Showing papers on "Air entrainment published in 1993"

Properties of polypropylene fiber reinforced concrete

Abstract: An experimental research investigation of the fresh and hardened material properties of the fibrillated polypropylene fiber reinforced concrete is reported. Fiber lengths were 1/2 and 1/4 inch, and volume fractions were 0.1, 0.3, and 0.5%. Fiber effects on concrete properties were assessed. Properties studied were slump, inverted slump cone time, air content, compressive and flexural behaviors, impact resistance and rapid chloride permeability, and volume percent of permeable voids. An innovative method of characterizing the flexural behavior of fibrillated polypropylene fiber concrete was proposed. The new method was dependent on the post-peak flexural resistance of concrete. For impact resistance and flexural behavior, it was concluded that 1/4-inch-long fibers were more effective than 1/2-inch-long fibers for volumes of 0.3% or less, while 1/2-inch-long fibers were more effective for 0.5% volume.

Stepped Spillway Flows and Air Entrainment

Abstract: Stepped spillways have become a popular method for handling flood releases. The steps significantly increase the rate of energy dissipation taking place on the spillway face and reduce the size of ...

Linear Eddy Modeling of Entrainment and Mixing in Stratus Clouds

Abstract: Mixing of entrained air in stratus clouds is an important but poorly understood process It is a crucial ingredient of cloud-top entrainment instability (CEI) CEI has been proposed as a breakup mechanism for stratus clouds A recently developed model called the linear eddy model was used to simulate mixing of air entrained into stratus clouds The linear eddy approach involves stochastic simulation on a one-dimensional domain with sufficient resolution to include all physically relevant length scales In each realization, molecular diffusion is implemented explicitly, while a sequence of statistically independent “rearrangement events” represents the effect of turbulent eddies Inertial range scaling is incorporated The linear eddy model was used to simulate the mixing of one or more wisps of entrained air with a specified volume of cloud-topped boundary layer (CTBL) air The volume was idealized to be a horizontal slab of fluid that travels from the top of the CTBL down to the surface in the d

Mean spacing of air voids in hardened concrete

Abstract: In this study geometric probability concepts and stereological principles are used to derive an equation for the mean spacing of air voids in hardened concrete The equation is presented and it is shown that the input paramerers for the equation are the same as those obtained from the standard air-void system analysis of hardened concrete The equation is valid for all values of paste-to-air ratio The mean spacing yields a better estimate of the actual spacing of the air voids in hardened concrete than the standard spacing factor

Dispersant, setting retarder and air entrainment additive for cement

Abstract: The invention is a cement, water containing composition containing an organic polyglycoside. The composition has at least one of an increased air entraining capacity, a reduced viscosity and a retarded setting time.

Correlation Between Strength of Small- and Standard-Size Concrete Cylinders

Abstract: The paper presents an analysis of the influence of specimen size on the measured compressive strengths of plain and fly ash concretes exposed to standard and cold-curing. Strength tests were performed on 3 classes of air entrained concrete using two subbituminous fly ashes at three replacement levels. Analysis of the present experimental results, combined with other results in the literature shows that the compressive strength indicated by 75 mm cylinders is statistically identical to that indicated by 150 mm cylinders. Analysis suggests that this one-to-one relationship may be valid for concrete strengths up to 50 MPa. Further analysis of published data leads to the conclusion that the equality of strength among specimen sizes also includes 100 mm diameter cylinders. The type of mold used to cast test cylinders, whether carboard, steel or tin does not have a significant effect on the trends observed.

An Experimental Study of Air Entrainment by Breaking Waves.

Abstract: : This work reports on a series of laboratory and field experiments on the measurement of air entrainment by breaking waves. The first part of this thesis addresses the measurement of high volumetric concentrations of air (0.3% to 100% void-fraction) found immediately beneath breaking waves. Instrumentation based on the change of electrical impedance of the bubbly mixture is developed. Laboratory and field measurements are conducted. Maps of the evolution of the void-fraction distribution for various size breaking waves yield several bulk characteristics of the air entrainment process. In particular, up to 95% of the initially entrained air volume is lost in the first wave period after breaking and up to 50% of the energy dissipated by breaking is found to be expended in entraining bubbles against their buoyancy. The second part addresses the measurement of very low void-fractions. Instrumentation based on the propagation velocity of low-frequency acoustic pulses is developed. Simultaneous measurements of the sound-speed at several depths are conducted during two field experiments. Time-series of sound-speed and attenuation show dramatic fluctuations over time periods on the order of minutes or less which are attributed to bubble clouds. The time-averaged sound-speed profile is found to have lower velocities and to be shallower than previously reported. (MM)

Bubbles, Noise and Breaking Waves: A Review of Laboratory Experiments

Abstract: We review a series of laboratory experiments on the sound production and air entrainment by breaking waves. The acoustic radiation and microwave backscatter from individual breaking events are found to correlate with important kinematic and dynamic properties of the breaking waves. Air entrainment measurements using a conductivity technique show that the bubble plume characteristics evolve as simple functions of time. The void-fraction for time scales of the order of a surface wave period is found to be several orders of magnitude greater than time-averaged values reported from the field. A large fraction (30% to 50%) of the surface wave energy dissipated is found to be expended in entraining air against buoyancy forces. A simple model of the sound generated by breaking is shown to reproduce the sound spectrum from gently breaking waves measured by Medwin & Daniel (1990).

Polymer concrete electrical insulator and method and apparatus for making

Abstract: An improved electrical insulator for high voltage transmission lines is formed from certain polymer-based concrete compositions An apparatus provides for the molding of the polymer-based mixture to form a highly degassed product The method and apparatus include the ability to combine the constituents of the mixture in a mixer A vacuum is applied to the mixture to degas the mixture A mold is filled with the degassed mixture without introducing air thereinto in the absence of a vacuum The resulting insulator has highly reduced air entrainment

The Influence of Clear Air Entrainment on the Droplet Effective Radius of Warm Maritime Convective Clouds

Abstract: The influence of clear air entrainment on the droplet effective radius of cloudy air parcels is investigated theoretically and experimentally by using data collected in 16 warm maritime tropical cumuli during the Joint Hawaii Warm Rain Project (1985). The theoretical study consists of calculations of the droplet spectrum, droplet effective radius, and liquid water content performed by an entraining cloud parcel model for different entrainment-mixing scenarios. The numerical simulation results are interpreted by means of an analytic equation of the droplet effective radius expressed as a function of both the liquid water content and the droplet concentration. In the experimental study, the behavior of the effective radius is examined at all scales as a function of the liquid water content, used as a dilution degree indicator. At a given cloud level, in the absence of secondary droplet activation, the effective radius of the droplet spectrum of small-scale parcels (10-Hz data) is roughly independen...

Droplet activation as related to entrainment and mixing in warm tropical maritime clouds

Abstract: Instrutnented aircraft 10-Hz data collected during the Joint Hawaii Warm Rain Project (1985) have been used to investigate CCN activation in clouds at levels far from cloud base and to infer the relation with clear air entrainment and mixing. The data analysis methodology is illustrated in a case study of a warm cumulus cloud. In this cloud, activation is observed at all levels. It occurs in parcels that have been previously diluted through clear air entrainment and have low or intermediate positive vertical velocities. Further, the activation sites are located near the interfaces separating ascending cloudy air regions from descending ones. These regions appear to have droplet concentration nonuniformities at scales below the measurement one. Both observations and calculations support the hypothesis that important activation occurs in ascending droplet-free parcels.

Air-entrained roller compacted concrete

Abstract: This article describes the Tennessee Valley Authority's first use of air-entrained roller compacted concrete (RCC). The effective air-entrainment into RCC, and the effective use of admixtures in RCC are discussed. The results of laboratory investigations and trial mixes are presented. Construction, and the problems encountered on site are discussed, as well as the control of air contents, the extraction cores, loss in air content in placement, and limited thermal cracking. It was found that the entrainment of air in RCC appeared to be limited to workable mixtures that will consolidate under vibration within 30 seconds without the application of a surcharge weight. This and other conclusions drawn from the study are presented.

Analysis of air entrainment by secondary fibers. I: Entrained air in coated recycled boxboard mill

Abstract: Entrained air was measured in the furnish of a mill producing a multi-ply coated boxboard from secondary fibers. Very high levels of air were found throughout the stock-preparation areas for the filler ply, bottom liner and top liner as well as in the headbox furnishes for the bottom liner and filler ply. The top-liner headbox contained the lowest levels of entrained air. Entrained-air levels in successive process steps showed no correlation. Moreover, the amount of entrained air in the headbox showed no correlation with the amount of air introduced during stock preparation.

Parameterization of the cloud optical thickness: Influence of clear air entrainment

Abstract: The effects of clear air entrainment and mixing on the cloud optical thickness are investigated. Three different approximate expressions of the cloud optical thickness are derived according to the type of mixing and the presence or absence of secondary droplet activation. They depend upon the liquid water path as well as upon the optical thickness and liquid water path of the corresponding adiabatic cloud. The influence of the cloud droplet concentration on the cloud optical thickness values is shown. These expressions are used to deduce a parameterization of the optical thickness, that is further validated with experimental data

Physical modeling of water sprays—comparison against field operations

Abstract: This paper discusses the procedure developed for simulating water sprays at a reduced scale (i.e., physical modeling) and, more impotantly, compares the model spray characterstics to corresponding full scale spray characteristics. An ideal physically modeled spray would simulate actual spray momentum, water volume flow rate, air entrainment rate, discharge water pressure, spray angle, spray pattern and drop size distribution. Full scale measurements or estimates of air discharge velocity, momentum, entrainment rate and drop size distributions were obtained for two types of sprays at two discharge pressures, various spray angles and at a 500 gpm flow rate. Physical modeling was conducted to simulate the full scale nozzles at a 1:50 scale reduction using commercially available spray nozzles. Measurements of spray momentum, volume flow rate and air entrainment rate were obtined for a subset of spray angles used in the full scale testing. Particle size distribution was not measured since it was considered of secondary importance in the simulation. The results of the study showed that model and full scale momentum, volume flow rate and air entrainment rates compared well with corresponding full scale observations (i.e., within 10 percent for momentum, 20 percent for volume flow rate and 25 percent for air entrainment rate). The results indicate that the method used for simulating water sprays is valid and that the model sprays adequately simulate the mechanical action of full scale sprays on vapor clouds. The results provide evidence that physical modeling can be used to evaluate the effectiveness of water sprays as a mitigation technique.

Flow resistance in high velocity supercritical flows

Abstract: On large chutes and spillways air is entrained through the free surface. When the air reaches the shear layers close to the invert the air bubbles interact with the turbulence mechanisms and induce drag reduction. This reduction of the friction factor was first described by WOOD (1983) using model data. In this paper a large number of prototype and model data are re-analyzed and confirm the drag reduction. A correlation between the reduction of friction factor and the quantity of air entrained is presented. The author suggests that the mechanisms of drag reduction are associated with the presence of rigid air bubbles next to the invert and linked with the air concentration boundary layer.

Plunge-Pool Aeration Due to Inclined Jets

Abstract: The present study quantifies how jet height and nozzle inclination angle and diameter affect the rate of air entrained by inclined circular jets. An air-collection hood and a vacuum pump were used to measure the amount of air entrainment. In high-velocity jets, the jet tends to break up and entrain air along the jet's path, as well as, at the point of impact with the pool. On the other hand, for low-velocity jets most of the air entrainment takes place at the point of impact with the water surface and the jet surface is relatively smooth along the jet's path. Expressions quantifying the air entrainment rate were obtained for both the low-velocity and the high-velocity jets.

Environmental Impact of Large Water Releases in Chutes: Oxygen and Nitrogen Contents due to Self-Aeration

Abstract: During storm and flood events, large water bodies are released in storm waterways and spillways, and induce supercritical flows with air entrainment at the free surface. This self-aeration contributes to the air-water transfer of oxygen and nitrogen since air bubbles within the flow increases substantially the air-water interface area. This paper develops a method to estimate accurately the air-water surface area in self-aerated flows as a function of the hydraulic flow properties. The results are applied to channels of constant slopes, and show that the air-water interface area and the aeration efficiency are primarily function of the discharge per unit width.

Pilot study on the effect of changes in total aggregate gradation on portland cement concrete performance. final report

Abstract: The primary objective of this study was to initially quantify the effect of changes in total aggregate gradation on concrete performance. The effect of total gradation was studied by monitoring for different mixes: ease of placement; unit weight of fresh PCC; change in w/c at constant slump; change in slump for constant w/c ratio; compressive strength; shrinkage; and possible segregation under vibration. These effects were assessed for bridge deck mix and pavement mix by field and laboratory investigation. Briefly, it was found that optimized gradations can result in the following: modest increases in compressive strength (10 to 20%); air content can be achieved with reduced amounts of air entraining agent (20 to 30%); potentially reduced water demand (up to 15%) as indicated by field results; potentially higher spacing factors in the air void system of hardened concrete; and reduced segregation and higher density surfaces following extended vibration (from 1 to 3 minutes). Some mixed findings and uncertainty continue to cloud the picture, however. Further testing is needed to confirm the trends observed and to establish the effect of gradation optimization on concrete freeze-thaw durability.

Mechanism of Air Entrainment in Spillway Aerators

Abstract: The mechanism of air entrainment in the Nappe over spillway aerators was investigated Turbulence was identified as the main agent for aeration It was found that Nappe underpressure influences the air-water behavior over the aerators in different ways

Analysis of initial concrete testing in relation to expected performance.

Abstract: : Portland cement concrete is unique among most construction materials in that its properties change substantially from when delivered to the job site to when the final produce made from the concrete is put to use. When delivered, the concrete must exhibit uniformity and workability sufficient to place and consolidate the fresh concrete. After sufficient curing the concrete must have strength and durability. Quality control testing must therefore consider both fresh and hardened properties to evaluate the acceptability of the concrete for its intended application. Standard quality control testing performed at the job site includes slump, air content, unit weight and yield. The information derived from the tests is believed to provide inspectors with an initial indication of the expected short and long term performance of the concrete. The purpose of this research was to investigate the policies and procedures of various agencies for quality control testing at the job site and to determine the influence of one of many important parameters affecting portland concrete, final aggregate gradation, on the test results. Results of this investigation were used to make an assessment of the adequacy of current job site procedures and to develop recommendations for improvement and identify areas requiring further research effort.

Numerical simulation of charge transport in ionographic printing

Abstract: The utilization of air entrainment for transport and control of charge deposition in ionographic printing is investigated numerically. The geometries considered can be thought of as a charge generation region followed by an air entrainment region followed by an ion projection region. The air entrainment region provides a zone in the ion flow path where moderate electrode voltages can be applied to achieve stable and accurate control and modulation of the ion current levels. Results of the numerical simulations showing effects of air flow and modulation voltage on ion trajectories are presented.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.