Properties of Shotcrete According to Accelerator Types and Mixing Ratios

Shotcrete should be attached to the ground and should have stable strength for a long-term. It should develop strength earlier for rapid work. Therefore, in this study, three types of accelerators—aluminate, cement mineral, and alkali-free—were selected and mixed to secure the initial strength. Depending on the type and mixing rate of each accelerator, slump, air amount, and compressive strength were used to evaluate the basic properties, boiling water absorption test, and chloride ion penetration resistance to conduct durability analysis. The mixing of aluminate-based and cement-mineral-based accelerators was effective in improving the initial strength, and alkali-free accelerator was effective in improving the long-term strength. The mixture to which accelerators were not mixed showed the highest water-tightness.

Tropical cyclone simulation with Emanuel’s convection scheme

ABSTRACT. A new convection parameterization scheme proposed by Emanuel (1991) is used to simulate the evolution of tropical cyclone. The numerical model used for this study is a 19 level axi-symmetric primitive equation, hydrostatic model in a z co-ordinate system. The vertical domain ranges from 0 to 18 km and the horizontal domain ranges upto 3114 km with a resolution of 20 km. in the central 400 km radius and with increasing radial distance thereafter. The evolution of an initially balanced vortex with an initial strength of 9 m/sec is studied. It is shown that Emanuel's convection scheme is successful in simulating the development of the initial vortex into a mature, intense cyclonic storm. At the mature stage, a minimum surface pressure of 930 hPa is attained with the associated low level maximum tangential wind speed of 70 m/sec. The simulated circulation features at the mature stage show the formation of an intense cyclone. Two different sensitivity experiments were performed. A set of experiments with the variation of sea surface temperature (SST) from 300.5° to 302° K in steps of 0.5° K have shown that the intensity of model cyclone increases with the increase of SST. Another set of experiments with variation of latitude has shown that the cyclonic storm is more intense at lower latitudes.

Performance of Recycled Concrete Made from Railway Sleeper: Experimental Study

Concrete railway sleeper has been used in Indonesia since about 1990�s. It has more advantage that is less maintenance, stabile, good quality, shapeable, onsite raw material, and higher loading ability [9-10, 20 - 22]. But It is prone to damage such as cracking and breaking during construction, so it is often thrown away as a solid waste that can contaminate the land and reduce soil fertility. Therefore, it should be utilized in order to be more useful that is used as recycle aggregate. The concrete waste that is taken from broken concrete railway sleeper will be crush as an aggregate as raw material in the concrete to substitute part or all of the normal concrete. It is called recycled aggregate and concrete that is made from recycled aggregate is called recycled concrete. Base on the testing of raw material, the recycled aggregate can be met to the specification as ASTM [1], so it can be used for concrete raw material. Recycled concrete compressive strength result lower the normal concrete compressive strength in the same initial strength design. The strength value of recycled concrete is decrease about 1 � 17% for 25 MPa and 10 � 18% for 30 MPa. It is also happened to tensile strength of recycled concrete that decrease about 2 � 13 % for 25 MPa and 7 � 22 % for 30 MPa.

An Application Process of Additive Manufacturing Based on Digital Simulation and BESO Topology Optimization

Additive manufacturing has now entered a wide range of areas and plays an important role. There are many factors affecting the application of additive manufacturing, such as the amount of printing supplies, print product strength, print speed and so on. These factors potentially hinder the application of additive manufacturing in some typical areas, such as spare parts producing for on-orbit maintenance in space environments. Based on the improvement of the above factors, an additive manufacturing application process based on topology optimization of variable density method and digital simulation was proposed. Print volume of product was used as an explicit constraint, and the design goal of the product, such as strength and modal, was transformed into implicit stress constraints in the topology optimization of three-dimensional model, then stress constraints were independently extracted for secondary verification, finally the checked model is put into print. This process saves computational resources during optimization calculations and printing time, reduces print product’s weight, conserves supplies, and meets initial strength or modal design goals. This process greatly exploited the advantages of additive manufacturing in product manufacturing and made up for the shortcomings of traditional manufacturing processes that can not directly output a relatively abstract model after topological optimization. Under the constraints of saving material and increasing strength, it becomes optimum solution in the manufacture of specific products.

How Efficient are LC3 and GGBFS-Contained Mortar Mixtures Submerged into Na2SO4 Solution against External Sulfate Attack at an early Age?

Ordinary Portland cement (OPC) is one of the most widely used construction materials in civil engineering infrastructure construction but it is susceptible to sulfate attack. One of the ways to improve the sulfate resistance of an OPC mortar/concrete is to replace a certain amount of OPC with different pozzolanic materials such as ground granulated blast furnace slag (GGBFS) and metakaolin. The use of pozzolanic materials to mortar/concrete not only enhances durability but also reduces carbon dioxide (CO2) emission due to the less usage of OPC at the initial construction state. As considering these aspects, limestone calcined clay cement (LC3) has been developed in recent decades. However, the influence of LC3 on sulfate attack resistance has not been fully evaluated. Therefore, this study investigated the efficiency of LC3 mortar mixtures against sulfate attack at an early age (approximately 4.5 months) after two different curing periods, namely 1-day and 3-day curing, since the strength of the LC3 mixture is lower than OPC mixtures. To evaluate the synergistic effect of a combination of LC3 and GGBFS on the sulfate resistance, the LC3 and OPC mixtures containing 25% GGBFS were also assessed in terms of density, porosity, compressive strength, volumetric expansion, and weight changes. The experiment results show that the expansion of the LC3 mixture regardless of the addition of GGBFS and an initial curing strength made a plateau after a rapid increase up to 7 days, while the expansion of the OPC mixture kept increasing throughout the period. Furthermore, the addition of GGBFS to OPC or LC3 mixture provides the synergistic effect on reducing the expansion due to sulfate attack. Therefore, if LC3 mixture has high initial strength (min. 15 MPa) and dense microstructure to minimize the penetration of sulfate ion into the mixture, it is expected that LC3 mixture is more efficient than OPC mixture against the sulfate attack.

Application of Supplementary Cementitious Materials in Precast Concrete Industry

Supplementary cementitious materials (SCMs) are increasingly incorporated into the concrete mix design. Silica fume, fly ash, and multi-wall carbon nanotubes are used to improve concrete mix properties. The objective of this chapter is to decipher the impact of different SCMs on the fresh and hardened concrete properties, including concrete flowing ability, initial strength, final strength, modulus of elasticity, and modulus of rupture. In addition, the impact of SCMs on mitigating the alkali-silica reactivity of concrete and increasing the hardened concrete long-term performance is investigated. Developed concrete mixes, incorporating SCMs, are used in fabricating different precast/prestressed bridge girders. The impact of improved concrete properties on precast girder performance in increased flexure, shear, and span-to-depth ratio significantly improves project sustainability and reduces the overall project life cycle cost.

Influence of stress history on undrained cyclic shear strength evolution

Offshore infrastructure often interacts cyclically with the seabed over the operational life of a project. Previous research on the evolution of soil’s undrained strength under long term, large-amplitude cyclic loading has focused on contractile clays and demonstrated that this cyclic interaction can lead to the initial generation and later dissipation of positive excess pore pressure in the soil. This process generally leads to an initial strength reduction, with subsequent densification and soil strength gains that can have consequences on the performance of seabed infrastructure during its design life. In this paper, new experimental data from T-bar penetrometer testing in reconstituted kaolin and Gulf of Mexico clays is presented. The data illustrate how the stress history, quantified via the overconsolidation ratio, affects soil strength changes during large-amplitude cyclic loading. The experiments explore both long-term continuous loading cycles and episodic loading with packets of undrained cycles followed by quiescent consolidation periods. A critical state-based framework is used to interpret the experimental data and provide predictions of the long-term steady-state strength of both soils as a function of the initial in situ state of the soil.

Mechanical Behavior and Healing Efficiency of Microcapsule-Based Cemented Coral Sand under Various Water Environments

The cracks in the cemented coral sand (CCS) would result in significant damage for the marine structures. In this study, the effective and efficiency of microcapsules in self-healing CCS under various water environments were investigated with a series of experimental tests. Firstly, a new preparation method was proposed to fabricate the microcapsules with a wide particle size distribution, which was adapted to the high porosity, large difference in pore size, and uneven distribution of CCS. Secondly, the mechanical properties of microcapsule-based CCS were examined by the uniaxial compressive tests and split Hopkinson pressure bar (SHPB) tests. The results indicated that the microcapsule could improve the initial strength of CCS. The CCS mixed with 3% of the microcapsule that synthesized under a rotating speed of 450 rmp had the highest compressive strength at the initial strain state. Finally, the healing efficiency of microcapsule for CCS was investigated in various environmental conditions, which were freshwater, seawater, and water of various pH values. The non-destructive experiment approach of the piezoelectric transducer (PZT) test was adopted to evaluate the healing efficiency of microcapsules. Experimental results indicated that the healing efficiency of microcapsules in freshwater and seawater were 75% and 59.56%, respectively. In contrast, the acid and alkali water environment would greatly reduce the healing efficiency of microcapsules in CCS.

Stress-based forecasting of induced seismicity with instantaneous earthquake failure functions: Applications to the Groningen Gas Reservoir.

The Groningen gas field is a natural laboratory to test stress-based forecasting models of induced seismicity due to the detailed knowledge of the reservoir geometry and production history, as well as the availability of surface subsidence measurements and high quality seismicity data. A specific feature of that case example is the exponential rise of seismicity that was detected nearly 30 years after the onset of production. In this study, the subsurface is represented as a homogeneous isotropic linear poroelastic half-space subject to stress changes in three-dimensional space due to reservoir compaction and pore pressure variations. The reservoir is represented with cuboidal strain volumes. Stress changes within and outside the reservoir are calculated using a simple convolution with semi-analytical Green functions. The uniaxial compressibility of the reservoir is spatially variable and constrained with surface subsidence data. Coulomb stress changes are maximum near the top and bottom of the reservoir where the reservoir is offset by faults. To assess earthquake probability, we use the standard Mohr-Coulomb failure criterion assuming instantaneous nucleation and a non-critical initial stress. The distribution of initial strength excess, the difference between the initial Coulomb stress and the critical Coulomb stress at failure, is treated as a stochastic variable and estimated from the observations. We calculate stress changes since the onset of gas production. The lag and exponential onset of seismicity are well reproduced assuming either a a generalized Pareto distribution of initial strength excess, which can represent the tail of any distribution, or a Gaussian distribution, to describe both the tail and body of the distribution. This representation allows to test if the induced seismicity at Groningen has transitioned to the steady-state where seismicity rate is proportional to the stressing rate. Our results indicate that the system has not yet reached such a steady-state regime. The forecast is robust to uncertainties about the ability of the model to represent accurately the physical processes. It does require in particular a priori knowledge of the faults that can be activated. The method presented here is in principle applicable to induced seismicity in any setting provided deformation and seismicity data are available to calibrate the model.

Biomechanical aspects of the initial stability of instrumental fixation in the treatment of subaxial cervical dislocations: an experimental study

Objective. To study the influence of thoracic inlet angle (TIA) and the fracture of the articular process on the initial strength of the fixation of the spinal segment during its anterior and circular instrumental surgical stabilization in an experiment on a model of the lower cervical spinal segment.Material and Methods. The material of the study was assembled models of C6–C7 spinal segments made using addictive technologies by 3D printing. After preliminary instrumentation, spinal segments were installed on the stand testing machine using specially manufactured equipment. A metered axial load simulating the native one was applied along the axis of the parameters SVA COG–C7 and C2–C7 SVA, which values were close to the value of 20 mm, at a rate of 1 mm/min until the shear strain was reached. The system’s resistance to displacement was measured, and the resulting load was evaluated. Four study groups were formed depending on the modeling of the T1 slope parameter, the integrity of the facets, and the type of instrumentation. Three tests were conducted in each group. The graphical curves were analyzed, and the values of the parameters of the neutral and elastic zones, the yield point, time to yield point, and the value of the applied load for the implementation of shear displacement were recorded. The data were subjected to comparative analysis.Results. In Group 1, anterior shear displacement of the C6 vertebra could not be induced in all series. In groups 2, 3, and 4 a shear displacement of ≥4 mm was noted in all series. In Group 3 where a fracture of the articular process was additionally modeled, the average value of the yield point was 423.5 ± 46.8 N. Elastic zone, the time to the onset of the yield point, the time at the end point or at a shear of C6 ≥4 mm did not differ significantly. In Group 4, a translational displacement of ≥4 mm was observed, though the average yield point was 1536.0 ± 40.0 N.Conclusion. The direction of the load applied to the fixed spinal segment, as well as the presence of damage to the articular processes, play a crucial role in maintaining resistance to shear deformation of the spinal segment during its instrumental stabilization. At high values of TIA (T1 slope) and the presence of fractures of the articular processes, the isolated anterior stabilization is less effective, circular fixation of 360° under these conditions gives a high initial stability to the spinal segment.