The Effect of Bacteria on Early Age Strength of CEM I and CEM II Cementitious Composites

Despite being associated with lower carbon emissions, CEM II cementitious materials exhibit reduced early age strength compared to that of CEM I. Several studies have demonstrated early age strength improvements by incorporating bacterial cells in concrete. In this study, live vegetative bacteria and dead bacteria killed in two different ways were used to explore whether changes in strength are related to the bacteria’s viability or their surface morphology. Compressive and flexural strength tests were performed at mortars with and without bacteria for both CEM I and CEM II cement. Their microstructure, porosity and mineralogy were also examined. No net strength gain was recorded for either CEM I or CEM II bacterial mortars compared to non-bacterial controls, although changes in the porosity were reported. It is proposed that two phenomena, one causing strength-reduction and one causing strength-gain, took place in the bacterial specimens, simultaneously. It is suggested that each phenomenon is dependent on the alkalinity of the cement matrix, which differs between CEM I and CEM II mortars at early age. Nevertheless, in neither case could it be recommended that the addition of bacteria is an effective way of increasing the early age strength of mortars.

Numerical Simulation and Design Recommendations for Web Crippling Strength of Cold-Formed Steel Channels with Web Holes under Interior-One-Flange Loading at Elevated Temperatures

This paper investigates the interior-one-flange web crippling strength of cold-formed steel channels at elevated temperatures. The stress-strain curves of G250 and G450 grade cold-formed steel (CFS) channels at ambient and elevated temperatures were taken from the literature and the temperatures were varied from 20 to 700 °C. A detailed parametric analysis comprising 3474 validated finite element models was undertaken to investigate the effects of web holes and bearing length on the web crippling behavior of these channels at elevated temperatures. From the parametric study results, it was found that the web crippling strength reduction factor is sensitive to the changes of the hole size, hole location, and the bearing length, with the parameters of hole size and hole location having the largest effect on the web crippling reduction factor. However, the web crippling strength reduction factor remains stable when the temperature is changed from 20 to 700 °C. Based on the parametric analysis results, the web crippling strength reduction factors for both ambient and elevated temperatures are proposed, which outperformed the equations available in the literature and in the design guidelines of American standard (AISI S100-16) and Australian/New Zealand standard (AS/NZS 4600:2018) for ambient temperatures. Then, a reliability analysis was conducted, the results of which showed that the proposed design equations could closely predict the reduced web crippling strength of CFS channel sections under interior-one-flange loading conditions at elevated temperatures.

Influence of Hydrate Exploitation On Stability of Submarine Slopes

The decomposition of natural gas hydrate will reduce the cementation effect of hydrate and produce ultra-static pore pressure, which will change the mechanical characteristics of the reservoir. Eventually, a series of geological disasters could be triggered, of which the submarine landslide is a typical example. In order to analyze the stability of hydrate-bearing submarine slopes and to explore the internal relationship between hydrate decomposition and submarine landslides, a “two-step reduction method” was described in this paper. This method was based on a strength reduction approach, which can be used to assess the effects of the initial geostress balance and hydrate decomposition on substrate strength reduction. This method was used to reveal the essence of hydrate decomposition and then a joint operation mode of multi-well was proposed. The internal relationship between hydrate decomposition and submarine landslides were analyzed in detail. And the development process and mechanism of submarine landslide were deeply discussed. The results showed that hydrate decomposition is a dynamic process of stress release and displacement, where the “stress inhomogeneity” distributed along the slope is transformed into “displacement inhomogeneity”. We concluded that hydrate decomposition could trigger a submarine landslide, especially along a sliding surface. The formation of submarine landslide is a gradual development process, and presents the dual characteristics of time and space.

Estimating the Basal Heave Stability of Narrow Braced Excavations

Engineering practices indicate that narrow braced excavation exhibits a clear size effect. However, the slip circle method in the design codes fails to consider the effect of excavation width on basal heave stability, causing waste for narrow excavation. In this paper, numerical simulation for basal heave failure of excavation with different widths was performed by FEM with SSRT (shear strength reduction technique). The results revealed that the failure mechanism of narrow excavation is different from the complete slip circle mode. In addition, the safety factor decreases increasingly slowly as the excavation widens and stabilizes when approaching the critical width. Subsequently, the corresponding computation model was presented, and an improved SCM (slip circle method) was further developed. Finally, the engineering case illustrated that it can effectively optimize the design, which exhibits clear superiority.

Concrete Microstructure Study on the Effect of Sisal Fiber Addition on Sugarcane Bagasse Ash Concrete

Background: Concrete made using sugarcane bagasse ash as a cement replacement is associated with a reduction in split tensile strength and therefore a need to establish the possible causes of tensile strength reduction and explore ways of mitigating that reduction. Objective: The aim of this study is to establish the possible causes of tensile strength reduction in sugarcane bagasse ash concrete and determine the effect of sisal fiber addition on its mechanical properties. Methods: Scanning Electron Microscopy was first done to analyse concrete microstructure in establishing the possible causes of tensile strength reduction in sugarcane bagasse ash concrete. Thereafter, sisal fiber addition was done by varying aspect ratios and percentages. The effect of the addition was determined on the mechanical properties of bagasse ash concrete accompanied by microstructure studies on extracted fibers and split surfaces of concrete. Results: Concrete microstructure studies revealed that wider cracks due to drying shrinkage and poor bonding properties of sugarcane bagasse ash are the possible causes of tensile strength reduction in bagasse ash concrete. Sisal fiber addition improved the mechanical properties of bagasse ash concrete. Microstructure studies portrayed effective bridging of cracks and good adhesive properties of the fibers. Conclusion: Sisal fibers can be used to improve on the mechanical properties of sugarcane bagasse ash concrete with 100 aspect ratio and 1.5% addition being the optimal combination.

Correlations of geotechnical monitoring data in open pit slope back-analysis -A mine case study

Geotechnical monitoring plays an important role in the detection of operational safety issues in the slopes of open pits. Currently, monitoring companies offer several solutions involving robust technologies that boast highly reliable data and the ability to control risky conditions. The monitoring data must be processed and analysed so as to allow the results to be used for several purposes, thereby providing information that can be used to manage operational actions and optimize mining plans or engineering projects. In this work we analysed monitoring data (pore pressure and displacement) and its correlation with the tension and displacement of the mass of an established failure slope calculated using the finite element method. To optimize the back-analysis, a Python language routine was developed using input data (point coordinates, parameter matrix, and critical section) to use software with the rock mass parameters (cohesion, friction angle, Young's modulus, and Poisson's ratio). For the back-analysis, the Mohr-Coulomb criterion was applied with the shear strength reduction technique to obtain the strength reduction factor. The results were consistent with both the measured displacements and the maximum deformation contours, revealing the possible failure mechanism, allowing the strength parameters to be calibrated according to the slope failure conditions, and providing information about the contribution of each variable (parameter) to the slope failure in the study area.

The Stability Analysis of Heituwan Tailings Pond Based on Strength Reduction Method

To analyze the stability of Heituwan tailings pond after vacuum well point dewatering treatment, a unit thickness numerical 3D model was built based on field survey data and physical mechanical properties tests; and the model was analyzed by FEM numerical software according to strength reduction method. The properties of stress and strain, the plastic region, and the deformation properties are acquired by numerical stimulation, and the simulation result was compared withe the in-situ monitoring data. The results show that the safety factor does not meet the requirements of the standard; and most of the landfill made of manganese tailing has developed into plastic status; the deformation is more obvious where the tailing store is higher and closer to roller compacted rockfill dam; the manganese tailing landfill near the roller compacted rockfill dam should be grouted by cement to meet the requirements of continued use.

Fatigue Characterization on a Cast Aluminum Beam of a High-Speed Train Through Numerical Simulation and Experiments

The cast aluminum beam is a key structure for carrying the body-hung traction motor of a high-speed train; its fatigue property is fundamental for predicting the residual life and service mileage of the structure. To characterize the structural fatigue property, a finite element-based method is developed to compute the stress concentration factor, which is used to obtain the structural fatigue strength reduction factors. A full-scale fatigue test on the cast aluminum beam is designed and implemented for up to ten million cycles, and the corresponding finite element model of the beam is validated using the measured data of the gauges. The results show that the maximum stress concentration occurs at the fillet of the supporting seat, where the structural fatigue strength reduction factor is 2.45 and the calculated fatigue limit is 35.4 MPa. Moreover, no surface cracks are detected using the liquid penetrant test. Both the experimental and simulation results indicate that the cast aluminum beam can satisfy the service life requirements under the designed loading conditions.

Structural Analysis of Backfill Highway Subgrade on the Lower Bearing Capacity Foundation Using the Finite Element Method

The present study is to investigate the stability of the backfill subgrade on the lower bearing capacity foundation. A finite element (FE) model was constructed to simulate the high-filled road subjected to the actual self-weight load. The strength reduction method was adopted to establish an analysis model of slope stability. At the same time, the sensitivity analysis of the factors affecting the slope stability was carried out through parametric studies, including the elastic modulus, cohesion, internal friction angle, and slope rate. The results showed that the slope stability analysis model established by the strength reduction method can characterize the stability of the slope by calculating the slope safety coefficient. The mutation point of the relationship curve between the displacement generated in the slope and the reduction coefficient can be used as the criterion. Under the condition of a given strength reduction coefficient, the calculated results obtained through FE modeling can show the development of the equivalent plastic zone in the form of cloud diagrams.