The effect of random porosity field on supercritical carbonation of cement-based materials

In this paper, the supercritical carbonation process of cement-based materials is modelled by introducing a random porosity field to simulate the heterogeneous geometry of the carbonation profile. The suitability of two different random fields of porosity, based on the probability density function (PDF) and the ellipsoidal autocorrelation function (EAF) methods, are investigated, respectively, in simulating the distribution of porosity in cement mortar. After incorporating the above random fields into an established supercritical carbonation model, it is found that with some modifications, the EAF method with consideration of spatial correlation produces better simulation of the irregularities of the carbonation zones that have been observed from experimental results. It is also found that for given average porosity and coefficient of variation, the predicted average and maximum carbonation depths have much smaller coefficients of variation. The validated EAF supercritical carbonation model is then used in parametric studies that are conducted to assess the effect of various factors on the carbonation depth of the chemical process

Experimental and statistical study on the irregularity of carbonation depth of cement mortar under supercritical condition

The heterogeneity of a cement-based material results in a random spatial distribution of carbonation depth. Currently, there is a lack of both experimental and numerical investigations aiming at a statistical understanding of this important phenomenon. This paper presents both experimental and numerical supercritical carbonation test results of cement mortar blocks. The carbonation depths are measured along the carbonation boundary by the proposed rapid image processing technique, which are then statistically studied by calculating, e.g., their probability density and power spectral density (PSD). The results indicate that the distribution of the carbonation depth can be approximately represented by a lognormal distribution function and the PSD has quantitative correlation with some of the statistic parameters used in the simulations. In particular, the effects of the autocorrelation lengths and the coefficient of variation of porosity, which are used to define the random porosity field, on the irregularity of carbonation depth are analyzed numerically in details and validated by experimental results. The study has shown that using a random field of porosity with due consideration of spatial correlation and variance, the irregularity of carbonation depth can be realistically captured by the numerical model. The numerical results confirm that lognormal distributions represent the random nature of carbonation depth well and the average and variance of the irregular carbonation depth increase with the increase of carbonation time, autocorrelation length and coefficient of variation of porosity. The study also offers a potential method to numerically calibrate some of the statistic parameters required by a numerical carbonation model through comparing the PSD with that from experimental tests. Overall the methodology adopted in the paper can provide a foundation for future investigations on probability analysis of carbonation depth and other similar work based on multi-scale and -physics modelling

Experimental research on influence of multi-level loading on axial compression behavior of concrete-filled circular steel tubular short columns

In order to study the mechanical properties of early-age concrete-filled steel tube (CFST) under continuous construction process, an experimental investigation on six concrete-filled circular steel tubular short columns under multi-level loading was carried out. The influence of multi-level loading with different initial stress degrees, loading time intervals and loading levels on the deformation progression was analyzed. The bearing capacity of the columns was then tested at 28 days after the multi-level loading. The results show that creep deformation of the concrete-filled steel tubes under multi-level loading is significant and cannot be neglected. The deformation of the columns is significantly affected by the applied loading scheme. An increase of the initial stress degrees and increment of loading levels, or a reduction of loading time interval will all result in an increase of the deformation of the columns. However, the multi-level loading process has only small impact on the bearing capacity of the CFST short columns after 28 days

Experimental and Numerical Investigation on the Irregularity of Carbonation Depth of Concrete Under Supercritical Condition

The heterogeneity of a cement-based material results in a random spatial distribution of carbonation depth, which may significantly affect the mechanical properties and durability of the material. Currently, there is a lack of both experimental and numerical investigations aiming at a statistical understanding of this important phenomenon. This paper presents both experimental and numerical supercritical carbonation test results of concrete blocks. The random fields of porosity and two-dimension random aggregate model of concrete were proposed for the simulation. The carbonation depths are measured and distributed along the carbonation boundary by the proposed rapid image processing technique, which are then statistically studied. The study has shown that considering the random distribution of coarse aggregates and using a random field of porosity with due consideration of spatial correlation and variance, the irregularity of carbonation depth can be realistically captured by the numerical model. Overall the methodology adopted in the paper can provide a foundation for future investigations on probability analysis of carbonation depth and other similar work based on multi-scale and –physics modelling

A Study of the Mechanism of the Congruence of Leader–Follower Power Distance Orientation on Employees’ Task Performance

Based on implicit leadership theory, we examine the congruence effect of leader–follower power distance orientation (PDO) on follower trust in supervisor and work engagement, which in turn influences employees’ task performance. Results of polynomial regressions on 526 dyads supported the congruence effect hypothesis. The results show that (1) the congruence of leader–follower PDO leads to better performance; (2) under the condition of congruence, subordinate task performance is higher when leader–follower PDO matching in low–low ratings congruence than it is in high–high ratings congruence; (3) under the condition of asymmetrical incongruence, the follower had higher task performance when a leader’s PDO is lower than a follower’s PDO; (4) trust in supervisor and the work engagement mediate the effect of congruence of leader–follower PDO on employees’ task performance; (5) trust in supervisor also mediates the effect of congruence of leader–follower PDO on employees’ work engagement

Performance and mechanism of sand stabilization via microbial-induced CaCO3 precipitation using phosphogypsum

Phosphogypsum is a solid waste generated during the production of phosphoric acid. Effective utilization of phosphogypsum resources is a complex challenge. In this research, an innovative and eco-friendly sand consolidation technique, i.e., microbial-induced CaCO3 precipitation using phosphogypsum (MICPP), is applied to achieve phosphogypsum mineralization and sand stabilization. Phosphogypsum is employed as a calcium source for sand consolidation. To elucidate the efficiency and the mechanism of sand consolidation through MICPP, a series of experimental tests on the sand columns using varying phosphogypsum dosages and consolidation methods are conducted. The results show a positive correlation between the increase in phosphogypsum dosage and the increase in the compressive strength of the specimens. Concurrently, As the amount of phosphogypsum increased, the permeability coefficient of the sand columns decreased and the production of CaCO3 increased. Notably, the immersion method exhibits a superior curing effect compared to the stirring method. The MICPP-treated specimens significantly mitigated the risk of environmental contamination. The CaCO3 precipitated by the microbial action is predominantly in the form of calcite that effectively fills the voids, bond surfaces, and bridge gaps in the sand columns, thereby substantially enhancing the performance of sand columns

Stress analysis of cross-ply composite laminates with transverse cracks

A boundary discontinuous Fourier expansion method to analyze the displacements and stresses in cross-ply composite laminates with transverse cracks is presented. The governing equations of the problem are derived on the basis of the generalized plane strain assumption and two-dimensional equations of elasticity. By employing the boundary-discontinuous Fourier expansion method, the governing equations in the form of coupled high-order ODEs are transformed to a set of systems of linear algebraic equations. The method is used to obtain solutions for which published results can be found for comparisons. Compared with the conventional numerical methods for solving coupled high-order ODEs, the method presented is more efficient. Further parametric studies are carried out for cracked laminates with various geometric and material properties

Experimental and Numerical Study on Mixed Lubrication Performance of Journal Bearing Considering Misalignment and Thermal Effect

The shaft misalignment under mixed lubrication is an important factor affecting the running performance of the bearing, which can occur under heavy load and unsatisfactory assembly. This paper presents a misaligned journal mixed lubrication model coupling for the asperity contact effect, elastic deformation, viscosity–temperature, and viscosity–pressure effect. The finite difference method was employed to calculate the model, and an experimental apparatus designed in this paper was used to test the friction and temperature characteristics of the specimens. The results show that the pressure field, film thickness, and elastic deformation of the bearing conformed to asymmetric distribution along the axial direction under misalignment conditions and there was a notable end side effect. In addition, the frictional force and side leakage flow were evidently enhanced with the increase in the inclination angle in a certain range. The experimental results showed that there was a visible wear phenomenon on the end sides of the bush and shaft. The research results are beneficial for understanding the mixed lubrication mechanism of misaligned journal bearing

Experimental and Numerical Study on Mixed Lubrication Performance of Journal Bearing Considering Misalignment and Thermal Effect

The shaft misalignment under mixed lubrication is an important factor affecting the running performance of the bearing, which can occur under heavy load and unsatisfactory assembly. This paper presents a misaligned journal mixed lubrication model coupling for the asperity contact effect, elastic deformation, viscosity–temperature, and viscosity–pressure effect. The finite difference method was employed to calculate the model, and an experimental apparatus designed in this paper was used to test the friction and temperature characteristics of the specimens. The results show that the pressure field, film thickness, and elastic deformation of the bearing conformed to asymmetric distribution along the axial direction under misalignment conditions and there was a notable end side effect. In addition, the frictional force and side leakage flow were evidently enhanced with the increase in the inclination angle in a certain range. The experimental results showed that there was a visible wear phenomenon on the end sides of the bush and shaft. The research results are beneficial for understanding the mixed lubrication mechanism of misaligned journal bearing

Performance evaluation of steel-polypropylene hybrid fiber reinforced concrete under supercritical carbonation

In this paper, systematic supercritical carbonation tests of steel-polypropylene hybrid fiber reinforced concrete (SPFRC) were carried out to evaluate the performance of SPFRC under supercritical condition. The effects of the length-diameter ratio of steel fiber, volume fraction of steel fiber, and polypropylene fiber on the carbonation depth and compressive strength of concrete under supercritical condition were studied. A one-dimensional mathematical model for the physical-chemical coupling process of supercritical carbonation of cement-based materials was established. The relational model between the equivalent porosity and the compressive strength of fully carbonated SPFRC was also proposed. Results indicate that whether the addition of steel fibers or polypropylene fibers or the inclusion of fibers can accelerate the carbonation process by the increase of porosity. The carbonation depths of SPFRC increase with the increase of the addition of steel fibers and polypropylene fibers. The compressive strength after carbonation is significantly increased. The maximum relative compressive strength was obtained when the volume fraction of steel fibers and polypropylene fibers were 1.5% and 0.0% and the length-diameter ratio of steel fiber was 60, respectively. Furthermore, a mathematical model was proposed to evaluate the equivalent initial porosity of SPFRC