Increased capabilities of the 30-cm diameter Hg ion thruster

Some space flight missions require advanced ion thrusters which operate at conditions much different than those for which the baseline 30-cm Hg thruster was developed. Results of initial tests of a 30-cm Hg thruster with two and three grid ion accelerating systems, operated at higher values of both thrust and power and over a greater range of specific impulse than the baseline conditions are presented. Thruster lifetime at increased input power was evaluated both by extended tests and real time spectroscopic measurements

Strength and Stiffness Characterization of Controlled Low-Strength Material Using Native High-Plasticity Clay

A research attempt was made to design a controlled low-strength material (CLSM) mix that can be used as bedding and haunch material for a pipeline by using the native soil as fine aggregate. Several CLSM mix designs were attempted using native high-plasticity clay as fine aggregate material. Comprehensive material characterization studies including flowability to strength tests were performed. These results were analyzed to address the applicability of each mix to serve as pipe bedding/backfilling zones in a pipeline construction. Both flowability and density test results are first evaluated, and as a result, several mixes are formulated. These mixes were further subjected to engineering characterization-related studies, and this paper presents these test results. Setting time, strength, and stiffness results as well as excavatability evaluations of these mixtures are covered as a part of these studies. These results indicate that the CLSMs can be produced using native high-plasticity soils with strength properties always matching specified requirements. Certain relaxation on setting time periods could further help in developing economical mix designs. CLSMs that meet project specifications are recommended for field implementation

Flowability and Density Characteristics of Controlled Low Strength Material (CLSM) Using Native High Plasticity Clay

In pipeline construction projects when high plastic clayey soils are encountered in the excavated trench material, they are typically landfilled and better quality materials are imported from outside quarry sources for use as bedding and haunch zone materials. This practice has detrimental environmental and cost impacts; therefore, an efficient reutilization of this high plastic excavated material to produce controlled low strength materials (CLSMs) to use as bedding and haunch zone materials will have major sustainability benefits. As a part of an on-going research study, novel CLSM mix designs were developed by utilizing native high plastic clayey soils from the excavated trench material. Due to the high plasticity nature of the soils, it is essential to address both flowability and density property requirements prior to validating them against other engineering properties. Hence, several CLSM mixtures with the native clayey soils as ingredients were initially designed as per flowability criterion to establish the optimum quantities of chemical binders and water quantities. Later, these mixes were verified for satisfying density property criterion. This technical note presents the step by step procedure followed in preparing these mixes along with test results obtained from various mixes designed as a part of the testing program. Based on these results it was evident that CLSM mixes with high plastic clays can be developed that meet both flowability and density criteria. The success of this research has enhanced the sustainability efforts in pipeline construction projects as this study showed excavated clayey soils can be successfully reused in CLSM applications than landfilling them

Stress-Crack Separation Relationship for Macrosynthetic, Steel and Hybrid Fiber Reinforced Concrete

An experimental evaluation of the crack propaga tion and post-cracking response of macro fiber reinforced concrete in flexure is c onducted. Two types of structur al fibers, hooked end steel fibers and continuousl y embossed macro-synthetic fibers are used in this study. A fiber blend of the two fibers is evaluated for spec ific improvements in the post peak residual load carrying response. At 0.5% volume fraction, both steel and macrosynthetic fiber reinforced concrete exhibits load recovery at large crack opening. The blend of 0.2% macrosynthetic fibers and 0.3% steel fibers shows a significa nt improvement in the immediate post peak load response with a significantly smaller load drop and a constant residual load carrying capacity equal to 80% of the peak load. An analytical formulation to predict fle xure load-displacement behaviour considering a multi-linear stress- crack separation (σ -w) relationship is developed. An inverse analysis is developed for obtaining the multi- linear σ -w relation, from the experimental response. The � -w curves of the steel and macrosynthetic fiber reinforced concrete exhibit a stress recovery after a significant drop with increa sing crack opening. Significant residual load carrying capacity is attained only at large crack separation. The fiber blend exhibits a constant residual stress with increasing crack sepa ration following an initial decrease. The constant residual stress is attained at a small crack separation

Status and projections of the NAS program

NASA's Numerical Aerodynamic Simulation (NAS) Program has completed development of the initial operating configuration of the NAS Processing System Network (NPSN). This is the first milestone in the continuing and pathfinding effort to provide state-of-the-art supercomputing for aeronautics research and development. The NPSN, available to a nation-wide community of remote users, provides a uniform UNIX environment over a network of host computers ranging from the Cray-2 supercomputer to advanced scientific workstations. This system, coupled with a vendor-independent base of common user interface and network software, presents a new paradigm for supercomputing environments. Background leading to the NAS program, its programmatic goals and strategies, technical goals and objectives, and the development activities leading to the current NPSN configuration are presented. Program status, near-term plans, and plans for the next major milestone, the extended operating configuration, are also discussed

Effect of Set Accelerator on Properties of Wet Sprayed Concrete

This paper describes sprayed concrete experiments varying the set accelerator dose. Literature on the hydration of cement with modern alkali-free set accelerators is reviewed and two full scale wet spraying experiments have been conducted, varying the dose of set accelerator in each. The effects on the properties of the hardening and hardened sprayed concrete were investigated by field and laboratory testing. Increasing the set accelerator dose was found to increase the rate of early age strength development but reduce density, long term strength and increase suction porosity of hardened sprayed concrete.publishedVersio

Freeze-Thaw Durability and Long-Term Performance Evaluation of Shotcrete in Cold Regions

This study’s aim was to evaluate the freeze-thaw durability of shotcrete in cold regions and predict its long-term performance. One benchmark mix design from the WSDOT was chosen to prepare samples for performance evaluation. Shotcrete specimens were conditioned in accordance with ASTM C666. The long-term freeze-thaw performance after certain cycles was evaluated using the dynamic modulus of elasticity test (ASTM C215), fracture energy test (RILEM 50-FMC), and X-ray CT microstructure imaging analysis. Probabilistic damage analysis was conducted to establish the relation between the durability life and the damage parameter for different probabilities of reliability using the three-parameter Weibull distribution model. The fracture energy test was found to be a more sensitive test method than the dynamic modulus of elasticity for screening material deterioration over time and for capturing accumulative material damage caused by rapid freeze-thaw action, because of smaller durability factors (degradation ratios) obtained from the fracture energy test. X-ray CT imaging analysis is capable of detecting microcracks that form and pore evolution in the aggregate and interface transition zone of conditioned samples. Moreover, the continuum damage mechanic-based model shows potential in predicting long-term material degradation and the service life of shotcrete

Effect of Set Accelerator on Capillary Suction and Porosity of Concrete – Cast Samples with Constant Water/Binder Ratio

Alkali-free set accelerators are added at the nozzle to ensure rapid set of wet sprayed concrete. The accelerator affects the strength development, porosity and transport properties, and hence the durability, of the sprayed concrete. We developed a method to cast samples with varying set accelerator doses to measure the effect of the accelerator on porosity, but with a constant effective water/binder ratio of 0.45 for each accelerator dose. Six cylinders of concrete were cast with set accelerator doses of 0, 2, 4, 6, 8 and 10 % of effective binder mass. High workability was achieved to enable mixing before rapid stiffening occurred, though this high workability led to some aggregate settlement in the cylinders. Porosity was measured by capillary suction on dried specimens of hardened concrete and subsequent pressure saturation of macro pores (PF test). The samples cast with higher doses of set accelerator had higher suction porosities and higher rates of capillary suction. Using a modified Powers equation gave very low calculated degree of hydration values for concrete with set accelerator, indicating that the equation is not applicable for concrete with set accelerators, due to the higher suction porosity in accelerated matrices, caused by different hydration products.Effect of Set Accelerator on Capillary Suction and Porosity of Concrete – Cast Samples with Constant Water/Binder RatiopublishedVersio