Microstructure and mechanical properties of accelerated sprayed concrete

Considering the different hydration processes of concrete without accelerator, sprayed concrete with low-alkali accelerator not only presents short setting times and high early-age mechanical properties but also yields different hydration products. This study presents an analysis of the mechanical properties of concrete with and without accelerator and sprayed concrete with three water–binder (w/b) ratios and four dosages of fly ash (FA) after different curing ages. It also examines the setting time, mineral composition, thermogravimetric–differential scanning calorimetry curves and microscopic images of cement pastes with different accelerator amounts. Furthermore, the setting time and microstructure of accelerated sprayed concrete with different w/b ratios and FA contents are examined. Results show that the retarded action of gypsum disappears in the accelerated cement–accelerator–water system. C₃A is quickly hydrated to form calcium aluminate hydrate (CAH) crystals, and a mesh structure is formed by ettringite, albite and CAH. A large amount of hydration heat improves the hydration rate of the cement clinker mineral and the resulting density, thereby improving mechanical properties at early curing ages. The setting times of the pastes increase with increasing w/b ratio and FA dosage. Thus, the hydration level, microstructure and morphology of the hydration products also change. Models of mechanical properties as functions of w/b, FA and curing age, as well as the relationship between compressive strength and splitting tensile strength, are established.     

Aggregate packing and -void saturation in mortar and concrete proportioning

Proportioning was studied by measuring aggregate packing (C) and filling of aggregate void space (1?C) with varying volumes of cement paste (\(V_{\rm p}\)) or matrix (\(V_{\rm matrix}\)), i.e., all material <0.125 mm. Eleven widely different normal density aggregates with C = 0.57 to 0.71, i.e., aggregate void content (1?C) = 0.29 to 0.43, were used at constant w/c = 0.38 and flowing consistency and varying dosage of water reducer and paste- and matrix volume. Analysing plots of four excess phase volumes (paste with/without air, matrix with/without air) versus aggregate void space showed constant aggregate void saturation ratios. Both paste- and matrix-aggregate void saturation ratio can be used with the best estimate (\(V_{\rm p}-V_{\rm air})/{(1-C)}=1.15\) per \(\hbox{m}^{3}\). Including air voids in paste- or matrix volume improved correlation so air void content must be included in the normalized paste aggregate void saturation ratio (\(k=V_{\rm p}\)/[(1?C) \(V_{\rm tot}\)]). Simple measurements of aggregate packing are thus very useful. Cost per unit material, per unit fresh (slump, flow diameter) or hardened (compressive strength) property were used to show the cost efficiency of the mixes. The ranking of cost/MPa strength and cost/mm consistency is similar to ranking of total concrete cost for the 11 aggregates with a certain scatter though.     

Influence of moisture condition on chloride diffusion in partially saturated ordinary Portland cement mortar

Experiments have been carried out to study the influence of moisture condition, including moisture content and its distribution, on the chloride diffusion in partially saturated ordinary Portland cement mortar. The mortar samples with water-to-cement (w/c) ratios of 0.4, 0.5 and 0.6, cured for 1 year, were preconditioned to uniform water saturations ranging from 18 to 100%. The interior relative humidities of these partially saturated cement mortars, i.e. water vapour desorption isotherm (WVDI), were measured. The WVDI results in relation to the pore structures obtained from the mercury intrusion porosimetry tests of paste samples with the same w/c ratios were analyzed, which provided a basic insight into the moisture distribution in the non-saturated cement mortars. The relative chloride diffusion coefficients of cement mortars at various water saturations were determined based on the Nernst-Einstein equation and conductivity technique. It is found that the relative chloride diffusion coefficient D_rc depends on the degree of water saturation S_w and WVDI. At a given S_w level, the D_rc is larger for a higher w/c ratio. The role of the w/c ratio in the D_rc–S_w relation, however, becomes less pronounced with increasing w/c ratio. There exists a critical saturation, below which the water-filled capillary pores are discontinuous and the D_rc-value tends towards infinitely small. An increase of the w/c ratio results in a decrease of the critical saturation level.     

Temperature-Dependent Thermal Conductivity of High Strength Lightweight Raw Perlite Aggregate Concrete

Twenty-four types of high strength lightweight concrete have been designed with raw perlite aggregate (PA) from the Erzincan Mollaköy region as new low-temperature insulation material. The effects of the water/cement ratio, the amount of raw PA, and the temperature on high strength lightweight raw perlite aggregate concrete (HSLWPAC) have been investigated. Three empirical equations were derived to correlate the thermal conductivity of HSLWPAC as a function of PA percentage and temperature depending on the water/cement ratio. Experimentally observed thermal conductivities of concrete samples were predicted 92 % of the time for each set of concrete matrices within 97 % accuracy and over the range from 1.457 W · m^?1 · K^?1 to 1.777 W · m^?1 · K^?1. The experimental investigation revealed that the usage of raw PA from the Erzincan Mollaköy region in concrete production reduces the concrete unit mass, increases the concrete strength, and furthermore, the thermal conductivity of the concrete has been improved. The proposed empirical correlations of thermal conductivity were considered to be applicable within the range of temperatures 203.15 K ≤ T ≤ 303.15 K in the form of λ = a(PAP ^b ) + c(T ^d ).     

The problem of non-compliant concrete and its influence on the reliability of reinforced concrete columns

The discussion about the fact that the concrete provided by concrete companies for the Brazilian construction sites is not meeting the compressive strength specified in the structural design is growing rapidly. These are the so-called non-compliant concretes. This paper discusses the influence of non-compliance of the compressive strength specified in the design (f_ck) on the structural safety level of reinforced concrete columns. The simulation of the column behavior was carried out by a numerical model based on the finite element method. The nonlinear finite element model used was presented and validated by comparison with experimental results obtained by different researchers. The Monte Carlo method was adopted for the determination of the reliability index (β) of reinforced concrete columns. The results show that many factors should be considered in the assessment of the actual damage caused by a non-compliant concrete to the column safety level.     

Multiscale structural characterization of methyltriethoxysilane-based silica aerogels

A series of methyltriethoxysilane-based silica aerogel monoliths were prepared by ambient pressure drying with various volume ratios of water to ethanol (R). The pore volumes and average pore sizes of silica aerogels were obtained by Barrett–Joyner–Halenda (BJH) method from nitrogen adsorption–desorption isotherms. The stress–strain curves of the cylindrical aerogel specimens were measured by performing uniaxial compressive tests. The particle size distributions and the average particle sizes of silica aerogels were also evaluated based on scanning electron microscopic observations. The experimental data revealed that the average particles size increased from 0.115 to 3.08 μm as R varied from 0.7 to 1.5, and that the silica aerogels exhibited two characteristic types of the compressive stress–strain responses. By proposing a multiscale structural model to describe microstructures of silica aerogels, a structural parameter, defined as the slenderness L/D of the cube column length L and the average particle diameter D, was related to the specific volume and the BJH volume of the silica aerogel monoliths, as well as the specific volume of silica. Accordingly, the two types of the compressive stress–strain responses may be distinguished by the critical value (L/D)_c.     

Preparation and characterization of monodisperse silica nanoparticles via miniemulsion sol–gel reaction of tetraethyl orthosilicate

Monodisperse silica nanoparticles were prepared via miniemulsion sol–gel reaction of tetraethyl orthosilicate (TEOS). Hexadecane (HD) or hexadecyltrimethoxysilane was used as costabilizer to effectively retard the Ostwald ripening process involved in TEOS miniemulsion. The Ostwald ripening behavior was characterized by dynamic light scattering (DLS), and it was adequately described by the modified Kabal’nov equation. The miniemulsion sol–gel reaction of TEOS/HD with a volume fraction (φ _c) of 0.024 at 80 °C is stable in the pH range 6–10. By contrast, gelation of reacting miniemulsions occurs at 70 and 100 min at pH 4 and 5, respectively. The weight-average silica particle size (d _w) of colloidal products prepared at 80 °C and pH 7 decreases from 59 to 36 nm with low polydispersity index (PDI, in the range 1.02–1.03), determined by transmission electron microscopy, when the φ _c of HD increases from 0.024 to 0.23. At constant φ _c (0.024), the resultant silica nanoparticles show larger d _w (83 nm) and PDI (1.35) for the TEOS/HD system at pH 10 as compared to the counterpart of pH 7. Furthermore, for the TEOS/HD system at pH 7 and low φ _c (0.024), d _w increases significantly with temperature being increased from 25 to 80 °C. By contrast, the effect of temperature on silica nanoparticle size becomes insignificant when a high level of HD (φ _c = 0.23) is used. Zeta potential measurements and field emission scanning electron microscopy were used to characterize the surface charge density and morphology of resultant silica nanoparticles.     

Copper-containing glass polyalkenoate cements based on SiO<Subscript>2</Subscript>–ZnO–CaO–SrO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses: glass characterization,physical and antibacterial properties

A series of copper (Cu)-containing glasses were synthesized and incorporated into a SiO₂–ZnO–CaO–SrO–P₂O₅-based glass system. Additions of 6 and 12 mol% CuO retained the amorphous character, and glasses were processed to possess similar particle sizes and surface areas. Glass characterization using X-ray photoelectron spectroscopy and magic angle spinning nuclear magnetic resonance determined that the addition of 12 mol% CuO increased the fraction of Q⁴-speciation and the concentration of bridging oxygens. Each glass presented solubility profiles for the release of Si⁴⁺ (18–31 mg/L), Ca²⁺ (13–16 mg/L), Zn²⁺ (<3 mg/L) and Sr²⁺(2–10 mg/L); however, no Cu²⁺ or P⁵⁺ were released. Cu-GPCs were formulated, and the working time (T _w) and setting times (T _s) were found to be dependent on both polyacrylic acid concentration and CuO addition. The mechanical properties, i.e. the compressive strength (18–30 MPa) and the adhesive bond strength (0.79–1.32 MPa), were relative low which is likely due to the glass structure. Antibacterial properties were evaluated in E. coli (4 mm), S. epidermidis (10 mm), S. aureus (UMAS-1) and vancomycin resistant S. aureus (2 mm) and presented antibacterial effects in each microbe tested.     

Reversible phase transformation in boron nitride under pulsed mechanical action

It is established that hexagonal boron nitride (h-BN) exhibits transformation to high-pressure diamondlike phases (w-, c-BN) under conditions of treatment in a planetary ball mill. After a 12-h processing, the yield of these phases (?20%) no longer grows with further increasing the process duration, which is evidence for a reversible character of the transformation. This conclusion is confirmed by the w-BN → h-BN transformation observed under the same process conditions. In addition to h-, w-, and c-BN, we have also found several new modifications of boron nitride.     

Predicting the tensile behaviour of adhesively bonded sheets using equivalent geometrical heterogeneities

In the present work, the formability of adhesively bonded sandwichsteel sheets is predicted during tensile tests, with the help of equivalent geometrical heterogeneities in the base sheets, without incorporating adhesive layer and properties during modelling simulations. This will help us mainly to overcome the difficulties while incorporating adhesion/adhesive properties during formability simulation and prediction. The limit strains during the tensile test are predicted and validated with experiments. The effect of hardener to resin (H/R) ratios of adhesive on the forming limit strains has been predicted with a set of thickness heterogeneity factor ‘f’ and size (w) of the defect, located in the base material. In other words, a relationship between defect morphology (f, w) and adhesive properties has been evaluated. For example, in the case of rectangular-infinite groove of width, w?=?0.5 mm, the limit strains of DDQ steel evaluated during tensile tests at f?=?0.912, f?=?0.9137, f?=?0.924, f?=?0.927, and f?=?0.95 are equivalent to the H/R ratios of 0.6:1, 0.7:1, 0.8:1, 0.9:1, and 1:1 of adhesive, respectively. The limit strains are predicted in the same manner with a square hole and finite square defect in the base material. It is suggested that the overall forming (tensile) behaviour of adhesively bonded blanks (ABB) can be predicted with the help of a thickness heterogeneity factor (or similar geometrical heterogeneities) that is equivalent to all the geometrical and structural heterogeneities in the whole bonded sheets.     

Low-temperature acoustic properties of single-crystal hafnium

Five independent adiabatic elastic tensor components c _ij of single-crystal Hf (0.3% Zr) have been precisely measured in the range 78–300 (c ₁₁ and c ₃₃) or 78–160 K (c ₁₂, c ₁₃, and c ₄₄) at 50 MHz using an ultrasonic pulse technique. The corresponding linear absorption coefficients α _ij (T) have been measured in the same temperature ranges. The c _ij data have been used to determine adiabatic compressibilities β_∥ and β_⊥, and Young’s moduli E _∥ and E _⊥ along and across the c axis, respectively. Near 140 K, we have revealed an anomaly in c ₃₃(T) due to changes in the phonon spectrum of single-crystal α-Hf. The observed anomalies in temperature-dependent α₁₁ and α₃₃ are tentatively attributed to grown-in dislocations.     

Seismic performance and prediction equations of sandwich beam-column joints subjected to skew cyclic loads

To study the seismic performance of sandwich beam-column joints constructed with high strength concrete column and normal strength concrete floor system and joint regions, four specimens with different column to beam concrete strength ratios (α) were tested under skew cyclic loads. The performance indices of the specimens including the failure mode, ductility, energy dissipation were compared and analyzed. The results show that the failure modes of the sandwich joints are in form of joint shear failure after yielding of the beam, while the ductility coefficient is found to be greater than 3.0. Compared to the joints with low concrete strength ratios α, the specimen with a high concrete strength ratio α features larger deformation at the joint. Based on the softened strut-and-tie model, a set of shear strength prediction equations for the sandwich beam-column joint, taking into account the effects of the concrete strength ratio α, floor slabs and plastic region of beams, is proposed. Comparison of the present tests against the published literature confirms that the shear strength of the sandwich joints can be well predicted by the proposed model.     

Influence of coronene addition on some superconducting properties of bulk MgB<Subscript>2</Subscript>

This study reports the effect of coronene (C₂₄H₁₂) addition on some superconducting properties such as critical temperature (T_c), critical current density (J_c), flux pinning force density (F_p), irreversibility field (H_irr), upper critical magnetic field (H_c2), and activation energy (U₀), of bulk MgB₂ superconductor by means of magnetisation and magnetoresistivity measurements. Disk-shaped polycrystalline MgB₂ samples with varying C₂₄H₁₂ contents of 0, 2, 4, 6, 8, 10 wt%, were produced at 850 °C in Ar atmosphere. The obtained results show an increase in field-J_c values at 10 and 20 K resulting from the strengthened flux pinning, and a decrease in critical temperature (T_c) because of C substitution into MgB₂ lattice, with increasing amount of C₂₄H₁₂ powder. The H_c2(0) and H_irr(0) values are respectively found as 144, 181, 172 kOe, and 128, 161, 145 kOe for pure, 4 wt% and 10 wt% C₂₄H₁₂ added samples. The U₀ depending on the magnetic field curves were plotted using thermally activated flux flow model. The maximum U₀ values are respectively obtained as 0.20, 0.23 and 0.12 eV at 30 kOe for pure, 4 wt% and 10 wt% C₂₄H₁₂ added samples. As a result, the superconducting properties of bulk MgB₂ at high fields was improved using C₂₄H₁₂, active carbon source addition, because of the presence of uniformly dispersed C particles with nanometer order of magnitude, and acting as effective pinning centres in MgB₂ structure.     

Moment redistribution in two-span prestressed NSC and HSC beams

This paper presents a numerical investigation into two-span continuous prestressed normal-strength concrete (NSC) and high-strength concrete (HSC) beams, focusing on aspects of behavior related to moment redistribution. A comparative study is performed by using an experimentally validated computer model. The concrete cylinder compressive strength of investigated beams covers from 40 to 90 MPa, while the prestressing reinforcement ratio (ρ _p) ranges from 0.15 to 1.29%. The results show that the tendon tensile strength can be better exploited by HSC than by NSC at moderate to high ρ _p levels. At a given level of ρ _p, HSC generally mobilizes smaller neutral axis depth, higher strain in nonprestressed steel and a bit larger moment redistribution at ultimate than NSC. Typical code recommendations (ACI, CSA and EC2) for permissible moment redistribution are examined. The effect of concrete strength on moment redistribution in prestressed beams is well reflected in ACI but inadequately reflected in CSA and EC2. A simplified equation for calculating moment redistribution in continuous prestressed beams is proposed.     

Synthesis,structure, and properties of the non-centrosymmetric borate Rb<Subscript>2</Subscript>CaB<Subscript>8</Subscript>O<Subscript>26</Subscript>H<Subscript>24</Subscript>

Single crystals of Rb₂CaB₈O₂₆H₂₄, a new non-centrosymmetric borate material, have been grown with sizes up to 8 × 5 × 3 mm³ by the slow evaporation of water solution at room temperature. The structure of the compound was determined by single-crystal X-ray diffraction. It crystallizes in the orthorhombic system, space group P2₁2₁2₁ with a = 11.5288(3) Å, b = 12.6334(4) Å, c = 16.6966(4) Å, Z = 4 and R ₁ = 0.0405, wR ₂ = 0.1043. Ultraviolet (UV)–vis spectrum transmission is performed on the Rb₂CaB₈O₂₆H₂₄, which shows an absorption edge about 195 nm in the UV region. Thermal properties were investigated by TG–DSC analysis. The powder second-harmonic generation (SHG) intensity measured by the Kurtz-Perry method indicates that Rb₂CaB₈O₂₆H₂₄ has about one-third of KDP (KH₂PO₄).The influence of different molar ratios and evaporation speed of water solution on crystal quality and size was also performed on the reported material.     

Comparative Experimental and Density Functional Theory (<Emphasis Type="Italic">DFT</Emphasis>) Study of the Physical Properties of MgB<Subscript>2</Subscript> and AlB<Subscript>2</Subscript>

In the present study, we report an intercomparison of various physical and electronic properties of MgB₂ and AlB₂. In particular, the results of phase formation, resistivity ρ(T), thermoelectric power S(T), magnetization M(T), heat capacity (C _P ), and electronic band structure are reported. The original stretched hexagonal lattice with a=3.083 Å, and c=3.524 Å of MgB₂ shrinks in c-direction for AlB₂ with a=3.006 Å, and c=3.254 Å. The resistivity ρ(T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB₂. Superconductivity is not observed for AlB₂. Interestingly, the sign of S(T) is +ve for MgB₂ the same is ?ve for AlB₂. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB₂ to Debye–Einstein model and estimated the value of Debye temperature (Θ _D) and Sommerfeld constant (γ) for electronic specific heat. Further, from γ, the electronic density of states (DOS) at Fermi level N(E _F) is calculated. From the ratio of experimental N(E _F) and the one being calculated from DFT, we obtained value of λ to be 1.84, thus placing MgB₂ in the strong coupling BCS category. The electronic specific heat of MgB₂ is also fitted below T _c using α-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB₂ is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB₂, and hence it is not superconducting. DFT calculations demonstrated that for MgB₂, the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB₂ is absent. This leads to a weak electron phonon coupling and also to hole deficiency as π bands are known to be of electron type, and hence obviously the AlB₂ is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB₂ and AlB₂.     

Study of effect of soft inclusion on the mechanical behavior of soilcrete mixtures: experimental and modeling approaches

The influence of the volume fraction and size of soft soil inclusions on the mechanical behavior of soilcretes was experimentally and numerically investigated. Spherical kaolinite chunks with a specific water content and volume were added to the soilcrete samples during the filling phase of the molds. Different mechanical properties such as the effective static elastic modulus (E) and the unconfined compressive strength (UCS) of soilcrete specimens with different volume fraction of inclusions were determined. E and UCS of soilcrete specimens with different volume fraction of inclusions were also determined using 2D (unit cell) and 3D (digital specimens) finite element models. UCS and E decrease considerably as the volume fraction of soft inclusions increases. Experimental results show that these UCS and E drops are, respectively, about 47% and 20% for a volume fraction of inclusions f_v = 9%. A good agreement between the experimental and simulated data was observed, especially for elastic modulus.     

The Use of Acoustic Emission Intensity Analysis for the Assessment of Cover Crack Growth in Corroded Concrete Structures

In this paper, acoustic emission (AE) intensity analysis was utilized to assess the concrete cover cracking due to steel corrosion in reinforced concrete structures. A total of 30 reinforced concrete prism samples were tested under an accelerated corrosion test coupled with continuous AE monitoring using attached AE sensors. The samples were cast with three concrete cover thicknesses (20, 30 and 40 mm) around steel bars and were exposed to five percentages of steel mass loss: 1, 2, 3, 4, and 5 %. The cover cracking was monitored daily by visual inspection to detect and measure crack widths. Different AE signal parameters were continuously recorded during the tests, including number of hits, signal strength, energy, and amplitude. The acquired AE events were subjected to an intensity analysis of signal strength to estimate historic index (H (t)) and severity (\(S_r)\). In addition, a b value analysis was conducted on all AE data and the results were compared to those obtained from the intensity analysis. The results showed that increasing the cover thickness had no significant impact on AE parameters (number of hits, cumulative signal strength, cumulative energy, amplitude, H (t), and \(S_{r})\) at similar values of crack growth. Nonetheless, varying the cover thickness from 20 to 40 mm resulted in lower crack widths and slightly higher b values at the same levels of steel mass loss. It was also found that both H (t) and \(S_r\) showed a more evident correlation with the values of crack growth than did b values, regardless of cover thickness or percentage of steel mass loss. Finally, an intensity classification chart was developed to quantify the cover crack growth based on the values of H (t) and \(S_{r}\).     

Preparation and structural study of Mg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> ceramics and their dielectric properties from 1 Hz to 7.7 GHz

A series of Mg_1?x Zn _x TiO₃, x = 0–0.5 (MZT0–MZT0.5) ceramics was synthesised and characterised. The dielectric properties of the samples in the frequency range of 1 Hz–7.7 GHz were explored using three different methods: a contacting electrode method, a parallel-plate method and a perturbed resonator method. The electrical properties in the space charge and dipolar polarisation frequency ranges are discussed in relation to the phase composition and microstructure data. Differences in the zinc substitution divided the dielectrics into two groups, namely MZT0–MZT0.2 and MZT0.3–MZT0.5, each with different amount of a main Mg_1?x Zn _x TiO₃ solid solution phase and a secondary solid solution phase. Zinc substitution promoted the density of the ceramics, improved the purity of the main phase and increased the permittivity for frequencies up to 10⁸ Hz, but reduced the permittivity in the microwave range. In the MZT0.3–MZT0.5 samples, for frequencies less than 1 MHz the quality (Q × f) factors were lower and log σ _a.c, the AC conductivity, was higher than for the MZT0–MZT0.2 samples. Above 10 MHz, the (Q × f) factors and log σ _a.c of the two groups were similar.