strength (UTS) gradually decreased upon solutionizing from 300 to 550 °C while further increase in the temperature followed by a sharp decrease in UTS up to 580 °C solutionizing temperature. It was found that the elongation has become three times greater in comparison to the as-cast state. Elongation results showed an increase up to 500 °C and then reduced temperatures of 550 and 580 °C. Fractographic analysis revealed a cellular nature for the fracture surface. On the cellular fracture surface, the features of both brittle and ductile fracture were present simultaneously. As a result of solution treatment the potential sites for stress concentration and crack initiation areas were reduced due to softening of the sharp corners and break up of eutectic network respectively, while increase in the number of fine dimples rendered the nature of fracture to ductile and also increased elongation.     

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.     

Simultaneously enhanced tensile and compressive response of AZ31–NanoAl<Subscript>2</Subscript>O<Subscript>3</Subscript>–AA5052 macrocomposite

New bimetal AZ31–Al₂O₃/AA5052 macrocomposite comprising (a) Al₂O₃ nanoparticle-reinforced magnesium alloy AZ31 shell and (b) aluminum alloy AA5052 millimeter-scale core reinforcement was fabricated using solidification processing followed by hot coextrusion. Microstructural characterization revealed more rounded intermetallic particle of decreased size, reasonable Al₂O₃ nanoparticle distribution, and non-dominant (0 0 0 2) texture in the longitudinal and transverse directions in the AZ31–Al₂O₃ nanocomposite shell. Interdiffusion of Mg and Al across the core–shell macrointerface into each other was also significant. Compared to monolithic AZ31, the AZ31–Al₂O₃ shell exhibited significantly higher hardness (+33%). In tension, the presence of Al₂O₃ nanoparticles (in the AZ31 shell) and AA5052 core significantly increased stiffness (+39%), yield strength (0.2% TYS) (+9%), ultimate strength (UTS) (+19%), average failure strain (+7%), and work of fracture (WOF) (+27%) of AZ31. In compression, the presence of Al₂O₃ nanoparticles (in the AZ31 shell) and AA5052 core significantly increased yield strength (0.2% CYS) (+58%), ultimate strength (UCS) (+4%), average failure strain (+11%), and WOF (+49%) of AZ31. The effect of joint presence of (a) Al₂O₃ nanoparticles (in the AZ31 shell) and (b) AA5052 millimeter-scale core on tensile and compressive properties of AZ31 is investigated in this article.     

Effect of freeze–thaw cycles on the stress–strain curves of unconfined and confined concrete

An experimental research was performed on the complete compressive stress–strain relationship for unconfined and confined concrete after exposure to freeze–thaw cycles. For the unconfined concrete, tests were carried out on three series of prisms specimens (100 mm × 100 mm × 300 mm) with water/cement ratio of 0.60, 0.54 and 0.48 respectively. While for confined concrete, two series of tied columns (150 mm × 150 mm × 450 mm prisms) with confinement index of 0.317 and 0.145 were prepared. Analytical models for the stress–strain relationship of frozen-thawed unconfined and confined concrete were empirically developed respectively. Through the regression analysis, formulations for the main parameters were established, including the compressive strength, peak strain and elastic modulus. Compared with the available experimental data, the proposed models were shown to be applicable to concrete after different numbers of freeze–thaw cycles.     

Effect of alkaline and acidic solutions on the tensile properties of glass–polyester pipes

The influence of liquids on the state of stresses and tensile strengths in the longitudinal and circumferential direction of glass–polyester pipes is the subject of this paper. The pipes were manufactured in Corporation “Poliester” Priboj, and had a definite structure and known fabrication process. These analyses are of great importance for the use of glass–polyester pipes in the chemical industry.     

Effects of Zr and B on the structure and tensile properties of Al–20%Mg alloy

In current research, the effects of different Zr and B contents on the structure and tensile properties of Al–20%Mg alloy have been investigated by using Al–15Zr and Al–8B master alloys. Optical and scanning electron microscopy (SEM) were utilized to study the microstructures and fracture surfaces. Microstructural analysis of the cast alloy showed dendrites of primary α-phase within the eutectic matrix which consists of β-Al₃Mg₂ intermetallic and α-solid solution. After tensile testing, the optimum amounts for both Zr and B were found to be 0.5 wt.%. Ultimate tensile strength (UTS) value of the unrefined alloy increased from 168 MPa to 243 MPa and 236 MPa by adding 0.5% Zr and 0.5%B, respectively. The main mechanism for UTS enhancement was found to be due to the refinement of grains and also altering large dendrites of Al(α)-phase to finer structure. The study of fracture faces revealed that B/Zr addition changes the mode of fracture from brittle to rather ductile.     

The effect of B-site cations on the properties of KTaxNb1−xO3 [1 0 0] surface: A study of density functional theory

Ferroelectric material and piezoelectric properties are different in potassium tantalite niobate (KTa_xNb_1−xO₃) crystal according to differences in x. Here, we show the results of KTa_xNb_1−xO₃ (x = 0.25, 0.5, 0.75) crystal [1 0 0] surface properties calculated by density functional theory (DFT). Using local density approximation (LDA) and generalized gradient approximation (GGA), DFT has been employed to determine the structural, electronic, and optical properties of chemically ordered ferroelectric KTa_xNb_1−xO₃ crystal [1 0 0] surfaces. Based on the research, comparison and analysis of the results show that all kinds of properties of KTa_xNb_1−xO₃ [1 0 0] surfaces are vary with respect to x. It is found, for instance, that the ordering of B-site cations obviously influences the properties of the KTa_xNb_1−xO₃ surface. At the same time, this allows the experimental studies to be supervised effectively.