Occupational stress affects the health and wellbeing of people who work, and the construction industry is recognized as a high-stress working environment. The relationship between job demands, job ...control, workplace support, and experiences of stress in the South African construction context is investigated, using hierarchical regression, factor analysis and structural equation modeling to explore the strength of thirteen factor relationships with perceived stress. Data were gathered from an on-line questionnaire survey response sample of 676 architects, civil engineers, quantity surveyors, and project and construction managers. Predictors displaying a significant relationship with occupational stress are the presence of work–life imbalance, the need to ‘prove’ oneself, hours worked per week, working to tight deadlines, and support from line managers in difficult situations at work. Existing theories of occupational stress are confirmed but not completely supported. The construction industry should give attention to how the need to work long hours is justified. Organizations should look to improving managerial and collegial support for construction professionals, but be careful in engaging in socializing and project team-building activities. Further research will need to focus more deeply on construction-specific job demand factors; explore why women professionals appear to experience more stress than men; and aim to develop reliable early-warning detection techniques for construction professionals.
•Investigates occupation stress and job demand, control and support factors.•Uses hierarchical regression, factor analysis and structural equation modelling.•Identifies job demand factors as the most critical job stressors.•Calls for research into gender differences and management intervention techniques.
Efforts to tune the bulk physical properties of concrete are hindered by a lack of knowledge related to the atomic-level structure and growth of calcium silicate hydrate phases, which form about ...50–60% by volume of cement paste. Here we describe the first synthesis of compositionally uniform calcium silicate hydrate phases with Ca:Si ratios tunable between 1.0 and 2.0. The calcium silicate hydrate synthesized here does not contain a secondary Ca(OH)2 phase, even in samples with Ca:Si ratios above 1.6, which is unprecedented for synthetic calcium silicate hydrate systems. We then solve the atomic-level three-dimensional structure of these materials using dynamic nuclear polarization enhanced 1H and 29Si nuclear magnetic resonance experiments in combination with atomistic simulations and density functional theory chemical shift calculations. We discover that bridging interlayer calcium ions are the defining structural characteristic of single-phase cementitious calcium silicate hydrate, inducing the strong hydrogen bonding that is responsible for stabilizing the structure at high Ca:Si ratios.
Despite use of blended cements containing significant amounts of aluminum for over 30 years, the structural nature of aluminum in the main hydration product, calcium aluminate silicate hydrate ...(C-A-S-H), remains elusive. Using first-principles calculations, we predict that aluminum is incorporated into the bridging sites of the linear silicate chains and that at high Ca:Si and H2O ratios, the stable coordination number of aluminum is six. Specifically, we predict that silicate-bridging AlO2(OH)45– complexes are favored, stabilized by hydroxyl ligands and charge balancing calcium ions in the interlayer space. This structure is then confirmed experimentally by one- and two-dimensional dynamic nuclear polarization enhanced 27Al and 29Si solid-state NMR experiments. We notably assign a narrow 27Al NMR signal at 5 ppm to the silicate-bridging AlO2(OH)45– sites and show that this signal correlates to 29Si NMR signals from silicates in C-A-S-H, conflicting with its conventional assignment to a “third aluminate hydrate” (TAH) phase. We therefore conclude that TAH does not exist. This resolves a long-standing dilemma about the location and nature of the six-fold-coordinated aluminum observed by 27Al NMR in C-A-S-H samples.
Single amino acids are present in blood plasma and are the building blocks of larger organic residues. Their interaction with surfaces is therefore crucial for biomedical applications in contact with ...blood. In this work, we use well-tempered metadynamics to study the adsorption of six amino acids, with nonpolar (Ala and Leu), polar (Ser), positively charged (Arg and Lys), and negatively charged (Asp) side groups, on a negatively charged rutile (110) surface. The free energy of adsorption and the desorption barriers were determined for all the amino acids under different adsorption conformations. When using the center of mass as the collective variable in well-tempered metadynamics, results for different amino acids were difficult to interpret because of different adsorption conformations on the surface overlapping in collective variable space. After projecting onto separate collective variables for the backbone and the side group, much clearer trends were observable. We show that, on the negatively charged surface of rutile, adsorption via the backbone occurs for all the amino acids irrespective of their side group. Adsorption, driven via the side group, only occurs for the polar and charged side groups as opposed to the nonpolar side groups. This points to the importance of interactions of the side group with the strongly structured water layer rather than direct side group–surface interactions in determining the adsorption behavior.
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Simulated body fluid (SBF) is widely used as part of an in vitro method to evaluate implant materials such as their apatite forming ability (AFA), a typical indication of potential ...bone-bonding ability in vivo. We report the use of carbonate-buffered SBFs as potential solutions for implant evaluation and the effect of proteins, represented by bovine serum albumin (BSA) in SBFs on the formation of hydroxyapatite (HA). These solutions are buffered by the thermodynamic equilibrium with 5% CO2 in an incubator, and result in a deposition of carbonated HA. Using several titanium-based surfaces, these solutions were studied in comparison with the widely-used SBF (ISO 23317). The presence of BSA strongly inhibited the formation of HA in traditional SBF, while HA can still be observed in carbonate-buffered SBFs. A kinetic study reveals that the inhibitory effect is concentration dependent with 0.1g/L and 1g/L of BSA having little effect on HA growth but a complete inhibition of HA formation at 5g/L of BSA, as tested using NaOH treated titanium with a known positive AFA. The decrease in solution pH and free calcium concentrations in SBFs due to the addition of BSA is not significant, suggesting other causes for the strong inhibitory effect.
The successful use of simulated body fluids (SBFs) to evaluate potential bioactive implants relies on the better understanding of the heterogeneous nucleation and growth of hydroxyapatite in solution. Although a standardized recipe for SBF was developed over a decade ago, a few key issues remain to be understood, i.e. the behavior of carbonate-buffered SBFs having similar buffering mechanism as human blood, and the effect of proteins on hydroxyapatite formation on bioactive materials. This paper addresses these two issues and would help the reader better understand the subtleties in this domain and better interpret the results generated using SBFs.
The prediction of implant behavior in vivo by the use of easy-to-perform in vitro methods is of great interest in biomaterials research. Simulated body fluids (SBFs) have been proposed and widely ...used to evaluate the bone-bonding ability of implant materials. In view of its limitations, we report here a rapid in vitro method based on calcium titration for the evaluation of in vivo bioactivity. Using four different titanium surfaces, this method identifies that alkaline treatment is the key process to confer bioactivity to titanium whereas no significant effect from heat treatment is observed. The presence of bioactive titanium surfaces in the solution during calcium titration induces an earlier nucleation of crystalline calcium phosphates and changes the crystallization pathway. The conclusions from this method are also supported by the standard SBF test (ISO 23317), in vitro cell culture tests using osteoblasts and in vivo animal experiments employing a pelvic sheep model.
Alloy 709 austenitic stainless steel is being investigated as a candidate structural material for the next generation fast neutron reactors at service temperature of 500–550 °C. However, the study of ...deformation mechanisms on Alloy 709 and of tensile response of aged Alloy 709 is lacking. In this study, thus, the tensile behaviour of as-received and aged Alloy 709, their deformation microstructures and failure mechanisms, have been investigated at room temperature (RT), 550, 650 and 750 °C. Aging brought about the formation of particles at grain boundaries and interior of grain, thus leading to enhancement of yield strength but reduction in ductility. The ultimate strength of both materials is strongly temperature dependent, which clearly decreases with temperature. It is caused by the decreasing strain hardening ability, dynamic strain aging and dynamic recovery together with dynamic recrystallisation at different temperatures.
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•DSA effect was observed at 550°C in both as-received and aged materials.•Yield strength of the as-received alloy reduces with increasing temperature but no clear reduction in range of 550-750°C.•Aging increases the yield strength but reduces the ultimate tensile strength and ductility.
We studied systematically aqueous suspensions of amorphous well-characterized silica particles by potentiometric titration, electrophoretic mobility, and time-resolved light scattering. Their ...charging behavior and aggregation rate constants were measured as a function of pH and ionic strength in KCl electrolytes for three types of particles of approximately 30, 50, and 80 nm in diameter. The charging behavior was consistent with the basic Stern model; the silica particles carry a negative charge, and its magnitude gradually increases with increasing pH and ionic strength. On the other hand, their early-stage aggregation (or coagulation) behavior is complex. The aggregation of the largest particles shows features resembling predictions of the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. On one hand, the rate constant decreases sharply with increasing pH at low ionic strengths and attains fast aggregation conditions at high ionic strengths. On the other hand, we observe a characteristic slowing down of the aggregation at low pH and high ionic strengths. This feature becomes very pronounced for the medium and the small particles, leading to a complete stabilization at low pH for the latter. Stabilization is also observed at higher pH for the medium and the small particles. From these aggregation measurements we infer the existence of an additional repulsive force. Its origin is tentatively explained by postulating hairy layers of consisting of poly(silicilic acid) chains on the particle surface.
This study examined the construct validity and internal consistency of modified versions of the job autonomy and control, job pressure, work contact, work-family conflict, psychological distress, and ...sleep problems scales developed by Schieman and Young (2013) among construction professionals through confirmatory factor analysis and tests of internal consistency.
Using a cross-sectional design, survey data were collected from 942 South African construction professionals, of which 630 responses were considered for analysis. Confirmatory factor analysis was used to examine construct validity. Cronbach's coefficient alpha was used to determine the internal consistency, and convergent validity was tested using correlation analysis.
The final CFA indicated very good model fit to the data (χ2 /df ratio = 2.11, IFI = .95, CFI = .95, RMSEA = .06, and Hoelter (95%) = 176). The scales demonstrated satisfactory internal consistency: .82; .91; .83; .90; .90; and .73, respectively. Convergent validity was largely demonstrated with respect to direction of association, but not in relation to magnitude. A limitation of the validation study was the lack of available data for a more robust examination of reliability beyond internal consistency, such as test-retest.
The six scales developed by Schieman and Young (2013) hold promise as measures of work contact, work-family conflict, psychological distress, and sleep problems in relation to working conditions of construction professionals.
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