Showing papers on "Air entrainment published in 2016"

Effect of hydrophobic surface treatment on freeze-thaw durability of concrete

Abstract: The effect of surface treatment using silanes on the frost durability is investigated on both laboratory and field specimens in an accelerated laboratory test. Measurements include moisture uptake during the pre-saturation and F-T stages, cumulative mass loss and internal bulk cracking under frost salt/water exposure. It is found silane treatment substantially reduces surface scaling, but cannot prevent bulk moisture uptake or the occurrence of the internal frost damage when concrete is insufficiently air entrained. Salt scaling is dominated by the capillary suction process in the thin surface region under freezing which can be curtailed by the pore lining effect from silanes creating a hydrophobic barrier to the ingress of external liquid. This in turn suppresses ice growth in the surface region, evidenced by the complete elimination of sub-freezing dilation in a length-change measurement of small-scale concrete specimens with surface treatment. However, internal frost damage is controlled by the universal degree of pore saturation which in turn is dependent on the bulk moisture uptake.

Air-void structure, strength, and permeability of wet-mix shotcrete before and after shotcreting operation: The influences of silica fume and air-entraining agent

Abstract: It is well known that the air-void structure of hardened concrete has substantial effects on the mechanical properties and durability of concrete. In this study, laboratory evaluations were conducted to quantify the effects of air-entraining agent (AEA) and silica fume on the air-void characteristics of wet-mix shotcrete (WMS) before and after shotcreting process. For this purpose, a high-resolution image analyzer capturing elaborate graphical layouts of air-void structure using the linear transverse method was employed. Also, this study examined the effects of air-void characteristics, such as air content and spacing factor, on the strength and permeability of WMS. Based on the findings of this study, it can be concluded that: (1) shotcreting process considerably reduces overall air contents in WMS; (2) incorporating AEA with a 4.5% silica fume replacement ensures both satisfactory spacing factor and good retention of small entrained air bubbles even after shotcreting, which may improve the freeze-thaw and scaling resistance; (3) the compressive and flexural strengths of WMS were reduced as the air content increased and average spacing factor decreased; and (4) the air content affected the permeability of WMS, but no consistent correlation was found between spacing factor and permeability.

Universal mechanism for air entrainment during liquid impact

Abstract: When a millimetre-sized liquid drop approaches a deep liquid pool, both the interface of the drop and the pool deform before the drop touches the pool. The build-up of air pressure prior to coalescence is responsible for this deformation. Due to this deformation, air can be entrained at the bottom of the drop during the impact. We quantify the amount of entrained air numerically, using the boundary integral method for potential flow for the drop and the pool, coupled to viscous lubrication theory for the air film that has to be squeezed out during impact. We compare our results with various experimental data and find excellent agreement for the amount of air that is entrapped during impact onto a pool. Next, the impact of a rigid sphere onto a pool is numerically investigated and the air that is entrapped in this case also matches with available experimental data. In both cases of drop and sphere impact onto a pool the numerical air bubble volume Vb is found to be in agreement with the theoretical scaling Vb=Vdrop=sphere St

Air Entrainment in a Tall Plunging Flow Dropshaft

Abstract: A physical model study was conducted to investigate the mechanisms of air entrainment in a tall plunging dropshaft of 7.7 m in height. The water flow in the dropshaft was observed to break up into small drops of approximately a few millimeters diameter at a drop height of 5 m. The dominant water drops were found to be approximately 2 mm and they fell at an average speed of approximately 6 m/s. An analytical model was developed to predict the amount of air drag and air flow needed on the basis of the momentum transfer from the water drops. The drag force induced a vertical air pressure gradient, and the model prediction compared well with the measurements. The entrained air flow rate increased with the water flow rate, but the ratio of the air to water flow rate decreased from approximately 20 to 5 times when the water flow rate increased from 3.9 to 47.7 L/s. The air pressure was negative at the top of the dropshaft to allow the ambient air to be entrained, and it increased to the atmospheric pr...

Self-similarity and scale effects in physical modelling of hydraulic jump roller dynamics, air entrainment and turbulent scales

Abstract: A physical study of hydraulic jump is often undertaken using down-scaled Froude-similar models with Reynolds numbers much smaller than in prototype (e.g. spillway stilling basins). The potential viscous scale effects may affect a number of physical processes including turbulence development and air entrainment, thus challenging the extrapolation of laboratory data to the prediction of prototype conditions or justification of numerical modelling. This paper presents an experimental study of hydraulic jumps with a particular focus on the scale effects in terms of free-surface fluctuation and deformation, bubble advection and diffusion, bubble-turbulence interaction and turbulence dissipation. A broad range of free-surface, air–water flow and turbulence properties were measured systematically for Froude numbers from 3.8 to 10 and Reynolds numbers from 2.1 × 104 to 1.6 × 105. Based upon self-similarities in the longitudinal evolution of a number of characteristic flow properties, the analytical expressions of time-averaged roller surface profile, void fraction distribution and longitudinal velocity distribution were derived for given Froude number. The roller surface dynamics were found free of scale effects in terms of fluctuation amplitudes but the characteristic frequencies were scale-sensitive. While some air–water flow parameters such as bubble count rate, bubble chord time distribution and bubble grouping behaviour could only be correctly quantified at full-scale prototype conditions, the aeration level and turbulent scales might be estimated with satisfactory accuracy for engineering applications given a model Reynolds number no less than 4 × 10 to 6 × 104.

Calibration of an Air Entrainment Model for CFD Spillway Applications

Abstract: Air entrained has become one of the main variables in the study of large spillways performance since it can help avoiding cavitation. Moreover, high rates of air concentration produce significant bulking of the flow as well as a water–solid friction reduction, generating flow acceleration and increasing maximum velocities at the inlet of the energy dissipation structure. Air entrained also affects turbulence inside the flow producing different energy dissipation rates. Aerated spillways physical models are affected by scale effects, with Weber and Reynolds numbers being usually too low to adequately reproduce observed flows. Alternatively, simulation of air–water flows can be carried out by means of Computational Fluid Dynamics techniques in 1:1 scale. However, 3D numerical simulations of spillway flows are time expensive and air–water interfaces need fine resolution meshes which would require extensive computing. Thus, the use of a subgrid scale in air entrainment models can be useful to predict the inception point and the air concentration profile of the flow along the spillway. Computational techniques can handle a more accurate momentum distribution over the spillway sections with affordable costs. In this research, FLOW-3D® routine for turbulent air entrainment is used, coupled with variable density evaluation. VOF and κ-e RNG turbulence model are also employed. Over 200 spillway flow simulations have been carried out to obtain optimal values of the air-entrainment model parameters, which can be used in future spillway simulations. The calibration of the model is carried out employing prototype data. Interesting conclusions are obtained concerning air entrainment model performance.

Air entraining agents

Abstract: Entrained air modifies the rheology of concrete and its durability to freezing and thawing cycles. An air-entrained network is characterized by the total volume of entrained air expressed, the diameter of the bubbles, the spacing factor (which represents the average half-distance between two adjacent air bubbles), and the average surface volume. Like cement particles, entrained air bubbles have a diameter varying from 1 to 100 μm; on the contrary, coarse-entrapped air bubbles have a diameter similar to that of sand particles. Parameters of formulation, cement type and fineness, concrete consistency, superplasticizer dosage, presence of supplementary cementitious materials, use of other types of admixtures, use of fibers, mixing, and transportation techniques influence air entrainment. Entrained air bubbles must remain stable during transportation, placing, pumping, and finishing. Usually, it is not difficult to fix the formulation of a mix by trial and error and produce a network of entrained air bubbles that satisfy design requirements.

Hydraulic performance of stepped spillway aerators and related downstream flow features

Abstract: To protect spillways against cavitation damage, adding a small air concentration to the flow close to the invert is efficient. Aerator performance on smooth chutes was therefore well studied in terms of air entrainment and downstream air concentration development. Since bottom aerators are built upstream of regions exposed to cavitation, no damages have been observed on spillways. The introduction of roller compacted concrete (RCC) dams in the 1980s promoted the use of stepped spillways rarely used until then. Compared to conventional smooth spillways, they have the advantage of a higher energy dissipation rate, and of a self-aeration point located higher upstream. However, the non-aerated flow upstream of the inception point is exposed to an increased cavitation risk due to flow separation on the steps. Until recently, the uncertainty about the conditions required for cavitation inception motivated conservative unit discharges. Today, the cavitation potential on stepped spillways is better known and is significant, so that techniques are necessary to safely use stepped spillways under increased unit discharges. This research includes a physical model investigation of a deflector aerator on a stepped spillway. The key parameters influencing aerator performance and stepped spillway flow are systematically varied. The air concentration downstream of the aerator is measured at regularly spaced profiles by means of a fiber optical probe. The flow field downstream of the stepped spillway aerator can be described in three main zones: (i) thejet zone where air is entrained on the lower and upper surfaces, (ii) the spray and reattachment zone where spray is produced by the jet impact and where there are rapid variations of the average and bottom air concentrations, and (iii) the far-field zone where the flow depth as well as the average and bottom air concentrations gradually tend towards quasi-uniform conditions. The lower and upper surfaces of the jet issued by the deflector were considered to derive the effective takeoff angles. With the takeoff velocity, it allows to describe the lower and upper jet surfaces with projectile motion trajectories. The maximum jet elevation, the jet length, and the jet impact angle on the pseudo-bottom can then be determined. Similarly to smooth spillways, the air entrainment coefficient of the aerator is described as a function of the relative jet length. Besides, a relation for the air entrainment coefficient in function of the Froude number and the deflector geometry is presented. The average and bottom air concentration developments show a minimum shortly after the jet impact, followed by a maximum in the spray zone. These extrema are quantified and are related to the relative jet length. In the far-field zone, unlike smooth chutes, no continuous detrainment is observed for the bottom air concentration. Both the average and the bottom air concentrations gradually converge to quasi-uniform flow values. Tests with an increased approach flow bottom roughness showed a large increase of air entrainment due to the higher flow turbulence, but only small average and bottom air concentration differences downstream of the jet impact result. A pre-aerated approach flow leads to slightly higher average and bottom air concentrations downstream of the aerator. The design of a stepped spillway aerator is presented in the end to summarize the results obtained and their practical application.

Entrained air in concrete: Rheology and freezing resistance

Abstract: Any concrete should contain some entrained air in order to improve its rheology, as well as to decrease the risks of bleeding and segregation. When concrete is submitted to freezing and thawing cycles, the spacing factor of the bubble network should be a concern. Of course, when adding some air to an ordinary concrete, its compressive strength decreases, as does the amount of mixing water due to the lubrication action of the bubbles. Therefore, for a given workability, air-entrained concrete has a lower water–cement ratio (w/c) than a corresponding non-air-entrained concrete, which means that it will be more durable. A good network of entrained air bubbles with a low spacing factor has been found to make concrete freeze–thaw resistant, even in the case of low w/c concretes. When freeze–thaw cycles occur with deicing salts, the specification on the spacing factor has to be smaller.

Use of Air Circulation Pipes in Deep Dropshafts for Reducing Air Induction into Sanitary Sewers

Abstract: Falling water in dropshafts can induce large amounts of air into the airspace of sewers, and the subsequent release of the pressurized air can cause sewer odor concerns. The construction of a vertical airshaft that is connected to a dropshaft via pipes is expected to circulate the air and reduce air induction into sewers. In this study, two deep dropshafts with drop heights of 24 and 11 m were retrofitted sequentially with air circulation pipes in a sanitary sewer system. Air pressure inside the sewer line and air flow rates inside the air circulation pipes were monitored in 2006–2011. The results show that the retrofits reduced the manhole air pressure by about 10–47%. Air pressures in the manholes and the dropshafts, as well as air flow rates in the air circulation pipes, were noticed to have diurnal patterns, which appeared to relate to wastewater discharge and air pipe elevation. Overall, the ratios of air flow rate to water flow rate were found to be 160±78 and 144±113 for the two dropshafts,...

Numerical simulation of air entrainment and suppression in pump sump

Abstract: The dynamics of air entrainment and suppression schemes in a pump sump are investigated. Four different turbulence models (standard k-e model, realizable k-e model, renormalization group (RNG) k-e model and shear-stress transport (SST) k-e model) and the volume of fluid (VOF) multiphase model are employed to simulate the three-dimensional unsteady turbulent flow in a pump sump. The dynamic processes of air entrainment are simulated under conditions of relatively high discharge and low submergence; the mechanism of air entrainment is discussed in detail. Then suppression means for air entrainment is adopted by placing a circular plate on the intake pipe at three different heights. The results show: the position and structure of the free-surface vortices, sidewall-attached vortices, back wall-attached vortices, and floor-attached vortices calculated by SST k-e turbulence model agree well with the experimental data. The two main contributors for air entrainment are pressure difference and vortex strength. By placing a circular plate in the middle of the intake pipe under water, air entrainment is suppressed because vortex strength is reduced.

Mechanical Behavior of Concrete, Made with Micro-Nano Air Bubbles

Abstract: Nano materials have been widely used in laboratory and industrial scales in order to improve various properties of concrete and concrete mixture. The mainstream practice of the researches in this field is to add metallic nano-particles into the concrete mixture. The present research focuses on adding Micro-Nano Air Bubbles (MNAB) into water before mixing it with aggregate and cement mixtures. It studies the compressive and tensile strength as well as other engineering properties of the concrete such as the initial and final setting time and the variation in temperature during the setting. The ratio of water/cement was 0.6 with three specimens, prepared for each mixed design to ensure the data quality. Results showed that MNAB-made concrete had 19% higher compression and 16% tensile strength, while the initial and final setting times were significantly shorter (approximately a half) and hydration temperature was notably lower than ordinary concrete.

Studies of Two-Phase Flow at a Chute Aerator with Experiments and CFD Modelling

Abstract: The chute aerator of a spillway is a structure in such a sense that air is, in the intense emulsification, entrained into the high-velocity water flow. Correctly predicting the air entrainment and two-phase flow pattern at the aerator would contribute to reliable spillway operation. Based on experimental data, 2D numerical simulations are preformed to predict streamwise air concentrations in the aerated flow, in which a two-fluid model is used. Depending on the air bubble size, relatively good agreement is seen with the experiments in the air cavity zone. The simulations give rise to higher air concentration downstream of the cavity, which is presumably due to underestimation of the interfacial forces in the two-fluid model.

Influence of Freeze-Thaw Damage on the Steel Corrosion and Bond-Slip Behavior in the Reinforced Concrete

Abstract: This paper mainly studies the behavior of steel corrosion in various reinforced concrete under freeze-thaw environment. The influence of thickness of concrete cover is also discussed. Additionally, the bond-slip behavior of the reinforced concrete after suffering the freeze-thaw damage and steel corrosion has also be presented. The results show that the freeze-thaw damage aggravates the steel corrosion in concrete, and the results become more obvious in the concrete after suffering serious freeze-thaw damage. Compared with the ordinary concrete, both air entrained concrete and waterproofing concrete possess better resistance to steel corrosion under the same freeze-thaw environment. Moreover, increasing the thicknesses of concrete cover is also an effective method of improving the resistance to steel corrosion. The bond-slip behavior of reinforced concrete with corroded steel decreases with the increase of freeze-thaw damage, especially for the concrete that suffered high freeze-thaw cycles. Moreover, there exists a good correlation between the parameters of bond-slip and freeze-thaw cycles. The steel corrosion and bond-slip behavior of reinforced concrete should be considered serious under freeze-thaw cycles environment, which significantly impact the durability and safety of concrete structure.

Cellular Concrete & its facets of application in Civil Engineering

Abstract: Cellular Concrete is a cementitious paste of neat cement or cement & fine sand with a multitude of micro / macroscopic discrete air cells uniformly distributed throughout the mixture to create a lightweight concrete. The concrete is manufactured in two methods viz. First by mixing a pre- formed foam (surfactant) or mix-foaming agents mixture into the cement & water slurry with the hardening of concrete, the bubbles disintegrate leaving air voids of similar sizes. Second designated as Autoclaved Aerated Concrete (AAC), consisting of a mix of lime, sand, cement, water & an expansion agent. The bubble is made by adding expansion agents (aluminum powder or hydrogen peroxide) to the mix during the mixing process. This creates a chemical reaction that generates gas, either as hydrogen or as oxygen to form a gas-bubble structure within the concrete to be molded. Each mold is filled to one- half of its depth with the slurry. The gasification process begins & the mixture expands to fill the mold above the top similar to baking a cake. After the initial setting, it is cured under high-pressured-steam (180° to 210°C / 356°to 410°F) "autoclaved" for a specific amount of time to produce the final micro / macro-structure. HPCC has is an excellent insulator & significantly reduces the transfer of heat through concrete member. With a low water absorption, high tensile strength, high fire resistance & sound retention, this corrects deficiencies in the sand reducing bleeding. Besides structural forms cellular concrete finds application in as Flowable Fill or Controlled Low Strength Material (CLSM), Low Density Controlled Low Strength Material (LD-CLSM) which is a an engineered backfill material used as an alternative to compacted fill that can make backfill faster, being self- leveling, making total compaction within a few hours of placement. Compressive strengths can be adjusted according to the project requirements. Placing as a permanent material or permitting re-excavation at a later date is the paramount advantage of this material.